JP3271787B2 - How to make stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic-austenite duplex stainless steel having excellent hot workability, in particular, small edge cuts during hot rolling and excellent toughness.

[0002]

2. Description of the Related Art Duplex stainless steels such as SUS329J4L have excellent resistance to stress corrosion cracking as compared with austenitic stainless steel and have excellent weldability as compared with ferritic stainless steel. Utilizing these properties, duplex stainless steel has been used in a wide range of fields as various structural materials. In particular, recently, it has attracted attention as a material for stainless steel water storage tanks.

[0003] Although duplex stainless steel has such excellent properties, it has poor hot workability. This is because the composition of the duplex stainless steel is a mixed composition of a ferrite phase and an austenite phase. That is, since two phases having different deformation resistances coexist, the stress applied during hot rolling is concentrated on the boundary between the ferrite phase and the austenite phase, and defects such as cracks and fractures occur at the phase interface. These defects appear as cut edges on the hot-rolled sheet and cause a reduction in product yield.

In order to improve the hot workability, various methods have been conventionally proposed. For example,
Japanese Patent No. 15660 discloses that the content of impurities such as S and O, which adversely affect the deformability of the austenite phase, is reduced to reduce the concentration of impurities segregated at the phase interface. In Japanese Patent Publication No. 59-14099, the S content which causes segregation at grain boundaries and causes cracks is reduced to 0.005% by weight or less, and hot workability is improved by adding B. .

Further, Japanese Patent Publication No. 1-19465 discloses that
It is introduced that the addition of Ca to a duplex stainless steel whose S content is reduced to 0.003% by weight or less significantly suppresses the solid solution S, and improves the hot workability by adding Al.

[0006] These prior arts all reduce the S content, and as a means for that, the rare earth metal R
Desulfurization elements such as EM and Ca are added. Alternatively, since it is difficult to reduce S alone, means such as reduction of the O content and addition of Al and Ca, which are deoxidizing agents, are employed.

However, when a duplex stainless steel equivalent to SUS329J4L, in which the S content is regulated to 0.003% or less, is melted and subjected to hot rolling, there are cases in which ear breaks occur and cases in which they do not occur. Was. That is, it was difficult to obtain stable hot workability. Conversely, when a duplex stainless steel having an S content exceeding 0.003% is hot-rolled, there are cases in which ear breaks occur and cases in which ears do not occur. Furthermore, when the duplex stainless steel produced by adding the third element disclosed in the above-mentioned prior art document was hot-rolled, a slight improvement in hot workability was observed as compared with the non-added material. However, stable hot workability was not always obtained.

The inventors of the present invention have studied to solve such a problem, and as a result, by quantitatively adjusting the balance between the ferrite phase and the austenite phase in relation to the S content, It has been found that hot workability can be stably improved, and hot work can be performed with good yield, and this is disclosed in Japanese Patent Application No. 2-204427.

[0009]

SUMMARY OF THE INVENTION SUS32 satisfying the composition requirements introduced in the prior application and having an S content of 23 ppm.
About 9J4L worth of duplex stainless steel is actually
0 tons were melted, and the resulting slab was hot rolled by a tandem hot rolling mill. As a result, coarse cuts of 70 mm or more were generated in any of the hot coils. This means that when hot-rolled in a laboratory, even if the steel has a component composition that shows sufficient hot workability, coarse strips may occur when hot-rolled in actual lines. Show.

The present invention has been devised in order to obtain a hot coil having the same good appearance as that of a laboratory even in an actual machine, and by reducing the S content in steel more strictly, the heat of the material can be improved. It is an object of the present invention to obtain a hot coil which is excellent in toughness even in an actual machine and has no coarse cuts by hot rolling with a reduction in a width direction during hot rolling while further improving hot workability.

[0011]

According to the present invention, there is provided a method for producing a stainless steel, comprising the steps of:
It is characterized in that it is heated at a temperature of 1200 ° C. or more for 150 minutes or more, and hot-rolled in the width direction as well as the thickness direction. Hot rolling is preferably performed at a temperature of 900 ° C. or higher. Further, the winding is preferably performed at a temperature of 600 ° C. or less.

