EP0535238A1 - Hochfestes stahlblech für die umformung und dessen herstellung - Google Patents

Hochfestes stahlblech für die umformung und dessen herstellung Download PDF

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Publication number
EP0535238A1
EP0535238A1 EP92906722A EP92906722A EP0535238A1 EP 0535238 A1 EP0535238 A1 EP 0535238A1 EP 92906722 A EP92906722 A EP 92906722A EP 92906722 A EP92906722 A EP 92906722A EP 0535238 A1 EP0535238 A1 EP 0535238A1
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European Patent Office
Prior art keywords
effective
less
steel sheet
strength steel
ratio
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EP92906722A
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English (en)
French (fr)
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EP0535238A4 (en
Inventor
Susumu Kawasaki Steel Corporation Masui
Kei Kawasaki Steel Corporation Sakata
Fusao Kawasaki Steel Corporation Togashi
Masahiko Kawasaki Steel Corporation Morita
Toshiyuki Kawasaki Steel Corporation Kato
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JFE Steel Corp
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Kawasaki Steel Corp
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Priority to EP19920906722 priority Critical patent/EP0535238A4/en
Publication of EP0535238A1 publication Critical patent/EP0535238A1/de
Publication of EP0535238A4 publication Critical patent/EP0535238A4/en
Withdrawn legal-status Critical Current

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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
    • C21D8/041—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 involving a particular fabrication or treatment of ingot or slab
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • 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

