JPH10287954A - Coating/baking curable type cold rolled steel sheet excellent in strain aging resistance, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To industrially and stably produce a coating/baking curable type cold rolled steel sheet excellent in aging resistance by specifying the composition of a steel sheet and also specifying the value determined from the relation between the aging index of the steel sheet and the aging index of the heat treated steel sheet. SOLUTION: A composition, consisting of, by weight, <=0.005% C, <=1.0% Si, <=3.0% Mn, <=0.15% P, <=0.05% S, 0.01-0.20% Al, <=0.01% N, 0.01-0.2% Ti, and the balance Fe with inevitable impurities, is provided. Further, the value of A represented by equation is >=0.4, and the value of AIQUENCH is >=30, where the value of AIQUENCH is an aging index (MPa) after 500 deg.C×40 sec heating and water quenching treatment is applied to the steel sheet and AI is an aging index (MPa) of the cold rolled and annealed sheet. Moreover, the aging index is the amount of increase (MPa) of yield stress, as a difference between the amounts of yield stress before and after the undermentioned heat treatment, in the case where 7.5% tensile prestrain is applied to the steel sheet and then heat treatment is applied at 100 deg.C for 30 min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled thin steel sheet, and particularly to a cold-rolled thin steel sheet, mainly for a car body.
The present invention relates to a good cold-rolled thin steel sheet used for applications such as press forming, drawing and the like, followed by baking.

[0002]

2. Description of the Related Art In order to reduce the weight of an automobile body, a reduction in the thickness of a steel sheet used has been demanded, and studies have been made to increase the strength of an automobile steel sheet. However, increasing the strength of a steel sheet tends to degrade the press formability of the steel sheet, and there has been a demand for the development of a high-tensile steel sheet having excellent press formability.

[0003] Paint-bake hardening type steel sheets for automobiles have been developed as steel sheets having both high press formability and high strength. This steel sheet is a steel sheet whose yield stress increases when a paint baking treatment including holding at a high temperature of usually 100 to 200 ° C. is performed after press working. The presence of solid solution C in the steel causes the solid solution C to adhere to the dislocations introduced during the press working by heating during the coating baking process, hinder the movement of the dislocations, and increase the yield stress. In such a paint bake hardening type automotive steel sheet, a paint bake hardening amount (BH amount) of 30 MPa or more is required.

[0004] However, in this paint bake hardening type automotive steel sheet, some dislocations may be fixed by solid solution C before working, so that a wavy shape called a stretcher strain due to yield point elongation at the time of press working. There is a problem that surface defects occur and product characteristics are significantly deteriorated. To solve such a problem of deterioration of aging resistance, a paint bake hardening type cold-rolled steel sheet having improved aging resistance has been proposed. For example, Japanese Patent Publication No. Sho 61-12008 discloses that the C content is 2%.
Hot-rolling at a low temperature of 550 to 200 ° C on ultra-low carbon steel with a combined addition of up to 10 times Nb and B at least 0.3 times the N content, and quenching after annealing in the α-γ2 phase region The present invention discloses a method for producing a high-strength dual-phase steel sheet for deep drawing, which obtains a high r-value and bake hardenability by performing a combination of the following steps. This manufacturing method is characterized in that a two-phase structure of acicular ferrite and ferrite is formed by heating to the α-γ2 phase region and rapidly cooling. Although this structure contains solid solution C and has a high bake hardenability (BH property), most of the solid solution C is trapped in the acicular ferrite having a high dislocation density, so that almost no yield elongation occurs even after annealing. Not shown. However,
According to this method, it is necessary to perform annealing at a high temperature of the α-γ2 phase region, and it is difficult to stably secure the material as the process production because the α-γ2 phase region of the ultra-low carbon steel is very narrow. And left a problem.

[0005]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and can be industrially produced stably.
An object of the present invention is to provide a paint bake hardening type cold rolled steel sheet having excellent aging resistance.

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies to obtain high BH properties and excellent aging resistance characteristics in ultra-low carbon steel. It has been newly found that the place where it exists is different from that of the solute C which contributes. That is, the solid solution C that expresses the BH property is all the solid solution C in the steel present in the grain boundaries and in the grains.
On the other hand, the only solute C that contributes to the aging at room temperature is the solute C existing in the grains, and the solute C existing in the grain boundaries has no effect.

