JP3370930B2 - Manufacturing method of high-strength cold-rolled steel sheet excellent in perforation corrosion resistance and deep drawability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a deep drawability suitable for use in steel plates for automobiles and the like, and further has 340 N / mm.
^{The present} invention relates to a method for producing a cold-rolled steel sheet having high-grade ^second strength and excellent perforation corrosion resistance.

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight and safety of automobile bodies, the tensile strength is 310 to 440 N / mm.
² and a cold-rolled steel sheet having excellent deep drawability has been required. Low carbon Al killed steel and ultra low carbon steel with Si added to the base,
There are many proposals for deep-drawing high-strength steel sheets in which Mn, P, and Cr are added to increase the strength. For example, Japanese Patent Publication Sho 57
No. 57945 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 extremely low-carbon Ti-added steel. There is. These are techniques for simultaneously imparting deep drawability and strength to a steel sheet for automobiles, and as a result, attempt to achieve thinning, that is, weight reduction of the steel sheet for automobiles.

On the other hand, steel sheets used for automobiles are used in severe corrosive environments such as running in the rain and large temperature changes, so that corrosion resistance is required in addition to strength and workability. Especially in North America, Northern Europe, etc., antifreeze materials (NaCl, K
(Cl, MgCl, etc.) and gravel for non-slip application, the gravel that destroys the coating film and the Cl ⁻ ion that promotes corrosion of the steel sheet are repeatedly dried and wet, resulting in extremely excellent resistance to corrosion. Perforated and corrosive 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 automobile steel sheets. Particularly, the problem of pitting corrosion has become important, and a high-strength cold-rolled steel sheet having excellent deep drawability and further excellent pitting corrosion resistance has been required.

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

This technique relates to an alloyed hot-dip galvanized steel sheet of a hot-rolled original sheet intended to be applied to an automobile undercarriage part, in which P is added in an amount of 0.05% or more to improve corrosion resistance, and In order to suppress the generation of FeS and MnS which are the starting points, S is 0.01% or less, and is substantially 0.
It is regulated to 005% or less. Since a large amount of P is added, it is difficult to obtain a steel sheet of the strength class intended by the present invention. Further, as for S, the technique of the publication is Ti.
Since it is based on low carbon steel that does not contain, it is presumed that the above-mentioned regulations will be required.

On the other hand, as a method for improving the corrosion resistance of high-strength steel sheets for automobiles of the strength class intended by the present invention, for example, JP-A-4-246128 (Japanese Patent Publication No. 7-57893) and JP-A-5-195078 are disclosed. As disclosed, it is well known that the combined addition of P and Cu improves the corrosion resistance.

[0008]

However, in the technique disclosed in the above publication, although the claimed range is 0.03% or more with respect to P addition, 0.06% or more in the embodiment base. It is the fact that they are 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 Laid-Open No. 9-67626, the amount of solid solution Ti is 0.02 to 0.02 based on ultra low carbon steel.
It is described that when the content is 0.3%, the corrosion resistance is improved even if the P content is restricted to 0.03% or less due to the interaction with Cu, but the level of the corrosion resistance by refraining from the addition of P Since the strength class that can be achieved is not described, it is not clear whether it can be applied as an actual steel sheet for automobiles.

As described above, the problem of the prior art is that it lacks consideration for comprehensively satisfying the necessary properties such as required strength, deep drawability, corrosion resistance and the like as a steel sheet for automobiles. Therefore, an object of the present invention is to establish a technique capable of overcoming such problems.

[0011]

Means for Solving the Problems As a result of intensive studies for realizing the above-mentioned object, the present inventor has found that when the following steel composition and manufacturing conditions are followed, extremely excellent resistance to perforation corrosion and deep drawability are obtained. The inventors have completed the present invention by finding that it becomes possible to manufacture a high-strength cold-rolled sheet having the same. That is, the manufacturing method of the high-strength cold-rolled steel sheet according to the present invention is C:
0.01% or less, Si: 0.5% or less, Mn: 0.1
1.5%, S: 0.005 to 0.015%, Al:
0.01-0.10%, 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 relation of the following formula (1), and contain Ni: 0.1 to 0.5%, and the balance is Fe and inevitable impurities at 1125 ° C. After heating as above, hot rolling, winding in the range of 550 to 750 ° C, pickling, cold rolling at a rolling rate of 60% or more, and subsequent re-heating in the temperature range of 750 to 950 ° C. Crystal annealing.
Cu% / 64 + (P% / 31) × (3 × 10 ³ × effective Ti%
/ 48) ≤ 0.01% (1)

First, an experimental study that led to the present invention will be described. 1 sheet bar made of steel with the following ingredients
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 reduction rate of 80% to 0.8 mm, and at 850 ° C, 60
s recrystallization annealing was performed. 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:
0.03-0.08%, Cu: 0.15-0.45%,
Ni: 0.15%

