WO2023243730A1

WO2023243730A1 - High-strength steel member which comprises medium-carbon chromium–molybdenum steel and which has high-frequency-tempered refined layer in surface layer section, and method for manufacturing same

Info

Publication number: WO2023243730A1
Application number: PCT/JP2023/022512
Authority: WO
Inventors: 重信濱中
Original assignee: 濱中ナット株式会社
Priority date: 2022-06-17
Filing date: 2023-06-17
Publication date: 2023-12-21

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

High-strength steel member made of medium-carbon chromium-molybdenum steel having an induction-tempered tempered layer on the surface layer, and its manufacturing method

The present invention relates to a high-strength steel member manufactured from medium-carbon chromium-molybdenum steel having an induction tempered tempered layer on the surface layer, and a method for manufacturing the same. In particular, in steel members made of chromium molybdenum steel that has a high strength of 1100 MPa or more, an induction tempered and tempered layer is formed near the surface layer of 1.2 mm to maintain the tensile strength of the core while making it susceptible to delayed fracture. The present invention relates to a high-strength steel member that has a reduced capacity, and a high-strength bolt and PC steel bar that uses the same to roll the threaded part and has a structure that prevents screws from coming off.

PC steel bars and high-strength bolts are known as typical examples of high-strength steel members.

PC steel rod refers to a tension material for prestressed concrete (PC) that has a higher tensile strength than reinforcing steel materials such as reinforcing bars. The purpose of using high-strength steel for the PC is to ensure sufficient prestress even after the prestress decreases over time due to concrete creep and drying shrinkage. In order to reduce delayed fracture in this PC steel material having a tensile strength of 1100 MPa or more, a method has been proposed in which 1.2% or more of Si element is added to the steel material to improve this (Patent Document 1). However, there have been reports of breakage accidents due to delayed breakage during use.

Therefore, in order to reduce delayed fracture by adding Si to the steel material, a special ultra-high frequency tempering method has been proposed in which the quenched steel material is heated above the Ac1 transformation point and then rapidly cooled within 1 second. It has been reported that characteristics that cannot be obtained by high-frequency tempering treatment can be obtained (Patent Document 2).

However, the improvement method disclosed in Patent Document 2 is a processing technology for reducing delayed fracture by adding Si element to so-called boron-added medium carbon steel, and in general-purpose medium carbon chromium molybdenum steel, the amount of Si added is within 0.35. limited and cannot be applied. Moreover, since the technology 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.

Therefore, in order to improve the delayed fracture resistance of general-purpose medium carbon chromium molybdenum steel whose Si content is limited to within 0.35, it is necessary to consider (1) a bolt surface layer of 0.2 mm within the JIS standard; Although a small decarburized layer has a large effect on delayed fracture properties, it does not affect fatigue fracture properties. (2) The delayed fracture characteristics of a bolt depend more on the surface hardness than the internal hardness, and the higher the surface hardness, the worse the delayed fracture characteristics. (3) There is a report that mild decarburization of the bolt surface layer significantly extends the delayed fracture life in a corrosive environment (Non-Patent Document 1), but since the decarburized layer is extremely shallow at 0.2 mm in carbon It was discovered that fatigue failure progresses when the decarburized layer is dissolved by the anode reaction.

Japanese Patent Application Publication No. 1984-219447 Patent No. 5337792 (November 6, 2013) Publication

Therefore, in the present invention, in chromium molybdenum steel, instead of a thin decarburized layer of 0.2 mm on the surface layer, a relatively deep hardness gradient tempered layer reaching 1.2 mm or more on the surface layer is provided, and fatigue failure due to dissolution of the decarburized layer in an anode reaction is provided. When trying to avoid the progression of the problem, problems arose in that high-strength steel members of 1,100 MPa or higher, especially high-strength bolts and PC steel bars, are susceptible to screws coming off and damage to incompletely threaded parts. . Therefore, an object of the present invention is to develop a high-strength steel member that can prevent high delayed fracture resistance while maintaining high strength of 1100 MPa or more in chromium-molybdenum steel, and a method for manufacturing the same.

