JP3336079B2 - High-strength cold-rolled steel sheet excellent in deep drawability and chemical conversion property and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet having excellent deep drawability and chemical conversion property, which is useful for use in automotive steel sheets and the like.

[0002]

2. Description of the Related Art Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order for the steel sheet to exhibit excellent deep drawability as described above, as the mechanical properties of the steel sheet, a high r value (Rankford value) and a good ductility (El.
) Is necessary.

Various methods have been proposed for improving the deep drawability. For example, JP-B-44-17268, JP-B-44-17268
No. -17269 and Japanese Patent Publication No. Sho 44-17270, low-carbon rimmed steel is subjected to cold-rolling-annealing twice to increase the r value to 2.18.
A method for producing a cold-rolled steel sheet having an increased temperature is disclosed. However, these methods involve two steps of cold rolling and recrystallization annealing.
It has to be done every time, and the energy and cost required for it are enormous.

On the other hand, in recent years, for the purpose of reducing the weight of a vehicle body and improving safety, a tensile strength of 35 to 60 kgf / mm ^{2 has been used.}
The momentum for using higher strength steel sheets has rapidly increased. Even with such a high-strength steel sheet,
It is needless to say that during press forming, it is required to exhibit excellent deep drawability, and therefore, a steel plate having higher strength and a r value equal to or higher than that of conventional steel and excellent ductility. R & D is progressing.

[0005] In order to manufacture such a high-strength cold-rolled steel sheet for deep drawing, a low-carbon steel containing Si, Mn, P, etc. as a reinforcing component is used.
Generally, Al-killed steel is subjected to ordinary hot rolling, then to cold rolling, and subsequently to recrystallization annealing. However, in order to obtain high strength, a large amount of the above-described reinforcing component must be contained, and therefore, a texture that is not favorable for deep drawability is formed, and only a steel sheet having a low r-value has been obtained. Therefore, the tensile strength (TS)
Strength and to evaluate the product of the r value - processability balance, in the conventional 35 kgf / mm ² or more high strength cold rolled steel sheet, obtained only the value of less than 100, therefore provided with a sufficient press formability I didn't. In order to press-form a high-strength cold-rolled steel sheet having a tensile strength of 35 kgf / mm ² or more, the product of the tensile strength and the r value (TS × r) needs to be 100 or more, and this relationship is satisfied. Unless a stable steel sheet is obtained, press forming of a high-strength cold-rolled steel sheet cannot be satisfied.

[0006] Although a chemical conversion property is required as a property of a steel sheet for automobiles, as described above, a conventional high-strength cold-rolled steel sheet contains a large amount of Si, Mn, P, etc. in order to increase the strength. Therefore, there is a problem that the chemical conversion property is significantly deteriorated.

[0007]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and regulates the composition, crystallographic orientation, and carbon concentration of the steel sheet surface to reduce the tensile strength.
It is an object of the present invention to propose a high-strength cold-rolled steel sheet having characteristics of 35 kgf / mm ² or more and TS × r of 100 or more, and excellent in chemical conversion treatment.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to improve the deep drawability, and as a result, as described below, a high-strength cold-rolled steel sheet in which the steel composition, crystal orientation, and carbon concentration on the steel sheet surface are limited as follows. Has excellent deep drawability and chemical conversion treatment properties.

The gist of the present invention is as follows. (1) C: 0.01 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt%
%, B: 0.0001 to 0.0080 wt%, Al: 0.01 to 0.20 wt%
%, P: 0.01 to 0.20 wt%, S: 0.05 wt% or less, and N: 0.
01 wt% or less, and further Ti: 0.01 to 0.2 wt% and Nb:
0.001 to 0.2 wt% of one or two kinds, and
i, Mn, and P represent their contents [% Si], [% Mn],
The value of E defined by the relationship of [% P] is within the range satisfying the following equation: E = 6 + 2.5 [% Si] +0.5 [% Mn] +10.5 [% P] ≧ 7, with the balance being Fe and unavoidable And hot finish rolling at the Ar ₃ transformation point and below
The upper part of the steel sheet obtained in the manufacturing process under lubrication in the lower temperature range
E value and {222} parallel to the plate surface in the plate thickness center plane
{200} plane strength I parallel to the plate surface of plane strength I ₍₂₂₂₎
X-ray diffraction intensity ratio to ₍₂₀₀₎ I = I ₍₂₂₂₎ / I ₍₂₀₀₎
With I × E ≧ 240 , tensile strength (TS) ≧ 35
kgf / mm ² and TS × r value ≧ 100, the difference between the average carbon concentration of the layer from the surface of at least one of the steel sheets up to 100 μm in the thickness direction and the average carbon concentration on the inner side in the thickness direction (ΔC) Is a high-strength cold-rolled steel sheet having a surface high C layer having an average carbon concentration of 20% or more of the total thickness and having excellent deep drawability and chemical conversion treatment properties (first invention). (2) In the above item 1, the steel sheet further contains Mo: 0.01 to 1.5 wt.
%, Cu: 0.1 ~1.5 wt% and Ni: 0.1 to 1.5 become wt% of one or composition containing two or more, the deep drawability and chemical conversion excellent in high-strength cold-rolled steel sheet (second invention ).

