JP2002080932A - Cold rolled steel sheet and plated steel sheet having excellent strength increasing heat treatability after forming and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet and a plated steel sheet in which tensile strength effectively increases by press forming-heat treatment while maintaining excellent deep drawability in press forming. SOLUTION: The steel composition is controlled so as to contain <0.0050% C, 0.005 to 1.0% Si, 0.01 to 1.5% Mn, 0.005 to 0.050% Nb, 0.005 to 0.030% Al, 0.005 to 0.040% N, 0.0005 to 0.0015% B, <=0.1% P and <=0.01% S in the ranges satisfying the inequalities (1) and (2): N%>=0.0015+14/93.Nb%+14/27.Al%+14/11.B%...(l) and C%<=12/93.Nb%...(2) or further to contain one or more kinds selected from Cu, Ni and Mo, and the balance substantially Fe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in a place where structural strength, particularly strength and / or rigidity at the time of deformation, is required, such as a construction member, a part for mechanical structure and a part for automobile structure. TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet and a plated steel sheet which are excellent as a material steel sheet of a formed body subjected to a heat treatment for increasing strength after working by forming and the like, and have excellent heat treatment ability for increasing strength after molding, and a method for producing them.

[0002]

2. Description of the Related Art In the production of a press-formed body of a thin steel sheet, before press-forming, it is soft and easy to press-form, and as a method of hardening after press-forming to increase the part strength, paint baking is performed at less than 200 ° C. There is a method, and a BH steel sheet has been developed as such a steel sheet for paint baking.

For example, in Japanese Patent Application Laid-Open No. 55-141526, Nb is added in accordance with the contents of C, N, and Al in steel, and Nb /
A method of controlling the solid solution C and the solid solution N in the steel sheet by limiting (solid solution C + solid solution N) to a specific range and controlling a cooling rate after annealing is disclosed in JP-B-61-45689. Japanese Patent Laid-Open Publication No. H11-146131 discloses a method of improving bake hardenability by adding a combination of Ti and Nb.

However, since the above-mentioned steel sheet is made of a material having excellent deep drawability, the strength of the material steel sheet is low, and is not always sufficient as a structural material. Further, Japanese Unexamined Patent Publication No.
No. 25549 discloses a method of improving bake hardenability by adding W, Cr, Mo alone or in combination to steel. In the prior art described above, the increase in strength due to bake hardening is caused by a slight amount of solid solution C and solid solution N in the steel sheet.
As is well known, in the case of a BH steel sheet, only the yield strength of the material is increased, but not the tensile strength.

[0005] Therefore, the effect of increasing the deformation starting stress of the part is only sufficient, and the effect of increasing the stress (tensile strength after molding) required for deformation over the entire deformation range from the start of deformation to the end of deformation is not sufficient. As a cold-rolled steel sheet whose tensile strength increases after forming, for example, JP-A-10-310847 discloses that
There is disclosed an alloyed hot-dip galvanized steel sheet whose tensile strength increases by 60 MPa or more in a heat treatment temperature range of 00 to 450 ° C.

This steel sheet has a mass percentage of C: 0.01 to 0.2.
08%, Mn: 0.01 to 3.0%, and one of W, Cr and Mo
Species or two or more kinds are contained in a total of 0.05 to 3.0%, and if necessary, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%,
V: A composition containing one or more of 0.005 to 0.1% of steel, and the microstructure of the steel is mainly composed of ferrite or ferrite.

However, this technique increases the amount of strain by forming fine carbides in the steel sheet by heat treatment after forming, effectively growing dislocations with respect to the strain given during pressing, and increasing the amount of strain. However, heat treatment must be performed in the temperature range of 220 to 370 ° C., and the required heat treatment temperature is higher than a general bake hardening treatment temperature.

Among the painted and baked steel sheets of the press-formed body, hot rolled steel sheets are disclosed, for example, in Japanese Patent Publication No. 8-23048, in which tensile strength which is soft at the time of working and effective for improving fatigue properties by baking coating after working is described. A method for producing a hot-rolled steel sheet whose resilience is greatly increased is disclosed. In this technology, the amount of C is 0.02-
0.13 mass%, N is added in a large amount of 0.0080-0.0250 mass%, and the finish rolling temperature and winding temperature are controlled to allow a large amount of solid solution N to remain in the steel, and the metal structure is changed to ferrite and martensite. By making it a composite organization mainly composed of
It is disclosed that an increase in tensile strength of 100 MPa or more is achieved at a heat treatment temperature of 170 ° C. after molding.

Japanese Patent Application Laid-Open No. 10-183301 discloses that, among the steel components, C and N are particularly 0.01 to 0.12 mass% in C and N: 0.
0001-0.01 mass% and the average grain size is 8
A hot-rolled steel sheet with excellent bake hardenability and room temperature aging resistance capable of securing a high BH amount of 80 MPa or more and controlling an AI amount of 45 MPa or less by controlling the thickness to not more than μm has been proposed. .

However, since these steel sheets are hot-rolled sheets, the texture of ferrite is randomized by austenite / ferrite transformation after finish rolling, so that it is difficult to obtain a high r value, and it is difficult to obtain a high r value. It is hard to say that it has deep drawability. Moreover, even if cold rolling and recrystallization annealing are performed using the hot-rolled steel sheet obtained by these techniques as a starting material, it is not necessarily the same as the hot-rolled steel sheet, that is, the tensile strength increases after forming and heat-treating, or 80 MPa or more. High BH is not always obtained. This is because the steel structure has a microstructure different from that during hot rolling due to cold rolling and recrystallization annealing, and large strain accumulation occurs during cold rolling, so that carbides, nitrides or carbonitrides are formed. This is because the state of solid solution C and the state of solid solution N change easily.

