JP2000119749A - Method of manufacturing Cr-Mo seamless steel pipe for machine structure - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To produce a Cr-Mo based seamless steel pipe excellent in toughness without variation in strength in a simplified production process. SOLUTION: C: 0.10 ~ 0.25%, Si: 1.0% or less, Mn: 0.2 ~
2.0%, P: 0.020% or less, S: 0.010% or less, Cr: 0.2-1.5%, M
o: 0.1-0.5%, Ti: 0.005-0.020%, V: 0.01-0.10%, N: 0.
006% or less, if necessary Ni: 1.0% or less, Cu: 0.5% or less, Nb: contains one or more of 0.01% or less,
After the billet consisting of Fe and unavoidable impurities is heated and pierced and rolled, and then subjected to finish rolling consisting of stretching and constant diameter processing, at a finishing temperature of 900 ° C or higher, the cross-section reduction rate is 40% or more. Gives a large processing strain. Furthermore, immediately after finishing rolling, without cooling, 900-100
It is soaked in the temperature range of 0 ° C and quenching is performed online. Thereafter, it is maintained at a predetermined temperature so as to have a predetermined strength,
Perform tempering.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a process for producing a Cr-Mo seamless steel tube for a machine structure, which is capable of producing a Cr-Mo seamless steel tube for a machine structure having excellent toughness at a low cost as compared with the conventional method. About the method.

[0002]

2. Description of the Related Art In the production of steel that requires huge equipment,
From the viewpoint of process saving and energy saving, simplification of the process by applying online thermomechanical treatment has been studied and has been put into practical use. In the production of seamless steel pipes, from the viewpoint of high reliability and high quality, the fact is that a considerable number of products are still produced by off-line heat treatment. For example, a high-toughness seamless steel pipe for a machine structure is formed by drilling a billet into a hollow shell by punching, drawing by a plug mill or a mandrel mill, then performing a constant diameter process with a sizer or a reducer, and cooling. It is manufactured by reheating and performing heat treatment of quenching and tempering.

[0003] For this reason, the off-line heat treatment is not economical because it is necessary to install a heat treatment furnace separately from the pipe production line and perform quenching and tempering heat treatment. On the other hand, online heat treatment is a direct quenching process in which quenching is performed immediately by using the heat of the material after hot working, and a large cost reduction can be obtained industrially.

[0004] For example, C: 0.10 to 0.45%, Si: 0.10 to 0.50%, Mn:
0.50 to 1.50%, P: 0.020% or less, S: 0.015% or less, Cr: 0.10 to
2.00%, Mo: 0.05-0.50%, Al: 0.01-0.10%, and N
b: 0.01 to 0.10% and V: 0.01 to 0.15%, containing 1 or 2 types, and, if necessary, Cu: 0.40% or less, Ti: 0.050% or less, B: 0.0030% or less 1 or 2 containing more species, the balance is hot processed into a steel tube to the steel material consisting of Fe and unavoidable impurities, and water directly quenched by the inner and outer surfaces immersed quenching immediately the steel pipe from the a _C3 transformation point or above the temperature, and then the steel pipe that Production of a high-strength seamless steel pipe tempered at a temperature T (° C) and a tempering time t (hour) that satisfies both formulas of T ≦ A _C1 and 18000 ≦ (T + 273) (20 + log t) ≦ 20000 A method (JP-A-60-75523) has been proposed.

[0005] Further, C: 0.20 to 0.35%, Si: 0.10 to 0.50%, Mn: 1.
0 to 1.6%, P: 0.03% or less, S: 0.02% or less, Cr: 0.2 to 1.0%,
V: 0.02-0.08%, Al: 0.02-0.10%, Ti: 0.005-0.050%,
B: 0.0005-0.0050%, Ni: 0.10-0.30 if necessary
% Or Nb: One or two types of 0.015 to 0.035%, the balance being Fe and unavoidable impurities.
C. Quenching from a temperature of ₃ or more points, followed by tempering at a temperature of 450 to 580 ° C.
No. 2) has been proposed.