[0007] Regarding the experimental results on which the present invention is based,
First, a description will be given. C: 0.0020 wt%, Si: 0.02 wt%, Mn: 0.
1 wt%, P: 0.01 wt%, S: 0.005 to 0.015 wt%, Al:
0.04 wt%, N: 0.002 wt%, Ti: 0 to 0.075 wt%, Nb:
A sheet bar having a composition of 0 to 0.025 wt%
After heating and soaking to 300 ° C, the plate was rolled in three passes to a finish temperature of 900 ° C to form a hot-rolled sheet with a thickness of 3.5 mm.
The coil was wound for 1 hour. Thereafter, these hot-rolled sheets are subjected to cold rolling at a rolling reduction of 80%, and then 800 to 880 ° C.
× 40 sec recrystallization annealing was performed. Among these cold-rolled annealed sheets, those showing characteristics of BH = 35 to 45 MPa,
After aging at 100 ° C for 10 hours, the yield point elongation was investigated. An aging treatment at 100 ° C. for 10 hours corresponds to an aging treatment at room temperature for about 6 months, and it has been found that there is no problem with the aging resistance if the yield point elongation is 0.2% or less after this treatment.

As shown in FIG. 1, the yield point elongation after aging treatment at 100 ° C. for 10 hours is 0.2% when A value is 0.4 or more.
The following result was obtained. That is, a steel sheet having an A value of 0.4 or more exhibits excellent aging resistance.
The A value is a value calculated from the following equation: A = (AI _QUENCH -AI) / AI _QUENCH . Here, AI _QUENCH is the aging index (MPa) after heating and water quenching the cold-rolled annealed sheet at 500 ° C. for 40 _seconds , and AI is the aging index of the cold-rolled annealed sheet. The aging index was determined from the increase in yield stress (MPa) before and after heat treatment when a steel sheet was subjected to a heat treatment at 100 ° C. for 30 minutes with a tensile prestrain of 7.5%.

Then, the solid solution C existing in the grains and at the grain boundaries
Is proportional to AI _QUENCH and corresponds to the BH amount. The amount of solute C present at the grain boundary is proportional to AI _QUENCH -AI. Therefore, the grain boundary abundance ratio of solid solution C is determined by the A value (A =
(AI _QUENCH -AI) / AI _{QUEN CH} ). From this fact, the present inventors have succeeded in producing a steel sheet having excellent aging resistance while securing high paint bake hardenability by controlling the grain boundary of solid solution C and the presence state in the grains. I thought.

[0010] That is, the present invention relates to a method for preparing C: 0.00% by weight.
5% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15
%, S: 0.05% or less, Al: 0.01 to 0.20%, N: 0.01
% Or less, Ti: 0.01 to 0.2%, has a chemical composition consisting of the balance Fe and inevitable impurities, and A = (AI
_QUENCH- AI) / AI _QUENCH (where AI _QUENCH is the aging index (MPa) after heating and water quenching the cold-rolled annealed sheet at 500 ° C. for 40 _seconds , and AI is the aging index of the cold-rolled annealed sheet. The aging index adds 7.5% tensile prestrain to the steel sheet,
This is the increase in yield stress (MPa) before and after the heat treatment at 100 ° C for 30 min. ) A paint bake hardening type cold rolled steel sheet having excellent aging resistance characterized by having a value of 0.4 or more and an AI _QUENCH value of 30 or more.

In the present invention, in addition to the above chemical composition, Nb: 0.001 to 0.2% or B: 0.00% by weight.
01-0.0080%, and Nb: 0.001-0.2
% And B: 0.0001 to 0.0080%. In addition, the present invention relates to a method for producing C: 0.005% or less by weight,
i: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less,
S: 0.05% or less, Al: 0.01 to 0.20%, N: 0.01% or less,
Ti: A material containing 0.01 to 0.2%, the balance consisting of Fe and unavoidable impurities is formed into a cold-rolled sheet by a hot rolling step and a cold rolling step, and the AI _QUENCH value is 30 or more and the A value is 0.4 This is a method for producing a paint-bake-hardened cold-rolled steel sheet having excellent aging resistance characterized by annealing as described above. Further, in the method of the present invention, in addition to the chemical composition of the material, Nb: 0.001 to 0.2% or B: 0.0001 to 0.0080% by weight may be contained.
1 to 0.2% and B: 0.0001 to 0.0080% may be contained in combination.