(3) C: 0.01 wt% or less, Si: 2.0 wt% or less
Below, Mn: 3.0 wt% or less, B: 0.0001-0.0080 wt%, Al:
0.01 to 0.20 wt%, P: 0.01 to 0.20 wt%, S: 0.05 wt% or less
Below and N: 0.01 wt% or less, and further Ti: 0.01 to 0.2
wt% and Nb: Contains 1 or 2 types of 0.001 to 0.2 wt%
And the above-mentioned Si, Mn, and P are contained in their respective contents [% S
i], [% Mn], and [% P] in a range that satisfies the following equation: E = 6 + 2.5 [% Si] +0.5 [% Mn] +10.5 [% P] ≧ 7 Contained, balance is Fe and inevitable impurities
Rolled material with composition of less than Ar ₃ transformation point and more than 500 ℃
The total draft is more than 50% while lubricating in the temperature range of 95
% Hot finish rolling, and then the winding temperature 6
Winding process at 50 ° C or higher or annealing temperature: 650 to 950 ° C
Hot rolled sheet for crystal annealing After recrystallization annealing, reduction
Rate: 50-95% cold rolling, then annealing temperature: 700
After recrystallization annealing at ~ 950 ° C, in the temperature range of 500 ° C or more
Whether the atmosphere during heating or cooling is a carburizing atmosphere,
Alternatively, once cooled, carburize at a temperature of 500 ° C or more
Excellent deep drawability and chemical conversion property characterized by applying
(3rd invention). (4) In the above item 3, the steel sheet further contains Mo: 0.01 to 1.5 wt.
%, Cu: 0.1-1.5 wt% and Ni: 0.1-1.5 wt%
Or a composition containing two or more, deep drawability and chemical conversion
Method of manufacturing high-strength cold-rolled steel sheet with excellent processability (4th development)
Akira).

Hereinafter, the results of the research on which the present invention was developed will be described. Experiment 1 C: 0.002 wt%, Si: 1.0 to 1.8 wt%, Mn: 0.2 to 2.5
wt%, P: 0.01-0.18wt%, S: 0.01wt%, Al: 0.05wt
%, N: 0.002 wt%, Nb: 0.03 wt% and B: 0.0015 wt%
Various steel slabs having the following composition were heated and soaked at 1150 ° C., subjected to hot rough rolling at 950 ° C. and a reduction of 75%, and then subjected to lubricating rolling at a finishing temperature of 620 to 980 ° C. Subsequently, the obtained hot-rolled sheet was subjected to recrystallization annealing at 750 ° C. for 5 hours, and then cold-rolled at a reduction of 75% to a sheet thickness of 0.7 mm.
Recrystallization annealing at 20 ° C. for 20 seconds was performed.

FIG. 1 shows the results of examining the effects of the steel composition and the texture of the center plane of the sheet thickness on the r-value and tensile strength (TS) of the thus obtained cold-rolled steel sheet. As is clear from FIG. 1, the r value after cold rolling and annealing, TS (kgf / mm ² ) depends on the steel composition and the texture of the center plane of the sheet thickness, and the steel composition is E = 6 +
2.5 Si + 0.5 Mn + 10.5P ≧ 7, and the {222} plane strength I ₍₂₂₂₎ parallel to the sheet plane at the center plane of the sheet thickness with respect to the {200} plane strength I ₍₂₀₀₎ parallel to the sheet plane When the X-ray diffraction intensity ratio I = I ₍₂₂₂₎ / I ₍₂₀₀₎ satisfies I × E ≧ 240 in relation to the E value, the tensile strength becomes 35.
It has been found that characteristics of kgf / mm ² or more and TS × r of 100 or more can be obtained.