[0011]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above-mentioned circumstances, and the tensile strength is increased by press-forming-heat treatment while maintaining excellent deep drawability during press-forming. Specifically, after processing under the condition of processing strain: about 10%, even if heat treatment is performed in the low temperature range of 120 to 200 ° C, in addition to the conventional heat treatment at a high temperature exceeding 200 ° C, An object of the present invention is to propose a cold-rolled steel sheet and a plated steel sheet which can achieve a strength increase of 60 MPa or more (increase in tensile strength) and are excellent in heat treatment capability after forming, together with their advantageous production methods.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and have obtained the following findings. 1) Molding-To increase the tensile strength after heat treatment,
Due to the interaction between the dislocations introduced by molding and the interstitial elements or precipitates, it is necessary that the dislocations introduced by pre-deformation do not move even when the upper yield stress is reached. 2) In order to obtain the above-mentioned interaction by forming carbides, nitrides, or carbonitrides such as W, Cr, Mo, Ti, Nb, and Al, the heat treatment temperature after molding is increased to 200 ° C. or more. There is a need. Therefore, active utilization of interstitial elements or utilization of Fe carbide or Fe nitride is more advantageous in lowering the heat treatment temperature after molding. 3) Among the interstitial elements, solid solution N has a larger interaction with dislocations introduced by forming even if the heat treatment temperature after forming is lowered, and solid solution N has higher interaction with solid dissolution C than solid solution C even if the upper yield stress is reached. Dislocations introduced in pre-deformation are difficult to move. 4) The presence of solute N in steel is within the crystal grains and at the crystal grain boundaries. The increase in strength after heat treatment after forming is larger when the crystal grain boundary area is larger. That is, it is advantageous that the crystal grain size is small. 5) From the viewpoint of increasing the grain boundary area, by adding Nb and B in combination, and cooling immediately after the completion of hot rolling, normal grain growth of ferrite grains after the completion of hot rolling is suppressed, and It is advantageous to suppress grain growth during recrystallization annealing following cold rolling.

The present invention is based on the above findings. That is, the gist configuration of the present invention is as follows. (1) In mass%, C: less than 0.0050%, Si: 0.005 to 1.0
%, Mn: 0.01 to 1.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.005 to 0.030%, N: 0.005 to 0.040%, N
b: 0.005 to 0.050%, B: 0.0005 to 0.0015%, the following formulas (1) and (2): N% ≧ 0.0015 + 14/93 · Nb% + 14/27 · Al% + 14/11 · B% …… (1) C% ≦ 12/93 · Nb% …… (2) (where N%, Nb%, Al%, B%, C%: Content of each element (mass%)) A cold-rolled steel sheet having an excellent heat treatment ability after forming, characterized in that it is contained in the range and the balance substantially comprises Fe. (2) In addition to the above composition, Cu, Ni, Mo
The cold-rolled steel sheet according to (1), wherein one or two or more kinds selected from the above are contained in a total of 1% or less. (3) The cold-rolled steel sheet according to (1) or (2), wherein the average crystal grain size is 20 μm or less. (4) Heat treatment temperature: in a low temperature range of 120 to 200 ° C., the tensile strength increase ΔTS after forming is not less than 60 MPa, and the cold as set forth in any one of (1) to (3). Rolled steel sheet. (5) Strength after forming, characterized in that the surface of the cold-rolled steel sheet according to any of (1) to (4) is provided with an electroplating layer, a hot-dip coating layer, or an alloyed hot-dip layer. Plated steel sheet with excellent ascending heat treatment ability. (6) The plated steel sheet according to (5), wherein the heat treatment temperature: in a low temperature range of 120 to 200 ° C., a tensile strength increase ΔTS after forming: 60 MPa or more. (7) By mass percentage, C: less than 0.0050%, Si: 0.005 to
1.0%, Mn: 0.01 to 1.5%, P: 0.1% or less, S: 0.01
% Or less, Al: 0.005 to 0.030%, N: 0.005 to 0.040
%, Nb: 0.005 to 0.050%, B: 0.0005 to 0.0015%,
The following formulas (1) and (2) N% ≧ 0.0015 + 14/93 · Nb% + 14/27 · Al% + 14/11 · B% …… (1) C% ≦ 12/93 · Nb% …… (2) (here, N%, Nb%, Al%, B%, C%: each element content (mass%)) is contained within the range that satisfies, or further, among Cu, Ni, and Mo One or two selected
A steel slab containing 1% or less in total and the remainder being substantially Fe in composition is hot-rolled. At this time, cooling is started immediately after finishing rolling, and a winding temperature: 400 to 800 C., then cold-rolled at a draft of 60-95%,
A method for producing a cold-rolled steel sheet having an excellent heat treatment ability for increasing strength after forming, wherein recrystallization annealing is performed at a temperature of 650 to 900 ° C. (8) In the heating process in the recrystallization annealing, 500
(7) The method for producing a cold-rolled steel sheet according to (7), wherein the temperature is raised in a temperature range from 1 ° C to a recrystallization temperature at a rate of 1 to 20 ° C / s. (9) In mass percentage, C: less than 0.0050%, Si: 0.005
~ 1.0%, Mn: 0.01 ~ 1.5%, P: 0.1% or less, S: 0.
01% or less, Al: 0.005 to 0.030%, N: 0.005 to 0.040
%, Nb: 0.005 to 0.050%, B: 0.0005 to 0.0015%,
The following formulas (1) and (2) N% ≧ 0.0015 + 14/93 · Nb% + 14/27 · Al% + 14/11 · B% …… (1) C% ≦ 12/93 · Nb% …… (2) (here, N%, Nb%, Al%, B%, C%: each element content (mass%)) is contained within the range that satisfies, or further, among Cu, Ni, and Mo One or two selected
A steel slab containing 1% or less in total and the remainder being substantially Fe in composition is hot-rolled. At this time, cooling is started immediately after finishing rolling, and a winding temperature: 400 to 800 C., then cold-rolled at a draft of 60-95%,
A recrystallization annealing at a temperature of 650 to 900 ° C, followed by electroplating or hot-dip plating, or a further heat alloying treatment. Production method. (10) In the temperature rising process in the recrystallization annealing, 50
The method for producing a plated steel sheet according to claim 9, wherein a temperature range from 0 ° C to a recrystallization temperature is increased at a rate of 1 to 20 ° C / s.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, experimental results on which the present invention is based will be described. (Experiment 1) In mass%, C: 0.0015%, B: 0.0010%, S
i: 0.01%, Mn: 0.5%, P: 0.03%, S: 0.008% and N: 0.011%, Nb in the range of 0.005 to 0.05% and Al in the range of 0.005 to 0.03%, the balance being Fe And a bar with a composition of unavoidable impurities (thickness: 30mm)
Was uniformly heated at 1150 ° C., hot-rolled in three passes so that the finishing temperature was 900 ° C., which was equal to or higher than the Ar ₃ transformation point, and was water-cooled 0.1 seconds after the rolling was completed. Thereafter, a heat treatment equivalent to coil winding maintained at 500 ° C. for 1 hour was performed.

