JPH1161332A - Coating/baking hardened type cold rolled steel sheet superior in press formability and aging resistance characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the cold rolled steel sheet, which is large in coating/baking hardening quantity, which is high in uniform elongation, and which is superior in press formability and age resistance characteristic, by specifying chemical composition consisting of C, Si, Mn, P, S, Al, N, Ti and Fe and making a ratio more than constant between the average intergranular angle and the average crystal grain size. SOLUTION: The steel contains 0.005 wt.% or less C, 1.0% or less Si, 3.0% or less Mn, 0.15% or less P, 0.02% or less S, 0.01-0.20% Al, 0.01% or less N, and 0.01-0.2% Ti and also contains, as necessary, 0.001-0.2% Nb and/or 0.0001-0.0080% B, with the balance Fe and inevitable impurities; this steel is coiled after hot rolling and accelerated cooling. The hot rolled sheet having crystallite grains thus obtained is cold rolled at a reduction rate of about 70% or above and then annealed at about 880-750 deg.C. As a result, a coating/baking hardened type cold rolled steel sheet is stably obtained whose ratio M/G is 0.8 or above between the average intergranular angle M and the average crystal grain size G and whose coating/baking hardening quantity BH is 30 MPa or above.

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, which is mainly used for an automobile 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]

Further, since such a paint-baked hardened cold-rolled steel sheet is mainly used as an outer panel of an automobile, it has a uniform elongation property related to press formability, particularly stretch-forming property. It needs to be good. Conventionally, it has been known that the uniform elongation increases as the yield strength decreases or the work hardening index n increases. However, paint bake hardening steel sheets are more advantageous as the yield strength after paint baking is higher, and thus it has been difficult to obtain high uniform elongation characteristics with paint bake hardening steel sheets.

The present invention advantageously solves the above problems,
An object of the present invention is to provide a paint bake hardening type cold rolled steel sheet which can be industrially stably produced, has high uniform elongation, is excellent in press formability, and has excellent aging resistance.

[0007]

SUMMARY OF THE INVENTION The present inventors have developed a high uniform elongation, excellent aging resistance and high B
As a result of intensive studies to obtain the H property, by setting the ratio between the grain boundary angle and the crystal grain size of the steel sheet to a certain value or more, even in the case of a steel sheet having a high BH property, high uniform elongation and excellent It has been found that aging resistance can be obtained.

[0008] First, the experimental results on which the present invention is based will be described. C: 0.0025 wt%, Si: 0.02 wt%, Mn: 0.
1wt%, P: 0.01wt%, S: 0.003wt%, Al: 0.04wt
%, N: 0.002 wt%, Ti: 0 to 0.075 wt%, after heating and soaking at 1050 ° C.
A 3.5 mm thick hot rolled sheet was formed by three-pass rolling at 900 ° C., and coiled at 600 ° C. for 1 hour.
Thereafter, these hot-rolled sheets are subjected to cold rolling at a rolling reduction of 80%,
Next, recrystallization annealing was performed at 750 to 880 ° C for 40 seconds.
About the obtained cold-rolled annealed sheet, uniform elongation and BH amount were measured. The BH amount was determined as a stress difference before and after the heat treatment at 170 ° C. for 20 minutes after a tensile prestrain of 2% was applied to the cold-rolled annealed sheet.

[0009] Among these cold rolled annealed sheets, BH amount ≧ 30 MPa
For those exhibiting the characteristics of, uniform elongation, yield point elongation,
Average grain size G (μm) and average intergranular angle M (degree)
Was measured. The uniform elongation was determined by a tensile test using a JIS No. 5 tensile test piece. The yield point elongation is 100
This is the yield point elongation after aging treatment at 10 ° C. for 10 hours. 100
The aging treatment at 10 ° C. for 10 hours is equivalent to the aging treatment at room temperature for about 6 months. After this treatment, if the yield point elongation is 0.2% or less,
It has been found that there is no problem with the aging resistance. The average crystal grain size G (μm) was obtained by measuring the crystal grain size of a steel sheet in a thick section using an optical microscope with respect to a test piece taken from any three places of the steel sheet, and taking the average value as the average crystal grain size. In addition, the average intergranular angle M (degree) is calculated by Electron Back Scattering D
Using iffraction Patern (EBSD), measure the crystal orientation of the steel sheet thick section for each crystal grain, find the orientation difference (tilt angle) between adjacent crystal grains for 50 or more crystal grains, and calculate the average value did.

As shown in FIG. 1, the yield point elongation after aging treatment at 100 ° C. for 10 hours is 0.2% when M / G is 0.8 or more.
% And the uniform elongation is 34% when M / G is 0.8 or more.
That is the result. That is, M / G is 0.8
The steel sheet having the above has obtained a new finding that even when the BH content is as high as 30 MPa or more, it exhibits excellent aging resistance and high uniform elongation, that is, high press formability.

