JP2007196237A - Method for producing seamless steel tube for machine structural component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a seamless steel tube free from variation in strength, having excellent toughness, further having satisfactory surface skin and optimum as a machine structural component in a simplified production process at a low production cost. <P>SOLUTION: In the production method, a billet having a composition comprising, by mass, 0.10 to 0.25% C, ≤1.00% Si, 0.20 to 2.00% Mn, ≤0.030% P, ≤0.020% S, 0.1 to 1.5% Cr, ≤0.5% Mo, 0.005 to 0.030% Ti and 0.01 to 0.10% V, satisfying Ti/N: ≤3.5, and the balance Fe with impurities is subjected to piercing, is thereafter subjected to finish rolling composed of drawing and sizing at a reduction in area of ≥40% and also at a finishing temperature of ≥900°C, is directly soaked at 900 to 1,000°C without being cooled, and is continuously subjected to quenching/tempering heat treatment on-line. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a seamless steel pipe that employs a simplified manufacturing process as compared to conventional methods, has no strength variation, has excellent toughness, and has a good surface skin, and is optimal as a machine structural component. .

  In the manufacture of seamless steel pipes, from the viewpoint of high reliability and high quality, a significant amount of products are still subjected to offline heat treatment for manufacturing products that require strength and toughness. For example, steel pipes having a tensile strength of 600 MPa or 800 MPa used for cylinders or the like as mechanical structural parts are subjected to offline quenching and tempering (QT) treatment in order to ensure strength and toughness.

  In other words, seamless steel pipes for machine structures with high strength and toughness are made into hollow core pipes by drilling billets with a drilling machine, and after drawing with a plug mill or mandrel mill, they are subjected to constant diameter processing with a sizer or reducer. After cooling, it is reheated and subjected to heat treatment such as quenching and tempering.

  For this reason, offline heat treatment requires the installation of a heat treatment furnace separately from the pipe production line, and it is necessary to perform quenching and tempering heat treatment, which is not economical and raises the manufacturing cost, and the lead time becomes long. is there. On the other hand, on-line heat treatment is a direct quenching process that uses the heat of the raw material after hot working and immediately quenches, resulting in a large cost reduction industrially.

For example, in Patent Document 1, a steel material having a specific component system is hot-worked into a steel pipe, and the steel pipe is immediately water quenched by inner and outer surface immersion quenching from a temperature equal to or higher than the Ac ₃ transformation point. Proposing a method for producing a high-strength seamless steel pipe tempered at a temperature T (° C.) and a tempering time t (hours) satisfying both T ≦ Ac ₁ and 18000 ≦ (T + 273) · (20 + logt) ≦ 20000 Yes.

JP 60-33312 A

  If the manufacturing method proposed in Patent Document 1 is adopted, even if there is a difference for each pipe in the temperature after constant diameter processing due to the pipe making dimensions, or even if there is no temperature difference for each pipe, There is a temperature difference between the central portion and the central portion, and due to temperature variation during quenching, the uniformity of the structure in the longitudinal direction and circumferential direction of the tube cannot be obtained, and strength variation and toughness degradation may occur.

Further, in the production of the structural seamless steel pipe, in order to ensure high strength and high toughness in the online heat treatment, after cooling to Ar ₁ temperature or lower subsequent to hot working, heated at a predetermined heating rate, A method of tempering at a temperature of Ac ₁ point or less after quenching was also proposed, but after hot working and cooling to Ar ₁ temperature or less, it takes time for cooling, lengthening the pipe production line, While measures such as lowering the efficiency are required, the lead time becomes longer.

  The present invention has been made in view of the above-described problems, and is a manufacturing process simplified from the conventional method. There is no variation in strength, excellent toughness, good surface skin, and optimal as a machine structural component. It aims at providing the manufacturing method of a seamless steel pipe.

  In order to solve the above-mentioned problems, the present inventors have conducted various studies on the characteristics in online heat treatment and the influence of alloy elements on the characteristics. As a result, even in the case of a direct on-line quenching process, the component system is specified, and the conditions for finish rolling consisting of stretching and constant diameter processing after drilling are limited, and appropriate quenching and tempering heat treatment is performed. As a result, it has been found that the crystal grain size can be reduced and the same characteristics as those obtained by offline quenching / tempering heat treatment can be secured.

  Based on the above findings, the production method of the seamless steel pipe of the present invention is in mass%, C: 0.10 to 0.25%, Si: 1.00% or less, Mn: 0.20 to 2 0.000%, P: 0.030% or less, S: 0.020% or less, Cr: 0.1-1.5%, Mo: 0.5% or less, Ti: 0.005-0.030% and V: containing 0.01 to 0.10%, Ti / N: 3.5 or less, and using a billet composed of Fe and impurities as the balance, after piercing and rolling, finishing consisting of stretching and constant diameter processing Rolling is performed at a cross-section reduction rate of 40% or more and at a finishing temperature of 900 ° C. or higher, immediately heated to 900 to 1000 ° C. without cooling, and subjected to quenching / tempering heat treatment continuously online. .

