Steel for high-frequency hardening

Abstract

To provide steel for high-frequency hardening which is excellent in cold workability and high-frequency hardenability.SOLUTION: Steel for high-frequency hardening contains 0.50 mass% or more and 0.85 mass% or less C, 0.10 mass% or more and 0.40 mass% or less Si, 0.20 mass% or more and 0.80 mass% or less Mn, 0.030 mass% or less S, 0.10 mass% or more and 0.90 mass% or less Cr, and 0.050 mass% or less Al. A carbon equivalent Ceq calculated by Expression is 0.60 or more. The metallic structure of the steel contains cementite. A product (Ceq As) of the carbon equivalent Ceq and a needle ratio As of the cementite is 4.00 or more and 9.00 or less. Expression: Ceq=C%+Si% / 7+Mn% / 5+CR% / 9. In the expression, C% represents a C mass percentage content, Si% represents an Si mass percentage content, Mn% is a Mn mass percentage content, and Cr% represents a Cr mass percentage content.SELECTED DRAWING: None

Description

Technical Field

[0001] This specification discloses steel suitable for rough materials, bars, tubes, wires, etc. to be subjected to high-frequency quenching.

Background Art

[0002] General rough materials are obtained through hot working. Cold working is performed on this rough material to obtain an intermediate product. For the purpose of surface hardening, high-frequency quenching is performed on this intermediate product to obtain a steel product. An example of steel suitable for high-frequency quenching is disclosed in Japanese Patent Application Laid-Open No. 2014-37592.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Rough materials require excellent cold workability. Rough materials further require high-frequency quenchability. In conventional steels, the compatibility between cold workability and high-frequency quenchability is insufficient. Similar problems are also found in steels for bars, tubes, wires, etc.

[0005] What the applicant intends is to provide steel with an excellent balance between cold workability and high-frequency quenchability.

Means for Solving the Problems

[0006] The steel for high-frequency quenching disclosed in this specification is C: 0.50 mass% or more and 0.85 mass% or less Si: 0.10 mass% or more and 0.40 mass% or less Mn: 0.20 mass% or more and 0.80 mass% or less S: 0.030 mass% or less Cr: 0.10 mass% or more and 0.90 mass% or less and Al: 0.050 mass% or less It contains the following. The carbon equivalent Ceq of this steel, calculated by the following formula, is 0.60 or more. The metallographic structure of this steel contains cementite. The product (Ceq·As) of the carbon equivalent Ceq and the acicular ratio As of cementite is 4.00 or more and 9.00 or less. Ceq = C% + Si% / 7 + Mn% / 5 + CR% / 9 In this formula, C% represents the mass content of C, Si% represents the mass content of Si, Mn% represents the mass content of Mn, and Cr% represents the mass content of Cr.

[0007] Preferably, the ratio (Ceq / C) of the carbon equivalent Ceq to the content of C (mass%) is 1.20 or more and 1.50 or less.

[0008] The material of the rough-shaped material disclosed in this specification is C: 0.50 mass% or more and 0.85 mass% or less Si: 0.10 mass% or more and 0.40 mass% or less Mn: 0.20 mass% or more and 0.80 mass% or less S: 0.030 mass% or less Cr: 0.10 mass% or more and 0.90 mass% or less and Al: 0.050 mass% or less It is a steel containing the following. The carbon equivalent Ceq of this steel, calculated by the following formula, is 0.60 or more. The metallographic structure of this steel contains cementite. The product (Ceq·As) of the carbon equivalent Ceq and the acicular ratio As of cementite is 4.00 or more and 9.00 or less. Ceq = C% + Si% / 7 + Mn% / 5 + CR% / 9 In the above formula, C% represents the mass content of C, Si% represents the mass content of Si, Mn% represents the mass content of Mn, and Cr% represents the mass content of Cr.

