JPH0617504B2 - Method for producing high-carbon martensitic stainless steel suitable for quenching treatment - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high carbon content martensitic stainless steel suitable for quenching treatment by warm rolling.

For martensite stainless steel for knives, razors, knives, cutters and other household or industrial knives, and medical knives, mainly 13% Cr-0.3% C (SUS420J
2) Steel is applied. However, the sharpness of blades made of this steel is not necessarily good, which is one of the reasons why the reputation of "stainless steel blades cannot be cut."
By the way, since the sharpness and durability of the blade are proportional to the hardness of the cutting edge, it is natural that the hardness of the blade is insufficient with steel having a C content of about 0.3%. If the amount is increased to the level of carbon steel cutlery, even stainless steel can undergo sufficient quality evaluation. However, high carbon stainless steel containing 0.5% or more of C, which can be expected to have high quality, tends to leave huge eutectic carbides in the product blade, which causes insufficient quenching hardness and blade spillage. Furthermore, since the workability is extremely poor, only an inefficient method can be applied to the production of thin plates. In particular, since the cracking susceptibility in cold rolling is abnormally high as compared with other stainless steels, etc., the rolling reduction cannot be increased in cold rolling, which makes production difficult. As a result, it is inevitably a very expensive material, and although it has high quenching hardness and excellent quality, only a part of it is used as a high-grade blade. Therefore, it has been highly demanded industrially to simplify the manufacturing process of high carbon stainless steel used for high-grade cutlery and improve the material.

[Conventional technology]

The martensitic stainless steel used for blades has finely dispersed and precipitated carbides in the steel before quenching, from the viewpoint of improving quenching and tempering hardness and preventing spilling of the blades due to the loss of carbides in the steel. There is a need. However,
High carbon content martensitic stainless steel containing 0.5% or more of C crystallizes a huge eutectic carbide when solidification is completed, and forms a large carbide with a grain size of several tens of μm in the hot rolled annealed plate. To remain. For this reason, in the conventional case, a huge eutectic carbide in the steel is solid-dissolved by subjecting the cast steel ingot to soaking for a long time, and a uniform fine carbide structure is obtained by the subsequent spheroidizing treatment. ing. However, due to a long soaking process, a large amount of oxide scale and a decarburized layer are generated on the surface layer of the steel ingot, which significantly reduces the product yield and at the same time requires a large number of man-hours for the maintenance of the steel slab in each process. have. For this reason, for example, in JP-A-58-189322, after producing a steel ingot by electroslag melting, hot rolling / annealing is performed while controlling the hot rolling heating temperature to obtain a uniform fine spherical carbide structure. A method has been proposed. However, this manufacturing method requires special melting equipment, and the process is complicated, resulting in an increase in manufacturing cost.

Furthermore, high carbon content martensitic stainless steels are inferior in ductility and toughness compared to other stainless steels, and during cold rolling, giant carbides in the steel are the starting point, and edge cracks and plate fracture due to their progress occur. easy. Therefore, in the conventional case, cold rolling is performed by sandwiching a number of intermediate softening anneals to obtain a sheet having a predetermined plate thickness. But,
This conventional method requires a great deal of labor and cost,
Moreover, since the cumulative rolling reduction in one cold rolling is small, the giant carbides precipitated in the steel cannot be crushed, and it is not possible to obtain a finely dispersed carbide structure.

[Problems to be solved by the invention]

The present invention has been made by paying attention to the conventional problems as described above, and does not require special melting equipment or long-term soaking treatment, and further, inserts intermediate softening annealing a number of times during cold rolling. However, the present invention discloses a method for producing a high carbon content martensitic stainless steel having a uniform and fine spherical carbide structure suitable for quenching treatment.

