Quenched and tempered round steel bars

Abstract

The present invention provides a steel for quenching and tempering that reduces production costs and has excellent corrosion resistance, wear resistance, hot workability, and toughness. The round steel bar for quenching and tempering of the present invention has a predetermined chemical composition, and is characterized in that, in a cross section perpendicular to the longitudinal direction of the round steel bar, the concentration distributions of C, Si, Mn, and Mo in a region from the surface of the round steel bar toward the center to a position corresponding to a depth of 10% of the diameter of the cross section satisfy predetermined relationships.

Description

[Technical Field]

[0001] The present invention relates to a round steel bar for quenching and tempering. [Background technology]

[0002] Due to their nature, mechanical properties such as impact resistance are important for machine structural parts. These mechanical properties are ensured by quenching and tempering processes performed after machining into the product shape. Depending on the operating environment, corrosion resistance may also be required. For example, in machine structural parts used in coastal or offshore areas, corrosion-induced thinning can lead to a deterioration in mechanical properties, potentially resulting in breakage. Furthermore, some machine structural parts are subject to sliding, which can lead to a deterioration in mechanical properties due to wear and peeling. For these reasons, machine structural parts are regularly inspected and repaired during actual use. Therefore, to reduce inspection and repair costs, machine structural parts are required to have longer life through further improvements in corrosion resistance and wear resistance. Furthermore, the steel used for machine structural parts must also have excellent toughness.

[0003] Patent Document 1 discloses stainless steel with improved wear resistance and corrosion resistance achieved by appropriately controlling the ratio of C to Cr. Patent Document 2 discloses a method for producing stainless clad steel that achieves both corrosion resistance and wear resistance by reducing the coefficient of dynamic friction through a polishing process and electrolytic treatment (or pickling treatment). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2-243741 [Patent Document 2] International Publication No. 2013 / 132863 Summary of the Invention [Problem to be solved by the invention]

[0005] However, while the techniques described in Patent Documents 1 and 2 achieve both corrosion resistance and wear resistance, they require the steel material itself or the surface of the component to be made of stainless steel. Patent Document 1 requires the addition of a large amount of Cr, which poses a problem of poor hot workability, which is required when processing the steel material into desired machine structural parts. Furthermore, Patent Document 2 requires processes such as polishing and electrolytic treatment. As such, in both Patent Documents 1 and 2, a significant increase in manufacturing costs is unavoidable in terms of the steel material and processes.

[0006] In view of the above problems, an object of the present invention is to provide a steel for quenching and tempering that reduces production costs and has excellent corrosion resistance, wear resistance, hot workability, and toughness. In the present invention, "excellent corrosion resistance and wear resistance" means excellent corrosion resistance and wear resistance evaluated after the steel for quenching and tempering is subjected to quenching and tempering treatment. [Means for solving the problem]

[0007] The present inventors conducted research focusing on the chemical composition, hardness, and distribution of alloying elements of a steel material in order to simultaneously achieve and improve corrosion resistance, wear resistance, hot workability, and toughness in a steel material that can be produced at low cost. As a result, they discovered that it is important to satisfy the following conditions. That is, it is important to have an appropriate chemical composition and to make the C, Si, Mn, and Mo concentrations of the steel material uniform in the near-surface region when used as a machine structural part.

[0008] That is, the gist and configuration of the present invention are as follows.

[0009] [1] In mass%, C: 0.10% or more and 0.50% or less, Si: 0.002% or more and 1.200% or less, Mn: 1.80% or more and 5.00% or less, P: 0.050% or less, S: 0.050% or less, Cu: 0.005% or more and 2.000% or less, Ni: 0.002% or more and 5.000% or less, Cr: 0.10% or more and 3.00% or less, Al: 0.0010% or more and 1.0000% or less, Mo: 0.20% or more and 2.00% or less, V: 0.0010% or more and 1.0000% or less, and N: Contains 0.0010% or more and 0.0250% or less, A round steel bar for quenching and tempering having a chemical composition with the balance being Fe and unavoidable impurities, A round bar steel for quenching and tempering, characterized in that in a cross section perpendicular to the longitudinal direction of the round bar steel, the concentration distribution of C, Si, Mn, and Mo in a region from the surface of the round bar steel toward the center to a position corresponding to a depth of 10% of the diameter of the cross section satisfies the following formula (1):