Advantages of the Invention

[0009] This high-frequency quenching steel is excellent in cold workability and high-frequency hardenability. [Embodiment for Carrying Out the Invention]

[0010] The high-frequency quenching steel according to this embodiment is, for example, the material of a rough-shaped material. Cold working is performed on this rough-shaped material or the like to obtain an intermediate product. High-frequency quenching is performed on this intermediate product to obtain a steel product.

[0011] [Composition] The main component of this high-frequency quenching steel is Fe. This high-frequency quenching steel C: 0.50 mass% or more and 0.85 mass% or less Si: 0.10 mass% or more and 0.40 mass% or less Mn: 0.20 mass% or more and 0.80 mass% or less S: 0.030 mass% or less Cr: 0.10 mass% or more and 0.90 mass% or less And Al: 0.050 mass% or less contains. Preferably, the balance is Fe and inevitable impurities. Details of each element will be described below.

[0012] [Carbon (C)] C dissolves in Fe by high-frequency quenching. By this high-frequency quenching, high hardness on the surface of the steel product can be achieved. From this viewpoint, the content of C is preferably 0.50 mass% or more, more preferably 0.53 mass% or more, and particularly preferably 0.55 mass% or more. Excessive C inhibits the cold workability of the rough-shaped material. From the viewpoint of excellent cold workability, the content of C is preferably 0.85 mass% or less, more preferably 0.75 mass% or less, and particularly preferably 0.70 mass% or less.

[0013] [Silicon (Si)] Si contributes to deoxidation in the steelmaking process. Si also contributes to high-frequency hardenability. From these viewpoints, the Si content is preferably 0.10 mass% or more, more preferably 0.13 mass% or more, and particularly preferably 0.15 mass% or more. Excessive Si inhibits the cold workability of the rough-shaped material. From the viewpoint of excellent cold workability, the Si content is preferably 0.40 mass% or less, more preferably 0.35 mass% or less, and particularly preferably 0.32 mass% or less.

[0014] [Manganese (Mn)] Mn can contribute to the high-frequency hardenability of the rough-shaped material. From this viewpoint, the Mn content is preferably 0.20 mass% or more, more preferably 0.40 mass% or more, and particularly preferably 0.50 mass% or more. Excessive Mn inhibits the cold workability of the rough-shaped material. From the viewpoint of excellent cold workability, the Mn content is preferably 0.80 mass% or less, more preferably 0.75 mass% or less, and particularly preferably 0.72 mass% or less.

[0015] [Sulfur (S)] S is an impurity. S can combine with Mn to form MnS. MnS promotes the propagation of cracks during the cold working of the rough-shaped material. From the viewpoint of excellent cold workability, the S content is preferably 0.030 mass% or less, more preferably 0.025 mass% or less, and particularly preferably 0.020 mass% or less. S is not an essential element for this high-frequency hardening steel. In other words, the S content may be less than the detection limit.

[0016] [Chromium (Cr)] Cr can contribute to the high-frequency hardenability of the rough-shaped material. From this viewpoint, the Cr content is preferably 0.10 mass% or more, more preferably 0.15 mass% or more, and particularly preferably 0.17 mass% or more. Excessive Cr inhibits the cold workability of the rough-shaped material. Furthermore, excessive Cr promotes the remaining of Cr carbides after high-frequency hardenability. The remaining of Cr carbides causes low hardness of the steel product due to the lack of C. From the viewpoints of excellent cold workability and high hardness, the Cr content is preferably 0.90 mass% or less, more preferably 0.60 mass% or less, and particularly preferably 0.40 mass% or less.

[0017] [Aluminum (Al)] Al is present in the steel as an oxide. This oxide inhibits the cold workability of the steel. From the perspective of cold workability, the Al content is preferably 0.050 mass% or less, more preferably 0.040 mass% or less, and particularly preferably 0.035 mass% or less. Al is not an essential element for this high-frequency quenching steel. In other words, the Al content may be less than the detection limit.

