US3904446A - Process of making high strength cold rolled steel having excellent bake-hardening properties - Google Patents

Process of making high strength cold rolled steel having excellent bake-hardening properties Download PDF

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US3904446A
US3904446A US487653A US48765374A US3904446A US 3904446 A US3904446 A US 3904446A US 487653 A US487653 A US 487653A US 48765374 A US48765374 A US 48765374A US 3904446 A US3904446 A US 3904446A
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Prior art keywords
temperature
steel
range
strip
reheating
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US487653A
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Kuniki Uchida
Kenzi Araki
Hirosi Narita
Shiro Fukunaka
Takao Kurihara
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JFE Engineering Corp
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Nippon Kokan Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling

Definitions

  • ABSTRACT A high tension steel of which the yield point is more increased by the ensuring heat treatment after pressforming than that of the properties in which the tensile strength is above 40 to Kglmm is made by the following steps;
  • the chemical composition of steel is substantially controlled at the range of 0.04 to 0.12% C and 0.1 to 1.60% Mn,
  • the above steel is hot-rolled with a finishing temperature of more than 800C and a coiling temperature of less than 700C,
  • This invention relates to a process of making an inexpensive high tension cold-reduced steel, and more in particular a process of offering cold-reduced steel excellent in accelerated aging properties wherein remarkable improvement is strength is seen by maintaining the tensile strength of 40 to 80 Kg/mm and particularly, further increasing its yield point by the coming heat treatment after press forming in actual use.
  • the process (1) is defective in its high cost, while the process (2) causes dispersions in the longitudinal direction as the differences in the annealing temperature occur in the inner and the outer peripheries of the coil.
  • the process (3) does improve the stength, but its elongation is not improved proportionately, thus leaving a problem unsolved.
  • the process (4) is suitable for obtaining inexpensive high tension cold-reduced steel and various proposals have been made recently in sucession on said process, namely in Japanese Pat. Nos. 40-3020, 469541 and 469542.
  • the art disclosed in Japanese Pat. No. 40-3020 is well known as a BISRA process (in England).
  • the art heats up the coldreduced strip to 740 to 850C, quenches the strip down to 150 to 250C, coils the same and then performs an overaging treatment through selfannealing of said coil.
  • the steel manufactured by said process has no accelerated aging property, i.e., AA property, caused by the heat treatment such as baking the coat after press forming, which causes an excessive lowering of the strength. Accordingly, it becomes necessary to further increase C content if the strength is to be enhanced.
  • the continuous process is interrupted by the above mentioned self-annealing, consequently decreasing the productivity.
  • the steels made by these processes are defective in that their strength is lowered by about 15 Kg/mm by the coating and its baking after press forming, and that it has a poor ductility compared to its strength. This means that the steel is very difficult to handle since it is hard when being subjected to press forming and then it becomes soft when finished.
  • the present invention was developed specifically to offer a solution comprising a method for overcoming the aforementioned difficulties and it is mainly characterized in that the manufacture is carried out by a con tinuous annealing process.
  • Heating and soaking in this art is selected from among the range of 700 to 900C at which the solution treating of cold-reduced steel is achieved, and the steel is subjected to quenching by a jet stream of water from the above temperature to the room temperature, then said steel is reheated to to 400C and is given the aging treatment at the said latter temperature.
  • This aging treatment is not for completing the precipitation of C in the steel, but for causing partial remaining solution carbon therein.
  • the steel is then cooled down to the room temperature and then coiled.
  • the tensile strength of 40 to 80 Kg/mm is stably obtained.
  • its yield strength is improved by about 7 to 11 Kg/mm in comparison with the initial strength before press-forming.
  • an object of this invention is to provide a process for manufacturing high tension steel sheet imparting safety to vehicles when used therein by improving of the accelerated aging property of said sheet.
  • Another object of this invention is to provide an inexpensive manufacturing process in a continuous annealing furnace of a high tension steel sheet excellent in accelerated aging property without addition of any special element.
  • the FIGURE is a graph showing the variation of accelerated aging property dependent to the pre-heating requirements of travelling strip.
  • the steel to which the present invention may be applied consists of substantially 0.04 to 0.12% C and 0.10
  • any type of furnace for manufacturing steel znown in the art today may be used for this invention 1nd any preferred processes after steel manufacture nay be employed including ingot making blooming, :ontinuous casting, etc. After scarfing the steel is leated up to about 1,250C or more, and is hot rolled [t the finishing temperature of 800C or more and a :oiling temperature of 700C or less. Ordinary cold re lucing process may be employed after pickling. The lnnealing in the ensuing step is a complete continuous mnealing process. In this case, said strip is heated up to to 900C and is maintained for 10 to 120 seconds at said temperature. The heating temperature should )referably be A to 850C and should be controlled rigirously since it will affect the strength of the final prodnot very much.