Experiment 2 C: 0.002 wt%, Si: 1.0 wt%, Mn: 1.0 wt%, P: 0.
05 wt%, S: 0.005 wt%, Al: 0.05 wt%, N: 0.002 wt
%, Nb: 0.03% by weight and B: 0.0030% by weight of a steel slab are heated at 1150 ° C, soaked, and reduced at 950 ° C at 75%.
, And lubricated rolling was performed at a finishing temperature of 700 ° C. Subsequently, the obtained hot-rolled sheet is subjected to recrystallization annealing at 750 ° C. for 5 hours, and then cold-rolled at a reduction of 75% to obtain a sheet thickness.
After 0.7 mm, recrystallization annealing was performed at 890 ° C. for 20 seconds.
During the annealing of the cold-rolled sheet, the carbon concentration in the surface layer of the steel sheet was variously changed by adjusting to a carburizing atmosphere. In addition,
Carburizing was performed at the time of soaking after recrystallization was completed.

With respect to the cold-rolled steel sheet thus obtained, the carbon concentration and the chemical conversion property of the surface layer of the steel sheet were examined. Regarding the carbon concentration, the difference between the average carbon concentration of the layer from the steel sheet surface on one side to 100 μm and the average carbon concentration on the inner side in the thickness direction therefrom was evaluated by (ΔC). In addition, chemical conversion treatment,
The steel sheet was degreased and washed with water, subjected to a phosphate treatment, and evaluated by a pinhole area ratio when a pinhole test described below was performed. In other words, in the pinhole test, a filter paper soaked with a reagent (ferrooxyl solution) that reacts with iron ions to form a color is brought into close contact with the test surface, and the portion of the steel sheet surface where phosphoric acid crystals are not adhered is detected, and the image is detected. This was analyzed and evaluated as the pinhole area ratio. FIG. 2 is a graph showing the effect of the carbon concentration in the surface layer of the steel sheet on the chemical conversion property of the steel sheet after cold rolling and annealing. From FIG. 2, it was found that when ΔC was 20% or more, the pinhole area ratio was less than 2% even when a large amount of Si, Mn, and P was contained, and the chemical conversion treatment property was significantly improved.

[0015]

[Action]

(1) Steel component As described above, in the present invention, the steel component is important, and unless the component composition range described above is satisfied, excellent deep drawability cannot be secured. Hereinafter, the reason for limiting the range of each component will be described.

(A) C: 0.01 wt% or less C is preferable as the content of C is smaller, since the deep drawability is improved. However, when the content of C is 0.01 wt% or less, C is not so adversely affected. % Or less. More preferably, it is 0.008 wt% or less.

(B) Si: 2.0 wt% or less Si has an effect of strengthening steel and contains a necessary amount according to a desired strength. The content is 2.0 wt%.
Exceeding the range will adversely affect deep drawability and surface properties.
It is limited to 2.0 wt% or less, preferably 1.5 wt% or less. In order to exert the above-mentioned effect, it is preferable to contain about 0.1% by weight or more. (c) Mn: 3.0 wt% or less, Mn has the effect of strengthening steel and contains a necessary amount according to the desired strength.
If the content exceeds the limit, deep drawability is adversely affected, so the content is limited to 3.0 wt% or less. In order to exhibit the above-mentioned effect,
It is preferable to contain about 0.5 wt% or more.

(D) B: 0.0001 to 0.0080 wt% B is contained in order to improve the resistance to secondary working brittleness. If the content is less than 0.0001wt%, there is no effect,
On the other hand, if the content exceeds 0.0080 wt%, the deep drawability deteriorates, so the content is limited to 0.0001 to 0.0080 wt%.

(E) Al: 0.01 to 0.20 wt% Al is deoxidized and is contained as necessary in order to improve the yield of carbonitride-forming components, but the content is reduced to 0.01 wt%. If the content is less than this, there is no effect. On the other hand, if the content exceeds 0.20 wt%, a further deoxidizing effect cannot be obtained, so the content is limited to 0.01 to 0.20 wt%.

