CA2693787C

CA2693787C - Method for producing low yield strength cold rolled steel sheet excellent in uniformity

Info

Publication number: CA2693787C
Application number: CA2693787A
Authority: CA
Inventors: Yoshihiko Ono; Hideyuki Kimura; Kaneharu Okuda; Takeshi Fujita; Michitaka Sakurai
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2007-07-11
Filing date: 2008-07-10
Publication date: 2012-02-07
Anticipated expiration: 2028-07-10
Also published as: JP5272548B2; KR101164471B1; CN101688265B; EP2169083B1; US20100326572A1; EP2169083A1; CA2693787A1; CN101688265A; KR20100027209A; EP2169083A4; JP2009035816A; WO2009008548A1

Abstract

Producing a high-strength cold-rolled steel sheet with low YP and excellent uniformity by hot-rolling and cold-rolling steel containing, by % by mass, 0.01 to 0.08 C, 0.2 or less Si, 0.8 to 1.7 Mn, 0.03 or less P, 0.02 or less S, 0.3 or less sol. A1, 0.01 or less N, and 0.4 to 2 Cr, and where 1.9 < [Mneq] < 3 and 0.34 <= [%Cr] / [%Mn]. The steel is heated at less than 3 °C/sec to 680 to 740°C, annealed at 740 to 820°C, cooled at 2 to 30 °C/sec to 650°C; cooled at 10 °C/sec or more to 650 to Tc°C and cooled at 0.2 to 10 °C/sec to Tc°C to 200°C. Tc = 410 - 40 x [%Mn] - 30 [%Cr] and [Mneq] is [%Mn] + 1.3 x [%Cr].

Description

= CA 02693787 2010-01-11 DESCRIPTION
METHOD FOR PRODUCING LOW YIELD STRENGTH COLD ROLLED STEEL

SHEET EXCELLENT IN UNIFORMITY
Technical Field The present invention relates to a method for producing a high-strength cold-rolled steel sheet for press forming which is used for automobiles, home electric appliances, and the like through a press forming process.

Background Art BH steel sheets with 340MPa grade in tensile strength (bake-hardenable steel sheets, simply referred to as "340BH"
hereinafter) and IF steel sheets with 270MPa grade in tensile strength (Interstitial Free steel sheets, simply referred to as "270IF" hereinafter), which is ultra-low-carbon steel containing carbide/nitride-forming elements such as Nb and Ti to control the amount of dissolved C, have been applied to automotive outer panels, such as hoods, doors, trunk lids, back doors, and fenders, which are required sufficient dent resistance. In recent years, regarding the increasing requirement of further weight reduction of car bodies, new attempt of applying steel sheet with higher strength and superior dent resistance has been carried out to reduce thickness of the steel sheet for outer panel. Also investigations to improve dent resistance and to decrease the temperature and time of a baking finish process while maintaining the current thickness have been progressed in view of applying higher strength steel sheet.

However, when a solution-hardening element such as Mn, P, or the like is further added to 340BH with a yield strength YP of 230 MPa or 270IF with a YP of 180 MPa to strengthen and thin a steel sheet, surface distortion occurs.
The term "surface distortion" represents micro wrinkles or wavy patterns produced in a press-formed surface due to an increase in YP. The occurrence of surface distortion impairs the design or design property of a door, a trunk lid, or the like. Therefore, the steel sheet for this application is desired that YP after press forming and baking finish treatment is increased more than YP of conventional steel sheet while maintaining extremely low YP
before press forming.

In such a background, for example, Patent Document 1 discloses a method for producing a steel sheet having low YP, high work-hardenability WH, and high BH by appropriately controlling the cooling rate after annealing of steel to form a dual phase mainly composed of ferrite and martensite, the steel containing 0.005 to 0.15% of C, 0.3 to 2.0% of Mn, and 0.023 to 0.8% of Cr. In addition, Patent Document 2 discloses a method for producing a high-strength cold-rolled steel sheet having high BH by annealing steel which contains 0.01% to 0.04% of C, 0.3 to 1.6% of Mn, 0.5% or less of Cr, and 0.5% or less of Mo and which satisfies 1.3 <_ Mn + 1.29Cr + 3.29Mo <_ 2.1% and cooling at a cooling rate of 100 C/sec or more in the temperature range of at least 550 C or lower to increase the amount of dissolved C in the steel. Patent Document 3 discloses a method for producing a high-strength cold-rolled steel sheet including ferrite and a low-temperature transformed phase and having high BH and excellent surface appearance quality after press forming, the method including annealing steel which contains 0.0025%
to less than 0.04% of C, 0.5 to 2.5% of Mn, and 0.05% to 2.0% of Cr, cooling at a cooling rate of 15 to 200 C/sec in the temperature range of 650 C to 450 C, and further cooling at a cooling rate of less than 10 C/sec in the temperature range of 200 C to near 300 C.