The present invention applies quenching and tempering to medium-carbon chromium-molybdenum steel containing a chemical composition of C: 0.30 to 0.50, and then induction-tempering the steel member that has been tempered to have an average cross-sectional tensile strength of 1100 MPa or more. While maintaining a hardness of 340Hv or more on the surface and a hardness of 400Hv on the core, a high-frequency tempering part with a depth of 1.2mm or more is provided on the surface layer to create a straight or almost linear hardness gradient of 40Hv/mm or more. It was discovered that by applying this, delayed fracture resistance can be imparted while maintaining the tensile strength of the core of the steel member, and the product was completed and is a chromium-molybdenum steel with a chemical composition of C: 0.30 to 0.50. It is a high-strength steel member that has been heat-treated and tempered to have an average cross-sectional tensile strength of 1100 MPa or more, and has a core with a hardness of 340 Hv or more on the surface by high-temperature induction tempering at 630°C or higher, which is below the A1 transformation point. While maintaining a hardness of Hv400 or more in the steel member core, a tempered layer having a linear or almost linear hardness gradient of Hv40/mm or more is formed from the surface layer to the core part 1.2 mm or more away, and A high-strength steel member characterized by having delayed fracture resistance while maintaining tensile strength.

The present invention also provides a method for manufacturing the above-mentioned high-strength steel member. That is, quenching and tempering is performed on chromium molybdenum steel containing a chemical component of C: 0.30 to 0.50, and delayed fracture resistance is imparted to steel members tempered to have an average cross-sectional tensile strength of 1100 MPa or more. The processing method includes tempering a steel member tempered to have a cross-sectional average tensile strength of 1100 MPa or more by high-frequency induction heating at a temperature within the range of 630° C. or higher below the A1 transformation point, particularly at a rate of 10 mm to 30 mm/sec. By heating only the surface at the speed of movement within the coil and allowing it to cool, the hardness of the surface is maintained at 340Hv or higher and the core hardness is 400Hv, while the distance from the surface to the core is approximately 1.2mm or more at 40Hv/ The object of the present invention is to provide a high-strength steel member that has a linear or nearly linear hardness gradient of mm or more and provides delayed fracture resistance while maintaining the tensile strength of the steel member core.

According to the present invention, in chromium molybdenum steel containing a chemical composition of C: 0.30 to 0.50, the high frequency range is 10 to 40 kHz, which is slightly higher than that of the conventional method, preferably 15 kHz to 35 kHz, and particularly 10 to 30 mm/sec. When only the surface layer of a high-strength steel member is tempered by induction tempering at a high temperature of 630°C below the A1 transformation point using induction heating at a rate of It has been found that the above linear hardness or almost linear hardness is formed, and that this unique tempered layer provides the core of the steel member with resistance to breakage while maintaining tensile strength. Here, the term "substantially linear gradient" refers to a case where the linear gradient includes a curved line that is separated by a maximum of ±20% in the intermediate portion, and is preferably about ±10% at the maximum.
Moreover, when a threaded part is formed by final rolling using a high-strength steel member having this tempered layer on its surface as a blank material, plastic deformation during rolling of the tempered layer with a certain hardness gradient causes the screw bottom to reach the cutting edge. It has been found that as a result of the hardness increasing in the direction, a structure is formed to prevent screws from coming off.

In the chromium molybdenum steel used in the present invention. When the chemical component contains C: 0.30 to 0.50, a steel member tempered to have a cross-sectional average tensile strength of 1100 MPa or more can be provided. This is because C is necessary for high-strength steel members mainly having a tempered martensitic structure or a tempered bainite structure. Therefore, as a chromium molybdenum steel, SCM435H is a medium carbon chromium molybdenum steel containing C: 0.30 to 0.50, Cr: 0.85 to 1.25, and Mo: 0.15 to 0.35. 440 and 445 chromium molybdenum steels are preferred steel types for the present invention.

