Steel round bar for quenching and tempering

Abstract

Provided is a steel for quenching and tempering, which is reduced in production cost and has excellent corrosion resistance, wear resistance, hot workability, and toughness. A steel round bar for quenching and tempering according to the present invention has a specific component composition, and is characterized in that, in a cross-section perpendicular to the longitudinal direction of the steel round bar, the concentration distribution of C, Si, Mn, and Mo in a region from the surface of the steel round bar to a position corresponding to a depth of 10% of the diameter of the cross-section from the surface toward the center satisfies a specific relationship.

Description

Round bar steel for quenching and tempering

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

[0002] Due to their nature, mechanical properties such as impact resistance are emphasized for parts used in mechanical structures. These mechanical properties are ensured by performing quenching and tempering treatments after processing into the product shape. Also, depending on the usage environment, not only mechanical properties but also corrosion resistance, which is resistance to corrosion, may be required. For example, in parts used in mechanical structures along the coast or at sea, thinning due to corrosion can lead to a decrease in mechanical properties and may result in a fracture accident. Furthermore, some parts used in mechanical structures involve sliding, and in this case, a decrease in mechanical properties is caused by wear, peeling, etc. due to sliding. From the above, parts used in mechanical structures are regularly inspected and repaired during actual use. Therefore, in order to reduce the costs required for inspection and repair, parts used in mechanical structures are required to have a longer lifespan through further improvement of corrosion resistance and wear resistance. Also, the steel materials used for parts in mechanical structures are required to have excellent toughness.

[0003] Patent Document 1 discloses a stainless steel in which wear resistance and corrosion resistance are enhanced by appropriately controlling the ratio of the amount of C to Cr. Patent Document 2 discloses a method for manufacturing a stainless steel clad steel in which the coefficient of dynamic friction is reduced by performing a polishing process and an electrolytic treatment (or pickling treatment), achieving both corrosion resistance and wear resistance.

[0004] Japanese Patent Application Laid-Open No. 2-243741 International Publication No. 2013 / 132863

[0005] However, although the technologies described in Patent Documents 1 and 2 achieve both corrosion resistance and wear resistance, it is necessary to configure the steel material itself or the surface of the member with stainless steel. In Patent Document 1, since a large amount of Cr addition is required, there is also a problem in that the hot workability required when processing the steel material into a desired part for a mechanical structure is low. Also, in Patent Document 2, it is necessary to perform processes such as polishing and electrolytic treatment. Thus, in both Patent Documents 1 and 2, a significant increase in manufacturing costs in the steel material and the process cannot be avoided.

[0006] In view of the above issues, the present invention aims to provide a quenching and tempering steel that reduces manufacturing costs and has excellent corrosion resistance, wear resistance, hot workability, and toughness. In this invention, excellent corrosion resistance and wear resistance means that the corrosion resistance and wear resistance are excellent when evaluated after the quenching and tempering treatment has been applied to the quenching and tempering steel.

[0007] The inventors of the present invention conducted research focusing on the component composition of steel, hardness, and distribution of alloy components in steel materials in order to achieve and improve both corrosion resistance, wear resistance, hot workability, and toughness in steel materials that can be manufactured at low cost. As a result, they found that it is important to satisfy the following conditions. That is, after achieving an appropriate component composition, it is important to make the C concentration, Si concentration, Mn concentration, and Mo concentration of the steel uniform in the near-surface region when used as a mechanical structural component.

[0008] In other words, the gist of the present invention is as follows:

[0009] [1] A round steel bar for quenching and tempering having a composition in mass%, containing 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, with the remainder being Fe and unavoidable impurities, A round steel bar for quenching and tempering, characterized in that, in a cross section perpendicular to the longitudinal direction of the round steel bar, the concentration distribution of C, Si, Mn, and Mo in the region from the surface of the round steel bar to a position corresponding to a depth of 10% of the diameter of the cross section, extending from the surface toward the center, satisfies the following formula (1). ... (1) Here, [X] is the content of element X in the above component composition, and [X] ａｔＣｍｉｎ and [X] ａｔＣｍａｘThese are the concentrations of element X at the positions where the C concentration is minimum and maximum in the aforementioned concentration distribution, respectively.

[0010] [2] The above component composition is further defined as follows in mass percent: 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 The round steel bar for quenching and tempering according to claim 1, containing at least one element selected from the group consisting of: 0.100% or less and REM: 0.1000% or less.