The obtained hot-rolled sheet having a thickness of 4 mm was subjected to a rolling reduction:
After cold rolling at 82.5%, recrystallization annealing was performed at 800 ° C. for 40 seconds, followed by temper rolling at a reduction of 0.8%. From the cold-rolled sheet thus obtained, a JIS No. 5 tensile test piece was sampled in the rolling direction, and the strain rate was determined using a normal tensile tester:
The tensile strength was measured at 0.02 / s. Separately, a 10% JIS No. 5 tensile test specimen taken in the rolling direction from these cold-rolled sheets
, And subjected to a heat treatment at 120 ° C. for 20 minutes, and then subjected to a normal tensile test. After applying the tensile strength of the test specimen taken from the cold rolled sheet and the tensile strain of 10%,
The difference from the tensile strength of the test piece that had been heat-treated at 20 ° C for 20 minutes was defined as the strength increase after molding (ΔTS).

FIG. 1 shows a steel composition (N% -14 / 93 · Nb% -14).
/ 27 · Al% -14 / 11 · B%) and ΔTS are shown. As shown in the figure, (N% -14 / 93
When the value of (Nb% −14 / 27 · Al% −14 / 11 · B%) satisfied 0.0015% by mass or more, it was found that ΔTS became 60 MPa or more. (Experiment 2) In mass%, C: 0.0010%, Si: 0.02%, Mn:
0.6%, P: 0.01%, S: 0.009%, N: 0.012%, A
l: 0.01% and Nb: 0.015%, and B: 0.00005
The content is within the range of ~ 0.0025%, and the remainder is a sheet bar (thickness: 30 mm) having a composition of Fe and unavoidable impurities.
After uniform heating at ℃, the finishing temperature is higher than the Ar ₃ transformation point.
Perform 3 pass rolling at 0 ° C.
After a second, water cooling was performed, and a heat treatment equivalent to coil winding, which was maintained at 450 ° C. for 1 hour, was performed.

The obtained hot-rolled sheet having a thickness of 4 mm was subjected to a rolling reduction:
After cold rolling at 82.5%, recrystallization annealing was performed at 820 ° C. for 40 seconds, and then temper rolling was performed at a rolling reduction of 0.8%. From the cold-rolled sheet thus obtained, a JIS No. 5 tensile test piece was sampled in the rolling direction, and the strain rate was set to 0.02 using a normal tensile tester.
The tensile strength was measured at / s. Separately, a tensile test of 10% was applied to the tensile test pieces collected from these cold-rolled sheets,
After heat treatment at 20 ° C. for 20 minutes, it was subjected to a normal tensile test.

FIG. 2 shows the results of a study on the relationship between the B content in steel and ΔTS. As shown in FIG.
High Δ of 60 MPa or more when contained in 0005 to 0.0015 mass%
It can be seen that TS is obtained. Further, it was found from microstructure observation that the crystal grain was refined by adding Nb and B in combination, and a high ΔTS was obtained.

That is, if the B content is less than 0.0005% by mass, Nb
Has a small effect of crystal grain refinement due to the addition of the compound. Conversely, B
If the amount exceeds 0.0015% by mass, the amount of B segregating at and near the grain boundary increases, and the B atoms have a strong interaction with N atoms, so that the effective amount of solute N decreases. △ It is inferred that TS decreased. (Experiment 3) By mass%, C: 0.0010%, N: 0.012%,
B: 0.0010%, Si: 0.01%, Mn: 0.5%, P: 0.03%,
Steel A containing S: 0.008%, Nb: 0.014% and Al: 0.01%, the balance being Fe and unavoidable impurities
And C: 0.010%, N: 0.0012%, B: 0.0010%, Si:
0.01%, Mn: 0.5%, P: 0.03%, S: 0.008%, Nb:
Each sheet bar of steel B containing 0.014% and Al: 0.01%, with the balance being Fe and unavoidable impurities (thickness:
30mm) is uniformly heated at 1150 ° C, and then three-pass rolling is performed so that the finishing temperature is 910 ° C, which is equal to or higher than the Ar ₃ transformation point. After rolling is completed, gas cooling is started 0.1 seconds later, and then 600 ° C.
, A heat treatment equivalent to coil winding, which was held for one hour, was performed.

The obtained hot-rolled sheet having a thickness of 4 mm was subjected to a rolling reduction of 8 mm.
After cold rolling at 2.5%, recrystallization annealing was performed at 880 ° C. for 40 seconds, and then temper rolling was performed at a rolling reduction of 0.8%. From the cold-rolled sheet thus obtained, a JIS No. 5 tensile test piece was sampled in the rolling direction, and the strain rate was set to 0.
The tensile strength was measured at 02 / s. Separately, 10% tensile strain was applied to tensile test specimens collected from these cold-rolled sheets,
After being subjected to heat treatment at various temperatures for 20 minutes, they were subjected to a normal tensile test.

