JPH07207409A - High-strength hot rolled steel sheet having excellent corrosion resistance and its production - Google Patents

Abstract

PURPOSE:To obtain a high-strength hot rolled steel sheet which is excellent in both of corrosion resistance and workability. CONSTITUTION:This high-strength hot rolled steel sheet contains 0.08 C, 0.5 to 2.5% Sr, 0.10 to 3.5% Mn, 0.03 to 0.20% P, 0.010% S, 0.05 to 2.0% Cu, 0.005 to 0.1% Sol. Al, <=0.008% N and (48/12XC+48/14XN+48/32XS) to 0.20% Ti and further, contains one or >=2 kinds of 0.05 to 6.0% Cr, 0.005 to 2.0& Ni, 0.05 to 3.0% Mo and 0.0003 to 0.0020% B. This slab is heated to 1100 to 1300 deg.C, is subjected to hot rolling having an end temp. at the Ar3 transformation point or above and 800 to 950 deg.C and is in succession coiled at 400 to 700 deg.C, by which the hot rolled steel sheet described above is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet which is excellent in corrosion resistance, particularly corrosion resistance against perforation, and which is suitably used as an underbody member for automobiles, a reinforcing member and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Steel plates having excellent corrosion resistance are used as structural materials for automobiles. Particularly for steel sheets used for underbody members, there is a strong demand for improvement in corrosion resistance including piercing corrosion resistance. The suspension members are exposed to, for example, splashed seawater in coastal areas and corrosion caused by salt sprayed for snow melting to prevent freezing of the road surface in cold regions, and therefore, superior corrosion resistance is required. As a typical method for improving the corrosion resistance of a steel material, a hot-dip Zn-plated steel sheet is used instead of a solid steel sheet. Against this background, many plated steel sheets having excellent corrosion resistance have been developed. For example, Japanese Patent Laid-Open No.
JP-A-310354 discloses a method for producing a hot-dip galvanized steel sheet having excellent workability.

Most of the underbody members and reinforcing members of automobiles are manufactured by arc welding of individual parts. In the case of using a plated steel sheet as a material, defects such as blowholes are likely to occur during welding, and it is difficult to obtain a sound weld. Problems caused by Zn plating can be suppressed to some extent by forming a Zn plating layer having a light weight. For example, JP-A-2-22416 discloses an alloyed Z that improves the corrosion resistance of the plating base plate itself and exhibits excellent corrosion resistance even with a light weight.
N-plated steel sheet is introduced. However, even with a light weight, since the Zn plating layer exists on the surface of the steel material, the occurrence of defects during welding cannot be avoided. On the other hand, in order to save energy and protect the global environment, it is required to improve the fuel efficiency of automobiles. In order to reduce the weight of a vehicle body in order to improve fuel efficiency, use of high-strength steel material that can achieve a predetermined strength even if the plate thickness is reduced is being promoted. However, the reduction of the plate thickness with the increase in strength is disadvantageous in terms of piercing corrosion.

[0004]

DISCLOSURE OF THE INVENTION The inventors of the present invention have conducted various investigations and studies to solve such problems, and have been able to endure hot rolling as a suspension member, a reinforcing member, etc. for automobiles. A method for manufacturing a steel sheet was proposed in Japanese Patent Application Nos. 5-266256 and 5-266257. A detailed investigation and study of the obtained hot rolled steel sheet revealed that although it had good corrosion resistance to some extent even in the bare form of use,
It was found that the corrosion resistance was slightly inferior and the workability was not sufficient in a severe corrosive environment. The present invention further improves the corrosion resistance of the hot-rolled steel sheet proposed in the prior application, exhibits very excellent corrosion resistance even in a more severe corrosive environment, and provides a high-strength hot-rolled steel sheet with improved workability. With the goal.

