Costsaving steels and their manufacturing processes
WANG Guodong LIU Xianghua WU Di
（The State Key Laboratory of Rolling & Automation （Northeastern University） Shenyang 110004 China）
Abstract Resourcesconserving manufacturing is becoming an important tendency in materials production Costsaving steels and their manufacturing processes were explored during which the viewpoint in energy saving and sourcessaving to realize costsaving production was discussed There were about four aspects in detail design and production technology for costsaving steels costsaving production processes costsaving production lifecycle and environmentfriendly production
Keywords Costsaving steels Manufacturing process Environmentfriendly production
China has issued the mediumterm and longterm development planning compendium which is a guiding document for the development of science and technology It has been pointed out that manufacturing process for new generation steels needs to be developed and three major functions need to be realized during the production including raw iron and steel supplying energy and disposing waste transferring To realize costsaving manufacturing processes of new generation steels will promote continuous socioeconomic development of our country
1 Designing and manufacturing process for costsaving steels
Iron and steel materials are widely used for two reasons On the one hand they are inexpensive and in large scale production on the other hand they may possess different strengths in a large range by applying suitable controlled rolling and controlled cooling or the socalled TMCP The chief goal of controlled rolling is to control the state of the austenite while controlled cooling is to control the phase transformation of deformed austenite and to refine ferrite grain or improve the steel performance by controlling the phase proportion to form complex phase structure Intensively utilizing the characteristics of phase transformation in iron and steel materials plays an important role in tapping the materials potential and realizing costsaving manufacturing [1]
11 Development of 400500MPa grade strips
New generation iron and steel materials are developed to save more resources That is to add none or less alloyingmicroalloying elements to produce the steels with high performance For example the strength and toughness of 200MPa grade carbon steel can reach the same level of 400MPa steels by improving and optimizing processing parameters According to the research on static recrystallization of low carbon steel deformed austenite does not recrystallize when the interpass time is less than 5s after hot deformation at the deformation temperature between 850℃and 900℃ Considering the time controlled cooling carried out in the nonrecrystallization zone in several minutes after deformation is the key to refine the grains of CMn and low CMn steels Therefore the cooling after rolling （including laminar current cooling device of high density and large flux curling at low temperature） plays an important role in the development of costsaving strip
According to the thoughts above by optimizing rolling schedules checking the rolling load lowering finishing rolling temperature to 800℃ increasing cooling rate to be higher than 30℃s low carbon steels can possess the same overall properties as 400MPa grade steels Its structure analyses indicate that the increasing of strength is due to not only grain refinement but also the formation of bainite Owning to the combined effect of refinement and transformation it reaches the need of customer with the yielding strength exceeding 400MPa the tensile strength exceeding 540MPa elongation percentage exceeding 27 Expensive alloying elements have been saved to substitute microalloy steels and costsaving steel manufacturing process is realized
12The exploitation of costsaving 400－500MPa grade wire
Wire rolling belongs to highspeed rolling process the deformation heat leads to higher temperature for the workpiece the finishing temperature can reach to 1000℃ so low temperature finishing rolling can not be executed as in plate rolling In wire rolling the total reduction is high rolling speed is high direction of deformation varies continuously and interpass time is short Such a continuous large deformation is prone to realize strain accumulation and leads to hardening the austenite
In allusion to the feature of wire rolling in order to realize deformation hardening a large strain continuous accumulation deformation is used in wire rolling crosswater quick cooling begins immediately after rolling and water spray cooling is used in order to get high cooling rate and necessary final cooling temperature to limit recrystallization of austenite and keep the hardening state of austenite By finishing cooling at a proper temperature and controlling phase transformation process the quenching organization is prevented to occur According to such a technical route by using C022025－Si033042－Mn0708 （mass fraction） as material HRB400MPa finegrained bar wire is produced the stable and largescale industrial production has been realized （Fig1） [3]
Fig1 Route of reduction wire production technology
13The exploitation of costsaving 400－500MPa grade bars
Screw thread steel is a steel product with the largest production in China and has a drastic competing market 400MPa grade hot strip finegrained bar HRB is usually added definite amount of microalloy elements such as Nb V and Ti on the base of 20MnSi which has increased the cost of production and consume plenty of microalloy elements
In order to use HRB335 （viz 20MNSi II grade steel bar） as the basic material the overall standard quantity III grade steel bar （HRB400） is obtained by controlling production process on the basis of no added microalloy element A new technical thought of conventional high temperature continuous rolling + ultrafast cooling after cooling + stop cooling at proper temperature is brought forward for bar super steel production This optimized process system fully utilizes high temperature high speed intensive deforming hardening and phasechange during the course of finegrained austenite cooling to control material performance Not only mechanical property can reach national standards but also welding performance and ageing performance can also satisfy the requirements This technology has been applied to a passel of steel mills and acquires very good effects