FIG. 3 shows the results of an investigation on the effect of the post-molding heat treatment temperature on ΔTS. As shown in the figure,
In the region where the heat treatment temperature after forming is relatively low at 200 ° C or lower, steel A, which is a steel with very low carbon and high N content, has a semi-low carbon and low nitrogen content.
It shows a higher ΔTS than steel B, which is steel, and shows a similar ΔTS in a high-temperature region. From these experimental results, it was found that ΔTS
It can be seen that it is effective to utilize solid solution N in order to secure the value.

FIG. 4 shows the effect on the amount of decrease in elongation (ΔEl) and the increase in tensile strength after molding (ΔTS) due to aging at normal temperature.
Grain size d and steel composition (N% -14 / 93 · Nb% -14 / 27 · Al
% -14 / 11 · B%). The decrease in elongation (ΔEl) was determined by measuring the total elongation measured with a JIS No. 5 test piece taken from the cold-rolled sheet in the rolling direction, and using a separately taken test piece at 100 ° C, which is a treatment for accelerating aging at room temperature.
Was evaluated by the difference from the total elongation measured after holding for 8 hours. In addition, the above-mentioned crystal grain size means an average crystal grain size, and for a cross section perpendicular to the rolling direction of each steel sheet,
From the micrograph of the structure, of the value calculated by the quadrature method specified by ASTM and the nominal particle size similarly determined by the cutting method (for example, Umemoto et al .: Heat treatment, 24 (1984), 334),
Whichever is larger was adopted.

As shown in the figure, (N% −14 / 93 · Nb
% -14 / 27 · Al% -14 / 11 · B%) is 0.0015 mass%
It can be seen that when the crystal grain diameter d is 20 μm or less, it is possible to achieve both high ΔTS and low ΔEl. Next, the reason why the component composition of the steel sheet is limited to the above range in the present invention will be described. Hereinafter,% in the composition means mass%.

C: less than 0.0050% The smaller the C content, the better the deep drawability and the more advantageous the press formability. Further, in the annealing process after cold rolling, the re-dissolution of NbC progresses, solute C in crystal grains increases, and the normal temperature aging resistance is likely to be lowered. Therefore, the C content needs to be suppressed to less than 0.0050%. In addition, it is preferably 0.0030% or less. In the current manufacturing technology, the lower limit of the amount of C that can be achieved without an extreme increase in cost is 0.0005.
%.

Si: 0.005 to 1.0% Si is a useful component that suppresses the decrease in elongation and improves the strength. However, if the content is less than 0.005%, the effect of its addition is poor, while the content exceeds 1.0%. Therefore, Si is limited to the range of 0.005 to 1.0% because it deteriorates the surface properties and lowers the ductility. In addition, it is preferably in the range of 0.01 to 0.75%.

Mn: 0.01 to 1.5% Mn is not only useful as a strengthening component of steel, but also has the effect of forming MnS to suppress embrittlement due to S.
If less than 0.01%, the effect of the addition is poor, while 1.5%
If Mn exceeds Mn, deterioration of the surface properties and decrease in ductility are caused.
Was contained in the range of 0.01 to 1.5%. In addition,
Preferably it is 0.01-1.0%. More preferably 0.10 ~
0.75%.

P: 0.1% or less P effectively contributes to the strengthening of steel as a solid solution strengthening component.
It is preferable to contain 001% or more. On the other hand, if the content exceeds 0.1%, phosphides such as (FeNb) xP are formed, so that the deep drawability decreases. Therefore, P was limited to 0.1% or less. In addition, it is preferably 0.05% or less.

S: 0.01% or less If a large amount of S is contained, the amount of inclusions increases and the ductility is reduced. Therefore, it is desirable to avoid the incorporation of S as much as possible.
Up to 0.01% is acceptable. Al: 0.005 to 0.030% Al is added as a deoxidizing agent and to improve the yield of carbonitride forming components. However, if the content is less than 0.005%, there is no sufficient effect, while if it exceeds 0.030%, In addition, an increase in the amount of N to be added to steel is caused, and slab defects are easily generated during steelmaking. Therefore, Al is contained in the range of 0.005 to 0.030%.

N: 0.005 to 0.040% N is an important element in the present invention that plays a role in imparting a post-forming strength increasing heat treatment ability to a steel sheet. However, if the content is less than 0.005%, a sufficient heat treatment capability for increasing the strength after molding cannot be obtained, while a large amount exceeding 0.040% causes a decrease in press formability. Therefore, N is 0.005-0.
Limited to the 040% range. In addition, preferably 0.008-0.
015%.

Nb: 0.005 to 0.050% Nb contributes effectively to the refinement of the hot-rolled structure and the cold-rolled recrystallization-annealed structure by the combined addition with B.
Has the effect of fixing. Further, Nb forms a nitride such as NbN, and contributes to refinement of a cold rolled recrystallization annealing structure. However, if the Nb content is less than 0.005%,
Not only becomes difficult to precipitate and fix, but also the hot rolling structure and the cold rolling recrystallization annealing structure become insufficiently fine,
On the other hand, if it exceeds 0.050%, ductility is reduced. Therefore, Nb
Is limited to the range of 0.005 to 0.050%. The content is preferably in the range of 0.010 to 0.030%.

B: 0.0005 to 0.0015% B, when added in combination with Nb, has the effect of effectively refining the hot-rolled structure and the cold-rolled recrystallized structure and improving the resistance to secondary working embrittlement. However, the content is 0.0005
%, A sufficient refining effect cannot be obtained, while 0.0015%
If it exceeds, not only the amount of deposited BN increases, but also hinder solution in the slab heating step. Therefore, B
Was limited to the range of 0.0005 to 0.0015%. In addition, it is preferably 0.0007 to 0.0012%.

