Case hardened steel having excellent cracking resistance

Abstract

The purpose of the present invention is to provide a case hardened steel with which it is possible to obtain a steel material that has excellent cold workability, particularly excellent cracking resistance, while ensuring hardenability. The present invention provides a case hardened steel which contains, in mass%, 0.15% to 0.30% inclusive of C, 0.05% to 1.00% inclusive of Si, not less than 0.10% but less than 0.50% of Mn, 1.3% to 2.5% inclusive of Cr, 0% to 0.5% inclusive of Ni, 0% to 0.5% inclusive of Mo, 0% to 0.050% inclusive of Al, 0% to 0.20% inclusive of Ti, 0% to 0.10% inclusive of Nb, 0% to 0.0050% inclusive of B, and 0% to 0.0300% inclusive of N, with the balance being made up of Fe and unavoidable impurities, wherein: P and S in the unavoidable impurities are 0.030% or less and 0.025% or less, respectively; and the score of a ferrite band in the structure of a cross-sectional surface that is parallel to the hot rolling direction is 1-3.

Description

Case-hardened steel with excellent crack resistance

[0001] The present invention relates to a case-hardening steel that is suitable for use as a material for mechanical parts such as gears and shafts in transportation equipment such as automobiles and industrial machinery such as construction machinery, and which has excellent cold workability and particularly excellent crack resistance.

[0002] Patent Document 1 describes a material containing, by mass%, C: 0.1-0.3%, Si: 0.01-0.15%, Mn: 0.2-0.65%, S: 0.005-0.15%, Cr: 0.4-1.25%, B: 0.0005-0.005%, Al: 0.015-0.05%, Nb: 0.002-0.035%, and N: less than 0.005% (including 0%), with Ti added in amounts ranging from 0.015 to 3% depending on the N content. The composition contains 4N + 0.02%, with P limited to 0.025% or less (including 0%) and O limited to 0.0025% or less (including 0%), with the remainder consisting of iron and unavoidable impurities. The amount of AlN precipitate is limited to 0.005% or less, and the matrix contains a total of 20 precipitates per 100 μm of Nb, Ti, or a composite composition of Nb and Ti with a diameter of 0.2 μm or less. ２ The present invention describes a case-hardening steel having the above characteristics, characterized by limiting the bainite microstructure fraction to 15% or less, and having a score of 1 to 5 for the ferrite bands in the microstructure of a cross-section parallel to the hot-rolling direction, exhibiting excellent cold workability and low carburizing strain properties.

[0003] Patent Document 2 describes a steel for cold-working machine structures, characterized in that it contains, by mass%, C: 0.3-0.6%, Si: 0.05-0.5%, Mn: 0.2-1.7%, P: greater than 0-0.03%, S: 0.001-0.05%, Al: 0.01-0.1%, and N: 0-0.015%, with the remainder being iron and unavoidable impurities, the steel's microstructure has protereminate ferrite and pearlite, the total area ratio of protereminate ferrite and pearlite to the whole microstructure is 90% or more, the area ratio of protereminate ferrite to the whole microstructure is (1-1.25 × [C%]) × 80% or more, and the average block size of the pearlite is 25 μm or less.

[0004] Japanese Patent Publication No. 11-335777 Japanese Patent Publication No. 2018-003106

[0005] The case-hardened steel described in Patent Document 1 did not take hardenability into consideration. Furthermore, the steel described in Patent Document 2 was intended to shorten the spheroidizing process, and therefore the annealing process could not be omitted.

[0006] The present invention aims to provide a case-hardening steel that ensures hardenability while also providing excellent cold workability, particularly crack resistance.

[0007] Conventionally, ferrite bands have not been considered, nor have variations in microstructure been adequately taken into account. As a result of diligent research, the inventors have found that ferrite bands affect cold workability, and further, that focusing on variations in microstructure is useful in improving cold workability, particularly crack resistance.

