JPH06279947A - Austenitic stainless steel excellent in press formability - Google Patents

Abstract

PURPOSE:To obtain an austenitic stainless steel excellent in press formability by providing a composition containing specific amounts of C, Si, Mn, S, Cr, Ni, Mo, Al, and N and further containing P and Cu in specific relations. CONSTITUTION:This steel is an austenitic stainless steel which Has a composition containing, by weight, 0.02-0.10% C, 0.2-1.0% Si, 0.4-1.5% Mn, <=0.008% S, 15.0-19.0% Cr, 6.0-8.0% Ni, 0.01-3.0% Mo, <=0.05% Al, and 0.01-0.06% N, further combinedly containing P and Cu by the amount within the range enclosed with lines represented by equations (1) to (5), and having the balance Fe with inevitable impurities. This steel has superior press formability, small plastic anisotropy, and excellent season cracking resistance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel having excellent press formability, which is characterized by excellent press formability, small plastic anisotropy, and excellent crack resistance. It is intended to provide a steel plate.

[0002]

2. Description of the Related Art As an austenitic stainless steel used by press forming, SUS304 has been widely used, and it has been adopted for kitchen appliances and tableware. However, this SUS304 has a large plastic anisotropy, and an earing (ear) occurs at the edge after deep drawing. Since such earrings must be cut and removed after molding, the number of steps is complicated, and the yield of materials is reduced.

Further, this SUS304 has a problem of cracking due to placement. That is, if left in place after deep drawing, this in-place crack will naturally occur due to residual stress or process-induced martensite generated during forming or hydrogen in steel. In order to prevent the above, strain relief annealing may be performed after the forming process, but naturally it causes an increase in manufacturing cost and cannot be desirable.

In order to solve the above-mentioned problems of SUS304, some kind of component system has been proposed. For example, in JP-A-1-301840, C: 0.05% or less,
N: 0.03% or less, Si: 1.00% or less, Mn: 1.00
~ 2.00%, Cu: 0.50 ~ 3.00%, Ni: 8.00 ~
It contains 10.50%, Cr: 18.00 to 2.00%, consists of the balance unavoidable impurities and Fe, and has the following formula: Mr (RT) = 386-311 (C + N) -6.2Si-5.4Mn-9.2 Cr-
Stainless steel satisfying 20 (Ni + Cu) ≦ 5 has a limiting drawing ratio of 3.1 or more,
It is said that it has high press formability and is excellent in resistance to time-related cracks (anti-deposition cracking).

In Japanese Patent Laid-Open No. 1-92342, C: 0.
001 to 0.10%, Si: 0.10 to 1.4%, Mn: 0.
1-3.0%, Cr: 15.0-30.0%, Ni: 8.0-2
5.0%, P: 0.050% or less, S: 0.020% or less,
N: 0.010 to 0.060%, Sol.Al: 0.070 to 0.20
Stainless steel containing 0% and O: 0.0025% or less (Ti, Ca, Mo, Cu, B in addition to this in some cases) has low oxygen content and few coarse inclusions.
Even if subjected to severe molding, no defects such as cracks or surface defects are generated, and it is said that it is extremely excellent in deep drawability.

Further, in Japanese Patent Publication No. 2-18382, C: 0.04% or less, Si: 0.8% or less, Mn: 2.2 as an austenitic stainless steel excellent in rust resistance and cold workability.
5% or less, P: 0.030% or less, S: 0.005% or less,
Ni: 10.0 to 18.0%, Cr: 17.0 to 19.0%, M
o: 0.05 to 2.0%, Cu: 0.05 to 3.0%, N: 0.0
4% or less, O: 0.006% or less, C + N: 0.045% or less, and Ca: 0.0005 to 0.008% are proposed. That is, with this component, rust resistance including the welded portion can be secured, and since work hardening is small, intermediate annealing in the cold working step can be omitted.

According to Japanese Patent Publication No. 59-33663, C: 0.10% to 0.17%, Si: 0.3% to less than 1.0%, Mn: 0.1% as an austenitic stainless steel excellent in formability and strength. 5 to 3.0%, Ni: 5.5 to 8.5%, Cr:
13.0 to less than 15.0%, Cu: 1.0 to 3.5%, Mo: 0.
2 to less than 1.0%, N: 0.025% or less, and Nb, T
A stainless steel containing a and Ti and having an austenite stability Md ₃₀ calculated by a predetermined formula of −20 to + 20 ° C. has been proposed. It is said that this steel has excellent composite formability for overhanging and deep drawing by controlling the austenite stability appropriately, has relatively high strength, and has good time crack resistance (deposition crack resistance). .

