JPH10298646A - Production of stainless steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a stainless steel plate having high impact value, free from the occurrence of porous scale at the time of hardening, increased in hardening velocity, and suitable as material for cutting tool, etc. SOLUTION: A stainless steel, which has a composition consisting of, by weight, 0.25-0.34% C, 0.4-1.0% Si, 0.1-1.0% Mn, <=0.04% P, <=0.01% S, 12.6-14.0% Cr, 0-0.5% Ni, <=0.2% V, and the balance Fe with inevitable impurities and satisfying inequality C(%)<=0.04286×Cr(%)+0.88 (the part under the line connecting the points C, D in the figure), is used. This stainless steel is hot-rolled at <=950 deg.C finishing temp. and then annealed at 750 to 950 deg.C.

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 stainless steel sheet suitable as a material for a knife or the like.

[0002]

2. Description of the Related Art Stainless steel, such as SUS420, is used as a material for blades such as knives because it is hardened by quenching to impart abrasion resistance and has excellent corrosion resistance. Normally, a stainless steel cutting tool is subjected to annealing at a temperature of 750 to 950 ° C. for 1 to 10 hours on a stainless steel plate processed to a thickness of 1 to 3 mm by hot rolling, and then to cold rolling. After processing into the shape, it is punched into the shape of the product, and about 1050
After being kept at 10 ° C. for 10 to 20 minutes, air quenching or oil quenching is performed, and thereafter, the scale on the surface generated during heating is removed by polishing.

[0003] In the process of manufacturing a blade, problems arising from the characteristics of stainless steel frequently occur. The first problem is that the steel sheet cracks during cold rolling or die punching. Cracks during cold rolling are so-called edge cracks that occur at both width ends of the steel sheet and propagate toward the center of the width. When this crack occurs, the product yield decreases, and the edge crack increases, which may cause the steel sheet to break, which significantly impairs the safety of work. In order to prevent edge cracks, it is necessary to sufficiently increase the impact value (toughness) of the steel sheet before cold rolling.

A second problem is that a porous scale (or nodule-like scale) having a thickness of several tens μm is locally formed on the surface of a steel sheet during quenching. When this scale occurs, not only does the man-hour in the polishing process increase, but also the difference in the degree of decarburization reaction between the point where the porous scale occurs and the point where the normal scale occurs causes a lattice distortion difference in the amount of martensite transformation. This may cause the steel sheet to warp, thereby lowering the product yield. In addition, insufficient quenching and hardening may occur due to a decarburization reaction at a location where a porous scale is generated.

[0005] The third problem is that the condition of holding at about 1050 ° C. for 10 to 20 minutes, which is a heating condition for ordinary quenching, is to increase the line speed of the quenching step.
When the holding time is shortened to 3 to 5 minutes, the steel sheet becomes insufficiently hardened.

The following techniques have already been proposed as technical means for solving the above problems.

[0007] As a technical means for preventing the first problem, there is a method of improving the impact value by subjecting a steel sheet after hot rolling to a long-time batch annealing to soften the steel sheet. However, although this technique is an effective means for improving the impact value, it cannot solve the second and third problems at the same time. Further, as another technical means, the temperature of the steel sheet at the time of performing the cold rolling is not normal room temperature, but 50 to 1 mm.
There is also a method of preventing the occurrence of ear cracks by setting the temperature to 50 ° C. However, these technical means require additional equipment for long-time processing and heating, and increase the manufacturing cost.

As a technical measure for preventing the second problem, Japanese Patent Application Laid-Open No. 57-19362 discloses a technique for preventing the presence of Si in steel.
A steel sheet in which the generation of nodule-like scale is prevented by regulating the content to 0.5% by weight or more and 1.5% by weight or less and the N content to 0.035% by weight or less is disclosed. JP-A-57-19329 discloses that a steel having a specific composition is hot-rolled, annealed in a specific atmosphere and temperature, and quenched after polishing the surface roughness of the steel sheet to 11 μm or less with Hmax. A method for preventing nodule-like scale by performing a treatment is disclosed. But,
The steel sheet described in Japanese Patent Application Laid-Open No. 57-19362 does not sufficiently prevent the nodule-like scale, and can also solve the problem of occurrence of edge cracks and insufficient hardening when the quenching line speed is increased. Not. Further, in the method described in Japanese Patent Application Laid-Open No. 57-19329, the control of the atmosphere for annealing is complicated, and special polishing is performed.