14 The exploitation of costsaving pipeline steels
Pipeline steel is the most important steel for energy generation from the end of the 80s on pipeline steel was exploited in China which has reached X80 strength grade and can be supplied as different specifications or varieties such as hot strips medium plates In order to assure the mechanical properties TMCP techniques are employed in pipeline steels exceeding X70 to form acicular ferrite structure granular bainite structure and lower bainite structure according to different requirements On the design of elements Mo is used as alloy elements extensively but the price of Mo is very high so people start to search for new alloy element design and corresponding production technology
Nb Corporation of Brazil exploited new series of microalloy elements by using Nb instead of precious Mo to produce pipeline steels The design of high Nb element series （including Nb about 01） and corresponding HTP （High Temperature Processing） manufacturing techniques [5] were developed with international cooperation from several different national mills and X80 grade pipeline steel has been produced so far
15 Exploit high strength steel for vehicle by transformation strengthen and cooling route control〔AHHS〕
Transformation strengthening mainly results from the refined ferrite to increase yield strength Martensite and bainite hard phases distributed on the matrix of ferrite are mainly used for improving tensile strength which can decrease yield ratio of material The State Key Lab of Rolling and Automation Northeastern University （RAL） establish a set of control cooling experimental facility including ultrafast cooling （UFC〕and accelerating cooling （ACC） as shown in Fig 2 UFC can acquire cooling rate more than 400℃s for a 4 mm thick plate Nbbearing microalloy steel whose chemical constitution is 014C （mass percent） has been used in trials for studying the effects of transformation strengthening
Head UFC
Back UFC
ACC
Mill
4500mm
500mm
500mm
Fig2 Layout of experimental mill and UFC equipment Fig3 Structure and mechanical property of steel
With the decreasing of temperature during the transformation from austenite to ferrite the transformation products are different Therefore we can obtain different transformation products with different performances by controlling the cooling route of transformation To make use of the feature different microstructures and performance of steels are obtained by adjusting UFC and ACC cooling rate of different parts the interpass time on ACC and the temperature of UFC subsequently Fig 3 shows the mechanical property and corresponding phase composition of steels by different cooling processes in the experiment It is obvious that the control of the cooling processes provide a powerful means to obtain different performance of steels and that it can play an important role on the reduction of steels
2 New process of Reduction production
21 DP and TRIP steels produced through TSCR
The stability during rolling and cooling is an important characteristic for TSCR process In other words the temperature and cross dimensions on the line of rolling do not change with time Then the disturbance which is brought about by changing speed rolling in the production of hot strip can be avoided and the process window of rolling and cooling is enlarged without alloying agent during the stable controlling of cooling route An exploratory experiment about CMn DP steel was investigated on the line of CSP in Baotou Iron & Steel Group Co The mechanical properties of the first DP steels which are industrially produced and F+P steels with the same components manufactured by conventional process are shown in Table 1 It can be seen that as compared with the FP steel their yield strength is almost same but the tensile strength of DP steel is obviously improved so yield to tensile ratio is reduced greatly Furthermore the strip can own excellent uniformity of structure and property due to well stability of CSP controlling The mechanical property of tested steel is equivalent to DP540 hot rolling dual phase steel
Table 1 Mechanical properties of the test DP steel and the conventional F+P steel
Test piece number
Type of steel
thickness
width
Rp02
Rm
A50
Rp02 Rm
n
mm
mm
MPa
MP a
1#
Tested DP
60
1500
365
545
31
067
021
2#
Tested DP
50
1500
375
570
29
066
021
3#
Common F+P steel
50
1500
370*
450
43
082
—
22 TSCR short route for producing silicon steel
MnS or AlN is used for the inhibitor in the traditional production process of orientation silicon steel which restrains the growth of disadvantageous orientation recrystallized grains and promotes the growth of advantageous oriented grains In order to form the needed inhibitor traditional process must reheat materials into very high temperature （13501400℃） however which not only consumes more energy but also increases the quantity of emitted CO2 and the difficulty of processing In recent years the new production process of silicon steel has been invented continually
Recently continuous casting process of slabs attained great promotion China has had more than ten short route lines Some foreign factories exploited the new continuous casting and tandem rolling process for producing oriented silicon steels TSCR process is that pouring the molten steel （MnS and AlN naturally completely dissolve in the molten steel） into the mold then the slab is directly put in tunneling furnace for soaking and heating preservation finally the slab is loaded to the mills for rolling Because the billet do not appear the step of ferrite transformation and do not deform before loading to the mills so most of MnS and AlN still maintain the dissolved state which superfinely precipitate during the subsequent finishing rolling These precipitates can be used for inhibitors At the same time taking advantage of NH3 for extracting nitrogen to supply a part of inhibitors under comparatively low temperature （for example 900℃） after the subsequent PR and decarbonization Because the process that TSCR roll orientation silicon steel need no reheat into high temperature which reduces energy consumption greatly and decreases the building difficulty of production and facilities so TSCR is a promising new process for orientation silicon’s costsaving production