[0005]

In order to achieve the object, the high-strength hot-rolled steel sheet of the present invention has C: 0.008% by weight or less, Si: 0.5 to 2.5% by weight, and Mn: 0. .10
3.5% by weight, P: 0.03 to 0.20% by weight, S:
0.010% by weight or less, Cu: 0.05 to 2.0% by weight, acid-soluble Al: 0.005 to 0.1% by weight, N: 0.
008% by weight or less and Ti% / 48 ≧ C% / 12 + N%
Under the condition of / 14 + S% / 32, Ti: 0.20 wt% or less is contained, and the balance has a composition of Fe and unavoidable impurities. This high-strength hot-rolled steel sheet further had Cr: 0.05-6.
0% by weight, Ni: 0.05 to 2.0% by weight, Mo: 0.
05 to 3.0 wt% and Nb: 0.005 to 0.05 wt% may be included alone or in combination. If necessary, B: 0.0003 to 0.0020 wt% may be contained. The high-strength hot-rolled steel sheet of the present invention heats a steel slab having the above composition to a temperature range of 1100 to 1300 ° C.,
Hot rolling is carried out at a temperature above the r ₃ transformation point and in the temperature range of 800 to 950 ° C., and then 400 to 700 ° C.
It is manufactured by winding in the temperature range of.

[0006]

[Function] Japanese Patent Application No. 5-266256 and Japanese Patent Application No. 5-26
As introduced in No. 6257, the combined addition of P, Cu and Si improves the corrosion resistance of the steel sheet. However, when the present inventors conducted a detailed investigation and research on corrosion resistance,
It was found that the corrosion resistance deteriorates when a large number of coarse carbides exist in the steel even when P, Cu and Si are added in combination. It is considered that this deterioration in corrosion resistance is caused by the fact that the carbide forms a local battery with the steel matrix, and becomes an environment in which corrosion easily progresses. Therefore, it is expected that the corrosion resistance will be further improved when C in the steel is reduced to form a ferrite single phase, and the improvement of the corrosion resistance was also confirmed by experiments. The present invention has been completed as a result of such findings and experiments. Further, the reduction of the C content is also effective for improving the workability. Generally, the strength of steel decreases as the C content decreases, but since the hot-rolled steel sheet of the present invention contains solid solution strengthening elements such as Si and Mn, the C content is low. Nevertheless, it has a sufficiently high strength.

The contents of alloying elements and hot rolling conditions specified in the present invention will be described below. C: 0.008% by weight or less Although it is an alloying element effective for increasing strength, it causes an increase in the amount of carbides in steel and deteriorates corrosion resistance. Therefore, in the present invention, by restricting the C content to 0.008% by weight or less, the generation of carbides is suppressed and the corrosion resistance is improved. Further, the workability is also improved by reducing the C content. Si: 0.5 to 2.5 wt% It is an effective alloying element for improving the strength of the steel material without impairing the workability. Si, in the coexistence of Cu and P, also exhibits a function of significantly improving corrosion resistance, particularly puncture corrosion resistance. In order to obtain such an action, it is necessary to contain 0.5% by weight or more of Si. However, 2.
If a large amount of Si is contained in excess of 5% by weight, the steel material hardens and the ductility deteriorates.

Mn: 0.1 to 3.5 wt% It is an alloy element effective in preventing hot embrittlement due to S, and also effectively acts to improve strength. Such an effect is remarkable when the Mn content is 0.1% by weight or more. However, when a large amount of Mn is contained in excess of 3.5% by weight, the strength of the steel material becomes too high and the ductility deteriorates. P: 0.03 to 0.20 wt% By the combined addition with Cu, the corrosion resistance of the steel material is remarkably improved. For this purpose, it is necessary to contain 0.03% by weight or more of P. However, the P content exceeding 0.20% by weight is
Not only is the effect of improving corrosion resistance saturated, but ductility deteriorates. S: 0.010 wt% or less It is a harmful element that forms a sulfide-based non-metallic inclusion and reduces workability and corrosion resistance. In this respect, the lower the S content, the more preferable. In the present invention, the upper limit of the S content is set to 0.010% by weight.

Cu: 0.05-2.0 wt% By adding P in combination, the corrosion resistance of the steel material is significantly improved. The effect of Cu addition becomes remarkable when the content is 0.05% by weight or more. However, as with P, a Cu content exceeding 2.0% by weight not only saturates the effect of improving corrosion resistance, but also causes deterioration of ductility. Acid-soluble Al: 0.005 to 0.1% by weight It is an alloying element added as a deoxidizer, and needs to be 0.005% by weight or more. However, when the Al content exceeds 0.1% by weight, inclusions such as Al ₂ O ₃ increase, which deteriorates workability and surface quality. N: 0.008% by weight or less It is a harmful element that produces inclusions such as nitrides and carbonitrides and deteriorates the properties of steel materials. In this respect, the lower the N content, the more preferable, and in the present invention, the upper limit is 0.008.
It was set to% by weight. Ti: 0.20 wt% or less Fixing the harmful elements C, N and S to improve the workability of the steel sheet. To completely fix C, N, S, etc., [48
/ 12 × C + 48/14 × N + 48/32 × S] or more Ti is required to be contained. But 0.20
When a large amount of Ti in excess of weight% is contained, not only the effect is saturated, but also the manufacturing cost is increased.