As a technical measure for preventing the third problem, Japanese Unexamined Patent Publication No. 5-39547 discloses a stainless steel razor steel having a carbide density of 140 to 600 particles / 100 μm ^2. A method is _{disclosed in} which annealing is performed at a temperature equal to or higher than the A _c1 transformation point during hot rolling to define the dispersion density of chromium carbide in steel and increase the quench hardening rate. However, this method requires that the steel sheet be once brought into a heating furnace for annealing during the cold rolling, and then returned to the cold rolling device after the annealing, so that the manufacturing process becomes complicated and the manufacturing cost increases. Will rise.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a stainless steel sheet which has a high impact value, does not generate a porous scale at the time of quenching, and has a high quench hardening speed, and is suitable as a material for cutting tools. It is to provide a method of manufacturing without raising.

[0011]

The gist of the present invention resides in the following method for manufacturing a stainless steel plate.

"% By weight, C: 0.25 to 0.34%,
Si: 0.4 to 1.0%, Mn: 0.1 to 1.0%,
P: 0.04% or less, S: 0.01% or less, Cr: 1
2.6 to 14.0%, Ni: 0 to 0.5%, V: 0.2
% Or less and the following formula is satisfied, with the balance being stainless steel consisting of Fe and unavoidable impurities having a finishing temperature of 950.
A method for producing a stainless steel sheet, comprising a step of performing hot rolling at a temperature of at most ℃ and then annealing at 750 to 950 ° C.

C (%) ≦ −0.04286 × Cr (%) + 0.88... The method of the present invention solves the above three problems as follows.

[0014] The first occurrence of ear cracks occurs when the impact value of the steel plate is V
Occurs when the notch is less than 75 J / cm ² in the Charpy impact test. In order to improve the impact value, it is effective to disperse and precipitate chromium carbide such as Cr ₇ C ₃ generated in the steel sheet with a diameter of 3 μm or less. Coarse chromium carbide having a diameter of more than 3 μm is formed at a stage of being wound around a coil and gradually cooled after hot rolling. This diameter 3μm
In order to prevent the formation of coarse chromium carbides exceeding, the finishing temperature of hot rolling is specified to be 950 ° C. or lower in the method of the present invention. By setting the finishing temperature below this temperature, the crystal grain size of the steel sheet becomes smaller and the grain boundary area increases, and at the same time, many dislocations are introduced into the crystal grains. These grain boundaries and dislocations become chromium carbide nucleation sites, and the individual chromium carbides start to be dispersed and precipitated from a large number of nucleation sites, resulting in fine particles having a diameter of 3 μm or less.

The problem of the formation of porous scale on the surface of the second steel sheet occurs when the amount of Cr dissolved in the steel sheet during heating for quenching is small. The porous scale is composed of Cr dissolved in a matrix near the surface of the steel sheet.
(Not Cr forming carbides), Fe is oxidized instead of Cr, and Fe ₂ O ₃ or Fe ₃ O ₄
Which can be raised locally on the surface of the steel sheet. On the other hand, the scale that is desirably formed at the time of quenching is one in which Cr is oxidized to Cr ₂ O ₃ and can be uniformly and densely formed on the entire surface of the steel sheet. If the amount of Cr dissolved in the steel sheet during quenching and heating is large, formation of a porous scale can be prevented and a normal scale can be generated. In order to increase the amount of Cr that forms a solid solution, the amount of Cr that precipitates as chromium carbide in the steel sheet may be increased to form a solid solution during quenching and heating. In the method of the present invention, by adjusting the contents of C and Cr so as to satisfy a specific relational expression (the above expression), and by setting the finishing temperature of hot rolling to 950 ° C. or less, the diameter at the time of coil winding can be improved. Fine chromium carbide of 3 μm or less is precipitated. Fine chromium carbides dissolve at a higher rate during quenching and heating than coarse ones. If there is much chromium carbide to dissolve, Cr
Since the amount increases, a porous scale is not generated even by heating during quenching, and a normal scale such as Cr ₂ O ₃ is generated to cover the surface of the steel sheet.