[0011] According to the present invention, it is possible to provide a quenching and tempering steel that reduces manufacturing costs and has excellent corrosion resistance, wear resistance, hot workability, and toughness.

[0012] This is a drawing of the specimen used in the hot tensile test for evaluating hot workability in the embodiment of the present invention. This is the temperature profile used in the hot tensile test for evaluating hot workability in the embodiment of the present invention. This is a drawing of the Charpy test specimen used in the toughness evaluation in the embodiment of the present invention.

[0013] The following describes a steel for quenching and tempering according to one embodiment of the present invention. Note that the embodiments described below are examples that embody the present invention, and these specific examples do not limit the configuration of the present invention.

[0014] First, we will explain the chemical composition of steel used for quenching and tempering. Unless otherwise specified, the "%" indicating the content of constituent elements refers to "mass percent".

[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 should be 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 decreases. Therefore, the C content should be 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 an effective element for imparting wear resistance. Here, if the Si content is less than 0.002%, the above effects are insufficient. Therefore, the Si content should be 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%, the hot workability decreases. Therefore, the Si content should be 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 an effective element for imparting wear resistance. Here, if the Mn content is less than 1.80%, the above effects are insufficient. Therefore, the Mn content should be 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%, the hot workability decreases. Therefore, the Mn content should be 5.00% or less, preferably 4.50% or less, and more preferably 4.00% or less.

[0018] [P: 0.050% or less] While phosphorus (P) is an effective element for increasing the strength of steel, it is also an element that reduces toughness, particularly segregation at grain boundaries, as an impurity. Here, if the P content exceeds 0.050%, the toughness decreases significantly. Therefore, the P content should be 0.050% or less. Furthermore, since a lower P content is better in terms of toughness, a P content of 0.040% or less is preferable, and 0.030% or less is more preferable. On the other hand, there is no particular lower limit to the P content, but from the viewpoint of steelmaking costs, a P content of 0.005% or more is preferable.

[0019] [S: 0.050% or less] S is an element that forms compounds 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, reducing corrosion resistance. Therefore, the S content should be 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. On the other hand, there is no particular lower limit to the S content, but from the viewpoint of steelmaking costs, it is preferable that the S content be 0.009% or more.

[0020] [Cu: 0.005 to 2.000%] Cu is an effective element for improving corrosion resistance. However, if the Cu content is less than 0.005%, the above effect is insufficient. Therefore, the Cu content should be 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 more likely to occur during steel manufacturing, increasing the cost of maintenance. Therefore, the Cu content should be 2.000% or less, preferably 1.000% or less, and more preferably 0.800% or less.

[0021] [Ni: 0.002 to 5.000%] Ni is an effective element for improving corrosion resistance. However, if the Ni content is less than 0.002%, the above effect is insufficient. Therefore, the Ni content should be 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 should be 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 effective element for improving corrosion resistance. However, if the Cr content is less than 0.10%, the above effect is insufficient. Therefore, the Cr content should be 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 decreases. Therefore, the Cr content should be 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

[0023] [Al: 0.0010 to 1.0000%] Al is a deoxidizing element and, by combining with N in steel to form nitrides, is an effective element for improving toughness through grain refinement. Here, if the Al content is less than 0.0010%, the above effect is insufficient. Therefore, the Al content should be 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%, the hot workability decreases. Therefore, the Al content should be 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 effective element that greatly improves the hardenability of steel materials with only a small amount of addition, and also provides wear resistance. However, if the Mo content is less than 0.20%, the above effects are insufficient. Therefore, the Mo content should be 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 decreases. Therefore, the Mo content should be 2.00% or less, preferably 1.80% or less, and more preferably 1.50% or less.

[0025] [V: 0.0010 to 1.0000%] V is an element that is effective in improving toughness by refining the crystal grains by bonding with N in steel to form nitrides. Here, if the V content is less than 0.0010%, the above effect is insufficient. Therefore, the V content should be 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 are formed, and the hot workability decreases. Therefore, the V content should be 1.0000% or less, preferably 0.7000% or less, and more preferably 0.6000% or less.

