JP2001140017A - Method for producing martensitic stainless steel hot rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple method for producing a martensitic stainless steel hot rolled steel sheet capable of suppressing the generation of cracking caused by a bandlike abnormal structure in a hot rolled steel sheet and reducing the generation of cracking at the time of the subsequent working. SOLUTION: At the time of subjecting a martensitic stainless steel slab containing, by mass, 0.10 to 0.40% C and 11 to 15% Cr to hot rolling, the draft in the hot rolling is controlled so as to satisfy R>=100 1-t1/f ([C], [P])}, (where R: the draft (%) in the hot rolling, t1: the finish sheet thickness (mm) of the hot rolled steel sheet, f([C], [P])=a×[P]+b×[C]+c, [C]: the C content (mass %) in the slab, [P]: the P content (mass %) in the slab, and a, b, c: constants).

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 hot-rolled steel sheet of low-C and medium-C martensitic stainless steel, and particularly to the prevention of cracks generated during hot-rolling and subsequent working such as cold rolling and shearing. And a method for producing a hot-rolled steel sheet having a thickness of 10 mm or less. The steel sheet referred to in the present invention includes a steel sheet and a steel strip.

[0002]

2. Description of the Related Art Low-C martensitic stainless steel represented by SUS410 has good corrosion resistance and machinability, and is used for general household goods such as forks and spoons.
Utilizing its high hardness, it is widely used for knives such as knives. The medium C martensitic stainless steel represented by SUS420J1 and SUS420J2 is SUS410J.
Since it has a higher C content as compared with U.S.C., it is harder and has good corrosion resistance, so that it is increasingly used for cutting tools such as knives.

[0003] These low-C martensitic stainless steels and medium-C martensitic stainless steels are generally melted by a converter, an electric furnace, or the like, or are further melted by AOD.
The slab is formed by continuous casting after secondary refining such as the VOD method and the RH method. Next, these slabs are formed into a hot-rolled steel sheet by hot rolling, and are usually softened and annealed after hot rolling.

[0004] Such a martensitic stainless hot-rolled steel sheet is often made into an end product by secondary working such as various cold workings represented by cold rolling and blanking. Conventionally, during this secondary processing, during the cold rolling process, the steel plate cracks and the plate cannot be passed. During the cold working process, the plate cracks and the operation is interrupted. In some cases, a two-plate-like defect is generated and the yield is reduced.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present inventors have observed the cross-sectional structures of low-C and medium-C martensitic stainless steel hot-rolled steel sheets and examined the relationship with the above-mentioned cracks and defects. We studied diligently. As a result, it was found that the above-mentioned cracks and defects were caused by a band-shaped abnormal structure formed almost at the center of the hot-rolled steel sheet and elongated in the rolling direction. This abnormal band-like structure is presumed to be related to the center segregation at the time of solidification of the slab. The band-like abnormal structure is elongated in the rolling direction by hot rolling, is thinly extended, and remains to some extent. Under such circumstances, there has been a demand for a method for reducing the occurrence of cracks and defects described above.

The present invention advantageously solves the above-mentioned problems of the prior art, reduces the extent of residual abnormal band-like structure, and suppresses the occurrence of cracks during secondary working. The purpose is to propose a manufacturing method.

[0007]

Means for Solving the Problems In order to achieve the above object, the present inventors have studied in more detail the abnormal band-like structure formed on a hot-rolled martensitic stainless steel sheet. As a result, precipitation of Cr carbide was observed in this band-shaped abnormal structure, and it was found that the precipitation was particularly remarkable when the P content was high. It was considered that when the P content was high, the complete solidification temperature of the molten steel was lowered, so that P, C, and the like were easily segregated at the final solidification position, and as a result, Cr carbide was likely to be precipitated.

