JPH11293346A - Manufacture of high strength cold rolled steel sheet excellent in pitting corrosion resistance and deep drawability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a high strength cold rolled steel sheet synthetically satisfying the characteristics required for an automobile steel sheet such as required strength, deep drawability and corrosion resistance. SOLUTION: Steel having a composition which consists of, by weight, <=0.01% C, <=0.5% Si, 0.1-1.5% Mn, 0.005-0.015% S, 0.01-0.10% Al, <=0.006% N, 0.030-0.006% P, 0.1-0.5% Cu, 0.02-0.15% of effective Ti [where (effective Ti)=(total Ti%)-(48/12)C%-(48/32)S%$-(48/14)N% is satisfied], 0.1-0.5% Ni and the balance Fe with inevitable impurities and in which P, Cu and effective Ti satisfy the relation of inequality Cu%/64+(P%/31)×[3×10<3> ×(effective Ti%)/48]>=0.01%, is used. After the steel is heated to >=1125 deg.C, it is hot-rolled, coiled at 550 to 750 deg.C, pickled, cold-rolled at a >=60% rolling rate and successively subjected to recrystallization-annealing at 750 to 950 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention has a deep drawability suitable for use as a steel plate for automobiles and the like.
^{The present} invention relates to a method for producing a cold-rolled steel sheet having a ^second- class high strength and having excellent perforated corrosion resistance.

[0002]

2. Description of the Related Art In recent years, for the purpose of reducing the weight of an automobile body and improving safety, a tensile strength of 310 to 440 N / mm is required.
² , and a cold-rolled steel sheet having excellent deep drawability has been required. Conventionally, low-carbon Al-killed steel or ultra-low-carbon steel based on Ti,
There are many proposals for a high-strength steel sheet for deep drawing in which Mn, P, and Cr are added to increase the strength. For example,
No. 57579 discloses a cold-rolled steel sheet obtained by adding P to a low-carbon Ti-added steel, and Japanese Patent Publication No. 58-29129 discloses a cold-rolled steel sheet obtained by adding Mn to an ultra-low carbon Ti-added steel. I have. These are techniques for simultaneously imparting deep drawability and strength to an automotive steel sheet, and as a result, are intended to achieve a reduction in the thickness of the automotive steel sheet, that is, a reduction in weight.

[0003] On the other hand, steel sheets used in automobiles are used in severe corrosive environments such as running in rain and large temperature changes. Therefore, corrosion resistance is required in addition to strength and workability. Especially in North America and Northern Europe, antifreeze materials (NaCl, K
Resistance for the wet and dry repeatedly state in the presence of ions, was very good ^- Cl, in areas where spraying gravel for MgCl, etc.) or a non-slip, Cl to promote corrosion of gravel and the steel sheet to break the coating Perforated corrosion is required.

As described above, in recent years, high-strength cold-rolled steel sheets have been adopted from the viewpoint of reducing the weight of automobiles, and it has become possible to reduce the thickness of automotive steel sheets. In particular, the problem of pitting corrosion has become important, and there has been a demand for a high-strength cold-rolled steel sheet having excellent deep drawability and further excellent pitting corrosion resistance.

Under such circumstances, techniques for improving the corrosion resistance of the steel sheet itself, particularly the corrosion resistance against perforation, have been studied in addition to the improvement of the corrosion resistance such as galvanizing the steel sheet for automobiles. For example, JP-A-2-22416 discloses a technique for improving corrosion resistance by forming a dense rust layer by adding P or Cu alone or in combination.

This technique relates to a hot-rolled hot-dip alloyed hot-dip galvanized steel sheet intended for application to underbody parts of automobiles. In order to improve corrosion resistance, P is added in an amount of 0.05% or more, and pitting corrosion is reduced. In order to suppress the generation of FeS or MnS as a starting point, S is set to 0.01% or less, substantially 0.1%.
005% or less. Since a large amount of P is added, it is difficult to obtain a steel sheet having a strength class intended by the present invention.
Since it is based on low-carbon steel that does not contain, it is presumed that the above provisions will be required.

