JPH11310820A - Production of cold tool steel stabilized in residual austenite - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide cold tool steel in which the problem as for the secular changes of the dimension and shape caused by the progress of martensitic transformation is solved by stabilizing residual austenite contained in the steel while maintaining the hardness thereof. SOLUTION: Cold tool steel examplified by 1C-1Si-0.4Mn-8Cr-2Mo-0.3V-Fe as the representive alloy comspn. is used as the material, which is quenched to form into a structure in which residual austenite occupies by 3 to 30 volume ratio, thereafter, heating to a temp. less than the AC1 transformation point and also the temp. enough for decomposing the half of the residual austenite is executed for one or >= two times, and net, the steel is heated at the range of 150 to 550 deg.C and also at the temp. lower than that in the above heat treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the improvement of cold tool steel, and provides a cold tool steel capable of stabilizing retained austenite, suppressing aging, and processing precision products.

[0002]

2. Description of the Related Art In recent years, materials to be used for cold forming have become increasingly difficult to process, and products have been required to have high precision. The tempering temperature of the tool is selected so that a higher tempering height is obtained and a more stable structure is obtained.

[0003] However, under the heat treatment conditions under which a high hardness can be obtained by a known heat treatment, the amount of austenite remaining during quenching by high-temperature quenching increases, and in many cases, the retained austenite does not completely disappear even by subsequent tempering. Tempering is also often performed near the temperature at which the tempering hardness reaches a peak in order to obtain high hardness. When using a tool manufactured by performing such a heat treatment, aging becomes a problem. As is well known, retained austenite tends to transform to martensite at room temperature. Since the volume changes with the transformation, a phenomenon occurs in which the shape and dimensions of the tool change, which causes problems such as inferior product accuracy and mismatching of the tool mounting portion.

[0004] In order to avoid such aging, a measure may be taken in which the material after heat treatment is left for a while to cause a change in the structure and then processed into a tool, but since the aging continues for more than half a year, Attempting to achieve a sufficient effect would take an unreasonably long period of time and is not practical.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a heat treatment for stabilizing retained austenite contained in steel while maintaining the hardness of the cold work tool steel.
An object of the present invention is to provide a cold tool steel that solves the above-mentioned problem of aging.

[0006]

According to the present invention, there is provided a method for producing a cold tool steel in which retained austenite is stabilized, comprising the steps of quenching cold tool steel and occupying 3 to 30% by volume of retained austenite. After that, _one or more heat treatments at a temperature below the AC ₁ transformation point and sufficient to decompose at least half of the retained austenite are performed, and then within a range of 150 to 500 ° C. And heating to a lower temperature than the above heat treatment.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention is generally applicable to tempered martensitic steel and tempered bainite steel, but is most effective when applied to steels having the following alloy compositions.

The basic one is C: 0.4 by weight.
~ 2.0%, Si: 2% or less, Mn: 2% or less, Cr:
It is an alloy steel containing 5 to 20% and V: 3% or less, with the balance being Fe and unavoidable impurities.

One preferred alloy composition contains one or more of Mo: 5% or less and W: 3% or less by weight in addition to the basic alloy components described above, with the balance being Fe. And alloy steel comprising unavoidable impurities.

Another preferred alloy composition is, in addition to the basic alloy components described above, Nb: 3% or less by weight, N
It is an alloy steel containing one or more of i: 3% or less and Co: 5%, with the balance being Fe and unavoidable impurities.

The problem of the aging of cold tool steel is how to reduce the amount of austenite present and how to promote the stabilization of the remaining austenite. On the other hand, the inventors chose a strategy in which austenite was repeatedly decomposed, and retained austenite was once made unstable and stabilized. The discovery of the fact that austenite obtained by stabilizing highly unstable austenite is more stable than that obtained by stabilizing moderately unstable austenite is the starting point of the present invention. The retained austenite during quenching is unstable, but by maintaining it at a temperature that causes carbide precipitation, it becomes more unstable and also after a significant portion of the small amount of retained austenite has transformed, The discovery that the retained austenite is highly unstable, and that the presence of stabilized austenite is useful in ensuring toughness, an important property for tool steel, has led to the present invention. Was.

The amount of austenite remaining when quenching cold tool steel is determined by the martensitic transformation start temperature Ms determined by the solid solution amount of the austenitized alloy element.
It is roughly determined by the relationship between the martensitic transformation end temperature Mf and the quenching end temperature. The alloy composition of the tool steel is a given condition, whereby Ms and M
Since f is determined, in practicing the present invention, first, the retained austenite has a volume ratio of 3 to 3 in accordance with the alloy composition.
The quenching end temperature is selected so as to account for 0%. 3
%, The problem of aging is eliminated, but the amount of stabilized austenite is too small to contribute to the toughness described above. On the other hand, if a large amount of austenite exceeding 30% remains, it is difficult to decompose more than half of it.