[0008] To solve the above problems, the present invention provides the following case-hardening steel. [1] A case-hardening steel comprising, by mass%, C: 0.15% or more and 0.30%, Si: 0.05% or more and 1.00%, Mn: 0.10% or more and less than 0.50%, Cr: 1.3% or more and 2.5%, Ni: 0% or more and 0.5%, Mo: 0% or more and 0.5%, Al: 0% or more and 0.050%, Ti: 0% or more and 0.20%, Nb: 0% or more and 0.10%, B: 0% or more and 0.0050%, N: 0% or more and 0.0300%, and the remainder being Fe and unavoidable impurities, wherein the P content in the unavoidable impurities is 0.030% or less and the S content is 0.025% or less, and the score of the ferrite band in the microstructure of the cross-section parallel to the hot-rolling direction is 1 to 3. [2] The case-hardening steel according to [1], wherein the case-hardening steel contains one or more of the following: Ni: greater than 0% and 0.5% or less, Mo: greater than 0% and 0.5% or less, Al: 0.020% or more and 0.050% or less, Ti: greater than 0% and 0.20% or less, Nb: greater than 0% and 0.10% or less, B: greater than 0% and 0.0050% or less, and N: 0.0040% or more and 0.0300% or less. [3] The case-hardening steel has a diameter D, and the total area fraction A of ferrite and pearlite at the D / 4 position. ４ The total area fraction A of ferrite and pearlite at position D / 2 ２Ratio A with respect to ４ / A ２ is 0.80 to 1.10, the skin-burning steel according to [1] or [2]. [4] The skin-burning steel is represented by the following formula 1: 0.6 ≦ 0.59[Cr] + 0.26[Si] - 0.81[Mn] ≦ 1.4 [where, [Cr] represents the content rate (mass%) of Cr, [Si] represents the content rate (mass%) of Si, and [Mn] represents the content rate (mass%) of Mn.], the skin-burning steel according to [1] or [2]. [5] The skin-burning steel has a diameter D, and the total area fraction A of ferrite and pearlite at the D / 4 position ４ of the total area fraction A of ferrite and pearlite at the D / 2 position ２ Ratio A with respect to ４ / A ２ is 0.80 to 1.10, and the skin-burning steel is represented by the following formula 1: 0.6 ≦ 0.59[Cr] + 0.26[Si] - 0.81[Mn] ≦ 1.4 [where, [Cr] represents the content rate (mass%) of Cr, [Si] represents the content rate (mass%) of Si, and [Mn] represents the content rate (mass%) of Mn.], the skin-burning steel according to [1] or [2].

[0009] According to the skin-burning steel of the present invention, it is possible to obtain a steel material excellent in cold working properties, particularly crack resistance, while ensuring hardenability. Therefore, the skin-burning steel according to the present invention can be suitably applied to cold forging in which spheroidizing annealing treatment is omitted. Therefore, if the skin-burning steel according to the present invention is applied to drive system parts such as automobiles manufactured by cold forging, it is possible to reduce the manufacturing cost by omitting spheroidizing annealing.

[0010] FIG. 1 is a micrograph of a metal structure for explaining a criterion for quantitatively evaluating the degree of a ferrite band (banded structure). FIG. 2 is an optical micrograph of the developed steel 5 having a ferrite band rating of 1. FIG. 3 is an optical micrograph of the comparative steel 3 having a ferrite band rating of 6. FIG. 4 is a schematic diagram for explaining a cross section parallel to the hot rolling direction, the D / 4 position, and the D / 2 position, taking a bar-shaped steel material as an example.

[0011] The chemical composition of the skin-burning steel according to the present invention will be described below. "%" used as the unit of the content of each element means mass%.

[0012] The skin-burning steel according to the present invention contains C, Si, Mn, and Cr as essential elements. The skin-burning steel according to the present invention may contain one or more selected from Ni, Mo, Al, Ti, Nb, B, and N as optional elements. In the skin-burning steel according to the present invention, the balance other than the essential elements and the optional elements consists of Fe and inevitable impurities. "Inevitable impurities" means components mixed from raw materials of steel (for example, ore, scrap, etc.), the environment of the manufacturing process, etc., and are not components intentionally contained in the steel.

