JPH11199985A - Superhigh hardness and high toughness steel and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel combining superhigh hardness of >=600 HB and excellent toughness and to provide a method for producing it. SOLUTION: In a steel having a compsn. contg., by weight, 0.4 to 0.55% C, 0.03 to 0.15% Si, <=0.5% Mn, 5 to 13% Ni, 0.5 to 3% Cr, 0.2 to 2% Mo, 0.05 to 0.2% V, and the balance Fe with inevitable impurities, it is composed of a microstructure essentially consisting of martensite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-hard and tough steel and a method for producing the same.

[0002]

2. Description of the Related Art High-hardness steel is widely used for construction machines such as bulldozers and power shovels, and steel materials which require strong wear resistance, such as cutters for cutting industrial waste. Since the wear resistance of the steel material increases as the hardness increases, it is desirable that the hardness be as high as possible in such applications. However, most of the currently used high hardness steels have a Brinell hardness of about 500 or less. The reason for this is that the higher the hardness of the steel material, the lower the toughness generally, so that breakage and cracks are likely to occur, so that the service life is rather shortened. In particular, in the case of being used for an application that receives a strong impact in an environment below the freezing point of a cold region, there is a great concern. Examples of high-hardness steel are disclosed in JP-A-63-83225,
21846 JP, there are such Hei 8-41535, JP-Korare relates steel both Brinell hardness H _B 600 less hardness. Also, Japanese Patent Application Laid-Open No.
The 9427 JP, H _B 600 on the surface by carburizing
Although a method of obtaining a layer having a hardness higher than that has been proposed, since the obtained ultra-high hardness layer is limited to an extremely thin layer, there is a concern that abrasion resistance during use is reduced due to wear during use. The cost is higher because it is necessary.

[0003]

The Brinell hardness H _B is 6
Since ultra-high hardness steel of 00 or more shows extremely excellent wear resistance and large deformation resistance to a large instantaneous impact,
Although its utility value is extremely high, including wear-resistant materials for civil engineering and construction machinery, none has sufficient toughness with low possibility of breakage or cracking, and it has been practically used in such fields. There was no example.

[0004] The present invention, in order to solve such problems, it is an object to provide a steel and a manufacturing method thereof bring together excellent toughness and H _B 600 or more ultra-high hardness.

[0005]

The inventors of the present invention have conducted various studies on the effects of alloying elements and manufacturing methods on the toughness of ultra-high hardness steel. a Ni steel plate, by performing the cryogenic process and the low temperature tempering subsequent to normal quenching heat treatment, H _B 600
It has been found that a method can be obtained by ultra-high hardness and high toughness steel having both the above hardness and excellent toughness and its production.

The present invention has been made based on this finding, and the gist thereof is as follows: (1) C: 0.4 to 0.55% by weight, Si:
0.03 to 0.15%, Mn: 0.5% or less, Ni: 5
１３13%, Cr: 0.5-3%, Mo: 0.2-2%,
V: An ultra-high hardness and toughness steel containing 0.05 to 0.2% and having a composition comprising the balance of Fe and unavoidable impurities, wherein the steel has a microstructure mainly composed of martensite.

(2) Co: 3 to 10% by weight
%, Nb: 0.02-0.2%, Ti: 0.01-0.
The ultra-high hardness and high toughness steel according to the above (1), which contains one or more of a toughness improving element group consisting of 2%.

(3) By weight%, B: 0.00
The ultra-high hardness and high toughness steel according to the above (1) or (2), containing 0.05 to 0.005%.

(4) The microstructure mainly composed of martensite is a mixed structure of martensite and retained austenite, and the microstructure fraction of retained austenite is 3% or less. ) Or the ultra-hard and tough steel according to (3).