[Means for Solving Problems] The present inventors set a hot-rolled sheet of high-carbon-containing martensitic stainless steel to a predetermined sheet thickness by rolling once without sandwiching intermediate annealing a number of times, Various studies were conducted on a method of obtaining a carbide structure suitable for the subsequent quenching treatment. As a result, the high carbon content martensitic stainless steel having a (ferrite + carbide) metallographic structure has improved impact characteristics and ductility as the working temperature rises, and by performing warm rolling, edge cracks and the resulting sheet It was found that no fracture occurred. Furthermore, it was found that when warm rolling with a cumulative reduction of 20% or more was performed, the giant carbides precipitated in the steel were crushed. The present invention is based on the above findings, that is, Cr: 10 to 20% by weight, C:
After forming a hot-rolled sheet of high-carbon martensitic stainless steel containing 0.5 to 1.2% into a (ferrite + carbide) structure, the cumulative rolling reduction is 2 in the temperature range of A ₁ transformation point to 150 ° C.
By performing 0% or more warm rolling, it is possible to obtain a predetermined plate thickness in one rolling without causing edge cracks and plate breakage due to its progress, and moreover, uniform fine spherical carbides suitable for quenching heat treatment. It is a method for producing a high carbon content martensitic stainless steel having a structure. The method of forming the steel of the component according to the present invention into a (ferrite + carbide) structure is performed by spheroidizing annealing in which the steel is heated at 700 ° C. to 850 ° C. for 30 minutes or more and gradually cooled, and A ₁ transformation point. Around the temperature A ₁
Known means such as spheroidizing annealing in which heating and cooling are performed a plurality of times between a transformation point temperature of + 100 ° C. and an A ₁ transformation point temperature of −200 ° C. and cooling is applicable.

[Work]

Hereinafter, the method of the present invention will be described in detail based on the results of experiments conducted mainly using 17% Cr martensitic stainless steel as an example. Fig. 1 shows 0.85% C, 1.0
17% Cr stainless steel containing 5% C and normal SU
S420J2 steel, 13% Cr stainless steel containing 0.35% C, was used as a test material with (ferrite + carbide) structure by spheroidizing annealing, and V notch shearpy impact value and edge crack or plate rupture were used. It shows the relationship with the cumulative rolling reduction that occurs. From Fig. 1, the impact value is 1.5kg
It has been found that when m / cm ² or more, rolling with a cumulative rolling reduction of 70% or more is possible without interposing softening annealing on the way.
Next, FIG. 2 shows the V-notch Charpy impact test results of annealed materials of 17% Cr stainless steel and SUS420J2 steel containing 0.85% C and 1.05% C. The impact value of 17% Cr stainless steel containing 0.85% C and 1.05% C is 20 to 100 ° C. which is a normal cold rolling temperature range.
Shows that it is extremely low. As shown in FIG. 2, the present inventors contain 0.85% and 1.05% of C 1
The impact value of 7% Cr stainless steel is 1.5 at 150 ° C or higher.
It has been found that the value is kg · m / cm ² or more, which is the same as or higher than the impact value of SUS420J2 steel at room temperature. In other words, this indicates from the results shown in FIG. 1 that by rolling in a temperature range of 150 ° C. or higher, rolling with a cumulative rolling reduction of 70% is possible in one rolling without interposing softening annealing. There is.

Next, the present inventors have studied in detail the influence of rolling conditions on the morphology of carbides, and by performing warm rolling with a cumulative reduction of 20% or more, giant carbides with a grain size of 20 μm or more were crushed. I was found to be done. High-carbon martensitic stainless steel forms voids around the giant carbides by cold rolling, but since the ductility of the matrix ferrite near room temperature is low, edge cracks and plate rupture occur before crushing the giant carbides. Cause However, when it is rolled at a warm temperature, the ductility of the matrix ferrite is improved, crack propagation is suppressed, and the plastic flow of the metal is sufficiently performed around the carbide, so that the giant carbide is crushed without cracking. It As a result, the rate of solid solution of the carbide is increased in the subsequent quenching treatment by increasing the surface area ratio of the carbide, the quenching / tempering hardness is improved, and the carbide structure after the quenching becomes uniform and fine, which makes it difficult to cause blade spillage.

From the above results, C: 0.5 to 1.2%, Cr: 10
After forming a hot-rolled sheet of a high carbon content martensitic stainless steel containing 20% to 20% into a (ferrite + carbide) structure, the cumulative rolling reduction is 20 in the temperature range of A ₁ transformation point to 150 ° C.
Invented is a method for producing a high carbon content martensitic stainless steel which is free from rolling cracks and which is suitable for the subsequent quenching treatment, which is characterized by performing warm rolling of not less than 10%.

Next, the reasons for limiting the components and manufacturing conditions will be described.

As a reason for limiting the C content, if the C content is less than 0.5%, it is not possible to obtain a high hardness that is desirable as a material for cutting tools, so 0.5% was made the lower limit. But,
If the C content exceeds 1.2%, the precipitation amount of carbides increases and the corrosion resistance decreases, and at the same time, the cutting edge becomes brittle, which easily causes chipping and chipping of the blade, and is therefore excluded from the present invention.

