Manufacturing method of chromium-molybdenum vanadium steel

Abstract

To improve mechanical properties of a standard type of chromium molybdenum vanadium steel when a cross-section diameter of a bar is large.SOLUTION: A chromium molybdenum vanadium steel has a component composition consisting of C: 0.35-0.50%, Si: 0.40% or less, Mn: 0.40-0.85%, P: 0.040% or less, S: 0.040% or less, Cr: 0.80-1.20%, Mo: 0.45-0.65%, V: 0.25-0.35%, in mass%, and the balance of Fe and impurities. In a production method of the chromium molybdenum vanadium steel, a steel bar having a circular equivalent diameter and a cross section of 170-330 mm is subjected to quenching at a quenching temperature of 880-1050°C, and a cooling rate from the quenching temperature to 550°C is set to 0.4-1.1°C / second at the center temperature of the cross section of the steel bar. After quenching, tempering is performed at a tempering temperature of 455-730°C.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a chromium-molybdenum-vanadium steel. [Background technology]

[0002] Conventionally, chromium-molybdenum-vanadium steel is required to have mechanical properties such as yield strength, tensile strength, elongation, reduction of area, and toughness after quenching and tempering, and many standard steel types have been proposed.

[0003] For example, "SNB16" of JIS-G-4107 (high-temperature alloy steel bolt material) has a composition, by mass, of 0.36-0.44% C, 0.20-0.35% Si, 0.45-0.70% Mn, 0.040% or less P, 0.040% or less S, 0.80-1.15% Cr, 0.50-0.65% Mo, 0.25-0.35% V, with the balance being Fe and impurities. After quenching, the steel is tempered at a temperature of 650°C or higher and provided as a steel bar with a cross-sectional diameter of 180 mm or less. Furthermore, "Grade B16" of ASTM-A193, which corresponds to SNB16, has a composition, in mass%, of 0.36-0.47% C, 0.15-0.35% Si, 0.45-0.70% Mn, 0.035% or less P, 0.040% or less S, 0.80-1.15% Cr, 0.50-0.65% Mo, 0.25-0.35% V, with the balance being Fe and impurities (including 0.015% or less Al). Similarly, after quenching, the steel is tempered at a tempering temperature of 650°C or higher, and is provided as a steel bar with a cross-sectional diameter of, for example, more than 100 mm and 180 mm or less. If it is "40CrMoV4-7" of DIN 17240, which corresponds to SNB16, it has a composition, by mass%, of C: 0.36 to 0.44%, Si: 0.40% or less, Mn: 0.45 to 0.85%, P: 0.030% or less, S: 0.030% or less, Cr: 0.90 to 1.20%, Mo: 0.50 to 0.65%, V: 0.25 to 0.35%, with the balance being Fe and impurities. Then, it is quenched (oil or water quenched) at a quenching temperature of 880 to 930°C, and then tempered at a tempering temperature of 670 to 730°C.

[0004] Additionally, "SNB21" has been standardized in JIS-G-4108 (special purpose alloy steel bars for bolts), which has a composition, in mass%, of C: 0.36 to 0.44%, Si: 0.20 to 0.35%, Mn: 0.45 to 0.70%, P: 0.025% or less, S: 0.025% or less, Cr: 0.80 to 1.15%, Mo: 0.50 to 0.65%, V: 0.25 to 0.35%, with the balance being Fe and impurities. The corresponding ASTM-A540 "Grade B21" has a composition, in mass%, of C: 0.35-0.44%, Si: 0.15-0.35%, Mn: 0.45-0.70%, P: 0.025% or less, S: 0.025% or less, Cr: 0.80-1.15%, Mo: 0.50-0.65%, V: 0.25-0.35%, with the balance being Fe and impurities (including Al: 0.015% or less). These standard steel grades are tempered at a tempering temperature of 455°C or higher after quenching, and for example, in the case of Class 5, they are provided as steel bars with a cross-sectional diameter of more than 150mm and not more than 200mm.