Definitions

  • This invention proposes high-strength steel sheets having a tensile strength of not less than 40 kgf/mm2, which are mainly suitable for use in inner and outer panels of an automobile, as well as a method of producing the same.
  • High-strength steel sheets have widely been used from the old time as a steel sheet for use in body constituting members, outer panels and the like of the automobile in order to reduce the vehicle weight.
  • Such high-strength steel sheets for the vehicles are required to have not only a good formability for press forming or the like but also a sufficient strength for ensuring a safeness of the automobile.
  • these steel sheets are frequently subjected to a heat treatment above 900°C or heated to a higher temperature by welding, brazing or the like for removing a work strain after the forming or improving a resistance to cold work embrittlement, so that they are desirable to have a property hardly causing the softening against the high temperature heating.
  • large-scale formable cold rolled steel sheets having an improved rigidity (high Young's modulus) and a method of producing the same are disclosed in, for example, Japanese Patent laid open No. 57-181361, and a method of producing deep drawable cold rolled steel sheets having slow aging property and small anisotropy is disclosed in Japanese patent laid open No. 58-25436.
  • a slight amount of Nb, Ti or the like is added to a ultra-low carbon steel as a base, and continuous annealing conditions are controlled, and P having less material degradation and large solid-solution strengthening property is used as a component for raising tension.
  • the tensile strength of such P-added ultra-low C steel is about 40 kgf/mm2 at most, so that such a composition system based on the ultra-low C steel and added with the solid-solution strengthening component is clearly difficult to cope with the demand for raising the strength of the steel sheet together with the reduction of body weight of the automobile rapidly advanced in future.
  • transformation strengthening steel sheets dual-phase steel sheets
  • precipitation strengthening steel sheets as a high-tensile steel sheet having a different strengthening mechanism.
  • the transformation strengthening steel sheets are unsuitable for deep drawing because r-value is low though low yield ratio and excellent elongation can easily be obtained.
  • the precipitation strengthening steel sheets or so-called HSLA (high strength low alloy) steel sheets are steels added with Si, Mn, Nb and the like to utilize solid-solution strengthening of Si and Mn and the strengthening based on precipitation of carbonitride of Nb and the formation of fine crystal grains accompanied therewith, and are used for not only automobiles but also domestically electric appliance and the like.
  • the yield ratio is high, so that the use conditions are restricted.
  • Japanese Patent Application Publication No. 54-27822 discloses a method of producing precipitation strengthening high-strength cold rolled steel sheets
  • Japanese patent Application Publication No. 55-16214 discloses a method of producing deep drawable high-strength cold rolled steel sheets. In these steel sheets, the yield ratio exceeds 70% and a greater part of the sheets exhibit a high value of not less than 80%.
  • Japanese Patent laid open No. 55-152128 is disclosed a method of producing low-yield ratio and high-strength cold rolled steel sheets having an excellent formability through continuous annealing as a method for the production of precipitation strengthening steel sheets, but the deep drawability is not mentioned at all.
  • Japanese patent laid open No. 57-35662 discloses super-deep drawable cold rolled steel sheets having an excellent secondary formability
  • Japanese Patent laid open No. 60-92453 discloses brazable cold rolled steel sheets having an excellent drawability.
  • the tensile strength described in Examples of the above Japanese Patent laid open No. 57-35662 is less than 40 kgf/mm2 and does not arrive at the strength level aimed at the present invention because the tensile strength aimed at the present invention is not less than 40 kgf/mm2.
  • Si is an essential component and is restricted to a range of 0.1-1.2 wt%
  • the claim of the above Japanese Patent laid open No. 60-92453 does not disclose the use of Si and the Si content is not more than 0.09 wt% as seen from the description of Examples, which is essentially different from the present invention effectively utilizing the Si effect.
  • This invention is to propose high-strength steel sheets which are based on a low C steel having a C content larger than the conventional ultra-low C steel and have a tensile strength of not less than 40 kgf/mm2 by attaining IF with Ti and carefully selecting additive components and are low in the yield ratio (less than 70%) as compared with the conventional precipitation strengthening steel and small in the plane anisotropy and hardly cause the softening due to abnormal grain growth by reheating treatment, as well as a method of producing the same.
  • the inventors have made various studies and experiments and found that an Si-added low C - high Ti steel composition is rendered into complete IF to provide a high-strength steel sheet having a low yield ratio and a small plane anisotropy, and as a result the invention has been accomplished.
  • Fig. 1 is shown a relation between tensile property values and Si content from the above measured results.