Patent Document 1: Japanese Examined Patent Application Publication No. 62-40405 Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-233294 Patent Document 3: Japanese Unexamined Patent Application Publication No. 2006-52465 Disclosure of Invention However, the high-strength cold-rolled steel sheets produced by the methods described in Patent Documents 1 to 3 have the following problems:

i) YP is not sufficiently decreased, and thus press-forming into a door panel or the like produces a large amount of surface distortion as compared with 340BH.

ii) In such dual phase high-strength cold-rolled steel sheets, hard martensite is dispersed as a second phase for strengthening, and thus fluctuations of mechanical properties are essentially easy to occur. For example, the volume fraction of a second phase significantly influenced by changes in the C content of several tens ppm in steel and the annealing temperature of 20 to 50 C, and thus mechanical properties significantly vary as compared with conventional 340BH and 270IF which are solid-solution-hardened with Mn and P.

The present invention has been achieved in the above-described situation, and an object is to provide a method for producing a high-strength cold-rolled steel sheet with low YP and excellent uniformity.

The inventors of the present invention conducted close investigation on a method for further decreasing YP while maintaining high BH equivalent to or higher than a general value and decreasing variation of mechanical properties with respect to a dual phase high-strength cold-rolled steel sheet. As a result, the following findings were obtained:

(I) By appropriately controlling the composition ranges of Mn and Cr and performing slow heating in a predetermined temperature range during annealing, an attempt can be made to coarsely and uniformly disperse a second phase, thereby decreasing YP and suppressing YP variation with annealing temperature.

(II) By appropriately controlling the composition ranges of Mn and Cr, excessive decrease in the amount of dissolved C can be suppressed, thereby achieving high BH.

The present invention has been achieved on the basis of the above-mentioned findings and provides a method for producing a high-strength cold-rolled steel sheet, the method including hot-rolling and cold-rolling steel having a composition which contains, by % by mass, over 0.01% to less than 0.08% of C, 0.2% or less of Si, 0.8% to less than 1.7%
of Mn, 0.03% or less of P, 0.02% or less of S, 0.3% or less of sol. Al, 0.01% or less of N, and over 0.4% to 2% of Cr, and which satisfies 1.9 < [Mneq] < 3 and 0.34 <_ [%Cr]/[%Mn], the balance being composed of iron and inevitable impurities; heating at an average heating rate of less than 3 C/sec in a temperature range of 680 C to 740 C; annealing at an annealing temperature of over 740 C to less than 820 C; cooling at an average cooling rate of 2 to 30 C/sec in the temperature range of the annealing temperature to 650 C; cooling at an average cooling rate of 10 C/sec or more in the temperature range of 650 C to Tc C represented by the equation (1) below; and cooling at an average cooling rate of 0.2 to 10 C/sec in the temperature range of Tc C to 200 C.

Tc = 410 - 40 x [%Mn] - 30 x [%Cr] ... (1) Here, [Mneq] represents the Mn equivalent shown by [Mneq] = [%Mn] + 1.3 x [%Cr] and [%Mn] and [%Cr] represent the contents of Mn and Cr, respectively.

In the method for producing the high-strength cold-rolled steel sheet of the present invention, heating is preferably performed at an average heating rate of less than 2 C/sec in the temperature range of 680 C to 740 C during annealing.

Further, preferably, steel satisfying 0.55 <_ [%Cr]/[%Mn] is used, and 0.005% by mass or less of B is contained. In addition, at least one of 0.15% by mass or less of Mo and 0.2% by mass or less of V is preferably contained. Further, at least one of less than 0.014% by mass of Ti, less than 0.01% by mass of Nb, 0.3% by mass or less of Ni, and 0.3% by mass or less of Cu is preferably contained.

According to the present invention, a high-strength cold-rolled steel sheet with low YP and excellent uniformity can be produced. The high-strength cold-rolled steel sheet produced by the method of the present invention has excellent resistance to surface distortion and excellent dent resistance and is thus suitable for strengthening and thinning automotive parts.