[0006] Further, C: 0.07 to 0.16%, Si: 0.1 to 1.0%, Mn: 0.
5 to 1.5%, Cr: 0.3 to 1.0%, Mo: 0.1 to 0.7%, Cu: 0.30% or less, Al: 0.07 to 0.11%, N: 0.0020 to 0.0040%, B: 0.0005 to
Steel containing 0.0020%, balance consisting of Fe and unavoidable impurities, or C: 0.07 to 0.16%, Si: 0.1 to 1.0%, Mn: 0.5
~ 1.5%, Cr: 0.3 ~ 1.0%, Mo: 0.1 ~ 0.7%, Ni: 0.2 ~ 1.5
%, Cu: 0.30% or less, Al: 0.07 to 0.11%, N: 0.0020 to 0.0040
%, B: 0.0005 to 0.0020%, the balance being Fe and unavoidable impurities, formed by a normal hot rolling process, and subsequently cooled to a temperature of Ar ₁ temperature or less, 850 to
Heating to a temperature of 1000 ° C. at a temperature rising rate of 1 to 30 ° C./sec, then quenching and then tempering at a temperature not higher than A _C1 point. Has been proposed.

[0007]

The methods disclosed in Japanese Patent Application Laid-Open Nos. 60-75523 and 60-33312 disclose differences in the temperature after the constant diameter processing for each pipe depending on the pipe manufacturing dimensions. Or even if there is no difference in the temperature of each tube, the difference occurs between the temperature at the tube end and the center, that is, the uniformity of the structure in the tube longitudinal direction and circumferential direction due to temperature variation during quenching. Properties may not be obtained, and strength variation and toughness may decrease.

[0008] Further, the method disclosed in JP-A-58-91123,
After hot working, it is necessary to cool to Ar ₁ temperature or less, so it takes time to cool down, lengthening the pipe production line,
It is necessary to take measures such as reducing the production efficiency.

[0009] An object of the present invention is to provide a Cr-Mo seamless steel pipe for machine structure capable of producing a Cr-Mo seamless steel pipe having no variation in strength and excellent in toughness in a simplified manufacturing process as compared with the conventional method. It is to provide a manufacturing method.

[0010]

Means for Solving the Problems The present inventors have intensively studied and studied to improve the above problems. As a result, even in the process of direct quenching, by specifying the component system, further limiting the conditions of finish rolling consisting of stretching after drilling and constant diameter processing, by performing appropriate quenching and tempering heat treatment, The present inventors have found that sized crystal grains can be obtained, and that the same performance as that obtained by performing ordinary off-line quenching and tempering heat treatment can be obtained, thereby completing the present invention.

[0011] The method for producing a Cr-Mo series seamless steel pipe for a machine structure according to the present invention comprises the following steps: C: 0.10 to 0.25%;
1.0% or less, Mn: 0.2 to 2.0%, P: 0.020% or less, S: 0.010% or less, Cr: 0.2 to 1.5%, Mo: 0.1 to 0.5%, Ti: 0.005 to 0.020%,
V: 0.01 to 0.10%, N: 0.006% or less, the balance is a billet composed of Fe and inevitable impurities, or C: 0.10 to
0.25%, Si: 1.0% or less, Mn: 0.2 to 2.0%, P: 0.020% or less, S:
0.010% or less, Cr: 0.2 to 1.5%, Mo: 0.1 to 0.5%, Ti: 0.005 to
0.020%, V: 0.01 to 0.10%, N: 0.006% or less, and Ni:
A billet containing one or more of 1.0% or less, Cu: 0.5% or less, and Nb: 0.01% or less, with the balance being Fe and unavoidable impurities is used.

[0012] The above-mentioned Cr-Mo-based billet is heated and pierced and rolled, and then subjected to finish rolling consisting of stretching and constant diameter processing. Gives large processing strain. Furthermore, after finish rolling,
Without cooling, it is immediately soaked in the temperature range of 900-1000 ° C, and quenching is performed online. After that, a predetermined temperature is maintained so as to have a predetermined strength, and a tempering process is performed.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical components of steel in the present invention are as follows. C is an essential element for ensuring hardenability, but if its content is less than 0.10%, its effect is not sufficient, and if it exceeds 0.25%, cracking during quenching and reduction in toughness and welding in products Since the workability and workability deteriorate, the content is set to 0.10 to 0.25%.