As described above, Nb has an effect of fixing solid solution C as NbC. Nb forms a nitride such as NbN. Similarly, Al and B are AlN, B respectively.
Form N. Therefore, in order to ensure a sufficient amount of solid solution N and sufficiently reduce the solid solution C, it is necessary to use N, C, Nb, A
It is important that the contents of l and B be within the above-mentioned ranges and satisfy the following relations (1) and (2).

N% ≧ 0.0015 + 14/93 · Nb% + 14/27 · Al% + 14/11 · B% (1) C% ≦ 12/93 · Nb% (2) N%, Nb%, Al%, B%, C%: Content of each element
(% By mass) In the present invention, in addition to the above-described composition,
It is preferable that one or two or more selected from Ni and Mo are contained in a total of 1% or less.

Each of Cu, Ni and Mo is an element for increasing the strength of the steel sheet, and can be selectively used alone or in combination as necessary. This effect is more than 0.05% for each Cu,
Ni: 0.05% or more, Mo: 0.05% or more contained. However, if one or more selected from Cu, Ni, and Mo exceeds 1%, hot This results in an increase in deformation resistance, a decrease in chemical conversion property, a deterioration in surface treatment property in a broad sense, and a deterioration in weld formability due to hardening of the weld. Therefore, one selected from Cu, Ni and Mo
It is preferable that the content of the species or two or more species is 1% or less in total.

In the present invention, in order to obtain high strain aging characteristics and prevent aging deterioration, it is preferable to reduce the crystal grain size. Here, the crystal grain size means the average crystal grain size obtained by the above-described measurement method. That is, as shown in FIG. 4 described above, by reducing the crystal grain size d to 20 μm or less, (N% −14 / 93 · Nb% −14 / 27 · Al% −14 / 11 ·
(B%) ≧ 0.0015% Even when a relatively large amount of solute N is contained, ΔEl can be suppressed to 2.0% or less. It is more preferable to reduce the crystal grain size d to 15 μm or less. This is because, as shown in FIG. 4, when the crystal grain size d is reduced to 15 μm or less, ΔEl can be suppressed to 1.5% or less.

The cold-rolled steel sheets having the above-mentioned composition or the above-mentioned structure and the plated steel sheets formed by forming an electroplating layer, a hot-dip coating layer, or an alloyed hot-dip layer on the surface of these cold-rolled steel sheets are excellent. In addition to having excellent deep drawability, it has an excellent post-molding strength increasing heat treatment ability in which the tensile strength is increased by press molding-heat treatment. Hereinafter, for reference, the forming conditions when the steel sheet of the present invention is subjected to forming processing such as press forming and the subsequent heat treatment conditions for increasing the strength will be described.

When the steel sheet of the present invention is subjected to press working such as drawing, for example, the strain introduced by press working is several percent to several tens of percent. Although the amount of distortion varies depending on the molded part, about 5 to 10% of distortion is introduced into the inner plate and the structural member in the automobile field. These formed parts are subjected to a heat treatment such as a paint baking treatment. However, with the steel sheet of the present invention, the strength of the formed parts can be effectively increased after the heat treatment.

In the present invention, JIS 5 is used as a method for evaluating such a post-molding strength increase heat treatment ability in a laboratory.
A tensile test piece of size No. is sampled in the rolling direction, 10% tensile strain is given by a tensile tester, and after a heat treatment is applied, a tensile test is performed again. The tensile strength TS _HT after applying the 10% tensile strain and the heat treatment, and the tensile strength of the steel sheet before applying the 10% tensile strain (the tensile strength of the product sheet) thus obtained.
The difference between TS and the tensile strength increase ΔTS (= TS _HT −TS) due to molding and heat treatment is defined as the post-molding strength increase heat treatment ability. Note that, in the present invention, the heat treatment after the tensile strain is applied is, for example, a treatment equivalent to painting and baking.
When the conditions are 20 ° C for 20 minutes, or when the characteristics after the heat treatment in a low temperature range are particularly evaluated, the heat treatment conditions are 120 ° C for 20 minutes. This test evaluates the properties of the completed part after heat treatment following press molding.

It should be noted that the cold-rolled steel sheet of the present invention can be used even if the heat treatment after the press forming is performed in a low temperature range of 120 to 200 ° C.
It has a ΔTS (heat treatment capacity for post-molding strength increase: tensile strength rise after molding) of MPa or more. In general, in order to increase the strength of a molded product, it is preferable that the amount of strain introduced by molding or the heat treatment temperature after processing be higher.

However, the steel sheet of the present invention has a sufficient heat treatment temperature after forming, that is, even if the heat treatment temperature is 200 ° C. or lower, when the applied strain is about 5 to 10% as described above. It is possible to achieve a strong increase in strength.
However, if the heat treatment temperature is lower than 120 ° C., a sufficient strength increasing effect cannot be obtained when the strain is low. On the other hand, when the heat treatment temperature after molding exceeds 350 ° C, softening proceeds. Therefore, the heat treatment temperature after the molding is preferably set to about 120 to 350 ° C.

The heating method of the heat treatment after molding includes:
Methods such as hot air heating, infrared furnace heating, hot bath heat treatment, electric heating, and high frequency heating can be applied, and are not particularly limited. Also,
It is also possible to selectively heat only the portion where the strength is to be increased. Next, the manufacturing conditions according to the present invention will be described. The steel having the above-mentioned preferred component composition is smelted by a known smelting method such as a converter, and is made into a billet by an ingot-making method or a continuous casting method.

Next, after heating and soaking the steel slab, it is subjected to hot rolling to obtain a hot rolled sheet. In the present invention, the heating temperature of hot rolling is not particularly specified, but it is advantageous to fix solid solution C and precipitate it as carbide in order to improve deep drawability. The heating temperature of the hot rolling is preferably set to 1300 ° C. or lower. In order to further improve the workability, the heating temperature is more preferably set to 1150 ° C. or lower. However, the heating temperature is 900 ℃
If it is less than 30, the improvement in workability is saturated, and conversely, the rolling load during hot rolling increases, and the risk of occurrence of rolling trouble increases. Therefore, the lower limit of the heating temperature is preferably 900 ° C.