[0026] [N: 0.0010 to 0.0250%] N is an element that is effective in improving toughness by refining the crystal grains by forming nitrides with nitride-forming elements in steel and acting as a grain boundary pinning particle. Here, if the N content is less than 0.0010%, the above effect is insufficient. Therefore, the N content should be 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%, the toughness decreases. Therefore, the N content should be 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 contains the above elements, with the remainder being Fe and unavoidable impurities. Preferably, the round steel bar for quenching and tempering contains the above elements, with the remainder being Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the purpose of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap.

[0028] Furthermore, the composition of round steel bars for quenching and tempering includes, in addition to the above basic components, 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% It may contain at least one element selected from the group consisting of %, 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. If any element is included below the preferred lower limit described later, that element shall be included as an unavoidable impurity.

[0029] [Mg: 0.0100% or less] Mg forms MgO with O in steel and acts as a grain boundary pinning particle, making it an effective element for improving toughness through grain refinement. To obtain this effect, a Mg content of 0.0001% or more is preferable. On the other hand, if the Mg content exceeds 0.0100%, a large amount of MgO is generated, and toughness decreases. Therefore, when Mg is included, the Mg content should be 0.0100% or less.

[0030] [Ti: 0.100% or less] Ti is an element that is effective in improving toughness by refining the crystal grains by bonding with N in steel to form nitrides. To obtain this effect, a Ti content of 0.001% or more is preferable. On the other hand, if the Ti content exceeds 0.100%, a large amount of Ti-based inclusions will be generated in the steel, and the toughness will decrease. Therefore, when Ti is included, the Ti content should be 0.100% or less.

[0031] [Nb: 0.100% or less] Nb is an element that is effective in improving toughness by refining the crystal grains, as it combines with C in steel to form carbides. To obtain 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 are generated, and the toughness decreases. Therefore, when Nb is included, the Nb content should be 0.100% or less.

[0032] [Ta: 0.1000% or less] Ta is an element effective in improving toughness through grain refinement. To obtain this effect, a Ta content of 0.0001% or more is preferable. On the other hand, if the Ta content exceeds 0.1000%, toughness decreases. Therefore, when Ta is included, the Ta content should be 0.1000% or less.

[0033] [Pb: 0.500% or less] Up to a certain amount of Pb addition does not adversely affect any of the properties required for mechanical structural parts; therefore, if the content is 0.500% or less, it is not a problem to have it mixed in. However, if the Pb content exceeds 0.500%, the strength of the steel decreases. Therefore, the Pb content should be 0.500% or less. On the other hand, there is no particular lower limit to the Pb content, but the Pb content may be 0.001% or more.

[0034] [Bi: 0.100% or less] Bi is an element effective in improving toughness through grain refinement. To obtain this effect, a Bi content of 0.001% or more is preferable. On the other hand, if the Bi content exceeds 0.100%, toughness decreases. Therefore, when Bi is included, the Bi content should be 0.100% or less.

[0035] [O: 0.0100% or less] O is an element effective in improving toughness through grain refinement. To obtain this effect, an O content of 0.0001% or more is preferable. On the other hand, if the O content exceeds 0.0100%, toughness decreases. Therefore, when O is included, the O content should be 0.0100% or less.

[0036] [As: 0.0100% or less] As is an element effective in improving toughness through grain refinement. To obtain this effect, an As content of 0.0001% or more is preferable. On the other hand, if the As content exceeds 0.0100%, toughness decreases. Therefore, when As is included, the As content should be 0.0100% or less.

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

[0038] [B: 0.0100% or less] B is an effective element for increasing the toughness of steel. To obtain this effect, a B content of 0.0001% or more is preferable. On the other hand, if the B content exceeds 0.0100%, the effect of adding B will saturate. Therefore, when B is included, the B content should be 0.0100% or less, and preferably 0.0050% or less.

[0039] [Zr: 0.0500% or less] Zr is an effective element for increasing the toughness of steel. To obtain this effect, a Zr content of 0.0001% or more is preferable. On the other hand, if the Zr content exceeds 0.0500%, the toughness will actually decrease. Therefore, when Zr is included, the Zr content should be 0.0500% or less.

[0040] [Hf: 0.0500% or less] Hf is an element effective in enhancing the toughness of steel materials. To obtain such an effect, the Hf content is preferably 0.0001% or more. On the other hand, when the Hf content exceeds 0.0500%, the effect saturates. Therefore, when Hf is contained, the Hf content shall be 0.0500% or less.