In manufacturing steel members including high-strength bolts and PC steel rods that impart delayed fracture resistance that have been tempered to have a cross-sectional average tensile strength of 1100 MPa or more, Tempering the steel member by high-frequency induction heating at a temperature within the range of 630°C or higher below the A1 transformation point, moving the workpiece at a moving speed within the coil of 10 mm to 30 mm/sec, heating only the surface for a short time, By cooling, it is possible to maintain a hardness of 340Hv or more on the surface and 400Hv on the core, while imparting a carbon concentration gradient of 40Hv/mm from the surface to the core in the vicinity of 1.2mm or more. good. By using this as a blank material and performing final rolling to form a threaded portion, it is possible to further provide a screw removal prevention structure (FIG. 7(b)).

It is a figure showing the hardness distribution of a test material. FIG. 3 is a diagram showing the hardness distribution after softening treatment. It is a figure showing an example of the high frequency induction heating device used for softening processing. This is the hardness distribution when induction tempering was performed at 650°C, 700°C, and 750°C after preliminary treatment. FIG. 3 is a diagram showing the breaking load, tensile strength, yield load, yield strength, elongation, and rupture location when heat-treated at 650° C. and 700° C. after the pre-processing as is. When examining changes in tensile strength due to softening treatment temperature, as-pretreated (Photo 1), softened at 650°C after pretreatment (Photo 2), and softened at 700°C after pretreatment (Photo 3) FIG. 2 is a photograph and a schematic diagram showing a fractured form of the material. FIG. 3 is a schematic cross-sectional view (a) and (b) of a tooth portion of a blank material of the present invention before and after final rolling treatment.

High-frequency induction heating is used to manufacture steel parts such as automobile parts, bolts, and PC steel bars, and surface hardening by high-frequency induction heating is widely adopted. Heating depth and heating temperature can be adjusted by power and coil design. In the cited document 2, a method is adopted in which boron-added medium carbon steel containing 1.2% or more Si element is heated to the A1 transformation point or higher using an ultra-high frequency of 50 kHz, but in the present invention, the Si element is 0.35% or more. A medium carbon chromium molybdenum steel having the following limits was used, and induction tempering was performed at a temperature below the A1 transformation point.

In the present invention, SCM435, 440 with C of 0.3 or more and 0.5 or less is used as a PC steel material that uses chromium molybdenum steel with limited Si element and is tempered to a tensile strength of 1100 MPa or more up to the core by quenching and tempering. 445 chromium molybdenum steel can be used, but was prepared using SCM435.
Thereafter, high-frequency tempering is performed using a high-frequency induction heating apparatus shown in FIG.
In the high-frequency induction heating device, Cited Document 2 applies a predetermined high frequency of 50 kHz to the induction coil, but since it is difficult to control the heating time, it is usually It is preferable to conduct induction heating at a frequency of 10 kHz to 40 kHz higher than the 9.35 kHz used, 10 kHz to 670 to 720 ℃, 20 kHz to 650 to 700 kHz, 30 kHz to 630 to 680 kHz, and 40 kHz to 650 kHz. It is preferable to heat at a temperature of 700°C or higher.

An experiment was conducted to confirm this effect.
Test material: SCM435 Prepared material φ25.47 x 200mm
Temperature conditions for test material (pretreatment)
Quenching: 855°C gas heating water cooling, tempering: 540°C gas heating water cooling

Figure 1 shows the hardness distribution of the test material.