The samples thus obtained were examined for pitting corrosion resistance and deep drawability. To evaluate the resistance to pitting corrosion, after subjecting the steel sheet to phosphate treatment, insert 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 a corrosion acceleration test with 50 cycles of humidity (85% humidity for 4 hours) as one cycle.
Cycling was performed to measure the maximum corrosion depth (maximum perforation depth) of the cross cut portion. Further, the deep drawability was obtained by taking test pieces from 0, 45, and 90 degrees with respect to the rolling direction, and measuring the ratio r0 of the plate width strain and the plate thickness strain at 15% tensile deformation,
r45 and r90 were measured, the average r value was calculated from the following formula, and this value was used for evaluation. Average r value = (r0 + 2r45 + r90) / 4

According to the investigation result, the effective Ti amount is 0.03.
%, 0.05% and 0.08% P
1 to 3 show graphs in which the effects of Cu content and Cu content on perforation corrosion resistance and deep drawability are arranged. 1 to
In Fig. 3, the numbers attached in the figure indicate the maximum perforation depth and the average r value.

As can be seen from FIGS. 1 to 3, the corrosion resistance becomes better as the amount of P and Cu added increases, but the required amount depends on the amount of Ti added, and the required amount decreases as the amount of Ti added increases. . That is, if the amount of P, Cu and Ti added is increased, the corrosion resistance is not simply improved by the total amount thereof, but if the amount of Ti added is increased, good corrosion resistance can be obtained unless the amount of P added is particularly specified. It became clear that there is no.

In FIGS. 1 to 3, hatched regions in the drawings show regions having good perforation corrosion resistance and deep drawability,
The formula below is the lower right limit line of this area.
It is (2). For reference, the limit line (two-dot chain line) when the effective Ti amount is 0.02% and 0.15% is also added to each figure. Cu% / 64 + (P% / 31) × (3 × 10 ³ × effective Ti% / 48) = 0.01% (2)

The reason why these relationships are established has not been fully clarified, but is considered as follows. Ultra low carbon Ti
When the amount of P added is increased during the addition, precipitates such as FeTiP are formed in the steel, and P and solid solution T which are effective in improving corrosion resistance are formed.
It is settled because the i amount rather decreases. Also, Fe
The formation of TiP also deteriorates the deep drawability.

Further, steel types No. 23 to 28 in Table 3 described later
4 is used to manufacture a cold-rolled steel sheet under the same conditions, and FIG. 4 shows a graph in which the effects of the S content on the perforation corrosion resistance and the deep drawability are arranged. From FIG. 4, the above characteristics depend on the S content in the steel, and the best corrosion resistance can be obtained when the S content is 0.005 to 0.015%, preferably 0.006 to 0.013%. Recognize.

Up to now, it has been considered that the lower the S content, the more excellent the corrosion resistance is, and in most of the prior arts, it is regulated to 0.005% or less at the practical level, and in the claims, it is a little higher. Although it may be regulated by the range, the S allowable range has not been sufficiently studied, and in the low carbon steel base containing no Ti disclosed in JP-A-2-22416, the S Is trapped in the technical knowledge that it produces FeS and MnS and deteriorates corrosion resistance.

FeS and MnS are not produced in the ultra low carbon Ti-added steel of the present invention, and TiS and Ti ₄ C ₂ are produced.
Since S ₂ is generated and the size thereof is finer than that of FeS or MnS, the adverse effect on corrosion resistance is small. Therefore, not only the upper limit of the amount of S is significantly eased, but 0.01
If it is 5% or less, it shows better corrosion resistance than that 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 lowered, they are not produced and solid solution S is present, or Ti produces precipitates such as FeTiP to improve 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 be explained here. Hereinafter, the unit of the amount of elements means% by weight.

C: 0.01% or less C is an unavoidable element in mass-produced steel, and it is desirable that C is as small as possible from the viewpoint of workability and corrosion resistance of steel sheet. When the content is 0.01% or less, it does not exert a bad influence so much, 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 the required amount according to the desired strength. However, if the amount exceeds 0.5%, deep drawability and heat resistance are increased. Since the surface properties after rolling and the plating adhesion during galvannealing are deteriorated, the upper limit is set to 0.
5%. It is preferably 0.3% or less.

Mn: 0.1 to 1.5% Mn has the effect of strengthening the steel, so it is added in the required amount according to the desired strength, but 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 it exceeds 1.0, the deep drawability and corrosion resistance deteriorate, so the upper limit was made 1.5%. It is preferably 0.2 to 1.0%.