[0012]

First, the reasons for limiting the chemical composition of the steel sheet of the present invention will be described. C: 0.005% or less C is an element which has an adverse effect on the deep drawability and is preferably reduced as much as possible. However, C is limited to 0.005% or less because it is acceptable up to 0.005%.

Si: 1.0% or less Si has the effect of increasing the strength of steel, and is added according to the desired strength. However, when the addition amount exceeds 1.0%, the deep drawability decreases. For this reason, Si was limited to 1.0% or less. Mn: 3.0% or less Mn has the effect of increasing the strength of steel, and is added according to the desired strength. However, when the addition amount exceeds 3.0%, the deep drawability decreases. For this reason, Mn was limited to 3.0% or less.

P: 0.15% or less P has an effect of strengthening steel, and is added according to a desired strength. However, if the addition amount exceeds 0.15%, the deep drawability deteriorates, so P was limited to 0.15% or less. S: 0.05% or less S is an element which has an adverse effect on the deep drawability, and it is preferable to reduce it as much as possible. However, S is limited to 0.05% or less because it is acceptable up to 0.05%.

Al: 0.01 to 0.20% Al is added for deoxidation and for improving the yield of carbonitride forming elements. If the addition amount is less than 0.01%, the effect of the addition is small. On the other hand, if the addition amount exceeds 0.20%, the effect corresponding to the addition amount cannot be obtained. N: 0.01% or less C is an element which has an adverse effect on the deep drawability, and it is preferable to reduce it as much as possible. However, N is limited to 0.01% or less because it is acceptable up to 0.01%.

Ti: 0.01-0.2% Ti combines with C in steel and precipitates as carbides, so
Has the effect of preventing deterioration of the deep drawability due to If the amount of Ti is less than 0.01%, the effect of the addition is small, and
If the content exceeds 0.2%, the effect corresponding to the added amount cannot be obtained. Therefore, the content of Ti is limited to the range of 0.01 to 0.2%.

In addition to the above main components, the following elements can be added as required. Nb: 0.001 to 0.2% Nb has the effect of refining the structure of the hot-rolled sheet and improving the r-value of the cold-rolled annealed sheet. This has the effect of increasing the ratio of the solid solution C present in the grain boundaries and the ratio of the solid solution C present at the grain boundaries. These effects can be observed with the addition of 0.001% or more. However, if the addition exceeds 0.2%, no further effect can be obtained, and the deep drawability tends to deteriorate. For this reason, Nb is limited to the range of 0.001 to 0.2%.

B: 0.0001 to 0.0080% B has an effect of improving the secondary work embrittlement resistance of steel. In order to improve the secondary work embrittlement resistance, 0.0001% or more must be added, but if it exceeds 0.0080%, deep drawability deteriorates. For this reason, B is limited to the range of 0.0001 to 0.0080%. A = (AI _QUENCH -AI) / AI _QUENCH : 0.4 or more AI _QUENCH value: 30 or more The A value is a grain boundary abundance ratio of solid solution C, and A = (AI _QUENCH)
-AI) / AI _QUENCH . Here, AI
_QUENCH is the aging index (MPa) after heating and water-quenching the cold-rolled annealed sheet at 500 ° C. for 40 _seconds , and AI is the aging index of the cold-rolled annealed sheet. In the present invention, the aging index is 7.
It is defined as the increase in yield stress (MPa) before and after heat treatment when a 5% tensile prestrain is added and heat treatment is performed at 100 ° C for 30 minutes.

By _{setting the} AI _QUENCH value to 30 or more and the A value to 0.4 or more, a steel sheet having high paint bake hardenability (BH property) of 30 MPa or more and excellent aging resistance can be obtained. A
If I _{QUEN CH} value is 30 or more, high B of 30MPa or more
Although the H content can be obtained, if the A value is less than 0.4, it is not possible to have both a high BH content and excellent aging resistance even if the steel composition is optimized.

As described above, the inventors of the present invention differ from the solid solution C exhibiting the BH property and the solid solution C contributing to the aging at room temperature in different locations, Solid solution C existing in grains and at grain boundaries, that is, solid solution C in steel, which can be estimated by AI _QUENCH , while solid solution C contributing to room temperature aging is solid solution C existing in grains. And A
It has been found that estimation is possible with I _QUENCH -AI.

