JPH04232231A - High strength chromium-containing steel sheet excellent in corrosion resistance and workability - Google Patents

Abstract

PURPOSE:To increase the tensile strength of a Cr-contg. steel sheet to >=40kgf/ mm<2> while its corrosion resistance and workability are improved. CONSTITUTION:This is a high strength chromium-contg. steel sheet contg., by weight, >=0.030% C, <=3.0% Si, <=3.0% Mn, <=0.150% P, <=0.010% S, <=2.0% Ni, 5.0 to <11.0% Cr, <=0.030% N, 0.01 to 0.10% V and <=2.0% Cu, furthermore contg., according to demands, one or >= two kinds among 0.01 to 0.30% Ti, 0.01 to 0.30% Nb, 0.01 to 0.30% Zr, 0.01 to 0.20% Al and 0.0002 to 0.0200% B, in which the content of Si, Mn, P, Ni and Cu is regulated so as to satisfy the relationship of Si+Mn+10P+Ni+Cu>1.0% and the balance iron with inevitable impurities. The steel is excellent in corrosion resistance and workability.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength chromium-containing steel sheet that has excellent workability and good corrosion resistance and is used as steel sheets for automobiles and various forming materials. The steel of the present invention is provided on the market in the form of hot-rolled steel strip or hot-rolled steel plate, or cold-rolled steel strip or cold-rolled steel plate, and in the present invention, these are collectively referred to as steel plate.

0002

[Prior art] In recent years, automobile bodies have become highly rust-proof,
In order to improve the corrosion resistance and reliability of building materials, the amount of use of various surface-treated steel sheets, mainly zinc-based, is increasing, replacing conventional plain steel sheets.

[0003] Particularly in the automobile industry, in response to the growing global concern about environmental issues, reducing exhaust gas and improving fuel efficiency have become urgent issues, and the use of high-strength steel plates has led to lighter vehicle bodies. is being promoted as one of the effective measures. High-strength steel sheets for automobiles contain Si and Mn.
There are steels that are solid-solution strengthened by adding elements such as steel, transformation-strengthened steels that utilize secondary phases such as martensite and bainite, precipitation-strengthened steels that utilize precipitation, and even steels that combine these strengthening mechanisms. It is also used as the base plate for surface-treated steel sheets. In addition, in order to reduce the weight of an automobile, it is necessary to have a tensile strength of at least 40k.
A steel plate with a gf/mm2 or higher is required.

[0004] Surface-treated steel sheets include hot-dipped steel sheets and electroplated steel sheets, but the corrosion resistance level required of these surface-treated steel sheets is becoming increasingly strict.
Furthermore, as a means to improve corrosion resistance, increasing the basis weight,
Alloy plating, multilayer plating, etc. are being used. However, when surface-treated steel sheets are subjected to press processing such as deep drawing, peeling of the plating layer called powdering or flaking may occur, causing problems during press processing, and spot weldability and arc weldability may not always be sufficient. This becomes a particular problem when the plating thickness is increased to improve corrosion resistance.

[0005] Therefore, steel sheets have been proposed in which the corrosion resistance of the material is improved without using surface treatment. , Japanese Patent Publication No. 1-53344 discloses a chromium-containing corrosion-resistant steel sheet with improved formability by adding Ti-Al, and Japanese Patent Application Publication No. 2-50940 discloses a similar method. Nb-Al
A chromium-containing corrosion-resistant steel sheet is disclosed in which the deep drawability is improved by adding chromium.

[0006]

[Problems to be solved by the invention] JP-A-2-1560
The steel disclosed in Publication No. 48 is made of Cu, which has solid solution strengthening ability.
Although it contains Ni, Cu and Ni are added for the purpose of improving corrosion resistance, and the tensile strength of the steels shown in the examples are all 38 kgf/mm2 or less, which is not sufficient in terms of strength. Also, in Japanese Patent Publication No. 1-53344,
Steels to which some Cu and Ni are added have been disclosed, but they still do not have sufficient strength.

On the other hand, as disclosed in Japanese Patent Publication No. 1-53344 and Japanese Unexamined Patent Publication No. 2-50940, Ti,
Although adding carbon/nitride forming elements such as Nb and Al is effective in improving formability, it is not necessarily sufficient for improving strength and corrosion resistance. Therefore, there has been no steel sheet that has corrosion resistance without surface treatment, and at the same time has high strength and good workability.

