JP3948825B2 - Method for producing austenitic stainless steel sheet free from wrinkles during hot rolling - Google Patents

Description

【００３２】
【表２】

【００３３】
【発明の効果】
本発明によれば、割れが発生しやすいＭｏ、Ｎを多量に含有したオーステナイト系ステンレス鋼の熱間圧延時のオーステナイト粒界割れに起因する疵を防止できるため、歩留りの向上が達成でき、安価なオーステナイト系ステンレス鋼板の生産に大きく寄与する。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a production method in which wrinkles are not generated during hot rolling when producing an austenitic stainless steel sheet.
[0002]
[Prior art]
Since austenitic stainless steel is a high alloy, hot workability is poor, and cracks occur during hot rolling, and various studies have been conducted to prevent this. In particular, large cracks that occur at the hot-rolled plate edge, which are called ear cracks, are serious problems in manufacturing, such as cases that are related to whether or not manufacturing is possible, and that the yield is greatly reduced.
With regard to large cracks such as ear cracks generated in the hot rolling process, it is becoming less likely that production is impossible due to optimization of components and the like today.
[0003]
On the other hand, apart from such large cracks related to the availability of production, wrinkles such as hege wrinkles occur in the portion of about 200 mm from the edge of the hot-rolled steel sheet, and the generated portion cannot be commercialized. Yield may be lost.
[0004]
In particular, in the case of a high Mo and high N steel such as SUS316LN steel in which austenite is stable and austenite crystallizes in the early stage of solidification, the slab is once heated and rolled (breakdown) to a predetermined thickness, and the surface is cleaned. In addition, a so-called two-heat method is performed in which the heel portion is removed and heated again to perform rolling, or the production size is determined in consideration of the occurrence of wrinkles.
[0005]
On the other hand, as a conventional technique, for example, from the viewpoint of preventing cracks because heels are small cracks in the hot rolling process, Japanese Patent Application Laid-Open No. 57-16153 discloses Cr equivalent of steel composition, Ni Equivalent amount is regulated, and δ (cal) = 3 (Cr + Mo + 1.5Si + 0.5Nb) −2.8 (Ni + 0.5Mn + 0.5Cu) −84 (C + N) −19.8 is set to 4 or less. Discloses a technique for ensuring hot workability.
[0006]
From the viewpoint of the slab structure, Japanese Patent Application Laid-Open No. 57-127554 controls the relationship between the N content of austenitic stainless steel and the tundish temperature (ΔT) during casting to prevent crystal grains from becoming coarse. Thus, a technique for improving hot workability is disclosed.
[0007]
Furthermore, from the viewpoint of improving the structure of the surface layer, Japanese Examined Patent Publication No. 2-9651 introduces a processing layer to the surface layer by performing shot blasting before inserting the heating furnace into the slab in which the Si content of the austenitic stainless steel is regulated. Discloses a technique for preventing cracking by recrystallizing during heating to refine the crystal grains of the slab surface layer.
[0008]
However, with the above-described technology alone, although the occurrence rate is reduced, it has not yet been completely prevented, which is a major cause of cost increase.
[0009]
[Problems to be solved by the invention]
The present invention improves the generation of wrinkles without increasing the process load for preventing wrinkles in order to improve the fine cracks and the wrinkles that are caused by hot rolling of the austenitic stainless steel containing Mo and N described above. It aims at providing the austenitic stainless steel plate which prevents.
[0010]
[Means for Solving the Problems]
The present inventors investigated in detail the relationship between the hot rolling conditions and the wrinkles when rolling a continuous cast slab of austenitic stainless steel containing a large amount of Mo and N. As a result, it was found that the drought greatly changes depending on the rolling reduction, the time between passes and the rolling temperature at the beginning of rolling. The present invention is based on such knowledge and has the following configuration.
That is, in mass %,
C: 0.03% or less, Si: 0.1-2.0%,
Mn: 0.5 to 2.0%, P: 0.04% or less,
S: 0.0030% or less, Cr: 16.0-25.0%,
Ni: 11.0-20.0%, Mo: 2.0-7.0%,
Cu: 1.0% or less, Al: 0.01 to 0.05%,
O: 0.005% or less, Ca: 0.0010 to 0.0050%,
N: 0.15-0.30%