23 Flexible production process
Modernized large production should be largescale customization namely the production is largescale at the same time products can satisfy the demand of consumer’s special requriements Therefore how to enhance the production flexibility is very important Changing the condition of rolling and cooling especially controlling the cool path can refine the grain in wide range and influence the microstructure leading to different properties to satisfy different requirements We can take full advantage of it to research the socalled flexible rolling technology The technology just is that using the same material to produce steel with different strengths structure and properties through the controled rolling and cooling Thus these can well simplify the operating of steelsmelting and continuous cast process be good for production’s organization and scheduling and moreover establish the basic for achieving the largescale customization of rolling production
24 The online thermal treatment process for plate
Generally speaking the thermal treatment is operated in offline situation using normalizing furnace or modified furnace therefore it needs reheating Being different from traditional offline thermal treatment JFE FuSan Plate Company installed the online thermal treatment equipment behind leveler which cooperated with the new supercooling facility SuperOLAC （Fig 4） in order to harden and temper the plate and realize the production of costsaving high quality steel plate This equipment applys a few of high frequency electrical sources to heat by inductance coil and could online heat the plate with maximal width 45m When the coil has current it makes the plate occur induced current and emit heat The heat transfer rate is about 105107w㎡ The heat capability of this equipment is equivalent to 100 times of gas heating
Because the online thermal treatment equipment applies the new online treatment process it has some new characteristics as compared with traditional offline thermal treatment including that 1） thermal treatment process is completed at the same step with rolling and cooling process short delivery time and high production 2） by cooperating with SuperOLAC it can control phase transformation and precipitation leading to generate new structure and property and finding a new method to improve the steel’s property
Inductive heater
Fig 4 New production of on –line heat treatment
25 New forming process of higher strength of automotive steels
Fig 5 Process of heat treatment strengthening
The automotive steels are continually developed towards high strength In recent years the strength of AHSS which includes DP TRIP and other steels has already arrived at 1200MPa grade Along with the improvement of strength there are more and more difficulties in forming property production and application of steels especially in shape Therefore work has begun to explore other ways to improve the strength of steels For example in recent years the process of strengthening heat treatment is a rapidly developing new technology [11]Fig 5 shows the main form of strengthening heat treatment They can greatly reduce the difficulty of processing deformation After processing shape production will be in dies or special quenching devices for heat treatment The strength of the material needs to be raised to the level by request Currently Europe and Japan have applied the process of strengthening heat treatment and they produced of components such as roof strengthening pieces of automotive in industrial trails At present the hot forming accessories have been developed and used and the tensile strength of 15002000MPa can be obtained
3 Reductions throughout the life cycle of products
Steel materials are manufactured to be steel products It is to be transported to the users for further processing of final products Then the products are to be waste disposal after being used Throughout the life cycle of steel materials there are a certain number of CO2 pollutants and waste Steel manufactories should take into account that the manufacturing processes meet the reduction and green not only in the producing process but also in the followup processing We had better meet the needs of human civilization for the smallest environmental load The following are a few examples of achieving reductions throughout the entire life cycle of steel materials
31 TRIP steel DP steel used for high strength fastener
The general product process of high strength fastener is shown in Fig6 After annealing and drawing process the hotrolled wires are machine processed （ including upsetting and processing the whorls） Then the thermal leveling is proceeded The stocks are heattreated twice which needs more labor and equipment and increased the cost To solve this problem a new type DP steel used for high strength fastener was developed by CORUS[12] the chemical composition of which is C008Si10Mn17Cr0016 （by weight） Through this the simplifying of process is realized
Rod
rolling
Drawing
Annealing
Head &
Thread
Q & T
Descale
Clean
Plate
Rod
rolling
Drawing
Head &
Thread
Plate
Conventional Route
New Route
Fig6 Process of fastener produce
The RAL in NEU also has developed a TRIP steel used for high strength fastener for which the thermal process is not necessary either
32 LP steel
If the opening of the rollers changes continuously during rolling process to make the thickness of plates change continuously at the length direction the LP steel plates are produced [14] Because the LP steel can be used to optimize the crosssection designs of bridges and ships through changing the thickness of beams according to the load LP steel is a kind of costsaving steel
Figs 7 and fig 8 is samples of LP steel’s applications in the bridges and ships
Fig7 Usage of LP steel in bridges Fig8 Usage of LP steel in ships
33 Materials used in large inner heat welding