[0005] Furthermore, AMS standardizes "6304," which has a composition, in mass%, of C: 0.40-0.50%, Si: 0.15-0.35%, Mn: 0.40-0.70%, P: 0.025% or less, S: 0.025% or less, Cr: 0.80-1.10%, Mo: 0.45-0.65%, V: 0.25-0.35%, with the balance being Fe and impurities (including Ni: 0.25% or less, Cu: 0.35% or less). 6304 is, for example, a material with a cross-sectional area of ​​36 in 2 (approx. 232cm2 . Equivalent diameter of circle is approximately 170 mm or more 133 in 2 (approx. 858cm 2 It is also available as a steel bar with a diameter of approximately 330 mm or less (equivalent to a circle). The evaluation criteria for hardenability stipulate that the bar must be quenched (air-cooled) at a quenching temperature of 954±14°C, followed by tempering at a tempering temperature of 593±8°C.

[0006] Although the chemical compositions of the above-mentioned standard steel grades are generally the same, in recent years, in order to improve various mechanical properties of these standard steel grades after quenching and tempering, in addition to improvements to the above-mentioned standard steel grades, various quenching and tempering processes have been carried out on the above-mentioned standard steel grades themselves. The quenching and tempering conditions at these times are not limited to the above standards, and for example, the quenching temperature is 1050°C followed by oil quenching, or 940°C. The tempering temperature is selected from a wide range of 525 to 700°C (Patent Documents 1 to 6). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2021-179222 [Patent Document 2] Patent Publication No. 2021-092289 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-328032 [Patent Document 4] Japanese Patent Application Laid-Open No. 2000-119818 [Patent Document 5] Japanese Patent Application Publication No. 06-158170 [Patent Document 6] Japanese Patent Publication No. 130456 / 1983 Summary of the Invention [Problem to be solved by the invention]

[0008] Ideally, the mechanical properties of chromium-molybdenum vanadium steel are expected to improve when its quenched and tempered structure is fully martensite. This is also true for the above-mentioned existing standard steel types. However, when the chromium-molybdenum vanadium steel is quenched, its mechanical properties after quenching and tempering may deteriorate. An object of the present invention is to provide a method for producing chromium-molybdenum-vanadium steel that can improve the mechanical properties of steel bars having large cross-sectional diameters, targeting the above-mentioned standard steel types. [Means for solving the problem]

[0009] That is, the present invention provides a method for producing a chromium-molybdenum vanadium steel having a chemical composition, by mass%, of C: 0.35 to 0.50%, Si: 0.40% or less, Mn: 0.40 to 0.85%, P: 0.040% or less, S: 0.040% or less, Cr: 0.80 to 1.20%, Mo: 0.45 to 0.65%, V: 0.25 to 0.35%, with the balance being Fe and impurities, and wherein the steel bar has a cross-sectional size of 170 to 330 mm in equivalent circle diameter and is quenched at a quenching temperature of 880 to 1050°C, wherein the quenching is performed at a cooling rate of 0.4 to 1.1°C / second from the quenching temperature to 550°C at the center temperature of the cross section of the steel bar. After the above quenching, tempering is carried out at a tempering temperature of 455 to 730°C in this method for producing chromium molybdenum vanadium steel. [Effects of the Invention]

[0010] According to the present invention, it is possible to improve the mechanical properties of steel bars having large cross-sectional diameters for standard steel grades of chromium-molybdenum-vanadium steel. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing a continuous cooling transformation point diagram of a chromium-molybdenum-vanadium steel having a chemical composition according to the present invention, and cooling curves by quenching of examples of the present invention and comparative examples. [Figure 2]1 is an optical microscope photograph showing the quenched and tempered structure of the center and surface of a chromium-molybdenum-vanadium steel having a chemical composition according to the present invention, when the steel bar is subjected to quenching and tempering according to an example of the present invention and a comparative example. [Figure 3] FIG. 1 is a graph showing the mechanical properties at a position midway between the surface and the center of a chromium-molybdenum-vanadium steel having a chemical composition according to the present invention, when the steel bar is subjected to quenching and tempering according to an example of the present invention and a comparative example. [Figure 4] FIG. 1 is a graph showing the mechanical properties at a position midway between the surface and the center of a chromium-molybdenum-vanadium steel having a chemical composition according to the present invention, when the steel bar is subjected to quenching and tempering according to an example of the present invention and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0012] (1) The method for producing a chromium-molybdenum vanadium steel of the present invention is such that the chromium-molybdenum vanadium steel has a composition, by mass%, of C: 0.35 to 0.50%, Si: 0.40% or less, Mn: 0.40 to 0.85%, P: 0.040% or less, S: 0.040% or less, Cr: 0.80 to 1.20%, Mo: 0.45 to 0.65%, V: 0.25 to 0.35%, with the balance being Fe and impurities. As described above, the chemical composition of the chromium-molybdenum-vanadium steel according to the present invention is that of a conventionally proposed standard steel type, such as the above-mentioned JIS SNB16 and SNB21, ASTM B16 and B21, DIN 40CrMoV4-7, and AMS 6304.

