High-strength steel component made of medium-carbon chromium-molybdenum steel having a high-frequency tempered layer on its surface, and method for manufacturing the same.

Abstract

Provided is a high-strength steel member that has reduced delayed fracture sensitivity at 1100 MPa or greater.　During the manufacture, using a high-strength steel member in which chromium–molybdenum steel that includes 0.30–0.50% of carbon as a chemical constituent is used, of a steel member refined to a full-section mean tensile strength of at least 1100 MPa, high-frequency tempering is performed at a temperature within the range from 630°C up to the A1 transition point, producing a refined structure with a linear or substantially linear hardness gradient of at least HV 40 / mm from a surface to a core section, so as to provide a high-strength steel member that, while maintaining the tensile strength of the core section of the steel member, has reduced delayed fracture sensitivity. By forming a threaded section through final rolling of the steel member, a high-tension bolt and a PC steel bar can be provided that do not come unscrewed.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel member manufactured from medium-carbon chromium-molybdenum steel having a high-frequency tempering refined layer in a surface layer portion and a method for manufacturing the same. In particular, the present invention relates to: a high-strength steel member made of chromium-molybdenum steel having a high strength of 1100 MPa or more, in which a high-frequency tempering refined layer is formed up to the vicinity of 1.2 mm of the surface layer portion, and which has reduced delayed fracture susceptibility while maintaining the tensile strength of the core portion; and further a high-strength bolt having a screw slip-out prevention structure obtained by subjecting a screw portion to rolling using the same, and a PC steel bar. [Background Art]

[0002] As typical examples of high-strength steel members, PC steel bars and high-strength bolts are known.

[0003] A PC steel bar refers to a tensioning material for prestressed concrete (PC) that has higher tensile strength than reinforcing steel materials such as reinforcing bars. The reason why high-strength steel materials are used for PC is to ensure sufficient prestress even after the prestress decreases over time due to creep and drying shrinkage of concrete. In this PC steel material having a tensile strength of 1100 MPa or more, in order to reduce delayed fracture, a method of improving this by adding 1.2% or more of Si element to the steel material is pro posed (Patent Document 1). However, there have been reports of fracture accidents due to delayed fracture during use.

[0004] Accordingly, when adding Si to the steel material to reduce delayed fracture, a special ultra-high-frequency tempering method has been proposed in which quenched steel is heated to a temperature equal to or higher than the Ac1 transformation point and then rapidly cooled within 1 second, and it has been reported that characteristics that cannot be obtained by ordinary high-frequency tempering treatment can be obtained (Patent Document 2).

[0005] However, the improvement method described in Patent Document 2 is a processing technique for reducing delayed fracture by adding Si elements to so-called boron-added medium-carbon steel, and it cannot be applied to commonly used medium-carbon chromium-molybdenum steel because the amount of Si added is limited to within 0.35. Moreover, since it is a technique that involves heating at an ultra-high frequency of 50 kHz and rapid heating within 1 second after heating, there is a problem that it cannot be applied without special heating equipment.

[0006] Therefore, in considering the improvement of delayed fracture resistance in general-purpose medium-carbon chromium-molybdenum steel, where the Si content is limited to within 0.35, we found that (1) the small decarburized layer of 0.2 mm on the surface layer of bolt, which is within the JIS standard, has a large effect on delayed fracture characteristics, but does not affect fatigue fracture characteristics. (2) The delayed fracture characteristics of bolt are more influenced by surface hardness than internal hardness. (3) Weakness of the surface layer There are reports that charcoal significantly extends the delayed fracture life in corrosive environments (Non-Patent Literature 1), but we have learned that because the decarburized layer is extremely shallow at 0.2 mm of carbon, fatigue fracture progresses when the decarburized layer dissolves in the anodic reaction. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 1984-219447 [Patent Document 2] Patent No. 5337792 (November 6, 2013) Publication [Non-patent literature]