In the case of hot rolling a continuous cast slab of austenitic stainless steel, the balance of which is Fe and inevitable impurities, and δcal represented by the following formula is less than 0, heating is performed at 1150 ° C. or more, rolling Rolling at 1050 ° C or more with a reduction rate of 3% or less and a time between passes of 15 seconds or more from the start, and then rolling at 1000 ° C or more with a time between passes of 10 seconds or more until the cumulative reduction rate of 50% Is a method for producing an austenitic stainless steel sheet free from wrinkles during hot rolling.
δcal = 3 (Cr + 1.5Si + Mo) −2.8 (Ni + 0.5Mn
+ 0.5Cu) -84 (C + N) -19.8
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors conducted a hot rolling experiment using a 190 mm continuous cast slab of SUS316LN containing 18% Cr-12% Ni-3% Mo-0.17% N, and The relationship between hot rolling conditions was examined in detail, and a method for preventing cracking by controlling heating conditions and hot rolling conditions was confirmed.
[0012]
The present invention is described in detail below.
The reason why the heating temperature is set to 1150 ° C. or higher is that at a heating temperature lower than this, the temperature decreases during hot rolling, which is disadvantageous from the viewpoint of hot workability. In the present invention, the higher the heating temperature, the more desirable. The upper limit of the heating temperature is not particularly defined, but the surface layer portion of the continuous cast slab of austenitic stainless steel has coarse austenite columnar crystals as in the present invention, where δcal is less than 0. However, the austenite grains are remarkably large, and when heated above 1300 ° C., the austenite grains become coarse and cracks cannot be prevented by the present invention, and surface defects due to abnormal scale occur. Therefore, 1300 ° C. is considered the upper limit.
[0013]
In addition, it is necessary to perform rolling at 1050 ° C. or more after taking 3 seconds or more from the start of rolling at a light reduction rate of 3% or less with a time between passes of 15 seconds or more.
This is due to light rolling at the beginning of rolling to avoid strain concentration at coarse columnar grain boundaries with high cracking sensitivity, and to soften and recrystallize by recovery by setting the time between passes to 15 seconds or more. By repeating this three or more passes, it is considered that the occurrence of cracks at the initial stage of rolling can be prevented due to the movement of coarse columnar grain boundaries and the disappearance of coarse columnar grain boundaries due to recrystallization.
[0014]
When the rolling reduction ratio exceeds 3% at the beginning of rolling, cracks occur at the grain boundaries of the coarse grains in the surface layer portion, and when the time between passes is less than 15 seconds, high pressure is maintained even if light rolling is repeated due to the cumulative strain effect. Cracks occur at the grain boundaries of coarse grains in the same manner as the rolling at lower rate.
[0015]
In the present invention, in order to prevent cracking in rolling following the initial light reduction, up to half of the slab thickness (50% in cumulative reduction ratio) is rolled at 1000 ° C. or more and the time between passes is 10 seconds or more. It is necessary. The reduction ratio in this case does not need to be a light reduction ratio, and a normal pass schedule may be used. However, cracking is likely to occur at a low temperature, so that the temperature is set to 1000 ° C. or higher, and the time between passes from the viewpoint of preventing the cumulative strain effect. Requires more than 10 seconds.
[0016]
As a result of expanding the component range and examining the above relationship, it has been found that the present invention is composed of the following component systems.
That is, the austenitic stainless steel of the present invention is in mass %,
C: 0.03% or less, Si: 0.1-2.0%,
Mn: 0.5 to 2.0%, P: 0.04% or less,
S: 0.0030% or less, Cr: 16.0-25.0%,
Ni: 11.0-20.0%, Mo: 2.0-7.0%,
Cu: 1.0% or less, Al: 0.01 to 0.05%,
O: 0.005% or less, Ca: 0.0010 to 0.0050%,
N: 0.15-0.30%
And an austenitic stainless steel having a δcal of less than 0 in the following formula.
δcal = 3 (Cr + 1.5Si + Mo) −2.8 (Ni + 0.5Mn
+ 0.5Cu) -84 (C + N) -19.8
[0017]
The reasons for limiting the components are described below.
C: C is harmful to the corrosion resistance of stainless steel, and is set to 0.03% or less. If it is added in excess of this, the corrosion resistance deteriorates.
[0018]
Si: Si is used as a deoxidizing element for stainless steel and is effective at 0.1% or more. Even if it is added over 2.0%, the deoxidation effect is saturated, the hot workability is deteriorated, and the frequency of soot generation is increased.