[0018] [Carbon equivalent] In this specification, the carbon equivalent Ceq is calculated by the following formula. Ceq = C% + Si% / 7 + Mn% / 5 + CR% / 9 In this formula, C% represents the mass content of C, Si% represents the mass content of Si, Mn% represents the mass content of Mn, and Cr% represents the mass content of Cr. The above formula is set considering the ratio of the influence of Si, Mn, and Cr on high-frequency hardenability to the influence of C on high-frequency hardenability.

[0019] The carbon equivalent Ceq is preferably 0.60 or more. Rough materials with the carbon equivalent Ceq within this range are excellent in high-frequency hardenability. From this perspective, the carbon equivalent Ceq is more preferably 0.65 or more, and particularly preferably 0.68 or more. Rough materials with an excessive carbon equivalent Ceq are inferior in cold workability. From the perspective of excellent cold workability, the carbon equivalent Ceq is preferably 1.20 or less, more preferably 1.05 or less, and particularly preferably 0.95 or less.

[0020] [Metallographic structure] The main metal structure of the rough-shaped material is pearlite. This metal structure contains a plurality of plate-like cementites. The acicular ratio As of these cementites is measured by the method to be described in detail later. The acicular ratio As is preferably 3.00 or more and 11.50 or less. For a rough-shaped material with an acicular ratio As of 3.00 or more, even with short-time heating in high-frequency quenching, carbides can be sufficiently dissolved in the base material. By this dissolution, sufficient hardness (for example, 750 HV or more) of the steel product can be achieved. From this viewpoint, the acicular ratio As is more preferably 5.00 or more, and particularly preferably 6.50 or more. In a rough-shaped material with an acicular ratio As of 11.50 or less, lamellar carbides are moderately segmented. In this rough-shaped material, a hardness of 260 HV or less can be achieved. From this viewpoint, the acicular ratio As is more preferably 11.40 or less, and particularly preferably 11.30 or less.

[0021] The product (Ceq·As) of the carbon equivalent Ceq and the acicular ratio As is preferably 4.00 or more and 9.00 or less. In a rough-shaped material with a product (Ceq·As) of 4.00 or more, a sufficient amount of C is dissolved in Fe by high-frequency quenching. From this rough-shaped material, a steel product with high surface hardness can be obtained. From this viewpoint, the product (Ceq·As) is more preferably 5.00 or more, and particularly preferably 6.00 or more. In a rough-shaped material with a product (Ceq·As) of 9.00 or less, the lamellar structure is less. This rough-shaped material has excellent cold workability. From this viewpoint, the product (Ceq·As) is more preferably 8.90 or less, and particularly preferably 8.80 or less.

[0022] The product (Ceq·As) of the conventional high-frequency quenching steel was less than 4.00 or more than 9.00. In the present embodiment, by adjusting the temperature, holding time, and cooling rate in the heat treatment, and the composition, a product (Ceq·As) of 4.00 or more and 9.00 or less is achieved. In the high-frequency quenching steel having the above-described composition, an example of the heat treatment conditions for achieving a product (Ceq·As) of 4.00 or more is as follows. Temperature: 650 °C or more and 750 °C or less Holding time: 0.5 hours or more and 4.0 hours or less Cooling rate: 0.10 °C / second or more and 0.80 °C / second or less In the high-frequency quenching steel having the above composition, an example of the heat treatment conditions for achieving a product of (Ceq·As) of 9.00 or less is as follows. Temperature: 840°C or higher and 870°C or lower Holding time: 1.0 hour or longer and 4.0 hours or shorter Cooling rate: 0.10°C / second or higher and 0.80°C / second or lower

[0023] The ratio (Ceq / C) of the carbon equivalent Ceq to the content (%) of C is preferably 1.20 or higher and 1.50 or lower. A rough-shaped material with a ratio (Ceq / C) of 1.20 or higher is excellent in high-frequency hardenability. From this perspective, the ratio (Ceq / C) is more preferably 1.25 or higher, and particularly preferably 1.30 or higher. A rough-shaped material with a ratio (Ceq / C) of 1.50 or lower is excellent in cold workability. From this perspective, the ratio (Ceq / C) is more preferably 1.45 or lower, and particularly preferably 1.40 or lower.