In low temperature aging, such as room temperature aging,
It is impossible for the solid solution C existing at the grain boundaries to diffuse into the grains while being trapped by the grain boundaries. On the other hand, in a heat treatment at a high temperature such as paint baking, the solid solution C existing at the grain boundaries is Can also be diffused into the grains and contribute to BH properties. The method for producing a steel sheet of the present invention is such that, after the raw material having the above-mentioned chemical composition is formed into a cold-rolled sheet by a hot rolling step and a cold rolling step, the A value is 0.4 or more and the AI _QUENCH value is 30 or more. Anneal.

In order to adjust the AI _QUENCH value to 30 or more, there is a method of adjusting the chemical composition within the scope of the present invention and dissolving fine carbides in the annealing process or leaving solid solution C in the hot-rolled sheet. The former is more advantageous from the viewpoint of deep drawability. In order to dissolve fine carbides, the annealing temperature must be 780
It is preferable to control the temperature to at least ° C. In order to set the A value to 0.4 or more, the annealing temperature should be set low, preferably 88%.
It is better to control the temperature below 0 ° C to 780 ° C or more. When the annealing temperature is high, the energy difference between the grain boundary and the grain is almost eliminated,
The solute C present at the grain boundaries diffuses into the grains, and the A value decreases. In order for a large amount of solid solution C to be present at the grain boundaries, it is necessary to set the annealing temperature low.

A hot rolling step in the production method of the present invention,
Although the cold rolling step is not particularly limited, conditions suitable for imparting excellent workability will be described. The material is heated to a temperature of 1300 ° C. or less for hot rolling. In order to improve the deep drawability by fixing solid solution C and N as precipitates, it is desirable that the heating temperature be as low as possible. However, if the temperature is lower than 900 ° C., the workability is not improved, and rather, a trouble due to an increase in the rolling load at the time of hot rolling tends to occur. From this, the heating temperature of the hot rolling is in the range of 900 ° C. to 1300 ° C., more preferably 950 ° C. to 1150 ° C.

In order to make the crystal grains of the hot-rolled sheet finer, it is preferable that the rolling reduction in hot rolling is 70% or more in total. Further, the rolling finishing temperature (FDT) may be either of the Ar ₃ transformation point or more γ region or Ar ₃ following α zone transformation point, since the rolling load during rolling and finishing rolling temperature is too low increases, Preferably, the temperature is at least 600 ° C. Although the coil winding temperature after hot rolling is higher, the higher the temperature, the more advantageous the carbonitride coarsening is. However, if it is too high, the scale thickness formed on the surface of the steel sheet increases, so that it is preferably 800 ° C. or lower.

Further, the hot-rolled sheet is subjected to cold rolling. This step is necessary for obtaining a high r-value, and for this purpose, the rolling reduction is preferably set to 50% or more. If the rolling reduction is less than 50%, a high r value cannot be expected, and excellent deep drawability cannot be obtained. The cold rolled steel sheet that has been subjected to the cold rolling step is then subjected to recrystallization annealing. The annealing method may be either a box annealing method or a continuous annealing method. In the present invention, the annealing temperature is preferably set low, preferably less than 880 ° C.
The temperature should be 780 ° C or higher. The annealing time is preferably set to 5 seconds or more. When the annealing temperature is 880 ° C. or more, the A value does not become 0.4 or more, the proportion of solute C existing in the grain boundary is low, and the aging resistance is deteriorated. When the temperature is lower than 780 ° C., the deep drawability deteriorates and the BH amount decreases.

The annealed steel sheet may be subjected to a temper rolling of 10% or less for adjusting shape correction, surface roughness and the like. It goes without saying that the cold-rolled steel sheet of the present invention can be used not only as a cold-rolled steel sheet for processing but also as an original sheet of a surface-treated steel sheet for processing. Examples of the surface treatment include zinc plating containing a zinc alloy, tin plating, enamel, and the like.

After annealing or galvanizing, the steel sheet of the present invention may be subjected to a special treatment, for example, Ni plating to improve chemical conversion treatment, weldability, press formability, corrosion resistance and the like.

[0028]

EXAMPLE A steel material (slab) having the chemical composition shown in Table 1 was used.
Under the hot rolling conditions shown in Table 2, a 3.5 mm thick hot rolled sheet was obtained. These hot-rolled sheets were cold-rolled into cold-rolled steel strips having a thickness of 0.8 mm. Next, these steel strips were continuously annealed to 750-880.
Recrystallization annealing was performed at a temperature of ° C. The obtained steel strip was further temper-rolled by 0.8% to obtain a product sheet.