[0012]

As the duplex stainless steel, for example, C
The basic composition of r: 20 to 30% by weight and Ni: 4 to 9% by weight is used, and the S content is adjusted to 0.001% by weight or less. This basic composition comprises 2-4% by weight of Mo
Can also be included.

[0014]

[Function] The hot workability of duplex stainless steel with reduced S content is divided into good and bad cases, and the hot workability between the laboratory and the actual machine is significantly different from the hot workability. It means that the influencing factor is not only the S content. Then, the present inventors investigated factors affecting hot workability other than the S content in steel in order to stably improve hot workability in an actual machine. As a result, the following findings were obtained.

What influences the hot workability of the duplex stainless steel is not the total S content, but the S segregated at the ferrite-austenite grain boundary. The binding energy of the ferrite phase and the austenite phase is weakened by S segregated at the grain boundaries, and the stress applied during hot rolling is concentrated on the boundary between the two phases having different deformability. As a result, defects such as cracks and breaks occur at the phase interface. From this, it is presumed that the hot workability of the duplex stainless steel is improved by reducing the S content precipitated at the grain boundaries.

The ferrite phase has a higher solid solubility of S than the austenite phase. Therefore, when the composition is adjusted to a composition in which a large amount of ferrite phase is formed, the content of S segregating at the ferrite-austenite grain boundary is reduced, and a duplex stainless steel excellent in hot workability is obtained.

On the premise of such a situation, the present inventors have described the above-mentioned Japanese Patent Application No. 2-204427 (Japanese Patent Application Laid-Open No.
No. 8151), a reduction in the S content and an increase in the amount of ferrite, which are conventionally qualitatively known, quantitatively determine the synergistic action of improving the hot workability of the duplex stainless steel. Figured out. That is, the phase ratio (austenite amount) of the duplex stainless steel can be quantitatively grasped by using the A value represented by the following equation obtained experimentally. A = 210 (C%)-5.8 (Si%) + 3.5 (Mn%) + 11.5 (Ni%)-5.8 (Cr%)-6 (Mo%) + 4.5 (Cu%) + 235 (N%) +100 Further, the relationship between the A value and the S content is expressed as 20 ≦ A ≦ 70−67.
By specifying the relational expression of 00 × (S%), it became possible to stably prevent ear breakage in a laboratory.

However, even when steel is rolled in a laboratory and has a component composition exhibiting sufficient hot workability, coarse strips may be formed when hot rolled in an actual line. The major differences between the hot rolling of the laboratory and the actual machine are that the total rolling ratio increases due to the difference in the slab thickness before hot rolling, the rolling speed increases, and the compression by rolling for rolling in coil form It is conceivable that stress and tensile stress between stands are also applied. In any case, much more strain is applied to the ferrite and austenite phases and their grain boundaries than in the laboratory. Therefore, the level of the amount of S segregation allowed for the reduction of the binding energy, and hence the total amount of steel in the steel, against the distortion when hot rolling in a laboratory and the distortion when rolling in a real hot rolling mill, The level of S content should be different.

An important finding in the present invention is that the width reduction H
Is controlled in the range of 5 to 20 mm. Width reduction H
The term “is” indicates a value obtained by subtracting the width of the hot coil after hot rolling (measured at room temperature) from the width of the slab before hot rolling (measured at room temperature). When the width reduction H becomes positive, it means that rolling is performed not only in the thickness direction but also in the width direction during hot rolling. The improvement of the hot workability by controlling the width reduction H was found by examining the difference between the hot rolling in the laboratory and the actual machine described below.

In the case of laboratory hot rolling, the thickness of the slap or ingot before hot rolling is 30 to 50 mm, and the thickness after hot rolling is 3 to 5 mm.
mm hot rolled sheet. With this amount of reduction, the spread of the plate in the width direction is slight. In the rolling of the actual machine, the thickness of the original slab becomes 200 mm or more, and plastic flow of the metal occurs in the width direction. At this time, a tensile stress is applied to the tip of the ear, and cracks are likely to occur due to a decrease in the binding energy of the ferrite-austenite grain boundary due to S segregation.