If the heating time for the third quenching is shortened to about 3 to 5 minutes, the problem of insufficient hardening of the steel sheet occurs when the amount of C dissolved in the quenching heating is small. The quenching hardness of the steel sheet is not the C content in the steel, but is proportional to the amount of C dissolved during heating before quenching, and the quenching hardness of the steel sheet increases as the amount of dissolved C increases. Upon heating for quenching, the steel becomes a face-centered cubic austenitic phase and forms a solid solution of C between lattices. Transformation to martensite by quenching is accompanied by non-diffusion shear deformation, and the greater the amount of C between lattices, the greater the amount of strain and the greater the hardness. In the method of the present invention, the C content of the steel sheet is specified in order to increase the amount of C that forms a solid solution during heating for quenching. As described above, the finishing temperature of the hot rolling is specified to be 950 ° C. or less,
Increasing the dissolution rate of chromium carbide by finely dispersing and depositing chromium carbide during coil winding also contributes to increasing the solid solution C. If the dissolution rate of chromium carbide is high, even if heating for quenching is performed in a shorter time than before, the amount of C dissolved in the steel sheet increases, so that the problem of insufficient hardness of the steel sheet due to insufficient quenching does not occur.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION

(1) Chemical composition of steel to which the method of the present invention can be applied The percentages used in the description of the chemical composition mean weight%.

C: 0.25 to 0.34% C is one of the important elements that determine the impact value, scale type and quenching hardness of the steel sheet. C must be contained at 0.25% or more. If it is less than this, hardening by quenching becomes insufficient, and the Rockwell hardness according to JIS Z 2245 becomes HRC 55 or less. HRC55
Is the minimum hardness necessary for maintaining sharpness and durability as a blade. On the other hand, when the content exceeds 0.34%, coarse chromium carbides are generated, a porous scale is generated, and the impact value of the steel sheet is reduced.

Si: 0.4 to 1.0% Si is an element necessary for deoxidizing steel. Also Si
Is also an element that contributes to preventing the generation of porous scale. In order to obtain these effects, it is necessary to contain 0.4% or more. On the other hand, if the content of Si exceeds 1.0%, the impact value of the steel sheet deteriorates.

Mn: 0.1 to 1.0% Mn is an element necessary for deoxidizing steel. In order to obtain the effect, it is necessary to contain 0.1% or more.
However, when the Mn content is 1.0% or more, the phase balance of the steel changes, and after the quenching, the γ phase remains, thereby lowering the hardness of the steel sheet.

P: 0.04% or less P is an element that is inevitably mixed in the steel refining process, but it is preferable that P is as small as possible. If it exceeds 0.04%, the impact value and corrosion resistance of the steel sheet are deteriorated.

S: 0.01% or less S is an element that is inevitably mixed in the steel refining process. If it exceeds 0.01%, the impact value and corrosion resistance of the steel sheet are deteriorated. Therefore, the content is 0.01% or less,
And it is better to have as little as possible.

Cr: 12.6 to 14.0% Cr is an essential component for securing the basic characteristics (corrosion resistance and oxidation resistance) of stainless steel. Further, it is one of the important elements that determine the impact value, scale type and quenching hardness of the steel sheet. Cr must be contained at 12.6% or more. If it is less than this, a porous scale is generated during quenching. On the other hand, 14.0
%, The impact value and quenching hardness of the steel sheet are insufficient in the C content range of the present invention.

Ni: 0 to 0.5% Ni may not be added. If added, γ is generated to adjust the phase balance of the steel and improve the quench hardness. In order to reduce the variation in quenching hardness,
It is preferable to add 0.2% or more. However, 0.5%
If it exceeds, other α-forming elements must be added,
Increases component costs.