(F) P: 0.01 to 0.20 wt% P has an effect of strengthening steel and contains a necessary amount according to a desired strength. The content is 0.01 wt%.
If less than the above, there is no effect of increasing the strength. On the other hand, if it exceeds 0.20 wt%, the deep drawability is adversely affected, so the content was limited to 0.01 to 0.20 wt%.

(G) S: 0.05% by weight or less S is preferably as small as possible because the deep drawability is improved. However, if the content of S is 0.05% by weight or less, the content is not so adversely affected. Limited.

(H) N: 0.01 wt% or less N is preferably as small as possible because the deep drawability is improved. However, if the content of N is 0.01 wt% or less, the content is not so adversely affected. Limited.

(I) Ti: 0.01 to 0.2 wt% Ti is an important component in the present invention, and the solid solution (C, N) in the steel is reduced by precipitation and fixation as carbonitride, and the deep drawability is improved. This has the effect of preferentially forming the {111} orientation, which is advantageous for. Therefore, in the present invention, this Ti and the following
One or two of Nb are contained. Ti content 0.01wt
%, The effect is not obtained. On the other hand, if the content exceeds 0.2 wt%, no further effect is obtained, and instead, the steel sheet surface property is deteriorated. Therefore, the content is limited to 0.01 to 0.2 wt%.

(J) Nb: 0.001 to 0.2 wt% Nb is an important component in the present invention, and is reduced by precipitation-fixing solid solution C in steel as carbide and {111} which is advantageous for deep drawability. This has the effect of forming the orientation preferentially.
In this respect, Nb is the same active ingredient as Ti, and in this invention, Ti and Nb
One or two types are contained. Furthermore, by the inclusion of Nb,
The structure before finish rolling is refined, and as a result, there is also an effect of preferentially forming the {111} orientation advantageous for deep drawability after finish rolling and recrystallization. If the Nb content is less than 0.001 wt%, the effect is not obtained. On the other hand, if the Nb content is more than 0.2 wt%, no further effect is obtained and the ductility is deteriorated. Limited.

In the second invention, Mo: 0.01 to 1.5 wt%, Cu:
One or more of 0.1 to 1.5 wt% and Ni: 0.1 to 1.5 wt% are contained. These components are all reinforcing components of steel. (k) Mo: 0.01 to 1.5 wt% Mo has the effect of strengthening steel and is contained according to the desired strength. However, if the content is less than 0.01 wt%, there is no effect. If the content exceeds 1.5 wt%, the deep drawability is adversely affected, so the content is limited to 0.01 to 1.5 wt%.

(L) Cu: 0.1-1.5 wt% Cu has the effect of strengthening steel and is contained in accordance with the desired strength. However, if the content is less than 0.1 wt%, the effect is not obtained. On the other hand, if the content exceeds 1.5 wt%, the deep drawability is adversely affected, so the content was limited to 0.1 to 1.5 wt%.

(M) Ni: 0.1 to 1.5 wt% Ni has an effect of strengthening the steel, and is effective for improving the surface properties of the steel sheet when Cu is contained. The content is 0.1 wt%
If it is less than 1.5 wt%, there is no effect. On the other hand, if it exceeds 1.5 wt%, deep drawability is adversely affected, so it was limited to 0.1-1.5 wt%.

(N) E = 6 + 2.5 [% Si] +0.5 [% Mn]
+10.5 [% P] ≧ 7 The content of Si, Mn and P is within the above-mentioned range, and E = 6 + 2.5 [% Si] +0.5 [% Mn] +10.5 [% P] ≧
7 must be satisfied. As described above, Si, Mn, and P all have the effect of strengthening the steel, and contain a necessary amount according to the desired strength. However, the content is 6 + 2.5 [% Si] +0. 5 [% Mn] When +10.5 in E values computed in [% P] is less than 7, the tensile strength of the resulting steel sheet does not reach ^{35kgf / mm 2, E = 6} + 2.5 [% Si] +0.5 [% Mn] +10.5 [% P] ≧
7 shall be satisfied.

