JPH09256054A - Production of surface layer hardened steel good in toughness with high efficiency - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel excellent in toughness by subjecting a steel contg. specified wt.% of specified metals to specified heat treatment by a specified method. SOLUTION: This is a method for producing a surface layer hardened steel good in toughness with high efficiency in which a steel having a compsn. contg., by weight, 0.02 to 0.25% C, 0.03 to 2.0% Si, 0.30 to 3.5% Mn and 0.002 to 0.10% Al, furthermore contg. one or two kinds of 0.002 to 0.10% Nb and 0.002 to 0.10% Ti or moreover contg. one or >= two kinds among 0.05 to 3.0% Cu, 0.05 to 10.0% Ni, 0.05 to 10.0% Cr, 0.05 to 3.5% Mo, 0.05 to 10.0% Co and 0.05 to 2.0% W, and the balance Fe with inevitable impurities is heated to the temp. region in which the temp. in the surface layer part of the steel is regulated to the AC1 point or above and also the temp. in the center part is regulated to less than the AC1 point and is immediately quenched. Furthermore, >= two thick steel plates are laminated, and similarly, heating and quenching are executed, by which a one side surface layer hardened thick steel plate good in toughness can be produced with high efficiency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for efficiently hardening a surface layer portion of a steel material such as a thick steel plate and a shaped steel.

[0002]

2. Description of the Related Art As a method for hardening the surface layer of a steel material, a carburizing method, a nitriding method and the like have been conventionally performed. In these methods, heat treatment for a long time is required, productivity is low, and manufacturing cost is high. On the other hand, as a method of hardening the entire steel material, a method of causing martensite or lower bainite transformation by quenching after reheating is manufactured. However, such a manufacturing method also requires a long manufacturing time and a large manufacturing cost.

In recent years, in order to make up for the drawbacks of such a normal quenching process, a direct quenching technique has been developed in which quenching is carried out as it is after rolling. This method has produced some effects in terms of reducing manufacturing costs and strengthening steel.

Examples of such a manufacturing method include Japanese Patent Publication No. 53-6616, Japanese Patent Publication No. 55-49131, and Japanese Patent Publication No. 58-3011. However, such a technique cannot cure only the surface layer portion of the steel material, and does not meet the original purpose. For this reason, there has been a strong demand for a heat treatment method for efficiently hardening the surface layer portion of a steel material such as thick steel plate and shaped steel.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a heat treatment method for efficiently hardening a surface layer portion of a steel material such as a thick steel plate or a shaped steel in a short time and without enormous cost. To do.

[0006]

The present invention is a heat treatment method capable of advantageously eliminating the above-mentioned drawbacks of the conventional method and hardening only the surface layer portion of the steel material by a short time heat treatment. Yes, the summary is as follows.

(1)% by weight, C: 0.02% to 0.2
5%, Si: 0.03% to 2.0%, Mn: 0.30%
.About.3.5%, Al: 0.002% to 0.10%, and the balance of the steel material consisting of Fe and unavoidable impurities so that the temperature rising rate of the surface becomes 0.5.degree. The temperature of the steel surface layer is Ac ₁ point or higher and the temperature of the center is Ac ₁
A high-efficiency method for producing a surface-hardened steel material having good toughness, which comprises immediately heating after heating to a temperature range below the point.

(2) The method of highly efficient production of a single-sided surface hardened thick steel plate having good toughness according to (1), characterized in that two thick steel plates are stacked and heated. (3) Further, in% by weight, Nb: 0.002% to 0.10.
%, Ti: 0.002% to 0.10%, or a combination of two or more thereof, (1) or (2), which is a high-efficiency method for producing a surface hardened steel material having good toughness.

(4) Further, by weight%, Cu: 0.05% to
3.0%, Ni: 0.05% to 10.0%, Cr: 0.
05 to 10.0%, Mo: 0.05% to 3.5%, C
o: 0.05% to 10.0%, W: 0.05% to 2.0
%, 1 type, or 2 or more types are contained, The highly efficient manufacturing method of the surface hardening steel material with good toughness as described in any one of (1) to (3).