number

[0010] [2] The component composition further includes, in mass %, Mg: 0.0100% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ta: 0.1000% or less, Pb: 0.500% or less, Bi: 0.100% or less, O: 0.0100% or less, As: 0.0100% or less, Ca: 0.0100% or less, B: 0.0100% or less, Zr: 0.0500% or less, Hf: 0.0500% or less, W: 1.0000% or less, Co: 0.5000% or less, Zn: 0.0100% or less, Te: 0.100% or less, Sn: 0.100% or less, Sb: 0.100% or less, and REM: 0.1000% or less, 2. The round steel bar for quenching and tempering according to claim 1, which contains at least one element selected from the group consisting of: [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a steel for quenching and tempering which is excellent in corrosion resistance, wear resistance, hot workability and toughness while reducing production costs. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a drawing of a test piece when a hot tensile test was carried out in an evaluation of hot workability in an example of the present invention. [Figure 2] 1 shows a temperature profile when a hot tensile test was carried out in an evaluation of hot workability in an example of the present invention. [Figure 3] 1 is a diagram of a Charpy test specimen used in toughness evaluation in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, a steel for quenching and tempering according to one embodiment of the present invention will be described. Note that the embodiment described below is an example of a specific embodiment of the present invention, and the configuration of the present invention is not limited to this specific example.

[0014] First, the chemical composition of the steel for quenching and tempering will be described. Note that "%" representing the content of the component elements means "% by mass" unless otherwise specified.

[0015] [C: 0.10~0.50%] C is an element added to ensure wear resistance. If the C content is less than 0.10%, sufficient wear resistance cannot be ensured. Therefore, the C content is set to 0.10% or more, preferably 0.18% or more, and more preferably 0.20% or more. On the other hand, if the C content exceeds 0.50%, hot workability deteriorates. Therefore, the C content is set to 0.50% or less, preferably 0.35% or less, and more preferably 0.30% or less.

[0016] [Si: 0.002 to 1.200%] Si is an element necessary for deoxidation and is also effective in imparting wear resistance. If the Si content is less than 0.002%, the above effects are insufficient. Therefore, the Si content is set to 0.002% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Si content exceeds 1.200%, hot workability deteriorates. Therefore, the Si content is set to 1.200% or less, preferably 1.000% or less, and more preferably 0.900% or less.

[0017] [Mn: 1.80~5.00%] Mn is an element necessary for deoxidation and is also effective in imparting wear resistance. If the Mn content is less than 1.80%, the above effects are insufficient. Therefore, the Mn content is set to 1.80% or more, preferably 2.20% or more, and more preferably 2.51% or more. On the other hand, if the Mn content exceeds 5.00%, hot workability deteriorates. Therefore, the Mn content is set to 5.00% or less, preferably 4.50% or less, and more preferably 4.00% or less.

[0018] [P:0.050% or less] While P is an effective element for increasing the strength of steel, it also acts as an impurity, segregating at grain boundaries and reducing toughness. If the P content exceeds 0.050%, toughness is significantly reduced. Therefore, the P content is set to 0.050% or less. Furthermore, the lower the P content, the better the toughness, so the P content is preferably 0.040% or less, and more preferably 0.030% or less. While there is no particular lower limit for the P content, from the viewpoint of steelmaking costs, the P content is preferably 0.005% or more.

[0019] [S:0.050% or less] S is an element that forms a compound with Mn in steel to produce MnS. If the S content exceeds 0.050%, the large amount of MnS produced becomes the starting point for corrosion, resulting in a decrease in corrosion resistance. Therefore, the S content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. On the other hand, although there is no particular lower limit for the S content, from the viewpoint of steelmaking costs, the S content is preferably 0.009% or more.

[0020] [Cu: 0.005 to 2.000%] Cu is an element effective in improving corrosion resistance. If the Cu content is less than 0.005%, the above effect is insufficient. Therefore, the Cu content is set to 0.005% or more, preferably 0.008% or more, and more preferably 0.010% or more. On the other hand, if the Cu content exceeds 2.000%, surface defects are likely to occur during steel production, increasing maintenance costs. Therefore, the Cu content is set to 2.000% or less, preferably 1.000% or less, and more preferably 0.800% or less.