On the other hand, as a method of improving the corrosion resistance of a high-strength steel sheet for automobiles of a strength class intended by the present invention, for example, Japanese Patent Application Laid-Open Nos. Hei 4-246128 (JP-B-7-57893) and JP-A-5-195078 disclose the method. As disclosed, it is well known that the combined addition of P and Cu improves corrosion resistance.

[0008]

However, in the technology disclosed in the above-mentioned publication, although the range of claims regarding P addition is 0.03% or more, it is 0.06% or more in the embodiment. The fact is that it is added, and the corrosion resistance depends on the amount of P added, and if it is less than 0.06%, it is determined that sufficient corrosion resistance is not obtained.

On the other hand, in Japanese Patent Application Laid-Open No. Hei 9-67626, the amount of solute Ti
It is described that when 0.3% is set, the corrosion resistance can be improved even if the P content is restricted to 0.03% or less due to the interaction with Cu. It is not clear whether or not it can be applied as an actual steel sheet for automobiles because it does not describe the strength class that can be achieved.

As described above, the problem with the prior art is that there is a lack of consideration for comprehensively satisfying the properties required for an automotive steel sheet such as the required strength, deep drawability, and corrosion resistance. Therefore, an object of the present invention is to establish a technique capable of overcoming such a problem.

[0011]

The inventor of the present invention has conducted intensive studies to achieve the above-mentioned object. As a result, when the following steel components and manufacturing conditions are satisfied, extremely excellent perforated corrosion resistance and deep drawability are obtained. It has been found that it is possible to produce a high-strength cold-rolled sheet having the same, and the present invention has been completed. That is, the method for producing a high-strength cold-rolled steel sheet according to the present invention uses C:
0.01% or less, Si: 0.5% or less, Mn: 0.1 to
1.5%, S: 0.005 to 0.015%, Al:
0.01 to 0.10%, N: 0.006% or less, P
: 0.030 to 0.060%, Cu: 0.1 to 0.5
%, Effective Ti: 0.02 to 0.15% (however, effective Ti =
Total Ti%-(48/12) C%-(48/32) S%-(48/14) N
%) And P, Cu, and effective Ti satisfy the relationship of the following formula (1), and contain Ni: 0.1-0.5%, and the balance is Fe and unavoidable impurities. After the above heating, hot rolling, winding in the range of 550 to 750 ° C., pickling, cold rolling at a rolling rate of 60% or more, and then re-rolling in the temperature range of 750 to 950 ° C. Anneal the crystal.
Cu% / 64 + (P% / 31) × (3 × 10 ³ × Effective Ti%
/48)≦0.01%… (1)

First, an experimental study which led to the present invention will be described. 1 sheet bar made of steel with the following components
After heating to 180 ° C and soaking, hot rolling was performed at a finishing temperature of 920 ° C and a winding temperature of 680 ° C. Subsequently, after pickling, cold rolling was performed at a rolling reduction of 80% to 0.8 mm, and at 850 ° C., 60%.
s was subjected to recrystallization annealing. Steel composition (% by weight, balance substantially Fe) C: 0.002%, Si: 0.01%, Mn: 0.30
%, P: 0.015 to 0.07%, S: 0.008%,
Al: 0.04%, N: 0.0025%, effective Ti:
0.03 to 0.08%, Cu: 0.15 to 0.45%,
Ni: 0.15%

Using the obtained sample, the corrosion resistance against perforation and the deep drawability were examined. To evaluate the pitting corrosion resistance, after subjecting the steel sheet to phosphate treatment, put a cross cut that reaches the substrate, spray salt water (spray salt water at 50 ° C for 16 hours), and dry (hold at 70 ° C for 4 hours) ), And 50 cycles of a corrosion promotion test using 1 cycle of wetness (holding at 85% humidity for 4 hours).
The cycle was performed, and the maximum corrosion depth (maximum hole depth) of the cross cut portion was measured. The deep drawability was determined by taking test pieces from 0, 45, and 90 degrees with respect to the rolling direction, and measuring the ratio of the sheet width distortion to the sheet thickness distortion at 15% tensile deformation r0,
r45 and r90 were measured, an average r value was calculated from the following equation, and evaluation was made based on this value. Average r value = (r0 + 2r45 + r90) / 4

According to the investigation result, the effective Ti amount was 0.03
%, 0.05% and 0.08%
FIGS. 1 to 3 show graphs in which the effects of the Cu content and the Cu content on the pitting corrosion resistance and the deep drawability are arranged. In addition, FIG.
In FIG. 3, the numbers given in the figure indicate the maximum perforated depth and the average r value.