[0014] Si is an element effective in deoxidizing and improving the strength of steel, but if it is contained in excess of 1.0%, the steel becomes embrittled. Therefore, to secure good toughness, the content is set to 1.0% or less.

[0015] Mn is necessary for deoxidation and desulfurization of steel, and is a useful element for improving strength and hot workability and obtaining an appropriate structure. But not
If the content exceeds 2.0%, the strength of the high-strength Cr-Mo steel increases, but the weldability and workability deteriorate.

[0016] P is an element inevitably present in steel as an impurity, but if it exceeds 0.020%, it segregates at grain boundaries and deteriorates toughness, so the upper limit of the impurity was made 0.020%.

S is an element inevitably present in steel as an impurity like P, but forms coarse inclusions and deteriorates the toughness particularly in the direction perpendicular to the rolling direction (T direction). The upper limit as an impurity was set to 0.010%.

[0018] Cr is an element necessary for ensuring hardenability like C, but if its content is less than 0.2%, its effect is not sufficient.
Further, if it exceeds 1.5%, the weldability and workability are reduced, so the content is set to 0.2 to 1.5%.

Mo is an element necessary for securing hardenability and toughness by high-temperature tempering. If less than 0.1%, its effect is not sufficient, and if it exceeds 0.5%, its effect is saturated and segregation occurs. 0.1-0.5% to reduce workability during pipe making
And

[0020] Ti is an element that has the effect of reducing the crystal grain size and improving the toughness, but if less than 0.005%, the effect is not sufficient, and if added in a large amount exceeding 0.020%, coarse carbides To reduce toughness, 0.005 to 0.0
20%.

V is an element having the effect of improving the toughness by high-temperature tempering, but if it is less than 0.01%, its effect is not sufficient, and if it is added in a large amount exceeding 0.10%, it becomes a coarse carbide, 0.01 to 0.10% to reduce
And

N is inevitably present in steel as an impurity,
It is an element that lowers toughness, and in the in-line heat treatment process, it causes product strength variation, especially due to the ubiquitous precipitation of nitrides and the like.
0.006%.

By limiting the chemical components in steel,
Cr for machine structural use with fine grain size and excellent toughness and strength
-Mo series seamless steel pipes can be obtained, but if you want to improve them further, Ni, Cu,
It is effective to add one or more of Nb. The reasons for limiting the contents of these added chemical components are as follows.

[0024] Ni is an element having the effect of improving hardenability and improving toughness. However, since it is expensive, the upper limit is set to 1.0% in terms of economy.

[0025] Cu is an element effective for increasing the strength and improving the corrosion resistance. However, if it exceeds 0.5%, many defects occur on the surface of the steel pipe, so the upper limit was made 0.5%.

Nb is an element having the effect of refining the crystal grain size and improving the toughness, similar to Ti. Therefore, the upper limit was made 0.01%.

[0027] The heating temperature of the Cr-Mo-based billet in the present invention may be any temperature at which hot drilling can be performed by a drilling machine. The optimum temperature depends on the material, and is determined in consideration of high-temperature ductility and high-temperature strength. Usually, it is heated to the range of 1100 to 1300 ° C.
The piercing step is a step of piercing a solid billet with a piercing machine to form a hollow shell. In the punching step, a cross-piercing machine with a cone-type roll is used in order to facilitate the strong working in the finish rolling, which is the sum of the following stretching and sizing.

[0028] In the present invention, the finish rolling, which is a combination of the elongation processing and the constant diameter processing of the pierced and rolled hollow shell, is carried out by a cross-sectional reduction rate of 40%.
% And a finishing temperature of 900 ° C. or more. If the cross-sectional reduction rate is less than 40%, recrystallization does not proceed smoothly, the effect of reducing the crystal grain size cannot be obtained, and crystal grains sometimes grow abnormally. Although the upper limit of the cross-sectional reduction rate in finish rolling differs depending on the material to be pipe-formed and the capacity of the mill, it is not particularly limited,
If the cross-section reduction rate is too large, flaws are likely to occur,
It is preferably set to 80%.