When a width reduction is applied to the slab in this state,
Compressive stress is applied and cracks are less likely to occur. Further, strain is applied by the width reduction, dynamic recrystallization occurs during hot rolling, and S
A new ferrite-austenite grain boundary in which is not segregated is generated. Under a width pressure of less than 5 mm, it is difficult to accumulate enough strain to cause dynamic recrystallization. However, in order to apply a width reduction exceeding 20 mm, a very large load is applied in the width direction, and the slab is deformed and the coil shape is deteriorated. From the above points, the width reduction H was set in the range of 5 to 20 mm.

It is known that the balance between the ferrite and austenite phases of a duplex stainless steel is not uniquely determined by the components but changes with the heat treatment temperature. In other words, at high temperatures, a structure close to a ferrite single phase,
Austenite precipitates in the temperature range around 800 to 900 ° C. in a nose to form a two-phase structure. This means that, when considering from the viewpoint of the difference between the S solid solubility and the deformability of the two-phase structure, it is more advantageous to extract and process at a high temperature of 1200 ° C. or more, which is rich in ferrite in hot workability. Indicates that there is. On the other hand, hot rolling at a temperature lower than 900 ° C. at which austenite precipitates is disadvantageous. Further, since the size of the slab to be handled is different between the hot rolling in the laboratory and the hot rolling in the actual machine, it is necessary to perform sufficient heating for 150 minutes or more before hot rolling in order to obtain a uniform structure up to the center.

Further, in a duplex stainless steel, an austenite phase is precipitated from a ferrite phase in a process of cooling from a high temperature and lowering the temperature. At this time, due to the difference in solid solubility of elements such as Cr in both phases, those elements are concentrated at the interface, and a sigma phase is precipitated in an extremely short time. Therefore, after hot rolling, the temperature around 700 ° C., which is the precipitation temperature range of the sigma phase, is avoided, and winding at a temperature of 600 ° C. or less can improve the toughness of the coil.

[0024]

The present invention has been completed on the basis of the above findings, and aims at strictly reducing the total S content in steel and improving the structure of the slab side by reducing the width. Further, control hot rolling under various hot rolling conditions is performed to improve the hot workability of the duplex stainless steel. Therefore, the regulation based on the A value disclosed in Japanese Patent Application No. 2-204427 is no longer an essential condition. In the case of duplex stainless steel, Mo, C
u, N, Ni, etc. may be added. Again,
The effect of the present invention is maintained as it is.

Hereinafter, alloy components and contents of the stainless steel used in the present invention will be described. S: A harmful element that segregates at the ferrite-austenite grain boundary and reduces the hot workability of steel. In addition, it forms sulfides with Mn in steel, and reduces pitting corrosion resistance and crevice corrosion resistance. From this point, the S content is preferably as low as possible. In the present invention, the upper limit is set to 0.001% by weight.

Ni: An alloy element that effectively acts on improving corrosion resistance and is indispensable for forming a two-phase structure. Regarding the Ni content, in order to obtain an appropriate structure in relation to the Cr content and the like, the content range is set to 4 to 9% by weight.

Cr: an alloy element indispensable for improving corrosion resistance. If the Cr content is less than 20% by weight, the corrosion resistance is not sufficient in a severe corrosive environment where a duplex stainless steel is used, and the ratio of the ferrite phase is reduced. Conversely, if the Cr content exceeds 30% by weight, a sigma phase is likely to precipitate, and the toughness, weldability and the like are deteriorated. Therefore, the Cr content is specified in the range of 20 to 30% by weight.

As for the slab heating conditions, it is more advantageous to perform hot working for a long time at a high temperature at which the amount of ferrite generated in the duplex stainless steel increases. When heat treatment is performed at a temperature lower than 1200 ° C., austenite is generated from the beginning of hot rolling, and the hot workability is reduced. In addition, in order to form a uniform structure of the slab up to the center in the actual hot rolling, it is necessary to sufficiently heat the hot slab over 150 minutes or more.
From the above points, the slab heating conditions were 1200 ° C. or more and 150 minutes or more.

The hot rolling is preferably performed in a region where the amount of austenite generated at a higher temperature is small. In other words, from the viewpoint of the difference between the S solid solubility in austenite and the deformability of the two-phase structure, it is disadvantageous to perform hot rolling under a condition of less than 900 ° C. at which the amount of austenite is maximized, and the hot rolling finish temperature 9
It is preferably at least 00 ° C.