Softening treatment conditions 1. Heating element: Water-cooled φ10mm copper tube with 2 turns 2. Coil inner diameter φ40mm, gap between coil and test material 7.25mm (=(40-25.5)/2)
3. Output 15KW, frequency 30KHz
4. Coil moving speed: 20 mm/sec The high-frequency induction heating device (manufactured by Fuji Electronics Industries) used is shown in FIG. A high frequency is applied to the coil through which the workpiece passes, allowing induction heating, and the heating depth and heating temperature can be adjusted by changing the frequency, input power, and coil design.

Figure 2 shows the hardness distribution after softening treatment. According to FIG. 2, it was possible to soften up to 1.2 mm of the surface from Hv400 to Hv359 by heating at 650°C.

Furthermore, it was investigated to what extent the softening of the surface of the PC steel bar affects the tensile strength of the PC steel bar. Class C No. 1 (SBPR1080/1230) was used as the PC steel bar. The relationship between the treatment temperatures of 650°C, 700°C, and 750°C and the hardness distribution from the surface was determined. The results are shown in FIG.
From FIG. 4, it was found that the surface softening progressed as the treatment temperature increased to 650°C, 700°C, and 750°C.

Next, changes in tensile strength due to treatment temperature were investigated. A tensile test was conducted using a PC steel bar with M27 rolled screws. The treatment temperature was 650°C and 700°C, and the breaking load KN, tensile strength N/mm ² , yield load KN, yield strength N/mm ² , elongation %, and fracture morphology were examined. The results are shown in FIG. It was found that the decrease in tensile strength due to surface softening remained at 2% or less, and that there was no decrease in yield strength due to surface softening.

Photo 1 in Figure 6 shows the fracture form as it is pre-heated, Photo 2 in Figure 6 shows the fracture form when softened at 650°C after pre-heating, and Figure 6 shows the fracture form at 700°C after pre-heating. It is shown in Photo 3. In both cases, there was no abnormality in the fitting part with the nut, and it was confirmed that the breakage occurred at the play screw part. After testing, the nut could be removed by hand.
As a result, it was found that softening treatment at 650° C. and 700° C. after preliminary heat refining is preferably performed at a temperature of 630° C. or more and A1 transformation point of 723° C. or less.

The first object of the present invention is to provide a high-strength steel member tempered to have a cross-sectional average tensile strength of 1100 MPa or more using chromium-molybdenum steel containing C0.30 to 0.50, which has an A1 transformation point or lower. By high-temperature induction tempering at 630°C or higher, the surface hardness is 340Hv or more and the core has a hardness of Hv400 or more, while the core part that is 1.2mm or more away from the surface has a straight line or almost Hv40/mm or more. The objective is to provide a high-strength steel member that forms a tempered layer with a linear hardness gradient and has delayed fracture resistance while maintaining the tensile strength of the core of the steel member. In the case of a steel bar, it is preferable that the bar be further provided with a screw removal structure.
In the normal bolt manufacturing process, as shown in Fig. 7(a), the thread is formed by a partially decarburized layer 2 covering the non-decarburized raw metal 4, and a completely decarburized layer 1 remaining on the surface. When the final rolling process is performed, changes occur due to plastic deformation. In other words, in 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 cover the adjacent tempered layer, forming a thread, as shown in Fig. 7(b). In this case, a decarburized layer with a high carbon concentration remains at the bottom of the screw, while an incompletely decarburized layer moves toward the top of the thread due to plastic deformation, and the metal structure of the non-decarburized dough rises, increasing the height of the non-decarburized part of the thread. There is a tendency for the number to increase. This seems to be the result of an increase in hardness at the beginning of shearing when a screw is pulled out, where the top of the female thread on the nut comes into pressure contact with the thread on the bolt. As in the conventional case, when the surface layer is decarburized to provide resistance to delayed fracture (Fig. 7 (a)), although the hardness decreases from the bottom of the screw toward the middle, in the case of Fig. 7 (b), Hardness increases from the bottom of the screw to the middle of the screw. This is presumed to be the result of the thread configuration changing from Fig. 7(a) to Fig. 7(b) due to plastic change.