For these product plates, AI, A
I _QUENCH and A value were determined, and tensile properties, r value,
BH property and room temperature aging were investigated. Tensile properties are JIS5
The yield point, tensile strength, and elongation were measured using the No. 1 tensile test piece. The r value was measured by a three-point method after 15% tensile prestrain was applied, and the r value was measured in the L direction (rolling direction) and the D direction (45 directions in the rolling direction).
Degree direction) and C direction (90 degrees in the rolling direction).
= (R _L + 2r _D + r _C ) / 4).

The BH amount was determined as the increase in the upper yield stress before and after the heat treatment at a temperature of 170 ° C. for 20 minutes after a 2% tensile prestrain was applied to the product sheet. Room temperature aging
The product plate was subjected to an aging treatment at 100 ° C. for 10 hours and evaluated by elongation at yield point. As long as the yield point elongation is 0.2% or less, there is no problem in the aging resistance at room temperature. Table 2 shows the results.

[0031]

[Table 1]

[0032]

[Table 2]

From Table 2, it can be seen that the product plates of the present invention (No. 1, No. 4,
No. 6 and No. 7) show a higher BH content and a lower yield point elongation after aging treatment at 100 ° C. for 10 hours of not more than 0.2% as compared with the comparative examples out of the range of the present invention. It turns out that it has high BH property and excellent aging resistance. Comparative Example No.2
Since the annealing temperature is high, the A value is as low as less than 0.4 and the yield point elongation after aging treatment is as high as 0.60. Also, Comparative Example No. 3
Since the steel composition is out of the range of the present invention and the annealing temperature is high, A
The value is as low as less than 0.4, and the yield point elongation after aging treatment is as high as 0.70.

In Comparative Example No. 5, the annealing temperature was too low.
The AI _QUENCH is less than 30 MPa and the BH amount is as low as 10 MPa. In Comparative Example No. 8, since the steel composition was out of the range of the present invention, AI
_QUENCH is less than _30MPa and BH amount is as low as 7MPa.

[0035]

Industrial Applicability According to the present invention, there is an industrially outstanding effect that a paint-baked hardened cold-rolled steel sheet having excellent aging resistance can be industrially stably manufactured as compared with the conventional art.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of A value on yield point elongation.

Continued on the front page (72) Inventor Takashi Ohara 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (72) Inventor Tetsuya Kiyasu 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Prefecture Kawasaki Steel Works, Mizushima Works

Claims (3)

[Claims] 1. In% by weight, C: 0.005% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.05% or less, Al: 0.01 to 0.20%, N: 0.01% The following contains Ti: 0.01 to 0.2%, has a chemical composition consisting of the balance of Fe and inevitable impurities, and has the following A value of 0.4 or more and the following AI _QUENCH
A paint bake hardenable cold rolled steel sheet having excellent aging resistance characterized by having a value of 30 or more. A = (AI _QUENCH -AI) / AI _QUENCH where AI _{QUENCH is} an aging index (MPa) after heating and water quenching a steel sheet at 500 ° C. × 40 _seconds . AI: Aging index (MPa) of the steel sheet. Aging index: 100% after adding 7.5% tensile prestrain to steel sheet
Increase of yield response <br/> force before and after the heat treatment when the heat treatment of ℃ × 30min (MPa). 2. In addition to the chemical composition, Nb:
The baking-hardened cold-rolled steel sheet according to claim 1, wherein the steel sheet contains 0.001 to 0.2% and / or B: 0.0001 to 0.0080%. 3. In% by weight, C: 0.005% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.05% or less, Al: 0.01 to 0.20%, N: 0.01% Hereinafter, a material containing 0.01 to 0.2% of Ti, or 0.001 to 0.2% of Nb and / or 0.0001 to 0.0080% of B, and the balance of Fe and unavoidable impurities is subjected to a hot rolling step and a cold rolling. After cold-rolled by the rolling process, the following AI _QUENCH value is set to 30
As described above, a method for producing a paint-bake hardenable cold-rolled steel sheet having excellent aging resistance, wherein annealing is performed so that the following A value becomes 0.4 or more. A = (AI _QUENCH -AI) / AI _QUENCH where AI _{QUENCH is} an aging index (MPa) after heating and water quenching a cold-rolled annealed plate at 500 ° C. for 40 _seconds . AI: Aging index of cold-rolled annealed sheet. Aging index: 100% after adding 7.5% tensile prestrain to steel sheet
Increase in yield stress (MPa) before and after heat treatment when heat treatment is performed at ℃ 30min.