JPH04329824A - Production of martensitic stainless steel for cold forging - Google Patents

Abstract

PURPOSE:To produce a martensitic stainless steel for cold forging capable of making the most of the superior characteristics of SUS42011 and SUS42012 materials by means of a new heat treatment method capable of providing low hardness and tensile strength by annealing. CONSTITUTION:The steel can be produced by repeatedly subjecting, twice or more, a steel which has a composition consisting of, by weight ratio, 0.16-0.40% C, <=1.00% Si, <=1.00% Mn, <=0.030% S, 12.0-14.0% Cr, and the balance Fe with impurity elements and further containing, if necessary, one or >=2 kinds among 0.05-1.50% Mo, 0.01-0.20% Ti, 0.01-0.20% V, and 0.01-0.20% Nb to heat treatment consisting of heating up to 800-950 deg.C, holding for 2-16hr, and cooling down to a temp. between 600 deg.C and the Ar1 transformation point.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is used for bolts, nuts, shafts, etc. for home appliances, roof plate fixings, etc., and has excellent cold forging properties in the annealed state, as well as corrosion resistance and quench hardenability. The present invention relates to a method for producing martensitic stainless steel for cold forging which has excellent properties.

0002

[Prior Art] Among bolts, nuts, shafts, etc. for home appliances, roof panels, etc., parts that are processed into a predetermined shape by cold forging and that require high strength and excellent corrosion resistance are Processable hardness can be easily obtained during machining,
Materials that can provide high strength when used after processing are required. Conventionally, SU was used as a material to meet the above requirements.
13% Cr martensitic stainless steels such as S410, SUS420J1, and SUS420J2 are used.

[0003] These materials are made hard enough to be cold-worked by annealing after hot rolling, and after being cold-worked into a predetermined shape, they are quenched and tempered to give them high strength and toughness. ing.

0004

[Problems to be Solved by the Invention] Among the above-mentioned bolts, nuts, shafts, etc., parts that require particularly high strength are made using SU, which has superior strength compared to other stainless steels.
S420J1 and SUS420J2 are used. However, although SUS420J1 and SUS420J2 can obtain high strength after quenching, they are annealed under the commonly used annealing conditions specified in JIS G4303.
With heat treatment methods such as heating to 0°C and then slow cooling, or heating to 750°C and then air cooling, it is not possible to obtain a tensile strength of 57kgf/mm2 or less, which is considered an easy standard for cold forging, and it is not possible to obtain a tensile strength of 60kgf/mm2 or more. This results in a high tensile strength.

[0005] In particular, SUS420J2 has an extremely high tensile strength of about 65 to 70 kgf/mm2 when subjected to normal annealing, which places a heavy burden on the mold, punch, etc. during cold forging, making cold working virtually impossible. The current situation is close to that.

[0006]However, SUS420J1, SUS
420J2 has higher hardness in the hardened state than other stainless steels and provides excellent strength, so there has been a strong desire to take advantage of this and develop a new heat treatment method that can reduce the hardness after annealing.

The present invention has been made to solve the above-mentioned problems, and provides a new heat treatment method that can obtain low hardness and tensile strength by annealing.
The purpose of the present invention is to provide a method for manufacturing martensitic stainless steel for cold forging that can make the most of the excellent characteristics of S420J2 material.

[0008]

[Means for Solving the Problems] The present inventors have repeatedly studied various annealing conditions that can efficiently reduce the annealing hardness of the steel, and have obtained the following findings. Got an invention.

That is, by repeating the process of heating and holding at a temperature just above the A1 transformation point, then cooling to a temperature just below the A1 transformation point, and heating and holding again at a temperature just above the transformation point, coarse dispersion of carbides progresses. It has been found that uniformly dispersed spherical carbide can be obtained, and as a result, it is highly effective in reducing tensile strength.

[0010] It was also confirmed that by appropriately setting the heating holding time and heating temperature, the crystal grain size could be easily adjusted to a grain size of about 5 to 10, which has a high limit processing rate and is optimal in terms of strength. .

[0011] As a result of searching for the optimal heat treatment conditions based on the knowledge obtained above, we found that even when the same steel is heat treated, it is approximately 10kgf lower than the conventional heat treatment method specified by JIS.
We have found heat treatment conditions that can reduce the tensile strength by about /mm2, and succeeded in suppressing the tensile strength of SUS420J1 and SUS420J2 after heat treatment to 57 kgf/mm2 or less, which is easy to cold forge.

