JPH073327A - Production of metastable austenitic stainless steel sheet excellent in season cracking resistance at the time of production - Google Patents

Abstract

PURPOSE:To improve the season cracking resistance at the time of production in low austenite stability as a steel sheet by subjecting a steel whose chemical compsn. and Ni equivalent are specified to hot rolling, thereafter holding it under heating to a specified temp. and next controlling the cooling conditions. CONSTITUTION:The compsn, of steel is incorporated with, by weight, 0.09 to 0.30% C, <=2.0% Si, <=2.0% Mn, 10.0 to 20.0% Cr, 5.0 to 10.0% Ni and <=0.10% N and is furthermore incorporated with one or two kinds among 0.1 to 3.0% Mo, <=1.0% Nb, <=1.0% Ti and <=1.0% V. Furthermore, the Ni equivalent shown by the formula is regulated to the range of 19.0 to 24.0%. This steel slab is subjected to hot rolling, is thereafter held under soaking to 1000 to 1300 deg.C and is next subjected to controlled cooling in the temp. range of 950 to 550 deg.C at 1 to 10 deg.C/min cooling rate. In this way, the metastable austenitic steel excellent in the season cracking resistance can be produced by simple means.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a technique for improving the time crack resistance during production of a metastable austenitic stainless steel which is a component system in which martensite is prone to induced cracking due to its low austenite stability. To suggest.

[0002]

2. Description of the Related Art It is known that metastable austenitic stainless steel undergoes a kind of delayed fracture called age cracking or age cracking due to induced martensite during manufacturing, especially when left to stand naturally after cold rolling. ing. Therefore, for this metastable austenitic stainless steel sheet, after cold rolling, continuous annealing is performed within a certain period of time to reduce stress and control the strength of martensite, thereby preventing delayed fracture from occurring. It was being done. Conventionally, it has been very difficult to omit the continuous annealing treatment to produce a metastable austenitic stainless steel strip having a low austenite stability. That is, in the conventional technique, it is indispensable to anneal immediately after cold rolling, which has a problem that the productivity of this type of stainless steel is significantly restricted.

[0003]

SUMMARY OF THE INVENTION The object of the present invention is to reduce the productivity as described above, which is a problem of the prior art, that is, annealing is required to prevent cracking during manufacture. To propose a technology that can overcome the problem.

[0004]

In order to achieve the above-mentioned object, in the present invention, in the production of a metastable austenitic stainless steel sheet, after hot rolling, prior to cold rolling, it is peculiar only to the present invention. A method for heat treatment is proposed. As a result, it is possible to effectively prevent time cracking after cold rolling. As a result, it is no longer necessary to perform continuous annealing immediately after cold rolling as in the conventional method, and a metastable austenitic stainless steel sheet having excellent austenite stability and low time cracking resistance during production is produced while maintaining high productivity. be able to.

That is, according to the present invention, C: 0.09 to 0.30 wt.
%, Si: 2.0 wt% or less, Mn: 2.0 wt% or less, Cr: 10.0 to
20.0wt%, Ni: 5.0-10.0wt%, N: 0.10 or less,
Furthermore, Mo: 0.1-3.0 wt%, Nb: 1.0 wt% or less, Ti: 1.
Ni containing 0 or less wt% and V: 1.0 wt% or less, and one or more kinds selected from the following formula.
The equivalent weight is in the range of 19.0-24.0wt%,

[Equation 2] A steel slab whose balance consists of Fe and unavoidable impurities is hot-rolled, after which it is uniformly heated to a temperature of 1000 to 1300 ° C, and then cooled in the temperature range of 950 to 550 ° C at a cooling rate of 1 to 10 ° C / min. The method for producing metastable austenitic stainless steel having excellent resistance to time cracking during production.

[0006]

[Operation] Generally, a so-called metastable austenitic stainless steel in which the martensite phase is induced by cold working is low, so-called metastable austenitic stainless steel, when it is manufactured by a normal process, is subject to time cracking and aging after cold rolling. It is well known that delayed fracture such as cracking occurs. Further, the occurrence of this time cracking involves martensite induced by plastic deformation, and the larger the amount of martensite to be induced and the higher the strength of this martensite phase, the higher the time cracking susceptibility. Is known to be. Therefore, the present invention does not perform continuous annealing or cold rolling immediately after hot rolling a steel material having a predetermined composition, but at this time, by subjecting it to a heat treatment peculiar to the present invention, plastic deformation is caused. The strength of the induced martensite phase is controlled so that the time cracking that occurs in the steel sheet can be prevented without annealing within a certain time after cold rolling.