Welding is an important property for steel used in ships bridges and vessels The large inner heat wielding that can largely improve the manufacture efficiency is important to these industries The initial idea is that the TiN that has high melting point is used to pin the austenite grain boundary But during the large inner heat wielding the temperature of HAZ can reach 1400℃ At this temperature some TiN particles do not have the abilities to pin the grain boundary because they are solved or agglomerated So these years Mg and Ca which has strong affinity with S and O is added to the molten steel by some researchers abroad to form same heatstable particles that are neither solved nor prone to growing up These particles the sizes of which are tens or hundreds nm are thin and uniform And they are nucleus in the solidify process which can refine the solid textures Furthermore the fine particles can prohibit the grain growth in HAZ during welding [15] The growth of two kinds of steel at 1400℃ which contain TiN or oxide and sulfide of Mg and Ca particles are shown in Fig9 It can be concluded that grains of steel that contain oxide and sulfide particles did not grow and showed good heat endurance
Fig9 Growth of grain at 1400℃
4 Environmentfriendly products
The application of new generational iron and steel materials should meet the requirements of the environmental protection which satisfy the social demands with the least pressure to the environment Then manufacture of environmentfriendly products should be the responsibility for the material processing industries Taking Cr ions and Pb as examples the serious problems that the iron and steel industries are facing and the progress that has been made are explained in this section
41 Galvanized and colorcoated sheets without chrome ions
In order to improve the performance of resistance to white rusting and the painting performance for painted coat steel sheets the various painted coat steel sheets were previously passivated with Cr ions However Cr ions which are included in the electrolytic and products relating to passivation treatment can do harm to the environment during its using process and also after the disposal So a draft about A policy with respond to the vehicle waste〞 in which Cr ions was forbidden was brought forward by European Parliament in 1997 subsequently the use of materials including the above harmful substances was also forbidden in the electronic products according to the law WEEE〞 and RoHS〞 about the electronic and home appliance which will be carried out in 2007 In our country the same strict request was also put forward and legislated at the same time
To improve the formability of galvanized sheet used for vehicle a really thin inorganic MnP lubricating film （100mgm2） is investigated and produced According to the experiment the formability is surely improved with the use of this film which does not do any harm to the process of vehicle production and the corrosion resistant performance is still good after painting ZnNi alloyed coat with a thickness of 4um is developed by Kawasaki Japan Rail system which possesses performances of aestheticformability （selflubrication）corrosion resistance[16]
42 Unleaded steel used for fuel tank
Pb8Sn coated steel sheets are widely used under the conditions of servere corrosion However Pb is apparently forbidden for its harmness and several new environmentfriendly products have been developed for fuel tanks [17] in these years They include 1） hotdip SnZn alloyed steel sheet 2） hotdip AlZn alloyed steel sheet 3） GIdouble Ni plate 4） organic coating steel sheet All of them are widely used for their good inner corrosion resistance outer corrosion resistance good formability and weldability
Reference
[1] Xiaozhijunfu Controlled Rolling and Controlled coolingDevelopment of Rolling Technology for Improving Steel Properties Metallurgical Industry Publishing Company 2002 Beijing
[2] Wang GD et al The industrial rolling trials of SuperSS400Iron and Steel36〔2001〕No53943
[3] Zhao D W et alThe experiments and industrialization of hot rolled super steel wires Iron and Steel40〔2005〕 No12
[4] Wu DiThe Exploitation of Low Cost Highpowered HRB400HRB500 ribbed bar to be published
[5] Klaus Hulka Production Experience of Low Carbon HSLA Steels with Nb Content from 006 to 010 Microalloying Technology Vol4（2004） No4 113
[6] Liu Yanchun and etc Experiment research on Development of High Strength Hot Rolled Multi Phase Strip Steel with Ultrafast Cooling Technology Employed to be published
[7] Le Degang and etc Development on 540MPa Hot Rolled Dual Phase steel on CSP Product Line in Baotou Steel 2006’ International Conference on Thin Slab Casting and Rolling 2006Guangzhou
[8] Klaus Gunther etc Recent Development on Manufacturing Technology of Orientation electric steel World Iron and Steel 2005 No5 18
[9] Liu Xianghua etc Flexible Rolling Technology and the Trend in DevelopmentChinese Conference on Nonferrous Metals Processing 2005 Sanya
[10] FUJIBAYASHI Akio et al JFE Steel’s Advanced Manufacturing Technologies of Leading High Performance Steel Plates JFE TECHNICAL REPORT 2004 No 5 8–12
[11] Nobusato Kojima Strengthening by Heat Treatment for Steel Sheet Parts JSTP Vol46（2005） No534 595599
[12] AJ TROWSDALE & SB PRITCHARD DUAL PHASE STEEL HIGH STRENGTH FASTENERS WITHOUT HEAT TREATMENT CORUS CONSTRUCTION & INDUSTRIAL UK
[13] Li Zhuang etc Research on Heat treatmentfree High Strength Low Yield Ratio Riveted Steel Iron and Steel to be published
[14] Osamu Tanigawa et al Development of High Performance Steel Plates in Terms of Reliability and Economy of Steel Structure Kawasaki Steel Technical Report 32（2000）3 198204
[15] Plate Sales Division Super High HAZ Toughness Technology with Fine Microstructure Imparted by Fine Particles Nippon Steel Technical Report 2002 （376）82
[16] Hiroshi KANAI et al Electro and Hotdip Galvanized Steel Sheets Having Crfree Treatment Layer on the surface and Crfree Prepainted Electrogalvanized Steel Sheets NSC TECHNICAL REPORT No371（1999） 43461
[17] Masao KUROSAKI et al Pbfree Coated Steel Sheets for Fuel Tanks NIPPON STEEL TECHNICAL REPORT No 88 July 2003 5156