S: 0.005-0.015% S is an element that plays an important role in the present invention. Conventionally, it has been considered that the lower the S, the higher the corrosion resistance, but in the ultra low carbon Ti-added steel as in the present invention, the F
It does not produce eS or MnS, but produces TiS or Ti ₄ C ₂ S ₂ , its size is finer than that of FeS or MnS, and the potential difference from the base iron is small, so it does not easily become the starting point of corrosion, and corrosion resistance The adverse effect on The reason for this is that if the S content is less than 0.005%, TiS and Ti ₄ C are thermodynamically determined.
_This is because the binding energy of ₂ S ₂ is reduced and these precipitates are not formed, and solid solution S is present, or Ti forms precipitates such as FeTiP.
Therefore, good corrosion resistance and formability 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 TiS and Ti ₄ C ₂ S ₂ will increase in quantity, and they will become the starting point of corrosion due to the potential difference with the base iron, and the corrosion resistance will deteriorate.
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. The lower limit. On the other hand, if it exceeds 0.10%, this effect is saturated, so this is the upper limit. It is preferably 0.02 to 0.07%.

N: 0.006% N combines with Ti to form TiN in a steel containing Ti. Such a precipitate not only serves as a starting point of corrosion but also reduces effective Ti that contributes to the improvement of pitting corrosion resistance, so it is desirable to reduce the amount of N as much as possible, and the upper limit thereof is 0.006. %. Preferably, 0.
It is 004% or less.

P: 0.030 to 0.060% P is an element that plays an important role in the present invention and has the effect of strengthening the steel and the effect of improving the corrosion resistance of the steel sheet by the combined addition of Cu. So add the required amount. If the addition amount is less than 0.030%, the effect is too small, so 0.030% is made the lower limit. On the other hand, P
If the addition amount exceeds 0.060%, FeTiP precipitates are formed in the steel, and P or solid solution Ti that is effective in improving corrosion resistance is formed.
The amount is rather decreased, and the perforation corrosion resistance and deep drawability are deteriorated, so 0.060% was made the upper limit. Preferably, it is 0.035 to 0.055%. Furthermore, T
Beyond the P content determined by the condition of formula (1), which also depends on the i and Cu contents, there is no effect on the improvement of pitting corrosion resistance, and the required strength and excellent deep drawability are obtained. Since it cannot be obtained, the equation (1) is limited to the range satisfying.

Cu: 0.1 to 0.5% Like P, Cu is an additive element that plays an important role in the present invention, and even if added alone, the rust layer formed on the surface of the steel sheet is densified and corrosion resistance. However, the coexistence of P and solid solution Ti can further impart excellent perforation 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 perforation corrosion resistance is saturated and the workability is deteriorated, so this is made the upper limit. It is preferably 0.2 to 0.4%. Further, since the effect of improving the corrosion resistance is not remarkable outside the range satisfying the formula (1) defined by the relationship between P and the solid solution Ti content, the formula (1)
To a range that satisfies.

Effective Ti (sometimes referred to as Ti *): 0.02 to 0.15% Ti is a carbonitride forming element, and solid solution C and N in steel are T
iC and TiN are precipitated and fixed, and added to generate {111} crystal orientation 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, it is also specified from the viewpoint of further improving perforation corrosion resistance. Is an important additional element. That is, if 0.02% or less, even if the P and Cu contents are within the predetermined ranges, good perforation corrosion resistance cannot be obtained, so 0.02% is made the lower limit.
The mechanism of improvement of pitting corrosion resistance by solid solution Ti is not clear at present, but it is due to synergistic effect of elution of Ti during corrosion progress, stabilization of rust layer by preferential oxidation and densification of Cu rust layer. It is presumed that. On the other hand, Ti *
The upper limit of is basically determined by the relationship of the formula (1) defined for the P and Cu contents. This is because if Ti in an amount exceeding the amount specified by the formula (1) is added, precipitates such as FeTiP are formed and the pitting corrosion resistance and formability are deteriorated. On the other hand, if it exceeds 0.15%, the contents of P and Cu satisfying the relation of the formula (1) become small and the resistance to pitting corrosion cannot be obtained, so this is made the upper limit. 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 found in steel having a high Cu content, but also from the viewpoint of improving the resistance to pitting corrosion in the present invention. It is an additive element. That is, Ni is an element that easily concentrates on the surface, and the surface concentration of Ni suppresses the surface enrichment of Cu, and increases the effective solid solution Ti to improve the resistance to perforation corrosion.
If it is less than 0.1%, the effect is too small, so this is made the lower limit. On the other hand, if it exceeds 0.5%, not only these effects are saturated, but also the cost increases, so this is made the upper limit. It is preferably 0.2 to 0.4%.

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

Nb: 0.003 to 0.050% Nb is a carbide forming element, and solid solution C in steel is precipitated and fixed as NbC to preferentially generate {111} crystal orientation advantageous for deep drawability. Add to allow. If the added amount is less than 0.003%, the effect is too small, while if 0.05
%, The grain size of the annealed steel sheet becomes finer and rather deep drawability deteriorates, so the lower limit is made 0.003% and the upper limit is made 0.050%. Preferably 0.005-0.0
40%.