[0021] [Ni: 0.002~5.000%] Ni is an element effective in improving corrosion resistance. If the Ni content is less than 0.002%, the above effect is insufficient. Therefore, the Ni content is set to 0.002% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ni content exceeds 5.000%, the raw material cost due to the addition increases significantly. Therefore, the Ni content is set to 5.000% or less, preferably 3.000% or less, and more preferably 2.000% or less.

[0022] [Cr: 0.10~3.00%] Cr is an element effective in improving corrosion resistance. If the Cr content is less than 0.10%, the above effect is insufficient. Therefore, the Cr content is set to 0.10% or more, preferably 0.50% or more, and more preferably 0.60% or more. On the other hand, if the Cr content exceeds 3.00%, the hot workability deteriorates. Therefore, the Cr content is set to 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

[0023] [Al: 0.0010~1.0000%] Al is a deoxidizing element and also effectively improves toughness by bonding with N in steel to form nitrides, thereby refining crystal grains. Here, if the Al content is less than 0.0010%, the above effect is insufficient. Therefore, the Al content is set to 0.0010% or more, preferably 0.0020% or more, and more preferably 0.0025% or more. On the other hand, if the Al content exceeds 1.0000%, hot workability deteriorates. Therefore, the Al content is set to 1.0000% or less, preferably 0.9000% or less, and more preferably 0.8000% or less.

[0024] [Mo: 0.20-2.00%] Mo is an element that significantly improves the hardenability of steel materials with a small amount of addition and is effective in imparting wear resistance. Here, if the Mo content is less than 0.20%, the above effects are insufficient. Therefore, the Mo content is set to 0.20% or more, preferably 0.22% or more, and more preferably 0.26% or more. On the other hand, if the Mo content exceeds 2.00%, the hardenability becomes excessive and the hot workability deteriorates. Therefore, the Mo content is set to 2.00% or less, preferably 1.80% or less, and more preferably 1.50% or less.

[0025] [V:0.0010~1.0000%] V combines with N in steel to form nitrides, which is effective in improving toughness by refining crystal grains. If the V content is less than 0.0010%, the above effect is insufficient. Therefore, the V content is set to 0.0010% or more, preferably 0.0050% or more, and more preferably 0.0070% or more. On the other hand, if the V content exceeds 1.0000%, a large amount of V-based precipitates is formed, which deteriorates hot workability. Therefore, the V content is set to 1.0000% or less, preferably 0.7000% or less, and more preferably 0.6000% or less.

[0026] [N:0.0010~0.0250%] N forms nitrides with nitride-forming elements in the steel and acts as grain boundary pinning particles, thereby effectively improving toughness by refining crystal grains. Here, if the N content is less than 0.0010%, the above effect is insufficient. Therefore, the N content is set to 0.0010% or more, preferably 0.0020% or more, and more preferably 0.0025% or more. On the other hand, if the N content exceeds 0.0250%, toughness decreases. Therefore, the N content is set to 0.0250% or less, preferably 0.0200% or less, and more preferably 0.0150% or less.

[0027] [Remaining components] The round steel bar for quenching and tempering has a composition containing the above elements with the balance being Fe and inevitable impurities. Furthermore, the round steel bar for quenching and tempering preferably has a composition containing the above elements with the balance being Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are allowed to be present to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[0028] Furthermore, the chemical composition of the round bar steel for quenching and tempering may contain, in addition to the above basic components, at least one element selected from the group consisting of Mg: 0.0100% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ta: 0.1000% or less, Pb: 0.500% or less, Bi: 0.100% or less, O: 0.0100% or less, As: 0.0100% or less, Ca: 0.0100% or less, B: 0.0100% or less, Zr: 0.0500% or less, Hf: 0.0500% or less, W: 1.0000% or less, Co: 0.5000% or less, Zn: 0.0100% or less, Te: 0.100% or less, Sn: 0.100% or less, Sb: 0.100% or less, and REM: 0.1000% or less. When an optional element is contained in an amount less than the preferable lower limit value described below, the element is considered to be contained as an unavoidable impurity.

[0029] [Mg:0.0100% or less] Mg is an element that combines with O in the steel to form MgO, acting as grain boundary pinning particles, thereby improving toughness through grain refinement. To achieve this effect, the Mg content is preferably 0.0001% or more. On the other hand, if the Mg content exceeds 0.0100%, a large amount of MgO is generated, reducing toughness. Therefore, if Mg is to be added, the Mg content should be 0.0100% or less.