[0008] [Non-Patent Document 1] The effects of decarburization and charring on the surface of high-strength bolts on delayed fracture and fatigue fracture. "Materials" Vol. 38, No. 425: February 1989. [Overview of the project] [Problems that the invention aims to solve]

[0009] Therefore, in chromium-molybdenum steel, instead of a thin decarburized layer of 0.2 mm on the surface, a relatively deep hardness gradient tempering layer extending to 1.2 mm or more on the surface is provided to avoid the progression of fatigue fracture due to the dissolution of the decarburized layer in the anodic reaction. However, in high-strength steel members with a strength of 1100 MPa or more, particularly high-strength bolts and PC steel bars, problems arise such as a tendency for threads to come loose and for incomplete thread portions to break easily. For this reason, the objective of this invention is to develop a high-strength steel member and a method for manufacturing the same that can prevent high delayed fracture resistance while maintaining a high strength of 1100 MPa or more in chromium-molybdenum steel. [Means for solving the problem]

[0010] This invention relates to a steel member made of medium-carbon chromium-molybdenum steel containing C:0.30-0.50, which has been subjected to quenching and tempering to achieve an average cross-sectional tensile strength of 1100 MPa or higher. This member is then subjected to high-frequency tempering to create a surface hardness of 340 Hv or higher while maintaining a core hardness of 400 Hv, and a high-frequency tempered tempered layer with a depth of 1.2 mm or more in the surface layer, resulting in a linear or almost linear tempered surface with a hardness of 40 Hv / mm or higher. We discovered that applying a linear hardness gradient provides delayed fracture resistance while maintaining the tensile strength of the core of a steel member, and have completed the process. The chemical composition contains chromium with C:0.30~0.50. This high-strength steel member is made of ribdenum steel that has been heat-treated to achieve an average cross-sectional tensile strength of 1100 MPa or higher. The high-temperature high-frequency tempering at 630°C or higher, below the A1 transformation point, maintains a hardness of 340 Hv or higher on the surface and a hardness of 400 Hv or higher in the core, while forming a tempered layer with a linear or nearly linear hardness gradient of 40 / mm or higher extending from the surface layer to the core at a distance of 1.2 mm or more, thereby maintaining the tensile strength of the steel member's core while providing resistance to delayed fracture.

[0011] Furthermore, the present invention provides a method for manufacturing the above-mentioned high-strength steel member. Specifically, it is a treatment method for imparting delayed fracture resistance to a steel member that has been tempered to an average cross-sectional tensile strength of 1100 MPa or more by quenching and tempering chromium-molybdenum steel containing C:0.30 to 0.50 in its chemical composition, wherein the steel member tempered to an average cross-sectional tensile strength of 1100 MPa or more is tempered by high-frequency induction heating at a temperature of 630°C or higher below the A1 transformation point, particularly at a coil movement speed of 10 mm to 30 mm / sec, heating only the surface and then allowing it to cool, thereby maintaining a hardness of 340 Hv or more on the surface and 400 Hv in the core, while imparting a straight or nearly linear hardness gradient of 40 Hv / mm or more from the surface to the core, extending approximately 1.2 mm or more. This invention provides a high-strength steel member that maintains the tensile strength of the part while providing delayed fracture resistance. [Effects of the Invention]

[0012] According to the present invention, in chromium-molybdenum steel with a chemical composition of C:0.30-0.50, when only the surface layer of a high-strength steel member is tempered by high-frequency induction heating at a high temperature of 630°C below the A1 transformation point, particularly at a speed of 10-30 mm / sec, using induction heating in a high-frequency range of 10 to 40 kHz, preferably 15 kHz to 35 kHz, which is slightly higher than conventional methods, a linear hardness of HV40 / mm or more or a nearly linear gradient is formed in the core portion extending to approximately 1.2 mm from the surface. We found that this unique tempered layer imparts delayed fracture resistance to the core of the steel member while maintaining tensile strength. Here, a nearly linear gradient refers to a case where a curved line is included that is separated by a maximum of ±20% in the intermediate part from a linear gradient, and a maximum of about ±10% is preferable. Furthermore, we discovered that when a high-strength steel member having this tempered layer on its surface is used as a blank material and a screw thread is formed by final rolling, the plastic deformation of the tempered layer, which has a certain hardness gradient, during rolling causes the hardness to increase from the screw root towards the cutting edge, resulting in the formation of a screw pull-out prevention structure.