  Furthermore, the manufacturing method of the seamless steel pipe of this invention is 1 mass% or less, Cu: 0.5% or less, Ni: 1.0% or less, and Nb: 0.01% or less. It is desirable to use a billet containing. Further, the crystal grain size is reduced, and a seamless steel pipe for machine structural parts having excellent strength and toughness can be obtained.

  According to the seamless steel pipe manufacturing method of the present invention, even if it is an online quenching and tempering heat treatment, it can ensure the same performance as that of the conventional offline quenching and tempering heat treatment, compared with the conventional method, Seamless steel pipe that is optimal as a machine structural component because it is low in cost because it can simplify the manufacturing process, improve pipe manufacturing efficiency, and save energy, and also has no strength variation, excellent toughness and good surface skin. Can be obtained.

The reason for defining the production method of the present invention as described above will be explained by dividing it into billet chemical composition and production conditions. In the following description, the chemical composition is indicated by “mass%”.
1. Billet chemical composition C: 0.10 to 0.25%
C is an essential element for increasing the strength and ensuring the hardenability, but if its content is less than 0.10%, its effect is not sufficient, while if it exceeds 0.25%, Generation of cracks and toughness, and weldability and workability of the product deteriorate. For this reason, C content was made into 0.10 to 0.25%. Furthermore, in order to reduce the alloy elements when increasing the strength, it is desirable to set the upper limit of the C content to 0.16%.

Si: 1.00% or less Si is an element effective for deoxidation and strength improvement of steel, but if its content exceeds 1.0% and is excessively contained, the steel is embrittled. Therefore, in order to ensure good toughness, the content was made 1.0% or less.

Mn: 0.20 to 2.00%
Mn is an element that is necessary for deoxidation and desulfurization of steel and is useful for improving strength and hot workability and obtaining an appropriate structure. However, if its content is less than 0.20%, The effect is not sufficient. On the other hand, if it exceeds 2.00%, the strength increases, but the weldability and workability deteriorate. Therefore, the Mn content is set to 0.20 to 2.00%.

P: 0.030% or less P is an element unavoidably present in steel as an impurity. However, if the content exceeds 0.030%, it segregates at grain boundaries and deteriorates toughness, so the upper limit as an impurity. Was 0.030%.

S: 0.020% or less S is an element that is unavoidably present in steel as an impurity like P, but forms coarse inclusions, and particularly has a toughness in a direction perpendicular to the rolling direction (T direction). Deteriorate. For this reason, the upper limit of the S content as an impurity is set to 0.020%.

Cr: 0.1 to 1.5%
Cr is an element necessary for ensuring hardenability like C. However, if its content is less than 0.1%, its effect is not sufficient, while if it exceeds 1.5%, weldability and Reduces workability. For this reason, Cr content was made into 0.2 to 1.5%.

Mo: 0.5% or less Mo is an element necessary for securing hardenability and toughness by high-temperature tempering. However, when its content exceeds 0.5%, its effect is saturated and during pipe making due to segregation or the like. Reduces the workability. For this reason, the upper limit of Mo content was 0.5%.

Ti: 0.005-0.030%
Ti is an element having the effect of reducing the grain size and improving the toughness, but if its content is less than 0.005%, the effect is not sufficient, and more than 0.030% in large amounts. When added, coarse carbides are formed and the toughness is reduced. For this reason, Ti content was made into 0.005-0.030%.

  Furthermore, even if the Ti content is in the range of 0.005 to 0.030%, as described later, Ti / N: 3.5 or less in order to control the generation of TiC by fixing with TiN by N It is necessary to satisfy the relationship.

V: 0.01-0.10%
V is an element having an effect of improving toughness by high-temperature tempering, but if its content is less than 0.01%, its effect is not sufficient, and if added in excess of 0.10%, it is coarse. Carbides and lower toughness. For this reason, the V content is set to 0.01 to 0.10%.

Ti / N: 3.5 or less When Ti is added excessively, TiC precipitates and deteriorates the steel. In order to suppress the precipitation of TiC, it is effective to combine Ti with N to form TiN. From the ratio of the atomic weight of Ti and N, precipitation of TiC can be suppressed if the Ti / N relationship is 3.5 or less.

