Classifications

• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling

Definitions

• the present invention relates to a high strength cold-rolled steel sheet, favorable for use in a structural member of machine, particularly in a structural member of automobile, which has a tensile strength of 780 MPa or more, and a manufacturing method thereof.
• stress-flangeability means a property resisting to generation of cracks on a blank end face of steel sheet when it is press-formed and is evaluated, based on a hole-expanding ratio measured by means of hole-expanding test defined by the Japan Iron and Steel Federation Standard: JFST 1001-1996.
• JP-B No. 7-59726 JP-A Nos. 2001-226741, 10-60593 and 9-263838
• high strength cold-rolled steel sheets which have each aimed for improving stretch-flangeability by controlling structure through optimizing steel compositions and manufacturing conditions, and manufacturing methods thereof are disclosed. More specifically, for example, in JP-A No. 9-263838, a cold-rolled steel sheet is slowly cooled from soaking temperature at the time of annealing to allow second phase to be uniformly dispersed in ferrite phase and, then, bainite phase is allowed to be uniformly dispersed in the ferrite phase as a main component by adjusting cooling rate and overaging temperature, thereby aiming for enhancing strength and improving stretch-flangeability.
• JP-A No. 2001-355044 a high strength cold-rolled steel sheet in which ferrite phase is allowed to have higher strength and from 2% to 20% of residual austenite phase is formed in the ferrite phase to aim for simultaneously achieving strength enhancement and stretch-flangeability improvement is disclosed.
• JP-A No. 11-350038 a method for producing a complex phase type high strength cold-rolled steel sheet which is excellent in ductility and stretch-flangeability and has a tensile strength of about 980 MPa by controlling compositions and producing conditions is disclosed.
• JP-A No. 9-41040 a method for manufacturing a high strength cold-rolled steel sheet which is excellent in stretch-flangeability by subjecting a cold-rolled steel sheet to annealing in an ⁇ + ⁇ region, cooling the resultant steel sheet by holding it in a temperature range of from 650° C. to temperature to stop pearlite transformation for 10 seconds or more and, then, cooling the cooled steel sheet by holding it in a temperature range of from temperature to stop pearlite transformation to 450° C. for 5 seconds or less is disclosed.
• JP-B No. 58-55219 and Japanese Patent No. 2545316 a method for producing a high strength cold-rolled steel sheet by more strictly defining compositions and performing annealing under specified continuous annealing conditions is disclosed.
• JP-B No. 7-68583 a method for manufacturing a dual phase type high strength cold-rolled steel sheet which is excellent in mechanical characteristics, spot-weldability and phosphatability by specifying content of C, Si, and Mn, reheating conditions before hot rolling, soaking conditions, atmosphere and the like in continuous annealing after cold rolling is disclosed.
• JP-B No. 8-30212 a method for manufacturing a high strength cold-rolled steel sheet having high ductility and excellent bending property by allowing structure after hot rolling to be uniformly finer such that band structure is not generated therein and, then, allowing the resultant structure after continuous annealing to be that in which ferrite phase and martensite phase are uniformly distributed is disclosed.
• JP-B No. 5-57332 a method for producing a high strength cold-rolled steel sheet which has a yield ratio of 0.65% or less and is excellent in both surface property and bending property by heating steel containing Si and a comparatively large amount of Mn to austenite single phase zone which is higher than Ac3 transformation temperature and, then, allowing complex phase structure comprising ferrite phase and second phase such as martensite phase to be formed in a cooling step is disclosed.
• JP-B Nos. 1-35051 and 1-35052 a method for manufacturing a high strength cold-rolled steel sheet which is excellent in ductility by controlling heating temperature in continuous annealing, water-quenching start temperature, and overaging treatment temperature is disclosed.
• JP-B Nos. 7-74412 and 3-68927 a method for producing a high strength cold-rolled steel sheet which is excellent in bending property, deep drawability, and resistance to seasoned crack by allowing condensation of C to be low to thereby set austenite phase to be 5% or less by means of performing annealing in a high temperature range after cold rolling is disclosed.
