JP2006045592A - Steel material excellent in fatigue characteristic and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a steel material excellent in fatigue characteristic and a producing method therefor. <P>SOLUTION: After heating a steel plate or a steel tube having a galvanization of ≥10 g/m<SP>2</SP>and formed into the shape having a curving part of ≤5 times the thickness in a radius of curvature, or the steel plate or the steel tube coated with a coat containing zinc powder so as to stick the zinc on a part of or the whole surface at ≥10 g/m<SP>2</SP>and having the curving part of ≤5 times the thickness in a radius of curvature, at 800-1000°C, this steel material is rapidly cooled to form a carbon concentrated layer on the steel surface just below the iron-zinc alloy layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a steel material excellent in fatigue characteristics used for a reinforcing member, a foot circumference member, and the like of an automobile, and a manufacturing method thereof.

  In recent years, a reduction in weight of automobiles has been demanded for fatigue members such as legs. Fatigue properties are improved according to the strength of the base material, but the design limit of low-strength steel is low because formability is reduced. Therefore, attention is paid to the technique of heating and quenching low-strength materials. For example, Patent Document 1 describes a method in which a steel sheet is coated with an alloy such as Zn, heated and then quenched at the same time as stamping, or a method of quenching after cold working. According to these methods, since the strength of the entire steel material is increased, it is considered that the fatigue characteristics are also improved.

However, in order to quench at the same time as stamping, a special cooling structure is required for the mold, which greatly increases the cost. Further, when forming a steel pipe by hydraulic bulge forming, it is necessary to form the steel pipe at a temperature below the boiling point of the forming water, and it is practically impossible to quench the mold at the same time as stamping.
In addition, when quenching is performed after cold working, surface defects may occur in parts that are severely bent. In the fatigue member, fatigue progresses from these surface defects, and thus there is a problem that the fatigue characteristics as expected from the strength increase due to quenching cannot be obtained.

It is well known that the fatigue characteristics strongly depend on the surface condition of the steel material. For example, as described in Non-Patent Document 2, carburizing treatment and nitriding treatment increase the surface hardness of the steel material and greatly improve the fatigue characteristics. However, these techniques also require strict furnace atmosphere management, and the equipment cost is high, so the scope of application has been limited.
JP 2001-353548 A Edited by Japan Iron and Steel Institute: Steel Handbook IV, Maruzen Co., Ltd. (1982)

  The present invention has been made to solve the above-mentioned problems and to provide a method for producing a steel material having excellent fatigue characteristics that are quenched after cold working and a steel material having excellent fatigue characteristics obtained by this method.

The present invention made to solve the above problems
(1) Zinc-plated steel plate or steel pipe of 10g / m ² or more is formed into a shape with a bent part whose radius of curvature is 5 times or less of the plate thickness, and then heated to a temperature of 800 ° C or higher and 1000 ° C or lower and then rapidly cooled. A method for producing a steel material with excellent fatigue properties, which forms a carbon enriched layer on the steel immediately below the iron-zinc alloy layer on the surface,
(2) After forming a steel sheet or steel pipe into a shape with a bent part with a radius of curvature of 5 times or less of the plate thickness, a paint containing zinc powder is applied so that 10 g / m ² or more of zinc adheres to part or the entire surface of the steel material. After applying, heating to a temperature of 800 ° C or more and 1000 ° C or less and then rapidly cooling to form a carbon concentrated layer on the steel immediately below the iron-zinc alloy layer on the surface, a method for producing a steel material with excellent fatigue properties,
(3) It has a bent portion whose radius of curvature is less than 5 times the plate thickness, and the peak of the carbon concentration from the interface between the iron-zinc alloy layer on the surface and the steel to the inner surface of the steel is up to 50 μm. A steel material with excellent fatigue characteristics, characterized by being at least 1.3 times the content concentration,
It is.

  According to the present invention, a steel material having excellent fatigue characteristics can be provided at a low cost, which can greatly contribute to the promotion of weight reduction and safety improvement of automobiles and industrial machines, and the industrial contribution is great.

  In order to obtain a steel material with excellent fatigue characteristics, the present inventors have focused on increasing the hardness in the vicinity of the surface like carburizing. As a result of repeated studies, the inventors have baked steel with zinc plated on the surface. It was found that the fatigue characteristics were improved even though the outermost surface was softer than the center of the base metal. When the cross section of the hardened steel material is observed in detail, the surface layer is a soft iron-zinc alloy layer, but the hardness of the steel material directly below the iron-zinc alloy layer is greatly increased compared to the hardness estimated from the carbon concentration of the steel material I found out. This is because when the steel is heated to 800 ° C or higher, the surface zinc diffuses toward the inner surface of the steel, but since zinc is a strong ferrite stabilizing element, a soft ferrite phase is generated on the surface of the steel, and the inner surface It was thought that carbon was discharged into the austenite phase adjacent to the side to form a concentrated layer of carbon, and the hardness of martensite generated by the subsequent rapid cooling increased with the concentration of carbon.

  Furthermore, as a result of repeated bending fatigue tests with test pieces bent in a square shape, the fatigue characteristics are significantly greater when quenched after galvanizing than when steel without galvanizing is quenched. Found to improve. This is because bending with a curvature radius of 5 times the plate thickness or less causes defects of several μm to several tens of μm in the steel surface layer and reduces fatigue strength. However, if zinc is present on the surface, During heating, the iron-zinc alloy layer progresses to the inner surface of the steel material and creates a new smooth interface. This was thought to be due to being superimposed. Defects are less likely to occur in bending molding exceeding 5 times the plate thickness, so the effect is small.