[0013] In the chromium-molybdenum steel used in this invention, the chemical composition is C:0.30~0.50 The inclusion thereof provides a steel member tempered to have a cross-sectional average tensile strength of 1100 MPa or more. This is because C is necessary for high-strength steel members mainly composed of a tempered martensite structure or a tempered bainite structure. Therefore, as chromium molybdenum steel, C: 0.3 0 to 0.50, which is a medium-carbon chromium molybdenum steel containing Cr: 0.85 to 1.25, M o: Chromium molybdenum steels SCM435H, 440 and 445 containing 0.15 to 0.35 are listed as suitable steel types for the present invention.

[0014] In manufacturing steel members including high-strength bolts and PC steel bars that are tempered to a cross-sectional average tensile strength of 1100 MPa or more and impart delayed fracture resistance, tempering by high-frequency induction heating is performed on the steel member tempered to a cross-sectional average tensile strength of 1100 MPa or more at a temperature within the range of 630°C or higher below the A1 transformation point, in particular, by moving the workpiece at an in-coil moving speed of 10 mm / sec to 30 mm / sec, heating only the surface for a short time and then allowing it to cool, it is preferable to provide a carbon concentration gradient of 40 Hv / mm up to around 1.2 mm or more from the surface to the core while maintaining a hardness of 340 Hv or more on the surface and a hardness of 400 Hv in the core. Using this as a blank, final rolling is performed to form a threaded portion, and a screw escape prevention structure can be further imparted (Fig. 7(b)). BRIEF DESCRIPTION OF THE DRAWINGS

[0015] [Figure 1] It is a figure showing the hardness distribution of a test material. [Figure 2] It is a figure showing the hardness distribution after softening treatment. [Figure 3] It is a figure showing an example of the high-frequency induction heating apparatus used for softening treatment. [Figure 4] It is a hardness distribution diagram in a case where high-frequency tempering is performed at 650°C, 700°C, and 750°C after preliminary treatment. [Figure 5] It is a diagram showing breaking load, tensile strength, yield load, yield strength, elongation, and fracture position in a case where heat treatment is performed at 650°C and 700°C after preliminary treatment while maintaining the state after preliminary treatment. [Figure 6] These are photographs and schematic diagrams showing fracture morphologies when investigating changes in tensile strength depending on softening treatment temperature: as-pretreated (Photo 1), softened at 650°C after pretreatment (Photo 2), and softened at 700°C after pretreatment (Photo 3). [Figure 7] These are schematic cross-sectional views (a) and (b) of tooth portions of the blank material according to the present invention before and after the final rolling treatment. DETAILED DESCRIPTION OF EMBODIMENTS

[0016] High-frequency induction heating is used in the manufacture of steel members such as automobile parts, bolts, and PC steel bars, and surface hardening by high-frequency induction heating is widely employed. In a high-frequency induction heating apparatus used for high-frequency hardening, the heating depth and heating temperature can be adjusted depending on the frequency used, input power, and coil design. In Citation Document 2, a method is adopted in which a boron-added medium carbon steel containing 1.2 mass% or more of Si element is heated above the A1 transformation point using an ultra-high frequency of 50 kHz, whereas in the present invention, a medium carbon chromium molybdenum steel in which the Si element content is limited to 0.35 mass% or less is used, and high-frequency tempering is performed below the A1 transformation point.