  • the travelling strip hus quenched down to the room temperature is re ieated up to 150 to 400C, and more preferably to 180 to 300C and is to be maintained for some suitable ength of time.
  • the optimum period of maintaining deaends on the reheating temperature, and more in paricular the reheating temperature selected from among he range of 150 to 180C will take 15 to 300 seconds; [80 to 300C, 4 to 300 sec.
  • TS (kg/mm' E] is used as an index .0 evaluate the balance between the two since the in- :reased stength usually lowers the elongation which 'epresents ductility.
  • the present invention steel thus :reated will bring a remarkable improvement in the ;trength, particularly in the yield point of at least 7 (g/mm when given the heat treatment of 100 to 200C after being pressed and formed into a shape in- .ended. This naturally is attributable to the aging effectss of the solution carbon partially left by the abovenentioned low temperature aging treatment.
  • the present invention process is further elaborated in 'espect of its respective steps.
  • Tensile strength C in steel is an element which plays an important part in the present invention. Its lower limit is set at 0.04 percent in view of the stable operation of the converter used in most steel making stages and of the need to maintain required strength for steel.
  • the upper limit was set at 0.12 percent in view of the press-formability and weldability. But more concretely, the upper limit should be selected from within the above range depending upon the levels of the required strength of steel.
  • the lower limit of Mn content is set at 0.10 percent because of the red shortness.
  • the upper limit thereof was set at 1.60 percent in view of the stable operations for the ingot making. Actually, Mn content is selected from within the range of 0.10 to 1.60 percent as is the case for determination of C content.
  • the hot rolling conditions for obtaining a uniform hot rolled structure should be such that the finishing temperature is more than 800C and preferably 830C or more.
  • the coiling temperature may be on the same level as those for ordinary cold reduced sheets, that is below 700C or 500 to 650C on an industrial level. No special problems are existent if normal pickling and cold rolling after the hot rolling are performed.
  • the present invention is most remarkably characterized in its annealing process.
  • the said annealing process is performed in one continuous line starting with paying-off of the cold rolled coils and finishing with coiling of annealed strip.
  • the heating requirements should be such that the strip must be heated up to a temperature where a great amount of solution carbon is formed in order to obtain the required strength since the hardening mechanism of the present invention steel depends greatly on the solution carbon in steel, fine precipitation therefrom and hardening thereby. That is to say, the heating-temperature should be at least 700C or more and more than the A point practically.
  • the raising of said heating temperature in this way will increase quenching structure of martensite system thus increasing the strength of steel.
  • this at the same time causes lowering of elongation since the differences of hardness between the ferrite matrix and the quenching structure as the second phase particles become larger. That is why there should be an upper limit imposed on the heating temperature such as 900C at most and 850C industrial-wise.
  • the minimum period of time for holding the travelling strip at the said heating temperature should be such that it will be sufficient for the cold rolled strip to complete recrystallization and for C in steel to become solution state. That is the reason for the lower limit of 10 seconds.
  • the longer the holding time becomes the more the softening acceleration caused by the grain grown after recrystallization occurs which at the same time requires extending of heating-soaking zone in the continuous treating line and lowering of the line speed.
  • the upper limit is placed at seconds or less.
  • the travelling strip thus heated and soaked is quenched down to the room temperature by a jet stream of water.
  • the jetting of water is used to speed up said cooling and display its full cooling effect irrespective of place, i.e., in the air or under water. That is to say, when the travelling strip heated is quenched into water, there is instantaneously produced a boiling steam film on the surface of said strip which damages the thermal conductivity and slows 6
  • too high a reheating temperature will soften a part of ferrite in the two phases structure wherein ferrite matrix and quenched structure coexist, thus causing inevitable lowering of steel strength. In addown the cooling speed excessively.
  • the cooling rate in the present invention is because the 50- upper limit of the reheating temperature is set at lution carbon obtained through the process of heating 400C. There are no advantages obtained at the temand soaking is to held there at room temperature. The perature exceeding 400C.
  • the quenching to the room temperature as above menmost preferable temperature is set at 180 to 300C, tioned has determinative influences on the low temper- The optimum range of the holding time for such reheatature aging treatment which is the next step. ing temperatures varies depending upon said temperaln reheating as the successive process of low temper ture.
  • the time should preferably be tation nucleus for time carbide of the solution carbon in 15 to 300 seconds; if within 180 to 300C, 4 to 300 steel is first formed. Fine carbide thus formed is quite seconds; if 300 to 400C, 2 to 200 seconds respecuseful for improving the strength, but will act to curb tively. If the said low temperature aging treatment is the lowering of strength caused by said solution carbon performed with such temperature at time requirements to a minimum. This is the first reason why the strip then all the problems that have been discussed heretomust be quenched down to the room temperature.