• JP-B 7-59726 it is essential to perform averaging treatment at such a high temperature as 350° C. or more and, in order to compensate decrease of tensile strength to be caused by such high temperature averaging treatment, a large amount of C which is a reinforcing element has been added (in steel Nos. 9, 10, and 13 according to the invention in Table 1, in order to have a tensile strength of 980 MPa or more, 0.17% or more of C has been added.). For this reason, when the steel is spot-welded at the time of assembling an automobile, tenacity of spot-welded portion is deteriorated and, as a result, joint strength thereof is decreased.
• JP-A No. 2001-226741 it is essential to perform austempering treatment after soaking in continuous annealing in order to generate bainite phase, but there is a problem in that consistent characteristics of steel sheet can not be obtained in this treatment.
• JP-A No. 2001-355044 since residual austenite phase is allowed to exist, it is essential to generate bainite phase, thereby decreasing strength.
• the tensile strength shown in an example is as low as from 600 MPa to 800MPa, thereby being incapable of consistently obtaining a tensile strength of 780 MPa or more.
• JP-A Nos. 9-41040 and 9-263838 since structure comprises ferrite phase and pearlite phase, or ferrite phase and bainite phase, tensile strength is as low as from 400 MPa to 700 MPa.
• JP-A No. 10-60593,-JP-B Nos. 58-55219 and 7-68583, and Japanese Patent No. 2545316 tensile strength of from 400 MPa to 700 MPa can only be obtained.
• An object of the present invention is to provide a high strength cold-rolled steel sheet having an elongation of 18% or more, a hole-expanding ratio of 60% or more, and a tensile strength of 780 MPa or more and a manufacturing method thereof.
• This object can be achieved by a high strength cold-rolled steel sheet consisting essentially of, in terms of percentages by mass, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, and the balance being iron and inevitable impurities and having a structure substantially comprising ferrite phase and martensite phase.
• this high strength cold-rolled steel sheet can be realized by a method for manufacturing a high strength cold-rolled steel sheet comprising the steps of: producing a steel sheet by hot rolling a steel slab having the aforementioned compositions, followed by cold rolling; heating the cold-rolled steel sheet at from 750° C. to 870° C. for 10 seconds or more; cooling the heated steel sheet down to from 550° C. to 750° C.; and cooling the cooled steel sheet down to 300° C. or less at a cooling rate of over 100° C./sec.
• the present inventors have studied on a steel sheet which, even though an amount of C is reduced, has a tensile strength of 780 MPa or more and, further, excellent ductility in which an elongation is 18% or more, and excellent stretch-flangeability in which a hole-expanding ratio is 60% or more, and found that the steel sheet can be realized by a steel sheet consisting essentially of, in terms of percentages by mass, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, and the balance being iron and inevitable impurities and having a structure substantially comprising ferrite phase and martensite phase.
• C is an important element for giving a great influence on tensile strength, and reinforcing martensite phase which is generated at quenching.
• an amount of C is less than 0.04%, a tensile strength of 780 MPa or more can not be obtained, while, when it is over 0.10%, stretch-flangeability and spot-weldability are remarkably deteriorated. Accordingly, the amount of C is set to be 0.04 to 0.10%.
• the amount of C in order to obtain a tensile strength of from 780 MPa to less than 980 MPa without deteriorating stretch-flangeability or spot-weldability, it is preferable to set the amount of C to be 0.04% to less than 0.070% and, further, in order to obtain a tensile strength of from 980 MPa to less than 1180 MPa, it is preferable to set the amount of C to be 0.070 to 0.10%.
• Si is effective in enhancing ductility of dual phase type steel sheet comprising ferrite phase and martensite phase.
• an amount of Si is less than 0.5%, effectiveness thereof becomes insufficient, while, when it is over 1.5%, a large amount of Si oxide is formed on a surface of steel sheet in a hot rolling step, thereby generating surface defects.
• the amount of Si is set to be 0.5 to 1.5%. Further, from the point of view of phosphatability, the amount of Si is desirably set to be 1.0% or less.
• Mn:.Mn is an important element for suppressing generation of ferrite phase in a cooling step of continuous annealing.
• an amount of Mn is set to be 1.8 to 3%.
• the amount of Mn is desirably set to be 2.0 to 2.5%.
• P when an amount of P is over 0.02%, spot-weldability is remarkably deteriorated and, accordingly, the amount of P is set to be 0.02% or less.