[0008] Only from this fact, it is considered that the occurrence of band-like abnormal structure can be prevented by reducing the P content of the molten steel. If the amount of P in the molten steel is reduced, the solidification temperature of the molten steel is increased, segregation of P, C, etc. is reduced, and the precipitation of Cr carbide can be suppressed. However, lowering the P of molten steel has led to a longer refining time, lowering productivity and resulting in a rise in manufacturing costs, leaving a problem economically.

[0009] Further, it was considered that the band-like abnormal structure of the hot-rolled steel sheet can be eliminated by holding the slab at a high temperature for a long time and dissolving and diffusing the Cr carbide. It takes a long time of several days or more for solid solution diffusion, and also requires a special heating and holding furnace, and has many problems in terms of delivery time, manufacturing cost, and manufacturing equipment.

In view of the above, the present inventors did not prevent the center segregation from occurring, but divided the center segregation by adjusting the rolling reduction of hot rolling, thereby improving the adhesion. It was thought that the occurrence of cracks during secondary processing could be suppressed. By adjusting the rolling reduction in hot rolling to a predetermined value or more determined according to the C and P contents, the center segregation of the slab can be effectively separated, and the band-like abnormal structure of the hot-rolled steel sheet remains. Can be suppressed.

First, the results of basic experiments performed by the present inventors will be described. Cr with different C and P contents: 11 to 15
% Of martensitic stainless steel, and slabs having different thicknesses were hot-rolled into hot-rolled steel strips having a thickness of 10 mm. These hot-rolled steel strips were cut and test specimens were collected and subjected to a Charpy test. After the test, the cross-sectional structure was observed and the presence or absence of cracks caused by the abnormal band-like structure was adjusted. The result is shown in FIG. FIG. 1 shows the presence or absence of abnormal band-like tissue.
It is shown by the relationship between the slab thickness and the P content in the slab. Since all of the cracks exhibited a two-plate fracture surface at the center of the thickness, it was presumed that the fracture occurred due to the abnormal band-like structure.

From FIG. 1, as the P content increases, unless the slab thickness is increased, that is, unless the rolling reduction of hot rolling is increased, cracks due to the abnormal band-like structure are observed. Further, when compared with the same P content, as the slab thickness is not increased (the reduction ratio of hot rolling is increased) as the C content increases, cracks due to the abnormal band-like structure are observed.

That is, cracks caused by an abnormal band-like structure are not observed when the rolling reduction of hot rolling is increased to a predetermined value or more in accordance with the increase of the C content and the P content. From FIG. 1, from the viewpoint of preventing the occurrence of cracks caused by the abnormal band-like structure, t _s ≧ f ([C], [P]), where f ([C]
, [P]) = a × [P] + b × [C] + c, (a = 20 × 1
^{0 3/7, b = 2.5} × 10 2, c = 85), so as to satisfy the slab thickness t _S (i.e., knowledge was obtained that it is necessary to select the rolling reduction). Here, [C] and [P] indicate the mass% of C and P, respectively.

The present invention has been completed based on the above findings and further studies. That is, in the present invention, when the martensitic stainless steel slab containing 0.10 to 0.40% by mass and C: 11 to 15% by mass is formed into a hot-rolled steel sheet by hot rolling, The rolling reduction is a method for producing a hot-rolled martensitic stainless steel sheet characterized by adjusting the C content and the P content of the martensitic stainless steel slab. Hot rolling reduction R (%)
Is calculated by the following equation (1): R ≧ 100 {1−t ₁ / f ([C], [P])} (1) (where, R: rolling reduction (%) in hot rolling, t ₁ :
Finished thickness of hot rolled steel sheet (mm), f ([C], [P]) = a x
[P] + b × [C] + c, [C]: C content in slab (% by mass), [P]: P content in slab (% by mass),
a, b, c: constants) are preferably adjusted. ^{Incidentally, a = 20 × 10 3/} 7, b = 2.5 × 10 2, c =
85.