[0041] [W: 1.0000% or less] W is an element effective in enhancing the toughness of steel materials. To obtain such an effect, the W content is preferably 0.0001% or more. On the other hand, when the W content exceeds 1.0000%, the effect saturates. Therefore, when W is contained, the W content shall be 1.0000% or less.

[0042] [Co: 0.5000% or less] Co is an element effective in enhancing the toughness of steel materials. To obtain such an effect, the Co content is preferably 0.0001% or more. On the other hand, when the Co content exceeds 0.5000%, the effect saturates. Therefore, when Co is contained, the Co content shall be 0.5000% or less.

[0043] [Zn: 0.0100% or less] Zn is an element effective in improving the toughness of steel materials. To obtain such an effect, the Zn content is preferably 0.0001% or more. On the other hand, when the Zn content exceeds 0.0100%, the effect saturates. Therefore, when Zn is contained, the Zn content shall be 0.0100% or less.

[0044] [Te: 0.100% or less] Te is an element effective in enhancing the toughness of steel materials. To obtain such an effect, the Te content is preferably 0.001% or more. On the other hand, when the Te content exceeds 0.100%, the toughness decreases. Therefore, when Te is contained, the Te content shall be 0.100% or less.

[0045] [Sn: 0.100% or less] Sn is an element effective in enhancing the corrosion resistance of steel materials. To obtain such an effect, the Sn content is preferably 0.001% or more. On the other hand, when the Sn content exceeds 0.100%, the effect saturates. Therefore, when Sn is contained, the Sn content shall be 0.100% or less.

[0046] [Sb: 0.100% or less] Sb is an element effective in enhancing the corrosion resistance of steel materials. In order to obtain such an effect, the Sb content is preferably 0.001% or more. On the other hand, when the Sb content exceeds 0.100%, the toughness decreases. Therefore, when Sb is contained, the Sb content shall be 0.100% or less.

[0047] [REM: 0.1000% or less] REM is an element effective in enhancing the corrosion resistance of steel materials. In order to obtain such an effect, the REM content is preferably 0.0001% or more. On the other hand, when the REM content exceeds 0.1000%, the effect saturates. Therefore, when REM is contained, the REM content shall be 0.1000% or less. Here, REM refers to the general name of 17 elements, which are 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. Also, the REM content in the present invention means the total content of these 17 elements.

[0048] [In a cross section perpendicular to the longitudinal direction of the round steel bar, the concentration distribution of C, Si, Mn, and Mo in the region from the surface of the round steel bar to a depth corresponding to 10% of the diameter of the cross section, moving from the surface toward the center, satisfies equation (1).] The following equation (1) specifies the local concentration distribution in a round steel bar for quenching and tempering. Mechanical structural parts that undergo quenching and tempering have an internal structure based on martensite. If equation (1) is not satisfied, variations in hardness will occur even within the same martensite structure, and when mechanical structural parts come into contact with each other, the parts with lower hardness will wear preferentially and rapidly. Therefore, in order to suppress wear of round steel bars for quenching and tempering, in a cross section perpendicular to the longitudinal direction of the round steel bar, the concentration distribution of C, Si, Mn, and Mo in the (ring-shaped) region from the surface of the round steel bar to a depth corresponding to 10% of the diameter of the cross section, moving from the surface toward the center, must satisfy equation (1). While a smaller value on the left side of equation (1) is preferable, and therefore its lower limit is not particularly limited, in the present invention, the value on the left side is generally 0.25 or greater. ... (1) Here, [X] is the content of element X in the above component composition, and [X] ａｔＣｍｉｎ and [X] ａｔＣｍａｘ These are the concentrations of element X at the positions where the C concentration is minimum and maximum in the aforementioned concentration distribution, respectively.

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

[0050] One embodiment of the present invention relates to a round steel bar for quenching and tempering, which has the shape of a steel strip, i.e., a steel bar or wire.

[0051] (Method for manufacturing round steel bars for quenching and tempering) Next, a method for manufacturing round steel bars 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 suitable 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 of obtaining a steel ingot by pouring molten steel having a predetermined component composition and cooling it. In casting, molten steel having a predetermined component composition is first prepared. The component composition of the molten steel is adjusted so that the component composition of the round steel bar for quenching and tempering is the component composition described above. A batch-type electric furnace or a continuous-type blast furnace can be used to manufacture the molten steel.