From the above, the present inventors have found that by controlling the ratio between the intergranular angle and the crystal grain size, high uniform elongation and excellent aging resistance can be obtained while ensuring high paint bake hardenability. I thought that I could manufacture a steel sheet with That is, in the present invention, C: 0.005% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.02% or less, Al: 0.01 to 0.20%, N: 0.01% by weight. % Or less, Ti: 0.01-0.
2%, having a chemical composition consisting of the balance of Fe and unavoidable impurities, and having a ratio of the average grain boundary angle M (degrees) to the average crystal grain size G (μm), M / G of 0.8 or more. A paint bake hardening type cold rolled steel sheet having excellent press formability and aging resistance characterized by having a paint bake hardening amount (BH amount) of 30 MPa or more. % By weight, Nb: 0.001 to 0.2%, or B: 0.00%
01-0.0080%, and Nb: 0.001-0.2
% And B: 0.0001 to 0.0080%.

Further, the present invention relates to a method for producing C: 0.005% by weight.
Below, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.02% or less, Al: 0.01 to 0.20%, N: 0.01% or less, Ti: 0.01 to 0.2%, or more. Nb: 0.00
A material containing 1 to 0.2% and / or B: 0.0001 to 0.0080%, and the balance consisting of Fe and unavoidable impurities is preferably heated to 1300 to 900 ° C, and preferably the cumulative rolling reduction.
After a rolling process of 70% or more, a hot-rolled plate having fine crystal grains is preferably formed by a hot rolling step of winding by applying accelerated cooling, and the hot-rolled plate is preferably cold-rolled with a rolling reduction of 70% or more. After forming a cold rolled sheet by the rolling process, the average grain boundary angle M
(Degree) and the average crystal grain size G (μm), M / G is 0.8
A method for producing a paint-bake hardenable cold-rolled steel sheet having excellent press formability and aging resistance characterized by performing annealing at a temperature of less than 880 ° C. and at least 750 ° C.

[0013]

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: 0.02% 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 0.02% or less because it is acceptable up to 0.02%.

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 N is an element that has an adverse effect on the deep drawability and is preferably reduced as much as possible. However, N is limited to 0.01% or less because it is acceptable up to 0.01%.

Ti: 0.01% to 0.2% Ti has an effect of combining with C in steel and precipitating it as carbide, thereby preventing deep drawability deterioration due to solid solution C. Ti
If the addition amount is less than 0.01%, the effect of the addition is small, and if it exceeds 0.2%, the deep drawability is deteriorated. Therefore, 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 are observed when the content is 0.001% or more. However, if the content exceeds 0.2%, no further effect is 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%. BH content: 30 MPa or more In order to provide high paint bake hardenability (BH property) having a BH content of 30 MPa or more, it is necessary to leave solid solution C in steel. As a method of leaving solid solution C in steel, there is a method of adjusting the chemical composition within the scope of the present invention and dissolving fine carbide in the annealing process or leaving solid solution C in the hot-rolled sheet. The former is more advantageous from a gender point of view.

High paint bake hardenability of 30 MPa or more (BH
) And the state of solid solution C are controlled in order to obtain a steel sheet having excellent aging resistance. The present inventors differ in the location where the solid solution C that expresses BH property and the solid solution C that contributes to aging at room temperature are present, and the solid solution C that expresses the BH property exists in grains and at grain boundaries. Solid solution C, that is, solid solution C in steel;
On the other hand, it has been found that the solute C that contributes to the aging at room temperature is the solute C existing in the grains. In aging at a low temperature such as room temperature aging, it is impossible for solid solution C existing at the grain boundaries to diffuse into the grains while being trapped at the grain boundaries.
In a heat treatment at a high temperature such as a paint baking treatment, the solute C present at the grain boundaries can also diffuse into the grains and contribute to the BH property. Therefore, in order to increase the aging resistance without decreasing the BH amount, it is important to increase the amount of solute C existing at the grain boundaries without decreasing the amount of solute C contributing to the BH property.

Average intergranular angle M (degrees) and average grain size G
(Μm) ratio M / G is 0.8 or more Average intergranular angle M (degree) is determined by Electron Back Scattering
Using Diffraction Patern (EBSD), measure the crystallographic orientation of the steel sheet thick section for each crystal grain, calculate the orientation difference (tilt angle) between adjacent crystal grains for 50 or more crystal grains, and calculate the average value I do. On the other hand, the average crystal grain size G (μm)
Is used to measure the crystal grain size of a steel sheet in a thick section using an optical microscope and use the average value.