(5) C: 0.4-0.55% by weight
%, Si: 0.03 to 0.15%, Mn: 0.5% or less, Ni: 5 to 13%, Cr: 0.5 to 3%, Mo:
0.2 to 2%, V: 0.05 to 0.2%, balance F
e and a slab having a composition consisting of unavoidable impurities are heated and, after hot rolling, are cooled from a temperature above the A ₃ transformation point to 100 ° C. or less at a cooling rate of 10 ° C./sec or more. A method for producing an ultra-hard and high-toughness steel, comprising performing a quenching heat treatment, a deep cooling treatment, and a tempering heat treatment at a temperature of 100 ° C to 300 ° C.

(6) By weight%, Co: 3-1
0%, Nb: 0.02 to 0.2%, Ti: 0.01 to
(5) The steel slab or the cast slab containing at least one of the toughness improving element group consisting of 0.2%.
A method for producing the ultra-hard and tough steel described.

(7) B: 0.00% by weight
The method for producing an ultra-hard and tough steel according to the above (5) or (6), wherein the steel contains 0.05 to 0.005%.

(8) The method for producing an ultra-hard and high-toughness steel according to the above (5), (6) or (7), wherein the cryogenic treatment temperature is -50 ° C or lower.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

In order to obtain an ultra-high hardness exceeding H _B 600, it is essential that the microstructure is mainly composed of martensite. Further, since the hardness of martensite is almost determined by the contained C content, it is indispensable to increase C in order to obtain high hardness. Meanwhile, Ni is an element effective in improving the toughness of the matrix, ultra high hardness steel odor H _B 600 class is not very effective large in Ni addition of about less than 3%. Further, even if Ni is simply added in a large amount, the toughness after quenching does not increase so much. The inventor believes that the martensitic transformation temperature is very low in a high-C high-Ni steel, and the martensitic transformation is incomplete by ordinary quenching heat treatment, and the fraction of retained austenite increases. That is, it is presumed that the presence of a large amount of retained austenite lowers the hardness and at the same time lowers the toughness, thereby offsetting the effect of improving the toughness of Ni. Further, the reduction of Si is also effective in improving the toughness, but in a high C-high Ni steel, lowering the Si also lowers the martensitic transformation temperature, thereby increasing the retained austenite after quenching. That is, sufficient hardness can be obtained by almost completely transforming the martensitic transformation of a high-C high-Ni steel with reduced Si during quenching, and at the same time, a large amount of Ni and an effect of improving toughness due to low Si can be effectively obtained. Can be used. As a method for completely transforming such steel into martensite, the present inventor is extremely effective to perform a deep cooling treatment in which the steel material is cooled to a low temperature of less than 0 ° C., following normal quenching. At this time, as shown in the relationship between the retained austenite fraction and the hardness and toughness in FIG. 1, the retained austenite fraction in the mixed structure of martensite mainly composed of martensite and retained austenite is set to 3% or less. The present invention was newly found to be effective for obtaining ultra-high hardness and high toughness, and achieved the present invention. Furthermore, the present inventor can easily and stably generate the microstructure in a steel having the composition of the present invention by combining low-temperature tempering at a predetermined temperature after the deep cooling treatment, Has been found to be able to be manufactured efficiently. The steel composition of the sample in FIG. 1 is C: 0.42%,
Si: 0.08%, Mn: 0.35%, Ni: 6.4
%, Cr: 2.02%, Mo: 0.35%, V: 0.1
1%.

Further, when the steel material is used for abrasion resistance, since the temperature of the steel material may increase due to frictional heat, it is desirable that the tempering softening resistance is high, and by containing appropriate amounts of Cr, Mo, and V, In addition, tempering softening resistance can be increased. As in the present invention, in a high C-low Si-high Ni component steel having a structure in which a predetermined retained austenite is mainly mixed with a martensite structure, even if Cr, Mo, and V are contained in desired amounts, While effectively serve as a hardenability improving element simultaneously, a Cr, Mo, V even invention on essential element in the invention since ultra high hardness (H _B 600 or hardness) and hardly inhibit both high toughness.

Next, the reasons for limiting the steel components of the present invention will be described.

