JP2013133510A - Cold-rolled steel sheet having excellent shape fixability and aging resistance, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet having excellent shape fixability and aging resistance without using rare metals, and to provide a method for manufacturing the same.SOLUTION: The cold-rolled steel sheet having excellent shape fixability and aging resistance has a chemical composition including, by mass, 0.0030% or less of C, 0.05% or less of Si, 0.30% or less of Mn, 0.05% or less of P, 0.03% or less of S, 0.01-0.10% of Al, and 0.0030% or less of N, and the balance Fe with inevitable impurities, and satisfies [Mn]×[C]≤4.0×10, wherein [M] represents the content (mass%) of an element M. Further, the steel sheet is characterized in that the aging index AI is 20 MPa or less and the yield strength YP after aging for 6 months at 25°C is 180 MPa or less.

Description

  The present invention relates to a cold-rolled steel sheet that is used by press-molding a part in the fields of automobiles, home appliances, and office automation equipment, and more particularly, to a cold-rolled steel sheet that can be applied to a part that requires shape accuracy and a manufacturing method thereof.

  Conventionally, steel materials have realized various characteristics according to applications by effectively utilizing rare metals. However, in recent years, the price of rare metals has soared, greatly affecting the price of steel materials, and concerns about depletion of rare metal resources are increasing, and expectations for steel materials that do not use rare metals are increasing.

As an example of a steel material that does not use a rare metal, a steel plate having excellent aging resistance can be given. For example, in Patent Document 1, an ultra-low carbon Al killed steel containing C: 0.006 wt% or less, Mn: 0.05 to 0.3 wt%, Al: 0.02 to 0.1 wt% is hot rolled at a temperature equal to or higher than the Ar ₃ transformation point. After final rolling, winding at a temperature of 630 ° C or less, cold rolling and annealing at a reduction rate of 76% or more, the crystal grain size is reduced, and the slow aging that fixes solid solution C to the grain boundaries A method for producing a cold-rolled cold rolled steel sheet is disclosed.

Japanese Unexamined Patent Publication No. Sho 63-266025

  However, the slow-aging cold-rolled steel sheet described in Patent Document 1 has a small crystal grain size and also has a high aging index AI and poor aging resistance. Therefore, there is a problem that the shape accuracy is lowered and the shape freezing property is inferior.

  An object of this invention is to provide the cold-rolled steel plate excellent in shape freezing property and aging resistance, and its manufacturing method, without using a rare metal.

  The inventors of the present invention conducted the following investigation on the cold-rolled steel sheet having the above object.

  After aging various ultra-low carbon Al-killed steel sheets (thickness 0.8mm) that do not contain rare metals for 6 months at 25 ° C, 140 x 100mm specimens were collected and used with punches and dies with the shape shown in Fig. 1. Then, with a wrinkle holding force of 200kN, a 4mm-high bulge is formed as shown in Fig. 2, the height profile is measured along the dotted line shown in Fig. 2, and the difference between the maximum and minimum height is warped. And the shape freezing property was evaluated. Similarly, a 140 × 100 mm test piece was taken from the steel sheet before aging treatment, and using a punch and die having the shape shown in FIG. 1, a wrinkle holding force of 200 kN and a height of 4 mm as shown in FIG. Overhang forming was performed, and the warp height of the steel sheet before aging treatment was determined. Then, the difference in warp height before and after aging at 25 ° C. for 6 months was obtained, and this was regarded as the variation in warp height, and the variation in shape freezing property was evaluated.

  In addition, AI of steel plate before aging was applied with JIS No. 5 test piece, pre-strained 7.5%, stress after applying pre-strain from YP obtained by conducting a tensile test after heat treatment at 100 ° C for 30 minutes It was calculated by subtracting Furthermore, YP after aging was obtained by conducting a tensile test using a JIS No. 5 test piece. In each case, the tensile test was performed according to JIS Z2241.

  Fig. 3 shows the relationship between YP after aging and warp height.If YP after aging exceeds 180 MPa, the warp height increases rapidly, while if YP after aging is 180 MPa or less, the warp height increases. It can be seen that the thickness can be reduced to 0.3 mm or less, and excellent shape freezing property can be obtained.

  Figure 4 shows the relationship between AI and warp height variation.If AI exceeds 20 MPa, the warp height variation increases rapidly, while if AI is set to 20 MPa or less, the warp height is increased. It can be seen that the variation in thickness can be reduced to 0.04 mm or less.

  In this way, by reducing the warp height, the shape freezing property can be improved, for example, the warp height after press molding is 0 mm, that is, even when an additional press is performed to eliminate the warp, it is easy to correct the warp. Become. Further, by reducing the variation in the warp height, it is possible to reduce the warp without variation under a predetermined condition corresponding to the warp height even when performing additional pressing.

The present invention has been made based on the above findings, and in mass%, C: 0.0030% or less, Si: 0.05% or less, Mn: 0.30% or less, P: 0.05% or less, S: 0.03% or less, Al: 0.01 to 0.10%, N: not more than 0.0030%, satisfying [Mn] × [C] ≦ 4.0 × 10 ⁻⁴ , the balance being Fe and inevitable impurities, and aging index AI The present invention provides a cold-rolled steel sheet excellent in shape freezing property and aging resistance, characterized in that the yield strength YP after aging at 25 ° C. for 6 months is 180 MPa or less.