[0030] [Ti:0.100% or less] Ti is an element that effectively improves toughness by bonding with N in steel to form nitrides, thereby refining crystal grains. To achieve this effect, the Ti content is preferably 0.001% or more. On the other hand, if the Ti content exceeds 0.100%, a large amount of Ti-based inclusions will be generated in the steel, reducing toughness. Therefore, if Ti is to be added, the Ti content should be 0.100% or less.

[0031] [Nb:0.100% or less] Nb is an element that combines with C in steel to form carbides, thereby improving toughness by refining crystal grains. To achieve this effect, the Nb content is preferably 0.001% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of Nb-based carbides is generated, reducing toughness. Therefore, if Nb is added, the Nb content should be 0.100% or less.

[0032] [Ta:0.1000% or less] Ta is an element that is effective in improving toughness by refining crystal grains. To achieve this effect, the Ta content is preferably 0.0001% or more. On the other hand, if the Ta content exceeds 0.1000%, the toughness decreases. Therefore, if Ta is contained, the Ta content is set to 0.1000% or less.

[0033] [Pb:0.500% or less] Pb does not adversely affect any of the properties required for machine structural parts when added up to a certain amount, so its inclusion at a content of 0.500% or less is acceptable. However, if the Pb content exceeds 0.500%, the strength of the steel material decreases. Therefore, the Pb content is set to 0.500% or less. While there is no particular lower limit for the Pb content, the Pb content can be 0.001% or more.

[0034] [Bi:0.100% or less] Bi is an element that is effective in improving toughness by refining crystal grains. To achieve this effect, the Bi content is preferably 0.001% or more. On the other hand, if the Bi content exceeds 0.100%, the toughness decreases. Therefore, if Bi is contained, the Bi content is set to 0.100% or less.

[0035] [O:0.0100% or less] O is an element that is effective in improving toughness by refining crystal grains. To achieve this effect, the O content is preferably 0.0001% or more. On the other hand, if the O content exceeds 0.0100%, the toughness decreases. Therefore, when O is contained, the O content is set to 0.0100% or less.

[0036] [As:0.0100% or less] As is an element that is effective in improving toughness by refining crystal grains. To achieve this effect, the As content is preferably 0.0001% or more. On the other hand, if the As content exceeds 0.0100%, toughness decreases. Therefore, when As is contained, the As content is set to 0.0100% or less.

[0037] [Ca:0.0100% or less] Ca is an effective element for improving the toughness of steel by dispersing oxygen-based inclusions into fine and spherical shapes and controlling the shape of sulfides. To achieve these effects, the Ca content is preferably 0.0001% or more. On the other hand, if the Ca content exceeds 0.0100%, the toughness decreases. Therefore, if Ca is added, the Ca content should be 0.0100% or less.

[0038] [B:0.0100% or less] B is an element effective in increasing the toughness of steel. To obtain this effect, the B content is preferably 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, the effect of adding B becomes saturated. Therefore, when B is added, the B content is set to 0.0100% or less, and preferably 0.0050% or less.

[0039] [Zr:0.0500% or less] Zr is an element effective in increasing the toughness of steel. To achieve this effect, the Zr content is preferably 0.0001% or more. On the other hand, if the Zr content exceeds 0.0500%, the toughness actually decreases. Therefore, if Zr is added, the Zr content is set to 0.0500% or less.

[0040] [Hf:0.0500% or less] Hf is an element effective in increasing the toughness of steel. To obtain this effect, the Hf content is preferably 0.0001% or more. On the other hand, if the Hf content exceeds 0.0500%, the effect saturates. Therefore, if Hf is to be contained, the Hf content should be 0.0500% or less.

[0041] [W:1.0000% or less] W is an element that is effective in increasing the toughness of steel. To obtain this effect, the W content is preferably 0.0001% or more. On the other hand, if the W content exceeds 1.0000%, the effect saturates. Therefore, if W is to be added, the W content should be 1.0000% or less.

[0042] [Co:0.5000% or less] Co is an element effective in increasing the toughness of steel. To obtain this effect, the Co content is preferably 0.0001% or more. On the other hand, if the Co content exceeds 0.5000%, the effect saturates. Therefore, if Co is to be added, the Co content should be 0.5000% or less.