[0017] In the present invention, as a PC steel material that uses a chromium molybdenum steel with limited Si content and is quenched and tempered to have a tensile strength of 1100 MPa or more up to the core, SCM435, SCM440, and SCM445 chromium molybdenum steels having a C content of 0.3 mass% or more and 0.5 mass% or less can be used, and adjustment was performed using SCM435. Thereafter, high-frequency tempering is performed using the high-frequency induction heating apparatus shown in Fig. 3. In Citation Document 2, a predetermined high frequency of 50 kHz is applied to the induction coil of the high-frequency induction heating apparatus, but controlling the heating time is difficult. Therefore, when the pass time through the induction coil is set to a level that can be controlled at 10 to 30 mm / sec, it is preferable to perform induction heating at a frequency of 10 kHz to 40 kHz, which is higher than the commonly used frequency of 9.35 kHz. It is preferable to perform heating at 670°C to 720°C at 10 kHz, at 650°C to 700°C at 20 kHz, at 630°C to 680°C at 30 kHz, and at 650°C to 700°C at 40 kHz.

[0018] An experiment was conducted to confirm this effect. Test material: SCM435 processed material, φ25.47 x 200 mm Heat treatment conditions for test materials (preliminary treatment) Hardening: 855°C gas heating, water cooling; Tempering: 540°C gas heating, water cooling

[0019] The hardness distribution of the test material is shown in Figure 1.

[0020] Softening treatment conditions 1. Heating element: 2 turns, length of water-cooled φ10mm copper tube. 2. Coil inner diameter φ40mm, gap between coil and test material 7.25mm (=(40-25.5) / 2) 3. Output 15KW, Frequency 30KHz 4. Coil movement speed: 20 mm / second Figure 3 shows the high-frequency induction heating device (manufactured by Fuji Electronics Industry) used. High frequency is applied to the coil through which the workpiece passes, enabling induction heating. The heating depth and heating temperature can be adjusted by changing the frequency, input power, and coil design.

[0021] Figure 2 shows the hardness distribution after softening treatment. According to Figure 2, heating at 650°C allowed the surface to be softened from Hv400 to Hv359 up to 1.2 mm.

[0022] Furthermore, we investigated the extent to which surface softening of PC steel bars affects their tensile strength. Type C No. 1 (SBPR1080 / 1230) PC steel bars were used. The relationship between treatment temperatures of 650°C, 700°C, and 750°C and the hardness distribution from the surface was determined. The results are shown in Figure 4. Figure 4 shows that surface softening progresses as the processing temperature increases to 650°C, 700°C, and 750°C.

[0023] Next, we investigated the change in tensile strength due to processing temperature. Tensile tests were performed on PC steel bars with M27 rolled threads. The processing temperatures were 650°C and 700°C, and the breaking load was measured in kN and the tensile strength in N / mm². 2 , Yield load KN, Proof strength N / mm 2The elongation percentage and fracture mode were investigated. The results are shown in Figure 5. It was found that the decrease in tensile strength due to surface softening was limited to 2% or less, and there was no decrease in yield strength due to surface softening.

[0024] Figure 6, Photo 1 shows the fracture pattern in the pre-heat-treated state, Figure 6, Photo 2 shows the fracture pattern after pre-heat-treated softening at 650°C, and Figure 6, Photo 3 shows the fracture pattern after pre-heat-treated softening at 700°C. In all cases, there were no abnormalities in the mating portion with the nut, and it was confirmed that the fracture occurred from the loose thread portion. After the test, the nut could be removed by hand. This revealed that softening treatment at 650°C and 700°C after pre-heating is preferable, resulting in a softening treatment between 630°C and the A1 transformation point of 723°C.