B: 0.0003 to 0.0050% B is added to improve the secondary work embrittlement resistance of steel.
If it is less than 0.0003%, the effect is too small, so this is made the lower limit. On the other hand, if it exceeds 0.0050%, the deep drawability is rather deteriorated, so this is made the upper limit. Preferably, it is 0.0005 to 0.0020%.

Next, the hot rolling conditions of the present invention will be described. In the present invention, the hot rolling condition is also important, and the desirable hot rolling condition is to secure the solid solution Ti necessary for improving the perforation corrosion resistance and to obtain the effect of surface enrichment of Ni. After securing 1125 ° C. or higher as the heating temperature of the continuous casting slab, rough rolling is performed. The finishing temperature for hot rolling is A
r It is preferable to carry out at a temperature of ₃ points or more for deep-deepness,
From the viewpoint of energy saving, low temperature hot rolling with Ar less than ₃ points 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 the solid solution C in the steel as TiC or NbC and fix it, and to generate the {111} crystal orientation advantageous for deep drawability. Therefore, the lower limit is 550 ° C. On the other hand, when the temperature exceeds 750 ° C, Ti is more F than TiC.
Since it forms eTiP and deteriorates pitting corrosion resistance and formability, this is the upper limit. Preferably 600-70
It is 0 ° C.

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

Although recrystallization annealing is carried out after cold rolling, the annealing conditions are important in the present invention, and it is necessary to anneal in the 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 the lower limit. On the other hand, if annealing is performed in a temperature range exceeding 950 ° C., the transformation from ferrite to austenite causes the texture to become random and the deep drawability will deteriorate, so this is the upper limit. The temperature is preferably 800 to 900 ° C.

In the present invention, annealing in a continuous annealing line or a continuous hot dip galvanizing line is suitable as the annealing step. As the above continuous hot dip galvanizing method, both non-alloyed hot dip galvanizing and alloyed hot dip galvanizing are suitable. Further, for the steel sheet of the present invention, after annealing or galvanizing, an appropriate treatment is applied to the chemical treatability,
Weldability, press formability and corrosion resistance may be improved.

[0039]

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

A tensile test and a corrosion resistance evaluation test were performed on the test materials thus obtained. The tensile properties were measured using JIS No. 5 tensile test pieces. Further, for deep drawability and corrosion resistance, the average r value and the maximum perforation 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 sheets (marked with * in the remarks column) that are suitable for the present invention have 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 the tensile test and the corrosion resistance evaluation test performed on the test materials manufactured using the steel type No. 7 in Table 1 under various hot rolling conditions and annealing conditions. It was confirmed that the cold-rolled steel sheet manufactured under the manufacturing conditions compatible with the present invention (marked with * in the remarks column) has excellent perforation corrosion resistance and deep drawability.

[0046]

[Table 5]

[0047]

According to the method for producing a cold rolled steel sheet of the present invention,
In particular, the P and Cu contents are regulated to the optimum amount satisfying the formula (1) according to the effective Ti amount, and the S content is 0.00
5 ~ 0.015%, so the tensile strength is 340N / 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 drawings]

FIG. 1 P and Cu at 0.003 wt% effective Ti
It is a graph which showed the influence which content has on pitting corrosion resistance and deep drawability.

FIG. 2 P and Cu at an effective Ti of 0.005 wt%.
It is a graph which showed the influence which content has on pitting corrosion resistance and deep drawability.

FIG. 3 P and Cu at 0.008 wt% effective Ti
It is a graph which showed the influence which content has on pitting corrosion resistance and deep drawability.

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

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-31547 (JP, A) JP-A-8-104945 (JP, A) JP-A-7-286240 (JP, A) JP-A-6- 287684 (JP, A) JP-A-6-287642 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. By weight%, C: 0.01% or less, S
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.
Steel containing 1 to 0.5% and the balance of Fe and unavoidable impurities is heated to 1125 ° C. or higher and hot-rolled,
Winding in the range of 550 to 750 ° C, pickling, cold rolling at a rolling ratio of 60% or more, and subsequently recrystallization annealing in the temperature range of 750 to 950 ° C. A method for producing a high-strength cold-rolled steel sheet having excellent drawability. Cu% / 64 + (P% / 31) × (3 × 10 ³ × effective Ti% / 48) ≦ 0.01% (1) 2. The component according to claim 1, further comprising Nb:
0.003-0.050%, B: 0.0003-0.
The method for producing a high-strength cold-rolled steel sheet excellent in perforation corrosion resistance and deep drawability according to claim 1, containing 1% or 2% of 0050%.