[0012] Also, SUS420J1, SUS420J
As a result of investigating the improvement of strength in the quenched and tempered state of 2, it was found that adding small amounts of Mo, Ti, V, and Nb is effective in improving strength without having almost any effect on the tensile strength during annealing. This is what I also found.

The first aspect of the present invention obtained based on the above findings
In the invention, the weight ratio of C: 0.16 to 0.40%, S
i: 1.00% or less, Mn: 1.00% or less, S: 0.
030% or less, Cr: 12.0 to 14.0%, and the balance is Fe and impurity elements.
After heating to a temperature of 0 to 950°C and holding for 2 to 16 hours, heating to 600°C to Ar1 at a rate of 5 to 50°C/hr.
A second invention is a method for manufacturing a martensitic stainless steel for cold forging, characterized in that a heat treatment of cooling to a transformation point temperature is repeated two or more times, and the second invention is a method for manufacturing a martensitic stainless steel for cold forging, which is characterized in that the steel is further subjected to Mo: 0.05 ~ 1.50%, T
i:0.01~0.20%, V:0.01~0.2
0%, Nb: 0.01 to 0.20%, and the strength in the quenched and tempered state is further improved compared to the first invention.

Next, the reason for limiting the target steel components in the method of manufacturing martensitic stainless steel for cold forging of the present invention will be explained.

C: 0.16-0.40% C is an element necessary to ensure the necessary strength, and in order to obtain excellent strength after quenching and tempering, it should be contained at 0.16% or more. It is necessary. Within the scope of the present invention, C
The higher the content, the higher the strength obtained, but if particularly high strength is required, for example, quench hardness of Hv600 or more is required, the content should be 0.30% or more. However, if it increases too much, the strength of the annealed state will increase,
Since cold forging becomes difficult, the upper limit was set at 0.40%.

Si: 1.00% or less Si is an element that is effective in deoxidizing, but it is also an element that increases strength through solid solution strengthening and reduces cold forgeability, so the upper limit is set at 1.00%. did. Therefore, when it is necessary to obtain particularly excellent cold forging properties, it is desirable to set the content to 0.30% or less.

Mn: 1.00% or less Like Si, Mn is an element that is effective in deoxidizing, and is also an element that increases strength through solid solution strengthening and reduces cold forgeability. Therefore, since a large amount of content is not desirable, the upper limit was set at 1.00%. Therefore, like Si, if particularly excellent cold forgeability is required, it is necessary to reduce it as much as possible, and it is desirable to keep it at 0.40% or less.

[0018] S: 0.015% or less S is an element that generates MnS, which becomes a starting point for cracks, during cold forging, and significantly reduces cold forging properties, as well as deteriorating corrosion resistance. %. In particular, when placing importance on cold forgeability, it is desirable that the content be 0.005% or less.

Cr: 12.00-14.00% Cr is a basic element that imparts excellent corrosion resistance to stainless steel in the steel subject to the present invention, and must be contained in an amount of 12.00% or more. However, Cr is a strong ferrite-forming element, and if contained in a large amount, it impairs hardenability, so the upper limit was set at 14.00%.

[0020] Mo: 0.05-1.50%, Ti: 0
．． 01 ~ 0.20%, V: 0.01 ~ 0.20%
, Nb: 1 or 2 of 0.01 to 0.20%
More than a species. Mo, Ti, V, and Nb are elements that contribute to improving the strength in the quenched and tempered state when added in small amounts.
Strength can be improved by adding it as necessary. In order to obtain the above effect, Mo should be at least 0.05%,
Ti, V, and Nb must each be contained at 0.01%. However, if it is contained in a large amount, hardenability will be impaired, so the upper limit is 1.50% for Mo and 0% for Ti, V, and Nb.
．． It was set at 20%.

Next, the reasons for limiting the heat treatment conditions in the present invention will be explained.

[0022] The heating temperature was set at 800 to 950°C because the grain size was set at 5 to 1 in order to obtain excellent cold forging properties.
control in the range of 0 to obtain uniformly dispersed spherical carbides,
This is because it is the optimum temperature for efficiently reducing hardness. If heat treatment is performed at a temperature below 800°C, the crystal grains will become too fine and the strength will not be sufficiently reduced, and if the temperature exceeds 950°C, uniformly dispersed spherical carbides will not be obtained.
The hardness does not decrease sufficiently, and the crystal grains become too coarse, making it easy to crack during cold forging and making processing difficult.