[0013] The essential elements (C, Si, Mn, and Cr) will be described below.

[0014] C: 0.15% or more and 0.30% or less. C is a component that increases the hardness of the material. If the content of C is less than 0.15%, the strength after cold forging is insufficient. Therefore, the content of C is 0.15% or more. On the other hand, if the content of C exceeds 0.30%, the hardness of the material increases too much and the cold workability decreases. Therefore, the content of C is 0.30% or less. The content of C is preferably 0.25% or less. The above lower limit value may be combined with any of the above upper limit values.

[0015] Si: 0.05% or more and 1.00% or less. Si is a component useful for deoxidation. Also, Si is a component that increases the hardness of the material. If the content of Si is less than 0.05%, deoxidation may be insufficient. Therefore, the content of Si is 0.05% or more. The content of Si is preferably 0.30% or more. On the other hand, if the content of Si exceeds 1.00%, the hardness of the material increases too much and the cold workability decreases. Therefore, the content of Si is 1.00% or less. The content of Si is preferably 0.80% or less. The above lower limit values may be combined with any of the above upper limit values respectively.

[0016] Mn: 0.10% or more and less than 0.50% Mn is a component that forms a ferrite band. If the content of Mn is less than 0.10%, the hardenability is insufficient. Therefore, the content of Mn is set to 0.10% or more. The content of Mn is preferably 0.15% or more. On the other hand, if the content of Mn is 0.50% or more, the cold workability deteriorates. Therefore, the content of Mn is set to less than 0.50%. The content of Mn is preferably 0.40% or less. Each of the above lower limit values may be combined with any of the above upper limit values.

[0017] Cr: 1.3% or more and 2.5% or less Cr is a component that increases the material hardness. If the content of Cr is less than 1.3%, the hardenability and strength decrease. Therefore, the content of Cr is set to 1.3% or more. The content of Cr is preferably 1.5% or more. On the other hand, if the content of Cr exceeds 2.5%, the material hardness increases too much and the cold workability deteriorates. Therefore, the content of Cr is set to 2.5% or less. The content of Cr is preferably 2.2% or less. Each of the above lower limit values may be combined with any of the above upper limit values.

[0018] The following describes inevitable impurities.

[0019] Examples of inevitable impurities include P, S, Cu, etc.

[0020] P: 0% or more and 0.030% or less P is a component that may be mixed as an inevitable impurity. If the content of P exceeds 0.030%, the deformation resistance increases during cold working and the cold workability deteriorates. Therefore, the content of P is set to 0.030% or less. The content of P may be 0% or more than 0%. The content of P may be, for example, 0.001% or more, 0.002% or more or 0.003% or more. The above upper limit value may be combined with any of the above lower limit values.

[0021] S: 0% or more and 0.025% or less. S is a component that can be mixed in as an unavoidable impurity. If the S content exceeds 0.025%, MnS is formed, and the cold workability decreases. Therefore, the S content should be 0.025% or less. The S content may be 0% or more. The S content may be, for example, 0.001% or more, 0.002% or more, or 0.003% or more. The above upper limit may be combined with any of the above lower limits.

[0022] Cu: 0% or more and 0.5% or less. Cu is a component that can be mixed in as an unavoidable impurity. If the Cu content exceeds 0.5%, the cold workability decreases. Therefore, it is preferable to keep the Cu content at 0.5% or less. The Cu content may be 0% or more. The Cu content may be, for example, 0.01% or more, 0.02% or more, or 0.03% or more. The above upper limit may be combined with any of the above lower limit values.

[0023] The following describes the optional elements (one or more selected from Ni, Mo, Al, Ti, Nb, B, and N).