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WANG Guodong LIU Xianghua WU Di
（The State Key Laboratory of Rolling & Automation （Northeastern University） Shenyang 110004 China）
Abstract Resourcesconserving manufacturing is becoming an important tendency in materials production Costsaving steels and their manufacturing processes were explored during which the viewpoint in energy saving and sourcessaving to realize costsaving production was discussed There were about four aspects in detail design and production technology for costsaving steels costsaving production processes costsaving production lifecycle and environmentfriendly production
Keywords Costsaving steels Manufacturing process Environmentfriendly production
China has issued the mediumterm and longterm development planning compendium which is a guiding document for the development of science and technology It has been pointed out that manufacturing process for new generation steels needs to be developed and three major functions need to be realized during the production including raw iron and steel supplying energy and disposing waste transferring To realize costsaving manufacturing processes of new generation steels will promote continuous socioeconomic development of our country
1 Designing and manufacturing process for costsaving steels
Iron and steel materials are widely used for two reasons On the one hand they are inexpensive and in large scale production on the other hand they may possess different strengths in a large range by applying suitable controlled rolling and controlled cooling or the socalled TMCP The chief goal of controlled rolling is to control the state of the austenite while controlled cooling is to control the phase transformation of deformed austenite and to refine ferrite grain or improve the steel performance by controlling the phase proportion to form complex phase structure Intensively utilizing the characteristics of phase transformation in iron and steel materials plays an important role in tapping the materials potential and realizing costsaving manufacturing [1]
11 Development of 400500MPa grade strips
New generation iron and steel materials are developed to save more resources That is to add none or less alloyingmicroalloying elements to produce the steels with high performance For example the strength and toughness of 200MPa grade carbon steel can reach the same level of 400MPa steels by improving and optimizing processing parameters According to the research on static recrystallization of low carbon steel deformed austenite does not recrystallize when the interpass time is less than 5s after hot deformation at the deformation temperature between 850℃and 900℃ Considering the time controlled cooling carried out in the nonrecrystallization zone in several minutes after deformation is the key to refine the grains of CMn and low CMn steels Therefore the cooling after rolling （including laminar current cooling device of high density and large flux curling at low temperature） plays an important role in the development of costsaving strip
According to the thoughts above by optimizing rolling schedules checking the rolling load lowering finishing rolling temperature to 800℃ increasing cooling rate to be higher than 30℃s low carbon steels can possess the same overall properties as 400MPa grade steels Its structure analyses indicate that the increasing of strength is due to not only grain refinement but also the formation of bainite Owning to the combined effect of refinement and transformation it reaches the need of customer with the yielding strength exceeding 400MPa the tensile strength exceeding 540MPa elongation percentage exceeding 27 Expensive alloying elements have been saved to substitute microalloy steels and costsaving steel manufacturing process is realized
12The exploitation of costsaving 400－500MPa grade wire
Wire rolling belongs to highspeed rolling process the deformation heat leads to higher temperature for the workpiece the finishing temperature can reach to 1000℃ so low temperature finishing rolling can not be executed as in plate rolling In wire rolling the total reduction is high rolling speed is high direction of deformation varies continuously and interpass time is short Such a continuous large deformation is prone to realize strain accumulation and leads to hardening the austenite
In allusion to the feature of wire rolling in order to realize deformation hardening a large strain continuous accumulation deformation is used in wire rolling crosswater quick cooling begins immediately after rolling and water spray cooling is used in order to get high cooling rate and necessary final cooling temperature to limit recrystallization of austenite and keep the hardening state of austenite By finishing cooling at a proper temperature and controlling phase transformation process the quenching organization is prevented to occur According to such a technical route by using C022025－Si033042－Mn0708 （mass fraction） as material HRB400MPa finegrained bar wire is produced the stable and largescale industrial production has been realized （Fig1） [3]
Fig1 Route of reduction wire production technology
13The exploitation of costsaving 400－500MPa grade bars
Screw thread steel is a steel product with the largest production in China and has a drastic competing market 400MPa grade hot strip finegrained bar HRB is usually added definite amount of microalloy elements such as Nb V and Ti on the base of 20MnSi which has increased the cost of production and consume plenty of microalloy elements
In order to use HRB335 （viz 20MNSi II grade steel bar） as the basic material the overall standard quantity III grade steel bar （HRB400） is obtained by controlling production process on the basis of no added microalloy element A new technical thought of conventional high temperature continuous rolling + ultrafast cooling after cooling + stop cooling at proper temperature is brought forward for bar super steel production This optimized process system fully utilizes high temperature high speed intensive deforming hardening and phasechange during the course of finegrained austenite cooling to control material performance Not only mechanical property can reach national standards but also welding performance and ageing performance can also satisfy the requirements This technology has been applied to a passel of steel mills and acquires very good effects