The composition of the steel and the heat treatment conditions necessary for producing the metastable austenitic stainless steel excellent in the time cracking resistance during the production through the heat treatment peculiar to the present invention will be described below.

C: 0.09 to 0.30 wt% C is an element that improves the strength of the martensite phase induced by working. However, when the amount of C is less than 0.09 wt%, the problem of time cracking does not occur because the strength of martensite induced by working is low, which is outside the scope of the present invention. On the other hand, if it exceeds 0.30 wt%, a large amount of coarse undissolved carbide remains, which significantly deteriorates the cold workability. Therefore, the C content is limited to the range of 0.10 to 030 wt%.

Si: 2.0 wt% or less Si is an element effective for deoxidation and strength increase during steel making, but if it exceeds 2.0 wt%, manufacturability is impaired. Therefore, the Si content should be 2.0 wt% or less.

Mn: 2.0 wt% or less Mn is an element effective as a deoxidizing agent and a desulfurizing agent during steel making, but if it is contained in excess of 2.0 wt%, hot workability and surface oxidation during annealing become remarkable. . Therefore, the Mn content is 2.0 wt% or less.

Ni: 5.0-10.0 wt% Ni is a strong austenite stabilizing element and is 5.0 wt%.
If it is less than%, rolling becomes difficult because martensite occurs due to annealing. If the content exceeds 10.0 wt%, martensitic transformation during cold working is hindered, and the occurrence of time cracking disappears. Therefore, the Ni content should be in the range of 5.0 to 10.0 wt%.

Cr: 10.0 to 20.0 wt% Cr is an essential component in stainless steel, and if it is less than 10.0 wt%, the corrosion resistance deteriorates significantly. If it is contained in excess of 20.0 wt%, the amount of δ-ferrite increases rapidly and the hot workability deteriorates. Therefore, the Cr content is set in the range of 10.0 to 20.0 wt%.

N: 0.10 wt% or less When the steel material contains strengthening components such as Nb, Ti and V, it is 0.10
If N is contained in excess of wt%, coarse nitrides are formed with these elements and cause surface defects. Therefore, the N content is 0.10 wt% or less.

Mo: 0.1 to 3.0 wt% Mo is an element effective for improving the corrosion resistance. If the content is less than 0.1 wt%, a sufficient effect cannot be obtained. On the other hand, if the content exceeds 3.0 wt%, the effect is small relative to the addition amount, and since it is an element that easily causes solidification segregation, δ The hot workability deteriorates due to the residual ferrite. Therefore, the Mo content is 0.1 to 3.0 wt%.

Nb: 1.0 wt% or less, Ti: 1.0 wt% or less,
V: 1.0 wt% or less Nb, Ti, and V all combine with C and N to form a stable carbonitride, and are dispersed in the crystal grains, and mechanical strength is obtained by refining and strengthening the crystal grains. Is a component element that contributes to the improvement of If these elements are contained in an amount of 1.0 wt% or more, the manufacturability deteriorates due to the coarsening of carbonitrides and the formation of δ-ferrite. Therefore, these components should be 1.0 wt% or less.

Ni equivalent: 19.0 to 24.0 wt% Ni equivalent is an index of austenite stabilization and is generally given by the following equation.

[Equation 3] In the present invention, the reason for limiting the range of this Ni equivalent is
If it is less than 19.0 wt%, a large amount of martensite will be present after annealing, making it difficult to cold work. On the other hand, if it exceeds 24.0 wt%, martensite transformation due to cold working will be suppressed and no time crack will occur. .

Next, in the present invention, the hot rolled steel sheet is soaked to a temperature of 1000 to 1300 ° C. after hot rolling. By this soaking treatment, Cr carbides and Nb, Ti, and V carbonitrides existing after hot rolling can be re-dissolved in the parent phase. If the soaking temperature is less than 1000 ° C, re-dissolution of Cr carbide and carbonitrides of Nb, Ti, and V is not sufficient. On the other hand, if it exceeds 1300 ° C, the strength of the steel sheet decreases and the shape is changed. Adversely affect. Therefore, the temperature of such soaking is 1000 to 1300.
It must be in the range of ° C.