[0018] C: Since determined almost the hardness of martensite, as described above C, to obtain a H _B 600 or more hardness C is required than 0.4%. However, C is 0.55%
Even with the above addition, the effect of improving martensite hardness is reduced, and the effect on the reduction of toughness is rather increased.
The content was limited to 0.4-0.55%.

Si: Si is a deoxidizing element, and this kind of steel usually contains about 0.2% to 0.3%, but by reducing this to less than 0.15%, the structure of the as-quenched steel is reduced. The toughness can be greatly improved. On the other hand, if the content is less than 0.03%, the effect is small, so the Si content is 0.03 to 0.3.
15%.

Mn: Mn is particularly detrimental to breakage resistance because it forms MnS to form inclusions that serve as starting points for destruction. At least Mn is preferably reduced to 0.5% or less.

Ni: Ni is an important element for improving toughness. However, in a high C component such as the steel of the present invention, addition of 5% or more is necessary for remarkable improvement in toughness. However, even if added over 13%, a remarkable effect of improving toughness is hardly expected, and the cost increases, so that the Ni content is 5 to 1%.
3%.

Cr: Cr is effective for improving hardenability and tempering softening resistance. If it is less than 0.5%, their effect is small, and if it exceeds 3%, on the other hand, it is detrimental to toughness. Therefore, the content of Cr is set to 0.5 to 3%.

Mo: Mo is also effective for improving the hardenability and the tempering softening resistance. If the content is less than 0.2%, their effects are small, and even if added over 2%, their effects are saturated. Therefore, considering the economy, the upper limit is set, and the Mo content is 0.2 to 2%. %.

V: V is also effective for improving the hardenability and the tempering softening resistance. If it is less than 0.05%, the effect is small, and if it exceeds 0.2%, coarse precipitates are formed, which is harmful to toughness. Therefore, the content of V is set to 0.05 to 0.2%.

The above are the basic components of steel in the present invention. In the present invention, in addition to the above components, one or more of the toughness improving element group Co, Nb, Ti and B can be added.

Co: Co is an element effective for improving toughness, and its effect is exhibited at 3% or more. But 10%
If more than Co is added, there is no further effect of improving toughness and the cost increases, so the Co addition amount is desirably 3% to 10%.

Nb: Nb contributes to the refinement of crystal grains by suppressing and increasing the austenite grains during heating before rolling, and is effective in improving toughness. In order to exhibit this effect, it is necessary to add 0.02% or more. However, if it is added in excess of 0.2%, coarse Nb precipitates are formed and the toughness is reduced. Therefore, the Nb content is 0.02
It is desirable to set it to 0.2%.

Ti: Ti is effective in refining crystal grains. For this effect, addition of 0.01% or more is preferable. However, if added in excess of 0.2%, toughness tends to decrease. It is desirable that the Ti content be 0.01 to 0.2%.

B: B enhances hardenability to increase hardness,
Particularly, the wear resistance at room temperature is improved. To achieve the effect, 0.0005% or more is necessary, but 0.005%
Above, the toughness tends to decrease. Therefore, the content of B is desirably 0.0005 to 0.005%.

As unavoidable impurities other than the above components,
Since P, S, N, and O are harmful elements that lower the toughness, the smaller the amount, the better. Preferably, P:
0.005% or less, S: 0.003% or less, N: 0.0
1% or less, O: 0.003% or less.

As described above, according to the present invention, an ultra-high hardness and high toughness steel is obtained for the first time by combining the steel composition and the microstructure, and the microstructure mainly composed of martensite is a mixture of martensite and retained austenite. It is a structure, and the structure fraction of retained austenite is 3% or less. Retained austenite is morphologically similar to the martensite structure, and it is not easy to discriminate between retained austenite and martensite from observation of a microstructure. The structural fraction of retained austenite can be quantitatively determined by X-ray analysis from the ratio of the integrated intensity of ferrite to the integrated intensity of austenite.

Next, the manufacturing method will be described.