  • a part of the hot rolled sheet is subjected to a descaling treatment, cold rolled at a rolling reduction of 75%, held at 800°C for 40 seconds, continuously annealed under a condition of cooling at a rate of 20°C/sec (no overaging), and subjected to a temper rolling at an elongation of 0.8% to obtain a cold rolled sheet of 0.75 mm in thickness.
  • the coarsening of crystal grains is not caused at the C content of not less than 0.01 wt% and the ratio of effective *Ti wt%/C wt% of not less than 4, and the crystal grain size number is 7 or more.
  • the crystal grain size number is 7 or more, the crystal grains after the heating do not cause the softening.
  • the C content of not less than 0.01 wt% and the ratio of effective *Ti wt%/C wt% of not less than 4 are required for preventing the abnormal grain growth (prevention of softening) in the reheating. This is guessed due to the fact that the resulting fine carbides of Ti are relatively stably existent even in the reheating and serve to suppress the abnormal grain growth.
  • Si content largely affects the plane anisotropy and r-value.
  • Figs. 3(a), (b), (c) and (d) show pole figures measured on four steel sheets after a cold rolled steel sheet containing C: 0.05 wt%, Si: 0 wt%, 1.0 wt%, 1.5 wt% or 2.0 wt%, Mn: 0.01 wt%, Ti: 0.206 wt%, B: 0.0008 wt%, Al: 0.04 wt%, P: 0.01 wt%, S: 0.001 wt% and N: 0.0014 wt% is subjected to a box annealing at 720°C, in which each of Figs.
  • the target tensile strength of not less than 40 kgf/mm2 is not obtained and also the sheet is apt to be softened at a high temperature.
  • the carbon content is not less than 0.01 wt% but less than 1.0 wt%.
  • Si: 0.1-1.2 wt% Si is an important element in the invention and has an effect of discharging C from ferrite and promoting precipitation of Ti carbide and agglomeration coarsening.
  • the Si content is limited to a range of 0.1 wt% to 1.2 wt%, but is it preferable within a range of 0.4 wt% to 1.0 wt% from a viewpoint of the improvement of plane anisotropy and r-value.
  • Mn up to 3.0 wt% Mn is useful as a strengthening element for steel. However, when the amount exceeds 3.0 wt%, the sheet is too hardened and the ductility is considerably degraded.
  • the content is up to 3.0 wt%.
  • the effective *Ti is less than 4xC, C can not completely be fixed and the crystal grains are coarsen by reheating to cause the softening as previously mentioned.
  • the effective *Ti exceeds 12xC, Ti excessively solutes to degrade the properties and damage the surface quality of the steel sheet. Therefore, the content is within a range satisfying the ratio of effective *Ti/C of 4 to 12.
  • P up to 0.1 wt%
  • P is a very excellent solid-solution strengthening element.
  • the content is up to 0.1 wt%.
  • P (wt%)/C (wt%) as a relation to C content is less than 1.5.
  • S up to 0.02 wt% S results in the occurrence of cracking in the hot rolling, so that the content is up to 0.02 wt%.
  • N up to 0.005 wt% As N amount becomes large, the effective *Ti amount decreases and the degradation of r-value and ductility is caused.
  • the content is favorable to be less, but its acceptable upper limit is 0.005 wt%.
  • at least one of V, Nb and Zr as a carbide forming element may be added. The effect is developed when each content is not less than 0.02 wt%, but when it exceeds 0.2 wt%, the degradation of ductility is caused. Therefore, the content of each of V, Nb and Zr is within a range of 0.02 wt% to 0.2 wt%.
  • at least one of Cr, Ni, Mo and Cu as a solid-solution strengthening element may be added.
  • each content is not less than 0.05 wt%, but when it is too large, the degradation of surface quality of the steel sheet is caused. Therefore, the content is limited to 0.05 wt% to 1.5 wt% in Cr, 0.05 wt% to 2.0 wt% in Ni, 0.05 wt% to 1.0 wt% in Mo and 0.05 wt% to 1.5 wt% in Cu.
  • Ca may be added for controlling the shape of inclusion.
  • the effect is developed when the content is not less than 0.0005 wt%, but when it exceeds 0.005 wt%, the effect is saturated and also the degradation of properties becomes conspicuous. Therefore, the content is within a range of 0.0005 wt% to 0.005 wt%.
  • the reason why low yield ratio is obtained though the strength is raised by using low C steel having a C content larger than ultra-low C steel is considered as follows.
  • the steel-making is sufficient to be carried out according to the usual manner, so that the restriction of conditions is not particularly required.
  • the slab heating temperature is lower than 1100°C, the formability is poor, while when it exceeds 1280°C, coarse grains appear to cause the scattering of properties at subsequent step. Therefore, the slab reheating temperature is within a range of not lower than 1100°C but not higher than 1280°C.
  • the continuously cast slab may be subjected to rough rolling immediately or after the temperature holding treatment of not lower than 1100°C but not higher than 1280°C without reheating or lowering temperature after continuous casting to lower than 1100°C from a viewpoint of energy-saving.
  • the finish temperature at hot rolling when it is too high, the final structure becomes coarse, which is unfavorable for the ductility. On the other hand, when it is too low, the stretching of the structure becomes conspicuous and the rolling load rapidly increases, which are unfavorable in the operation. Therefore, the finish temperature is preferable within a range of above Ar3 transformation temperature but not higher than Ar3 transformation temperature + 100°C.