[0029] The finishing temperature in the finish rolling depends on the material to be pipe-formed and the capacity of the mill, but if it is less than 900 ° C, the deformation resistance of the steel material becomes large, and it is difficult to perform strong working with a cross-sectional reduction rate of 40% or more. Therefore, the temperature was set to 900 ° C. or more. In addition,
The upper limit of the finishing temperature is not particularly limited because it varies depending on the material to be pipe-formed and the capacity of the mill, but is preferably 1100 ° C.

The major feature of the present invention is that the recrystallization treatment (normalization) is immediately performed without cooling between the finish rolling and the quenching / tempering heat treatment. Recrystallization is induced in combination with
Crystal grains can be sized.

[0031] In the method of the present invention, unlike the prior art, when reheating is performed between the stretching process during rolling and the constant diameter process,
Since there is no need to process after soaking, the soaking temperature can be set to the lowest temperature at which recrystallization proceeds, and sized recrystallized grains can be obtained. When the soaking temperature is less than 900 ° C. in the case of the Cr—Mo steel, which is the object of the present invention, a long time is required for recrystallization, and the pipe production efficiency is significantly reduced. Also, 10
If the temperature exceeds 00 ° C., the crystal grains become extremely coarse and the toughness is reduced, which causes cracks during secondary processing or the like. Therefore, the soaking temperature was in a temperature range of 900 to 1000 ° C. After soaking, quenching treatment is immediately performed.

In the case of the conventional off-line quenching treatment, since the temperature is raised from room temperature, the residence time in the heating furnace is prolonged, the economy is poor, and the growth of the surface scale is large. Requires processes such as pickling and shot blasting to remove scale from products. In contrast, in the method of the present invention, the steel pipe finished at a high temperature of 900 ° C. or more can be soaked in the reheating furnace as it is, so
It can be reduced to less than 30 minutes, and is economical in terms of energy costs.

In the tempering process of the present invention, tempering is performed at a predetermined temperature so as to secure a target strength. Cr-Mo that is the object of the present invention
In steel, since V is added, the toughness is deteriorated by VC precipitation at 500 to 600 ° C, so that tempering is usually performed at a temperature of 620 to 720 ° C.

[0034]

EXAMPLES Steels A to G having the component compositions shown in Table 1 were melted by a conventional method and slab-rolled into billets having a diameter of 225 mm. Each billet was heated to 1250 ° C., and then made into a hollow shell using a punch. Then, in the present invention examples of Test Nos. 1 to 10, finish rolling consisting of stretching and constant diameter processing was performed under the conditions shown in Table 2 to produce a seamless steel pipe having an outer diameter of 240 mm and a wall thickness of 8 to 30 mm. Immediately without cooling, the temperature was maintained soaked under the conditions shown in Table 2, followed by heat treatment of quenching and tempering.

In Comparative Examples of Test Nos. 11 to 16, finish rolling consisting of stretching and constant diameter processing was performed outside the range of the present invention as shown in Table 2, and the outer diameter was 240 mm and the wall thickness was 8 mm. A 3030 mm seamless steel tube was made. And each obtained seamless steel pipe,
Immediately without cooling, as shown in Table 2, immediately after reheating and holding outside the range of the present invention, heat treatment of quenching and tempering was performed. Furthermore, in the conventional examples of Test Nos. 17 to 20, finish rolling consisting of stretching and constant diameter processing was performed under the conditions shown in Table 2 to produce a seamless steel pipe having an outer diameter of 240 mm and a wall thickness of 8 to 30 mm. did. Each of the obtained seamless steel pipes was cooled to room temperature, charged in a quenching furnace, heated to the conditions shown in Table 2, and then subjected to a water-cooled quenching and tempering heat treatment. The temperature in the parentheses in the soaking temperature column in Table 2 indicates the temperature after finishing rolling, once cooling to room temperature, charging into a quenching furnace and raising the temperature and soaking.