The steel material used in the present invention may further contain one kind or two or more kinds of Mo, Cr, Ni and B, if necessary, in order to increase strength and improve corrosion resistance. Cr: 0.05 to 6.0% by weight It is an alloying element effective in improving the corrosion resistance, and is 0.05% by weight.
With the above contents, the effect of Cr addition becomes remarkable. But,
If the Cr content exceeds 6.0% by weight, the production cost becomes very high. Ni: 0.05 to 2.0 wt% Prevents hot embrittlement due to Cu and suppresses hot cracking during hot rolling. Further, the addition of Ni also improves the corrosion resistance. Such an effect is obtained by adding 0.05% by weight or more of N.
Remarkably appears with i content. However, when the Ni content exceeds 2.0% by weight, not only is the effect saturated, but
Steel material cost rises because a large amount of expensive Ni is used.

Mo: 0.05 to 3.0 wt% P is an alloying element effective in improving the strength of the steel sheet, and P
In the coexistence of Cu and Cu, it also effectively acts to improve the corrosion resistance. To obtain this effect, 0.05% by weight
It is necessary to contain the above Mo. But,
If the Mo content exceeds 3.0% by weight, not only the effect is saturated, but also the manufacturing cost becomes high. Nb: 0.005 to 0.05% by weight It is an alloying element that refines the metal structure and improves the workability of the steel sheet. The effect of adding Nb becomes remarkable when the content is 0.005% by weight or more. However, if a large amount of Nb exceeding 0.05% by weight is contained. Not only the effect of Nb is saturated, but also the manufacturing cost is increased. B: 0.0003 to 0.0020% by weight While improving the hardenability of the steel material, it exerts the action of strengthening the grain boundaries and improves the secondary work crack resistance. The effect of adding B becomes remarkable at a content of 0.0003% by weight or more,
It saturates at 0.0020% by weight.

In the present invention, a steel material containing such an alloy component is made into a thin steel sheet through a hot rolling process. The conditions of hot rolling are as follows. Heating temperature before hot rolling: 1100 to 1300 ° C. Steel pieces are heated to 1100 to 130 in a soaking furnace before hot rolling.
Heat to 0 ° C. If the heating temperature at this time is lower than 1100 ° C., it becomes difficult to maintain the end temperature of hot rolling at the Ar ₃ transformation point or higher. On the other hand, if the heating temperature exceeds 1300 ° C., not only the workability is deteriorated due to the coarsening of the crystal grains, but also a large amount of heat energy is required, which causes an increase in manufacturing cost.

Finishing temperature: above the Ar ₃ transformation point and 800
~ 950 ° C In order to obtain good workability, the end temperature of hot rolling is set to Ar.
_The temperature is set at ₃ transformation points or higher and in the range of 800 to 950 ° C. At the end temperature below the Ar ₃ transformation point or below 800 ° C., a metal structure in which the ferrite phase is coarse-grained is likely to be formed,
Workability, especially ductility, deteriorates. Similarly, at a finish temperature of higher than 950 ° C., a metal structure in which the ferrite phase is coarse-grained is likely to be formed, and workability, particularly ductility is deteriorated. Winding temperature: 400-700 ° C. In order to obtain good workability, the hot-rolled strip material is 400
It is necessary to wind at ~ 700 ° C. When the winding temperature is lower than 400 ° C, workability, particularly ductility is deteriorated due to hardening of the steel material, and the plate shape is deteriorated. Conversely, 70
At a winding temperature higher than 0 ° C, the scale layer on the surface of the steel sheet becomes thick and the pickling property deteriorates.

[0014]

【Example】

Example 1: A steel having the composition shown in Table 1 was melted, made into a slab by continuous casting, and then heated to 1230 ° C. The slab was hot-rolled under the conditions of a hot rolling finish temperature of 900 ° C and a winding temperature of 550 ° C to obtain a hot rolled steel sheet having a plate thickness of 2.6 mm.