Duplex stainless steel has a sigma phase precipitation temperature range near 800 ° C., and its deposition rate is high. In particular, when a strain is given as after hot rolling, the precipitation rate is further increased. Therefore, in order to avoid the sigma phase precipitation temperature range, it is important to wind at a temperature of 600 ° C. or less.

Other components contained in the stainless steel have the following effects. C: An element inevitably contained in steel. When the C content is reduced, generation of carbides is reduced, and workability is improved. Further, corrosion resistance and intergranular corrosion resistance are also improved. In this regard, it is desirable that the C content be 0.03% by weight or less.

Si: an element added to steel as a deoxidizing agent. However, when the Si content exceeds 1.0% by weight,
The sigma phase forming ability is increased, and the toughness and corrosion resistance are deteriorated. Therefore, it is desirable to set the upper limit of the Si content to 1.0% by weight.

Mn: an element useful for improving weldability. However, when the Mn content exceeds 2.0% by weight, the corrosion resistance decreases. Therefore, it is desirable to set the upper limit of the Mn content to 2.0% by weight.

P: An impurity that is unavoidably mixed and is an element harmful to hot workability and stress corrosion cracking resistance. However, keeping the P content extremely low leads to an increase in manufacturing costs from the viewpoint of steelmaking. Therefore, the upper limit of the P content is set to 0.
Desirably, the content is 04% by weight.

Mo: Mo is an element which increases the resistance to local corrosion in a corrosive environment containing Cl ions in cooperation with Cr. However, in a high Cr steel as in the present invention, Mo
If the content is less than 2% by weight, the synergistic effect of Cr and Mo is small, and the effect of improving the local corrosion resistance is not sufficient. However, 8% by weight of Mo, 4% in duplex stainless steel.
If it is added in excess of weight%, embrittlement due to sigma phase precipitation occurs, deteriorating workability, toughness and the like. So Mo
Is contained, it is desirable to regulate the content within a range of 2 to 8% by weight, preferably 2 to 4% by weight.

Cu: an element effective for improving stress corrosion cracking resistance, sulfurous acid gas corrosion resistance and the like. But,
When Cu is contained in excess of 1.0% by weight, hot workability is reduced. Therefore, when Cu is contained, the upper limit is desirably set to 1.0% by weight.

N: Similar to C, it is an austenite-forming element and forms a solid solution in the austenite phase to improve corrosion resistance. As a result, a decrease in the corrosion resistance of the austenite phase due to the slightly lower Cr and Mo contents compared to the ferrite phase is suppressed. In this respect, N is an essential component for maintaining the corrosion resistance of the duplex stainless steel in a well-balanced manner. Further, it also has the effect of suppressing the precipitation of the sigma phase and improving the toughness. In order to exert the above effects, it is necessary to maintain the N content at 0.08% by weight or more.
However, the N content exceeding 0.30% by weight exceeds the solid solubility, so that excess N causes bubbles to be generated in the steel and causes defects in the steel ingot. Therefore, the N content is set to 0.08
It is desirable to set it in the range of 0.30% by weight.

Ti and Zr: These elements exhibit a desulfurizing action together with a deoxidizing action, and are effective elements for reducing S which adversely affects hot workability. However, when Ti and Zr are added in excess of 0.5% by weight, they cause streak-like surface flaws. While avoiding this defect, Ti and Zr can be added up to 0.5% by weight.

Rare earth metal REM: Similar to Ti and Zr, has the effect of auxiliaryly reducing S. However, when the rare earth metal REM is added in excess of 0.1% by weight, a large number of non-metallic inclusions are formed in the steel, which reduces the cleanliness of the steel. Therefore, the content of the rare earth metal REM is limited to the upper limit of 0.1.
It is defined as 1% by weight.