As can be seen from FIGS. 1 to 3, the corrosion resistance becomes better as the added amount of P and Cu increases, but the required amount depends on the added amount of Ti, and the required amount decreases as the added amount of Ti increases. . That is, when the added amounts of P, Cu, and Ti are increased, the corrosion resistance is not simply improved in the sum thereof. When the added amount of Ti is increased, good corrosion resistance is obtained unless the P added amount is particularly small. It became clear that there was none.

In FIGS. 1 to 3, hatched areas in the figures indicate areas having good perforated corrosion resistance and deep drawability.
The lower-right limit line of this area is expressed by the following equation.
(2). In each drawing, the limit line (two-dot chain line) when the effective Ti amount is 0.02% and 0.15% is additionally shown for reference. Cu% / 64 + (P% / 31) × (3 × 10 ³ × effective Ti% / 48) = 0.01% (2)

The reason why these relationships hold is not fully understood, but is considered as follows. Extremely low carbon Ti
When the addition amount of P is increased in the addition, precipitates of FeTiP are formed in the steel, and P and solid solution T effective for improving corrosion resistance are added.
It is argued that the amount of i decreases rather. Also, Fe
The formation of TiP also deteriorates deep drawability.

Further, steel types Nos. 23 to 28 in Table 3 described later are used.
FIG. 4 is a graph in which the effects of the S content on the perforated corrosion resistance and the deep drawability are shown in FIG. FIG. 4 shows that the above characteristics also depend on the S content in steel, and that the best corrosion resistance is obtained when the S content is 0.005 to 0.015%, preferably 0.006 to 0.013%. Recognize.

Heretofore, it has been considered that the lower the amount of S, the better the corrosion resistance, the more the amount of S is specified to be 0.005% or less at the level of the working example in most prior arts, and it is a little higher in the claims. Although there are cases where the range is restricted, the S allowable range has not been sufficiently studied, and in the low carbon steel base not containing Ti disclosed in Japanese Patent Application Laid-Open No. 22416/1990, the S Is attributed to the technical knowledge of generating FeS and MnS and deteriorating corrosion resistance.

In the ultra-low carbon Ti-added steel as in the present invention, FeS or MnS is not generated, and TiS or Ti ₄ C ₂
For generates S _2, its size is fine as compared with FeS or MnS, is small adverse effect on the corrosion resistance. For this reason, not only is the upper limit of the amount of S greatly relaxed,
Below 5% it shows better corrosion resistance than those based on low carbon steel. On the other hand, if the amount of S is too small, T
Since the binding energy of iS and Ti ₄ C ₂ S ₂ is reduced, these are not generated, and solute S is present, or Ti forms precipitates such as FeTiP to reduce corrosion resistance and formability. Deteriorate.

The present inventors have completed the present invention as a result of various studies based on the above experimental results, and the reasons for limiting the components of the cold-rolled steel sheet of the present invention will now be described. Hereinafter, all units of elemental amounts mean weight%.

C: 0.01% or less C is an unavoidable element in mass-produced steel, and is desirably as small as possible from the viewpoint of workability and corrosion resistance of the steel sheet. If the content is 0.01% or less, there is no significant adverse effect, so the upper limit was made 0.01%. Preferably, it is 0.006% or less.

Si: 0.5% or less Since Si has the effect of strengthening steel, it is added in a necessary amount depending on the desired strength. However, if the amount exceeds 0.5%, deep drawability and heat Since the surface properties after the rolling and the adhesion of the galvannealed galvanized coating are deteriorated, the upper limit is set to 0.
5%. Preferably, it is at most 0.3%.

Mn: 0.1-1.5% Since Mn has the effect of strengthening steel, it is added in a necessary amount depending on the desired strength. However, if it is less than 0.1%, the effect is too small. The lower limit was 0.1%. On the other hand, 1.5%
If the ratio exceeds 1, the deep drawability and the corrosion resistance deteriorate, so the upper limit is made 1.5%. Preferably, it is 0.2 to 1.0%.