(5) Further, V: 0.002% by weight%
0.10% is contained, The highly efficient manufacturing method of the surface hardening steel material with good toughness as described in any one of (1) to (4). (6) Further, by weight%, B: 0.0002% to 0.002
5% is contained, The highly efficient manufacturing method of the surface hardening steel material with good toughness as described in any one of (1) to (5). (7) Further, by weight%, Rem: 0.002% to 0.10.
%, Ca: 0.0003% to 0.0030% is contained, The highly efficient manufacturing method of the surface hardening steel material with good toughness as described in any one of (1) to (6).

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The technical idea underlying the present invention is as follows. Generally, in order to harden steel, means such as solid solution strengthening, precipitation strengthening and transformation strengthening are known. In order to harden the entire steel material, it is an effective means to transform the metal structure into a martensite or bainite structure by heating it in the austenite temperature range and then quenching it. However, this method cannot cure only the surface layer of the steel material.

On the other hand, in order to harden only the surface layer of the steel material, a carburizing method or a nitriding method is often used. These methods are a kind of solid solution strengthening method, but when the steel material contains a carbonitride forming element, hardening by precipitation strengthening is also possible. However, this method requires a great deal of time for processing and is an extremely inefficient manufacturing method, which is a serious problem in practical use.

However, the present inventors set the heating conditions appropriately and combined with the quenching after heating,
It has been found that surface hardened steel can be efficiently manufactured. That is, if the steel material is heated at a predetermined temperature rising rate or more, the temperature difference between the surface layer portion and the central portion of the steel material becomes large. At some point during heating, the surface layer of the steel will have a temperature of Ac ₁ point or higher,
It becomes possible to utilize transformation strengthening by quenching after heating.

On the other hand, the temperature of the central portion of the steel material is less than the Ac ₁ point and is not hardened by quenching after heating.
As a result, it has become possible to efficiently manufacture a steel material in which only the surface layer portion is hardened. Based on these new discoveries, as a result of detailed investigation of the chemical composition of steel and the manufacturing conditions of steel, the present inventors have led to a method of manufacturing high-strength steel as set forth in the claims.

The reasons for limiting the manufacturing method will be described in detail below. First, the reasons for limiting the components of the starting material in the present invention will be described. C is an element advantageous for strengthening steel,
If it is less than 0.02%, sufficient strength cannot be obtained. On the other hand, if its content exceeds 0.25%, the weldability is deteriorated.

Si is effective as a deoxidizing element and as a strengthening element for steel, but if the content is less than 0.03%, it is not effective. On the other hand, if it exceeds 2.0%, the surface properties of steel are impaired. Mn is an element effective for strengthening steel and is 0.30
If it is less than%, a sufficient effect cannot be obtained. On the other hand, if its content exceeds 3.5%, the workability of steel deteriorates. Al is added as a deoxidizing element. If the content is less than 0.002%, there is no effect, and if the content exceeds 0.1%, the surface properties of steel are impaired.

Both Nb and Ti function effectively in terms of grain refinement and precipitation hardening when added in a small amount.
You may add in the range which does not deteriorate the toughness of a welded part. From this viewpoint, the upper limit of the addition amount of Nb and Ti is 0.10.
%. Both are ineffective if the addition amount is too small, so the lower limit of the addition amount is set to 0.002%.

Cu, Ni, Cr, Mo, Co and W are all elements for improving the hardenability of steel. In the case of the present invention, the addition thereof can increase the strength of the steel, but addition of an excessive amount impairs the weldability of the steel.
Cu ≦ 3.0%, Ni ≦ 10.0%, Cr ≦ 10.0
%, Mo ≦ 3.5%, Co ≦ 10.0%, W ≦ 2.0%
Limited to. In addition, since there is no effect if the addition amount is too small, the lower limit of the addition amount is set to 0.05% for each element.