14 The exploitation of costsaving pipeline steels
Pipeline steel is the most important steel for energy generation from the end of the 80s on pipeline steel was exploited in China which has reached X80 strength grade and can be supplied as different specifications or varieties such as hot strips medium plates In order to assure the mechanical properties TMCP techniques are employed in pipeline steels exceeding X70 to form acicular ferrite structure granular bainite structure and lower bainite structure according to different requirements On the design of elements Mo is used as alloy elements extensively but the price of Mo is very high so people start to search for new alloy element design and corresponding production technology
Nb Corporation of Brazil exploited new series of microalloy elements by using Nb instead of precious Mo to produce pipeline steels The design of high Nb element series （including Nb about 01） and corresponding HTP （High Temperature Processing） manufacturing techniques [5] were developed with international cooperation from several different national mills and X80 grade pipeline steel has been produced so far
15 Exploit high strength steel for vehicle by transformation strengthen and cooling route control〔AHHS〕
Transformation strengthening mainly results from the refined ferrite to increase yield strength Martensite and bainite hard phases distributed on the matrix of ferrite are mainly used for improving tensile strength which can decrease yield ratio of material The State Key Lab of Rolling and Automation Northeastern University （RAL） establish a set of control cooling experimental facility including ultrafast cooling （UFC〕and accelerating cooling （ACC） as shown in Fig 2 UFC can acquire cooling rate more than 400℃s for a 4 mm thick plate Nbbearing microalloy steel whose chemical constitution is 014C （mass percent） has been used in trials for studying the effects of transformation strengthening
Head UFC
Back UFC
ACC
Mill
4500mm
500mm
500mm
Fig2 Layout of experimental mill and UFC equipment Fig3 Structure and mechanical property of steel
With the decreasing of temperature during the transformation from austenite to ferrite the transformation products are different Therefore we can obtain different transformation products with different performances by controlling the cooling route of transformation To make use of the feature different microstructures and performance of steels are obtained by adjusting UFC and ACC cooling rate of different parts the interpass time on ACC and the temperature of UFC subsequently Fig 3 shows the mechanical property and corresponding phase composition of steels by different cooling processes in the experiment It is obvious that the control of the cooling processes provide a powerful means to obtain different performance of steels and that it can play an important role on the reduction of steels
2 New process of Reduction production
21 DP and TRIP steels produced through TSCR
The stability during rolling and cooling is an important characteristic for TSCR process In other words the temperature and cross dimensions on the line of rolling do not change with time Then the disturbance which is brought about by changing speed rolling in the production of hot strip can be avoided and the process window of rolling and cooling is enlarged without alloying agent during the stable controlling of cooling route An exploratory experiment about CMn DP steel was investigated on the line of CSP in Baotou Iron & Steel Group Co The mechanical properties of the first DP steels which are industrially produced and F+P steels with the same components manufactured by conventional process are shown in Table 1 It can be seen that as compared with the FP steel their yield strength is almost same but the tensile strength of DP steel is obviously improved so yield to tensile ratio is reduced greatly Furthermore the strip can own excellent uniformity of structure and property due to well stability of CSP controlling The mechanical property of tested steel is equivalent to DP540 hot rolling dual phase steel
Table 1 Mechanical properties of the test DP steel and the conventional F+P steel
Test piece number
Type of steel
thickness
width
Rp02
Rm
A50
Rp02 Rm
n
mm
mm
MPa
MP a
1#
Tested DP
60
1500
365
545
31
067
021
2#
Tested DP
50
1500
375
570
29
066
021
3#
Common F+P steel
50
1500
370*
450
43
082
—
22 TSCR short route for producing silicon steel
MnS or AlN is used for the inhibitor in the traditional production process of orientation silicon steel which restrains the growth of disadvantageous orientation recrystallized grains and promotes the growth of advantageous oriented grains In order to form the needed inhibitor traditional process must reheat materials into very high temperature （13501400℃） however which not only consumes more energy but also increases the quantity of emitted CO2 and the difficulty of processing In recent years the new production process of silicon steel has been invented continually
Recently continuous casting process of slabs attained great promotion China has had more than ten short route lines Some foreign factories exploited the new continuous casting and tandem rolling process for producing oriented silicon steels TSCR process is that pouring the molten steel （MnS and AlN naturally completely dissolve in the molten steel） into the mold then the slab is directly put in tunneling furnace for soaking and heating preservation finally the slab is loaded to the mills for rolling Because the billet do not appear the step of ferrite transformation and do not deform before loading to the mills so most of MnS and AlN still maintain the dissolved state which superfinely precipitate during the subsequent finishing rolling These precipitates can be used for inhibitors At the same time taking advantage of NH3 for extracting nitrogen to supply a part of inhibitors under comparatively low temperature （for example 900℃） after the subsequent PR and decarbonization Because the process that TSCR roll orientation silicon steel need no reheat into high temperature which reduces energy consumption greatly and decreases the building difficulty of production and facilities so TSCR is a promising new process for orientation silicon’s costsaving production