[0043] [Zn:0.0100% or less] Zn is an element effective in improving the toughness of steel. To obtain this effect, the Zn content is preferably 0.0001% or more. On the other hand, if the Zn content exceeds 0.0100%, the effect saturates. Therefore, if Zn is to be added, the Zn content should be 0.0100% or less.

[0044] [Te:0.100% or less] Te is an element effective in increasing the toughness of steel. To achieve this effect, the Te content is preferably 0.001% or more. On the other hand, if the Te content exceeds 0.100%, the toughness decreases. Therefore, if Te is contained, the Te content is set to 0.100% or less.

[0045] [Sn:0.100% or less] Sn is an element that is effective in improving the corrosion resistance of steel. To obtain this effect, the Sn content is preferably 0.001% or more. On the other hand, if the Sn content exceeds 0.100%, the effect saturates. Therefore, if Sn is to be contained, the Sn content should be 0.100% or less.

[0046] [Sb:0.100% or less] Sb is an element effective in improving the corrosion resistance of steel. To obtain this effect, the Sb content is preferably 0.001% or more. On the other hand, if the Sb content exceeds 0.100%, the toughness decreases. Therefore, if Sb is contained, the Sb content is set to 0.100% or less.

[0047] [REM:0.1000% or less] REM is an element effective in improving the corrosion resistance of steel materials. To achieve this effect, the REM content is preferably 0.0001% or more. On the other hand, if the REM content exceeds 0.1000%, the effect saturates. Therefore, when REM is contained, the REM content should be 0.1000% or less. Note that REM here refers to 17 elements, including 15 lanthanoid elements from La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71, Sc (scandium) with atomic number 21, and Y (yttrium) with atomic number 39. These 17 elements can be contained alone or in combination. In the present invention, the REM content refers to the total content of these 17 elements.

[0048] [In a cross section perpendicular to the longitudinal direction of a round steel bar, the concentration distribution of C, Si, Mn, and Mo in the region from the surface of the round steel bar toward the center to a position equivalent to a depth of 10% of the diameter of the cross section satisfies formula (1)] The following formula (1) defines the local concentration distribution in a round steel bar for quenching and tempering. Mechanical structural components to be quenched and tempered have an internal structure primarily composed of martensite. If formula (1) is not satisfied, variations in hardness will occur even within the same martensite structure, and when mechanical structural components come into contact with each other, the component with lower hardness will wear preferentially and more rapidly. Therefore, to suppress wear of a round steel bar for quenching and tempering, the concentration distribution of C, Si, Mn, and Mo in a (ring-shaped) region extending from the surface of the round steel bar toward the center, a position corresponding to a depth of 10% of the cross-sectional diameter, in a cross section perpendicular to the longitudinal direction of the round steel bar must satisfy formula (1). The smaller the value of the left side of formula (1), the better. Therefore, the lower limit is not particularly limited. However, in the present invention, the value of the left side is generally 0.25 or greater.

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[0049] In a cross section perpendicular to the longitudinal direction of a round steel bar, the concentration distributions of C, Si, Mn, and Mo in the region from the surface of the round steel bar toward the center to a position corresponding to a depth of 10% of the cross-sectional diameter can be determined as follows. A test piece of the round steel bar for quenching and tempering is cut perpendicular to the longitudinal direction to obtain an evaluation sample with a circular cross-section. Line analysis of the concentration profiles of C, Si, Mn, and Mo is measured using an electron probe microanalyzer (EPMA) along a line segment corresponding to the diameter of the circle in the cross section. The beam diameter is 50 μm, and the acceleration voltage is 20 kV. The minimum and maximum values ​​of the C concentration in the concentration profile from the surface of the round steel bar toward the center to a position corresponding to a depth of 10% of the cross-sectional diameter are determined, as well as the concentrations of Si, Mn, and Mo at the same measurement points, and the value of the left side of equation (1) is calculated.

[0050] The round steel bar for quenching and tempering according to one embodiment of the present invention has the shape of a steel section, that is, a steel bar or a wire rod.

[0051] (Method of manufacturing round steel bars for quenching and tempering) Next, a method for manufacturing a round steel bar for quenching and tempering according to one embodiment of the present invention will be described.