Brief Description of Drawings Fig. 1 is a graph showing a relationship between YP and the average heating rate in annealing.

Best Mode for Carrying Out the Invention The present invention will be described in detail below.
"%" indicating the content of a component represents "% by mass" unless otherwise specified.

1) Composition C: over 0.01% to less than 0.08%

C is an element necessary for securing a predetermined amount of a second phase. When the C content is excessively low, the second phase cannot be secured in a sufficient amount, and low YP cannot be achieved. Further, sufficiently high BH cannot be secured, and the anti-aging property is degraded. In order to secure a sufficient amount of the second phase, the C content is required to exceed 0.01%. On the other hand, when the C content is 0.08% or more, the ratio of the second phase is excessively increased with a result that YP increases. Therefore, the upper limit of the C content is less than 0.08%. The C
content is preferably less than 0.06% for achieving lower YP

and more preferably less than 0.04% for achieving further lower YP.

Si: 0.2% or less Si has the effect of delaying scale formation in hot rolling and improves surface appearance quality when added in a small amount, the effect of further homogenizing and coarsening the microstructure of a steel sheet, and the effect of improving seizing to a mold (mold galling) in press forming. Therefore, Si can be added from this viewpoint. However, Si has a large solution-hardening ability and thus has the large effect of increasing YP.
Therefore, the Si content is in the range of 0.2% or less which causes little influence on an increase in YP. The C
content is preferably 0.1% or less.

Mn: 0.8% to less than 1.7%

Mn can enhance hardenability and decrease the amount of dissolved C in a predetermined range by appropriately controlling the Mn content to decrease YP and increase BH.
When the Mn content is 0.8% or less, the amount of dissolved C is excessively increased in a cooling step of annealing, and a large amount of dissolved C precipitates in strain around martensite during overaging treatment in the temperature range of less than 400 C, causing difficulty in sufficiently decreasing YP. In addition, when the amount of dissolved C is excessively increased, anti-aging property is degraded. On the other hand, when the Mn content is 1.7% or more, the amount of dissolved C is excessively decreased, thereby decreasing BH. Further, solid-solution hardening of Mn is increased, and a second phase is made fine increasing YP and cause variation of YP with annealing temperature.
Therefore, the Mn content is 0.8% to less than 1.7%.

P: 0.03% or less P has a large solution hardening ability and is preferably added in as small an amount as possible from the viewpoint of decreasing YP. However, P has the effect of further coarsening the microstructure of a steel sheet and the effect of improving seizing to a mold (mold galling) in press forming. Therefore, the P content is 0.03% or less which has a small adverse effect on an increase in YP.

S: 0.02% or less S precipitates as MnS in steel but decreases the ductility of a steel sheet and decreases press formability when added in a large amount. In addition, hot ductility is decreased in hot rolling of a slab, and thus surface defects easily occur. Therefore, the S content is 0.02% or less but is preferably as low as possible.

sol. Al: 0.3% or less Al is used as a deoxidizing element or an element for improving the anti-aging property by fixing N as AlN.
However, Al forms fine A1N during coiling or annealing after hot rolling to suppress the growth of ferrite grains and inhibit reduction in YP. From the viewpoint of decreasing oxides in steel or improving anti-aging property, Al is preferably added in an amount of 0.02% or more. On the other hand, from the viewpoint of improving the grain growth property, the ferrite grain growth property is improved by increasing the coiling temperature to 620 C or more, but the amount of fine AlN is preferably as small as possible.
Therefore, preferably, the sol. Al content is 0.15% or more, and A1N is coarsely precipitated during coiling. However, since the cost is increased when the sol. Al content exceeds 0.3%, the sol. Al content is 0.3% or less. In addition, when the sol. Al content exceeds 0.1%, castability is impaired to cause deterioration of the surface appearance quality. Therefore, the sol. Al content is preferably 0.1%
or less for application to exterior panels which are required to be strictly controlled in surface appearance quality.

N: 0.01% or less N precipitates during coiling or annealing after hot rolling to form. fine AiN and inhibit the grain growth property. Therefore, the N content is 0.01% or less but is preferably as low as possible. In addition, an increase in the N content causes deterioration of the anti-aging property. From the viewpoint of improving the grain growth and anti-aging property, the N content is preferably less than 0.008% and more preferably less than 0.005%.