Next, it is preferable that the total rolling reduction in the hot rolling be 70% or more. Because the total reduction is 70%
This is because if the amount is less than the above, the grain refinement of the hot-rolled sheet tends to be insufficient. Finish rolling in hot rolling is 960-65.
Is preferably terminated at 0 temperature range of ° C., the hot rolling finishing temperature, even γ region above Ar ₃ transformation point, it may be less α region Ar ₃ transformation point. If the hot rolling finish temperature exceeds 960 ° C, the crystal grains of the hot rolled sheet become coarse, and the deep drawability after cold rolling and annealing tends to deteriorate. On the other hand, when the hot rolling finish temperature is lower than 650 ° C., the deformation resistance increases, so that the hot rolling load increases, and the rolling tends to be difficult.

After completion of the hot finish rolling, it is desirable to start cooling immediately to prevent normal grain growth. Here, the above cooling treatment conditions are not particularly limited, but the cooling start time is preferably 1.5 seconds or less, more preferably 1.0 seconds or less, and still more preferably 0.5 seconds or less after finishing rolling. Is desirable. This is because cooling immediately after the end of rolling increases the degree of supercooling in the state where strain is accumulated, so more ferrite nuclei are generated, ferrite transformation is accelerated, and the crystal grains of the hot-rolled sheet become finer. This is because it is easy to secure a high r value on the product plate.

The cooling rate is preferably set to 10 ° C./s or more in order to secure solid solution N. In particular, when the hot rolling finish temperature is equal to or higher than the Ar ₃ transformation point, it is more preferable to set the cooling rate to 50 ° C./s or higher in order to secure solid solution N. Next, the hot rolled sheet is wound around a coil. The higher the winding temperature, the more advantageous is the coarsening of carbides. However, if it exceeds 800 ° C, it will be difficult to achieve finer particles by adjusting the cooling conditions after finishing hot rolling.
It should be below ° C. If the temperature exceeds 700 ° C., the scale formed on the surface of the hot-rolled sheet becomes thicker, which not only increases the load of the scale removal operation, but also causes the formation of nitrides to fluctuate in the amount of dissolved N in the longitudinal direction of the coil. Preferably, the temperature is set to 700 ° C. or lower because the temperature is easily increased. On the other hand, if the winding temperature is less than 400 ° C., the winding operation becomes difficult. Therefore, the winding temperature of the hot-rolled sheet needs to be in the range of 800 to 400 ° C., and is preferably
700-400 ° C.

Next, the hot-rolled sheet is subjected to cold rolling. The rolling reduction in such cold rolling needs to be 60 to 95%. This is because if the rolling reduction of the cold rolling is less than 60%, a high r-value cannot be expected, while if it exceeds 95%, the r-value rather decreases. The cold rolled sheet that has been subjected to the above cold rolling is then subjected to recrystallization annealing. The annealing method may be either continuous annealing or batch annealing, but continuous annealing is more advantageous. This continuous annealing may be either a normal continuous annealing line or a continuous galvanizing line.

The annealing conditions are preferably set to 650 ° C. or more and 5 seconds or more. This is because if the annealing temperature is less than 650 ° C. and the annealing condition is less than 5 seconds, recrystallization is not completed, and the deep drawability is reduced. In order to further improve the deep drawability, annealing at 800 ° C. or more for 5 seconds or more to achieve a certain degree of grain growth is desirable for improving the r-value. Further, annealing in the two-phase region of ferrite (α) + austenite (γ) partially causes α → γ transformation, whereby {111} texture is developed and the r-value is improved. On the other hand, when the α → γ transformation has progressed completely, the texture is randomized, the r value decreases, and the deep drawability deteriorates. Therefore, it is more preferable to perform annealing in the α + γ2 phase region. The upper limit of the annealing temperature is preferably set to 900 ° C. This is because if the annealing temperature exceeds 900 ° C., the re-dissolution of carbides proceeds and the solute C excessively increases, so that the delayed aging property decreases.

Further, in the temperature rising process in the above-mentioned recrystallization annealing, the temperature range from 500 ° C. to the recrystallization temperature is gradually heated to sufficiently precipitate AlN and the like, thereby effectively reducing the crystal grain size of the steel sheet. Can be smaller. Here, the temperature range in which the above-described controlled heating is to be performed is AlN
The temperature should be between 500 ° C and the recrystallization temperature at which precipitation begins. Further, the heating rate is preferably in the range of 1 to 20 ° C./s. This is because if the heating rate is higher than 20 ° C./s, a sufficient amount of precipitation cannot be obtained, whereas if the heating rate is lower than 1 ° C./s, the precipitates are coarsened and the effect of suppressing grain growth is weakened.

The cooling rate after soaking in the recrystallization annealing is, for example, in the case of continuous annealing, a cooling rate of 500 ° C. or more is set to 50 ° C./50° C. in order to easily secure a favorable surface and shape with the current technology. s or less, more preferably
30 ° C / s or less. After the recrystallization annealing as described above, a temper rolling of 10% or less may be further performed for shape correction and surface roughness adjustment.

Following the recrystallization annealing, an electroplating process, a hot-dip coating process, or a heat-alloying process is performed as necessary, so that an electroplating layer, a hot-dip coating layer, It is preferable to form one of the alloyed hot-dip coating layers to obtain a plated steel sheet. A plated steel sheet on which any of the electroplating layer, the hot-dip coating layer, and the alloyed hot-dip coating layer is formed has a post-forming strength increase heat treatment ability similar to that of the steel sheet before plating. Further, as the type of plating, any of electrogalvanizing, hot-dip galvanizing, alloyed hot-dip galvanizing, electrotin plating, electrochromic plating, and electronickel plating is suitable. The plating method is not particularly limited, and may be performed according to a conventionally known method.