[0019]
Mn: Mn has a deoxidizing effect, and an effect is seen at 0.5% or more. Moreover, since the effect is saturated even if it adds exceeding 2.0%, it adds at 0.5 to 2.0%.
[0020]
P: P is a harmful element from the viewpoint of corrosion resistance and hot workability, and it is desirable to reduce it as much as possible, and its component range is 0.04% or less.
[0021]
S: S is an element harmful to corrosion resistance and hot workability, and since it greatly affects hot workability, the lower the content, the better. The content was made 0.0030% or less.
[0022]
Cr: Cr is a basic component of stainless steel and needs to be added in an amount of 16.0% or more from the viewpoint of corrosion resistance. However, even if added over 25.0%, the corrosion resistance is saturated, and further in order to promote precipitation of intermetallic compounds in terms of hot workability, the content was made 16.0 to 25.0%.
[0023]
Ni: Ni is a basic component of stainless steel together with Cr. In the present invention, 11.0% or more is added from the relationship with the amount of Cr, and the upper limit is 20.0%. However, the upper limit is set to 20%.
[0024]
Mo: Mo is an important additive element for ensuring corrosion resistance, and an effect is seen with addition of 2.0% or more. Further, even if it exceeds 7%, the corrosion resistance is saturated, and further, the hot workability is deteriorated to promote the precipitation of intermetallic compounds, and wrinkles cannot be prevented even by the method of the present invention, so the upper limit was made 7%. .
[0025]
Cu: Cu is added at 1.0% or less in order to improve the corrosion resistance of stainless steel. Even if it is added in excess of this, the effect is saturated.
[0026]
Al: Al is added as a strong deoxidizer at 0.01% or more. However, even if added over 0.05%, the effect is saturated, and surface flaws due to Al oxides are more likely to occur, so the added amount was made 0.05% or less.
[0027]
O: O is an element that is extremely harmful to hot workability, and it is desirable to reduce its content as much as possible. Therefore, its content is set to 0.005% or less.
[0028]
Ca: Ca is a powerful deoxidizing and desulfurizing agent and an element effective for improving hot workability, and the effect is remarkable at 0.0010% or more. Further, even if added over 0.0050%, the effect is saturated, so 0.0010 to 0.0050% was set.
[0029]
N: N is an element added from the viewpoint of γ phase stabilization, corrosion resistance, and strength, and is added at 0.15% or more. Further, if added over 0.3%, the hot workability deteriorates, so the upper limit was made 0.3%.
[0030]
【Example】
Using a slab obtained by continuous casting of austenitic stainless steel (molten steel) containing the components shown in Table 1, it was hot-rolled under the conditions shown in Table 2 to produce a steel plate. A 20 mm steel plate was manufactured for steel A having a slab thickness of 190 mm, and a 10 mm steel plate was manufactured for steel B having a slab thickness of 140 mm, and the occurrence of cracks was observed. As a result, the steel sheet produced according to the present invention did not show any cracks, whereas cracks were observed in the comparative examples that did not satisfy the hot rolling conditions of the present invention.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
【The invention's effect】
According to the present invention, it is possible to prevent defects caused by austenite grain boundary cracking during hot rolling of austenitic stainless steel containing a large amount of Mo and N which are prone to cracking. This greatly contributes to the production of austenitic stainless steel sheets.

Claims (1)

% By mass
C: 0.03% or less,
Si: 0.1 to 2.0%,
Mn: 0.5 to 2.0%
P: 0.04% or less,
S: 0.0030% or less,
Cr: 16.0 to 25.0%,
Ni: 11.0-20.0%,
Mo: 2.0-7.0%,
Cu: 1.0% or less,
Al: 0.01 to 0.05%,
O: 0.005% or less,
Ca: 0.0010 to 0.0050%,
N: 0.15-0.30%
In the case of hot rolling a continuous cast slab of austenitic stainless steel, the balance of which is Fe and inevitable impurities, and δcal represented by the following formula is less than 0, heating is performed at 1150 ° C. or more, rolling From the start, rolling is performed at 1050 ° C. or more with a reduction rate of 3% or less at a reduction rate of 3% or less, and then, at a cumulative reduction rate of 50%, rolling is performed at 1000 ° C. or more with a time between passes of 10 seconds or more. A method for producing an austenitic stainless steel sheet free from wrinkles during hot rolling.
δcal = 3 (Cr + 1.5Si + Mo) −2.8 (Ni + 0.5Mn
+ 0.5Cu) -84 (C + N) -19.8