Cr: over 0.4% to 2%

Cr is the most important element in the present invention. Since Cr has a small amount of solid-solution hardening and the effect of making fine martensite as the second phase and enhancing hardenability, Cr is an element effective in decreasing YP and decreasing variation in material quality. In order to exhibit these effects, it is necessary to control the Mn equivalent and the composition ratio to Mn, which will be described below, and the Cr content is necessary to exceed 0.4%. On the other hand, when the Cr content exceeds 2%, the cost is increased, and the surface appearance quality of a galvanized steel sheet is degraded. Therefore, the Cr content is 2% or less.

1.9 < [Mneq] < 3 When the Mn equivalent defined in the present invention, i.e., the [Mneq], is controlled to exceed 1.9 by controlling the cooling rate in annealing, the amount of dissolved C is decreased to a proper range, and the formation of pearlite and bainite is suppressed to decrease YP and increase BH.
Further, from the viewpoint of decreasing YP, [Mneq]
preferably exceeds 2.1 and more preferably exceeds 2.2. On the other hand, when [Mneq] is excessively increased, BH is decreased, and the cost is increased. Therefore, [Mneq] is less than 3.

0.34 S [%Cr]/[%Mn]

When the ratio of the Cr content to the Mn content, i.e., [oCr]/[%Mn], is controlled to 0.34 or more while [Mneq] is constant, the second phase can be coarsened and solid-solution hardening of Mn can be decreased, thereby decreasing YP and material quality variation. In order to further decrease YP and material quality variation, 0.55 <
[%Cr]/[%Mn] is preferred.

The balance includes iron and inevitable impurities, but the elements below may be contained at predetermined contents.

B: 0.005% or less Similarly, B is an element for enhancing hardenability and has the function to fix N as BN to improve the grain growth property. However, when B is excessively added, the second phase is inversely made fine by the influence of residual dissolved B. Therefore, the B content is preferably 0.005% or less. In the steel of the present invention, the effect of improving the ferrite grain growth property can be sufficiently exhibited by adding over 0.001%
of B, thereby achieving extremely low YP. Therefore, the B
content preferably exceeds 0.001%.

Mo: 0.1% or less Like Mn and Cr, Mo is an element for enhancing hardenability and can be added for the purpose of improving hardenability. However, when Mo is excessively added, like Mn, the second phase is made fine and hard, increasing YP.
Therefore, Mo is preferably added in the range of 0.1% or less which has the small influence on an increase in YP.
From the viewpoint of further decreasing YP and 4YP, the Mo content is preferably less than 0.02% (not added).

V: 0.2% or less Similarly, V is an element for enhancing hardenability.
However, when V is added in an amount exceeding 0.2%, the cost is significantly increased. Therefore, V is preferably added in the range of 0.2% or less.

Ti: less than 0.014%

Ti has the effect of improving the anti-aging property by fixing N and the effect of improving castability.
However, Ti forms fine precipitates of TiN, TiC, Ti(C, N), and the like in steel to inhibit the grain growth property.
Therefore, from the viewpoint of decreasing YP, the Ti content is preferably less than 0.014%.

Nb: less than 0.01%

Nb has the effect of delaying recrystallization in hot rolling controlling the texture and decrease YP in a direction at 45 degrees with the rolling direction. However, Nb forms fine NbC and Nb(C, N) in steel to significantly degrade the grain growth property and increase YP.

Therefore, Nb is preferably added in the range of less than 0.01% which has a small influence on an increase in YP.

Cu: 0.3% or less Cu is an element mixed when scraps or the like are positively utilized and a recycled material can be used as a raw material when Cu is allowed to be mixed, thereby decreasing the production cost. Cu has a small influence on the material quality, but mixing of excessive Cu causes surface flaws. Therefore, the Cu content is preferably 0.3%
or less.

Ni: 0.3% or less Ni also has a small influence on the material quality of a steel sheet, but Ni can be added from the viewpoint of decreasing surface flaws when Cu is added. However, when Ni is excessively added, surface defects due to heterogeneity of scales are produced. Therefore, the Ni content is preferably 0.3% or less.

2) Production condition As described above, the production method of the present invention includes hot-rolling and cold-rolling a steel slab having the above-described composition, heating at an average heating rate of less than 3 C/sec in the temperature range of 680 C to 740 C, annealing at an annealing temperature of over 740 C to less than 820 C, cooling at an average cooling rate of 2 to 30 C/sec in the temperature range of the annealing temperature to 650 C, cooling at an average cooling rate of 10 C/sec or more in the temperature range of 650 C to Tc C represented by the equation (1) described above, and cooling at an average cooling rate of 0.2 to 10 C/sec in the temperature range of Tc C to 200 C.