[0008] Stainless steel is a steel with excellent corrosion resistance, but it is not only disadvantageous in terms of cost because it contains a large amount of Cr, but also because of its excellent corrosion resistance, unlike general corrosion in ordinary cold-rolled steel sheets, Localized pitting-like corrosion may occur, which may pose a problem when evaluated based on erosion depth.

[0009]

[Means for Solving the Problems] The present inventors have conducted detailed studies on the influence of alloying elements on corrosion resistance and workability. As a result, a steel plate with excellent corrosion resistance and workability was obtained by reducing the amounts of C and N, extremely reducing S, and adding a small amount of V in addition to 5 to 11% Cr. Ti, Nb, Zr, A
The basic knowledge was obtained that processability is further improved by adding L and B. In addition, by adding appropriate amounts of Si, Mn, P, Ni, and Cu to such chromium-containing steel sheets, it is possible to increase the strength without deteriorating corrosion resistance and while minimizing deterioration in workability. I found out that. The present invention was completed based on this knowledge, and its gist is that in weight percent, C: 0.030% or less, Si: 3.0% or less, Mn: 3.0% or less, P: 0.150% or less, S: 0.010% or less, Ni: 2.0% or less, Cr: 5.0% to less than 11.0%, N: 0.030% or less, V: 0.01 to 0 The amounts of Si, Mn, P, Ni and Cu are adjusted so as to contain .10%, Cu: 2.0% or less, and satisfy the relationship of the following formula: Si+Mn+10P+Ni+Cu>1.0%, with the remainder being iron and A high-strength chromium-containing steel plate with a tensile strength of 40 kgf/mm2 or more and excellent corrosion resistance and workability, which is made of unavoidable impurities, and this steel further contains Ti: 0.01 to 0.30%, Nb:
0.01-0.30%, Zr: 0.01-0.30%
, Al: 0.01-0.20% or B: 0.000
A high-strength chromium-containing steel sheet containing 2 to 0.0200% of one or more chromium and having a tensile strength of 40 kgf/mm2 or more and excellent corrosion resistance and workability.

[0010] The workability target in the present invention is the average value of the Rankford value, which is an index of deep drawability for cold-rolled steel sheets (in the rolling direction, the 450 direction and the 900 direction with respect to the rolling direction). Average value) is 1.3 or more, elongation depends on the strength level, but for example, the tensile strength is 50 kgf/
At the mm2 level, the hole expansion ratio, which is an index of burring workability, should be 30% or more, and for hot rolled steel sheets, the hole expansion ratio should be 1.0 or more.

[0011]

[Function] The function of each component contained in the steel of the present invention and the reason for limiting its content will be explained below. Although C increases the strength of steel, it also degrades ductility and weldability, so it is an undesirable element. In addition, from the viewpoint of corrosion resistance and deep drawability of cold-rolled steel sheets, a lower value is preferable, and 0.0
3% or less. More preferably, it is 0.010% or less.

[0012]Si is an element that is effective in deoxidizing and increasing the strength of steel by solid solution strengthening of the ferrite phase. However, if the content is too high, the toughness will decrease and the susceptibility to cracking during cold rolling and processing will increase, so the content should be 3.0% or less.

Mn is an element that improves hot workability and toughness of welded parts. In addition, although its solid solution strengthening ability is small, since it is an austenite-forming element, it is effective in lowering the A1 and A3 transformation points and generating martensite and bainite phases in addition to the ferrite phase to increase strength through transformation strengthening. It is an element. However, if the content exceeds 3.0%, it becomes difficult to obtain even a partial ferrite phase, which impedes workability, so the upper limit is set at 3.0%.

[0014] P is a very effective element for increasing the strength of steel through solid solution strengthening. However, if the content is too high, the toughness will decrease, so the content should be 0.150% or less.

[0015] The regulation of S is an important point in the present application, and since it adversely affects the corrosion resistance, particularly the rusting resistance, of low chromium steel, the lower it is, the more preferable it is, and it is set to 0.010% or less. More preferably, it is 0.005% or less.

[0016] Like Mn, Ni and Cu are effective in improving the toughness of the welded joint and in increasing the strength by transformation strengthening. It also has solid solution strengthening ability. However, since adding large amounts increases costs, the upper limit is set at 2.0% in either case.

Cr is an essential element for improving corrosion resistance, and its effect becomes even more pronounced by reducing S and adding a small amount of V, which will be described later. The lower limit of 5.0% Cr is the minimum amount to ensure the necessary corrosion resistance. On the other hand, if the Cr content is too high, the cost will increase and the workability will decrease, and in terms of corrosion resistance, pitting-like corrosion will occur and the erosion depth will increase, so the upper limit is set at 11.0%.