1 Completely decarburized layer 2 Partially decarburized layer 3 Pitch line 4 Raw metal

Claims

A high-strength steel member made by heat-treating chromium-molybdenum steel containing a chemical component of C: 0.30 to 0.50 and tempering it to an average cross-sectional tensile strength of 1100 MPa or more, which is 630°C below the A1 transformation point. The above high temperature induction tempering maintains a hardness of 340Hv or more on the surface and 400 or more Hv on the core, while maintaining a straight or almost straight line of 40Hv or more on the core that is 1.2mm or more away from the surface. A high-strength steel member characterized by forming a tempered layer having a hardness gradient and having delayed fracture resistance while maintaining the tensile strength of the steel member core.
SCM435H, 440H, and 445H whose chemical components include C: 0.30 to 0.50, Cr: 0.85 to 1.25, and Mo: 0.15 to 0.35, at 630°C below the A1 transformation point. By induction tempering at a temperature above, the surface hardness is 340Hv or more and the core has a hardness of Hv400 or more, while the core part that is 1.2mm or more away from the surface layer has a straight line or almost straight line of Hv40/mm or more. A high-strength bolt and a PC steel rod characterized by forming a tempered layer having a hardness gradient and imparting delayed fracture resistance while maintaining the tensile strength of the core of the steel member.
SCM435H, 440 and 445 whose chemical components include C: 0.30 to 0.50, Cr: 0.85 to 1.25, Mo: 0.15 to 0.35, at 630°C below the A1 transformation point. By induction tempering at the above temperature, the hardness of the surface is 340Hv or more and the core has a hardness of Hv400 or more, while the hardness of Hv40/mm or more is maintained in a straight line or almost A high-strength bolt and a PC steel bar, characterized in that they are made of a blank material having a tempered layer with a linear hardness gradient, which is subjected to final rolling to provide a threaded portion with a screw-out prevention structure.
A treatment method for imparting delayed fracture resistance to steel members tempered to have an average cross-sectional tensile strength of 1100 MPa or more by subjecting chromium molybdenum steel containing a chemical component of C: 0.30 to 0.50 to quenching and tempering. A steel member tempered to have a cross-sectional average 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 at 10 mm to 30 mm/s in a coil. By induction heating only the surface using high-frequency induction heating at a moving speed of 10 to 40 KHz and allowing it to cool, the surface hardness is 340 Hv or more and the core hardness is 400 Hv, while the depth from the surface layer to the core is maintained. A high-strength steel product characterized by forming a tempered layer having a hardness gradient of 40 Hv/mm or more over 1.2 mm or more, and imparting delayed fracture resistance while maintaining the tensile strength of the steel member core. Method of manufacturing parts.
Hardening and tempering treatment is applied to SCM435H, 440, and 445 chromium molybdenum steels whose chemical compositions include C: 0.30 to 0.50, Cr: 0.85 to 1.25, and Mo: 0.15 to 0.35. A method for producing high-strength bolts and PC steel bars that have been tempered to have an average cross-sectional tensile strength of 1100 MPa or more and have delayed fracture resistance, the method being made of steel that has been tempered to have an average cross-sectional tensile strength of 1100 MPa or more. The blank member is tempered by high-frequency induction heating at a temperature within the range of 630°C to 720°C below the A1 transformation point, and only the surface is induction-heated using high-frequency waves of 10 to 40 kHz at a moving speed in the coil of 10 mm to 30 mm/sec. By cooling, the tempered layer maintains a hardness of 340 Hv or more on the surface and 400 Hv in the core, and has a hardness gradient of 40 Hv/mm or more from the surface to the core at a depth of 1.2 mm or more. While maintaining the tensile strength of the core of the steel member and imparting delayed fracture resistance, the high-strength steel blank member is rolled to form a threaded portion to provide a structure that prevents screws from coming off. A method for producing a high-strength bolt and a prestressed steel bar.