[0008]

Even if the above-mentioned conventional technique has some merits, it is not always preferable as the austenitic stainless steel sheet used by press working. That is, the above-mentioned ones cannot be preferable in terms of plastic anisotropy that causes earing, and cutting after molding and reduction in material yield cannot be avoided. In addition, Al added to reduce oxygen is present in the thin steel sheet as alumina and easily causes flaws during cold rolling, and it is difficult to obtain good surface properties.
Also, the plastic anisotropy is insufficient.

[0009] In the above-mentioned ones, neither the plastic anisotropy nor the anti-deposition cracking has been improved, and the plastic anisotropy in the above one is still unsolved, and some measures are required. However, a preferable technique for improving the press formability of austenitic stainless steel has not been established.

Austenitic stainless steel is used for products such as tableware, pots and bathtubs by taking advantage of its corrosion resistance and surface beauty. These products are manufactured by press-forming thin steel plates in multiple stages. On the other hand, on the other hand, the improvement of press forming technology for thin steel sheets in recent years has made it possible to perform more advanced forming processing than in the past. Along with this, the demand for formability of thin steel sheets is also increasing,
A steel sheet that can withstand a greater degree of workability is required, and if it is a steel sheet that has excellent formability, it can be formed without intermediate annealing, although it was impossible to avoid intermediate annealing in the forming process in the past. Can be simplified. Thus, there is a great demand for high press formability stainless steel sheets, but the problem is, as mentioned above, the earring (ear) after deep drawing due to the plastic anisotropy of the material and the failure after a certain time after forming. Since the former will cut the ears after molding, the yield of the material will be reduced. The latter occurs suddenly after molding, and if it occurs, it will impair the function of the product.
Although it is a serious problem of lowering the reliability of the material, it is not easy to obtain an austenitic stainless steel sheet having these characteristics sufficiently and also having high press formability.

[0011]

Means for Solving the Problems The present invention has been studied to solve the problems in the prior art as described above, and sufficiently improves the formability of austenitic stainless steel, and has a small plastic anisotropy. It has succeeded in developing a stainless steel having good resistance to cracking upon placement, and is as follows.

% By weight, C: 0.02 to 0.10%, S
i: 0.2-1.0%, Mn: 0.4-1.5%, S: 0.008
% Or less, Cr: 15.0 to 19.0%, Ni: 6.0 to 8.0
%, Mo: 0.01 to 3.0%, Al: 0.05% or less, N
i: 0.01 to 0.06%, and is described below in FIG.
Containing P and Cu in the range surrounded by the formulas (1) to (5),
An austenitic stainless steel excellent in press formability, characterized in that the balance is Fe and inevitable impurities. [P] = 0.05 ───────────────────── (1) [P] = 0.30 ───────────── ───────── (2) {[P] +0.04} ・ {[Cu] -0.05} = 0.087───── (3) {[P] −0.18 } ・ {[Cu] -1.75} = 0.006────── (4) [Cu] = 3.0 ───────────────────── (5) In the above formulas, [P] and [Cu] are P and C, respectively.
The amount of u added is shown.

[0013]

In the present invention as described above, the important components for solving the above-mentioned problems are Cu and P. According to the research conducted by the present inventors, mainly Cu improves the press formability. In addition, P has an effect of improving plastic anisotropy and improving resistance to deferred cracking. These Cu
The action of P and P will be described below based on the examination results. 0.06% C-0.55% Si-0.95 Mn-1
8.3% Cr-7.8% Ni-0.1% Mo-0.002% Sol.
Al-0.035% T.I. Cu and P with N as the basic composition
Austenitic stainless steel with varying content of smelting, hot rolling, annealing, 75% cold rolling, annealing, temper rolling to produce a thin steel sheet with a thickness of 0.7 mm, conical A cup test (blank diameter 36 mm) and a deep-drawing cup test (blank diameter 100 mm, punch diameter 50 mm, punch shoulder radius 5 mm) were conducted to investigate the formability, plastic anisotropy, and susceptibility to misplacement cracking. For the evaluation of plastic anisotropy, the earing rate He defined by the following formula 1 was used.

[0014]

He = 100 × (H _max −H _min ) / {(H _max + H _min ) / 2}

In the above formula, H _max and H _min are the heights from the cup bottom after the deep drawing cup test to the highest and lowest portions of the edge, respectively. The smaller this value is, the smaller the earing is and the smaller the plastic anisotropy is.

Further, the susceptibility to placing cracks was evaluated by inserting a deep-drawn cup into a thermostat at 80 ° C. and measuring the time until the occurrence of cracks. In addition to the thin steel sheet test, a round bar tensile test piece (parallel part: diameter 6 m)
m, length 25 mm) was sampled and hot workability was also investigated.
The parallel part of the test piece was once heated to 1200 ° C. by high frequency, then cooled to 950 ° C. and pulled at a strain rate of 10 s ⁻¹ ,
The hot workability was evaluated by the drawing value after breaking.