Similar to C, the content of N is preferably low from the viewpoint of workability, and is set to 0.030% or less. More preferably 0
．． 0.010% or less.

The addition of V is an important point of the present invention, and in the steel of the present invention, by adding a small amount of V in addition to Cr,
Furthermore, corrosion resistance is further improved. Although the reason for this is not clear at present, it is presumed that V has the effect of promoting the formation of a passive film of Cr. 0 to get that effect
．． 0.01% or more is required. On the other hand, when the V content is 0.1
If it exceeds 0%, the corrosion resistance improvement effect will be saturated, so 0.10
The upper limit is %.

Ti, Nb, Zr, Al, and B are elements that are particularly effective in improving the deep drawability of cold-rolled steel sheets. Further, B suppresses work embrittlement caused by deep drawing and effectively acts to improve secondary workability. To obtain these effects, Ti, Nb, Z
r, Al needs to be at least 0.01% or more, and B needs to be 0.0002% or more. On the other hand, even if these elements are added in excess, the above effects will be saturated and the surface quality will be impaired, resulting in a cost disadvantage.
The upper limits are regulated to 0.30% or less for Zr, 0.20% or less for Al, and 0.0200% or less for B.

In addition to the above-mentioned limitations on each component, in the present invention, the upper limit of the total amount of Si, Mn, P, Ni, and Cu is set as follows:
Regulate Si+Mn+10P+Ni+Cu>1.0%. This formula is an index based on the solid solution strengthening ability of each component to the ferrite phase, and although this does not necessarily apply to transformation strengthening, it is the minimum amount to stably obtain a tensile strength of 40 kgf/mm2 or more. be.

[0022] The manufacturing method of the steel of the present invention is not particularly specified, but it is made into continuous cast slabs through conventional steel manufacturing, and in the production of cold-rolled steel sheets, it is possible to
The slab is heated to an appropriate temperature of 500°C, the finishing pass of hot rolling is completed in the austenite single phase region, and the slab is appropriately controlled and cooled until it is coiled, and the slab is coiled at a temperature of 500°C or higher, and then hot rolled. In order to obtain a cold-rolled steel sheet with excellent deep drawability, it is preferable to pickle the steel strip after obtaining a fine-grained ferrite structure, cold-roll it at a rolling ratio as high as possible at 70% or more, and then anneal it. .

[0023] In addition, in the production of hot rolled steel sheets, 110
The slab is heated to an appropriate temperature between 0 and 1300℃, the finishing pass of hot rolling is completed in the austenite single phase region, and the slab is appropriately controlled and cooled before being rolled, and the slab is rolled at a temperature of 400℃ or higher. . In this case, by creating a fine-grained ferrite structure similar to the cold-rolled material described above, a hot-rolled steel sheet that is soft and has excellent burring workability can be obtained. Adopt a rolling pass schedule, controlled cooling after finish rolling, and coiling temperature sufficient to allow rolling to progress and complete. On the other hand, in order to increase strength while maintaining good burring workability, it is effective to create a fine composite structure consisting of a clean, recrystallized ferrite phase and other hard phases that have not undergone ferrite transformation. To achieve this, the cooling rate after finish rolling is increased and low-temperature winding is performed. Note that hot-rolled steel sheets are annealed and/or pickled as appropriate after coiling.

When annealing a cold-rolled steel sheet or a hot-rolled steel sheet, there are two methods: softening by annealing in the ferrite region, and heating to a region where high-temperature austenite exists in order to increase the strength. There is a method of obtaining a composite structure of ferrite and a transformed phase after cooling.

When the steel of the present invention has a composite structure, both cold-rolled steel sheets and hot-rolled steel sheets have a better balance of strength and ductility than the ferrite structure (solid solution strengthening) due to the high strength achieved by transformation strengthening.

The steel of the present invention has Zn, Ni, and C on its surface.
It can also be used as a base steel plate coated with u, Al, Pb, Sn, Fe, B, and alloys thereof in a single layer or multiple layers.

[0027]

EXAMPLES The present invention will be further explained below with reference to Examples. Steel having the composition shown in Table 1 was melted into steel slabs, divided into two parts, and one half was hot-rolled to form a hot-rolled plate with a thickness of 4 mm. After descaling this hot-rolled plate, it was cold-rolled (rolling rate: 80%) to a thickness of 0.8 mm, and then rolled at 780°C for 1 min.
The tensile properties, average r value, and corrosion resistance were investigated by annealing. The remaining steel slab was hot-rolled to 2.3 mm and annealed at 780°C for 10 minutes, and then subjected to a hole expansion test to investigate burring workability.