[0053] Next, the prepared molten steel is poured into a mold to perform casting and obtain a steel ingot. The temperature of the molten steel during pouring is preferably above the melting point of the molten steel's composition and no more than 100°C above the melting point. The molten steel may be poured into the mold 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 more from the viewpoint of manufacturability. The pouring speed is preferably 1.0 m / min or less, and more preferably 0.5 m / min or less. Furthermore, it is preferable to set the flow velocity of the molten steel to 10 cm / sec or more and 20 cm / sec or less by electromagnetic stirring inside the mold or the like. By satisfying these casting conditions, C, Si, Mn, and Mo contained in the molten steel can be dispersed evenly in the steel ingot.

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

[0055] For processes and conditions not described in this specification, conventional methods may be used.

[0056] The configuration and effects of the present invention will be specifically described below with reference to the following examples. However, the present invention is not limited by the following examples, and can be modified as appropriate within the scope that is consistent with the gist of the present invention, and all such modifications fall within the technical scope of the present invention.

[0057] Steel ingots with the component composition shown in Table 1 were melted and hot-rolled into round bars of φ50 or φ230 mm to produce round steel bars for quenching and tempering. Subsequently, the φ50 mm round bars were cut to a length of 100 mm, and the φ230 mm round bars were cut to a length of 300 mm.

[0058]

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

[0060] [Hot Workability Evaluation] A hot workability evaluation was conducted for a φ50 mm example. Specifically, a tensile test specimen with a parallel section of φ6 mm was taken from the D / 4 position of a φ50 mm quenched and tempered round steel bar (as-hot-rolled material), as shown in Figure 1. Next, a hot tensile test was performed with the thermal history shown in Figure 2, and the percentage reduction in cross-sectional area until fracture was determined. Note that T in Figure 2 was set to 900°C. In this evaluation, a cross-sectional area reduction of 41% or more indicates sufficient hot workability.

[0061] [Toughness Evaluation] For a φ50 mm example, toughness evaluation was performed in accordance with the Charpy impact test method for metallic materials specified in JIS Z 2242. A sample with the shape shown in Figure 3 was taken from the D / 4 position of a φ50 mm quenched and tempered round steel bar (as-hot-rolled material). The notched surface was taken so that it faced away from the center. The fracture surface of the sample after the test was observed and photographed with a digital microscope, and the ductile fracture area ratio at the fracture surface was determined. In this evaluation, a ductile fracture area ratio of 50% or more indicates sufficient toughness.

[0062] Next, the round steel bars for quenching and tempering were subjected to quenching and tempering treatment, and the surface scale was removed by shot blasting to prepare the test specimens for each evaluation described below.

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

[0064] [Abrasion Resistance Evaluation] Abrasion resistance was evaluated for each example. Specifically, two test pieces with the same component composition were placed perpendicular to each other, one was fixed, and the other was slid longitudinally while a 10kN load was applied to the fixed test piece and it was in contact with it. The evaluation was based on the number of sliding cycles until the amount of wear reached the radius of the test piece. The sliding was performed within a range of 25 mm on each side from the contact point between the test pieces. In this evaluation, a test piece with a diameter of φ50 mm can be said to have excellent abrasion resistance if it slid 22,000 times or more, and a test piece with a diameter of φ230 mm can be said to have excellent abrasion resistance if it slid 100,000 times or more.

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

[0066]

[0067] As shown in Table 2, all of the inventive examples exhibited excellent corrosion resistance, wear resistance, hot workability, and toughness. On the other hand, the comparative examples lacked sufficient corrosion resistance, wear resistance, hot workability, and toughness in at least one of these areas.

[0068] According to the present invention, it is possible to provide a quenching and tempering steel that reduces manufacturing costs and has excellent corrosion resistance, wear resistance, hot workability, and toughness. This quenching and tempering steel can be used as a mechanical structural component where the above characteristics are required. Specifically, it can be used as steel for wind turbine shafts, pins and pistons for industrial machinery and ships, and mooring chains used for mooring offshore structures.

Claims

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

2. The above component composition is further defined as follows in mass percent: 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 The round steel bar for quenching and tempering according to claim 1, containing at least one element selected from the group consisting of: 0.100% or less and REM: 0.1000% or less.

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