  In the production method of the present invention, by limiting the chemical composition of the billet as described above, the crystal grain size can be reduced, and a seamless steel pipe for machine structural parts excellent in strength and toughness can be obtained. When it is desired to improve the toughness, it is effective to contain one or more of Ni, Cu and Nb in the chemical composition. The reasons for limiting the contents of these optional additives are as follows.

Cu: 0.5% or less Cu is an element that increases strength and is effective for improving corrosion resistance. However, if its content exceeds 0.5%, defects occur frequently on the surface of the steel pipe. For this reason, when Cu is contained, the upper limit is made 0.5%.

Ni: 1.0% or less Ni is an element that has the effect of improving hardenability and improving toughness. However, since it is expensive, the upper limit is set to 1.0% in the case of inclusion from the economical aspect. It was.

Nb: 0.01% or less Nb is an element that has the effect of reducing the grain size and improving toughness in the same way as Ti, but in the online heat treatment process, the product strength caused by the uneven distribution of Nb carbonitrides It causes variation. For this reason, when Nb is contained, the upper limit was made 0.01%.
2. Manufacturing conditions The heating temperature of the billet in the manufacturing method of this invention should just be a temperature which can be hot drilled with a punch. The optimum temperature depends on the material and is determined in consideration of the high temperature ductility and high temperature strength. Usually, it heats in the range of 1100-1300 degreeC. The perforating step is a step of perforating a solid billet with a perforator to form a hollow shell. In the piercing process, a cone-type roll piercing machine is used in order to facilitate strong processing in finish rolling that includes the next drawing and constant diameter processing.

  In the production method of the present invention, it is necessary to perform finish rolling, which is the sum of the stretching process and the constant diameter process of the hollow core tube that has been pierced and rolled, at a cross-section reduction rate of 40% or more and a finishing temperature of 900 ° C. or more. When the cross-section reduction rate is less than 40%, recrystallization does not proceed smoothly, the effect of making the crystal grain size fine cannot be obtained, and the crystal grains may grow abnormally. The upper limit of the cross-section reduction rate in finish rolling is not particularly limited because it varies depending on the material to be pipe-made and the capability of the mill. However, if the cross-section reduction rate is too large, wrinkles are likely to occur, so 80% is recommended. desirable.

  The finishing temperature in finish rolling varies depending on the material to be pipe-made and the ability of the mill, but if it is less than 900 ° C, the deformation resistance of the material steel becomes large and it becomes difficult to perform strong processing with a cross-section reduction rate of 40% or more. To 900 ° C. or higher. Further, the upper limit of the finishing temperature varies depending on the material to be pipe-made and the capability of the mill, and thus is not particularly limited, but is preferably 1100 ° C.

  The production method of the present invention is characterized in that the recrystallization treatment (normalization) is performed immediately without cooling between the finish rolling and the quenching / tempering heat treatment. Recombination is induced by the combination, and the grain size can be adjusted.

  In the production method of the present invention, as in the prior art, it is not necessary to reheat between the stretching process and the constant diameter process in the intermediate process of finish rolling, and to perform the constant diameter process after soaking. For this reason, the soaking temperature can be set to the lowest temperature at which recrystallization proceeds, and recrystallized grains that have been sized can be obtained.

  In the case of the billet (Cr—Mo steel) targeted by the present invention, the soaking temperature is less than 900 ° C., requiring a long time for recrystallization, and the pipe making efficiency is significantly reduced. On the other hand, when the temperature exceeds 1000 ° C., the crystal grains are excessively coarsened and the toughness is lowered, which causes cracks during secondary processing. Therefore, the soaking temperature was set to a temperature range of 900 to 1000 ° C. Immediately after soaking, quenching is performed.

  When performing off-line quenching treatment, it is necessary to raise the temperature from room temperature, so the residence time in the heating furnace becomes longer, the economy is inferior, and the growth of the surface scale is large. Steps such as pickling and shot blasting for removing the scale are required. On the other hand, in the manufacturing method of the present invention, since the steel pipe finished at a high temperature of 900 ° C. or higher can be soaked in a reheating furnace as it is, the in-furnace time can be suppressed to less than 30 minutes, and in terms of energy cost It is excellent in economic efficiency and the surface skin becomes good.

  In the tempering process in the present invention, tempering is performed at a predetermined temperature so as to ensure the target strength. In the billet (Cr—Mo steel) targeted by the present invention, since V is added, since there is a deterioration in toughness due to VC precipitation at 500 to 600 ° C., tempering is usually performed at a temperature of 620 to 720 ° C. Do.

  Inventive steels A to G and comparative steels H to M having chemical compositions shown in Table 1 were melted by a commonly used method and subjected to ingot rolling to form billets having a diameter of 225 mm.