The reason why the heating holding time is limited to 2 to 16 hr is that if it is less than 2 hr, the growth of crystal grains will be insufficient and the strength in the annealed state will increase.
This is because, if it exceeds this, the grains become too large and the strength decreases, but it also becomes more likely to crack during cold forging, making cold working even more difficult.

The reason why the final cooling temperature was limited to the range of 600° C. to Ar1 transformation point is that if the cooling is incomplete, the transformation of γ → α + carbide necessary for coarse dispersion of carbide will not be completed. Once the transformation is completed, there is no change in the structure even if the material is cooled further than that, and if it is heated again, extra energy is consumed and the heat treatment time becomes longer, which is not suitable for practical use, so the lower limit was set at 600°C. Note that the Ar1 transformation point is approximately 750°C in the case of the steel targeted by the present invention.

The reason why the cooling rate is limited to 5 to 50°C/hr is because if the cooling rate is too fast, the transformation of γ→α+ carbide will not occur sufficiently, and if the cooling rate is less than 5°C/hr, the heat treatment will not be possible. This is because it takes too much time and is not practical.

[0026]

EXAMPLES Examples of the present invention will be explained in comparison with comparative examples and conventional examples to clarify the characteristics of the present invention. Table 1 shows the chemical components of the test materials used as examples.

[0027]

[Table 1]

In Table 1, Steels 1 to 13 are steels to which the present invention applies, Steels 1 to 6 correspond to the first invention, and Steels 7 to 13 correspond to the second invention. Also, steels 14 to 16 are comparative steels, of which steel 14 is a comparative steel with a high C content, and steel 15 is a comparative steel with a high C content.
A comparative steel with a low content, Steel 16, is a comparative steel with a low Cr content.

Test materials having the components shown in Table 1 were melted in an electric furnace and prepared by hot rolling to produce wire rods with a wire diameter of 20 mm. Using this sample material, heat treatment (annealing) was performed using the method described above, and the tensile strength, area of area, and crystal grain size in the annealed state were measured. In addition, in order to evaluate the strength during use, the hardness in the quenched state, the hardness and tensile strength in the quenched and tempered state were measured.

[0030] The annealing treatment was carried out by holding the sample material under the temperature and time conditions shown in Table 2 below, and cooling it in a furnace to 650°C at a cooling rate of 20°C/hr, which was repeated twice. . Quenching treatment was performed by holding at 980℃ for 1 hour, cooling with oil, and then heating at 750℃ for 1 hour.
It was tempered under these conditions and its strength characteristics were evaluated.

[0031] For tensile strength and reduction of area, a JIS No. 4 tensile test piece was prepared, and a 25t autograph manufactured by Shimadzu Corporation was used.
Measurement was performed at a tensile speed of 1 mm/min.

[0032] The grain size was determined by measuring the grain size number based on the method for measuring ferrite grain size of steel specified in JIS G0552.

[0033] In order to confirm that the necessary corrosion resistance was obtained, the corrosion loss was measured in the tempered state. Corrosion loss is 5% NaCl-2% H2O2
The measurement was performed under the condition of immersion in an aqueous solution at 0° C. for 24 hours. In the case of this test condition, a corrosion resistance of 3 g/m2·hr or less is a guideline for passing the corrosion resistance.

Table 2 shows the results measured by the above method.

As is clear from Table 2, when comparing the characteristics of comparative steel treated with the heat treatment method of the present invention with that of the subject steel of the present invention, steel No. 14 has a high C content, so it is not annealed. Even after the tensile strength is 57kgf/mm2
15 steel has a low C content, so its strength after annealing decreases and it has excellent cold forgeability, but the quenched state It has low hardness and is inferior in strength when made into a product. Further, since steel No. 16 has a low Cr content, it has excellent properties in terms of strength, but it has poor corrosion resistance.

On the other hand, steels 1 to 13 which are the subject of the present invention
The properties of steel treated by the method of the present invention are that the tensile strength after annealing is an excellent value of 57 kgf/mm2 or less, and the strength and corrosion resistance after quenching and after quenching and tempering are also excellent. We were able to obtain excellent results.

[0037] Furthermore, steels 7 to 13 containing Mo, Ti, V, and Nb have improved hardening and quenching properties without affecting the strength in the annealed state, compared to steels 1 to 6 that do not contain the above components. It was confirmed that the strength of the quenched and tempered state can be improved.