V is effective in increasing the strength of steel by precipitation hardening, but excessive addition impairs the toughness of the steel.
The upper limit is set to 0.10%. Further, if the addition amount is too small, there is no effect, so the lower limit of the addition amount is made 0.002%.

B is an element that improves the hardenability of steel. In the case of the present invention, the addition thereof can increase the strength of the steel, but excessive addition increases the precipitates of B and impairs the toughness of the steel, so the upper limit of the content is 0.0.
025%. Further, if the addition amount is too small, there is no effect, so the lower limit of the addition amount is made 0.0002%.

Rem and Ca are effective in detoxifying S, but if the addition amount is small, S remains harmful and excessive addition impairs toughness, so Rem: 0.002% to 0.10.
%, Ca: added in the range of 0.0003% to 0.0030%.

Next, the manufacturing conditions in the present invention will be described. Since the present invention is effective for steel products manufactured under any casting or rolling conditions, it is not necessary to specify the manufacturing conditions for steel products. Further, the heating method may be any heating method such as furnace heating or high frequency induction heating as long as heat is applied from the surface of the steel material, and there is no particular need to determine it.

When manufacturing a product having a large size or weight, a heat treatment method in which a steel material to be heated is continuously conveyed inside a furnace set to a high temperature is suitable. When the surface temperature of the steel material is heated at a heating rate of less than 0.5 ° C / s during heating, the temperature difference between the surface layer of the steel material and the inside is small, and it is difficult to harden only the surface layer of the steel material. 0.5 ℃ /
Heat at a temperature rising rate of s or more.

Further, when a heating method is applied such that the surface temperature is increased to 0.5 ° C./s or more, coarsening of the structure is suppressed and fine carbides are precipitated in the center portion of the steel material to be tempered. Therefore, toughness is improved. Further, since the austenite grain size in the surface layer portion of the steel material heated to the Ac ₁ point or higher is continuously quenched while being fine, there is an effect that the toughness of the surface layer portion is also improved.

The heating temperature is set according to the intended thickness of the cured layer. That is, by quenching a region whose temperature has risen to the Ac ₁ point or higher, that region is hardened,
Heating is performed until the temperature at the location inside the steel material, which is the thickness of the hardened layer, is Ac ₁ point. Since the internal temperature of the steel material is less than Ac ₁ point from that location, it does not harden by quenching.

According to this method, the entire circumference of the steel material is hardened, but in order to partially harden it, it is sufficient to apply a heat insulating material for preventing hardening, a fireproof paint or the like. Moreover, when hardening only one side of a thick steel plate, two thick steel plates may be piled up and the above-mentioned heating and hardening may be performed. At that time, it is desirable to temporarily weld two thick steel plates together by welding or banding to prevent warpage deformation during quenching. Performing heat treatment such as tempering after quenching the steel material of the present invention does not impair the effects of the present invention.

[0027]

Next, the present invention will be described in detail with reference to examples. First, the steel having the chemical composition shown in Table 1 is used as the dimensions of the thick plate or shaped steel shown in Table 2, and Table 3 (specification table of heat treatment furnace)
Heat treatment was performed using the furnace shown in. The set furnace temperature of the heat treatment furnace was changed in the range of 670 ° C. to 1300 ° C., the depth was changed from the steel material surface layer portion in the plate thickness direction, several thermocouples were embedded, and the heating rate at each position was measured.

FIG. 1 is a diagram showing the dimensions and temperature measurement positions of a thick plate to be heat treated. FIG. 2 is a diagram showing the dimensions and temperature measurement positions of the heat-treated shaped steel. The temperature was measured near the black star mark in both figures by changing the embedding depth and embedding the thermocouple.