(2) Texture The texture of the steel sheet is one of the most important requirements in the present invention.
X-ray diffraction intensity ratio of 2｝ plane intensity I ₍₂₂₂₎ to {200} plane intensity I ₍₂₀₀₎ parallel to the plate surface I = I ₍₂₂₂₎ / I
₍₂₀₀₎ needs to satisfy I × E ≧ 240 in relation to the E value calculated based on the contents of Si, Mn and P described above. Unless such a relationship is satisfied, excellent deep drawability in which TS × r is 100 or more cannot be obtained. Here, the {222} plane strength and the {200} plane strength refer to relative strengths based on a random texture.

(3) Carbon Concentration in Surface Layer of Steel Sheet The carbon concentration in the surface layer of steel sheet is one of the most important requirements in the present invention, and the average carbon concentration of the layer up to 100 μm in the thickness direction from the surface of at least one of the steel sheets. It is necessary that the difference (ΔC) from the average carbon concentration on the inner side in the thickness direction is 20% or more of the average carbon concentration in the entire thickness. If such amount of carbon does not exist in the surface layer of the steel sheet, excellent chemical conversion treatment property cannot be obtained. Thus, the relatively high C
The effect on the chemical conversion treatment by the concentration is as follows.Since the carbon on the surface layer of the steel sheet became the nucleus of the phosphate crystal, Si, Mn,
It is considered that even if P and the like are concentrated on the surface layer of the steel sheet, pinholes are hardly generated. As a specific processing means, a carburizing treatment may be mentioned. In the present invention, the high C layer from the surface of the steel sheet to 100 μm may be on at least one side, and may be on both sides. Then, the average carbon concentration near the center in the thickness direction, which is the basis for calculating ΔC, is calculated excluding the case where the surface high C layer is 100 μm from both surfaces of the steel sheet regardless of whether it is on one side or both sides. The average carbon concentration in the region.

(4) Manufacturing Method Hereinafter, a preferable manufacturing method of the steel sheet of the present invention will be described. Hot Rolling Step In the hot rolling step, it is necessary to perform finish rolling such that the total draft is 50% or more and 95% or less while performing lubrication in a temperature range of 500 ° C. or lower below the Ar ₃ transformation point. Here, in the temperature range higher than the Ar ₃ transformation point, the texture is randomized by the γ-α transformation no matter how much rolling is performed.
No 1｝ texture is formed, so that only a low r-value is obtained after cold rolling and annealing. On the other hand, even if the rolling temperature is lowered to less than 500 ° C., further improvement of the r value cannot be expected and only the rolling load increases, so that the rolling temperature is suitably 500 ° C. or lower below the Ar ₃ transformation point.

The reduction ratio of the finish rolling is, {111} texture is not formed hot-rolled sheet if less than 50%, whereas, 95
%, An unfavorable texture is formed in the hot-rolled sheet in terms of r value .
Need to be

Further, if the rolling at or below the Ar ₃ transformation point is referred to as non-lubricating rolling, shear deformation caused by the frictional force between the roll and the steel sheet is not preferable for deep drawability.
0 ° crystal grains are preferentially formed on the surface layer of the steel sheet, and r
Since no improvement in the value can be expected, it is necessary to perform lubrication rolling in order to secure deep drawability.

Here, the roll diameter, roll structure, type of lubricant and type of rolling mill in the above-mentioned rolling may be arbitrary. Further, the steps before the above-mentioned rolling are not particularly limited.For example, for a rolled material, after reheating or continuously casting a continuous casting slab, immediately without lowering the temperature below the Ar ₃ transformation point, or a heat retaining treatment. It is preferable to use what has been made into a sheet bar by rough rolling.
As such rough rolling conditions, it is preferable to set the rough rolling end temperature to the Ar ₃ transformation point to (Ar ₃ transformation point + 100 ° C.) for the purpose of refining the structure before finish rolling.

Hot Rolled Sheet Recrystallization Treatment Step Next, since the steel of the present invention has a hot rolled finishing temperature of not more than the Ar ₃ transformation point, the hot rolled sheet has a worked structure. for that reason,
It is necessary to perform a recrystallization treatment on this hot rolled sheet to form a {111} orientation. If the recrystallization treatment is not performed, the {111} orientation is not formed in the hot-rolled sheet, so that the subsequent improvement in the r-value cannot be expected by cold rolling and annealing. This hot rolled sheet recrystallization treatment is performed by a winding step or a recrystallization annealing step after hot rolling. When recrystallization treatment is performed from the winding step, the winding temperature should be 650 ° C or higher. If the winding temperature is lower than 650 ° C., the hot-rolled sheet is difficult to recrystallize and the {111} orientation is hard to be formed in the hot-rolled sheet. Therefore, the improvement of the r value cannot be expected even by the subsequent cold rolling and annealing. When recrystallization is performed in the recrystallization annealing step, either batch annealing or continuous annealing is suitable, and the annealing temperature must be 650 to 950 ° C.
You .