V: 0.2% or less V is an impurity element mixed in with the Cr raw material. If the V content exceeds 0.2%, the chromium carbide is less likely to be decomposed during quenching, which causes a reduction in the quench hardening rate of the steel sheet and the formation of a porous scale. V makes the carbide hard to decompose because the carbide is combined with V or Cr to form a composite carbide, and these composite carbides are not easily decomposed at the temperature of quenching and heating. In order to further reduce the adverse effect of V, the content is desirably 0.1% or less.

As described above, the content of each component must be adjusted to a predetermined range, and the content of C and Cr must satisfy the above formula. If this formula is not satisfied, the steel composition,
Even if the finishing temperature and annealing conditions of hot rolling are optimized, ear cracks during cold rolling and porous scale during quenching may occur, and the hardness after quenching may also be low.
FIG. 1 is a diagram showing the relationship between the contents of Cr and C in steel to which the method of the present invention can be applied. The area surrounded by oblique lines in the figure is C
And an appropriate content of Cr. The minimum Cr content of 12.6% is represented as a boundary line AD in the figure. C
14.0%, which is the maximum content of r,
Expressed as C. 0.25% which is the minimum content of C
Is represented as a boundary line AB in the figure. Boundary line DC
Is a straight line represented by the above equation.

(2) Hot Rolling and Annealing of the Method of the Present Invention Hot rolling: Hot rolling is performed on stainless steel slab formed by a continuous casting machine or a lump mill. The hot rolling conditions specified by the method of the present invention may be ordinary conditions except for the finishing temperature. For example, using a hot rolling device including a rough rolling mill and a finishing rolling mill, the steel sheet is finished to a thickness of about 1 to 3 mm. The finishing temperature of the hot rolling is 950 ° C. or less. The reason is as follows. During hot rolling (1100 ° C
Stainless steel is an austenitic single phase, and all chromium carbides in the steel are in solid solution. After hot rolling, precipitation of chromium carbide starts when the temperature of the steel sheet becomes 1000 ° C. or less. On the other hand, the reduction in hot rolling temperature reduces the crystal grain size of the steel and introduces many dislocations. As described above, the crystal grain boundaries and dislocations serve as nucleation sites when chromium carbide is generated. Therefore, as the hot rolling temperature is lower, the number of chromium carbide precipitation sites increases, and the carbides are finely dispersed and precipitated. If there are few grain boundaries and dislocations during the precipitation of chromium carbide, the number of precipitation sites is small, and the chromium carbide grows coarsely.

If the finishing temperature of the hot rolling exceeds 950 ° C., the crystals recover and grow by slow cooling after coil winding, so that many dislocations disappear and the crystal grains become coarse. In that situation, the number of precipitation sites is reduced, and the chromium carbide is coarsely dispersed and precipitated, grows, and becomes coarse. Therefore, in the method of the present invention, the finishing temperature of the hot rolling is set to 950 ° C. or less. Note that the lower limit of the finishing temperature is desirably about 700 ° C.

Annealing after hot rolling:
Since the wound coil is too hard, it cannot be developed and provided for a subsequent process. Annealing is performed for the purpose of reducing the amount of solute C in the steel sheet, softening the coil, and improving the toughness and workability of the steel sheet. The annealing conditions in the method of the present invention are as follows:
Normal conditions other than the heating temperature may be used. For example, annealing is performed at 750 to 950 ° C. in the air, in a hydrogen gas, in a mixed gas of hydrogen and nitrogen, or in an inert gas. Annealing at less than 750 ° C. cannot improve the toughness of the steel sheet. Annealing above 950 ° C. significantly shortens the life of the furnace refractory. The annealing time is preferably about 1 to 20 hours, and is preferably as short as possible from the viewpoint of reducing fuel cost and extending furnace life.

The main use of the stainless steel sheet produced by the method of the present invention is as a cutting tool. For example, pickling, cold rolling, and punching into a product shape are performed.
After heating to the extent, quenching is performed. The heating time at this time is
It can be much shorter than conventional ones. After quenching, the surface is polished to form a blade.