The subsequent treatment is to first heat to a temperature below the Ac ₁ point and to a temperature sufficient to decompose at least half of the original retained austenite, thereby reducing the amount of austenite and at the same time reducing the amount of retained austenite. Austenite becomes unstable. This treatment must be carried out such that more than half of the austenite present at a time decomposes. Otherwise, the remaining austenite cannot be destabilized to a sufficient degree. This austenite decomposition treatment, if necessary,
Repeat once or twice more. The austenite remaining after such treatment is the last 150-500
Stabilizes by maintaining at a temperature of ° C.

The method of the present invention is generally applicable to tempered martensitic and tempered bainite steels,
When applied to steel having the following alloy composition, it exerts its significance best.

The basic one is C: 0.4 by weight.
~ 2.0%, Si: 2% or less, Mn: 2% or less, Cr:
It is an alloy steel containing 5 to 20% and V: 3% or less, with the balance being Fe and unavoidable impurities.

One preferred alloy composition contains, in addition to the basic alloy components described above, one or more of Mo: 5% or less and W: 3% or less by weight, with the balance being Fe. And alloy steel comprising unavoidable impurities.

Another preferred alloy composition is, in addition to the above basic alloy components, Nb: 3% or less by weight,
It is an alloy steel containing one or more of i: 3% or less and Co: 5%, with the balance being Fe and unavoidable impurities.

The reason why such an alloy composition is suitable will be described below.

C: 0.4 to 2.0% C is an essential element for cold work tool steel. To secure HRC60 as the hardness of martensite,
Requires at least 0.4% presence. If it exceeds 2.0%,
This is disadvantageous because it increases the amount of retained austenite during quenching.

Si: 2% or less Si has the function of increasing the high-temperature tempering hardness, but too much Si lowers the toughness and the tool is liable to chip.
The upper limit is 2%.

Mn: 2% or less Mn improves hardenability. If the amount is too large, the amount of retained austenite at the time of quenching increases, which is also disadvantageous. Therefore, the addition is limited to 2%.

Cr: 5 to 20% Cr is an important element for enhancing hardenability, increasing the amount of secondary hardening of tempering, and ensuring wear resistance and toughness. Suitable amounts are usually in the range of 5-20%.

V: 3% or less V is an element for promoting secondary hardening, and exhibits an effect by high-temperature quenching. In addition to preventing crystal grains from coarsening,
It forms hard carbide and contributes to improvement of wear resistance.
Since a large amount of addition causes a decrease in toughness, the content is limited to 3%.

Mo: 5% or less and W: 3% or less One or two types of Mo are effective in improving hardenability and increasing high-temperature tempering hardness. If the amount is too large, M ₇ C ₃ type carbides crystallize out,
The amount of addition should be up to 5% because it is coarsened and the toughness is impaired. W improves the high temperature tempering hardness as a secondary hardening element. Large additions reduce toughness due to carbides that crystallize out. The upper limit of the W amount is also 3%.

Nb: 3% or less, Ni: 3% or less and C
o: 1% or 2% or more of 5% Nb also forms a hard carbide similarly to V and enhances wear resistance. It has the effect of expanding the appropriate quenching temperature range. If the amount of addition is excessive, coarse carbides are formed and the toughness is reduced, so the upper limit is also 3%.
Ni forms a solid solution in the matrix and improves quenching properties and toughness.
Since it is an austenite-forming element, when added in a large amount, the amount of retained austenite during quenching increases. From this viewpoint, the upper limit of the addition amount is set to 3%. Co enhances high temperature tempering hardness. However, when the amount is large, not only the hardenability is reduced to lower the hardness, but also the toughness is deteriorated. Appropriate loading should be selected from the range up to 5%.

[0026]

EXAMPLE A cold tool steel having an alloy composition shown in Table 1 was melted.