  • the coiling temperature after the hot rolling is sufficient within a range of from 400°C to 700°C from a view point of subsequent pickling and capacity of coiling machine.
  • the rolling reduction is preferable to be not less than 55% for providing a sufficient formability after the annealing.
  • the annealing after the cold rolling is required to have a temperature above recrsytallization temperature for the recrsytallization of the structure. However, it is preferable to be lower than A c3 transformation temperature for avoiding the formation of composite texture after the annealing.
  • the annealing process is not particularly restricted, but may be a continuous annealing process or a box annealing process.
  • the plating conditions in case of electroplating, it is sufficient that the plating is conducted at a given amount to each of the hot rolled sheet and cold rolled sheet in the usual manner.
  • the continuous hot-dip galvanization line may be applied to the annealing step in addition to a single hot-dip galvanization line.
  • these steel sheets may be subjected to a temper rolling at a rolling reduction (%) equal to a sheet thickness (mm) within common sense for correction of sheet shape or the like.
  • the steel sheet according to the invention after the annealing or plating may be subjected to a special treatment to improve phosphatability, weldability, press formability, corrosion resistance and the like.
  • Each of 31 continuously cast slabs in total of 26 acceptable steels and 5 comparative steels each melted in a converter and having a chemical composition as shown in Tables 1 and 2 is hot rolled to a thickness of 3.2 mm in case of steel symbols O, P, Q and R or 2.8 mm in case of all other steels. Furthermore, a part of the steels is subjected to a hot-dip galvanization.
  • the mechanical properties, aging index AI and crystal grain size number after the heat treatment (reheating) are measured with respect to the thus obtained steel sheets.
  • a part of the above hot rolled sheets (in which the slab re-heating temperature is within the range defined in the invention) is descaled, cold rolled at a rolling reduction of 75% to a thickness of 0.80 mm or 0.70 mm, subjected to a continuous annealing or a box annealing and then subjected to a temper rolling at a rolling reduction of 0.80% or 0.70%.
  • a part of thus treated sheets is subjected to an electroplating or a hot-dip galvanization.
  • the mean r-value, mechanical properties including ⁇ r as an indication of plane anisotropy, aging index AI and crystal grain size number after heat treatment are measured with respect to the thus obtained steel sheets.
  • each treating condition is as follows.
  • Zn-Ni plating is carried out at a plating amount of 30 g/m2.
  • Zn plating or Al plating is conducted, in which the Zn plating is carried out at a plating amount of 45 g/m2 under conditions of bath temperature: 475°C, passing sheet temperature: 475°C, immersion time: 3 seconds and alloying temperature: 485°C, while the Al plating is carried out at a plating amount of 30 g/m2 under conditions of bath temperature: 650°C, passing sheet temperature: 650°C and immersion time: 3 seconds.
  • the heat treatment is conducted under such a condition that the sheet is heated to 950°C, held at this temperature for 30 minutes and slowly cooled at a rate of 5°C/sec.
  • JIS No. 5 specimen is used in the tensile test, while YS, TS and El are measured in the rolling direction.
  • the r-value is determined by measuring widths at three points of a center and both positions separated from the center by 12.5 mm in the longitudinal direction of the specimen under 15% strain, and the mean r-value and ⁇ r are calculated according to the following equations, respectively.
  • mean r-value (r0 + r90 + 2r45)/4
  • ⁇ r (r0 + r90 - 2r45)/4
  • r0, r90 and r45 are r-values in the rolling direction (r0), a direction of 45° with respect to the rolling direction (r45) and a direction of 90° with respect to the rolling direction (r90), respectively.
  • the AI value is determined from a difference of deformation stress before and after the aging when the sheet is previously tensioned under a strain of 7.5% and then subjected to an aging treatment at 100°C for 30 minutes.
  • the tensile strength of not less than 40 kgf/mm2 is obtained even when the annealing process is box annealing or continuous annealing, and low yield ratio (not more than 70%) and high El and crystal grain size number after heat treatment of not less than 7 hardly causing the softening by reheating are exhibited.
  • the cold rolled steel sheets exhibit excellent properties that the high mean r-value is shown, and also ⁇ r as an indication of plane anisotropy is small, and the aging index AI is not more than 1 kgf/mm2 to ensure complete non-aging property.
  • the high-strength steel sheets having a small plane anisotropy, a low yield ratio and a complete non-aging property and hardly causing the softening by high temperature heating are obtained by completely fixing solid soluted C, S, N and the like, and further high-strength precipitation strengthening steels having a higher r-value are obtained as a cold rolled steel sheet. Therefore, the invention serves to enlarge the application of the precipitation strengthening steel sheet for automobile and the like from its utility.
EP19920906722 1991-03-13 1992-03-13 High-strength steel sheet for forming and production thereof Withdrawn EP0535238A4 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
EP19920906722 EP0535238A4 (en) 1991-03-13 1992-03-13 High-strength steel sheet for forming and production thereof