[0028] The tensile properties were determined using a JIS No. 5 test piece.
Measurements were made in three directions: the rolling direction, the rolling direction and the 450 direction, and the rolling direction and the 900 direction, and the average was calculated. The average r value was measured in the same three directions using a JIS No. 13B test piece, and the average value was determined (indicated by adding a - above r in Table 2). Corrosion resistance was determined by a 100-hour salt spray test (JIS Z 2371) to determine the rusting rate (area rate), and the maximum corrosion depth was taken as the average of 5 points with large erosion depths. In the hole expansion test, a punched hole was expanded with a conical punch using a beaded die and a wrinkle presser with no material flowing in, and the hole expansion ratio λ [= (d - d0) / d0, where d
0=10mm] was determined. These results are shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

[0031] As can be seen from Table 2, the steel of the present invention exhibits a tensile strength of 40 kgf/mm2 or more, good elongation, and an average r value of 1.3 or more in all cold rolled annealed sheets. High hole expansion ratio of 1.0 or more, ductility,
Excellent formability such as deep drawability and burring workability. In addition, the corrosion rate in salt spray tests is low, and the maximum corrosion depth is small, resulting in excellent corrosion resistance.

On the other hand, comparative steel No. No. 11 has a Cr content of 3.40%, which is below the range of the present invention, and although the workability is good, the rusting rate and erosion depth are both large, and the corrosion resistance is poor.

Comparative steel No. In Nos. 12 and 13, the Cr content is 12.25% and 12.43%, respectively, which is beyond the scope of the present invention, and the average r value of the cold rolled steel sheet is low, the deep drawability is poor, and the hole expansion of the hot rolled steel sheet is difficult. The ratio is also low.

Comparative steel No. 14 and 15 are respectively S
The amount of i+Mn+10P+Ni+Cu is 0.47, 0.
50, which is lower than the range of the present invention, and although it has excellent workability, its strength is low.

Comparative steel No. Steel Nos. 15 and 16 do not contain V, and steels No. 15 and 16 of the present invention contain V and have almost the same amount of other components. As can be seen by comparing 1 and 2, the rusting rate is high and the corrosion resistance is inferior.

Comparative steel No. No. 17 has poor corrosion resistance because S content is higher than the range of the present invention.

[0037] In the above examples, the properties were basically shown in the state of having a ferrite structure, but as mentioned earlier, the steel of the present invention can also be improved by increasing the strength by transformation strengthening. It has excellent processability and corrosion resistance.

[0038]

[Effects of the Invention] As described above, according to the present invention, a high-strength chromium-containing steel sheet with excellent corrosion resistance and workability is obtained, and progress is being made in making steel sheets for automobiles etc. highly rust-proof and highly corrosion-resistant. Materials suitable for various molding materials can be provided.

Claims (3)

[Claims] [Claim 1] In weight%, C: 0.030% or less, Si: 3.0% or less, Mn: 3.0% or less, P: 0.150% or less, S: 0.010% or less, Ni : 2.0% or less, Cr: 5.0% to less than 11.0%, N: 0.030% or less, V: 0.01 to 0.10%, Cu: 2.0% or less, In addition, the amounts of Si, Mn, P, Ni and Cu are adjusted so as to satisfy the relationship of the following formula, Si + Mn + 10P + Ni + Cu > 1.0%, the balance is iron and unavoidable impurities, and has a tensile strength of 40 kgf/mm2 or more High-strength chromium-containing steel plate. [Claim 2] In weight%, C: 0.030% or less, Si: 3.0% or less, Mn: 3.0% or less, P: 0.150% or less, S: 0.010% or less, Ni : 2.0% or less, Cr: 5.0% to less than 11.0%, N: 0.030% or less, V: 0.01 to 0.10%, Cu: 2.0% or less. , furthermore, Ti: 0.01-0.30%, Nb: 0.01-0.
30%, Zr: 0.01-0.30%, Al: 0.
01-0.20% or B: 0.0002-0.020
0% of one kind or two or more kinds, and the following formula, Si+
A high-strength chromium-containing product with a tensile strength of 40 kgf/mm2 or more, with the amounts of Si, Mn, P, Ni, and Cu adjusted to satisfy the relationship of Mn+10P+Ni+Cu>1.0%, with the remainder consisting of iron and unavoidable impurities. steel plate. 3. The high-strength chromium-containing steel sheet according to claim 1, wherein S: 0.005% or less.