S: 0.005 to 0.015% S is an element that plays an important role in the present invention. Conventionally, it has been thought that the lower S is, the higher the corrosion resistance is. However, in the ultra-low carbon Ti-added steel as in the present invention, F
It does not generate eS or MnS but generates TiS or Ti ₄ C ₂ S ₂ , and its size is smaller than that of FeS or MnS, and its potential difference from ground iron is small, so it is hard to become a starting point of corrosion and corrosion resistance The adverse effect on the environment is small. The reason is that if the S content is less than 0.005%, TiS or Ti ₄ C
The binding energy of ₂ S ₂ is reduced, so that these precipitates are not generated, so that solid-solution S is present, or that Ti is to generate precipitates such as FeTiP.
For this reason, good corrosion resistance and moldability can be obtained even with 0.005% or more, so 0.005% was made the lower limit. on the other hand,
If it exceeds 0.015%, even if it is TiS or Ti ₄ C ₂ S ₂ , the amount becomes large, and it becomes a starting point of corrosion due to a potential difference from the ground iron, so that the perforated corrosion resistance deteriorates.
The upper limit was 15%. Preferably, 0.006 to 0.0
13%.

Al: 0.01 to 0.10% Al is added for the purpose of deoxidation, but if it is less than 0.01%, its effect is too small to reduce the oxygen content in the steel. Lower limit. On the other hand, if it exceeds 0.10%, this effect is saturated, so this is made the upper limit. Preferably, it is 0.02 to 0.07%.

N: 0.006% N combines with Ti in steel containing Ti to form TiN. Such precipitates not only serve as starting points of corrosion but also reduce effective Ti which contributes to improvement of perforated corrosion resistance. Therefore, it is desirable to reduce the N content as much as possible, and the upper limit is 0.006. %. Preferably, 0.
004% or less.

P: 0.030% to 0.060% P is an element which plays an important role in the present invention, and has an effect of strengthening the steel and an effect of improving the corrosion resistance of the steel sheet by adding it in combination with Cu. So add the required amount. If the addition amount is less than 0.030%, such effect is too small, so 0.030% is made the lower limit. On the other hand, P
If the addition amount exceeds 0.060%, precipitates of FeTiP are formed in the steel, and P and solid solution Ti, which are effective for improving corrosion resistance, are formed.
The amount is rather reduced, and consequently, the corrosion resistance to perforation and the deep drawability are deteriorated. Therefore, the upper limit is set to 0.060%. Preferably, it is 0.035 to 0.055%. Furthermore, T
Exceeding the P content determined by the condition of the formula (1), which also depends on the i and Cu contents, has no effect on the improvement of perforated corrosion resistance, but also has the required strength and excellent deep drawability. Is not obtained, so that it is limited to the range satisfying the expression (1).

Cu: 0.1 to 0.5% Cu, like P, is an additive element which plays an important role in the present invention, and even when added alone, it densifies the rust layer formed on the surface of the steel sheet to increase corrosion resistance. However, the coexistence of P and solid solution Ti can further impart excellent perforated corrosion resistance to the steel sheet. If it is less than 0.1%, the effect is too small, so 0.1% is made the lower limit. −
On the other hand, if it exceeds 0.5%, the effect of corrosion resistance to perforation is saturated, and the workability is also deteriorated. Preferably, it is 0.2 to 0.4%. Further, the effect of improving the corrosion resistance is not remarkable outside the range satisfying the expression (1) defined by the relationship with the P and the solute Ti content, so that the expression (1)
To a range that satisfies.