First, a slab or a slab having the above-mentioned steel composition is heated and hot-rolled. Heating temperature is 1000-1
The temperature may be appropriately selected between 300 ° C. according to the steel composition and the required characteristics of the steel. Also, hot rolling is not particularly limited, and rolling in only a recrystallized region or rolling in both a recrystallized region and a non-recrystallized region may be performed. Then after cooling once Ac less than ₃ transformation point of hot rolled steel, put baked by heating again Ac ₃ transformation point or above the temperature (reheating quenching)
Alternatively, quenching from a temperature equal to or higher than the Ar ₃ transformation point immediately after hot rolling (direct quenching) may be used.
That is, the A ₃ transformation point is Ac _{3 in} the case of reheating quenching.
In the case of direct quenching, it refers to the Ar ₃ transformation point or higher. In any case of quenching, following the quenching, the steel sheet is subjected to a deep cooling treatment using an appropriate refrigerant (for example, denatured alcohol or the like). The temperature of the deep cooling process is 0 ° C or less,
Further, it is preferable that the temperature is lower than −50 ° C. or lower, but −130 ° C.
The effect is almost saturated on the low temperature side beyond
C or higher is desirable. In this deep cooling, the martensitic transformation is made almost completely, and in the low temperature tempering after the deep cooling, a martensite-based structure, that is, a martensite mixed structure having a predetermined amount of retained austenite, is obtained from the quenching temperature. Cooling rate below 100 ° C is 1
0 ° C./sec or more is required. The cooling rate is desirably fast, but the effect is almost saturated at 300 ° C./sec, so that it is sufficient within 300 ° C./sec.

It is desirable that the time from the start of quenching to the completion of the deep cooling process is within 300 seconds. Further, after the deep cooling, the steel sheet needs to be tempered at a temperature of 150 to 300 ° C. Figure 2 shows the tempering temperature and hardness,
The relation of toughness is shown. The test material of FIG. 2 (steel composition: C: 0.4
2%, Si: 0.08%, Mn: 0.35%, Ni:
6.4%, Cr: 2.02%, Mo: 0.35%, V:
(0.11%), after hot rolling, quenching is performed at a temperature equal to or higher than the Ar ₃ transformation point, and then a deep cooling treatment is performed at −80 ° C., followed by tempering. Tempering temperature is not obtained a high toughness is less than 0.99 ° C., ensuring more H _B 600 decreases the hardness when tempering temperature exceeds 300 ° C. is difficult, toughness decreases slightly. Therefore, the tempering temperature is limited to 100 ° C. to 300 ° C. as a method for producing ultra-high hardness and high toughness steel.

The steel obtained by such a manufacturing method, the residual austenite fraction is 3% or less of martensite structure, H _B 600 or more hardness and Charpy absorbed energy at _-40 ℃ (vE -40) is 40J It shows the above excellent toughness.

As the steel, any of a thick plate, a thin plate (a hot rolled plate, a cold rolled plate), a shaped steel bar, a bar steel, and a steel bar may be used.

[0037]

EXAMPLE A steel slab obtained by melting a steel having the composition shown in Table 1 was heated at 1150 ° C., and then hot-rolled at 850 ° C. or more. Was manufactured into a steel plate having a thickness of 10 mm to 40 mm based on the manufacturing conditions of the above.

For these, the Brinell hardness H _B and toughness of the base material were investigated. The hardness was measured by measuring the hardness at a 1 / 4t position in the thickness direction with the Brinell hardness test method (JIS Z2243).
Measured at 25 ° C. The toughness was evaluated by the average value of three absorbed energy values of the Charpy impact test at -40 ° C (JIS Z22014 test piece, sampling position in the thickness direction of the test piece: 1 / 4t, the sampling direction in the thickness direction). At right angles). The retained austenite fraction (%) was calculated from the integrated intensity ratio between the ferrite (211) plane and the austenite (220) plane using X-rays, and calculated by the equation (i).

Retained austenite volume fraction (%) = {1 / [(0.6Ia / Ir) +1]} × 100 (i) Ia: integrated strength of ferrite, Ir: integrated strength of austenite In the table, underlined The numerical values given above indicate that the component values, temperature conditions and characteristics outside the present invention are insufficient.