If the Cr content is less than 10%, the basic corrosion resistance of stainless steel is insufficient, so the lower limit is 10%. On the other hand, if the Cr content exceeds 20%, huge carbides are likely to be formed, and the hardness of quenching / tempering cannot be obtained. Therefore, they were removed from the claims of the present invention.

The reason why the hot-rolled sheet has a (ferrite + carbide) structure is that high-carbon content martensitic stainless steel has better hardenability than ordinary carbon steel. It becomes baked and becomes a martensite structure. Since the martensite structure has high hardness and extremely low toughness, it cannot be practically rolled. Therefore, it is necessary to perform a heat treatment such as spheroidizing annealing to form a structure having excellent ductility and toughness and low hardness (ferrite + carbide).

The reason why the rolling temperature during the warm rolling is set to the A ₁ transformation point or lower is that the temperature above the A ₁ transformation point causes austenite to precipitate in the parent phase and cause phase transformation during rolling, resulting in deterioration of hot workability. At the same time, after rolling, martensitic transformation is caused to significantly harden, and spheroidizing annealing for a long time is required again. Therefore, the upper limit temperature of rolling is set to the A ₁ transformation point. Further, as shown in FIG. 2, the temperature at which the high-carbon martensitic stainless steel can be rolled without causing edge cracks and plate breaks is 150 ° C. or higher, so the lower limit temperature of rolling is 150 ° C.
And

The reason why the cumulative reduction rate is set to 20% or more is that if the cumulative reduction rate is less than 20%, sufficient plastic flow of the metal is not performed to crush the giant carbide having a grain size of 20 μm in the steel.
This is because it does not form a uniform and fine spherical carbide structure suitable for quenching treatment.

〔Example〕

Steel with the chemical composition shown in Table 1 was hot-rolled and annealed at 800 ° C. for 1 hour to produce a hot rolled annealed sheet of high carbon content martensitic stainless steel with a structure of (ferrite + carbide). . When the size of carbides of this hot rolled annealed plate was measured and the presence or absence of giant carbides was examined, it was as shown in Table 1 together. In Table 1, ◯ indicates that giant carbides having a particle size of 20 μm or more were not deposited, and x indicates that they were deposited. No. 1 with low C content as shown in this table
There was no giant carbide in the hot rolled sheet. But C
No. 2, 3, 4, 5, whose content is 0.5-1.2%
In the high carbon content stainless steel of No. 6, huge carbide was present, which was an unfavorable structure.

When rolling was performed under the conditions shown in Table 2 using this hot-rolled annealed sheet, giant carbides were crushed in the rolling with a cumulative reduction of 20% or more in a temperature range of 200 ° C. or higher, and quenching
After the tempering treatment, the carbide was finely and uniformly dispersed. On the other hand, in rolling at a cumulative reduction of 20% or more at 20 ° C., edge cracks and plate breakage due to their progress occurred. Further, when the cumulative rolling reduction was 10%, edge cracking and plate breakage due to the progress thereof did not occur at any rolling temperature,
After the quenching and tempering treatment, huge carbide having a particle size of 20 μm or more remained.

[Effects of the Invention] As described above, since the high carbon content martensitic stainless steel has remarkably low ductility and high cracking susceptibility, conventionally, cold rolling was repeated many times with softening annealing in between. According to the method of the present invention, a high carbon content martensitic stainless steel having an improved crack susceptibility, a predetermined plate thickness obtained by one warm rolling, and a structure suitable for subsequent quenching treatment, which has a structure without a giant carbide, is obtained. It became possible to manufacture steel sheets.

As a result, not only the simplification of the process and the improvement of the material can be realized, but also the yield and the production efficiency can be expected to be greatly improved by the continuous rolling and the coil rolling.

As described above, the industrial merit of the present invention is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a V-notch Charpy impact value of an annealed high-carbon martensitic stainless steel and an edge crack and a cumulative reduction rate of occurrence of plate breakage due to its progress. In addition, FIG. 2 shows 0.85% C, 1.05% C
It is a V-notch Charpy impact test result of the annealed material of 17% Cr stainless steel and SUS420J2 steel containing the.

Claims (1)

[Claims] 1. Cr: 10-20% by weight, C: 0.5-
Hot rolled sheet of high carbon content martensitic stainless steel containing 1.2% by spheroidizing annealing (ferrite + carbide)
A method for producing a high carbon content martensitic stainless steel suitable for quenching, characterized by performing warm rolling with a cumulative reduction of 20% or more in a temperature range of A ₁ transformation point to 150 ° C after forming a structure.