Hot rolling The slab can be hot-rolled by a method of rolling the slab after heating, a method of directly rolling the slab without heating after continuous casting, or a method of rolling the slab by heating for a short time after continuous casting. The hot rolling may be performed according to a general method, for example, at a slab heating temperature of 1100 C to 1300 C, a finish rolling temperature of Ara transformation point or more, an average cooling rate after finish rolling of 10 to 200 C/sec, and a coiling temperature of 400 C to 720 C. In order to obtain beautiful plating appearance quality for an outer panel, preferably, the slab heating temperature is 1200 C or less, and the finish rolling temperature is 840 C or less. In addition, descaling is preferably sufficiently performed for removing primary and secondary scales formed on the surface of the steel sheet. From the viewpoint of decreasing YP, the coiling temperature is preferably as high as possible and 640 C or more. In particular, when the coiling temperature is 680 C or more, Mn and Cr can be sufficiently concentrated in the second phase in the state of the hot-rolled sheet, and stability of y in the subsequent annealing step is improved, contributing to a decrease in YP. In order to decrease in-plane anisotropy of r value of the steel sheet and suppress YP in a direction at 45 with the rolling direction, the cooling rate after finish rolling is preferably as large as 40 C/sec or more Cold rolling The rolling rate of cold rolling may be 50% to 85%.
Annealing Average heating rate in annealing: less than 3 C/sec In order to uniformly disperse the coarse second phase after annealing and decrease YP and variation in material quality, it is effective to control the heating rate in the temperature region of 680 C to 740 C. This is because in a component system with [Mneq] of over 1.9, the second phase after annealing is easily made fine. It is considered that, when the Mn content is high, the Acl transformation temperature is excessively decreased, and y grains are formed in unrecrystallized ferrite grain boundaries before the completion of recrystallization. Even when recrystallization is completed, y grains are produced in fine ferrite grain boundaries immediately after recrystallization. Therefore, YP of a steel sheet is easily increased.

Steel containing 0.028% of C, 0.01% of Si, 1.6% of Mn, 0.01% of P, 0.01% of S, 0.04% of sol. Al, 0.8% of Cr, and 0.003% of N was molten in a laboratory to produce a slab of 27 mm in thickness. The slab was heated to 1250 C, hot-rolled to 2.3 mm at a finish rolling temperature of 830 C, and then coiled for 1 hour at 620 C. The resultant hot-rolled sheet was cold-rolled to 0.75 mm with a rolling reduction of 67%. The resultant cold-rolled sheet was annealed at 780 C for 40 seconds at an average heating rate changed from 0.3 to 20 C/sec in the range of 680 C to 740 C, cooled at an average cooling rate of 7 C/sec in the temperate range from the annealing temperature to 650 C, cooled at 25 C/sec in the temperature range from 650 C to 300 C, cooled at 0.5 C/sec in the temperature range from 300 C to 200 C, and then air-cooled to room temperature. A
JIS No. 5 tensile test piece was collected from the resultant steel sheet and subjected to a tensile test (according to JISZ2241, the tensile direction perpendicular to the rolling direction) and SEM observation of the structure.

Fig. 1 shows a relation between YP and the average heating rate in the temperature region of 680 C to 740 C
during annealing. At the heating rate of less than 3 C/sec, YP of 200 MPa or less can be obtained, while at the heating rate of less than 2 C/sec, YP of 195 MPa or less can be obtained. In this case, it was confirmed by SEM that the second phase is more coarsely and uniformly dispersed.
Further, the influence on variation in material quality was examined for steel sheets annealed at various heating rates.
Namely, the annealing temperature of each steel sheet was changed from 760 C to 810 C to examine a variation DYP of YP
with a change of 50 C in the annealing temperature. As a result, it was found that in a sample subjected to annealing at a heating rate of 20 C/sec in the range of 680 C to 740 C, AYP is 20 MPa, while in a steel sheet subjected to annealing at a heating rate of less than 3 C/sec, AYP is decreased to less than 15 MPa. Therefore, a steel sheet having low YP and low AYP with annealing temperature can be obtained by controlling the heating rate in a predetermined range.