A steel sheet (dull-finished steel sheet, bright-finished steel sheet, surface-treated steel sheet) which has been subjected to temper rolling in order to improve workability and appearance after processing after forming a coated steel sheet such as the above alloyed hot-dip galvanized steel sheet. A steel sheet having a specific roughness pattern formed thereon), a steel sheet having an oil film layer such as a rust-preventive oil or a lubricating oil on the surface thereof, and a steel sheet which has been subjected to a surface treatment usually adopted as a thin steel sheet. If so, the effects of the present invention can be sufficiently enjoyed.

Thus, not only has excellent deep drawability, but also the tensile strength is increased by press molding-heat treatment.
It is possible to obtain a cold-rolled steel sheet and a plated steel sheet which are excellent in the heat treatment ability for increasing the strength after forming.

[0053]

EXAMPLES (Example 1) A steel slab having the composition shown in Table 1 was formed into a hot-rolled sheet having a thickness of 3.5 mm and a cold-rolled sheet having a thickness of 0.7 mm under the conditions shown in Table 2. , A continuous annealing line or a continuous annealing-alloyed hot-dip galvanizing line, recrystallization annealing, and further a galvannealed treatment, followed by temper rolling at a rolling reduction of 1.0% to obtain a cold-rolled steel sheet and one side. An alloyed hot-dip galvanized steel sheet plated on both sides at a basis weight of 45 g / m ² was produced. In Table 2, steel sheet N
In o.3 and No.8, the hot-rolling finish temperature is lower than the Ar ₃ transformation point, and the other temperatures are higher than the Ar ₃ transformation point. The cooling rate of the steel sheet shown in Table 2 to 500 ° C. after the completion of soaking was 10 to 30 ° C./s.

The tensile strength and r value of the cold-rolled steel sheet and the galvannealed steel sheet thus obtained, and changes in the tensile strength after forming and heat treatment (strength increase after forming heat treatment: tensile strength after forming) Table 3 shows the results of investigation on the rise allowance ΔTS). The tensile properties were measured by collecting JIS No. 5 test pieces from the product plate in the rolling direction.

The r value was measured by a three-point method after 15% tensile prestrain was applied to the product sheet. The r value was measured in the L direction (rolling direction), the D direction (45 ° direction in the rolling direction), and the C direction. (90 ° in rolling direction) average value (r value = (r _L + 2r _D +
r _C ) / 4). Furthermore, the tensile strength after forming and heat treatment was determined by taking a JIS No. 5 test piece from the product plate in the rolling direction, applying a 10% pre-strain, and then applying 120 ° C and the conventional heat treatment temperature equivalent to paint baking. At 170 ° C
Heat treatment was performed for 20 minutes, and the tensile strength was measured and determined. The amount of decrease in total elongation (ΔEl) due to aging at room temperature was calculated using the total elongation measured by taking a JIS No. 5 test piece from the product plate in the rolling direction and the JIS No. 5 test piece separately taken in the rolling direction. It was determined as a difference from the total elongation measured after the aging treatment (holding at 100 ° C. for 8 hours).

In each of the obtained steel sheets, the crystal grain size in a cross section perpendicular to the rolling direction was measured. As described above, the crystal grain size of each steel sheet was determined from AST
The larger of the value calculated by the quadrature method specified in M and the nominal particle size similarly obtained by the cutting method (for example, Umemoto et al .: Heat treatment, see 24 (1984), 334) was used. .

[0057]

[Table 1]

[0058]

[Table 2]

[0059]

[Table 3]

As is clear from Table 3, both the cold-rolled steel sheet and the galvannealed steel sheet obtained according to the present invention have a higher r value and an excellent post-forming strength increasing heat treatment ability as compared with the comparative example. Have been obtained. Further, among the suitable examples, those having a crystal grain size of 20 μm or less have a small decrease in elongation due to aging at room temperature of ΔEl of 2.0% or less. (Example 2) A slab heating temperature: 11 under the same manufacturing conditions as steel sheet No. 2 in Table 2 using a slab of steel symbol B shown in Table 1.
After hot rolling at 00 ° C. and a finishing hot rolling temperature of 900 ° C., the film was wound around a coil at a winding temperature of 550 ° C. The coil was cold-rolled at a rolling reduction of 80%, and then recrystallized at 840 ° C.

The product properties of the obtained cold-rolled steel sheet include tensile strength.
TS = 365 MPa and r value = 1.7. From this cold rolled steel sheet
Take a JIS No. 5 test piece in the rolling direction and use a tensile tester
After imparting a tensile strain of 10%, heat treatment was performed under the heat treatment conditions (temperature, time) shown in Table 4, and a tensile test was performed again. Table 4 shows the tensile strength of the product before applying strain (TS = 365 MP
Increase in tensile strength from a) (strength increase after molding heat treatment:
ΔTS).

[0062]

[Table 4]

As shown in Table 4, ΔTS (increase in tensile strength after forming) increases as the heat treatment temperature increases and the heat treatment time increases, but the steel sheet of the present invention has a low heat treatment temperature of 120 ° C. Even if the holding time is as short as 2 minutes, a sufficient increase in tensile strength of 82 MPa (85% or more compared to 20 minutes heat treatment) is obtained, and even after low-temperature and short-time heat treatment, good post-mold strength increase heat treatment It can be seen that the performance can be obtained.

It should be noted that there is no problem in performing a heat treatment at a normal temperature and time in order to obtain a stable strength increasing effect on structural members of an automobile. It has also been confirmed that the same results as in Table 4 can be obtained for an alloyed hot-dip galvanized steel sheet obtained by subjecting this cold-rolled steel sheet to hot-dip galvanizing and heat alloying treatments.