Table 1 No. C Si Mn Cr Mo W V Ni Nb Co Example 1 1.01 0.98 0.32 7.41 2.24 0.45 0.45---2 0.66 0.45 0.44 5.80 1.70 0.03 0.29 0.55 0.05-3 1.45 0.55 0.41 10.88 0.65-0.98 0.05-1.124 4 1.21 1.51 1.21 9.10 3.13 0.06 1.81 0.01 0.11 0.12 5 1.85 0.64 0.33 14.55 1.72 0.21 0.21 0.23 0.07 2.71 6 0.58 1.23 0.31 16.80 0.95 1.12 0.23 0.04 0.08 −7 2.00 0.09 0.77 6.80 0.92 0.51 0.44 − − −8 0.89 0.75 0.67 8.21 1.77 0.32 0.12 − − − 9 1.00 1.41 0.77 3.41 5.45 1.01 0.87 0.21 0.05 4.10 Comparative Example 11 1.39 0.37 0.44 11.45 1.01 − 0.36 0.01 − − 12 1.50 0.77 0.29 14.26 0.31 0.26 0.23 − − − 13 1.04 0.88 0.31 1.23 0.55 0.43 0.18 − − −14 0.89 0.79 0.44 4.43 4.81 6.03 1.81 0.04--15 2.31 0.09 0.48 5.88 1.38 0.55 0.27 0.08-0.65 16 0.99 0.67 0.46 21.04 1.75-0.42 0.15 0.42 1.46 17 2.70 0.64 0.88 18.00 3.70 0.88 0.34-0.03 1.40 wt%, balance Fe.

Each tool steel sample was quenched at the temperature shown in Table 2 and then heat-treated once or twice at the temperature shown in Table 2 to decompose the retained austenite. Finally,
A final heat treatment was performed. The amount of retained austenite during quenching and the amount of austenite remaining after being decomposed by each heat treatment were examined. The results are shown in Table 2.

Table 2 No. Quenching Residual primary heat treatment Secondary heat treatment Final heat treatment temperature γ amount Temperature γ decomposition rate Temperature γ decomposition rate Temperature (° C) (%) (° C) (%) (° C) (%) (° C) Example 1 1030 21 530 70 530 60 400 2 1000 12 530 68 ----- 350 3 1030 14 530 90 530 50 300 300 4 1050 16 530 80 ----- 400 5 1030 18 530 63 530 60 60 350 6 1030 4 525 90 405 6000 6000 6000 450 8 1030 9 530 92 ---- 250 9 1000 30 530 90 530 50 400 400 Comparative Example 11 1030 8 500 35 -40 -12 1030 4 530 80---1393036 200 40--14 1160 27 530 8565-15 1000 4 530 75 70 - 16 1050 3 530 80 50 - 17 1030 19 300 0 - 400 "γ" is O - means austenite phase.

The final heat treatment was partially applied to a thickness of 50
The plate was cut into a comb shape having a length of 300 mm, a length of 300 mm, and a width of 200 mm by wire electric discharge machining, and the movement of a standard point before and after the cut was measured. Another part was also made into a plate and left at room temperature, and the change in distance between two points at a distance of 300 mm defined on the plate was measured after 120 days. Table 3 shows the results.

[0031] In general, the dimensional stability of tool steel is such that, in the above test, distortion of wire electric discharge machining is within 30 μm and aging is 20 μm.
m. The heat-treated tool steel of the present invention generally satisfies this requirement.

[0032]

According to the cold work tool steel which has been subjected to the stabilization treatment according to the method of the present invention, since the retained austenite is highly stabilized, deformation and aging due to working are remarkably suppressed, and are required for practical use. Have reached the level you are. Therefore, the cold tool manufactured from this steel has high dimensional accuracy and can maintain the accuracy for a long time. Stabilized austenite increases toughness and extends tool life.

Claims (3)

[Claims] 1. Quenching a cold tool steel, the volume ratio is 3 to
After making the structure occupied by 30% of retained austenite, A
A process of heating to a temperature not higher than the C ₁ transformation point and sufficient to decompose at least half of the retained austenite is performed once or twice, and then within a range of 150 to 500 ° C. A method for producing a cold tool steel in which retained austenite is stabilized by heating to a lower temperature than heat treatment. 2. As a cold tool steel, C: 0.4 by weight
~ 2.0%, Si: 2% or less, Mn: 2% or less, Cr:
5-20%, and V: 3% or less, Mo: 5%
The method for producing a cold tool steel according to claim 1, wherein an alloy steel containing one or two of the following and W: 3% or less and the balance being Fe and unavoidable impurities is used. 3. As a cold tool steel, C: 0.4 by weight
~ 2.0%, Si: 2% or less, Mn: 2% or less, Cr:
5-20%, and N: 3% in addition to V: 3% or less
Hereinafter, one or two of Ni: 3% or less and Co: 5%
2. The method for producing a cold tool steel according to claim 1, wherein an alloy steel containing at least one species and the balance being Fe and unavoidable impurities is used.