A value: A value experimentally confirmed as an index for estimating the ratio of the ferrite phase to the austenite phase. It is desirable to adjust each alloy component in the above-mentioned range so that the A value is in the range of 20 to [70-6700 × (S%)]. If the A value is less than 20, the structure becomes close to a ferrite single phase, and a problem occurs in the toughness and corrosion resistance of the welded portion. On the other hand, if the A value is [70-6700 × (S
%)], Ear cuts may occur even when the S content is reduced to 0.001% by weight or less, and the hot workability becomes unstable. Therefore, the A value is set to 20 to [70-6].
700 × (S%)].

[0042]

EXAMPLE About 4 stainless steels having the components shown in Table 1
0 to 80 tons were melted in an electric furnace, and a slab was manufactured by continuous casting. After removing the surface flaw of the obtained continuous cast slab, it was hot rolled under the conditions shown in Table 2.

[0043]

Steel types A and B in Table 1 belong to a duplex stainless steel having a ferrite-austenite duplex structure. Steel type A is a steel of the present invention satisfying the S content of 0.001% by weight or less, and steel type B has an S content of 0.1% by weight.
0020% by weight of comparative steel.

[0045]

Table 2 shows the hot rolling conditions when the various stainless steels shown in Table 1 were hot rolled, the cut end state of the coil after hot rolling, and the Charbie impact value. A-1 to A-6 relate to the results of hot rolling of steel type A in Table 1. A-1 to A-3
Is within the scope of the present invention in terms of both the composition of the steel and the hot rolling conditions, and has a small edge cut of less than 20 mm and good toughness. A-4 has a small width reduction, so that the degree of edge cut after hot rolling is large. A-5 is the one in which the slab heating time was short, and A-6 was the one in which the heating temperature was low. Particularly, in the case of A-6, the component of the material was low S due to the small amount of reduction in width, but the cut-out state was poor. In addition, A-5 did not perform low-temperature winding after hot rolling, so that the toughness of the material was lowered and the Charby impact value was low.

B-1 to B-3 relate to the results of hot rolling of steel type B shown in Table 1. Since the S content of the steel exceeds 0.001% by weight, coarse ears are formed in each case. .

[0048]

[0049]

As can be seen from FIG. 1, which shows the results obtained by rearranging the maximum edge cut depth by the width reduction amount, in order to reduce the edge cut, a steel having a reduced S content was subjected to a width reduction of 5 mm or more. It turns out that it is necessary to add.

[0051]

As described above, in the present invention, the S content is reduced, and the hot workability of the duplex stainless steel is improved by applying a width reduction during hot rolling. Can be. Therefore, the occurrence of edge breaks during hot rolling is suppressed, and a hot-rolled sheet can be manufactured with a high yield.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of width reduction on the hot workability of a duplex stainless steel.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Toshiro Adachi 4976 Nomura Minami-cho, Shinnanyo-shi, Yamaguchi Prefecture Nissin Steel Works, Ltd. Steel Research Laboratory (56) References JP-A-58-3917 (JP, A) JP-A Sho 62-286604 (JP, A) JP-A-2-25203 (JP, A) JP-B-57-15660 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00 -8/10 C21D 9/46-9/48 C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. S: Duplex stainless steel slab whose composition is adjusted to 0.001% by weight or less at a temperature of 1200 ° C. or more.
Heating for 150 minutes or more, width reduction H becomes 5-20 mm
Hot rolling in the width direction as well as the thickness direction. 2. Cr: 20 to 30% by weight and Ni: 4 to
9% by weight as basic composition, S content 0.001% by weight
A duplex stainless steel slab adjusted as follows
Heat for 150 minutes or more at a temperature of 900 ° C or more
In the thickness direction so that the width reduction H in degrees is 5 to 20 mm.
Both are hot rolled in the width direction, and at a temperature of 600 ° C or less
A method for producing stainless steel, comprising winding . 3. Cr: 20 to 30% by weight, Ni: 4 to 9
% By weight, Mo: 2 to 4% by weight and Cu: 1.0% by weight or less
The following is the basic composition, and the S content is 0.001% by weight or less.
The adjusted duplex stainless steel slab is heated at a temperature of 1200 ° C. or more for 150 minutes or more, and hot rolled in a thickness direction and a width direction so that a width reduction H is 5 to 20 mm at a temperature of 900 ° C. or more. And further winding at a temperature of 600 ° C. or lower.