[0031]

EXAMPLE A stainless steel plate was manufactured by carrying out the method of the present invention and a method deviating from the conditions specified in the method of the present invention.
The steel plate was subjected to cold rolling and quenching, which is a process for manufacturing a blade or the like, and the impact value of the steel plate, edge cracking during cold rolling, quench hardening speed, and scale formed on the surface of the steel plate were investigated.

In a vacuum melting furnace, 25 kg of stainless steel having the chemical composition shown in Table 1 was melted, hot-rolled, and finished at various finishing temperatures shown in Table 2 to produce a hot-rolled sheet having a thickness of 3 mm. did. After annealing these steel sheets at various temperatures for 4 hours, they were pickled, cold rolled, and 1 mm thick.
Was prepared.

[0033]

[Table 1]

[0034]

[Table 2]

(1) Investigation of Impact Value and Edge Crack In order to measure the impact value of the hot-rolled sheet, a Charpy impact test specimen of 3 mm wide and 2 mm deep V-notch was measured from a part of the hot-rolled sheet having a thickness of 3 mm. The sample was collected and the impact value was measured at 25 ° C. Hot rolled sheets having an impact value of less than 70 J / cm ² were evaluated as having poor punchability.

Further, the maximum length of a crack generated when manufacturing a cold-rolled sheet having a thickness of 1 mm was measured, and a cold-rolled sheet having a crack having a length of 2 mm or more was determined to be defective.

Table 2 shows the results of the investigation and the manufacturing conditions.

The impact values of the stainless steel sheets produced by the method of the present invention all exceeded 70 J / cm ² , and no ear cracks of 2 mm or more occurred. Steel composition,
A steel sheet manufactured by a method in which the hot rolling finish temperature or the annealing temperature deviates from the conditions specified in the method of the present invention has an impact value of:
Some were less than 70 J / cm ² and some had ear cracks exceeding 2 mm.

(2) Investigation of scale generated during quenching and quench hardening rate 25 mm × 1 mm from a part of a 1 mm thick cold rolled steel sheet
A quenched test piece having a size of 35 mm was collected, a hole having a diameter of 3 mm was formed in the upper part of the test piece, and quenching treatment was performed. Hang the test piece with a steel wire and in air at 1050 ° C for 5 minutes or 1
After holding for 0 minutes, the sample was taken out into the atmosphere and air-quenched. In the scale generation investigation, a test piece heated for 10 minutes was used so as to clarify the state of scale generation. In order to investigate the effect of the method of the present invention for improving the quench hardening rate in a shorter heating time, a test piece heated for 5 minutes was used. The quench hardening rate was evaluated by examining the hardness.

A sample having a porous scale generated by quenching was regarded as defective. Further, those having a hardness of 54 or less by HRC were evaluated as having insufficient quench hardening speed.

Table 2 shows the results of the investigation. As is clear from Table 2, the steel sheet produced by the method of the present invention has no porous scale. These were densely covered with a normal Cr scale. Hardness is also HRC56 or more,
Hardening was sufficient after quenching after heating for 5 minutes. on the other hand,
Cold rolled stainless steel sheets produced by a method in which the steel composition, hot rolling finish temperature or annealing temperature deviates from the conditions specified in the method of the present invention are those having a porous scale and hardness of HRC54 or less. There was one.

[0042]

According to the method of the present invention, it is possible to produce a stainless steel sheet which has a high quench hardening rate and does not generate porous scales or edge cracks. Therefore, the productivity of the quenching step of the blade and the like can be increased, and the number of polishing steps can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the contents of Cr and C in steel to which the method of the present invention can be applied.

Claims (1)

[Claims] (1) C: 0.25 to 0.34% by weight, S
i: 0.4 to 1.0%, Mn: 0.1 to 1.0%, P:
0.04% or less, S: 0.01% or less, Cr: 12.6
To 14.0%, Ni: 0 to 0.5%, V: 0.2% or less, and hot stainless steel which satisfies the following formula and the balance is Fe and unavoidable impurities and the finishing temperature is 950 ° C or less. A method for producing a stainless steel sheet, comprising a step of rolling and then annealing at 750 to 950 ° C. C (%) ≦ −0.04286 × Cr (%) + 0.88...