[0025] The first object of the present invention is to provide a high-strength steel member that is tempered to have an average cross-sectional tensile strength of 1100 MPa or more using chromium-molybdenum steel containing C0.30 to 0.50, wherein high-temperature high-frequency tempering at 630°C or higher below the A1 transformation point maintains a hardness of 340 Hv or more on the surface and a hardness of Hv400 or more in the core, while forming a tempered layer having a straight or nearly linear hardness gradient of Hv40 / mm or more from the surface layer to the core at a distance of 1.2 mm or more, thereby providing a high-strength steel member that has delayed fracture resistance while maintaining the tensile strength of the core of the steel member. In the case of high-strength bolts and PC steel bars, it is preferable to further provide a thread pull-out structure. In a typical bolt manufacturing process, as shown in Figure 7(a), the threads are formed on top of the non-decarburized raw metal 4. A partially decarburized layer 2 covers the surface, while a fully decarburized layer 1 remains on its surface. However, this structure changes due to plastic deformation during the final rolling process. Specifically, during the final rolling process, the upper part of the tempered layer, which has a hardness gradient depending on the carbon concentration, moves by plastic flow so as to overlap the adjacent tempered layer, forming the thread. As shown in Figure 7(b), a decarburized layer with a high carbon concentration remains at the bottom of the thread, while the incompletely decarburized layer moves towards the thread crest side by plastic deformation. Simultaneously, the non-decarburized base metal structure bulges, and the height of the non-decarburized portion of the thread tends to increase. As a result, the hardness at the shear initiation point of thread pull-out, where the female thread crest on the nut side presses into contact with the thread on the bolt side, is thought to increase. In the conventional case where delayed fracture resistance is provided by surface decarburization (Figure 7(a)) ), despite the hardness decreasing from the base to the middle of the screw, in the case of Figure 7(b), the hardness increases from the base to the middle of the screw. This is because the thread structure undergoes plastic deformation from Figure 7(a) It is presumed that the result will be as shown in Figure 7(b). [Explanation of Symbols]

[0026] 1. Completely decarburized layer 2 Partially decarburized layer 3 Pitch wires 4. Raw Metal

Claims

1. A method for manufacturing a high-strength steel member, characterized in that, in producing a high-strength steel member to which the average cross-sectional tensile strength is 1100 MPa or higher by heat-treating chromium-molybdenum steel containing C: 0.30 to 0.50% in its chemical composition, the heat treatment is performed by high-frequency tempering at 630°C or higher below the A1 transformation point, maintaining a hardness of 340 Hv or higher on the surface and 400 Hv or higher in the core, while forming a tempered layer having a linear or nearly linear hardness gradient of 40 Hv / mm or higher extending from the surface to the core at a distance of 1.2 mm or more, thereby providing the high-strength steel member with delayed fracture resistance while maintaining the tensile strength of the core.

2. The method for manufacturing a high-strength steel member according to claim 1, wherein the high-strength steel member is a high-strength bolt or a PC steel bar.

3. A method for manufacturing a high-strength bolt or PC steel bar, characterized in that the high-strength steel member described in claim 1 is used as a blank material, and the blank material is subjected to final rolling to provide a screw thread retention structure to the threaded portion.

4. Chromium-molybdenum steel containing C: 0.30-0.50% is subjected to quenching and tempering treatment. A method for manufacturing a high-strength steel member, characterized in that, in order to impart delayed fracture resistance to a steel member that has been tempered to an average cross-sectional tensile strength of 1100 MPa or more, the steel member that has been tempered to an average cross-sectional tensile strength of 1100 MPa or more is tempered by high-frequency induction heating at a temperature in the range of 630°C to 720°C below the A1 transformation point, with a coil movement speed of 10 mm / s to 30 mm / s and high-frequency induction heating of 10 to 40 kHz, inductively heating only the surface, and allowing it to cool, thereby forming a tempered layer with a hardness of 340 Hv or more on the surface and 400 Hv or more in the core, and a hardness gradient of Hv 40 / mm or more from the surface to the core at a depth of 1.2 mm or more, thereby producing a high-strength steel member that has delayed fracture resistance while maintaining the tensile strength of the core of the steel member.

5. A method for manufacturing a high-strength bolt or PC steel bar, characterized in that the high-strength steel member described in claim 4 is used as a steel blank member, a threaded portion is formed on the steel blank member by rolling, and a high-strength bolt and PC steel bar having a screw-loosening prevention structure are manufactured.

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