  • AYP shows the increase in yield point the conditions for heating, soaking and cooling thereafof the steels which have been subjected to the coatingter should be optimumly provided for so that not all the baking process after press forming as versus those solution carbon be precipitated by reheating, but some being just before said press forming. remain in solution state.
  • Composition Heat treating requirements Steel C Mn Object Heating Quenching Reheating Baking l 0.07 0.39 Influence 800C 1min In jet water 250C lmin 180C 30min -continued Composition Heat treating requirements Steel C Mn Object Heating Quenching Reheating Baking 2 of In still water 3 cooling In 250C lead bath 250C lhr method 4 Influence of 690C 1min In jet of water 250C 1min 5 max.
  • Baking requirements after press forming are the same as those generally used in autombile manufacturing companies.
  • the steels l to 3 were investigated in respect of the impacts of the cooling methods.
  • the steel 1 was quenched in a jet water stream in accordance with the present invention.
  • the steel 2 was quenched in still water and the steel 3 was quenched by BISRA method.
  • the steel of the present invention shows the values of YP: 34.2 Kg/mm TS: 44.7 Kglmm El: 30.9 percent at the time of shipping out of the plant after skin pass; and the values of YP: 44.5 Kg/mm TS: 45.5 Kg/mm after coatingbaking treatment following press-forming indicating a remarkable improvement of AYP, namely by 10.3 Kg/mm
  • TS El index in the present invention teel l is as high as 75.6 compared to 66.5 of the steel 2 and 68.8 of the steel 3 also suggests excellent press formability.
  • the steels 4 to 8 were investigated in respect of the impact of the maximum heating temperatures, and the steels 4 and 8 are comparison steels while the steels 5, 6 and 7 are the present invention steels.
  • the maximum heating temperature is lower for the steel 4 than for the present invention steel while that for the steel 8 is higher conversely.
  • the difference is clearly manifested in the mechanical properties of the steel.
  • YP El index for comparison steels are all below 73 while those for the present invention steels exceed 73. This indicates that undesirable influences are seen in the press formability if the maximum heating temperature is outside the range of the present invention.
  • the heating temperature should be selected from among the range of 700 to 900C, and more preferably from A point to 850C.
  • the steels 9 to 13 were investigated in respect of the influences of reheating temperatures, and steels 9 to 10 and 13 are the comparison steels while steels l 1 and 12 are the present invention steels. On these steels, the steel 9 was not given any reheating treatment, but shows remarkable lower value of TS, that is ATS is as low as 20.1 Kg/mm exemplifying a typical steel which is hard at the time of press forming and becomes soft as goods.
  • the reheating temperature is 100C for the steel 10
  • TS at the time of press forming is as high as 58.2 Kg/mm
  • TS remarkably lowers to 47.5 Kg/mm i.e., ATS, to l0.5 Kg/mm by the coating-baking after press forming and the steel is not a well balanced steel even if its AYP showed 10.1 Kglmm
  • this steel 10 cannot be called one having good AA effects.
  • the steels 1 1 and 12 of the present invention maintain TS value as at the time of press forming while AYP rises to 10.9 Kg/mm and 7.9 Kg/mm respectively and TS El index also show good values of 75.6 and 73.4 respectively. It is evident that the steels are well balanced and satisfactory.
  • the reheating temperature should be selected from within the range of 150 to 400C and preferably 180 to 300C.
  • the steels 14 to 17 were investigated in respect of the influences of a holding time at the reheating temperature.
  • the steel 17 is the comparison steel while the steels 14 to 16 are the present invention steels.
  • the present invention steels 14 to 16 show well balanced properties and have sufficient AA effects.
  • the comparison steel 17 with a long treatment time of 10 minutes have a low YP value of 4.0 Kg/mm thus displaying absence of AA effects.
  • the holding time at reheating temperature is to be selected from within the range of 2 to 300 seconds, corresponding to the particular reheating temperature being employed.
  • the steels 18 to 20 were investigated in respect of the influences of the compositions, and the steel 20 is the comparison steel while the steels 18 to 19 are the present invention steels.
  • the steel 20 has been subjected to identical treating requirements, except for its higher C content by 0.15 percent than the present invention steels. As is seen from the table, it has a low TS El index of 71.5. This clearly will present difficulties in stable formability and in weldabilities. Accordingly, the steel should be without doubt avoided even if it presents satisfactory AA effects after baking.
  • the steels within the range of the present invention compositon all show excellent TS El index and AA effects and are most satisfactory. Therefore, it becomes known that C content in steel should be within 0.12 to 0.04 percent range and should be varied depending upon the level of the strength required.
  • the steels 21 to 23 are the present invention steels and were investigated in respect of Mn influences. They show that the strength improved by Mn increase does not necessarily invite the lowering of elongation and does not invite the deterioration in TS El index. Thus, Mn is an effective element in the present invention since it doesnt act to deteriorate the mechanical properties but improves the strength directly and its amount is selected according to the level of the strength required as in the case of determination of C content.
  • C content and Mn content in steel are controlled within the ranges of 0.04 to 0.12 percent and 0.1 to 1.60 percent respectively,
  • a travelling strip is heated up within the range of 700 to 900C and is held at the above temperature for to 120 seconds,
  • said heated and held strip is rapidly cooled by a jet stream of water to room temperature
  • said rapidly cooled strip is reheated to the range of 150 to 400C and held there for a period of time selected from the range of 2 to 300 seconds corresponding to the above reheating temperature, and