Effective Ti (may be referred to as Ti *): 0.02 to 0.15% Ti is a carbonitride forming element, and the solid solution C and N in steel are converted to T
It is added to precipitate and fix as iC and TiN and to generate {111} crystal orientation which is advantageous for deep drawability. For this purpose, if Ti * is 0% or more, the amount of solid solution C becomes 0, and good deep drawability can be obtained. However, in the present invention, the content is also specified from the viewpoint of improving the perforated corrosion resistance. Is an important additive element. That is, if the content of P and Cu is within a predetermined range at 0.02% or less, good pitting corrosion resistance cannot be obtained, so the lower limit is 0.02%.
The mechanism of the improvement of the pitting corrosion resistance by solid-solution Ti is not clear at present, but it is due to the synergistic effect of elution of Ti during the progress of corrosion, stabilization of the rust layer by preferential oxidation, and densification of the rust layer of Cu. It is inferred. On the other hand, Ti *
Is basically determined by the relationship of the formula (1) defined for the P and Cu contents. This is because, if Ti is added in an amount exceeding the amount defined by the formula (1), precipitates such as FeTiP are generated, and the corrosion resistance against pitting and the formability are deteriorated. On the other hand, if it exceeds 0.15%, the contents of P and Cu satisfying the relationship of the formula (1) decrease, and the corrosion resistance to perforation cannot be obtained. Preferably, it is 0.03 to 0.12%.

Ni: 0.1-0.5% Ni is indispensable not only from the viewpoint of preventing surface flaws observed in steel having a high Cu content, but also from the viewpoint of improving perforated corrosion resistance in the present invention. It is an additional element. That is, Ni is an element which is easily concentrated on the surface, and the surface concentration of Ni suppresses the surface concentration of Cu, and improves the corrosion resistance to pitting by increasing the effective solid solution Ti.
If the amount is less than 0.1%, the effect is too small. On the other hand, if it exceeds 0.5%, not only these effects are saturated, but also the cost is increased. Preferably, it is 0.2 to 0.4%.

The steel component of the present invention comprises the above-mentioned basic components, and may further contain one or two of the following Nb and B, and (1) a basic component + Nb, (2) a basic component or
The component + B of (1) can be used.

Nb: 0.003 to 0.050% Nb is a carbide forming element, and precipitates and fixes solid solution C in steel as NbC to preferentially generate a {111} crystal orientation advantageous for deep drawability. To be added. If the amount is less than 0.003%, the effect is too small.
%, The grain size of the steel sheet after annealing becomes finer and deep drawability is rather deteriorated. Therefore, the lower limit is made 0.003% and the upper limit is made 0.050%. Preferably, 0.005 to 0.0
40%.

B: 0.0003% to 0.0050% B is added to improve the resistance to secondary working brittleness of steel.
If the amount is less than 0.0003%, the effect is too small. On the other hand, when the content exceeds 0.0050%, the deep drawability is rather deteriorated. Preferably, it is 0.0005 to 0.0020%.

Next, the hot rolling conditions of the present invention will be described. In the present invention, hot rolling conditions are also important. Desirable hot rolling conditions are to secure solid solution Ti necessary for improving perforation corrosion resistance, and to obtain the effect of surface enrichment of Ni. Performs rough rolling after securing 1125 ° C. or higher as the heating temperature of the continuous casting slab. Finishing temperature of hot rolling is A
r It is preferable to perform at a temperature of ₃ points or more for deep learning.
From the viewpoint of energy saving, low-temperature hot rolling with less than ₃ points of Ar may be used.
The winding temperature is preferably in the range of 550 to 750 ° C. That is,
If the temperature is lower than 550 ° C., it is difficult to precipitate and fix solid solution C in the steel as TiC or NbC and to generate {111} crystal orientation advantageous for deep drawability. Therefore, the lower limit is set to 550 ° C. On the other hand, above 750 ° C., Ti is more F
Since eTiP is generated to deteriorate the perforated corrosion resistance and the formability, the upper limit is set. Preferably, 600 to 70
0 ° C.

After hot rolling, pickling is performed and cold rolling is performed.
In order to obtain good deep drawability, the cold rolling reduction is set to 60% or more. It is more preferably at least 70%.

Although recrystallization annealing is performed after cold rolling, annealing conditions are important in the present invention, and it is necessary to anneal in a temperature range of 750 to 950 ° C. in order to obtain excellent deep drawability. is there. Since an excellent deep drawability cannot be obtained at an annealing temperature of less than 750 ° C., this is set as the lower limit. On the other hand, when annealing is performed in a temperature range exceeding 950 ° C., the texture is randomized due to the transformation from ferrite to austenite, so that the deep drawability deteriorates. Preferably, it is 800 to 900 ° C.

