JP2636591B2 - Austenitic stainless steel with excellent press formability, plastic anisotropy, and anti-cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to press formability and plastic anisotropy
Austenitic stainless steel sheet with excellent press formability, small plastic anisotropy, and excellent place crack resistance It is.

[0002]

2. Description of the Related Art As an austenitic stainless steel used by press forming, SUS304 has been widely used in the past, and is used for kitchen appliances and tableware. However, this SUS304 has large plastic anisotropy, and an earring (ear) occurs at the edge after deep drawing. Since such earrings must be cut and removed after molding, the man-hours are complicated and the yield of the material is reduced.

[0003] Further, SUS304 also has a problem of cracking. That is, if this standing crack is left after deep drawing, it is supposed to generate cracks naturally due to residual stress or work-induced martensite generated during forming or hydrogen in steel. Can be prevented by performing strain relief annealing after the forming process, but this naturally increases the manufacturing cost and cannot be made desirable.

[0004] In order to solve the above-mentioned problems of SUS304, a proper 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
~ 2.000%, Cu: 0.50 ~ 3.00%, Ni: 8.00 ~
10.50%, Cr: 18.0 to 20.00%, the balance is composed of unavoidable impurities and Fe, and 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 critical drawing ratio of 3.1 or more,
It is said that it has high press moldability and excellent time cracking resistance (standing cracking resistance).

In Japanese Patent Application Laid-Open No. 1-92342, C: 0.
001 to 0.10%, Si: 0.10 to 1.4%, Mn: 0.1%
1 to 3.0%, Cr: 15.0 to 30.0%, Ni: 8.0 to 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
The stainless steel containing 0% and O: 0.0025% or less (other than that, Ti, Ca, Mo, Cu, B) has a low oxygen content and a small amount of coarse inclusions.
No severe defects such as cracks or surface flaws are generated even when severe molding is performed, and the material is extremely excellent in deep drawing workability.

Further, Japanese Patent Publication No. 18382/1990 discloses, as an austenitic stainless steel excellent in rust resistance and cold workability, C: 0.04% or less, Si: 0.8% or less, Mn: 2.
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
A component of 4% or less, O: 0.006% or less, C + N: 0.045% or less, and Ca: 0.0005 to 0.008% is proposed. That is, this component can ensure rust resistance including the welded portion, and can omit the intermediate annealing in the cold working step because the work hardening is small.

In Japanese Patent Publication No. 59-33663, as an austenitic stainless steel excellent in formability and strength, C: 0.10 to 0.17%, Si: 0.3 to less than 1.0%, and Mn: 0.3%. 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. By properly controlling the austenite stability, this steel is said to be excellent in overmolding and deep-drawing composite formability, has relatively high strength, and has good resistance to seasonal cracking (standing cracking resistance). .

[0008]

Although the above-mentioned prior art has a certain merit, it is not always preferable as an austenitic stainless steel sheet used by pressing. That is, the above-mentioned one cannot be preferable in terms of plastic anisotropy which causes earing, and it is inevitable that cutting after molding and a reduction in material yield can be avoided. In addition, Al added for oxygen reduction is present in the thin steel sheet as alumina, so that it is easy to generate flaws during cold rolling, and it is difficult to obtain good surface properties,
Also, the plastic anisotropy is insufficient.

No improvement has been made in the above-mentioned ones with regard to plastic anisotropy or resistance to standing cracks, and the plastic anisotropy is still unsolved in any of the above-mentioned ones, and some measures need to be taken. However, a preferable technique for improving the press formability of austenitic stainless steel has not been established.

Austenitic stainless steels are used for products such as tableware, pots, bathtubs, etc., taking advantage of their corrosion resistance and surface beauty, and these products are manufactured by press-forming thin steel sheets in multiple stages. However, on the other hand, recent improvements in the press forming technology of thin steel sheets have made it possible to perform more advanced forming processes than before. Along with this, the demand for formability of thin steel sheets has been increasing,
A steel sheet that can withstand a greater degree of work is required.In addition, if the steel sheet has excellent formability, a process that could not avoid intermediate annealing in the forming process in the past can be formed without intermediate annealing. Can also be simplified. Although there is a great demand for high press formability stainless steel sheets, the problem is that, as described above, the earring (ear) after deep drawing and the fracture after a certain time after forming due to the plastic anisotropy of the material. In the former, the ears are cut after molding, which leads to a decrease in material yield. The latter occurs suddenly after molding, and if it occurs, impairs the function of the product,
Although this is a serious problem that lowers 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]

SUMMARY OF THE INVENTION The present invention has been studied to solve the above-mentioned problems in the prior art, and has sufficiently improved the formability of austenitic stainless steel, and has low plastic anisotropy. The present invention has succeeded in developing a stainless steel having good anti-cracking resistance, and is as follows.