23 Flexible production process
Modernized large production should be largescale customization namely the production is largescale at the same time products can satisfy the demand of consumer’s special requriements Therefore how to enhance the production flexibility is very important Changing the condition of rolling and cooling especially controlling the cool path can refine the grain in wide range and influence the microstructure leading to different properties to satisfy different requirements We can take full advantage of it to research the socalled flexible rolling technology The technology just is that using the same material to produce steel with different strengths structure and properties through the controled rolling and cooling Thus these can well simplify the operating of steelsmelting and continuous cast process be good for production’s organization and scheduling and moreover establish the basic for achieving the largescale customization of rolling production
24 The online thermal treatment process for plate
Generally speaking the thermal treatment is operated in offline situation using normalizing furnace or modified furnace therefore it needs reheating Being different from traditional offline thermal treatment JFE FuSan Plate Company installed the online thermal treatment equipment behind leveler which cooperated with the new supercooling facility SuperOLAC （Fig 4） in order to harden and temper the plate and realize the production of costsaving high quality steel plate This equipment applys a few of high frequency electrical sources to heat by inductance coil and could online heat the plate with maximal width 45m When the coil has current it makes the plate occur induced current and emit heat The heat transfer rate is about 105107w㎡ The heat capability of this equipment is equivalent to 100 times of gas heating
Because the online thermal treatment equipment applies the new online treatment process it has some new characteristics as compared with traditional offline thermal treatment including that 1） thermal treatment process is completed at the same step with rolling and cooling process short delivery time and high production 2） by cooperating with SuperOLAC it can control phase transformation and precipitation leading to generate new structure and property and finding a new method to improve the steel’s property
Inductive heater
Fig 4 New production of on –line heat treatment
25 New forming process of higher strength of automotive steels
Fig 5 Process of heat treatment strengthening
The automotive steels are continually developed towards high strength In recent years the strength of AHSS which includes DP TRIP and other steels has already arrived at 1200MPa grade Along with the improvement of strength there are more and more difficulties in forming property production and application of steels especially in shape Therefore work has begun to explore other ways to improve the strength of steels For example in recent years the process of strengthening heat treatment is a rapidly developing new technology [11]Fig 5 shows the main form of strengthening heat treatment They can greatly reduce the difficulty of processing deformation After processing shape production will be in dies or special quenching devices for heat treatment The strength of the material needs to be raised to the level by request Currently Europe and Japan have applied the process of strengthening heat treatment and they produced of components such as roof strengthening pieces of automotive in industrial trails At present the hot forming accessories have been developed and used and the tensile strength of 15002000MPa can be obtained
3 Reductions throughout the life cycle of products
Steel materials are manufactured to be steel products It is to be transported to the users for further processing of final products Then the products are to be waste disposal after being used Throughout the life cycle of steel materials there are a certain number of CO2 pollutants and waste Steel manufactories should take into account that the manufacturing processes meet the reduction and green not only in the producing process but also in the followup processing We had better meet the needs of human civilization for the smallest environmental load The following are a few examples of achieving reductions throughout the entire life cycle of steel materials
31 TRIP steel DP steel used for high strength fastener
The general product process of high strength fastener is shown in Fig6 After annealing and drawing process the hotrolled wires are machine processed （ including upsetting and processing the whorls） Then the thermal leveling is proceeded The stocks are heattreated twice which needs more labor and equipment and increased the cost To solve this problem a new type DP steel used for high strength fastener was developed by CORUS[12] the chemical composition of which is C008Si10Mn17Cr0016 （by weight） Through this the simplifying of process is realized
Rod
rolling
Drawing
Annealing
Head &
Thread
Q & T
Descale
Clean
Plate
Rod
rolling
Drawing
Head &
Thread
Plate
Conventional Route
New Route
Fig6 Process of fastener produce
The RAL in NEU also has developed a TRIP steel used for high strength fastener for which the thermal process is not necessary either
32 LP steel
If the opening of the rollers changes continuously during rolling process to make the thickness of plates change continuously at the length direction the LP steel plates are produced [14] Because the LP steel can be used to optimize the crosssection designs of bridges and ships through changing the thickness of beams according to the load LP steel is a kind of costsaving steel
Figs 7 and fig 8 is samples of LP steel’s applications in the bridges and ships
Fig7 Usage of LP steel in bridges Fig8 Usage of LP steel in ships
33 Materials used in large inner heat welding