In the present invention, as the annealing step, annealing in a continuous annealing line or a continuous hot-dip galvanizing line is suitable. As the continuous hot-dip galvanizing method, both non-alloyed hot-dip galvanizing and alloyed hot-dip galvanizing are suitable. Further, regarding the steel sheet of the present invention, after annealing or galvanizing, appropriate treatment is performed to perform chemical processing,
Improvements such as weldability, press formability and corrosion resistance may be made.

[0039]

EXAMPLE A steel slab having the chemical composition shown in Tables 1 and 3 was heated and soaked at 1180.degree. C., hot-rolled at a finishing temperature of 920.degree. C., and wound up at 680.degree. After pickling the obtained hot-rolled sheet, it was cold-rolled at a rolling reduction of 78% to a sheet thickness of 0.8 mm, and then 850 ° C. in a continuous annealing line.
For 60 seconds for recrystallization annealing.

The specimens thus obtained were subjected to a tensile test and a corrosion resistance evaluation test. The tensile properties were measured using a JIS No. 5 tensile test piece. For the deep drawability and corrosion resistance, the average r value and the maximum hole depth were determined in the same manner as described above. Tables 2 and 4 show the material properties of the final product. It can be seen that the cold-rolled steel sheet conforming to the present invention (marked with * in the remarks column) has excellent perforation corrosion resistance and deep drawability. The steel type No. 7 in Tables 3 and 4 shows the same data as those in Tables 1 and 2.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

[Table 4]

Table 5 shows the results of a tensile test and a corrosion resistance evaluation test performed on test materials manufactured under various hot rolling conditions and annealing conditions using steel type No. 7 in Table 1. It was confirmed that the cold-rolled steel sheets manufactured under the manufacturing conditions conforming to the present invention (marked with * in the remarks column) have excellent perforated corrosion resistance and deep drawability.

[0046]

[Table 5]

[0047]

According to the method for producing a cold-rolled steel sheet of the present invention,
Particularly, the contents of P and Cu are regulated to the optimum amounts satisfying the expression (1) according to the effective Ti amount, and the S content is set to 0.00
5 to 0.015%, the tensile strength is 340 N / mm
It is possible to obtain a high-strength cold-rolled steel sheet having a ^second- class high strength and excellent in perforation corrosion resistance and deep drawability.

[Brief description of the drawings]

FIG. 1. P and Cu in effective Ti 0.003 wt%
4 is a graph showing the effect of the content on perforated corrosion resistance and deep drawability.

FIG. 2 P and Cu in effective Ti 0.005 wt%
4 is a graph showing the effect of the content on perforated corrosion resistance and deep drawability.

FIG. 3 P and Cu in effective Ti 0.008 wt%
4 is a graph showing the effect of the content on perforated corrosion resistance and deep drawability.

FIG. 4 is a graph showing the effect of S content on perforated corrosion resistance and deep drawability.

Claims (2)

[Claims] 1. C .: not more than 0.01% by weight,
i: 0.5% or less, Mn: 0.1 to 1.5%, S:
0.005 to 0.015%, Al: 0.01 to 0.10
%, N: 0.006% or less, P: 0.030-0.
060%, Cu: 0.1-0.5%, effective Ti: 0.0
2 to 0.15% (however, effective Ti = total Ti%-(48/12) C%
− (48/32) S% − (48/14) N%), and P, Cu and effective Ti satisfy the relationship of the following formula (1), and Ni: 0.
A steel containing 1 to 0.5%, the balance being Fe and unavoidable impurities, is heated to 1125 ° C or higher, and then hot-rolled,
Winding in the range of 550 to 750 ° C., pickling, cold rolling at a rolling rate of 60% or more, and subsequently recrystallization annealing in a temperature range of 750 to 950 ° C. Manufacturing method of high strength cold rolled steel sheet with excellent drawability. Cu% / 64 + (P% / 31) × (3 × 10 ³ × effective Ti% / 48) ≦ 0.01% (1) 2. The composition according to claim 1, further comprising Nb:
0.003-0.05%, B: 0.0003-0.
2. The method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein the steel sheet contains 0050% of one or two kinds.