[0013] (2) The method for producing chromium-molybdenum vanadium steel of the present invention is such that the standard steel type of the component composition of (1) is "a steel bar having a cross-sectional size of 170 to 330 mm in equivalent circle diameter." In the standard steel types (1) above, the size of the steel bar according to the present invention is also the same as that previously proposed, as mentioned above. To elaborate, for example, SNB16 provides steel bars with a cross-sectional diameter (i.e., equivalent to a circle diameter depending on the cross-sectional shape) of 180 mm or less, B16 provides steel bars with a cross-sectional diameter of more than 100 mm and less than 180 mm, SNB21 and B21 provide steel bars with a cross-sectional diameter of more than 150 mm and less than 200 mm, and 6304 provides steel bars with a cross-sectional area of ​​36 in. 2 (approx. 232cm 2 . Equivalent diameter of circle is approximately 170 mm or more 133 in 2 (approx. 858cm 2 Steel bars with an equivalent diameter of approximately 330 mm or less are provided. Since it is believed that such a size of steel bar will result in a decrease in mechanical properties after quenching and tempering, the steel bar targeted by the present invention has a cross-sectional size of 170 to 330 mm in equivalent circle diameter, more specifically, 200 mm or more or 300 mm or less.

[0014] (3) The method for producing a chromium-molybdenum vanadium steel of the present invention involves quenching the steel bar of (2) at a quenching temperature of 880 to 1050°C, and then tempering the bar at a tempering temperature of 455 to 730°C, for example. The heat treatment (quenching and tempering) conditions according to the present invention applied to the standard steel types (1) above have also been proposed in the past, as described above. Specifically, for example, a tempering temperature of 650°C or higher is recommended for SNB16 and B16, and a tempering temperature of 455°C or higher is recommended for SNB21 and B21. Furthermore, a quenching temperature of 880 to 930°C and a tempering temperature of 670 to 730°C are recommended for 40CrMoV4-7. For 6304, a quenching temperature of 954±14°C and a tempering temperature of 593±8°C are recommended as evaluation conditions for hardenability. In addition to the above standards, for example, the quenching temperature is set to 1050°C or 940°C, and the tempering temperature is set within the range of 525 to 700°C.

[0015] (4) In the method for producing a chromium-molybdenum vanadium steel of the present invention, the quenching in (3) is performed at a cooling rate of 0.4 to 1.1°C / sec from the quenching temperature to 550°C at the center temperature of the cross section of the steel bar. Conventionally, when a steel bar made of the standard steel type (1) was quenched and tempered, the desired mechanical properties were obtained under the heat treatment conditions proposed in various standards and prior art documents if the cross-sectional diameter of the steel bar was small. However, when the cross-sectional diameter of the steel bar was large, the desired mechanical properties were sometimes not obtained when the above-mentioned heat treatment conditions were applied, and a decrease in toughness in particular was confirmed. From the perspective of general technical knowledge, one factor behind this decrease in toughness is thought to be insufficient transformation of the quenched and tempered structure into martensite. However, if it is desired to increase the cooling rate during quenching to promote martensite transformation, it is easy to imagine that, even if it is easy to do so in the surface region, it is difficult to do so throughout the center when the cross-sectional diameter of the steel bar is large (i.e., when the steel bar has an equivalent circular diameter of 170 to 330 mm).