[0024] [Manufacturing method] Hereinafter, taking a rough-shaped material as an example, the manufacturing method of the steel will be described. In this manufacturing method, an untreated rough-shaped material is obtained from molten steel having a predetermined composition through casting, hot plastic working, etc. Heat treatment is performed on this untreated rough-shaped material to obtain a rough-shaped material. By adjusting the composition and heat treatment conditions, a rough-shaped material having a preferable metal structure can be obtained.

[0025] [Normalizing] As the heat treatment, normalizing is exemplified. In normalizing, the untreated rough-shaped material after hot plastic working is held at a high temperature and then slowly cooled. By normalizing, homogenization of crystal grains is achieved. In normalizing, the preferable temperature is 850°C or higher and 870°C or lower, and the preferable holding time is 0.5 hour or longer and 3 hours or shorter.

[0026] [Spheroidizing annealing] As another heat treatment, spheroidizing annealing is exemplified. It is preferable that the steel subjected to normalizing is further subjected to spheroidizing annealing. In spheroidizing annealing, the preferable temperature is 650°C or higher and 880°C or lower, and the preferable holding time is 0.5 hour or longer and 6 hours or shorter.

[0027] Cold working The rough material is subjected to cold working. Typical working processes are plastic working and cutting. By cold working, intermediate products are obtained. Since the rough material has excellent cold workability, cracks in the rough material during cold plastic working are less likely to occur.

[0028] [High-frequency hardening] This intermediate product is subjected to high-frequency hardening. In high-frequency hardening, the vicinity of the surface of the intermediate product changes to austenite by high-frequency induction heating. By subsequent rapid cooling, this austenite changes to martensite, and a steel product is obtained. The surface hardness of this steel product is high.

Example

[0029] The effects of the martensitic stainless steel according to the examples will be clarified below, but the scope disclosed in this specification based on the description of this example should not be construed in a limited manner.

[0030] [Example 1] In a vacuum induction melting furnace, an ingot having the composition shown in Table 1 below was melted. The mass of this ingot was 100 kg. This ingot was subjected to hot forging and elongation at 1150 °C to obtain a cylindrical bar with a diameter of 32 mm. This bar was annealed. The annealing temperature was 860 °C. The annealing time was 1 hour. This bar was subjected to the heat treatment under the following condition H1. The scale was removed from the surface of this bar to obtain a sample. Heat treatment condition H1 Temperature: 840 °C Holding time: 1.5 Hr Cooling: Air cooling

[0031] [Examples 2 - 11 and Comparative Examples 1 - 16] Samples were obtained in the same manner as in Example 1, except that the composition and heat treatment conditions were as shown in Tables 2 and 3 below. The details of the composition are shown in Table 1 below. The details of the heat treatment conditions H2 and H3 are as follows. Heat treatment condition H2 Temperature: 700 °C Holding time: 4.0 Hr Cooling: Air cooling Heat treatment condition H3 Temperature: 780 °C Holding time: 2.0 Hr Cooling: Slowly cool to the Ar1 transformation point and then air cool

[0032] [Acicular ratio As] The sample was cut to obtain a cross-section parallel to the rolling direction. This cross-section was mirror-polished. The intermediate position between the center and the surface of the cross-section was observed by SEM. The magnification was 10,000 times. The image obtained from a colony where the plate-like cementite is almost perpendicular to the observation surface was binarized with image processing software, and the major and minor diameters of the cementite were measured. Observation was carried out in 5 fields of view, and the average value of the ratio of the major diameter to the minor diameter was calculated. This result is shown in Tables 2 and 3 below.

[0033] [Internal hardness] The sample was cut to obtain a cross-section parallel to the rolling direction. This cross-section was mirror-polished. The Vickers hardness was measured at the intermediate position between the center and the surface of the cross-section. The measurement was carried out in accordance with the provisions of "JIS Z 2244 (2020)". The load was 200 gf. Steel with low internal hardness has excellent cold workability. This result is shown in Tables 2 and 3 below.