[0052] The round steel bar for quenching and tempering according to one embodiment of the present invention can be manufactured by any appropriate method known to those skilled in the art, depending on the shape of the final product. That is, two processes, casting and hot rolling, are carried out in this order. Casting is a process in which molten steel having a predetermined chemical composition is poured and cooled to obtain a steel ingot. In casting, molten steel having a predetermined chemical composition is first prepared. The chemical composition of the molten steel is adjusted so that the chemical composition of the round steel bar for quenching and tempering is the above-mentioned chemical composition. A batch-type electric furnace or a continuous blast furnace can be used to manufacture the molten steel.

[0053] Next, the prepared molten steel is poured into a mold to obtain a steel ingot. The temperature of the molten steel during pouring is preferably equal to or higher than the melting point of the molten steel's chemical composition, but not higher than 100°C above the melting point. Pouring the molten steel into the mold may be performed by continuous casting or by using a batch-type mold. In the case of continuous casting, the pouring speed, i.e., the speed at which the cooled steel ingot descends in the mold, is preferably 0.3 m / min or higher from the viewpoint of manufacturability. The pouring speed is preferably 1.0 m / min or lower, more preferably 0.5 m / min or lower. In addition, it is preferable to set the flow rate of the molten steel to 10 cm / sec or higher and 20 cm / sec or lower, for example, by electromagnetic stirring in the mold. By satisfying these casting conditions, the C, Si, Mn, and Mo contained in the molten steel can be evenly dispersed throughout the steel ingot.

[0054] Hot rolling is a process in which the steel ingot produced in the casting process is heated and rolled at high temperatures to form the steel ingot into the desired dimensions and cross-sectional shape of a round steel bar. A heating furnace can be used to heat the steel ingot. The temperature to which the steel ingot is heated is preferably 900°C or higher and 1250°C or lower. Furthermore, reduction rolls can be used for rolling. The finishing temperature for hot rolling is preferably 750°C or higher, followed by cooling.

[0055] For steps and conditions not described in this specification, conventional methods can be used. [Example]

[0056] The configuration and effects of the present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples, and appropriate modifications can be made within the scope of the gist of the present invention, and all such modifications are included in the technical scope of the present invention.

[0057] Steel ingots having the chemical compositions shown in Table 1 were melted and hot-rolled into round bars of φ50 or φ230 mm to prepare round bars for quenching and tempering. The φ50 mm round bars were then cut to lengths of 100 mm, and the φ230 mm round bars were cut to lengths of 300 mm.

[0058] [Table 1] TIFF0007810317000004.tif233137TIFF0007810317000005.tif233141

[0059] [Concentration distribution measurement] Using the method described above, the concentration distributions of C, Si, Mn, and Mo were determined in a cross section perpendicular to the longitudinal direction of each round steel bar in a region extending from the surface of the round steel bar to a position corresponding to a depth of 10% of the cross-sectional diameter from the surface toward the center. That is, for a φ50 mm test piece, the concentration distribution was determined in a region extending from the surface of the round steel bar to a position 5 mm from the surface toward the center. For a φ230 mm test piece, the concentration distribution was determined in a region extending from the surface of the round steel bar to a position 23 mm from the surface toward the center.

[0060] [Hot workability evaluation] For the φ50 mm example, hot workability was evaluated. Specifically, a tensile test specimen with a parallel section of φ6 mm, as shown in Figure 1, was taken from the D / 4 position of a φ50 mm round steel bar for quenching and tempering (as-hot-rolled material). Next, a hot tensile test was conducted using the thermal history shown in Figure 2, and the area reduction rate (%) until fracture was determined. Note that T in Figure 2 was set to 900°C. In this evaluation, an area reduction rate of 41% or more can be said to have sufficient hot workability.

[0061] [Toughness evaluation] For the φ50mm example, toughness was evaluated in accordance with JIS Z 2242, the Charpy impact test method for metallic materials. Samples with the shape shown in Figure 3 were taken from the D / 4 position of a φ50mm quenched and tempered round steel bar (as-hot-rolled material). The sample was taken with the notch surface facing away from the center. After the test, the fracture surface of the sample was observed and photographed with a digital microscope, and the ductile fracture ratio on the fracture surface was determined. In this evaluation, a ductile fracture ratio of 50% or more can be said to have sufficient toughness.

[0062] Next, the round steel bars for quenching and tempering were subjected to quenching and tempering treatment, and as a finishing step, surface scale was removed by shot blasting to obtain test specimens for the various evaluations described below.