C: 0.02 to 0.10% by weight, S
i: 0.2 to 1.0%, Mn: 0.4 to 1.5%, S:
0.008% or less, Cr: 15.0 to 19.0%, N
i: 6.0 to 8.0%, Mo: 0.01 to 3.0%, A
1: 0.05% or less, N: 0.01 to 0.06%, contains P and Cu in a range surrounded by the following formulas (1) to (5) in FIG. 1, and the balance is Fe Press formability and plastic variation characterized by being composed of unavoidable impurities
Austenitic stainless steel with excellent anisotropy and crack resistance . [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) , [P] and [Cu] are P,
Shows the amount of Cu added.

[0013]

In the present invention as described above, important components for solving the problems described in the preceding paragraph are Cu and P. According to the study of the present inventors, it has been found that Cu mainly improves press formability. P has the effect of improving the plastic anisotropy and improving the anti-cracking resistance. These Cu
The effect of P and P will be described below based on the results of the study. 0.06% C-0.55% Si-0.95Mn-1
8.3% Cr-7.8% Ni-0.1% Mo-0.002% Sol.
Al-0.035% T. As a basic composition containing N as a component of a hot-rolled sheet, austenitic stainless steel in which the contents of Cu and P are changed is melted, hot-rolled, annealed, and
After 5% cold rolling, annealing, and temper rolling, a thin steel sheet with a thickness of 0.7 mm was manufactured, and a conical 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 formability, plastic anisotropy, and susceptibility to placing cracks. For the evaluation of the plastic anisotropy, an earing rate He defined by the following equation 1 was used.

[0014]

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

In the above equation, H _max and H _min are the heights from the cup bottom after the deep drawing cup test to the highest part and the lowest part of the edge, respectively. A smaller value indicates that the earring is smaller and the plastic anisotropy is smaller.

The susceptibility to cracking was evaluated by inserting a deep-drawn cup into a thermostat at 80 ° C. and measuring the time until cracking. Furthermore, apart from the test of the thin steel plate, a round bar tensile test piece (parallel part: diameter 6m)
m, 25 mm in length) were also 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.

[0017] Cu and P added amount and conical cup value C
FIG. 2 shows the relationship between CVs. With the addition of Cu, the CCV sharply decreases, and the formability improves. Also, the addition of P slightly reduces CCV. Thus, Cu
And P have the effect of improving the formability, and in the region above the curve in FIG. 2, the CCV is 26 mm or less. FIG. 3 shows the relationship between the amounts of Cu and P added and the earring rate. P is 0.0
In the case of 2%, the earring rate becomes 6 with an increase in Cu.
% To nearly 10%. However, P is 0.05%
When added, the earring rate is reduced to 3% or less, and P has an earring reducing effect. FIG.
Indicates the time of occurrence of cracks. The effect of the addition of P was also observed here, and the addition of P at 0.05% or more increased the time for the occurrence of standing cracks to 600 minutes or more, and solved the problem of standing 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 placing crack. However, excessive addition of Cu and P impairs hot workability. FIG. 5 shows the results of the hot workability test. In the region of high Cu and P above the curve in the figure, the aperture is 60% or less,
Cracks occur during hot rolling, leading to a reduction in yield. From the above examination results, favorable results can be obtained by setting the addition amounts of Cu and P in the range surrounded by the five curves shown in FIG.

The reasons for limiting other components in the present invention are as follows. C governs austenitic stability. If the content is less than 0.02%, the stability decreases, and a martensite phase is formed during cold working, which impairs cold workability and causes cracks in storage. In addition, refining costs for high purification are increased. If it exceeds 0.10%, carbides are likely to precipitate at the grain boundaries after hot rolling or annealing, and the corrosion resistance is reduced due to sensitization. For the above reasons, C is
0.02 to 0.10%.

Si is used in an amount of 0.1 to reduce the amount of oxygen in the steel.
2% or more is required. Even when the content exceeds 0.9%, the deoxidizing power is saturated, and the δ ferrite increases and the hot workability is lowered. Further, inclusions in the steel increase, which causes surface defects during cold rolling. Therefore, Si is set to 0.2 to 0.9%.

Mn is an element for stabilizing the austenite phase, and if it is 0.4% or less, the generation of work-induced martensite during cold rolling increases, making cold rolling difficult. On the other hand, if the content exceeds 1.5%, the corrosion resistance deteriorates.
Is the upper limit.

S degrades hot workability, and Mn
It precipitates as S and lowers the formability of the cold rolled sheet, particularly the bending workability. Also, the corrosion resistance decreases. C of steel according to the invention
In order to eliminate these effects in the range of the addition amount of r and Ni, S needs to be 0.008% or less.

Cr is an essential element for ensuring the corrosion resistance of steel, and requires 15% or more. Since exceeding 19% leads to an increase in manufacturing cost, the upper limit is 19%.

Ni stabilizes the austenite phase,
As a result, anti-cracking resistance is improved. Therefore, Ni is 6.
It is necessary to add 0% or more. However, if the addition exceeds 8.0%, the production cost increases, so the upper limit is 8.0%.