Annealing temperature: over 740 C to less than 820 C
At the annealing temperature of 740 C or less, the second phase cannot be stably secured because of the insufficient solid solution of carbides. At the annealing temperature of 820 C or more, the y ratio is excessively increased in annealing, and elements such as Mn, C, and the like are not sufficiently concentrated in y grains, thereby failing to achieve sufficiently low YP. This is possibly because when elements are not sufficiently concentrated in y grains, strain is not sufficiently applied to the periphery of martensite, and pearlite and bainite transformation easily occurs in the cooling step after annealing. The holding time during annealing is preferably 20 seconds or more in the temperature range of over 740 C which corresponds to usual continuous annealing, and is more preferably 40 seconds or more.

Average cooling rate (primary cooling rate) in temperature range of annealing temperature to 650 C: 2 to 30 C/sec In order to concentrate Mn and Cr in y grains during cooling to enhance hardenability of the y grains and decrease YP, the average cooling rate in the temperature range of the annealing temperature to 650 C is necessary to be 2 to 30 C/sec.

Average cooling rate (secondary cooling rate) in temperature range of 650 C to Tc C represented by equation (1) described above: 10 C/sec or more When cooling is performed at an average cooling rate of C/sec or more in the temperature range from 650 C to Tc C near the Ms point in which pearlite and bainite are easily produced, the formation of pearlite and bainite is suppressed, thereby achieving sufficiently low YP.

Average cooling rate (tertiary cooling rate) in temperature range of Tc C to 200 C: 0.2 to 10 C/sec When cooling is performed at an average cooling rate of 0.2 to 10 C/sec in the temperature range from Tc C to 200 C, dissolved C which excessively remains in ferrite is precipitated to decrease YP and increase ductility.

The high-strength cold-rolled steel sheet produced by the method of the present invention has yield point elongation (YPEl) of less than 0.5% and sufficiently decreased YP in an annealed state and thus can be used directly as a steel sheet for press forming. However, from the viewpoint of controlling surface roughness and stabilizing press formability by flattening a shape of steel sheet, skin pass rolling may be performed. In this case, from the viewpoint of decreasing YP and increasing El and WH, the elongation is preferably 0.3% to 0.5%.

EXAMPLE
Steel of each of Steel Nos. A to BB shown in Table 1 was molten and continuously cast into a slab of 230 mm in thickness. The slab was reheated to 1180 C to 1250 C and hot-rolled at a finish rolling temperature of 830 C (Steel Nos. A to D, I, R to V, and X to BB) or 880 C (Steel Nos. E
to H, J to Q, and W). Then, the steel sheet was cooled at an average cooling rate of 20 C/sec and coiled at a coiling temperature of 540 C to 640 C. The resultant hot-rolled sheet was cold-rolled with a rolling reduction of 67% to 78%
after pickling to form a cold-rolled sheet of 0.75 mm in thickness. The resultant cold-rolled sheet was annealed at the average heating rate in the temperature range of 680 C
to 740 C, the annealing temperature, the primary average cooling rate in the temperature range of the annealing temperature to 650 C, the secondary average cooling rate in the temperature range of 650 C to Tc C, and the tertiary average cooling rate in the temperature range of Tc C to 200 C, which are shown in Tables 2 and 3. A JIS No. 5 test piece was collected from the resultant annealed steel sheet, i.e., the steel sheet not having undergone skin pass rolling, in each of the rolling direction and the perpendicular direction and subjected to a tensile test (according to JISZ2241) to evaluate YP and TS. In addition, the annealing temperature for the steel sheet with each of the compositions was changed in the range of 760 C to 810 C to measure the maximum and minimum of YP and determine variation AYP of YP. Further, prestrain of 2% was applied to the same test piece to determine an increase in YP after heat treatment at 170 C for 20 minutes, i.e., BH.

The results are shown in Tables 2 and 3.