[0015]

[Table 1]

After pickling the hot rolled steel sheet, test pieces used for the corrosion test and the tensile test were cut out. JIS Z2241 No. 5 test piece was used for the investigation of tensile properties.
The test piece used in the corrosion test had a size of 70 mm × 150 mm, and the end surface and the back surface were sealed with a polyester tape. As the corrosion test, a compound corrosion test was conducted in which the salt spray test of JIS Z2371 was 2 hours, the hot air was dried at 70 ° C. for 4 hours, and the wet test was for 2 hours. Then, the maximum erosion depth due to corrosion was measured. Also, test numbers A1, A2
The secondary work crack resistance of the steel sheets A5 and A6 was investigated. Hot-rolled steel sheet with a diameter of 96 mm
After punching into 96 mm diameter → 50 mm diameter → 4 diameter
A pipe-shaped test piece with a diameter of 40 mm and a height of 25 mm was used, in which the two-stage cup was squeezed to 0 mm and the ears and the bottom were removed. After cooling the test piece in a refrigerant to a predetermined temperature, the test piece was crushed by a punch, and the secondary work crack resistance was judged by the limit temperature at which cracking occurred.

[0017]

[Table 2]

As is clear from Table 2 showing the survey results,
It is understood that the steel of the present invention has a shallow maximum erosion depth and is excellent in corrosion resistance as compared with the comparative steel, and also has excellent workability because the strength-ductility balance is balanced. That is,
It was confirmed that the corrosion resistance and workability were improved by reducing the C content. Comparative steel B1 without Ti addition
Has a C content at the same level as the steel of the present invention, but is inferior in workability to the steel of the present invention. Also, B-added steel A
2, A5 and A6 have lower secondary work cracking limit temperatures than the steel A1 containing no B, and it is understood that the addition of B improves the secondary work crack resistance.

Example 2 Steel A2 in Table 1 was hot-rolled under the hot rolling conditions shown in Table 3 to a plate thickness of 2.6 mm and pickled.

[0020]

[Table 3]

From the obtained hot-rolled steel sheet, JIS Z2241
No. 5 test piece was cut out and the tensile properties were investigated. As is clear from Table 4 showing the investigation results, hot-rolled steel sheets excellent in workability and pickling property are obtained by hot rolling under the conditions specified in the present invention. On the other hand, the hot-rolled steel sheet that has been subjected to hot rolling that does not satisfy the conditions specified in the present invention has low workability particularly represented by strength-ductility balance,
In addition, some of them had poor pickling properties.

[0022]

[Table 4]

[0023]

As described above, the hot rolled steel sheet of the present invention is superior in both corrosion resistance and strength as compared with the conventional high strength hot rolled steel sheet. This hot-rolled steel sheet exhibits unprecedented durability when used as an underbody member for automobiles, a reinforcing member, etc. without requiring surface treatment such as plating.
Further, due to its excellent workability, it can be processed into a defect-free product shape, and since it has high strength, it is also used as a useful material for reducing the weight of automobiles.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsunari Hara 11-11 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd.

Claims (4)

[Claims] 1. C: 0.008% by weight or less, Si: 0.
5 to 2.5% by weight, Mn: 0.10 to 3.5% by weight,
P: 0.03 to 0.20 wt%, S: 0.010 wt%
Hereinafter, Cu: 0.05 to 2.0 wt%, acid-soluble Al:
0.005-0.1% by weight, N: 0.008% by weight or less and Ti% / 48 ≧ C% / 12 + N% / 14 + S% / 3
A high-strength hot-rolled steel sheet containing Ti: 0.20% by weight or less under the condition of 2, and the balance being Fe and unavoidable impurities and having excellent corrosion resistance. 2. Cr: 0.05-6.0% by weight, Ni:
0.05 to 2.0 wt%, Mo: 0.05 to 3.0 wt% and Nb: 0.005 to 0.05 wt%, 1 or 2
The high-strength hot-rolled steel sheet according to claim 1, containing at least one kind. 3. B: 0.0003 to 0.0020% by weight
The high-strength hot-rolled steel sheet according to claim 1 or 2, containing. 4. A steel slab having the composition according to claim 1 is heated to a temperature range of 1100 to 1300 ° C.
A high-strength hot-rolled steel sheet having excellent corrosion resistance, which is characterized by being hot-rolled at a transformation temperature of ₃ transformation points or higher and having an end temperature in a temperature range of 800 to 950 ° C., and then wound in a temperature range of 400 to 700 ° C. Production method.