[0063] [Corrosion resistance evaluation] For each example, corrosion resistance was evaluated by immersing the test specimens in artificial seawater at 50°C for 30 weeks. The composition of the artificial seawater used in the test was, in mass %, 2.2% sodium chloride, 0.75% 2-aminopyridine, 0.52% magnesium chloride hexahydrate, 0.41% sodium sulfate, 0.14% hydrochloric acid, 0.12% calcium chloride, 0.08% potassium chloride, and the remainder was water. After the corrosion test, the diameter loss of the test specimens was measured using a vernier caliper. The diameter was measured at three positions that divided the longitudinal length of the test specimen into four equal parts. That is, for 50 mm diameter test specimens, the diameters were measured at 25, 50 mm (at the center of the test specimen), and 75 mm from the longitudinal end. For 230 mm diameter test specimens, the diameters were measured at 75, 150, and 225 mm from the longitudinal end. The diameter was also measured perpendicular to the diameter measured at each of the above positions, and the corrosion resistance was evaluated using the average value of six positions per sample. In this evaluation, it can be said that the corrosion resistance is excellent when the diameter reduction is 0.070 mm or less for a φ50 mm test piece and 1.500 mm or less for a φ230 mm test piece.

[0064] [Wear resistance evaluation] Abrasion resistance was evaluated for each example. Two test pieces with the same component composition were arranged perpendicular to each other, one fixed, and the other was slid longitudinally against the fixed test piece under a load of 10 kN. The abrasion resistance was evaluated based on the number of times the abrasion reached the radius of the test piece ...

[0065] Table 2 shows the manufacturing conditions and the above evaluation results for each example.

[0066] [Table 2] TIFF0007810317000007.tif218170TIFF0007810317000008.tif216170

[0067] As shown in Table 2, all of the inventive examples were excellent in corrosion resistance, wear resistance, hot workability, and toughness. On the other hand, the comparative examples were insufficient in at least one of corrosion resistance, wear resistance, hot workability, and toughness. [Industrial Applicability]

[0068] According to the present invention, it is possible to provide a quenched and tempered steel that reduces production costs and has excellent corrosion resistance, wear resistance, hot workability, and toughness. The quenched and tempered steel can be used for machine structural parts that require these properties. Specifically, it can be used as a steel for wind power shafts, pins and pistons for industrial machinery and ships, and mooring chains used to moor marine structures.

Claims

1. In mass%, C: 0.10% or more and 0.50% or less, Si: 0.002% or more and 1.200% or less, Mn: 1.80% or more and 5.00% or less, P: 0.050% or less, S: 0.050% or less, Cu: 0.005% or more and 2.000% or less, Ni: 0.002% or more and 5.000% or less, Cr: 0.10% or more and 3.00% or less, Al: 0.0010% or more and 1.0000% or less, Mo: 0.20% or more and 2.00% or less, V: 0.0010% or more and 1.0000% or less, and N: 0.0010% or more and 0.0250% or less; A round steel bar for quenching and tempering having a chemical composition with the balance being Fe and unavoidable impurities, A round bar steel for quenching and tempering, characterized in that in a cross section perpendicular to the longitudinal direction of the round bar steel, the concentration distribution of C, Si, Mn, and Mo in a region from the surface of the round bar steel toward the center to a position corresponding to a depth of 10% of the diameter of the cross section satisfies the following formula (1): [Equation 1] ・・・（１） Here, [X] is the content of element X in the composition, and [X] ａｔＣｍｉｎ and [X] ａｔＣｍａｘ are the concentrations of element X at the positions where the C concentration is minimum and maximum in the concentration distribution, respectively.

2. The composition of the components is further expressed as follows in mass %: Mg: 0.0100% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ta: 0.1000% or less, Pb: 0.500% or less, Bi: 0.100% or less, O: 0.0100% or less, As: 0.0100% or less, Ca: 0.0100% or less, B: 0.0100% or less, Zr: 0.0500% or less, Hf: 0.0500% or less, W: 1.0000% or less, Co: 0.5000% or less, Zn: 0.0100% or less, Te: 0.100% or less, Sn: 0.100% or less, Sb: 0.100% or less, and REM: 0.1000% or less, 2. The round bar steel for quenching and tempering according to claim 1, containing at least one element selected from the group consisting of:

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