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
JP7219491 1991-03-13
JP72194/91 1991-03-13
EP19920906722 EP0535238A4 (en) 1991-03-13 1992-03-13 High-strength steel sheet for forming and production thereof

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EP0535238A1 true EP0535238A1 (de) 1993-04-07
EP0535238A4 EP0535238A4 (en) 1993-08-04

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0748877A1 (de) * 1995-06-15 1996-12-18 Sollac S.A. Verfahren zur Herstellung von warmgewalztem Stahlblech mit sehr hoher Elastizitätsgrenze und Stahlblech
EP0947590A1 (de) * 1998-03-31 1999-10-06 Sms Schloemann-Siemag Aktiengesellschaft Verfahren zur Herstellung von mikrolegierten Baustählen
EP1253209A2 (de) * 1998-12-30 2002-10-30 Hille & Müller GmbH Stahlband mit guten Umformeigenschaften sowie Verfahren zum Herstellen desselben
EP1335036A1 (de) * 2002-02-06 2003-08-13 Benteler Automobiltechnik GmbH & Co. KG Verfahren zur Herstellung eines Strukturbauteils für den Fahrzeugbau
CN103088255A (zh) * 2013-01-02 2013-05-08 河北钢铁股份有限公司邯郸分公司 一种汽车用高强塑积的低合金高强钢冷轧板的生产工艺
EP2799562A1 (de) * 2011-12-27 2014-11-05 JFE Steel Corporation Heissgewalztes stahlblech und verfahren zu seiner herstellung
US10301698B2 (en) 2012-01-31 2019-05-28 Jfe Steel Corporation Hot-rolled steel sheet for generator rim and method for manufacturing the same

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB720614A (en) * 1952-06-10 1954-12-22 Henry William Kirkby Improvements relating to ferritic creep-resisting steels
DE2156164A1 (de) * 1970-11-12 1972-06-15 Ovako Oy Verfahren zur Herstellung eines hitzebeständigen Stahles
SU424904A1 (ru) * 1972-04-03 1974-04-25 Литая сталь
EP0015154A1 (de) * 1979-02-23 1980-09-03 The Torrington Company Limited Verfahren zur Herstellung schalenartiger Halteringe für Lager durch Tiefziehen

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB720614A (en) * 1952-06-10 1954-12-22 Henry William Kirkby Improvements relating to ferritic creep-resisting steels
DE2156164A1 (de) * 1970-11-12 1972-06-15 Ovako Oy Verfahren zur Herstellung eines hitzebeständigen Stahles
SU424904A1 (ru) * 1972-04-03 1974-04-25 Литая сталь
EP0015154A1 (de) * 1979-02-23 1980-09-03 The Torrington Company Limited Verfahren zur Herstellung schalenartiger Halteringe für Lager durch Tiefziehen

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See also references of WO9216669A1 *

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0748877A1 (de) * 1995-06-15 1996-12-18 Sollac S.A. Verfahren zur Herstellung von warmgewalztem Stahlblech mit sehr hoher Elastizitätsgrenze und Stahlblech
FR2735498A1 (fr) * 1995-06-15 1996-12-20 Lorraine Laminage Procede de realisation d'une bande de tole d'acier laminee a chaud a haute limite d'elasticite utlisable notamment pour la mise en forme
US5716464A (en) * 1995-06-15 1998-02-10 Sollac Process for producing a strip of hot rolled steel sheet having a very high yield point and the steel sheet obtained
EP0947590A1 (de) * 1998-03-31 1999-10-06 Sms Schloemann-Siemag Aktiengesellschaft Verfahren zur Herstellung von mikrolegierten Baustählen
EP1253209A3 (de) * 1998-12-30 2005-03-02 Hille & Müller GmbH Stahlband mit guten Umformeigenschaften sowie Verfahren zum Herstellen desselben
EP1253209A2 (de) * 1998-12-30 2002-10-30 Hille & Müller GmbH Stahlband mit guten Umformeigenschaften sowie Verfahren zum Herstellen desselben
EP1335036A1 (de) * 2002-02-06 2003-08-13 Benteler Automobiltechnik GmbH & Co. KG Verfahren zur Herstellung eines Strukturbauteils für den Fahrzeugbau
EP2799562A1 (de) * 2011-12-27 2014-11-05 JFE Steel Corporation Heissgewalztes stahlblech und verfahren zu seiner herstellung
EP2799562A4 (de) * 2011-12-27 2014-12-31 Jfe Steel Corp Heissgewalztes stahlblech und verfahren zu seiner herstellung
US9534271B2 (en) 2011-12-27 2017-01-03 Jfe Steel Corporation Hot rolled steel sheet and method for manufacturing the same
US10301698B2 (en) 2012-01-31 2019-05-28 Jfe Steel Corporation Hot-rolled steel sheet for generator rim and method for manufacturing the same
CN103088255A (zh) * 2013-01-02 2013-05-08 河北钢铁股份有限公司邯郸分公司 一种汽车用高强塑积的低合金高强钢冷轧板的生产工艺
CN103088255B (zh) * 2013-01-02 2014-12-03 河北钢铁股份有限公司邯郸分公司 一种汽车用高强塑积的低合金高强钢冷轧板的生产工艺

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