  The cold-rolled steel sheet of the present invention is a steel slab having the above chemical composition, hot-rolled, pickled, cold-rolled at a reduction rate of 50% or more, and subjected to annealing at a soaking temperature. It can be produced by a method in which the temperature is set to 700 to 900 ° C., and the ratio t / ([Mn] / [C]) between the soaking time t seconds and ([Mn] / [C]) is 1.0 or more.

  However, [M] represents the content (mass%) of the element M.

  According to the present invention, a cold-rolled steel sheet having excellent shape freezing property and aging resistance can be produced without using rare metals. The cold-rolled steel sheet of the present invention is suitable for press-formed parts in the fields of automobiles, home appliances, and OA equipment.

It is a figure which shows the shape of the punch and die which were used in order to obtain | require curvature height. It is a figure which shows the measurement position of the height profile after bulge forming. It is a figure which shows the relationship between YP after aging and curvature height. It is a figure which shows the relationship between AI and the dispersion | variation in curvature height. It is a figure which shows the relationship between [Mn] x [C] and AI. It is a figure which shows the relationship between [Mn] x [C] and YP after aging. It is a figure which shows the relationship between t / ([Mn] / [C]) and AI. It is a figure which shows the relationship between t / ([Mn] / [C]) and YP after aging.

  Details of the present invention will be described below. The “%” below represents “% by mass” unless otherwise specified.

1) Chemical composition
C: 0.0030% or less
C, when present in the grain as solute C, raises AI. On the other hand, when solute C segregates at the grain boundaries, AI can be reduced compared to the case where it exists in the grains. However, if the amount of solute C at the grain boundary becomes too large, it will exist as cementite and raise YP. Therefore, in order to reduce the YP after aging at 25 ° C. for 6 months to 180 MPa or less, the C content needs to be 0.0030% or less, preferably 0.0020% or less, more preferably 0.0015% or less.

Si: 0.05% or less A large amount of Si deteriorates formability by hardening. Therefore, the Si content is 0.05% or less, preferably 0.03% or less.

Mn: 0.30% or less and [Mn] × [C] ≦ 4.0 × 10 ⁻⁴
Mn deteriorates moldability by hardening. Therefore, the Mn content is 0.30% or less, preferably 0.15% or less, more preferably 0.10% or less.

Furthermore, Mn has an interaction with C and raises AI in order to inhibit the diffusion of solute C and suppress the segregation of C grain boundaries. This effect becomes more prominent as the C content increases. As a result of investigations by the inventors, in order to make AI 20 MPa or less, the Mn amount is 0.30% or less, and [Mn] × [C] ≦ 4.0 × 10 ⁻⁴ , preferably [Mn] × [C] ≦ It is necessary to satisfy 3.0 × 10 ⁻⁴ , more preferably [Mn] × [C] ≦ 2.0 × 10 ⁻⁴ . In the present application, the amount of Mn is adjusted as described above to achieve grain boundary segregation of solute C. Therefore, it is possible to suppress the increase in YP compared to the case where grain boundary segregation of solute C is performed by refining. In addition to lowering AI, lowering YP can also be achieved, and as a result, YP can be lowered after aging.

P: 0.05% or less
P segregates at the grain boundaries and deteriorates ductility and toughness. Therefore, the P content is 0.05% or less, preferably 0.03% or less.

S: 0.03% or less
S significantly lowers the hot ductility, induces hot cracking, and significantly deteriorates the surface properties. Therefore, the S content is 0.03% or less, preferably 0.02% or less, more preferably 0.01% or less.

Al: 0.01-0.10%
Al can suppress age hardening due to solute N by fixing N as a nitride. Therefore, in order to reduce AI to 20 MPa or less, the Al content needs to be 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. On the other hand, a large amount of Al exceeding 0.10% causes an increase in production cost. Therefore, the Al content is 0.01 to 0.10%, preferably 0.03 to 0.10%, more preferably 0.05 to 0.10%.

N: 0.0030% or less
When N is contained in a large amount, there is a risk that surface flaws occur due to slab cracking during hot rolling. Moreover, when it exists as solid solution N, age hardening is caused. Therefore, the N content is 0.0030% or less, preferably 0.0020% or less.

  The balance is Fe and inevitable impurities.

2) AI: 20MPa or less, YP after aging: 180MPa or less As described above, AI must be 20MPa or less and YP after 6 months aging at 25 ° C must be 180MPa or less to obtain excellent shape freezing properties. .

3) Manufacturing method The cold-rolled steel sheet of the present invention can be manufactured by hot rolling a steel slab having the above chemical composition, pickling, and then performing cold rolling and annealing under the following conditions.

Cold rolling reduction: 50% or more When the rolling reduction is less than 50%, the crystal grains after annealing become large, solid solution C remains in the grains, and AI cannot be reduced to 20 MPa or less. Therefore, the rolling reduction of cold rolling is set to 50% or more. If the rolling reduction exceeds 90%, the rolling load increases, so the rolling reduction is preferably 90% or less, and more preferably 80% or less.