The steel sheet of an example of the present invention exhibits low YP, i.e., low YR, as compared with a material in the same TS level. The steel sheet of an example of the present invention also has AYP and is thus excellent in YP
stability. In particular, in the steel sheet in which [Mneq] and [%Cr]/[%Mn] are appropriately controlled to over 2.1 and 0.55 or more, respectively, and the heating rate of annealing is controlled to less than 3 C/sec, solution hardening by Mn and dissolved C is decreased, and the second phase is uniformly coarsened, thereby decreasing YP and AYP.
For example, in the steel of Steel Nos. B, C, and D, [Mneq]
is increased as compared with the steel of Steel No. A, but [%Cr]/[%Mn] is in the range of 0.34 to 0.41. Therefore, the amounts of pearlite and bainite produced are decreased with increase in [Mneq], and the amount of dissolved C is decreased. However, the second phase is made fine to exhibit YP in the range of 191 to 197 MPa and AYP in the range of 7 to 9 MPa with annealing temperature under the conditions with a heating rate of 1.5 C/sec and an annealing temperature of 780 C. On the other hand, in the steel of Steel Nos. E, F, G, and H in each of which [Mneq]
is increased to over 2.1 and [%Cr]/[%Mn] is controlled to 0.55 or more, YP and AYP with annealing temperature are in the range of 172 to 198 MPa and the range of 4 to 6 MPa, respectively, and very low under the same conditions as Steel Nos. A, B, C, and D. In addition, an increase in YP
due to an increase in C is extremely small, and Steel No. K
in which the C content is increased to 0.058% has TS of 490 MPa and YP of as low as 208 MPa. Further, Steel No. L in which the C content is increased to 0.072% has TS of 541 MPa and YP of as low as 230 MPa. Namely, even when the C
content is changed, a steel sheet with small AYP and low YR
can be stably obtained. Further, since the composition ranges of Mn and Cr are appropriately controlled, high BH is achieved in spite of low YP.

However, a steel sheet in which [Mneq] and the heating rate and cooling rate in annealing are not appropriately controlled has high YR as compared with the steel sheet of an example of the present invention in the same strength level. For example, Steel Nos. S and V in which [%Cr]/[%Mn]
is not appropriately controlled have the fine second phase and the large amount of solution hardening and thus have high AYP and YP and low BH. Steel No. T containing Mo has the tendency to form a fine microstructure, increasing YP
and IYP. With Steel No. U in which the C content is out of the predetermined range, and consequently the area ratio of the second phase is out of the predetermined range, low YR
cannot be achieved. In Steel Nos. X and Y containing large amounts of P and Si, the second phase is coarsened, but low YP cannot be achieved because the amount of solid-solution hardening is excessively increased. Therefore, with conventional steel, a steel sheet satisfying low YP, small AYP, and high BH cannot be obtained.

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Claims

1. A method for producing a high-strength cold-rolled steel sheet comprising hot-rolling and cold-rolling steel having a composition which contains, by % by mass, over 0.01% to less than 0.08% of C, 0.2% or less of Si, 0.8% to less than 1.7% of Mn, 0.03% or less of P, 0.02% or less of S, 0.3% or less of sol. Al, 0.01% or less of N, and over 0.4%

to 2% of Cr, and which satisfies 1.9 < [Mneq] < 3 and 0.34 <=
[%Cr]/[%Mn], the balance being composed of iron and inevitable impurities; heating at an average heating rate of less than 3 °C/sec in a temperature range of 680°C to 740°C;
annealing at an annealing temperature of over 740°C to less than 820°C; cooling at an average cooling rate of 2 to 30 °C/sec in a temperature range of the annealing temperature to 650°C; cooling at an average cooling rate of °C/sec or more in a temperature range of 650°C to Tc°C
represented by equation (1) below; and cooling at an average cooling rate of 0.2 to 10 °C/sec in a temperature range of Tc°C to 200°C, Tc = 410 - 40 x [%Mn] - 30 x [%Cr] ... (1) wherein [Mneq] represents the Mn equivalent shown by [Mneq] = [%Mn] + 1.3 x [%Cr] and [%Mn] and [%Cr] represent the contents of Mn and Cr, respectively.

2. The method for producing the high-strength cold-rolled steel sheet according to Claim 1, wherein during annealing, heating is performed at an average heating rate of less than 2°C/sec in a temperature range of 680°C to 740°C.

3. The method for producing the high-strength cold-rolled steel sheet according to Claim 1 or 2, wherein steel satisfying 0.55 <= [%Cr]/[%Mn] is used.

4. The method for producing the high-strength cold-rolled steel sheet according to any one of Claims 1 to 3, wherein steel further containing, by % by mass, 0.005% or less of B
is used.

5. The method for producing the high-strength cold-rolled steel sheet according to any one of Claims 1 to 4, wherein steel further containing, by % by mass, at least one of 0.1%
or less of Mo and 0.2% or less of V is used.

6. The method for producing the high-strength cold-rolled steel sheet according to any one of Claims 1 to 5, wherein steel further containing, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu is used.