• S when an amount of S is over 0.01%, spot-weldability is remarkably deteriorated and, accordingly, the amount of S is set to be 0.01% or less.
• Sol. Al is added for performing deoxidization of steel or precipitating N as AlN.
• an amount of Sol. Al is less than 0.01%, the deoxidization or the precipitation of AlN is not sufficiently performed, while, when it is over 0.1%, effectiveness thereof is saturated, thereby inviting a cost increase. Accordingly, the amount of Sol. Al is set to be 0.01 to 0.1%.
• N since N deteriorates formability of steel sheet, an amount of N is desirably as low as possible. However, when the amount thereof is reduced more than necessary, a refining cost is increased. Accordingly, the amount of N is set to be 0.005% or less such that it does not substantially deteriorate the formability.
• Structure of steel sheet substantially comprises two phases of: ferrite phase and martensite phase. Besides these two phases, bainite phase in which iron is a main constitutional element or austenite phase may not deteriorate effectiveness of the present invention, so long as it is contained in an amount of less than 2% in terms of volume fraction. Further, compounds containing iron such as cementite may be contained in the ferrite phase, the martensite phase or an interface between ferrite and martensite phases. Still further, compounds such as AlN and MnS may not impair the effectiveness of the present invention, so long as each of the composition elements or impurity elements is within the scope of the invention.
• a volume fraction of martensite phase is 30 to 45%, in the range of from 780 MPa to less than 980 MPa of tensile strength, or when it is 45 to 60%, in the range of from 980 MPa to 1180 MPa of tensile strength, more excellent stretch-flangeability can be obtained.
• a tempering treatment can appropriately be performed on the martensite phase.
• a slab having the aforementioned compositions is produced by continuous casting method or ingot making plus blooming method and, then, either after reheating or directly, the resultant slab is hot-rolled.
• a final rolling temperature (finishing temperature) at hot rolling is desirably from Ar3 transformation temperature to 870° C., in order to allow structure to be finer to thereby enhance ductility or stretch-flangeability.
• the hot-rolled steel sheet is cooled and, then, coiled.
• a coiling temperature is desirably 620° C. or less for the purpose of enhancing ductility or stretch-flangeability.
• the resultant steel sheet is cold-rolled to be in a desired thickness.
• a cold-rolling reduction rate is desirably 55% or more for the purpose of enhancing ductility or stretch-flangeability by allowing structure to be finer.
• the cold-rolled steel sheet is annealed under conditions as described below in a continuous annealing furnace.
• a cooling rate at this time is desirably set to be 20° C. in the range of from 550° C. to 750° C. depending on compositions such that a volume fraction of austenite phase can be adjusted to be from 30% to 45% or from 45% to 60%, namely, a volume fraction of martensite phase can ultimately be adjusted to be from 30% to 45% or from 45% to 60%.
• a cooling rate is 100° C./sec. or less, quenching becomes insufficient and, accordingly, high strength can not be aimed for.
• rapid cooling is desirably performed at a cooling rate of 500° C./sec or more.
• a cooling terminal temperature is over 300° C., either bainite phase is generated, or austenite phase remains, thereby deteriorating stretch-flangeability.
• the cooling terminal temperature is preferably set to be 100° C. or less.
• the resultant steel sheet may be held at the cooling terminal temperature for from 5 minutes to 20 minutes or subjected to tempering treatment at from 150° C. to 390° C. for from 5 minutes to 20 minutes.
• tempering treatment By performing the tempering treatment, the martensite phase which has been generated at the rapid cooling is tempered, thereby enhancing ductility and stretch-flangeability.
• a tempering temperature is less than 150° C., or a tempering time is less than 5 minutes, such effect as described above can not sufficiently be obtained.
• the tempering temperature is over 390° C., or the tempering time is over 20 minutes, strength is remarkably decreased, thereby being sometimes incapable of obtaining a tensile strength of 780 MPa or more.
• the obtained steel sheet is subjected to temper rolling at a reduction rate of from 0.1% to 0.7% to thereby eliminate yield elongation completely.
• the steel sheet according to the present invention can be electroplated, hot-dip galvanized or applied with solid lubricant.
• Steel Nos. 1 to 10 having respective compositions as shown in Table 1 were each cast into slab.