Soaking temperature for annealing: 700-900 ℃
If the soaking temperature is less than 700 ° C, an unrecrystallized structure remains or crystal grains become finer, and YP cannot be reduced to 180 MPa or less after aging. On the other hand, when the soaking temperature exceeds 900 ° C and enters the austenite region, ferrite transformation occurs due to cooling after soaking, so it becomes difficult to segregate solid solution C at the grain boundaries, and AI cannot be reduced to 20 MPa or less. Therefore, the soaking temperature for annealing is 700 to 900 ° C, preferably 750 to 900 ° C.

Annealing time t of annealing: Time for t / ([Mn] / [C]) to be 1.0 or more (second)
If t / ([Mn] / [C]) is less than 1.0 and the soaking time is short, diffusion of the solid solution C is suppressed, so that the solid solution C cannot be segregated at the grain boundaries, and AI Can not be less than 20MPa. Accordingly, the soaking time t of annealing is set so that t / ([Mn] / [C]) is 1.0 or more, preferably 3.0 or more, more preferably 5.0 or more, and further preferably 10.0 or more.

  Note that the relationship between the above t / ([Mn] / [C]) and AI is that when the soaking time is short, it is difficult to segregate the solid solution C to the grain boundary because the diffusion of the solid solution C is suppressed, In addition, as the amount of Mn increases with respect to the amount of C, the suppression of such solute C diffusion becomes more prominent, so the inventors have determined that the soaking time t, the amount of C in the steel, the amount of Mn, It was obtained as a result of examining the relationship with AI. Note that if the soaking time is too long, the particle size becomes coarse and the appearance after molding may be deteriorated.

  A normal converter method, an electric furnace method, or the like can be appropriately applied to the steel melting method. The molten steel is cast into a slab and then subjected to hot rolling as it is or after cooling and heating. The annealing after the cold rolling is preferably performed in a continuous annealing furnace in which a uniform material can be easily obtained. The cold-rolled steel sheet after annealing can be subjected to surface treatment such as temper rolling at a rolling reduction of about 0.5 to 3%, hot dip galvanizing treatment or chemical conversion treatment.

  After melting the steel with the chemical composition shown in Table 1 and making it into a slab, heat at 1200 ° C for 1 hour, finish rolling in the austenite region, wind up at 600 ° C, and make a hot rolled steel plate with a thickness of 2.6 mm, After pickling, cold rolling and annealing were performed under the conditions shown in Table 1, and then temper rolling was performed at a rolling reduction of 1.0% to obtain a cold rolled steel sheet. At this time, annealing was performed in a continuous annealing furnace. Then, a JIS No. 5 tensile test piece was cut out from the rolling direction, a tensile test was performed, and YP, tensile strength TS, and elongation El were measured. In addition, AI and post-aging YP were measured by the above methods.

  The results are shown in Table 2.

FIG. 5 shows the relationship between [Mn] × [C] and AI, FIG. 6 shows the relationship between [Mn] × [C] and post-aging YP, and FIG. 7 shows t / ([Mn] FIG. 8 shows the relationship between t / ([Mn] / [C]) and post-aging YP. 5 and 6, it can be seen that by setting [Mn] × [C] ≦ 4 × 10 ⁻⁴ , AI can be 20 MPa or less, and YP after aging is 180 MPa or less, and the shape freezing property is excellent. 7 and 8, it can be seen that by setting t / ([Mn] / [C]) to 1.0 or more, AI can be 20 MPa or less and YP after aging is 180 MPa or less, and the shape freezing property is excellent. On the other hand, as shown in Test Steel No. 7 in Table 2, when the rolling reduction of cold rolling is less than 50%, or as shown in Test Steel Nos. When the temperature is outside the scope of the invention or when the annealing temperature is outside the range of 700 to 900 ° C, as shown in Test Steel Nos. 24 and 25, AI exceeds 20 MPa, and after aging, YP exceeds 180 MPa. Thus, it can be seen that the shape freezing property is inferior.

Claims (2)

In mass%, C: 0.0030% or less, Si: 0.05% or less, Mn: 0.30% or less, P: 0.05% or less, S: 0.03% or less, Al: 0.01-0.10%, N: 0.0030% or less, and It has a chemical composition satisfying [Mn] × [C] ≦ 4.0 × 10 ⁻⁴ , the balance being Fe and inevitable impurities, an aging index AI of 20 MPa or less, and a yield strength YP after aging for 6 months at 25 ° C. Cold-rolled steel sheet excellent in shape freezing property and aging resistance characterized by being 180 MPa or less; provided that [M] represents the content (% by mass) of element M.   When the steel slab having the chemical composition according to claim 1 is hot-rolled, pickled, cold-rolled at a reduction rate of 50% or more, and annealed, the soaking temperature is 700 to 900 ° C. And the ratio of soaking time t seconds to ([Mn] / [C]) t / ([Mn] / [C]) is 1.0 or more. Excellent shape freezing and aging resistance. A method for producing a cold-rolled steel sheet; where [M] represents the content (% by mass) of element M.