Cu and P addition amount and conical cup value C
FIG. 2 shows the CV relationship. The addition of Cu sharply reduces CCV and improves moldability. CCV is also slightly reduced by the addition of P. In this way, Cu
And P have the effect of improving the formability, and CCV is 26 mm or less in the region above the curve in FIG. FIG. 3 shows the relationship between the added amounts of Cu and P and the earring rate. P is 0.0
In the case of 2%, the earring rate is 6 with the increase of Cu.
% To nearly 10%. However, P is 0.05%
When added in the above amount, the earing rate is reduced to 3% or less, and P has an earing reduction effect. Figure 4
Indicates the time of occurrence of misplacement cracking. Here too, the effect of P addition was recognized, and the addition of 0.05% or more of P extended the time of occurrence of laying cracks to 600 min or more, and solved the problem of laying cracks.

As described above, by adding P and Cu in combination, it is possible to secure the formability, reduce the plastic anisotropy, and eliminate the set crack. However, excessive addition of Cu and P hinders hot workability. FIG. 5 shows the results of the hot workability test. In the high Cu and P regions above the curve in the figure, the aperture is 60% or less,
Cracking occurs during hot rolling, leading to a reduction in yield. From the above examination results, preferable results can be obtained by setting the added amounts of Cu and P within the range surrounded by the five curves shown in FIG.

The reasons for limiting the other components in the present invention are as follows. C governs austenite stability. If it is 0.02% or less, the stability is lowered, a martensite phase is generated during cold working, which impairs cold workability and causes cracks during deposition. In addition, the refining cost for high purification increases. Further, if it exceeds 0.10%, carbides are likely to precipitate at the grain boundaries after hot rolling or annealing and the alloy becomes sensitized, resulting in a decrease in corrosion resistance. For the above reasons, C is
It was set to 0.02 to 0.10%.

Since Si reduces the amount of oxygen in steel,
2% or more is required. Even if it exceeds 0.9%, the deoxidizing power is saturated, rather the δ ferrite is increased and the hot workability is deteriorated. In addition, inclusions in the steel increase, causing surface defects during cold rolling. Therefore, Si is set to 0.2 to 0.9%.

Mn is an element that stabilizes the austenite phase, and if it is 0.4% or less, the formation of work-induced martensite during cold rolling increases, which makes cold rolling difficult. On the other hand, if it exceeds 1.5%, the corrosion resistance deteriorates, so 1.5%
Is the upper limit.

S not only deteriorates hot workability, but also Mn
It precipitates as S and reduces the formability of the cold-rolled sheet, especially the bending workability. Moreover, the corrosion resistance is also reduced. C of steel according to the invention
In order to eliminate these effects in the r and Ni addition amount ranges, S must be 0.008% or less.

Cr is an essential element for ensuring the corrosion resistance of steel, and requires 15% or more. If it exceeds 19%, the manufacturing cost will increase, so 19% is made the upper limit.

Ni stabilizes the austenite phase,
As a result, resistance to cracking due to placement is improved. Therefore, Ni is 6.
It is necessary to add 0% or more. However, if added in excess of 8.0%, the manufacturing cost rises, so 8.0% is made the upper limit.

Since Mo improves the corrosion resistance, Mo can be added in an amount of 0.01% or more depending on the intended use of the product. However, if it is added in excess of 3.0%, the intermetallic compound phase may be precipitated and the hot workability and corrosion resistance deteriorate, and the manufacturing cost increases, so the upper limit was made 3.0%.

Al has a strong deoxidizing effect and is effective in reducing the amount of oxygen in the steel, but on the other hand, it forms an oxide and remains as an inclusion in the thin steel sheet, which easily causes a surface flaw. . If it is added at 0.05% or less as Sol.Al, it will not form inclusions that affect the surface properties,
Can be effectively deoxidized. Therefore, Al is added in an amount of 0.05% or less.

N is an element that stabilizes austenite and is effective for securing strength, so N must be added in an amount of 0.01% or more. However, if it exceeds 0.06%, the ductility decreases as the strength increases, so 0.06% was made the upper limit.

[0028]

EXAMPLES Specific examples of the present invention will be described. The present inventors melted 26 stainless steel grades shown in the following Table 1 and hot rolled them into steel plates with a thickness of 2.8 mm. 1080 Annealing at ℃ and pickling were performed.