Based on the results, the temperature rising rate of the surface layer and the position heated to the Ac ₁ point or higher were measured. When the invention and comparative heat treatment conditions shown in Table 4 (heat treatment time and temperature distribution in steel material) were applied, the treatment was completed in the heat treatment time shown in Table 4, and the hardness distribution and toughness inside the steel material shown in Table 5 also. was gotten.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

[0034]

[Table 5]

[0035]

[Table 6]

According to Table 5, in the steel material of the present invention, only the hardness of the portion heated to the Ac ₁ point or higher is hardened, and the inside is not hardened. In addition, when the heating rate is 0.5 ° C./s or more, the surface hardened portion can be widened. When two thick steel plates are stacked and heated / quenched by the method of the present invention, only one side of each of the two thick steel plates is hardened.

Further, when the temperature rising rate of the surface layer portion is 0.5 ° C./s or more, the toughness is improved in both the central portion and the surface layer portion of the steel material. Further, according to Table 5, it can be seen that the surface hardening method of the present invention requires much shorter time than the conventional hardening methods such as carburizing and nitrifying which require several tens of hours for treatment. As described above, according to the present invention, a steel material having a hardened surface layer portion with a good toughness is obtained in a short time, and the present invention is effective.

[0038]

According to the present invention, the surface layer portion of a steel material can be efficiently hardened in a short time, and a steel material having excellent toughness can be obtained, which greatly contributes to industry.

[Brief description of drawings]

FIG. 1 is a diagram showing dimensions and temperature measurement positions of a thick plate to be heat treated.

FIG. 2 is a diagram showing dimensions and a temperature measurement position of a shaped steel to be heat-treated.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication C21D 8/00 9270-4K C21D 8/00 B C22C 38/00 301 C22C 38/00 301A 38/06 38/06 38/50 38/50 (72) Inventor Yoshiyuki Watanabe 1 Kimitsu, Kimitsu-shi Shin Nippon Steel Co., Ltd. Kimitsu Steel Works (72) Inventor Kiyoshi Nishioka 2-6-3 Otemachi, Chiyoda-ku, Tokyo Within Nippon Steel Corporation

Claims (7)

[Claims] 1. By weight%, C: 0.02% to 0.25%, Si: 0.03% to 2.0%, Mn: 0.30% to 3.5%, Al: 0.002. % To 0.10%, the balance of the steel material consisting of Fe and unavoidable impurities is adjusted so that the temperature rising rate of the surface is 0.5 ° C./s or more.
After the temperature of and the central portion at a temperature Ac ₁ point or more steel surface layer portion was heated to a temperature range which is Ac less than ₁ point, immediately efficient method for producing a toughness good surface curing steel, characterized in that to put baked. 2. The high-efficiency production method for a single-sided surface hardened thick steel plate having good toughness according to claim 1, wherein two thick steel plates are stacked and heated. 3. The composition further comprises, by weight, one or more of Nb: 0.002% to 0.10% and Ti: 0.002% to 0.10%. 1. A highly efficient method for producing a surface-hardened steel material having good toughness according to 1 or 2. 4. Further, by weight%, Cu: 0.05% to 3.0%, Ni: 0.05% to 10.0%, Cr: 0.05% to 10.0%, Mo: 0. 05% to 3.5%, Co: 0.05% to 10.0%, W: 0.05% to 2.0%, or a combination of two or more thereof. 3. A highly efficient method for producing a surface-hardened steel material having good toughness according to any one of 3 above. 5. Further, V: 0.002% to 0.
10% is contained, The highly efficient manufacturing method of the surface hardening steel material with good toughness as described in any one of Claim 1 thru | or 4 characterized by the above-mentioned. 6. Further, by weight%, B: 0.0002% to
0.0025% is contained, The highly efficient manufacturing method of the surface hardening steel material with good toughness as described in any one of Claims 1 thru | or 5 characterized by the above-mentioned. 7. The composition according to claim 1, further comprising Rem: 0.002% to 0.10% and Ca: 0.0003% to 0.0030% by weight. High-efficiency method for producing a surface-hardened steel material having good toughness as described in 1.