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Heat Treatment Of Strip Materials And Filament Materials (AREA)
US487653A 1973-07-12 1974-07-11 Process of making high strength cold rolled steel having excellent bake-hardening properties Expired - Lifetime US3904446A (en)

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JP7786573A JPS5441983B2 (cs) 1973-07-12 1973-07-12

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JP (1) JPS5441983B2 (cs)
DE (1) DE2433665B2 (cs)
FR (1) FR2236945B1 (cs)
GB (1) GB1435460A (cs)

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4050959A (en) * 1974-11-18 1977-09-27 Nippon Kokan Kabushiki Kaisha Process of making a high strength cold reduced steel sheet having high bake-hardenability and excellent non-aging property
US4113523A (en) * 1973-07-25 1978-09-12 Nippon Kokan Kabushiki Kaisha Process of making high tension cold-reduced al-killed steel excellent in accelerated aging property
US4323403A (en) * 1972-06-22 1982-04-06 Nippon Kokan Kabushiki Kaisha Continuous annealing method for cold reduced steel strip
EP0053913A1 (en) * 1980-12-04 1982-06-16 Uss Engineers And Consultants, Inc. Method for producing high-strength deep-drawable dual-phase steel sheets
US4336080A (en) * 1979-12-14 1982-06-22 Nippon Kokan Kabushiki Kaisha Method for manufacturing high-strength cold-rolled steel strip excellent in press-formability
US4336081A (en) * 1978-04-28 1982-06-22 Neturen Company, Ltd. Process of preparing steel coil spring
US4407680A (en) * 1980-01-18 1983-10-04 British Steel Corporation Dual-phase steels
US4407683A (en) * 1978-04-28 1983-10-04 Neturen Company, Ltd. Steel for cold plastic working
US6143100A (en) * 1998-09-29 2000-11-07 National Steel Corporation Bake-hardenable cold rolled steel sheet and method of producing same
US20090314394A1 (en) * 2007-01-17 2009-12-24 Outokumpu Oyj Method for manufacturing an austenitic steel object