  Each billet was heated to 1250 ° C. and then formed into a hollow shell using a punch. And test no. In Examples 1 to 7 of the present invention, finish rolling consisting of stretching and constant diameter processing is performed under the rolling conditions shown in Table 2, and a seamless steel pipe having an outer diameter of 240 mm and a wall thickness of 8 to 30 mm is formed and cooled. Without being immediately maintained at the soaking temperature shown in Table 2, quenching and tempering heat treatment was performed.

  Test No. In Comparative Examples 8-13, finish rolling consisting of stretching and constant diameter processing was performed under the rolling conditions shown in Table 2 using comparative steels outside the range specified in the present invention, and the outer diameter was 240 mm and the wall thickness was 8 A seamless steel pipe having a thickness of ˜30 mm was produced, and immediately heated under the conditions shown in Table 2 without cooling, followed by quenching and tempering heat treatment.

  In addition, Test No. In Comparative Examples 14 to 17, finish rolling consisting of stretching and constant diameter processing is performed under the rolling conditions shown in Table 2 (Test Nos. 15 and 16 are outside the range defined in the present invention). A seamless steel pipe having a thickness of 240 mm and a thickness of 8 to 30 mm was produced. And each obtained seamless steel pipe was quenched and tempered after being reheated and held under the conditions shown in Table 2 (Test Nos. 14 and 17 were outside the range defined in the present invention) immediately without cooling. Heat treatment was performed.

  Furthermore, test no. In the conventional examples of 18 to 20, finish rolling consisting of stretching and constant diameter processing was performed under the conditions shown in Table 2 to produce seamless steel pipes having an outer diameter of 240 mm and a wall thickness of 8 to 30 mm. Each obtained seamless steel pipe was cooled to room temperature, charged into an offline quenching furnace, heated to the conditions shown in Table 2, and then subjected to water-cooling quenching and tempering heat treatment. In Table 2, the soaking temperature in parentheses () indicates the temperature after the finish rolling, once cooled to room temperature, charged in a quenching furnace, and heated and soaked.

  Evaluation of each obtained seamless steel pipe was conducted using the 12C test piece specified for the mechanical material of JIS Z2201 as the metal material tensile test piece and performing a tensile test according to the metal material tensile test method specified in JIS Z2241. The target values were the tensile strength (TS) of 590 MPa or more and the yield strength (YS) of 490 MPa or more.

  As for toughness, a V-notch test piece with a width of 10 mm specified for a metal material impact test piece of JIS Z2202, a Charpy impact test was performed according to the metal material impact test method specified in JIS Z2242, and Absorbed energy (J) was measured, and 100 J or more was set as a target value.

  In the evaluation, a case where all of the tensile strength, yield strength, and absorbed energy satisfy the target value was evaluated as ◯, and a case where any of the tensile strength and yield strength did not satisfy the target value was evaluated as x. Test results and evaluation results are shown in Table 3.

  As shown in Tables 2 and 3, test no. In Examples 1 to 7 of the present invention, there is little variation in both mechanical characteristics and absorbed energy. The performance is almost the same as that of 18-20 conventional examples. Furthermore, the surface skin was in a good state. In contrast, test no. In Comparative Examples 8 to 17, the condition range of the present invention was not satisfied. The performance of conventional examples of 18 to 20 could not be ensured, and all were evaluated as x.

According to the seamless steel pipe manufacturing method of the present invention, even if it is an online quenching and tempering heat treatment, it can ensure the same performance as that of the conventional offline quenching and tempering heat treatment, compared with the conventional method, Seamless steel pipes that are optimal as machine structural parts are available because they can simplify the manufacturing process, improve pipe manufacturing efficiency, save energy, are inexpensive, have no strength variation, have excellent toughness, and have a good surface texture. Obtainable. Thereby, it can employ | adopt widely as a manufacturing method of the seamless steel pipe for machine structural components.

Claims (2)

In mass%, C: 0.10 to 0.25%, Si: 1.00% or less, Mn: 0.20 to 2.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.1 to 1.5%, Mo: 0.5% or less, Ti: 0.005 to 0.030% and V: 0.01 to 0.10%, Ti / N: 3. 5 or less, and the balance is Fe and impurities consisting of impurities,
After piercing and rolling, finish rolling consisting of stretching and constant diameter processing is performed at a cross-section reduction rate of 40% or more and a finishing temperature of 900 ° C. or more, and immediately heated to 900 to 1000 ° C. without cooling, A method for producing a seamless steel pipe for machine structural parts, wherein tempering heat treatment is continuously performed online. Furthermore, billets containing one or more of Cu: 0.5% or less, Ni: 1.0% or less, and Nb: 0.01% or less are used. Item 2. A method for producing a seamless steel pipe for machine structural parts according to Item 1.