[0034] [Limiting reduction rate] The sample was cut, and a test piece (No. 1, constrained type) with a diameter of 14 mm and a length of 21 mm including the center of this sample was obtained. This test piece was subjected to a cold broaching test, and the reduction rate at which cracks occur was measured. The minimum value among the reduction rates obtained from 6 test pieces is the limiting reduction rate. Steel with a large limiting reduction rate has excellent cold workability. This result is shown in Tables 2 and 3 below.

[0035] [Surface hardness] The sample was cut, and a test piece with a diameter of 20 mm and a length of 100 mm, which included the center of this sample, was obtained. High-frequency quenching was performed on this test piece using a ring-shaped coil. The frequency was 150 kHz and the time was 5 seconds. This test piece was cut to obtain a cross-section parallel to the rolling direction. Mirror polishing was performed on this cross-section. The Vickers hardness was measured at a position 0.1 mm from the surface. The measurement was carried out in accordance with the provisions of "JIS Z 2244 (2020)". The load was 300 gf. Steel with high surface hardness is excellent in high-frequency hardenability. These results are shown in Tables 2 and 3 below.

[0036] [Evaluation of Cold Workability] The samples were classified according to the following criteria. A: The internal hardness is 260 HV or less and the limiting compression ratio is 57% or more. B: The internal hardness is more than 260 HV or the limiting compression ratio is less than 57%. These results are shown in Tables 2 and 3 below.

[0037] [Evaluation of Cold Workability] The samples were classified according to the following criteria. A: The surface hardness is 750 HV or more. B: The surface hardness is less than 750 HV. These results are shown in Tables 2 and 3 below.

[0038]

Table 1

[0039]

Table 2

[0040]

Table 3

[0041] As shown in Tables 2 and 3, the high-frequency quenching steel of each example is excellent in various properties. From the above evaluation results, the superiority of this steel is clear.

Industrial Applicability

[0042] The above-mentioned high-frequency quenching steel is suitable for various applications such as rough-shaped materials, bars, tubes, and wires.

Claims

1. C: 0.50% by mass or more and 0.85% by mass or less Si: 0.10% by mass or more and 0.40% by mass or less Mn: 0.20% by mass or more and 0.80% by mass or less S: 0.030% by mass or less Cr: 0.10% by mass or more and 0.90% by mass or less and Al: 0.050% by mass or less containing the carbon equivalent Ceq calculated by the following formula is 0.60 or more, its metallographic structure contains cementite, a high-frequency quenching steel in which the product (Ceq·As) of the above carbon equivalent Ceq and the acicular ratio As of the above cementite is 4.00 or more and 9.00 or less. Ceq = C% + Si% / 7 + Mn% / 5 + CR% / 9 (In the above formula, C% represents the mass content of C, Si% represents the mass content of Si, Mn% represents the mass content of Mn, and Cr% represents the mass content of Cr.)

2. The high-frequency quenching steel according to claim 1, wherein the ratio (Ceq / C) of the above carbon equivalent Ceq to the content rate (% by mass) of C is 1.20 or more and 1.50 or less.

3. The material thereof is C: 0.50% by mass or more and 0.85% by mass or less Si: 0.10% by mass or more and 0.40% by mass or less Mn: 0.20% by mass or more and 0.80% by mass or less S: 0.030% by mass or less Cr: 0.10% by mass or more and 0.90% by mass or less and Al: 0.050% by mass or less a steel containing the carbon equivalent Ceq calculated by the following formula of the above steel is 0.60 or more, the metallographic structure of the above steel contains cementite, A rough-shaped material in which the product (Ceq · As) of the above carbon equivalent Ceq and the acicular ratio As of the above cementite is 4.00 or more and 9.00 or less. Ceq = C% + Si% / 7 + Mn% / 5 + CR% / 9 (In the above formula, C% represents the mass content of C, Si% represents the mass content of Si, Mn% represents the mass content of Mn, and Cr% represents the mass content of Cr.)

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