In the steel sheet of the present invention, the average grain boundary angle M
The ratio M / G between (degree) and the average crystal grain size G (μm) is set to 0.8 or more. By setting M / G to 0.8 or more, the amount of solute C existing at the grain boundary increases. Increasing M / G, that is, refining crystal grains and increasing the angle between crystal grain boundaries increases the crystal grain boundary area, allows solid solution C to easily move to the grain boundaries, and has a large amount of solid solution C. Can exist at the grain boundary. Further, when the angle between the crystal grain boundaries increases, even in the same crystal grain boundaries, a large amount of solid solution C can be present at the grain boundaries.

If M / G is 0.8 or more, BH amount is 30MP
a) Even if the solid solution C is present in the steel, the amount of solid solution C existing in the grain boundaries increases, and a cold rolled steel sheet having low yield elongation and excellent aging resistance is obtained. When M / G is 0.8 or more, the uniform elongation increases. In order to increase uniform elongation, it is important to uniformly transmit strain throughout the material during tensile deformation. For this purpose, the present inventors consider that the ratio between the crystal grain size and the angle between the crystal grain boundaries is important, and that the ratio between the crystal grain size and the angle between the crystal grain boundaries is increased. It has been found that, if the same angle is increased, the strain propagation becomes uniform when the same strain is applied. As a result of the experiment, the critical value of the ratio M / G between the crystal grain size and the angle between the crystal grain boundaries is 0.8. I figured it out. When M / G is less than 0.8, a steel sheet having a high BH amount of 30 MPa or more as described above and having both high uniform elongation and excellent aging resistance is not obtained.

In the method for producing a steel sheet according to the present invention, a material having the above-mentioned chemical composition is formed into a cold-rolled sheet by a hot rolling step and a cold rolling step, and then annealed so that M / G becomes 0.8 or more. Factors that affect M / G include the crystal grain size of the hot rolled sheet, the rolling reduction of the cold rolling, and the annealing temperature. In order to make M / G 0.8 or more, it is important to carry out annealing at a low temperature with a small crystal grain size of the hot-rolled sheet, a high rolling reduction ratio, and a low temperature. When the annealing temperature is high, the crystal grains grow quickly and the average crystal grain size G tends to be large, and the crystal grains are engaged with each other so that the grain boundary energy becomes low during grain growth. M decreases and M / G decreases. On the other hand, if the annealing temperature is too low, the amount of solid solution C in which the BH amount is 30 MPa or more cannot be obtained. Therefore, in order for the M / G to be 0.8 or more when the BH amount is 30 MPa or more, it is preferable to form fine carbides that melt at low temperature during hot rolling and melt at low temperature during annealing.

A hot rolling step in the production of the steel sheet of the present invention,
Although the cold rolling step is not particularly limited, conditions suitable for imparting excellent workability will be described. In the hot rolling step, it is advantageous to finely precipitate solid solution C and N as carbonitride from the viewpoint of improving deep drawability.
Therefore, in order to perform hot rolling, the heating temperature of the material is desirably as low as possible, and is preferably 1300 ° C. or less. However, when the temperature is lower than 900 ° C, the processability is not improved.
On the contrary, troubles associated with an increase in the rolling load during hot rolling are likely to occur. For this reason, the heating temperature of the hot rolling is preferably in the range of 900 ° C to 1300 ° C, more preferably in the range of 950 ° C to 1150 ° C.

The rolling reduction in hot rolling is represented by the cumulative rolling reduction.
It is preferably at least 70%. If the cumulative rolling reduction is less than 70%, the grain refinement of the hot-rolled sheet is insufficient. Finishing temperature of hot rolling is preferred both austenite region or Ar ₃ transformation point of the ferrite zone temperature range over Ar ₃ transformation point. However, the hot rolling finish temperature (FDT) is 600
If the temperature is too low, the FDT is preferably set to 600 ° C. or higher.

Further, in order to make the crystal grains of the hot-rolled sheet fine, the hot-rolled sheet may be accelerated and cooled immediately after finishing rolling.
As the refrigerant used for the accelerated cooling, any of water, air, mist and the like can be applied. In order to make M / G 0.8 or more, it is preferable to set the crystal grain size of the hot-rolled sheet to 50 μm or less without using or using such accelerated cooling after hot rolling.