Welding is an important property for steel used in ships bridges and vessels The large inner heat wielding that can largely improve the manufacture efficiency is important to these industries The initial idea is that the TiN that has high melting point is used to pin the austenite grain boundary But during the large inner heat wielding the temperature of HAZ can reach 1400℃ At this temperature some TiN particles do not have the abilities to pin the grain boundary because they are solved or agglomerated So these years Mg and Ca which has strong affinity with S and O is added to the molten steel by some researchers abroad to form same heatstable particles that are neither solved nor prone to growing up These particles the sizes of which are tens or hundreds nm are thin and uniform And they are nucleus in the solidify process which can refine the solid textures Furthermore the fine particles can prohibit the grain growth in HAZ during welding [15] The growth of two kinds of steel at 1400℃ which contain TiN or oxide and sulfide of Mg and Ca particles are shown in Fig9 It can be concluded that grains of steel that contain oxide and sulfide particles did not grow and showed good heat endurance
Fig9 Growth of grain at 1400℃
4 Environmentfriendly products
The application of new generational iron and steel materials should meet the requirements of the environmental protection which satisfy the social demands with the least pressure to the environment Then manufacture of environmentfriendly products should be the responsibility for the material processing industries Taking Cr ions and Pb as examples the serious problems that the iron and steel industries are facing and the progress that has been made are explained in this section
41 Galvanized and colorcoated sheets without chrome ions
In order to improve the performance of resistance to white rusting and the painting performance for painted coat steel sheets the various painted coat steel sheets were previously passivated with Cr ions However Cr ions which are included in the electrolytic and products relating to passivation treatment can do harm to the environment during its using process and also after the disposal So a draft about A policy with respond to the vehicle waste〞 in which Cr ions was forbidden was brought forward by European Parliament in 1997 subsequently the use of materials including the above harmful substances was also forbidden in the electronic products according to the law WEEE〞 and RoHS〞 about the electronic and home appliance which will be carried out in 2007 In our country the same strict request was also put forward and legislated at the same time
To improve the formability of galvanized sheet used for vehicle a really thin inorganic MnP lubricating film （100mgm2） is investigated and produced According to the experiment the formability is surely improved with the use of this film which does not do any harm to the process of vehicle production and the corrosion resistant performance is still good after painting ZnNi alloyed coat with a thickness of 4um is developed by Kawasaki Japan Rail system which possesses performances of aestheticformability （selflubrication）corrosion resistance[16]
42 Unleaded steel used for fuel tank
Pb8Sn coated steel sheets are widely used under the conditions of servere corrosion However Pb is apparently forbidden for its harmness and several new environmentfriendly products have been developed for fuel tanks [17] in these years They include 1） hotdip SnZn alloyed steel sheet 2） hotdip AlZn alloyed steel sheet 3） GIdouble Ni plate 4） organic coating steel sheet All of them are widely used for their good inner corrosion resistance outer corrosion resistance good formability and weldability
Reference
[1] Xiaozhijunfu Controlled Rolling and Controlled coolingDevelopment of Rolling Technology for Improving Steel Properties Metallurgical Industry Publishing Company 2002 Beijing
[2] Wang GD et al The industrial rolling trials of SuperSS400Iron and Steel36〔2001〕No53943
[3] Zhao D W et alThe experiments and industrialization of hot rolled super steel wires Iron and Steel40〔2005〕 No12
[4] Wu DiThe Exploitation of Low Cost Highpowered HRB400HRB500 ribbed bar to be published
[5] Klaus Hulka Production Experience of Low Carbon HSLA Steels with Nb Content from 006 to 010 Microalloying Technology Vol4（2004） No4 113
[6] Liu Yanchun and etc Experiment research on Development of High Strength Hot Rolled Multi Phase Strip Steel with Ultrafast Cooling Technology Employed to be published
[7] Le Degang and etc Development on 540MPa Hot Rolled Dual Phase steel on CSP Product Line in Baotou Steel 2006’ International Conference on Thin Slab Casting and Rolling 2006Guangzhou
[8] Klaus Gunther etc Recent Development on Manufacturing Technology of Orientation electric steel World Iron and Steel 2005 No5 18
[9] Liu Xianghua etc Flexible Rolling Technology and the Trend in DevelopmentChinese Conference on Nonferrous Metals Processing 2005 Sanya
[10] FUJIBAYASHI Akio et al JFE Steel’s Advanced Manufacturing Technologies of Leading High Performance Steel Plates JFE TECHNICAL REPORT 2004 No 5 8–12
[11] Nobusato Kojima Strengthening by Heat Treatment for Steel Sheet Parts JSTP Vol46（2005） No534 595599
[12] AJ TROWSDALE & SB PRITCHARD DUAL PHASE STEEL HIGH STRENGTH FASTENERS WITHOUT HEAT TREATMENT CORUS CONSTRUCTION & INDUSTRIAL UK
[13] Li Zhuang etc Research on Heat treatmentfree High Strength Low Yield Ratio Riveted Steel Iron and Steel to be published
[14] Osamu Tanigawa et al Development of High Performance Steel Plates in Terms of Reliability and Economy of Steel Structure Kawasaki Steel Technical Report 32（2000）3 198204
[15] Plate Sales Division Super High HAZ Toughness Technology with Fine Microstructure Imparted by Fine Particles Nippon Steel Technical Report 2002 （376）82
[16] Hiroshi KANAI et al Electro and Hotdip Galvanized Steel Sheets Having Crfree Treatment Layer on the surface and Crfree Prepainted Electrogalvanized Steel Sheets NSC TECHNICAL REPORT No371（1999） 43461
[17] Masao KUROSAKI et al Pbfree Coated Steel Sheets for Fuel Tanks NIPPON STEEL TECHNICAL REPORT No 88 July 2003 5156
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