Next, in order to clarify the optimum annealing conditions, another example will be shown in which the characteristics in the annealed state were investigated when the heat treatment conditions were changed.

Table 3 shows 1 and 4 of the test materials shown in Table 1.
, 6, 10, and 12 steels were used and heat treated under various annealing conditions, and the tensile strength, hardness, area of area, grain size, and limit workability were investigated. Test No. 1
-6 correspond to the present invention, and No. Nos. 7 to 12 are comparative examples that partially do not satisfy the conditions of the present invention; 13 and 14 are S
This is a conventional example in which US420J1 and SUS420J2 were subjected to normal heat treatment specified by JIS.

The values of tensile strength, hardness, area of area, and grain size shown in Table 3 were measured in exactly the same manner as in the examples shown in Table 2. Moreover, the measurement of the limit working rate was evaluated by performing a compression test (using a notched test piece) based on the standards of the Cold Forging Subcommittee of the Japan Society for Working of Plastics. The limit processing rate is ○, 55% when no cracks occur at a compression rate of 55%.
In the cases where cracks occurred at the following compression ratios, the compression ratios were such that the crack occurrence rate was 50%.

[0041]

[Table 3]

As is clear from Table 3, when the characteristics in the annealed state are compared between the comparative example, the conventional example, and the heat treatment according to the present invention, No. Samples 7 to 10 do not satisfy the conditions of the present invention in terms of heating temperature, number of heating times, cooling rate, and heating time, so the growth of crystal grains is insufficient, the strength is not sufficiently lowered, and the cold forging property is poor. It is No.
No. 11 has too long heating time, so the strength decreases but the crystal grains become coarser and the limit processing rate is inferior.
．． Since the heating temperature is high in No. 12, the limit working rate is inferior like No. 11, and uniform spherical carbides cannot be obtained, resulting in insufficient reduction in strength and extremely poor cold forging properties. In addition, the conventional example No. 13 and 14 have tensile strength of 6
The value is more than 3kgf/mm2, especially No. 14
exceeds 70 kgf/mm2, making cold forging virtually impossible.

On the other hand, No. 2 that satisfies the conditions of the present invention
．． In Nos. 1 to 6, the heating temperature, heating time, cooling temperature, and cooling rate were adjusted to appropriate conditions, and heating and cooling were repeated two or more times above and below the A1 transformation temperature, thereby uniformly dispersing spherical carbides. Since we were able to obtain the optimum grain size, we were able to obtain excellent cold forgeability (low strength and high limit workability).

[0044]

Effects of the Invention The method for manufacturing martensitic stainless steel for cold forging of the present invention has been developed by discovering a new heat treatment method in which heating and cooling are repeated twice or more above and below the Ar1 transformation temperature. The tensile strength of can be suppressed to 57 kgf/mm2 or less,
Cold working for forming parts is now much easier than when conventional annealing is performed. Therefore, the heat treatment method of the present invention can fully utilize the high strength of SUS420J1 and SUS420J2, and can be applied to parts that particularly require cold working and require both strength and corrosion resistance. It is something to do.

[Table 2]

Claims (2)

[Claims] [Claim 1] C: 0.16 to 0.40 in terms of weight ratio
%, Si: 1.00% or less, Mn: 1.00% or less,
S: 0.030% or less, Cr: 12.0 to 14.0%
A steel containing iron with the remainder consisting of Fe and impurity elements is heated to a temperature of 800 to 950°C, held for 2 to 16 hours, and then heated to 600°C at a rate of 5 to 50°C/hr.
Ar1 A method for producing martensitic stainless steel for cold forging, characterized by repeatedly performing heat treatment of cooling to a transformation point temperature two or more times. [Claim 2] C: 0.16 to 0.40 in terms of weight ratio
%, Si: 1.00% or less, Mn: 1.00% or less,
S: 0.030% or less, Cr: 12.0 to 14.0%
further contains Mo: 0.05 to 1.50%,
Ti: 0.01-0.20%, V: 0.01-0.
20%, Nb: 0.01 to 0.20%, and the remainder is Fe and impurity elements, heated to a temperature of 800 to 950°C,
A martensitic stainless steel for cold forging, characterized in that a heat treatment of holding the steel for 2 to 16 hours and then cooling it to a temperature of 600°C to Ar1 transformation point at a rate of 5 to 50°C/hr is repeated two or more times. manufacturing method.