Coil winding temperature after hot rolling (CT)
Is preferably 800 ° C. or less. When CT is high temperature,
Although it is advantageous for the precipitation of carbonitrides, if it is too high, the scale thickness formed on the steel sheet surface becomes large. Therefore, CT is 80
The temperature is preferably set to 0 ° C. or lower. Further, the hot-rolled sheet is subjected to cold rolling. This step is necessary to obtain a high r-value, and therefore, it is preferable that the rolling reduction is 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 cold-rolled steel sheet is then subjected to recrystallization annealing. As the annealing method, a continuous annealing method is suitable. In the present invention, a large amount of fine carbides formed in the hot rolling step are dissolved in the annealing step. However, the annealing temperature needs to be set low in order to allow a large amount of solute C to be present at the grain boundaries. Further, since the BH amount is 30 MPa or more and M / G is 0.8 or more, the annealing temperature is preferably set low according to the steel composition, and is preferably less than 880 ° C. and 750 ° C. or more. If the annealing temperature exceeds 880 ° C., the solid solution C existing at the grain boundaries diffuses into the grains, and crystal grains grow, and the M / G does not become 0.8 or more. On the other hand, the annealing temperature is 750 ℃
If it is less than 30, the fine carbides will not be dissolved, and the BH amount will not be 30 MPa or more. Therefore, the annealing temperature is less than 880 ℃, 750 ℃
That is all. In order to further improve the deep drawability, annealing is preferably performed at a temperature of 800 ° C. or more for 5 seconds or more.

The annealed steel sheet may be subjected to a temper rolling of 10% or less for adjusting shape correction, surface roughness and the like. In addition,
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 the chemical conversion property, weldability, press formability, corrosion resistance and the like.

[0032]

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. The crystal size of the hot-rolled sheet was adjusted by controlling the cooling conditions after completion of the hot finish rolling (water cooling by variously changing the cooling start time). These hot-rolled sheets were cold-rolled into cold-rolled steel strips having a thickness of 0.8 mm. Next, these cold rolled steel strips are
Recrystallization annealing was performed at a temperature of 780 to 880 ° C. The obtained steel strip was further temper-rolled by 0.8% to obtain a product sheet.

For these product plates, the average crystal grain size G,
The average intergranular angle M, tensile properties, r value, BH amount, and room temperature aging were investigated. The average crystal grain size G was determined by optical microscopic observation of a cross section of the plate thickness of a test piece taken from three places. The average intergranular angle M is obtained by measuring the crystal orientation of each crystal grain in the sheet thickness section using EBSD, obtaining the orientation difference (tilt angle) between adjacent crystal grains for 50 or more crystal grains, and calculating the average. Values were calculated.

The tensile properties were determined using a JIS No. 5 tensile test piece.
The yield point, tensile strength, total elongation and uniform elongation were measured. The r value is measured by a three-point method after 15% tensile prestrain, and is measured in the L direction (rolling direction), the D direction (45 degrees in the rolling direction) and the C direction (90 degrees in the rolling direction). Average value (r =
(R _L + 2r _D + r _C ) / 4). The BH amount was measured at 170 ° C. × 20 after giving a 2% tensile prestrain to the product plate.
It was determined as the increase in the upper yield stress before and after the heat treatment after the heat treatment of min.

The aging at room temperature was evaluated by elongation at yield point after aging treatment of a product plate at 100 ° C. × 10 hours. 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.

[0036]

[Table 1]

[0037]

[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) have uniform elongation, higher r-value and BH amount, and 100 ° C. ×
The yield point elongation after aging treatment for 10 hours shows a low yield point elongation of 0.2% or less, which indicates that the steel has high BH property, excellent workability and excellent aging resistance. In Comparative Example No. 2, the crystal grain size of the hot-rolled sheet was large and the annealing temperature was high.
G is less than 0.8, uniform elongation, r value is low, and yield point elongation after aging treatment is as high as 0.60. Comparative Example No. 3 is Ti
Since the content is out of the range of the present invention, the crystal grain size of the hot-rolled sheet is large, and the annealing temperature is high, the M / G is less than 0.8, and the yield point elongation after aging treatment is as high as 0.70. Further, Comparative Example No. 5, the C content is out of the range of the present invention, uniform elongation,
The r value is low and the yield point elongation after aging treatment is as high as 0.75.

[0039]

Industrial Applicability According to the present invention, a paint bake hardening type cold rolled steel sheet having high uniform elongation, excellent press workability, and excellent aging resistance can be industrially produced in a stable manner. It has a special industrial effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of M / G on uniform elongation and yield point elongation.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takashi Ohara 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Pref. Kawasaki Steel Research Institute Co., Ltd. (72) Inventor Tetsuya Kiyasu 1-chome, Mizushima-Kawasaki-dori None) Kawasaki Steel Corporation, Mizushima Works

Claims (2)

[Claims] 1. In weight%, C: 0.005% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.02% or less, Al: 0.01 to 0.20%, N: 0.01% Hereinafter, Ti: 0.01 to 0.2%, having a chemical composition consisting of the balance of Fe and unavoidable impurities, and having an average intergranular angle M (degree) and an average crystal grain size G
(Μm), M / G is 0.8 or more, and baking hardening amount (BH amount) is 30 MPa or more, and baking hardening type cold rolling with excellent press formability and aging resistance. steel sheet. 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%.