In the present invention, C: 0.10 to 0.1% by mass.
When a martensitic stainless steel containing 40% and Cr: 11 to 15% is used as a slab, and the slab is formed into a hot-rolled steel sheet by hot rolling, the thickness of the slab is set to the value of the martensitic stainless steel. A method for producing a hot-rolled martensitic stainless steel sheet characterized by being at least a predetermined value determined according to the C content and the P content,
In the present invention, the slab thickness t _s, the following equation (2) _{t S ≧ f ([C]} , [P]) ...... (2) ( here, t _S: Slab thickness (mm) , F ([C], [P])
= A × [P] + b × [C] + c, [C]: C content in steel (% by mass), [P]: P content in steel (% by mass), a,
b, c: constants).

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the composition range of a slab used as a starting material for manufacturing a hot-rolled martensitic stainless steel sheet of the present invention will be described. Hereinafter, mass% in the composition is simply expressed as%. C: 0.10 to 0.40% C is an element that increases the strength and hardness of a martensitic stainless steel sheet, and requires 0.10% or more to obtain a predetermined strength and hardness. On the other hand, when the content exceeds 0.40%, the increase in strength and hardness becomes remarkable, the ductility, workability, and toughness are reduced, and the secondary workability is reduced. For this reason, in the present invention, C is limited to the range of 0.10 to 0.40%.

Cr: 11 to 15% Cr is an element which forms a protective film and increases the corrosion resistance. In the present invention, the content of 11% or more is required. Meanwhile, 15
%, The ferrite phase increases, and a sufficient increase in hardness due to martensitic transformation cannot be obtained. For this reason, Cr is limited to the range of 11 to 15%.

The content of alloying elements other than C and Cr does not need to be particularly limited in the present invention.
n: 1.5% or less, P: 0.040% or less, S: 0.030% or less, N: 0.100% or less, Al: 0.05% or less, with the balance being Fe and unavoidable impurities. In addition, in order to improve ductility and corrosion resistance, if necessary, Ni: 0.70% or less,
In order to increase strength and improve corrosion resistance,
Mo: 1.0% or less may be contained. Also, Nb: 1.0% or less for increasing strength, and Cu: 1.% for improving corrosion resistance.
It may contain 0% or less.

The content of alloying elements other than C and Cr is as follows:
Apart from the above range, JIS SUS 403, SUS 41
0, SUS410S, SUS410J1, and SUS410J2 may be within the specified range. In the present invention, the molten steel having the above-described composition is smelted by a commonly known converter, an electric furnace, or the like, or further subjected to secondary refining such as a commonly known AOD method, a VOD method, and a RH method, and then to a generally known method. The slab is preferably formed by the continuous casting method.

In the present invention, the thickness of the slab is preferably not less than a predetermined value determined according to the C content and the P content of the martensitic stainless steel. The predetermined value of the slab thickness in the present invention is a thickness that satisfies the following expression (2) so that cracks due to the abnormal band-like structure do not occur. t _s ≧ f ([C], [P]) (2) where, t _s : slab thickness (mm), f ([C], [P]) = a × [P] + b × [ C] + c, [C]: C content (% by mass) in steel, [P]: P content (% by mass) in steel, a, b, c: constants when formula (2) is not satisfied In the case of, a reduction ratio of a predetermined amount or more in hot rolling cannot be secured, and cracks due to an abnormal band-like structure occur.

Next, these slabs are formed into a hot-rolled steel sheet by hot rolling. Heating conditions for hot rolling are not particularly limited, but in order to reduce the load during hot rolling, from the viewpoint of surface properties such as hot rolling properties and prevention of surface roughening of the product surface, the range is 1100 to 1300 ° C. Is preferred. In the present invention, the center segregation of the slab is divided by the reduction in hot rolling. In order to separate the center segregation, it is necessary to apply a strong reduction of a certain value or more in hot rolling. When the P content and the C content are large, the center segregation becomes strong. Therefore, in the present invention, the rolling reduction R of the hot rolling is adjusted according to the P content and the C content of the slab. The adjustment of the rolling reduction R is preferably performed by changing (thickening) the slab thickness when the finished plate thickness t ₁ is constant.