Next, the above-mentioned hot rolled steel sheet is soaked and then
Controlled cooling is performed in the temperature range of 950 to 550 ° C at a cooling rate of 1 to 10 ° C / min. The reason for performing such cooling is to control the precipitation of Cr carbide, Nb, Ti, and V carbonitride. Especially, the temperature range from 950 to 550 ℃, Cr carbide, N
b, Ti, V is a temperature range for precipitating carbonitrides, and if the cooling rate in this temperature range is less than 1 ° C / min, Cr
Since a large amount of carbides and carbonitrides of Nb, Ti, and V precipitate at the grain boundaries, Cr in the vicinity of the grain boundaries is deficient, resulting in martensite. In this case, the elongation is remarkably reduced, and cold rolling becomes difficult. On the other hand, when the cooling rate exceeds 10 ° C./min, the precipitation of Cr carbides, Nb, Ti, and V carbonitrides decreases, and the amount of C dissolved in the matrix increases. When this is cold-rolled, the strength of the martensite phase induced is significantly increased, and the susceptibility to time cracking is increased. Therefore, the cooling rate at the temperature of 950-550 ℃ is 1-10
C / min.

[0019]

EXAMPLES Steels A to C having the respective compositional compositions shown in Table 1 were prepared.
A hot-rolled steel sheet having a thickness of 5 mm was manufactured by melting 60 tons, continuously casting, and hot rolling. These hot-rolled steel sheets were heat-treated according to the present invention or continuously annealed, and then cold-rolled at a rolling reduction of 50% (sheet thickness 2.5 mm). These steel sheets were evaluated for elongation and hardness by a tensile test before cold rolling, hardness after cold rolling, and presence or absence of time cracking up to 48 hours after cold rolling. In addition, the tensile test piece used JIS13B, and the hardness test used the Vickers hardness meter. The evaluation of the occurrence of cracks is
The case where it occurred less than 24 hours after cold rolling was marked with X, the case where it occurred within 24 to 48 hours was marked with Δ, and the case where it did not occur after 48 hours was marked with ◯. The results of continuous annealing are shown in Table 2, and the results of those subjected to heat treatment are shown in Table 3.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

As is clear from the results shown in Tables 2 and 3, A-6, B-5, and A-6, which were continuously annealed according to the conventional method, were used.
In C-5, a time crack occurred after cold rolling. On the other hand, even in the case of the method of the present invention in which heat treatment was performed after hot rolling, when the holding temperature was outside the range of the present invention and was less than 1100 ° C., time cracking occurred. The cooling rate is 10 ℃ /
If it exceeds min, the hardness after 50% cold rolling becomes high, and cracking occurs. On the other hand, in an example completely compatible with the method of the present invention, it is not necessary to perform heat treatment after cold rolling,
A stainless steel plate with low austenite stability could be manufactured.

[0024]

As described above, according to the present invention,
A metastable austenitic steel having excellent resistance to time cracking can be produced by a simple means.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeru Hirata 4-2 Kojima-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa Nihon Metallurgical Industry Co., Ltd. Technical Research Center

Claims (1)

[Claims] 1. C: 0.09 to 0.30 wt%, Si: 2.0 wt% or less, Mn: 2.0 wt% or less, Cr: 10.0 to 20.0 wt%, Ni: 5.0 to 10.0 wt%, N: 0.10 or less, Further, it contains one or more selected from Mo: 0.1 to 3.0 wt%, Nb: 1.0 wt% or less, Ti: 1.0 wt% or less, and V: 1.0 wt% or less, and has the following formula: The Ni equivalent shown by is in the range of 19.0 to 24.0 wt%, and A steel slab whose balance consists of Fe and unavoidable impurities is hot-rolled, after which it is uniformly heated to a temperature of 1000 to 1300 ° C, and then cooled in the temperature range of 950 to 550 ° C at a cooling rate of 1 to 10 ° C / min. A method for producing a metastable austenitic stainless steel sheet having excellent resistance to time cracking during production.