[0024] Ni: 0% or more and 0.5% or less. Ni is a component that increases the hardness of the material. The Ni content may be 0% or more than 0%. When Ni is added (i.e., when the Ni content is greater than 0%), it is preferable that the Ni content be 0.07% or more. Increasing the Ni content increases the hardness of the material, but it also increases the cost. Therefore, it is preferable that the Ni content be 0.5% or less. The above upper limit may be combined with any of the above lower limit values.

[0025] Mo: 0% or more and 0.5% or less. Mo is a component that increases the hardness of the material. The Mo content may be 0% or more than 0%. When Mo is added (i.e., when the Mo content is more than 0%), it is preferable that the Mo content be 0.10% or more. Increasing the Mo content increases the hardness of the material, but it also increases costs. Therefore, it is preferable that the Mo content be 0.5% or less. The above upper limit may be combined with any of the above lower limit values.

[0026] Al: 0% or more and 0.050% or less. Al is a useful component as a deoxidizing agent. The Al content may be 0% or more than 0%. When Al is added (i.e., when the Al content is more than 0%), it is preferable that the Al content be 0.020% or more. If the Al content exceeds 0.050%, coarse nitrides are formed, and the processability decreases. Therefore, it is preferable that the Al content be 0.050% or less. The above upper limit may be combined with any of the above lower limit values.

[0027] Ti: 0% or more and 0.20% or less. The Ti content may be 0% or more than 0%. When Ti is added (i.e., when the Ti content is greater than 0%), it is preferable that the Ti content be 0.02% or more. Since Ti forms carbonitrides, if the Ti content exceeds 0.20%, excess carbonitrides will be formed, and the processability will tend to decrease. Therefore, it is preferable that the Ti content be 0.20% or less. The above upper limit may be combined with any of the above lower limit values.

[0028] Nb: 0% or more and 0.10% or less. The Nb content may be 0% or more than 0%. When Nb is added (i.e., when the Nb content is greater than 0%), it is preferable that the Nb content be 0.02% or more. Since Nb forms carbonitrides, if the Nb content exceeds 0.10%, excess carbonitrides will be formed, and processability will tend to decrease. Therefore, it is preferable that the Nb content be 0.10% or less. The above upper limit may be combined with any of the above lower limit values.

[0029] B: 0% or more and 0.0050% or less. B is a component that improves hardenability. The content of B may be 0% or more than 0%. When B is added (i.e., when the content of B is more than 0%), it is preferable that the content of B be 0.0010% or more. If the content of B exceeds 0.0050%, its effect will saturate. Therefore, it is preferable that the content of B be 0.0050% or less. The above upper limit may be combined with any of the above lower limit values.

[0030] N: 0% or more and 0.0300% or less. N is a component that forms fine nitrides and contributes to improving the mechanical properties of steel. The N content may be 0% or more than 0%. When N is added (i.e., when the N content is more than 0%), it is preferable that the N content be 0.0040% or more. On the other hand, if the N content exceeds 0.0300%, productivity will decrease. Therefore, it is preferable that the N content be 0.0300% or less. The above upper limit may be combined with any of the above lower limits.

[0031] The following describes the scoring of the ferrite band of the case-hardened steel according to the present invention.

[0032] In the case-hardening steel according to the present invention, the score of the ferrite band in the microstructure of the cross-section parallel to the hot-rolling direction is set to 1 to 3. This is because if the score exceeds 3, the probability of crack initiation originating from the ferrite band increases.

[0033] Ferrite bands are a type of striped structure. Figure 1 is a micrograph of a metal structure illustrating the criteria for quantitatively evaluating the degree of ferrite bands. The reference photograph for quantitatively evaluating the degree of ferrite bands is described in "Journal of the Japan Institute of Metals, Vol. 34, No. 9, p. 961," published by the Japan Institute of Metals in 1970, and Figure 1 is a reproduction of that reference photograph. In the above-mentioned document, ferrite bands (striped structures) are scored on a scale of 1 to 7. This invention also evaluates ferrite bands (striped structures) according to this scale. A lower score indicates that the formation of ferrite bands is minor, while a higher score indicates that the formation of ferrite bands is significant. If the score of the ferrite bands is 4 or higher, the ferrite structure will be continuously connected in a striped pattern. As a result, cracks will occur starting from the ferrite structure. Therefore, from the viewpoint of crack resistance, the score of the ferrite bands in the structure of a cross-section parallel to the hot rolling direction is set to 1 to 3.

[0034] The rating of the ferrite band can be evaluated by the method described in the examples.

[0035] The ratio A of the case-hardening steel according to the present invention is as follows: ４ / A２ I will explain this.

[0036] In the case hardening steel according to the present invention, ratio A ４ / A ２ It is preferable that the ratio A is between 0.80 and 1.10. ４ / A ２ This is an index for evaluating the probability of cracking due to structural heterogeneity. Ratio A ４ / A ２ This is the total area fraction A of ferrite and pearlite at position D / 4 (the intermediate part, i.e., the part between the surface and the center). ４ (%), the total area fraction A of ferrite and pearlite at position D / 2 (center). ２ This is a ratio to (%).

[0037] "D" represents the diameter of the case-hardening steel according to the present invention. If the cross-sectional shape perpendicular to the hot-rolling direction of the case-hardening steel according to the present invention is a circle, "Diameter" represents the diameter of that circle. If the cross-sectional shape perpendicular to the hot-rolling direction of the case-hardening steel according to the present invention is an ellipse, "Diameter" represents the major or minor axis of that ellipse. If the cross-sectional shape perpendicular to the hot-rolling direction of the case-hardening steel according to the present invention is a quadrilateral (square or rectangle), "Diameter" represents the diameter of the circle circumscribing that quadrilateral.

[0038] "D / 4 position" refers to the portion of the case-hardening steel according to the present invention that is D / 4 depth from the surface. "D / 2 position" refers to the portion of the case-hardening steel according to the present invention that is D / 2 depth from the surface.

[0039] In one embodiment, the case-hardening steel according to the present invention is a rod-shaped steel material (cylindrical steel material). In this embodiment, the shape of the cross section perpendicular to the hot-rolling direction of the case-hardening steel according to the present invention is a circle, and "diameter" refers to the diameter of that circle. For the cross section perpendicular to the hot-rolling direction, the cross section parallel to the hot-rolling direction, the D / 4 position and the D / 2 position when the case-hardening steel according to the present invention is a rod-shaped steel material, please refer to Figure 4.

[0040] Ratio A ４ / A ２ When the ratio falls below 0.80, the probability of cracking due to structural heterogeneity increases. Therefore, ratio A ４ / A ２ The ratio A shall be 0.80 or higher.４ / A ２ The ratio is preferably 0.90 or higher. ４ / A ２ When the ratio exceeds 1.10, the probability of cracking due to structural heterogeneity increases. Therefore, ratio A ４ / A ２ The value shall be 1.10 or less. The above upper limit may be combined with any of the above lower limits.

[0041] A ２ The lower limit is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The upper limit is 100% or less. The upper limit may be, for example, 95% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0042] A ４ The lower limit is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The upper limit is 100% or less. The upper limit may be, for example, 95% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0043] A ２ A ４ and ratio A ４ / A ２ This can be determined by the method described in the examples.

[0044] The following explains Equation 1.

[0045] The case-hardening steel according to the present invention preferably satisfies Formula 1: 0.6 ≤ 0.59 [Cr] + 0.26 [Si] - 0.81 [Mn] ≤ 1.4 [wherein [Cr] represents the Cr content (mass%), [Si] represents the Si content (mass%), and [Mn] represents the Mn content (mass%).]

[0046] Si and Cr are A ３ It is an element that raises the transformation point, and Mn is A ３ It is an element that lowers the transformation point. The value in Equation 1 (0.59 [Cr] + 0.26 [Si] - 0.81 [Mn]) is A ３This is an index for balancing the content of Cr, Si, and Mn, which are components that raise and lower the transformation point. Cr, Si, and Mn are all elements that contribute to the formation of ferrite bands in segregated regions. However, if the value of Equation 1 (0.59 [Cr] + 0.26 [Si] - 0.81 [Mn]) is greater than the upper limit (1.4), Cr and Si become excessive, and ferrite bands are formed. On the other hand, if the value of Equation 1 (0.59 [Cr] + 0.26 [Si] - 0.81 [Mn]) is less than the lower limit (0.6), Mn becomes excessive, and ferrite bands are formed. In order to provide crack resistance as a structural alloy steel, it is necessary to suppress ferrite band formation while ensuring hardenability. To achieve this, A ３ Among the components that raise and lower the transformation point, it is preferable to maintain the content of Cr, Si, and Mn within a specified range and to balance the content of these elements.

[0047] It is preferable that the value of Formula 1 (0.59 [Cr] + 0.26 [Si] - 0.81 [Mn]), which is an index for balancing the content of Cr, Si, and Mn, be between 0.6 and 1.4. More preferably, the value of Formula 1 (0.59 [Cr] + 0.26 [Si] - 0.81 [Mn]) is between 0.7 and 1.3, and even more preferably between 0.8 and 1.3. Each of the above lower limits may be combined with any of the above upper limits. Note that steel materials in which the content of Si, Mn, and Cr approaches 0 will have a value of Formula 1 (0.59 [Cr] + 0.26 [Si] - 0.81 [Mn]) that is smaller than the lower limit (0.6), but when case-hardened steel is used as structural steel for machinery, its hardenability will be insufficient, thus impairing its practicality as structural steel for machinery and therefore it is unacceptable.

[0048] The case-hardening steel of the present invention will be described based on examples.

[0049] To obtain the case-hardened steels of Development Steels 1-10 (Examples) and Comparative Steels 1-5 (Comparative Examples), steel consisting of the elements listed in Table 1, with the remainder being Fe and unavoidable impurities, was melted down. Steel billets were then obtained through processes such as refining, ingot formation, and bloc rolling. The obtained steel billets were heated to 850-1100°C in a heating furnace and hot-rolled to a diameter of 32 mm using an intermediate row rolling mill and a finish row rolling mill. The resulting intermediate products were air-cooled to obtain the case-hardened steels of Development Steels 1-10 and Comparative Steels 1-5 as bar-shaped steel materials (cylindrical steel materials). Each of the obtained bar-shaped steel materials was cut to a length of 200 mm to obtain test specimens of each bar-shaped steel material.

[0050]

[0051] <Evaluation Method> In order to evaluate the properties of the development steel and the comparative steel, (1) microstructural observation, (2) limit upsetting test, and (3) hardenability evaluation test were performed.

[0052] (1) Observation of microstructure [Ratio A ４ / A ２ Each test specimen was cut through its center with a plane parallel to the hot rolling direction, the cut surface was mirror-polished, and then etched with Nital solution. Subsequently, the D / 4 and D / 2 positions from the surface were observed with an optical microscope (100x magnification, 10 fields of view) to identify and distinguish between the ferrite and pearlite structures.

[0053] Each captured microscope image was processed using image processing software, and the total number of pixels in each microscope image, the number of pixels in the ferrite tissue in each microscope image, and the number of pixels in the pearlite tissue in each microscope image were counted.

[0054] For each optical microscope image taken at the D / 4 position, the ratio of the number of pixels of ferrite tissue to the total number of pixels (%) was calculated, and the average of these ratios was defined as the "area fraction of ferrite tissue at the D / 4 position (%)". For each optical microscope image taken at the D / 4 position, the ratio of the number of pixels of pearlite tissue to the total number of pixels (%) was calculated, and the average of these ratios was defined as the "area fraction of pearlite at the D / 4 position (%)". The area fraction of ferrite tissue (%) at the D / 4 position and the area fraction of pearlite (%) at the D / 4 position were added together to determine the total area fraction of ferrite and pearlite (%) at the D / 4 position.

[0055] For each optical microscope image taken at position D / 2, the ratio of the number of pixels of ferrite tissue to the total number of pixels (%) was calculated, and the average value of these ratios was defined as the "area fraction of ferrite tissue at position D / 2 (%)". For each optical microscope image taken at position D / 2, the ratio of the number of pixels of pearlite tissue to the total number of pixels (%) was calculated, and the average value of these ratios was defined as the "area fraction of pearlite at position D / 2 (%)". The area fraction of ferrite tissue (%) at position D / 2 and the area fraction of pearlite (%) at position D / 2 were added together to determine the total area fraction of ferrite and pearlite (%) at position D / 2.

[0056] The total area fraction of ferrite and pearlite at position D / 4 is "A ４ " and the total area fraction of ferrite and pearlite at position D / 2 is "A ２ " and ratio A ４ / A ２ We calculated the value. The results are shown in Table 2.

[0057] [Scoring of Ferrite Bands] Each test specimen was cut along a plane passing through its center and parallel to the hot rolling direction. The cut surface was mirror-polished, etched with Nital solution, and then observed with an optical microscope at a magnification of 100x. Figure 2 shows the optical microscope image of development steel 5, and Figure 3 shows the optical microscope image of comparison steel 3. The ferrite band score for each test specimen was determined by comparing it with the reference photograph of the microstructure shown in Figure 1. Figure 1 is a reference photograph for quantitatively evaluating the degree of ferrite bands (striped structure) (specifically, for scoring ferrite bands on a 7-point scale from 1 to 7), as described in "Journal of the Japan Institute of Metals, Vol. 34, No. 9, p. 961," published by the Japan Institute of Metals in 1970. The present invention also evaluated ferrite bands (striped structure) in accordance with this. The results are shown in Table 2. In Table 2, "L section" means a section parallel to the hot rolling direction.

[0058] The optical microscope image of the developed steel 5 shown in Figure 2 has a score of 1, while the optical microscope image of the comparative steel 3 shown in Figure 3 has a score of 6.

[0059] If the score is 4 or higher, the ferrite structure becomes continuously connected, increasing the risk of cracks starting from that point.

[0060] (2) A cold-up test specimen measuring 14 mm (diameter) x 21 mm (length) was prepared from the central portion of the specimen for the limit upsetting test. In the limit upsetting test, five specimens were statically cold-compressed in the longitudinal direction with the end faces fully constrained, and the compression ratio (%) at which cracking began to occur was determined using a magnifying glass. The minimum compression ratio (%) was defined as the "limit compression ratio (%)". The target value for the limit compression ratio was set at 70% or higher. The limit compression ratio (%) is shown as "limit cracking (%)" in Table 2.

[0061] (3) Hardenability evaluation test A Jominy test specimen with a flange, 25 mm in diameter and 100 mm in length was machined from a 32 mm diameter steel bar. A Jominy test was performed on each test specimen according to JIS G0561 (2011). After the test, the hardness J was measured at a position 11 mm from the water-cooled end. １１ The hardness J was measured and measured. １１ Hardenability was evaluated using the hardness J. １１The hardness was measured using a diamond cone indenter with a tip radius of 0.2 mm and a tip angle of 120 degrees under a pressure of 150 kgf. １１ If the Rockwell hardness (HRC) is 23 or higher, it was determined to have high hardenability. Hardness J １１ If the Rockwell hardness (HRC) was less than 23, it was judged to have poor hardenability and was rejected. The results are shown in Table 2.

[0062]

[0063] As shown in Tables 1 and 2, the developed steels 1 to 10 have the following characteristics: composition, ferrite band score, value of formula 1, ratio A ４ / A ２ All of these satisfies the range defined in this invention, and it was confirmed that the limit compressibility by cold working exceeds 70%, exhibiting excellent crack resistance, and that when quenched as case-hardened steel, a practical hardness of 23 HRC or higher, required for machine structural steel, can be obtained.

[0064] Comparative steel 1 had insufficient Cr, and the value of Equation 1 was low and outside the range, so a hardened hardness of 23 HRC or higher could not be obtained. Comparative steel 2 had excessive Cr, and ratio A ４ / A ２ The ratio exceeds 1.10, resulting in the formation of ferrite bands and an increased probability of cracking due to structural non-uniformity, leading to lower crack resistance. Comparative steel 3 has an excess of Mn, and ratio A ４ / A ２ The ratio was below 0.6, resulting in the formation of ferrite bands and an increased probability of cracking due to structural non-uniformity, thus lowering crack resistance. Comparative steel 4 had an excess of Si, and ratio A ４ / A ２ The value exceeded 1.10, leading to the formation of ferrite bands and an increased probability of cracking due to structural non-uniformity, resulting in lower crack resistance. Comparative steel 5 had insufficient Mn, and a hardened hardness of 23 HRC or higher could not be achieved.

[0065] 1. Cross section perpendicular to the rolling direction of the steel bar. 2. Center of the cross section perpendicular to the rolling direction of the steel bar. 3. Cross section parallel to the rolling direction of the steel bar. 4. Center position of the steel bar. 5. Position at a depth of D / 2 from the surface. 6. Position at a depth of D / 4 from the surface. D: Diameter of the steel bar.

Claims

1. A case-hardening steel comprising, by mass%, C: 0.15% to 0.30%, Si: 0.05% to 1.00%, Mn: 0.10% to less than 0.50%, Cr: 1.3% to 2.5%, Ni: 0% to 0.5%, Mo: 0% to 0.5%, Al: 0% to 0.050%, Ti: 0% to 0.20%, Nb: 0% to 0.10%, B: 0% to 0.0050%, N: 0% to 0.0300%, and the remainder being Fe and unavoidable impurities, wherein the P content in the unavoidable impurities is 0.030% or less, and the S content is 0.025% or less, and the score of the ferrite band in the microstructure of the cross-section parallel to the hot-rolling direction is 1 to 3.

2. The case-hardening steel according to claim 1, wherein the case-hardening steel contains one or more of the following: Ni: greater than 0% and 0.5% or less, Mo: greater than 0% and 0.5% or less, Al: 0.020% or more and 0.050% or less, Ti: greater than 0% and 0.20% or less, Nb: greater than 0% and 0.10% or less, B: greater than 0% and 0.0050% or less, and N: 0.0040% or more and 0.0300% or less.

3. The case-hardened steel has a diameter D, and the total area fraction A of ferrite and pearlite at the D / 4 position. ４ The total area fraction A of ferrite and pearlite at position D / 2 ２ Ratio A ４ / A ２ The case hardening steel according to claim 1 or 2, wherein the ratio is 0.80 to 1.

10.

4. The case-hardening steel according to claim 1 or 2, wherein the case-hardening steel satisfies the following formula 1: 0.6 ≤ 0.59 [Cr] + 0.26 [Si] - 0.81 [Mn] ≤ 1.4 [wherein [Cr] represents the Cr content (mass%), [Si] represents the Si content (mass%), and [Mn] represents the Mn content (mass%).] 5. The case-hardened steel has a diameter D, and the total area fraction A of ferrite and pearlite at the D / 4 position. ４ The total area fraction A of ferrite and pearlite at position D / 2 ２ Ratio A ４ / A ２ The case-hardening steel according to claim 1 or 2, wherein the ratio is 0.80 to 1.10, and the case-hardening steel satisfies the following formula 1: 0.6 ≤ 0.59 [Cr] + 0.26 [Si] - 0.81 [Mn] ≤ 1.4 [wherein [Cr] represents the Cr content (mass%), [Si] represents the Si content (mass%), and [Mn] represents the Mn content (mass%).

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