[0040] In Table 2 of the present invention Example 1-A~10-J, Brinell hardness H _B is 600 or more, and Charpy absorbed energy value vE-40 ° C. is 40J or more at -40 ° C., are both high level is there. On the other hand, in the comparative steels deviating from the chemical composition range limited by the present invention, Comparative Example 1 was used despite the fact that the production method was the method of the present invention.
Since 1-K has a low C content, Comparative Example 16-P has a low Cr content, and Comparative Example 18-R has a low Mo content.
Since 9-S has a low V amount, each has low hardness. Also,
Since Comparative Example 12-L has a high C content, Comparative Example 13-M shows S
Since the amount of i is high, Comparative Example 14-N has a high amount of Mn.
Comparative Example 15-O has a low Ni content, Comparative Example 17-Q has a high Cr content, and Comparative Example 20-T has a high V content, and thus has low toughness. In the comparative example which deviated from the production method of the present invention even in the case of the steel of the present invention, the comparative example 21-A has a low quenching reheating temperature, and the comparative example 22-A has not been subjected to the deep cooling treatment. Example 24-A has low hardness and toughness due to high tempering temperature. Comparative Example 23-A has low toughness due to low tempering temperature.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

According to the present invention, the steel and its manufacturing method combines excellent toughness and H _B 600 or more ultra-high hardness is obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the retained austenite fraction and the hardness and toughness in the steel composition of the present invention.

FIG. 2 is a diagram showing the relationship between tempering temperature, hardness, and toughness in the steel composition of the present invention.

Claims (8)

[Claims] C. 0.4 to 0.55% by weight, S
i: 0.03 to 0.15%, Mn: 0.5% or less, N
i: 5 to 13%, Cr: 0.5 to 3%, Mo: 0.2 to
2%, V: 0.05 to 0.2%, a steel having a composition comprising the balance of Fe and unavoidable impurities, characterized in that the steel has a microstructure mainly composed of martensite, and is characterized by being ultra-hard and tough. steel. 2. In% by weight, further Co: 3 to 10%, N
2. The ultra-hard, high-toughness steel according to claim 1, comprising one or more of a toughness improving element group consisting of b: 0.02 to 0.2% and Ti: 0.01 to 0.2%. . 3. The composition according to claim 2, wherein B: 0.0005 to
The ultra-high hardness and high toughness steel according to claim 1 or 2, which contains 0.005%. 4. The microstructure mainly composed of martensite is a mixed structure of martensite and retained austenite,
4. The ultra-high hardness and toughness steel according to claim 1, wherein the structure fraction of retained austenite is 3% or less. 5. C: 0.4 to 0.55% by weight, S
i: 0.03 to 0.15%, Mn: 0.5% or less, N
i: 5 to 13%, Cr: 0.5 to 3%, Mo: 0.2 to
2% V: includes 0.05% to 0.2%, heating the steel slab or cast slab having a composition the balance being Fe and inevitable impurities, after hot rolling, which A ₃ transformation point or above of An ultra-high hardness and high hardness characterized by performing a quenching heat treatment of cooling from a temperature to 100 ° C. or less at a cooling rate of 10 ° C./sec or more, then performing a deep cooling treatment, and further performing a tempering heat treatment at a temperature of 100 ° C. to 300 ° C. Manufacturing method for tough steel. 6. In% by weight, Co: 3 to 10%,
Nb: 0.02-0.2%, Ti: 0.01-0.2%
The method for producing an ultra-hard and high-toughness steel according to claim 5, wherein the steel slab or the slab contains one or more of the toughness improving element group consisting of: 7. In% by weight, B: 0.0005 to
The method for producing an ultra-hard and high-toughness steel according to claim 5 or 6, comprising 0.005%. 8. The method for producing ultra-high hardness and toughness steel according to claim 5, wherein the cryogenic treatment temperature is -50 ° C. or lower.