Method for producing steel material

Abstract

The present invention addresses the problem of providing a method for producing a steel material in which it can be expected that hot shortness is suppressed. A means for solving the problem is a method for producing a steel material including at least Cu and S, the method for producing a steel material including: a holding step for holding a steel raw material including at least Cu and S to a range from the temperature at which a sulfide of Cu enters solid solution to 1,300° C for 180 seconds or more; and a steel raw material cooling step for cooling the steel raw material after holding the steel raw material to an austenite area temperature for 180 seconds or more.

Description

Method for manufacturing steel material

[0001] The present disclosure relates to a method for manufacturing a steel material.

[0002] Recycled iron contains a large amount of Cu (copper). In carbon steel containing a large amount of Cu (copper) manufactured using such recycled iron, it is known that hot brittleness occurs during processing at high temperatures. For example, in steel containing a relatively large amount of Cu (copper), Cu (copper) becomes a fracture nucleus during processing, and problems such as cracks may occur in rolling. To solve the above problems, in low-carbon steel, Cu (copper) and S (sulfur) are made to precipitate as Cu ,

[0008] , ,

[0007] ,

[0006] ,

[0009] , S (copper sulfide) to suppress hot brittleness has been devised.

[0003] For example, in Patent Document 1, by weight%, Cu: 0.04 to 3%, S: 0.02 to 0.16%, and copper sulfide particles having a particle diameter of 1 nm to 30 nm are contained in a volume% of 0.1% to 0.8%, and a particle dispersion-strengthened steel characterized in that the copper sulfide particles are dispersed in steel is disclosed.

[0004] Japanese Patent Application Laid-Open No. 2000-345300

[0005] However, in the above Patent Document 1, sufficient consideration has not been given to the process of manufacturing carbon steel.

[0006] Therefore, an object of the present disclosure is to solve the above problems of the prior art and provide a method for manufacturing a steel material that can be expected to suppress hot brittleness.

[0007] The gist configuration of the present disclosure for solving the above problems is as follows.

[0008] [1] A method for manufacturing a steel material containing at least Cu and S, comprising: a holding step of holding a steel material containing at least Cu and S in a range of a temperature at which a sulfide of Cu is dissolved to 1300 ° C. for 180 seconds or more; and a steel material cooling step of cooling the steel material after holding it at an austenite region temperature for 180 seconds or more.

[0009] [2] The method for manufacturing a steel material according to [1], wherein in the steel material cooling step, the temperature range in which the steel material is held for 180 seconds or more is 1000 to 700 ° C.

[0010] [3] The method for manufacturing steel according to [1] or [2], wherein the steel contains 0.7 to 1.0% by mass of C, 0.1% by mass or more of Cu, and 0.01% by mass or more of S.

[0011] [4] A method for manufacturing steel according to any one of [1] to [3], wherein the steel material contains 50 ppm by mass or more of N.

[0012] [5] The method for manufacturing steel according to any one of [1] to [4], wherein the steel contains 0.05 to 0.3% by mass of Cr.

[0013] [6] A method for manufacturing steel according to any one of [1] to [5], wherein at least a portion of Cu is present in the steel material in the form of copper sulfide.

[0014] [7] The method for manufacturing steel according to any one of [1] to [6], wherein the cooling rate in the steel material cooling step is 20°C / sec or more.

[0015] [8] A method for producing steel according to any one of [1] to [7], comprising, prior to the holding step, a melting step of heating the steel raw material to a temperature above which the steel raw material melts to obtain molten steel, and a molten steel cooling step of cooling the molten steel at a cooling rate of 0.05°C / sec or more in at least the austenite region temperature.

[0016] According to this disclosure, it is possible to provide a method for manufacturing steel that can solve the problems of the above-mentioned prior art and is expected to suppress hot brittleness.

[0017] The method for manufacturing the steel material of this disclosure will be described in detail below, based on its embodiments.

[0018] <Definitions> The compounds described herein may be derived in part or in whole from fossil resources, from biological resources such as plant resources, or from recycled resources such as used tires. They may also be derived from a mixture of two or more of fossil resources, biological resources, or recycled resources.

[0019] In this specification, "low-carbon steel" refers to steel with a carbon (C) content of less than 0.25 mass%. "Medium-carbon steel" refers to steel with a carbon (C) content of 0.25 mass% or more and 0.60 mass% or less. "High-carbon steel" refers to steel with a carbon (C) content of more than 0.60 mass%.

[0020] In this specification, the carbon (C) and sulfur (S) content of the steel material is measured by non-dispersive infrared absorption spectroscopy, and the copper (Cu) content is measured by ICP emission spectroscopy.

[0021] In this specification, "steel material" refers to steel billets produced by casting molten steel. "Steel material" is heated in a furnace or rolled, and specifically includes slabs, blooms, billets, and ingots. "Steel raw materials" refers to materials used in the manufacture of steel material, such as iron scrap and iron ore.

[0022] <Method for Manufacturing Steel Materials> The method for manufacturing steel materials according to this embodiment is a method for manufacturing steel materials containing at least Cu and S, and is characterized by comprising: a holding step of holding a steel material containing at least Cu and S in the range of the temperature at which Cu sulfide solid solution occurs to 1300°C for 180 seconds or more; and a steel material cooling step of holding the steel material in the austenite region temperature for 180 seconds or more and then cooling it. According to the method for manufacturing steel materials according to this embodiment, suppression of hot embrittlement can be expected. It is generally known that metals have better physical properties when the crystal size is small. It is also known that the strength of steel materials increases when copper sulfide is finely dispersed in the steel material. In the method for manufacturing steel materials according to this embodiment, by holding at a specific temperature for a certain period of time and then cooling, nucleation of copper sulfide that was solid-dissolved occurs in the steel material and disperses, so that the Cu and S contained in the steel material can form copper sulfide and be finely dispersed (micro-dispersed). Therefore, it is expected that the physical properties of the steel (such as strength) will be superior, and that the conversion of Cu into copper sulfide will suppress hot brittleness.

[0023] The following describes each step in the steel manufacturing method of this embodiment. In this specification, temperature refers to the surface temperature of the steel material, etc. The cooling rate is the average cooling rate of the surface temperature. The surface temperature can be measured, for example, with a thermocouple.

[0024] (Holding step) The steel manufacturing method of this embodiment includes a holding step of holding a steel material containing at least Cu and S in a temperature range from the temperature at which Cu sulfide solid dissolves to 1300°C for 180 seconds or more. It is believed that by holding a steel material containing at least Cu and S in a temperature range from the temperature at which Cu sulfide solid dissolves to 1300°C for 180 seconds or more, copper sulfide solid dissolves in the steel material, and that subsequent holding at a specific temperature makes it easier for the copper sulfide to be finely dispersed in the steel material.

[0025] The steel material is not particularly limited as long as it contains at least Cu (copper) and S (sulfur). More specifically, it may have the component composition of the steel material produced by the steel material manufacturing method of this embodiment. The component composition of the final steel material obtained is the same as the component composition of the steel material used. Examples of the steel material that can be used are slabs, blooms, billets, and ingots. The steel material is preferably derived from recycled iron. That is, it is preferable that the steel material is made from recycled iron. The recycled iron that can be used as a raw material for the steel material of this embodiment is not particularly limited as long as it is recycled, and may be iron scrap or steel cord extracted from tires, etc. Note that "derived from recycled iron" means that recycled iron is used as at least a part of the raw material, and in addition to recycled iron, for example, iron ore may be used as a raw material.

[0026] In the holding process, the steel material is held in a temperature range from the temperature at which Cu sulfide solid dissolves to 1300°C. It is believed that holding the steel material in this temperature range facilitates the fine dispersion of Cu sulfide. The temperature at which Cu sulfide solid dissolves varies depending on the components contained in the steel material, but it can be, for example, 1100°C or higher. That is, the temperature range in the holding process can be between 1100°C and 1300°C. The presence of solid dissolution of Cu sulfide can be confirmed by X-ray diffraction or other methods.

[0027] In the holding process, the steel material is held in the above temperature range for a holding time of 180 seconds or more. By holding the steel material in the above temperature range for 180 seconds or more, copper sulfide is solid-dissolved in the steel material, and it is thought that the subsequent holding at a specific temperature makes it easier for the Cu sulfide to be finely dispersed. The upper limit of the holding time is preferably 5 hours or less, and more preferably 4 hours or less, from the viewpoint of oxide scale formation, etc. On the other hand, the lower limit of the holding time is preferably 300 seconds or more. By holding for 300 seconds or more, solid dissolution of copper sulfide into the steel material can be expected.

[0028] In the holding process, the temperature only needs to remain within the above temperature range for the duration of the holding time. The temperature may be maintained at a constant temperature within the above temperature range, or it may be maintained by changing the temperature at regular intervals within the above temperature range. Furthermore, the temperature may be brought into the above temperature range by heating to reach the lower limit of the above temperature range, or by heating to an upper limit of the above temperature range and then cooling to bring it into the above temperature range.

[0029] The above holding process is not limited to, but may be carried out, for example, in a heating furnace.

[0030] (Steel Material Cooling Step) The steel material manufacturing method of this embodiment includes a steel material cooling step in which the steel material is held at an austenite temperature for 180 seconds or more and then cooled. This steel material cooling step may be provided after the holding step described above. That is, the steel material manufacturing method of this embodiment may include a steel material cooling step in which the steel material that has undergone the above holding step is held at an austenite temperature for 180 seconds or more and then cooled. It is thought that by including such a steel material cooling step, the dissolved Cu precipitates as copper sulfide, and the Cu sulfide becomes easier to finely disperse in the steel material.

[0031] The austenite region temperature varies depending on the steel material used, but is preferably 1100 to 650°C, and more preferably 1000 to 700°C. That is, in the steel material cooling process, the temperature range in which the steel material is held for 180 seconds or more is preferably 1100 to 650°C, and more preferably 1000 to 700°C. It is sufficient to hold the material within this temperature range for 180 seconds or more; it may be held at a constant temperature within the above temperature range, or the temperature may be changed at regular intervals within the above temperature range. The austenite region temperature can be confirmed using a thermocouple.

[0032] Furthermore, in the steel material cooling process, the holding time at the austenite temperature is 180 seconds or more, preferably 300 seconds or more, and more preferably 900 seconds or more, from the viewpoint of copper sulfide precipitation. On the other hand, from the viewpoint of copper sulfide coarsening, the holding time is preferably 3 hours or less, and more preferably 1.5 hours or less. The range of the holding time is preferably 180 seconds or more and 3 hours or less, more preferably 300 seconds or more and 3 hours or less, and even more preferably 900 seconds or more and 1.5 hours or less.

[0033] In the steel material cooling process, the steel material is cooled after being held within the temperature range described above. Cooling can be carried out by any method, such as air cooling or accelerated cooling.

[0034] In the cooling process for steel materials, the cooling rate is preferably 18°C / sec or higher, more preferably 20°C / sec or higher, and even more preferably 25°C / sec or higher. It is presumed that a cooling rate of 18°C / sec or higher allows Cu sulfides to be dispersed in the steel material as finely milled crystals without agglomerating. Furthermore, it is believed that the dispersion of Cu sulfides as finely milled crystals can be expected to suppress hot brittleness. The above cooling rate is preferably 80°C / sec or lower, and more preferably 60°C / sec or lower. The above cooling rate range is preferably 18°C / sec or higher and 80°C / sec or lower, more preferably 20°C / sec or higher and 80°C / sec or lower, and even more preferably 25°C / sec or higher and 60°C / sec or lower.

[0035] Furthermore, the steel material may be cooled between the holding step and the steel material cooling step. In other words, it is preferable to include a further cooling step between the holding step and the steel material cooling step. In this cooling, it is preferable that the steel material be rapidly cooled from a temperature range of the temperature at which Cu sulfide solid solution occurs to 1300°C to the austenite region temperature. In this cooling, the cooling rate is preferably 20°C / sec or higher, and more preferably 25°C / sec or higher. It is presumed that a cooling rate of 20°C / sec or higher allows Cu sulfide to be dispersed in the steel material as finely granulated crystals without agglomerating. As a result, it is expected that hot embrittlement can be suppressed. Furthermore, the cooling rate is preferably 80°C / sec or lower, and more preferably 60°C / sec or lower. The range of the above cooling rate is preferably 20°C / sec or higher and 80°C / sec or lower, and more preferably 25°C / sec or higher and 60°C / sec or lower.

[0036] The steel material cooling process is not limited to any particular method, but it can be carried out, for example, in a heating furnace.

[0037] (Melting process) The steel manufacturing method of this embodiment preferably includes a melting process, prior to the holding process described above, in which the steel raw material is heated to a temperature above the melting point of the steel raw material to obtain molten steel.

[0038] The raw materials for steel production are not limited as long as they contain at least Cu (copper) and S (sulfur), but recycled iron such as iron scrap or steel cord extracted from tires is preferred. Using recycled iron eliminates the need for a carbon source required for reduction, thus leading to a reduction in carbon dioxide emissions compared to using iron ore.

[0039] In the melting process, the heating temperature is not limited as long as it is above the melting temperature of the steel raw material, but for example, it can be 1600°C or higher.

[0040] Furthermore, there is no particular limit to the heating time required to molten steel; it should be heated until it becomes molten steel.

[0041] The melting process only needs to heat the above steel material raw materials to a temperature above the melting temperature to obtain molten steel, and known methods for obtaining molten steel can be used. The melting process is not limited, but can be carried out in an electric furnace. For example, in the heating of an electric furnace, the heating rate can be set to 500 °C / h by the program control of the electric furnace and heated.

[0042] (Molten steel cooling process) The method for manufacturing steel materials according to this embodiment preferably includes a molten steel cooling process of cooling the molten steel obtained in the above melting process at a cooling rate of 0.05 °C / sec or more at least in the austenite region temperature. That is, the method for manufacturing steel materials according to this embodiment heats the steel material raw materials to a temperature above the melting temperature of the steel material raw materials to obtain molten steel in a melting process before the above holding process, and at least in the austenite region temperature, a molten steel cooling process of cooling the molten steel obtained in the above melting process at a cooling rate of 0.05 °C / sec or more is preferably included. By cooling the molten steel at a cooling rate of 0.05 °C / sec or more, copper sulfide is more likely to be formed.

[0043] From the perspective of making the crystals in the steel material finer, the cooling rate in the molten steel cooling process is more preferably 3 °C / sec or more, and even more preferably 40 °C / sec or more.

[0044] In the molten steel cooling process, as long as the cooling rate is 0.05 °C / sec or more at least in the austenite region temperature, the cooling method is not particularly limited. Examples of the cooling method include furnace cooling, air cooling, and water cooling.

[0045] The cooling method will be described based on several examples. In this embodiment, the cooling method is not limited to the following, and any cooling method with a cooling rate not less than the above lower limit value is acceptable.

[0046] -Furnace cooling- As one of the above cooling methods, furnace cooling may be used. For example, for the cooling of molten steel using furnace cooling, the cooling rate can be set to 500 °C / h by the program control of the electric furnace, and the molten steel can be cooled in the furnace. At this time, the cooling rate of the molten steel is about 0.1 °C / sec.

[0047] -Air Cooling- Air cooling may be used as one of the cooling methods. For example, when cooling molten steel using air cooling, the molten steel can be cooled by exposing the container holding the molten steel to the atmosphere and allowing it to cool in the atmosphere. In this case, the cooling rate of the molten steel is approximately 5°C / sec.

[0048] -Water Cooling- Water cooling may be used as one of the cooling methods. For example, when cooling molten steel using water cooling, the container holding the molten steel can be removed from the furnace and the container can be immersed in water to cool the molten steel. In this case, the cooling rate of the molten steel is approximately 50°C / sec.

[0049] In the molten steel cooling process, it is preferable to adsorb oxygen using an oxygen adsorbent having a melting point above 1600°C. By adsorbing oxygen with an oxygen adsorbent, the generation of bubbles can be suppressed even when the cooling rate is increased. In particular, in the case of high-carbon steel, unlike low-carbon steel, the amount of carbon element is relatively large, and it is thought that bubbles are generated by the reaction between carbon and oxygen. Therefore, it is presumed that the generation of bubbles can be suppressed by adsorbing oxygen with an oxygen adsorbent. The oxygen adsorbent is not particularly limited as long as it has a melting point above 1600°C and can adsorb oxygen. Examples of such oxygen adsorbents include Ti (titanium), Zr (zirconium), and Ta (tantalum). In this specification, "oxygen adsorbent" refers to a substance that has the property of adsorbing oxygen.

[0050] (Other) The steel manufacturing method of this embodiment may include the addition of sulfur (S). Although sulfur (S) is an element that exists as an impurity in steel, such a step makes it easier to adjust the amount of sulfur (S) to a desired range. The method of adding sulfur (S) is not particularly limited, but for example, sulfur (S) can be added by dissolving FeS in the steel.

[0051] Furthermore, the steel manufacturing method of this embodiment may include, in addition to the steps described above, one or more steps from among the following: an optional step such as adding components other than S, a casting step to produce steel material by casting molten steel, a rolling step, and a surface treatment step. These optional steps can be carried out by known methods.

[0052] (Steel Material) The steel material obtained by the steel material manufacturing method of this embodiment contains at least Cu and S. It is believed that in the steel material obtained by the steel material manufacturing method of this embodiment, Cu and S are in the form of copper sulfide and are finely dispersed in the steel material.

[0053] In the above-mentioned steel material, the amount of carbon (C) is preferably 0.7 to 1.0 mass%. Such carbon steel is high-carbon steel because the amount of carbon (C) is within the above range. Furthermore, the amount of carbon (C) is more preferably 0.72 mass% or more. Furthermore, the amount of carbon (C) is more preferably 0.98 mass% or less, and even more preferably 0.88 mass% or less. The range of carbon (C) in the steel material is more preferably 0.72 to 0.98 mass%, and even more preferably 0.72 to 0.88 mass%.

[0054] In the steel material of this embodiment, the amount of Cu (copper) element is preferably 0.1% by mass or more. If the amount of Cu (copper) element is above the lower limit, it becomes easier to form Cu sulfides. Furthermore, in order to obtain Cu sulfides, the amount of Cu (copper) element in the steel material is preferably 3.0% by mass or less, and more preferably 2.0% by mass or less. The range of the amount of Cu element in the steel material is preferably 0.1% by mass or more and 3.0% by mass or less, and more preferably 0.1% by mass or more and 2.0% by mass or less.

[0055] In the steel material of this embodiment, the amount of sulfur (S) is preferably 0.01% by mass or more. If the amount of sulfur (S) is above the lower limit, it becomes easier for Cu sulfides to form. Furthermore, since the amount of sulfur (S) in the steel material affects the properties of the steel material, it is preferably 0.050% by mass or less, and more preferably 0.030% by mass or less. The range of the amount of sulfur (S) in the steel material is preferably 0.010% by mass or more and 0.050% by mass or less, and more preferably 0.010% by mass or less and 0.030% by mass or less.

[0056] Preferably, the steel obtained by the steel manufacturing method of this embodiment contains 0.7 to 1.0% by mass of carbon, 0.1% by mass or more of copper, and 0.01% by mass or more of sulfur. The remainder may consist of Fe and unavoidable impurities.

[0057] The above-mentioned steel material preferably has a nitrogen (N) content of 50 ppm by mass or more. Generally, new iron such as iron obtained from a blast furnace has high purity and a nitrogen (N) content of less than 50 ppm by mass, whereas typical recycled iron (i.e., recycled iron that has not been highly refined) has a nitrogen (N) content of 50 ppm by mass or more. Furthermore, from the viewpoint of durability, the nitrogen (N) content of the above-mentioned steel material is preferably 200 ppm by mass or less. In other words, the range of the nitrogen (N) content is preferably 50 ppm by mass or more and 200 ppm by mass or less.

[0058] The above-mentioned steel material preferably has a chromium (Cr) content of 0.3 mass% or less, and more preferably 0.05 to 0.3 mass%. Generally, new iron such as iron obtained from a blast furnace has high purity and a chromium (Cr) content of less than 0.05 mass%, whereas typical recycled iron (i.e., recycled iron that has not been highly refined) has a chromium (Cr) content of 0.05 mass% or more.

[0059] In the above-mentioned steel material, it is preferable that at least a portion of the Cu (copper) exists in the form of copper sulfide. Steel materials containing a large amount of copper (Cu) may cause problems such as cracking during processing, for example, in rolling, because the copper (Cu) acts as a fracture nucleus. However, it is expected that such problems can be suppressed if at least a portion of the Cu (copper) exists in the form of copper sulfide. Whether at least a portion of the Cu (copper) in the steel material exists in the form of copper sulfide can be confirmed by observing the steel material prepared using SEM or TEM, by simple analysis using energy-dispersive spectroscopy (EDS), and by observing the segregation of copper and sulfur using electron beam microanalyzer (EPMA).

[0060] The above copper sulfide is not limited to, but for example, Cu ２ S, CuS, Cu ２－ｘ Examples include S.

[0061] In the above-mentioned steel material, it is preferable that copper sulfides are finely dispersed within the steel. Furthermore, it is presumed that it is preferable for the copper sulfides to exist in smaller particle sizes. To reduce the particle size of copper sulfides, for example, it is possible to increase the cooling rate during the cooling of molten steel.

[0062] The copper sulfide is preferably such that its diameter is in the range of 1.0 nm (0.001 μm) to 10 μm. Here, in this specification, "copper sulfide diameter" refers to the diameter obtained by measuring the major and minor axes of the copper sulfide's shape, with a portion of it being calculated as the equivalent diameter of a circle. Furthermore, as for the particle size distribution of the copper sulfide, it is preferable that copper sulfides with a diameter of less than 1.0 μm account for more than 50 percent of the total number of copper sulfides, and it is more preferable that copper sulfides with a diameter of less than 0.5 μm account for more than 50 percent of the total number of copper sulfides. The copper sulfide diameter can be measured by SEM observation and TEM observation, etc. Specifically, it can be determined by the method described in the examples.

[0063] The steel obtained by the steel manufacturing method of this embodiment can be used in applications where steel is normally used. For example, the steel of this embodiment can be subjected to wire drawing or the like to produce steel cord. Furthermore, the steel may contain components other than those listed above, such as impurities, to the extent that it does not impair the effects of this disclosure. Examples of such components include Si (silicon), Mn (manganese), P (phosphorus), Al (aluminum), Ti (titanium), Sn (tin), Ni (nickel), Mo (molybdenum), O (oxygen), and the like.

[0064] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited in any way to the examples described below.

[0065] <Measurement Method> - Observation of Precipitates - Precipitates of the prepared sample were observed using SEM and TEM (manufactured by JEOL Ltd.). The size of the precipitates (copper sulfide diameter) was also measured. The copper sulfide diameter was converted to the equivalent circle diameter using image analysis software (WinROOF2018) after binarizing the SEM image. For TEM observation, measurement software (Digital Micrograph, manufactured by Gatan) was used.

[0066] -Confirmation of the presence and dispersion state of copper sulfide- The presence or absence of copper sulfide was confirmed by a simple analysis using energy-dispersive spectroscopy (EDS). Table 1 below shows the presence or absence of copper sulfide. An electron beam microanalyzer (EPMA, manufactured by JEOL Ltd.) was used to observe the segregation of copper and sulfur and to measure the particle size distribution of copper sulfide. Table 1 shows the measured particle size distribution of copper sulfide. The state of presence of copper sulfide was evaluated according to the following criteria. A: A state in which a large number of copper sulfides are present, and copper sulfides with a particle size of less than 0.1 μm account for 50-something percent or more of the total number of copper sulfides. B: A state in which a large number of copper sulfides are present, and copper sulfides with a particle size of 0.1 μm or more and less than 0.5 μm account for 50-something percent or more of the total number of copper sulfides. C: A state in which copper sulfides are present, but in small numbers, and copper sulfides with a particle size of 0.5 μm or more and less than 1.0 μm account for 50-something percent or more of the total number of copper sulfides.

[0067] <Example 1> Fe-C alloy, Fe-S powder, electrolytic iron, and electrolytic copper, which were prepared in advance by arc melting to the predetermined composition, were weighed to obtain Cu: 0.3 mass%, S: 0.03 mass%, and C: 0.8 mass%, which were used as raw materials for molten steel. After melting the raw materials to obtain molten steel, the molten steel was cooled (air cooled) to obtain steel material. The obtained steel material was held at a holding temperature of 1250°C for a holding time of 900 seconds (holding step), and then cooled at a cooling rate of approximately 20°C / sec. Subsequently, the steel material was held at a holding time of 1100°C (austenite region temperature) for a holding time of 900 seconds, and then cooled at a cooling rate of 23.7°C / sec to obtain steel material (steel material cooling step). The temperature was measured using a thermocouple (manufactured by Furuuchi Chemical Co., Ltd.). The obtained steel material was observed and evaluated using the method described above. The results are shown in Table 1.

[0068] <Example 2> Steel material was obtained in the same manner as in Example 1, except that the holding temperature in the steel material cooling process was set to 900°C and the cooling rate in the steel material cooling process was set to 21.4°C / sec. The results are shown in Table 1.

[0069] <Example 3> Steel material was obtained in the same manner as in Example 1, except that the holding temperature in the steel material cooling process was set to 700°C and the cooling rate in the steel material cooling process was set to 18.0°C / sec. The results are shown in Table 1.

[0070] Observation using the above-mentioned electron beam microanalyzer (EPMA, manufactured by JEOL Ltd.) confirmed the dispersion state, and the dispersion of copper and sulfur was confirmed, confirming the presence of dispersed copper sulfide in all examples. The results are shown in Table 1.

[0071]

[0072] Table 1 shows that the steel material obtained by the steel material manufacturing method of this embodiment contains copper sulfide particles in small particle sizes. Therefore, it can be seen that the steel material of the example contains copper sulfide particles in a fine dispersion.

Claims

1. A method for manufacturing steel containing at least Cu and S, comprising: a holding step of holding a steel material containing at least Cu and S in a temperature range from the temperature at which Cu sulfide solid dissolves to 1300°C for 180 seconds or more; and a steel material cooling step of holding the steel material in the austenite region temperature for 180 seconds or more and then cooling it.

2. The method for manufacturing steel according to claim 1, wherein in the steel material cooling step, the temperature range in which the steel material is held for 180 seconds or more is 1000 to 700°C.

3. The method for manufacturing steel according to claim 1, wherein the steel contains 0.7 to 1.0% by mass of C, 0.1% by mass or more of Cu, and 0.01% by mass or more of S.

4. The method for manufacturing steel according to claim 1, wherein the steel contains 50 ppm by mass or more of N.

5. The method for manufacturing steel according to claim 1, wherein the steel contains 0.05 to 0.3% by mass of Cr.

6. The method for producing steel according to claim 1, wherein at least a portion of the Cu in the steel material is present in the form of copper sulfide.

7. The method for manufacturing steel according to claim 1, wherein the cooling rate in the steel material cooling step is 20°C / sec or higher.

8. A method for producing steel according to claim 1, comprising, prior to the holding step, a melting step of heating the steel raw material to a temperature above which the steel raw material melts to obtain molten steel, and a molten steel cooling step of cooling the molten steel at a cooling rate of 0.05°C / sec or more in at least the austenite region temperature.

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