The evaluation of each seamless steel pipe obtained was performed by
Using a No. 12C test piece specified as a metal material tensile test piece according to JIS Z2201, n = 26 tensile tests were performed according to a metal material tensile test method specified according to JIS Z2241, and an average value and a standard deviation were determined.
In addition, the toughness was determined using a 5 mm wide sub-size V-notch specimen as specified in JIS Z2202 metal material impact specimen.
The Charpy impact test was performed three times according to the metal material impact test method specified in, and the absorbed energy (J) at -40 ° C was measured. In the flatness test, a test tube piece is sandwiched between two flat plates, and the state of cracks in the tube wall when compressed to 5 times the wall thickness is examined. ○ indicates no cracks, △ indicates fine cracks, and × indicates cracks. Was evaluated.
The results are shown in Table 3.

[0037]

【table 1】

[0038]

[Table 2]

[0039]

[Table 3]

As shown in Tables 2 and 3, in the examples of the present invention in Test Nos. 1 to 10, the mechanical properties, the absorbed energy, and the flatness test were almost the same as the conventional examples in Test Nos. 17 to 20. Is shown. On the other hand, the comparative examples of Test Nos. 11 to 16 do not satisfy the condition range of the present invention, so that the performance of the conventional examples of Test Nos. 17 to 20 cannot be secured.

[0041]

The method for producing a Cr-Mo seamless steel pipe for machine structure according to the present invention is characterized in that the Cr-Mo billet is heated, pierced and rolled, and then subjected to finish rolling comprising stretching and sizing. Finishing with a cross-section reduction rate of 40% or more under conditions of a finishing temperature of 900 ° C or more gives a large processing strain. Furthermore, after finish rolling,
Without cooling, it is immediately soaked in the temperature range of 900-1000 ° C, and quenching is performed online. Thereafter, by maintaining the temperature at a predetermined temperature so as to have a predetermined strength and performing a tempering process, the same performance as that of the conventional off-line heat treatment can be ensured even in the case of online thermal processing.
Therefore, as compared with the conventional method, it is possible to simplify the manufacturing process, improve the pipe production efficiency and save energy, and to manufacture a Cr-Mo seamless steel pipe for a machine structure excellent in toughness at low cost.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K032 AA05 AA11 AA12 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 BA03 CC04 CD06 CE02

Claims (2)

[Claims] (Claim 1) C: 0.10 to 0.25%, Si: 1.0% or less, Mn: 0.2 to
2.0%, P: 0.020% or less, S: 0.010% or less, Cr: 0.2-1.5%, M
o: 0.1-0.5%, Ti: 0.005-0.020%, V: 0.01-0.10%, N: 0.
Containing 006% or less, the remainder is heated billet consisting of Fe and unavoidable impurities, after piercing and rolling, finish rolling consisting of stretching and constant diameter processing, with a cross-sectional reduction rate of 40% or more,
And Cr-Mo seamless steel pipe for machine structure characterized by performing at a finishing temperature of 900 ° C or higher, immediately equalizing the temperature to 900 to 1000 ° C without cooling, and continuously performing quenching and tempering heat treatment online. Manufacturing method. 2.C: 0.10 to 0.25%, Si: 1.0% or less, Mn: 0.2 to
2.0%, P: 0.020% or less, S: 0.010% or less, Cr: 0.2-1.5%, M
o: 0.1-0.5%, Ti: 0.005-0.020%, V: 0.01-0.10%, N: 0.
006% or less, and Ni: 1.0% or less, Cu: 0.5% or less, Nb:
Contains one or more of 0.01% or less, with the balance being F
e and heating the billet consisting of unavoidable impurities, after piercing and rolling, finish rolling consisting of stretching and constant diameter processing, at a cross-section reduction rate of 40% or more, and at a finishing temperature of 900 ° C or more, without cooling A method for producing a Cr-Mo series seamless steel pipe for machine structure, wherein the pipe is immediately soaked to a temperature of 900 to 1000 ° C and quenching and tempering heat treatment is continuously performed online.