[0029]

[Table 1]

Each of the steels 1 to 26 shown in Table 1 above was cold-rolled to 75% to a thickness of 0.7 mm, annealed at 1080 ° C., and temper-rolled to 1%, and then machined. It was subjected to a test of physical properties and moldability. As for mechanical properties, a test piece was sampled parallel to the rolling direction, and 0.2% proof stress (0.2% P
S), tensile strength (TS) and elongation (El) were measured. As the moldability test, a conical cup test, an Erichsen test and a deep drawing test were performed. Conical cup test is JIS Z 2249, Erichsen test is JI
It carried out according to the A method of SZ2247. In the deep drawing test, blanks having various diameters were prepared, and a punch having a diameter of 50 mm and a shoulder radius of 5 mm was used to perform cup depth to determine the limit drawing ratio. Further, the earing ratio He was obtained from the cup drawn by the maximum drawing ratio of each steel type, and the plastic anisotropy was evaluated. The calculation of the earing rate was performed by the above-mentioned formula 1.

Further, in order to evaluate the resistance to cracking by placing, a cup squeezed with a drawing ratio of 2 was immediately inserted into a thermostat at 80 ° C., and the time until cracking was measured.

In addition to the above, a round bar tensile test piece (parallel portion: diameter 6 mm, length 25 mm) was sampled from the ingot and hot workability was also investigated. The parallel part of the test piece was once heated to 1200 ° C. by high frequency, then cooled to 950 ° C., stretched at a strain rate of 10 s ⁻¹ , and the hot workability was evaluated by the drawing value after breaking. The results of these tests are shown in Table 2 below.

[0033]

[Table 2]

In Table 2 above, low P-low Cu N
o. 14 steel has CCV of 27.5 mm, Er of 12.7 mm, L
The DR is 2.0 and the moldability is poor. Earrings are also great. The No. 15 steel having a low P content and containing 1.9% Cu exhibited good formability, but the earring rate was as high as 7.8% or more, and cracking occurred in a very short time. In addition, P containing less than 0.05% and less Cu No. 19, 2
Steels 1, 25, and 26 have a small earing rate due to the effect of P and are excellent in resistance to cracking upon placement, but are inferior in formability. The No. 18, 20, 22 to 24 steels containing an excessive amount of P or Cu have a small drawing at high temperature and are poor in hot workability. As described above, these comparative steels have no problem with respect to strength and ductility, but have problems with respect to formability, plastic anisotropy, resistance to cracking, and hot workability.

On the other hand, in the steel Nos. 1 to 15 of the present invention containing appropriate amounts of P and Cu, the CCV was 26.0 mm in all cases.
Below, Er is 13.5 mm or more, LDR is 2.3 or more,
Shows excellent moldability. Furthermore, the earring rate is 2.5% or less, no cracks occur in the placement, and the hot workability is good.

[0036]

As described above, according to the present invention, an austenitic stainless steel having excellent formability as well as stable properties in both plastic anisotropy and anti-deposition cracking is provided. The present invention has the effects of facilitating the molding process and increasing the material yield so that the increase of the manufacturing cost can be appropriately suppressed, which is an industrially great effect.

[Brief description of drawings]

FIG. 1 is a chart showing the range of the austenitic stainless steel of the present invention in relation to P% and Cu%.

FIG. 2 is a table showing the relationship between Cu and P% and the conical cup value CCV.

FIG. 3 is a table showing a relationship between Cu and P amounts and earring rates.

FIG. 4 is a chart showing the relationship between the occurrence time of disposition cracking and the amounts of P and Cu.

FIG. 5 is a table summarizing the relationship between hot workability (drawing%) and Cu% and P%.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] November 8, 1991

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

[0008]

Even though the above-mentioned conventional technique has some merits, it is not always preferable as an austenitic stainless steel sheet used by press working. That is, the above-mentioned ones cannot be preferable in terms of plastic anisotropy that causes earing, and cutting after molding and reduction in material yield cannot be avoided. In addition, Al added to reduce oxygen is present in the thin steel sheet as alumina and easily causes flaws during cold rolling, and it is difficult to obtain good surface properties.
Also, the plastic anisotropy is insufficient.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Abe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan KK (72) Inventor Tomoyoshi Ohkita 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd.

Claims (1)

[Claims] 1. By weight%, C: 0.02 to 0.10%, Si: 0.2 to 1.0%, Mn:
0.4-1.5%, S: 0.008% or less, Cr: 15.0-1
9.0%, Ni: 6.0-8.0%, Mo: 0.01-3.0%,
Al: 0.05% or less, Ni: 0.01 to 0.06%, P and Cu in the range surrounded by the following formulas (1) to (5) in FIG. 1, and the balance Fe. And austenitic stainless steel excellent in press formability, which is characterized by comprising unavoidable impurities. [P] = 0.05 ───────────────────── (1) [P] = 0.30 ───────────── ───────── (2) {[P] +0.04} ・ {[Cu] -0.05} = 0.087───── (3) {[P] −0.18 } ・ {[Cu] -1.75} = 0.006────── (4) [Cu] = 3.0 ───────────────────── (5) In the above formulas, [P] and [Cu] are P and C, respectively.
The amount of u added is shown.