Note that the above phenomenon requires that zinc is sufficiently adhered, and if the amount of zinc deposited is less than 10 g / m ² , the progress of the iron-zinc alloy layer that only improves surface defects is not observed. Also, if the heating temperature is less than 800 ° C, not only does the steel transformation point not be exceeded, but the iron-zinc solid solution does not grow sufficiently, but if it exceeds 1000 ° C, the incidental costs of the heating equipment increase, so 800 ° C or more and 1000 ° C The following.

Therefore, a steel plate or steel pipe plated with 10g / m ² or more galvanized is formed into a shape with a bent portion whose radius of curvature is 5 times or less of the plate thickness, and then heated to a temperature of 800 ° C or higher and 1000 ° C or lower and then rapidly cooled. Then, after forming a carbon-enriched layer on the steel immediately below the iron-zinc alloy layer on the surface, or forming a steel plate or a steel pipe into a shape having a bent portion whose curvature radius is 5 times or less of the plate thickness, After applying a coating containing zinc powder so that zinc adheres to the entire surface at 10 g / m ² or more, it is heated to a temperature of 800 ° C or higher and 1000 ° C or lower and then rapidly cooled to concentrate the carbon in the steel immediately below the iron-zinc alloy layer on the surface. It is necessary to form a layer, and the steel material obtained by such a method has a bent portion whose radius of curvature is not more than 5 times the plate thickness, and the steel material is formed from the interface between the surface iron-zinc alloy layer and the steel. The peak of carbon concentration up to 50 μm toward the inner surface of the steel is 1.3 times the average carbon content of steel Te, and is excellent in fatigue characteristics.

Next, examples will be described.
In a mass%, C: 0.2%, Si: 0.18%, Mn: 1.2%, P: 0.1% or less, S: 0.02% or less, Cr: 0.3% steel was melted in the laboratory, and the plate thickness was 2.0 After making a steel plate of mm, a part is galvanized with a plating simulator, and the fatigue test piece shape shown in Fig. 1 is held at 900 ° C for 5 minutes in the air atmosphere. gave. The stress was fixed at 600 MPa, the load was applied at 20 Hz, and the number of times until fracture was measured. The test results are shown in FIG. When the bending radius of curvature exceeds 5 times the plate thickness, the difference in fatigue characteristics due to the amount of galvanized coating is unclear. However, when the radius of curvature of the bent part is less than 5 times the plate thickness, the number of repetitions until rupture of the galvanized sample with 10g / m ² or more is clear compared with the case where the amount of zinc adhesion is small or galvanized. Big.

  FIG. 3 shows the result of a line analysis of the cross section of the sample before the fatigue test on which galvanization has been performed, using EPMA. Clear enrichment of carbon can be confirmed in the steel in contact with the Fe-Zn solid solution on the surface. In addition, although the carbon in the surface Fe-Zn alloy layer shows an extremely high value, it is considered that this has detected dirt in a crack existing in the surface Fe-Zn alloy layer. .

  In a mass%, steel containing C: 0.3%, Si: 0.1%, Mn: 1.5%, P: 0.1% or less, S: 0.02% or less, Ti: 0.02%, B: 15ppm was melted in the laboratory. After forming a steel plate having a thickness of 2.5 mm, it was processed into the shape of a fatigue test piece shown in FIG. Thereafter, a paint partially containing zinc powder was applied with a brush. The amount of zinc powder applied was similarly determined by observing the cross-section of the coated sample. Thereafter, the sample was heat-treated under the conditions shown in Table 1 and then subjected to a bending fatigue test. The stress was fixed at 700 MPa, loaded at 20 Hz, and the number of times until fracture was measured.

表１に示すように、本発明例であるNo.3、No.4は破断までの繰り返し数が百万回を超えるが、No.1、No.2は亜鉛塗布量が少なく、No.5は加熱条件が不適切でいずれも少ない繰り返し数で破断する。

As shown in Table 1, No. 3 and No. 4 which are examples of the present invention have a number of repetitions up to 1 million times, but No. 1 and No. 2 have a small zinc coating amount. Breaks with a small number of repetitions.

It is the shape of the fatigue test piece used in Example 1. It is a graph which shows the result of a fatigue test. It is a graph which shows the cross-sectional EPMA analysis result before a fatigue test. It is the shape of the fatigue test piece used in Example 2.

Claims (3)

After forming a steel plate or steel pipe plated with galvanization of 10 g / m ² or more into a shape having a bent portion with a radius of curvature of 5 times or less of the plate thickness, it is heated to a temperature of 800 ° C. or more and 1000 ° C. or less and then rapidly cooled, A method for producing a steel material having excellent fatigue characteristics, in which a carbon-enriched layer is formed on steel immediately below a surface iron-zinc alloy layer. After forming a steel sheet or steel pipe into a shape with a bent part with a radius of curvature less than 5 times the plate thickness, and then applying a zinc powder-containing paint so that 10 g / m ² or more of zinc adheres to part or the entire surface of the steel material A method for producing a steel material having excellent fatigue properties, wherein a carbon enriched layer is formed on the steel immediately below the surface of the iron-zinc alloy layer by heating to 800 ° C. to 1000 ° C. and then rapidly cooling. It has a bend with a radius of curvature of 5 times or less the plate thickness, and the peak of the carbon concentration from the interface between the iron-zinc alloy layer and the steel to the inner surface of the steel is up to 50 μm. Steel material with excellent fatigue characteristics, characterized by 1.3 times or more.

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