Hot rolling reduction R (= (t _s −t ₁ ) /
t _s and t _s are the slab thickness, and t ₁ is the thickness of the hot-rolled steel sheet). It is preferable to adjust them so as to satisfy the following expression (1). R ≧ 100 {1-t ₁ / f ([C], [P])} (1) where, R: rolling reduction (%) in hot rolling t ₁ : finished sheet thickness of hot-rolled steel sheet ( mm) f ([C], [P]) = a × [P] + b × [C] + c [C]: C content (mass%) in slab [P]: P content (mass) in slab %) A, b, c: It is preferable to adjust so as to satisfy the constants.

The constants a, b, and c are appropriately determined depending on the thickness of the finished plate. For example, 10mm thick
The following hot-rolled steel ^{sheet, a = 20 × 10 3/} 7, b = 2.5 × 1
0 ² , c = 85. Under hot rolling conditions that do not satisfy the expression (1), a band-like abnormal structure occurs, and cracks and defects occur during secondary working such as cold rolling and cold working.

After the hot rolling, the steel sheet is usually softened and annealed to provide a hot rolled sheet (steel strip) for subsequent processing to obtain a final product. Subsequent processing includes various types of cold processing represented by cold rolling and blanking.

[0025]

EXAMPLES Martensitic stainless steels shown in Table 1 were melted by a converter-RH method and slabs of various thicknesses were produced by a continuous casting method. These slabs were heated and hot-rolled at a reduction ratio shown in Table 2 by hot rolling to obtain a hot-rolled steel sheet having a finished plate thickness shown in Table 2. Specimens were sampled from these hot-rolled steel sheets and subjected to a Charpy test, and the cross section was examined for the occurrence of cracks due to the abnormal band-like structure. Table 2 shows the results.

Furthermore, softening annealing (800
° C) and subjected to cold rolling (cold working) of 20 to 60% or shearing. The occurrence of cracks in the processed product was investigated. Table 2 shows the results.

[0027]

[Table 1]

[0028]

[Table 2]

In any of the examples of the present invention, no cracks due to the abnormal band-like structure were generated in the hot-rolled state, and no cracks or defects were generated by the subsequent cold rolling or shearing. On the other hand, in Comparative Examples outside the range of the present invention, cracks caused by the abnormal band-like structure were observed in the hot-rolled state, and cracks and defects were found in the secondary processing.

[0030]

According to the present invention, it is possible to suppress the occurrence of an abnormal band-like structure in a hot-rolled steel sheet, to prevent the occurrence of defects such as cracks in the subsequent processing, to improve the production efficiency and the product yield. It can be expected, and has a special industrial effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the slab thickness and the P content on the occurrence of cracks caused by an abnormal band-like structure.

Continued on the front page (72) Inventor Toshihiro Kasamo 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term inside the Chiba Works, Ltd. CA02 CA03 CB02 CF03 CG01 CG02

Claims (2)

[Claims] C .: 0.10 to 0.40%, Cr: 11 to 100% by mass
In making a martensitic stainless steel slab containing 15% into a hot-rolled steel sheet by hot rolling, the rolling reduction in the hot rolling is determined according to the C content and the P content of the martensitic stainless steel slab. And producing a hot-rolled martensitic stainless steel sheet. 2. The method for producing a hot-rolled martensitic stainless steel sheet according to claim 1, wherein the rolling reduction R in the hot rolling is adjusted so as to satisfy the following equation (1). Note R ≧ 100 {1-t ₁ / f ([C], [P])} (1) where, R: rolling reduction (%) in hot rolling t ₁ : finished sheet thickness of hot-rolled steel sheet (Mm) f ([C], [P]) = a × [P] + b × [C] + c [C]: C content in slab (% by mass) [P]: P content in slab ( A, b, c: constants