Cold Rolling Step This step is performed in order to obtain a high r-value, and the cold rolling reduction must be 50 to 95%. If the cold rolling reduction is less than 50% or more than 95%, excellent deep drawability cannot be obtained.

Annealing Step The cold-rolled steel strip or the cold-rolled steel sheet having undergone the cold rolling step needs to be subjected to recrystallization annealing. This recrystallization annealing may be either a box-type annealing method or a continuous-type annealing method. Annealing temperature 700 ~ 95
It must be in the range of 0 ° C. Subsequent to the annealing, carburizing is performed to increase the carbon concentration in the surface layer of the steel sheet.
This carburizing treatment is carried out by setting a carburizing atmosphere during heating holding or cooling in a temperature range of 500 ° C. or higher, or by once cooling and then reheating to 500 ° C. or higher.

It goes without saying that the steel strip after the annealing may be subjected to a temper rolling of 10% or less in order to correct the shape or adjust the surface roughness. Further, the cold-rolled steel sheet obtained by the present invention can be applied to an original sheet of a surface-treated steel sheet for processing. Examples of the surface treatment include zinc plating (including alloys), tin plating, and enamel.

[0039]

EXAMPLES Steel slabs having various component compositions shown in Table 1 were prepared. In Table 1, those whose numerical values are out of the range of the present invention are underlined.

[0040]

[Table 1]

These slabs were subjected to hot rough rolling and finish rolling, and then recrystallized. After pickling the obtained hot-rolled sheet, cold-rolled to give a cold-rolled steel strip with a sheet thickness of 0.7 mm,
Recrystallization annealing is performed at 890 ° C for 20 seconds in the continuous annealing equipment, and then, after the steel sheet is completely recrystallized in the carburizing equipment in the continuous annealing line, carburizing treatment is performed at 5% H ₂ -0.5 to 1.5%.
The treatment was performed at CO, a treatment temperature of 750 to 850 ° C, and a treatment time of 0 to 10 s. Table 2 shows these hot rolling conditions, hot rolled sheet annealing conditions, cold rolling conditions, and recrystallization annealing conditions.

[0042]

[Table 2]

Table 2 also shows the results of examining the material properties of the thus obtained cold rolled steel sheet. Note that ΔC was measured by performing C analysis on a sample obtained by chemically polishing a sample in the thickness direction. The tensile properties were measured using a JIS No. 5 tensile test piece. The r-value is measured by the three-point method after 15% tensile prestrain is applied, and the L-direction (rolling direction), D-direction (45 degrees from rolling direction) and C-direction (90 degrees from rolling direction).
The average value in (degree direction) was determined from the equation: average r value = (r _L + 2r _D + r _C ) / 4. Further, the texture was measured by measuring the {222} plane intensity and the {200} plane intensity parallel to the plate surface with respect to the center plane of the plate thickness by the X-ray diffraction method. Furthermore, the chemical conversion treatment is to degrease the steel sheet, wash it with water, and then perform phosphate treatment,
As a pinhole test, a filter paper soaked with a reagent (ferrooxyl solution) that reacts with iron ions and develops color is brought into close contact with the test surface to detect the phosphoric acid crystal-free portion remaining on the steel plate surface and analyze the image. The pinhole area ratio was evaluated by examining the results. The pinhole area ratio was evaluated as 面積 when it was less than 2%, Δ when it was 2 to 9%, and X when it exceeded 9%.

As is apparent from Table 2, each of the conforming examples according to the present invention has both excellent deep drawability and chemical conversion treatment properties as compared with the comparative example.

[0045]

The high-strength cold-rolled steel sheet of the present invention, by limiting the steel composition and crystal orientation, has a much better strength-workability balance than before, and also has excellent chemical conversion treatment properties. Which is particularly useful as a panel for an automobile.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of examining the effects of the steel composition and the texture of the center plane of the sheet thickness on the r-value and tensile strength (TS) of a cold-rolled steel sheet.