[0016] Therefore, in the present invention, the quenched and tempered structure of a steel bar in which a decrease in toughness was confirmed was first confirmed. Figure 2 shows the quenched and tempered structure of the center and surface (periphery) of a steel bar of the standard steel type (1) with a cross-sectional area of ​​280 mm in equivalent circle diameter, observed using an optical microscope at 400x magnification. The upper part of the figure shows the quenched and tempered structure when toughness decreased. Evaluation of this revealed that the structure of the surface was mainly martensite, but the formation of pearlite (lumpy areas darker than the surrounding area) was clearly observed in the structure of the center. Therefore, if one of the factors behind the decrease in toughness is an increase in pearlite rather than a decrease in martensite, then in order to compensate for the resulting decrease in toughness, it is not necessary to adjust the structure to be mainly martensite, but rather it is sufficient to adjust it to a structure mainly composed of bainite (the structure at the bottom of the figure).If this is the case, it is possible to improve toughness without having to increase the cooling rate during quenching to an "unrealistically" high rate.

[0017] Therefore, we next investigated the range of cooling rates required during quenching to achieve the target bainite-dominated structure of the quenched and tempered structure of the standard steel grade (1). Figure 1 shows a continuous cooling transformation diagram obtained by actual measurements using a test specimen having the chemical composition of the standard steel grade (1). According to Figure 1, the transformation temperature of bainite for this chemical composition is approximately 400°C. Furthermore, if the cooling curve when cooling a steel bar from the quenching temperature passes through the range of the approximate length of the thick solid line in the diagram, which corresponds to the transformation temperature, excessive pearlite formation can be avoided, and bainite can be formed, allowing the quenched structure after cooling to be adjusted to be primarily bainite. The cooling curve for obtaining such a quenched structure can be achieved by setting the cooling rate from the quenching temperature to 550°C at a rate of 0.4 to 1.1°C / s, and this cooling rate is also feasible. Preferably, the cooling rate is 0.5°C / s or higher, and more preferably, the cooling rate is 0.6°C / s or higher. Preferably, the cooling rate is 1.0°C / sec or less, more preferably 0.9°C / sec or less, even more preferably 0.8°C / sec or less, and even more preferably 0.7°C / sec or less. The above-mentioned "cooling rate from the quenching temperature to 550°C" can be calculated using the formula "(quenching temperature - 550°C) / time required from the start of cooling to reach 550°C." Alternatively, it can be calculated from the heat transfer from the center to the surface in the cross section of the steel bar. Calculations from heat transfer are typically performed by those skilled in the art, and can be performed using, for example, the heat transfer coefficient between the material of the steel bar and the quenching medium, the physical properties of the steel bar (specific gravity, specific heat, thermal conductivity), and the temperature of the quenching medium.

[0018] To obtain the effects of the present invention, it is difficult to achieve the cooling rate of "0.4 to 1.1°C / sec" according to the present invention at the center of a steel bar having a cross-sectional area of ​​170 to 330 mm in equivalent circle diameter using oil cooling. Even with water cooling, it is not easy to achieve this rate by simply immersing the steel bar in a water tank. However, this water cooling can be achieved by agitating the water with a fan installed in the water tank and rocking the steel bar immersed in water. If the time required is recorded, even if it is difficult to measure the temperature at the center of the steel bar during actual quenching (for example, if it is difficult to install a thermocouple at the center of the steel bar), the temperature at the center of the steel bar at that time can be indirectly determined by measuring the immersion time, making it possible to reproduce the present invention. Furthermore, if the time until the temperature at the center of the steel bar drops below 550°C and reaches the temperature at which the steel bar can be removed from the water tank is recorded, the present invention can be reproduced by monitoring the "total" immersion time required to quench the steel bar, making it easy to implement the present invention.

[0019] According to the above, it is possible to improve the mechanical properties after quenching and tempering of steel bars having a large cross-sectional diameter (or circle-equivalent diameter) for the standard steel grades of conventionally known chromium-molybdenum-vanadium steels. [Example]

[0020] A steel bar of conventional standard steel type (equivalent to DIN-40CrMoV4-7) was prepared. The size of the steel bar was set to a cross-sectional diameter of 280 mm.

[0021] After installing a thermocouple in the center of the steel bar, the bar was heated to the quenching temperature of 930°C in a heating furnace, in accordance with the conditions specified for this steel grade. After the entire bar reached this temperature and was held there for 90 minutes, it was removed from the furnace and immediately lifted by a chain and transported to a water tank where the water was stirred by a fan installed inside the tank. The bar was then cooled while being oscillated. When the surface temperature of the bar measured by a radiation thermometer reached 170°C, the bar was removed from the water tank, completing the quenching process. The time required from immersion to removal from the water tank was approximately 20 minutes. This resulted in a cooling rate of 0.5°C / s from the quenching temperature to 550°C at the center of the cross section of the steel bar (solid line cooling curve in Figure 1).