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS579831A (en) * 1980-05-21 1982-01-19 British Steel Corp Steel production

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3666452A (en) * 1969-07-16 1972-05-30 Jones & Laughlin Steel Corp High-strength low-alloy steels
US3666570A (en) * 1969-07-16 1972-05-30 Jones & Laughlin Steel Corp High-strength low-alloy steels having improved formability
US3671336A (en) * 1969-07-16 1972-06-20 Jones & Laughlin Steel Corp High-strength plain carbon steels having improved formability
US3787250A (en) * 1971-03-11 1974-01-22 Jones & Laughlin Steel Corp Corrosion-resistant high-strength low-alloy steels

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR1479039A (fr) * 1965-05-10 1967-04-28 Ass Elect Ind Procédé de traitement par la chaleur de feuilles et bandes d'acier destinées à la fabrication de boîtes de conserve

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3666452A (en) * 1969-07-16 1972-05-30 Jones & Laughlin Steel Corp High-strength low-alloy steels
US3666570A (en) * 1969-07-16 1972-05-30 Jones & Laughlin Steel Corp High-strength low-alloy steels having improved formability
US3671336A (en) * 1969-07-16 1972-06-20 Jones & Laughlin Steel Corp High-strength plain carbon steels having improved formability
US3787250A (en) * 1971-03-11 1974-01-22 Jones & Laughlin Steel Corp Corrosion-resistant high-strength low-alloy steels

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4323403A (en) * 1972-06-22 1982-04-06 Nippon Kokan Kabushiki Kaisha Continuous annealing method for cold reduced steel strip
US4113523A (en) * 1973-07-25 1978-09-12 Nippon Kokan Kabushiki Kaisha Process of making high tension cold-reduced al-killed steel excellent in accelerated aging property
US4050959A (en) * 1974-11-18 1977-09-27 Nippon Kokan Kabushiki Kaisha Process of making a high strength cold reduced steel sheet having high bake-hardenability and excellent non-aging property
US4336081A (en) * 1978-04-28 1982-06-22 Neturen Company, Ltd. Process of preparing steel coil spring
US4407683A (en) * 1978-04-28 1983-10-04 Neturen Company, Ltd. Steel for cold plastic working
US4336080A (en) * 1979-12-14 1982-06-22 Nippon Kokan Kabushiki Kaisha Method for manufacturing high-strength cold-rolled steel strip excellent in press-formability
US4407680A (en) * 1980-01-18 1983-10-04 British Steel Corporation Dual-phase steels
EP0053913A1 (en) * 1980-12-04 1982-06-16 Uss Engineers And Consultants, Inc. Method for producing high-strength deep-drawable dual-phase steel sheets
US6143100A (en) * 1998-09-29 2000-11-07 National Steel Corporation Bake-hardenable cold rolled steel sheet and method of producing same
US20090314394A1 (en) * 2007-01-17 2009-12-24 Outokumpu Oyj Method for manufacturing an austenitic steel object
US9441281B2 (en) 2007-01-17 2016-09-13 Outokumpu Oyj Method for manufacturing an austenitic steel object

Also Published As

Publication number Publication date
DE2433665B2 (de) 1978-05-11
FR2236945B1 (cs) 1978-09-22
FR2236945A1 (cs) 1975-02-07
JPS5441983B2 (cs) 1979-12-11
GB1435460A (en) 1976-05-12
JPS5026714A (cs) 1975-03-19
DE2433665A1 (de) 1975-02-13

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