Since Mo improves corrosion resistance, it can be added in an amount of 0.01% or more depending on the use of the product. However, if added in excess of 3.0%, intermetallic compound phases may be precipitated, resulting in reduced hot workability and corrosion resistance and increased production costs. Therefore, the upper limit was set to 3.0%.

Al has a strong deoxidizing effect and is effective in reducing the amount of oxygen in steel. On the other hand, Al forms oxides and remains in the thin steel sheet as inclusions, and easily causes surface defects. . If 0.05% or less of Sol.Al is added, without forming inclusions that affect the surface properties,
Deacidification can be performed effectively. Therefore, Al is added at 0.05% or less.

N is an element that stabilizes austenite and is effective in securing strength. Therefore, it is necessary to add 0.01% or more. However, if it exceeds 0.06%, the ductility decreases as the strength increases, so the upper limit was made 0.06%.

[0028]

The present invention will be described in more detail with reference to the following examples. The present inventors melted 26 stainless steels shown in Table 1 below and hot-rolled them into 2.8 mm thick steel plates. Annealing at ° C. and pickling were performed. Note that the component set
The formation is after hot rolling.

[0029]

[Table 1]

Each of the steels 1 to 26 in Table 1 described above was cold rolled at 75% to a thickness of 0.7 mm, then annealed at 1080 ° C., further subjected to temper rolling at 1%, It was subjected to the test of mechanical properties and moldability. For mechanical properties, test specimens were taken in parallel with the rolling direction and 0.2% proof stress (0.2% P
S), tensile strength (TS) and elongation (El) were measured. As a test of moldability, a conical cup test, an Erichsen test, and a deep drawing test were performed. Conical cup test conforms to JIS Z 2249, Erichsen test conforms to JIS
Performed in accordance with the method A of SZ2247. In the deep drawing test, blanks having various diameters were prepared, the cup was deepened using a punch having a diameter of 50 mm and a shoulder radius of 5 mm, and a limit drawing ratio was obtained. In addition, the earing ratio He was determined from the cup drawn by the cup at the maximum drawing ratio of each steel type, and the plastic anisotropy was evaluated. The calculation of the earring rate was performed by the above-described formula 1.

Further, as an evaluation of the anti-cracking resistance, the cup squeezed at a squeezing ratio of 2 was immediately inserted into a constant temperature bath 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 the hot workability was also examined. The parallel part of the test piece was once heated to 1200 ° C. by high frequency, then cooled to 950 ° C., pulled at a strain rate of 10 s ⁻¹ , and 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 described above, it is found that low P-low Cu N
o. 14 steel has CCV of 27.5mm, Er of 12.7mm, L
DR is 2.0 and moldability is inferior. The earrings are also large. No. 15 steel with low P and containing 1.9% Cu showed good formability, but the earring rate was as large as 7.8% or more, and cracking occurred in a very short time. No. 19, 2 containing 0.05% or more of P and containing little Cu
The 1, 25, and 26 steels have a small earing ratio due to the effect of P and are excellent in anti-cracking resistance, but are inferior in formability. No. 18, 20, 22 to 24 steels excessively containing P or Cu have a small drawing at high temperature and are inferior in hot workability. As described above, these comparative steels have no problem in strength and ductility, but have problems in formability, plastic anisotropy, crack resistance, and hot workability.

On the other hand, in the steels Nos. 1 to 15 of the present invention containing an appropriate amount of P and Cu, the CCV was 26.0 mm.
Hereinafter, Er is 13.5 mm or more, LDR is 2.3 or more,
Shows excellent moldability. Further, the earring rate is 2.5% or less, no breakage occurs, and the hot workability is good.

[0036]

According to the present invention as described above, the present invention provides an austenitic stainless steel having not only excellent formability but also stable properties in both plastic anisotropy and anti-cracking resistance. In addition, the present invention has an effect of facilitating the molding process, increasing the material yield and appropriately suppressing an increase in the manufacturing cost, and is an industrially significant invention.

[Brief description of the 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 chart showing a relationship between Cu and P% and a conical cup value CCV.

FIG. 3 is a table showing the relationship between the amounts of Cu and P and the earring rate.

FIG. 4 is a table showing the relationship between the time of occurrence of a pre-crack and the amounts of P and Cu.

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

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takashi Abe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Tomo Ohkita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan (56) References JP-A-1-92342 (JP, A) JP-A-61-67760 (JP, A)

Claims (1)

(57) [Claims] C .: 0.02 to 0.10%, Si: 0.2 to 1.0% by weight,
Mn: 0.4-1.5%, S: 0.008% or less, C
r: 15.0 to 19.0%, Ni: 6.0 to 8.0%,
Mo: 0.01 to 3.0%, Al: 0.05% or less,
N: contains 0.01 to 0.06%, contains P and Cu in a range surrounded by the following formulas (1) to (5) in FIG. 1, and the balance consists of Fe and unavoidable impurities. Austenitic stainless steel with excellent press formability , plastic anisotropy, and anti-cracking characteristics. [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) , [P] and [Cu] are P,
Shows the amount of Cu added.