[0022] The steel bars were also quenched using oil quenching. The procedure for heating the steel bars in the heating furnace, removing them from the furnace, and transporting them was the same as for water quenching. The transported steel bars were then immersed in an oil bath for cooling. When the surface temperature of the steel bars measured by a radiation thermometer reached 170°C, the steel bars were removed from the oil bath, completing the quenching process. The time required from immersion to removal from the oil bath was approximately 60 minutes. A fan was installed in the oil bath to agitate the oil, but the steel bars were not rocked during immersion. The cooling rate from the quenching temperature to 550°C at the center of the steel bar cross section was calculated from the radial heat transfer to be 0.3°C / s (the cooling curve shown by the dashed line in Figure 1).

[0023] Then, when the surface temperature of the steel bars after quenching had dropped below 100°C, they were tempered. Tempering was carried out in a heating furnace to a tempering temperature of 690°C, based on the conditions specified for this standard steel type, and after the entire steel bar had reached this temperature, it was held for 7 hours, after which the steel bar was removed from the furnace and allowed to cool (air-cooled). The quenched and tempered structures of the surface (periphery) and center of the chromium-molybdenum-vanadium steels A (quenched by water quenching) and B (quenched by oil quenching) thus produced were observed (Fig. 2), and the mechanical properties (tensile strength, elongation, reduction of area, V-notch Charpy impact value) were measured at a position midway between the surface and center of the steel bars. The results are shown in Fig. 3.

[0024] As can be seen from Figure 2, the quenched and tempered structure (upper structure) of chrome molybdenum vanadium steel B was mainly composed of martensite in the surface area, but significant pearlite formation was confirmed in the center. In contrast, the quenched and tempered structure (lower structure) of chrome molybdenum vanadium steel A was mainly composed of martensite in the surface area, and even in the center, pearlite formation was suppressed, with bainite being the main component. 3, the mechanical properties of chromium molybdenum vanadium steel A were maintained at sufficient values, and compared to the mechanical properties of chromium molybdenum vanadium steel B, the Charpy impact value was improved. [Example]

[0025] Steel bars of conventional standard steel grade (equivalent to DIN-40CrMoV4-7) were prepared. The steel bars had a cross-sectional diameter of 245 mm. Chromium-molybdenum vanadium steels C (quenched by water quenching) and D (quenched by oil quenching) were then fabricated in the same manner as in Example 1. The cooling rate from the quenching temperature to 550°C at the center of the steel bar cross section, calculated from the radial heat transfer, was 0.6°C / sec for chromium-molybdenum vanadium steel C and 0.3°C / sec for chromium-molybdenum vanadium steel D. The mechanical properties (tensile strength, elongation, reduction of area, and V-notch Charpy impact value) were then measured at a position midway between the surface and center of the fabricated steel bars. The results are shown in Figure 4. As can be seen from Figure 4, chromium-molybdenum vanadium steel C maintained sufficient mechanical properties while exhibiting an improved Charpy impact value compared to chromium-molybdenum vanadium steel D.

Claims

1. A method for producing chromium-molybdenum vanadium steel, comprising the steps of: (a) forming a steel bar having a cross-sectional area of ​​170 to 330 mm in equivalent circle diameter; (b) quenching the steel bar at a quenching temperature of 880 to 1050°C; (c) forming a steel bar having a composition, in mass%, of C: 0.35 to 0.50%, Si: 0.40% or less, Mn: 0.40 to 0.85%, P: 0.040% or less, S: 0.040% or less, Cr: 0.80 to 1.20%, Mo: 0.45 to 0.65%, V: 0.25 to 0.35%, the balance being Fe and impurities; and (d) forming a steel bar having a cross-sectional area of ​​170 to 330 mm in equivalent circle diameter. The method for producing a chromium-molybdenum-vanadium steel, wherein the quenching is performed at a cooling rate from the quenching temperature to 550°C of 0.4 to 1.1°C / second at the center temperature of the cross section of the steel bar.

2. 2. The method for producing a chromium-molybdenum-vanadium steel according to claim 1, wherein after the quenching, tempering is carried out at a tempering temperature of 455 to 730°C.

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