FIG. 2 is a graph showing the effect of the carbon concentration of the steel sheet surface layer on the chemical conversion property of the steel sheet after cold rolling and annealing.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corp. Technical Research Division (56) References JP-A-2-197528 (JP, A) JP JP-A-4-72036 (JP, A) JP-A-1-1422051 (JP, A) JP-A-3-202443 (JP, A) JP-A-60-149729 (JP, A) JP-A-4-285125 (JP) JP-A-6-2069 (JP, A) JP-A-3-28325 (JP, A) JP-A-2-149624 (JP, A) JP-A-2-163318 (JP, A) 5-112858 (JP, A) JP-A-2-47222 (JP, A) JP-A-3-150316 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 -38/60

Claims (4)

(57) [Claims] C: 0.01 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, B: 0.0001 to 0.0080 wt%, Al: 0.01 to 0.20 wt%, P: 0.01 to 0.20 wt%, S: 0.05 wt% or less and N: 0.01 wt% or less, further contains one or two of Ti: 0.01 to 0.2 wt% and Nb: 0.001 to 0.2 wt%, and the Si, Mn, P The E value determined by the relationship between their contents [% Si], [% Mn], and [% P] is given by the following equation: E = 6 + 2.5 [% Si] +0.5 [% Mn] +10.5 [% P ≧ 7, the balance being composed of Fe and unavoidable impurities, and hot finish rolling in a temperature range below the Ar ₃ transformation point.
The above E value and {222} plane strength I _{(222) of} the steel sheet obtained in the manufacturing process performed under sliding, which is parallel to the sheet surface in the center plane of the sheet thickness.
X of {200} plane strength I ₍₂₀₀₎ parallel to the plate surface
The product of the line diffraction intensity ratio I = I ₍₂₂₂₎ / I ₍₂₀₀₎ is I × E ≧
240 , tensile strength (TS) ≧ 35 kgf / mm ² and T
The difference (ΔC) between the average carbon concentration of the layer up to 100 μm in the thickness direction from the surface of at least one of the steel sheets and the average carbon concentration on the inner side in the thickness direction from the surface of at least one steel sheet satisfies S × r value ≧ 100. Carbon concentration of 20
% High-strength cold-rolled steel sheet having a surface high C layer of not less than 10% and excellent in deep drawability and chemical conversion treatment properties. 2. The method of claim 1, the steel sheet further Mo: obtain a composition containing one or more 0.1 ~1.5 wt%: 0.01~1.5 wt% , Cu: 0.1 ~1.5 wt% and Ni High strength cold rolled steel sheet with excellent drawability and chemical conversion properties. C: 0.01 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, B: 0.0001 to 0.0080 wt %, Al: 0.01 to 0.20 wt %, P: 0.01 to 0.20 wt%, S: 0.05 wt% or less and N: 0.01 wt% or less , further contains one or two kinds of Ti : 0.01 to 0.2 wt% and Nb: 0.001 to 0.2 wt% , and the Si, Mn, P ,It
These are determined by the relationship between their contents [% Si], [% Mn] and [% P].
In the range satisfying the following equation: E = 6 + 2.5 [% Si] +0.5 [% Mn] +10.5 [% P] ≧ 7 , with the balance being Fe and unavoidable impurities
Rolled material with composition of less than Ar ₃ transformation point and more than 500 ℃
The total draft is more than 50% while lubricating in the temperature range of 95
% Hot finish rolling, and then the winding temperature 6
Winding process at 50 ° C or higher or annealing temperature: 650 to 950 ° C
Hot rolled sheet for crystal annealing After recrystallization annealing, reduction
Rate: 50-95% cold rolling, then annealing temperature: 700
After recrystallization annealing at ~ 950 ° C, in the temperature range of 500 ° C or more
Whether the atmosphere during heating or cooling is a carburizing atmosphere,
Alternatively, once cooled, carburize at a temperature of 500 ° C or more
Excellent deep drawability and chemical conversion property characterized by applying
Manufacturing method of high strength cold rolled steel sheet. 4. The composition according to claim 3, wherein the steel sheet further contains one or more of Mo: 0.01 to 1.5 wt%, Cu: 0.1 to 1.5 wt%, and Ni: 0.1 to 1.5 wt%. , Deep drawability
Method for producing high- strength cold-rolled steel sheets with excellent chemical conversion properties .