[0065]

Thus, according to the present invention, a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet whose tensile strength is effectively increased by press forming-heat treatment while maintaining excellent deep drawability during press forming. Can be obtained stably, and its industrial value is extremely large.

[Brief description of the drawings]

Fig. 1 Steel composition (N% -14 / 93 Nb% -14 / 27Al%-
14 is a graph showing the relationship between the tensile strength rise (ΔTS) after molding and 14/11 · B%).

FIG. 2 is a graph showing the relationship between the B content and ΔTS in Nb, B composite added steel.

FIG. 3 shows a comparison of the difference in tensile strength increase ΔTS after forming in a low temperature range between steel B having a large amount of solid solution C (conventional steel) and steel A having a large amount of solid solution N (invention steel). FIG.

FIG. 4 shows the effect of crystal grain size d and steel composition (N% -14 / 93 · Nb% -14 / 27 · Al) on the decrease in elongation (ΔEl) and the increase in tensile strength after forming (ΔTS) due to normal temperature aging. %-14/11 · B%)
6 is a graph showing the effect of the above.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) C21D 6/00 C21D 6/00 S 9/46 9/46 FG C22C 38/12 C22C 38/12 38 / 16 38/16 (72) Inventor Akio Tosaka 1-term Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 4K037 EA01 EA02 EA04 EA15 EA17 EA18 EA19 EA20 EA27 EB01 EB06 EB08 EB01 FE01 FE02 FE03 FJ01 FJ04 FJ05 FJ06

Claims (10)

[Claims] 1. In mass percentage, C: less than 0.0050%, Si: 0.005 to 1.0%, Mn: 0.01 to 1.5%, P: 0.1% or less, S: 0.01% or less Al: 0.005 to 0.030%, N: 0.005 to 0.040%, Nb: 0.005 to 0.050%, B: 0.0005 to 0.0015% in a range satisfying the following formulas (1) and (2), with the balance being substantially Fe. And
Cold-rolled steel sheet with excellent heat treatment ability after forming. Note N% ≧ 0.0015 + 14/93 · Nb% + 14/27 · Al% + 14/11 · B% …… (1) C% ≦ 12/93 · Nb% …… (2) where N% , Nb%, Al%, B%, C%: Content of each element
(mass%) 2. The composition according to claim 1, further comprising:
The cold-rolled steel sheet according to claim 1, wherein one or more selected from u, Ni, and Mo are contained in a total of 1% or less. 3. The cold-rolled steel sheet according to claim 1, wherein the average grain size is 20 μm or less. 4. The cooling method according to claim 1, wherein a heat treatment temperature: in a low temperature range of 120 to 200 ° C., a tensile strength increase ΔTS after forming: 60 MPa or more. Rolled steel sheet. 5. The cold-rolled steel sheet according to any one of claims 1 to 4, further comprising an electroplating layer, a hot-dip coating layer, or an alloyed hot-dip layer.
Plated steel sheet with excellent heat treatment ability to increase strength after forming. 6. The plated steel sheet according to claim 5, wherein the heat treatment temperature: in a low temperature range of 120 to 200 ° C., a tensile strength increase ΔTS after forming: 60 MPa or more. 7. In mass percentage, C: less than 0.0050%, Si: 0.005 to 1.0%, Mn: 0.01 to 1.5%, P: 0.1% or less, S: 0.01% or less Al: 0.005 to 0.030%, N: 0.005 to 0.040%, Nb: 0.005 to 0.050%, B: 0.0005 to 0.0015% in a range satisfying the following formulas (1) and (2), or further selected from Cu, Ni, and Mo. One
A steel slab containing 1% or less in total of two or more species, and the remainder having a substantially Fe composition, is hot-rolled. At that time, cooling is started immediately after finishing rolling, and a winding temperature: 400 to 80
Winding at 0 ° C, then cold rolling at a reduction rate of 60 to 95%, and then recrystallization annealing at a temperature of 650 to 900 ° C. Manufacturing method of cold rolled steel sheet. Note N% ≧ 0.0015 + 14/93 · Nb% + 14/27 · Al% + 14/11 · B% …… (1) C% ≦ 12/93 · Nb% …… (2) where N% , Nb%, Al%, B%, C%: Content of each element
(mass%) 8. The cooling method according to claim 7, wherein in the temperature raising step in said recrystallization annealing, a temperature range from 500 ° C. to a recrystallization temperature is raised at a rate of 1 to 20 ° C./s. Manufacturing method of rolled steel sheet. 9. In terms of mass percentage, C: less than 0.0050%, Si: 0.005 to 1.0%, Mn: 0.01 to 1.5%, P: 0.1% or less, S: 0.01% or less Al: 0.005 to 0.030%, N: 0.005 to 0.040%, Nb: 0.005 to 0.050%, B: 0.0005 to 0.0015% in a range satisfying the following formulas (1) and (2), or further selected from Cu, Ni, and Mo. One
A steel slab containing 1% or less in total of two or more species, and the remainder having a substantially Fe composition, is hot-rolled. At that time, cooling is started immediately after finishing rolling, and a winding temperature: 400 to 80
Winding at 0 ° C, then cold rolling at a reduction rate of 60 to 95%, then recrystallization annealing at a temperature of 650 to 900 ° C, and then electroplating or hot dip plating, or A method for producing a plated steel sheet having excellent heat treatment ability after forming, characterized by performing a heat alloying treatment. Note N% ≧ 0.0015 + 14/93 · Nb% + 14/27 · Al% + 14/11 · B% …… (1) C% ≦ 12/93 · Nb% …… (2) where N% , Nb%, Al%, B%, C%: Content of each element
(mass%) 10. A temperature range from 500.degree. C. to a recrystallization temperature of 1 to 20.degree.
The method for producing a plated steel sheet according to claim 9, wherein the temperature is raised at a rate of: