JPH02170917A - Manufacture of thick high-tensile steel stock - Google Patents

Abstract

PURPOSE:To manufacture a thick high-tensile steel stock in which hardness difference in the thickness direction is infinitesimal and superior material uniformity is provided by subjecting a hot rolled steel stock of a specific thickness to heating and cooling under respectively specified conditions, applying hardening treatment to the above stock, and then subjecting the above stock to tempering treatment under specific conditions. CONSTITUTION:A hot rolled steel stock of >=50mm thickness is heated up to a temp. of the Ac3 transformation point or above, and then, the above steel stock is cooled until the surface temp. of the steel stock reaches the temp. between a temp. lower than the temp. in the center of the steel stock by 20 deg.C and the Ac3 transformation point. Subsequently, the above steel stock is subjected to hardening treatment and further to tempering treatment at a temp. between the Ac1 transformation point and 450 deg.C. By this method, the thick high-tensile steel stock in which the distribution of hardness and strength is uniformized can be obtained.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for manufacturing thick-walled, high-strength gamma -Relates to a method of manufacturing a thick hypertonic cuff 4t which requires uniform strength distribution.

(Prior art) Recently, thick wall, high tensile strength f+;1 tA has been put into practical use as a material for general steel structures and l'd valley structures due to the remarkable increase in the height of buildings, has been increasing.

The production of such thick-walled high-strength steel materials is subject to restrictions depending on the type of steel.
Although it is sometimes carried out by the 11I rolling method, in most cases it is carried out by a method in which hot-rolled steel material is once made and then the S'':11 material is quenched and tempered. The treatment method is to heat the hot-rolled steel material to a temperature above its transformation point, cool it, quench it, and then temper it.

In addition, since the controlled rolling method requires a high degree of control, it has the drawbacks of being technically difficult and increasing production costs, and the quenching and tempering treatment method is inferior in this respect, so it cannot be used in special cases. However, quenching and tempering treatment methods are often used.

(Problems to be Solved by the Invention) However, in the above-mentioned conventional method for manufacturing thick inner high-strength steel materials using quenching and tempering treatments, the hardness of the brocade IF O (f7) is high in the surface layer of the steel material, and the hardness of the steel material is high. Since it tends to be lower in the center and the difference is relatively large, there is a problem that the hardness 4I in the meat+7 direction tends to be very uneven.

This is due to the problem that the strength and toughness of the steel material vary greatly depending on the location, and the quality of the material tends to be non-uniform. In addition, the degree of non-uniformity of such materials is particularly noticeable in low-strength thick-walled high-tensile steel materials, and 8
Steels treated with 50kg/mm" copper are significantly more sensitive than those treated with 0kg/mm" copper, and for steels with the same strength, steels with a lower C ratio are more significant.

The present invention has been made in view of these circumstances, and its purpose is to solve the problems of the conventional ones, and to achieve a uniform material with minimal difference in hardness in the thickness direction. The present invention aims to provide a method for producing thick-walled high-strength steel materials that can produce thick-walled high-strength steel materials with excellent properties.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a method for manufacturing a thick high-strength steel material having the following configuration.

That is, the method described in the first Δ+1 requirement is that after heating a hot-rolled steel material with a wall thickness of 50 mm or more to a temperature equal to or higher than the AC3 transformation point, the surface temperature of the material is lower than or equal to a temperature 20° C. lower than the center temperature of the steel material. Ac, the steel material is cooled to a temperature equal to or higher than the transformation point, then the steel material is subjected to a quenching treatment, and the steel material is further subjected to a tempering treatment at a temperature lower than the Ac3 transformation point and 450 ° C. or higher. This is a manufacturing method for thick-walled, high-strength steel.

The method according to the second claim includes heating a hot-rolled steel material having a wall thickness of 501 or more to a temperature of Ac3 transformation point or more, and then adjusting the surface temperature of the steel material to a temperature that is 20° C. lower than the center temperature of the steel material,
Cool until the temperature reaches the Act transformation point or higher, and then
After the next quenching treatment, and then heating to a temperature not higher than 150"C higher than the Acm transformation point, and not less than 10"C higher than the Acm transformation point, the surface temperature of the steel is not more than 20"C lower than the center temperature of the steel. , Ac, 20℃ from the odd 5-sided point
The steel material is cooled to a temperature below 450 F, followed by a second mine treatment, and then heated to an Ac transformation point of 450 F.
I that the 4 yen version is subjected to tempering treatment at a temperature of ℃ or higher
This is a method for manufacturing TJ high tensile strength steel.

be.

(Function) Method of manufacturing thick-walled high-tension j'4t, t according to the present invention'tl
As explained above, = is the thickness of the target steel material by 5
It is set to be 0 mm or more. This is because when conventional hardening and tempering treatment methods are used, a problem arises in that when the thickness is 50 cm or more, the difference in hardness in the thickness direction becomes large. If it is less than 11゛0150III+1, during quenching,
Steel (because the difference in cooling rate depending on the part of E is relatively small,
Even when conventional hardening and tempering treatment methods are used, the difference in hardness in the thickness direction is relatively small.

Since the hot-rolled steel material with the wall thickness <r is first heated to a temperature higher than the Ac3 transformation point, the pre-tea (austenite u1
)a) becomes a).

After the above heating, the surface temperature of the steel material is 20° below the center temperature of the steel material.
The steel material is cooled to a temperature lower than 0.degree. C. and higher than the Aci transformation point, and then the steel material is subjected to a quenching treatment. This quenching treatment involves cooling the steel material, which is usually performed by air cooling, water cooling, or oil cooling. The temperature after heating and cooling is the temperature immediately before the cooling of the quenching process is started, and is called the quenching temperature.

Here, the reason why the temperature after heating and cooling, that is, the quenching temperature of the surface of the steel 11 is set to be higher than the Acz transformation point is to create the pre-tJl texture necessary for quenching both inside and on the surface of the steel material. In addition, the reason why the temperature is 20°C lower than the center temperature of the steel material is that by setting the quenching temperature of the surface of the steel material to a temperature that is 20°C lower than the quenching temperature of the center part of the steel material. This is to reduce the difference between the hardness and the hardness of the steel surface.It should be noted that the quenching temperature of the steel surface must be equal to or lower than the middle temperature of the center material of the first material. However, if l is less than 20°C, the difference between the hardness at the center of the steel material and the hardness at the surface of the steel material becomes large.

In this way, when the quenching temperature of the steel surface is lowered by 20°C or more than the quenching temperature of the center of the steel, the difference between the hardness of the center of the steel and the hardness of the steel surface becomes smaller because of the cooling rate of the surface of the steel. This is because the difference between the cooling rate at the center of the steel material and the cooling rate at the center of the steel material becomes smaller, and the structure after quenching becomes more uniform.In other words, the quenching temperature at the surface of the steel material is equivalent to the quenching temperature at the center of the steel material; Or, if the step is less than 20℃ even if it is lower than the quenching temperature at the center of the steel material, the cooling rate at the surface of the steel material will be greater than the cooling rate at the center of the steel material, and the step difference will be relatively large. A martensite structure with high hardness or a martensite group 1a exists in which only a small amount of heinite coexists, and in the center of the steel, there is relatively more bainite (relatively lower hardness than martensite) than in the surface layer of the steel. Therefore, the surface layer becomes extremely hard compared to the center of the steel, resulting in a large difference in hardness.On the other hand, the quenching temperature of the surface of the steel is adjusted to the quenching temperature of the center of the steel. When the temperature is lowered by 20°C or more than the input temperature, the difference between the cooling rate at the surface of the H material and the cooling rate at the center of the steel becomes small, and therefore a relatively large amount of bainite coexists with martensite in both the center and surface of the steel. Therefore, the difference in hardness between the center part and the surface part of the rope becomes small.

After the hardening treatment, in the method according to the first aspect, a tempering treatment is performed at a temperature of Ac, below the transformation point, and above 450°C. The temperature of this tempering process, that is, the tempering temperature, is set below the Ac + transformation point because the higher the tempering temperature, the more uniform the hardness distribution, but when the temperature exceeds the Act transformation point, the strength of the button 11 decreases. Because it does. The reason why the temperature is set at 450°C or higher is that if the temperature is lower than 450°C, the effect of tempering becomes smaller and the toughness decreases.

In the method according to claim 2, after the quenching treatment (i.e., the first quenching treatment), the temperature is lower than or equal to 150°C higher than the Ac3 transformation point, and at a temperature higher than or equal to 10"C higher than the Ac3 transformation point. Heating, then cooling until the surface temperature of the steel material is 20°C lower than the center temperature of the cough field material, and 20°C lower than the AC transformation point, followed by a second quenching treatment, and then is subjected to the same tempering treatment as above.If the second quenching treatment is performed between the first quenching treatment and the tempering treatment, the hardness in the thickness direction will increase. It becomes possible to make the difference more uniform, and to improve the uniformity and properties of the material.

Here, regarding the heating temperature after the first quenching treatment, this is A.
c. The reason why the temperature is set at 10° C. higher than the transformation point is to ensure uniform heating to the inside of the steel material and ensure the strength and toughness of the steel material. The reason why the temperature is 150°C higher than the Ac3 transformation point is that if heated to a temperature exceeding this temperature, the austenite crystal grains during heating will become thicker than those during heating before the primary quenching treatment (and the crystal grains will become thicker). This is because coarsening occurs, leading to a decrease in toughness.

Regarding the temperature after cooling before the secondary quenching treatment, that is, the quenching temperature of the surface of the steel material, the reason why this temperature is set to 20°C lower than the Ac1 transformation point is because both the inside and surface of the steel material are necessary for quenching. This is to ensure the strength and toughness of the steel. This is to make the structure after quenching uniform and reduce the difference in hardness between the center of the steel material and the surface.

Regarding the 1-2 weave after the secondary quenching, if the quenching temperature of the surface of the steel material is higher than the Ac3 transformation point, it becomes a mixed M-weave in which martensite and bainite coexist.

When the quenching temperature of the steel surface is lower than the Acz transformation point, mixture X, 1llI where haynite and ferrite coexist:
, ll.

(Example) The chemical compositions of the hot rolled steel materials shown in Tables 1 and 2 are Ac
+ and Ac, shown together with the transformation point. The R4 material is classified into three types (steel materials A, B, and C) in terms of composition. Compared to steel material B, steel material A has higher ■ and Mol, and steel material C has higher ■ and -0 stars than those. It is characterized by having a high amount of carbon dioxide and containing B. Hot rolling of the steel material is performed by a normal method, and the finished rolling thickness is 50.80, 100, 150 m1.
1, 4 types, 11.

The above-mentioned hot rolled steel material is subjected to the manufacturing method according to the present invention, that is, a method of subjecting it to a tempering treatment after heating, cooling and quenching treatment, or
A method of applying tempering treatment after drilling, cooling, and primary quenching, heating, cooling, and secondary quenching (hereinafter referred to as the methods of the embodiments of the present invention), thick-walled Manufactured high tensile strength steel. Also, in order to compare with this, the conventional addition, Su・
After the quenching treatment, a method of performing a tempering treatment, or a method of heating and
A thick-walled high-tensile steel material was manufactured by a method of performing a secondary quenching treatment, a heating/secondary quenching treatment, and then a tempering treatment (hereinafter, these are referred to as methods of comparative examples). These manufacturing conditions are 3rd to 4th.
Shown in the table.

A piece was taken from the manufactured thick-walled high-tensile steel material, and the hardness distribution in the wall thickness direction was measured and a tensile test was performed.The results are shown in Table 5.The hardness was measured using a Vickers hardness tester. The tensile test was carried out by applying force to the cross section of the copper material.
The test was conducted using a test piece that had been cut and processed from the surface layer and the center of the steel material so that the longitudinal direction of the test piece was in the rolling direction.

In Tables 3 to 5, Experiment No. 1.3, 5.7 and 9
is related to the method of the example of the present invention, experiment No. 2.4
．． 6.8 and 10 relate to methods of comparative examples.

Comparing Experiments No. 1 and No. 2, both have the same composition (steel material A), the same wall thickness (50 mm), and the same initial heating temperature (930°C), but the subsequent manufacturing conditions are different. It's different. In experiment No. 2, the steel was heated to 930°C, then quenched at that temperature (925°C at the center of the steel material), then heated to 640°C, cooled, and tempered. This is what I did. On the other hand, real! J.N.
o, 1 has a steel surface temperature of 86°C after heating to 930°C.
The steel material was cooled to 0°C until the center temperature of the material reached 905°C, then quenched at this temperature, then heated to 640°C, cooled, and tempered. be.

FIG. 1 shows the hardness distribution in the thickness direction of thick high-strength steel materials obtained under different manufacturing conditions as described above. In the figure, O is the experiment No., l, . is the result for experiment No. 2. In Experiment No. 2, the hardness at the surface layer of the steel material is higher than the hardness at the center of the steel material, and the difference is 11ν, which is 72. On the other hand, in experiments No. 1, the hardness distribution in the thickness direction was made uniform, and the difference between the hardness of the surface layer of the steel and the hardness of the center of the steel material was 24 in Ilv, which was extremely small. In addition, as shown in Table 5, the tensile test results show that the difference in tensile strength and yield strength between the steel 4A layer and the center of the steel material is 7 kg for Experiment No. 2. /mm2 and 5
kg/mm2, but experiment N.

, l are all 1.5 kg/mm",
It can be seen that it has become extremely small. 1) The results do not show that the method lh according to the present invention can minimize the difference in hardness and the difference in steel material strength in the wall thickness direction and improve the uniformity of the material compared to the conventional method.・This is an example.

Experiment No. 3 and No. 4 are the above experiments No. 1 and N.
o, the wall thickness is larger than that of 2 (10
0mm). Incidentally, the temperature of the tempering treatment was slightly lower than that in Experiment No. 1 and No. 2 above. Further, the quenching temperature for No. 3 was slightly higher than that for No. 1. Experiment No.
, 3 and Experiment No. 4, it is found that the steel surface layer and II
As shown in Table 5, the difference in hardness, tensile strength, and yield strength between the center part of t4 is that actual X2 No. 3 is better than experimental No. 4.
much smaller.

In experiments No. 9 and No. 10, the wall thickness was further increased L(1
5f) mm), and the initial heating temperature and quenching temperature are lowered. Comparing Experiment No. 9 and Experiment No. 10, the difference in hardness, tensile strength, and yield strength between the surface layer of the steel and the center of the steel is as shown in Table 5.
, 9 is extremely smaller than experiment No. 10.

Comparing Experiment No. 5 and No. 6, both h- have the same composition (I material B), the same wall thickness (50 mm), ? i
The initial heating temperature was also the same (910°C), but the subsequent manufacturing conditions were different. In experiment No. 6, after heating to 910°C, the first quenching process was performed using that temperature as the quenching temperature, and then the second quenching process was performed by heating to 790°C and using that temperature as the quenching temperature. , and then heated to 550°C, cooled and tempered. On the other hand, experiment No.
, 5, after heating to 910 °C, the steel surface temperature is 8
50℃, the steel material is cooled until the center temperature of the material reaches 890℃, followed by the first quenching treatment using this temperature as the quenching temperature, and then heated to 790℃, until the steel surface temperature reaches 750℃.
℃, cool the steel material until the center temperature of the steel material reaches 780°C,
Subsequently, this temperature was used as the quenching temperature to perform a second quenching treatment, and then the material was heated to 550° C., cooled, and tempered.

Ru.

Comparing the above two materials, the difference in hardness, tensile strength, and yield strength between the surface layer of the steel and the center of the steel is extremely smaller in Experiment No. 015 than in Experiment No. 006, as shown in Table 5.

Experiments No. 7 and No. 8 are the above experiments No. 095 and No.
Compared to 016, the wall thickness is thicker L (80+os), and
The initial heating temperature, the first quenching salinity, the heating temperature after the first quenching treatment, and the second quenching salinity are increased.

Comparing Experiment No. 7 and Experiment No. 8, the difference in hardness, tensile strength, and yield strength between the surface layer of the steel and the center of the steel is greater in Experiment No. 7, as shown in Table 5. is experiment N018
much smaller.

The above comparative study results show that the method for manufacturing thick-walled high-tension steel materials according to the present invention (hereinafter referred to as "margin") (hereinafter referred to as "margin") (hereinafter referred to as "margin") (hereinafter referred to as "margin") This proves that the difference in hardness and strength of high-strength steel in the thickness direction can be made extremely small.

(Effects of the Invention) According to the method for manufacturing Iγ macroscopic tensile steel materials according to the present invention, it is possible to manufacture thick high-strength steel materials in which the difference in hardness in the wall thickness direction is extremely small and the material uniformity is excellent. It becomes like this.

[Brief explanation of the drawing]

Figure 1 shows a thick-walled high-tensile steel material (experiment No. 1) manufactured by the method of the example of the present invention and a thick-walled high-tensile steel material (experiment No. 2) manufactured by the method of the comparative example. ) is a diagram showing each thickness direction hardness distribution. Patent applicant: Kobe Steel, Ltd. Representative Patent attorney: Akira Kanemaru

Claims (2)

[Claims] (1) After heating a hot-rolled steel material with a wall thickness of 50 mm or more to a temperature equal to or higher than the Ac_3 transformation point, the surface temperature of the steel material is 20°C lower than the center temperature of the steel material or lower, and continues until the temperature reaches the Ac_3 transformation point or higher. The steel material is cooled, and then the steel material is subjected to a quenching treatment, and the steel material is further heated to a temperature below the Ac_1 transformation point, at 450°C.
A method for producing thick-walled, high-strength steel material, characterized by subjecting the material to tempering treatment at a temperature of ℃ or higher. (2) After heating a hot rolled steel material with a wall thickness of 50 mm or more to a temperature above the Ac_3 transformation point, cool it until the surface temperature of the steel material is 20°C lower than the center temperature of the steel material and a temperature above the Ac_3 transformation point. Then, the steel material is subjected to a primary quenching treatment, and then heated to a temperature that is 150°C higher than the Ac_3 transformation point or 10°C higher than the Ac_1 transformation point, and then the surface temperature of the steel material is 20°C higher than the center temperature of the steel material. below low temperature,
The steel material is cooled to a temperature 20°C lower than the Ac_1 transformation point, then subjected to secondary quenching treatment, and then A
c_1 A method for producing a thick high-strength steel material, characterized by subjecting it to a tempering treatment at a temperature below the transformation point and above 450°C.