• the cast slab was reheated at 1250° C., hot-rolled at a finishing temperature of about 860° C., and slowly cooled at about 20° C. to produce a hot-rolled steel sheet having a thickness of 2.8 mm by simulating coiling at 600° C. for one hour.
• the hot-rolled steel sheet was cold-rolled to-produce a cold-rolled steel sheet having a thickness of 1.2 mm and, then, the cold-rolled steel sheet was subjected to heating treatment which simulated continuous annealing.
• the continuous annealing was performed under conditions that a temperature of the steel sheet was elevated at a heating rate of about 20° C.
• the steel sheet was soaked at 830° C. for 300 seconds, slowly cooled down to 700° C. at about 10° C.sec and, thereafter, rapidly cooled in jet-flowing water having a temperature of 20° C. A cooling rate of such rapid cooling was about 2000° C/sec.
• the steel sheet was subjected to tempering treatment at 300° C. for 15 minutes, cooled and, then, subjected to temper rolling of 0.3% to produce steel sheet Nos. 1 to 10. Thereafter, in regard to the steel sheet Nos. 1 to 10, respective tensile characteristics and hole-expanding ratios ( ⁇ ) were measured.
• JIS Z 2201 JIS No. 5 test piece was obtained along each of a rolling direction and a direction at a right angle thereto and subjected to a test in accordance with JIS Z 2241 to determine yield strength (YP), tensile strength (TS), and elongation (El).
• each of steel sheet Nos. 2, 3, 4, 9, and 10 which are examples according to the present invention satisfies the relations: TS ⁇ 780 MPa; El ⁇ 18%; and ⁇ 60%, and thus has high strength, and is excellent in ductility and retch-flangeability.
• steel sheet No. 1 is low in TS, due to small amount of C; steel sheet No. 5 is remarkably low in ⁇ , due to large amount of C and small amount of Mn; steel sheet No. 6 is low in ⁇ , due to small amount of Si; steel sheet No. 7 is low in TS and ⁇ , due to small amount of Mn; and steel sheet No. 8 is low in El, due to large amount Mn.
• each of steel sheet Nos. A, E, G, and H which are examples according to the present invention satisfies the relations: TS ⁇ 780 MPa; El ⁇ 18%; and ⁇ 60%, and thus has high strength, and is excellent in ductility and stretch-flangeability.
• Steel Nos. 1 to 9 having respective compositions as shown in Table 5 were each cast in to slab.
• the cast slab was subjected, under the same conditions as in Example 1, to hot rolling, cold rolling, continuous annealing, and temper rolling to produce steel sheet Nos. 1 to 9. Thereafter, yield strength (YP), tensile strength (TS), elongation (El), and hole-expanding ratio (k) were measured in the same manner as in Example 1.
• each of steel sheet Nos. 1, 2, 3, 8, and 9 which are examples according to the present invention satisfies the relations: TS ⁇ 780 MPa; El ⁇ 18%; and ⁇ 60%, and thus has high strength, and is excellent in ductility and stretch-flangeability.
• steel sheet No. 4 is low in El and ⁇ , due to large amount of C; steel sheet No. 5 is remarkably low in ⁇ , due to large amount of C and small amount of Mn; steel sheet No. 6 is low in ⁇ , due to small amount of Si; and steel sheet No. 7 is low in El and ⁇ , due to large amount of Mn.
• TABLE 5 Chemical compositions (mass %) Steel Sol. No.
• each of steel sheet Nos. B, G, I, and J which are examples according to the present invention satisfies the relations: TS ⁇ 780 MPa; El ⁇ 18%; and ⁇ 60%, and thus has high strength, and is excellent in ductility and stretch-flangeability.
• steel sheet No. A is low in El, due to low heating temperature
• steel sheet No. C is low in ⁇ , due to high heating temperature: this is considered to be caused by that structure having martensite phase as a main component has become coarse
• steel sheet No. D is low in ⁇ , due to short heating time: this is considered to be caused by that austenite phase was not sufficiently generated during heating and, after rapid cooling, a sufficient volume fraction of martensite phase was not obtained
• steel sheet No. E is low in El, due to high slow cooling terminal temperature

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Applications Claiming Priority (5)

Publications (1)

Family

ID=29738398

Family Applications (1)

Country Status (4)

Cited By (24)

Families Citing this family (3)

Citations (5)

Family Cites Families (24)

• 2003
  • 2003-06-04 US US10/485,780 patent/US20040238082A1/en not_active Abandoned
  • 2003-06-04 DE DE60335106T patent/DE60335106D1/de not_active Expired - Lifetime
  • 2003-06-04 WO PCT/JP2003/007062 patent/WO2003106723A1/fr active Application Filing
  • 2003-06-04 EP EP08159197A patent/EP2017363A3/fr not_active Withdrawn
  • 2003-06-04 EP EP03736017A patent/EP1514951B1/fr not_active Expired - Fee Related

Patent Citations (5)

Also Published As

Similar Documents

Legal Events