Steel material and method for manufacturing the same

Abstract

To provide: a steel material by which reduction of thermal shortness can be expected despite being high-carbon steel; and a method for manufacturing the steel material.SOLUTION: A steel material includes 0.6-1.2 mass% of C element content, 0.1 mass% or more of Cu element content, and 0.01 mass% or more of S element content. A method for manufacturing the steel material includes cooling molten steel in which steel including 0.6-1.2 mass% of C element is melted, at a cooling rate of 0.05°C / sec or more in at least an austenitic zone.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a steel material and a method for producing the same. [Background technology]

[0002] It has been known that carbon steels with a relatively high Cu (copper) content suffer from hot embrittlement when processed at high temperatures. For example, in steels with a relatively high Cu (copper) content, the Cu (copper) acts as a fracture nucleus during processing, which can cause problems such as cracks during rolling.

[0003] In order to solve the above problems, in low carbon steel, Cu (copper) and S (sulfur) are mixed into Cu x It has been devised to precipitate copper as S (copper sulfide) and suppress hot brittleness. For example, Patent Document 1 discloses a particle dispersion strengthened steel containing, by weight, 0.04 to 3% Cu and 0.02 to 0.16% S, and containing, by volume, 0.1 to 0.8% copper sulfide particles having a particle diameter of 1 to 30 nm, the copper sulfide particles being dispersed in the steel. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2000-345300 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, the steel material described in the above Patent Document 1 is a low-carbon steel material, and there has been no high-carbon steel material that can be expected to suppress hot brittleness, etc. Therefore, there is a demand for a high-carbon steel material that can be expected to suppress hot brittleness, etc.

[0006] Therefore, an object of the present invention is to solve the above-mentioned problems of the conventional technology and to provide a steel material which is expected to suppress hot brittleness even though it is a high carbon steel. Another object of the present invention is to provide a method for producing such a steel material. [Means for solving the problem]

[0007] The essential configuration of the steel material and the manufacturing method thereof according to the present invention that solves the above problems is as follows.

[0008] [1] Steel having a C element content of 0.6% by mass or more and 1.2% by mass or less, a Cu element content of 0.1% by mass or more, and a S element content of 0.01% by mass or more. The steel material described in [1] above is a steel material that is expected to suppress hot brittleness.

[0009] [2] The steel material according to [1], wherein the steel material is made from recycled iron. The steel material described in [2] above is made from recycled steel, and therefore contributes to the circular use of resources.

[0010] [3] The steel material according to [1] or [2], wherein the amount of N element is 60 ppm by mass or more. The steel material described in [3] above can be produced from ordinary recycled iron.

[0011] [4] The steel material according to any one of [1] to [3], wherein the Cr element content is 0.3 mass% or less. The steel material described in [4] above can be produced from ordinary recycled steel.

[0012] [5] The steel material according to any one of [1] to [4], wherein at least a portion of Cu (copper) is present in the form of copper sulfide. The steel material described in [5] above is a high carbon steel, but copper is present in the form of copper sulfide.

[0013] [6] A method for producing a steel material according to any one of [1] to [5], A method for manufacturing steel, comprising the steps of: cooling molten steel containing 0.6% by mass or more and 1.2% by mass or less of the C element at a cooling rate of 0.05°C / sec or more at least in the austenite region. According to the method for producing a steel material described in [6] above, it is possible to produce a steel material that is expected to suppress hot brittleness even in the case of high carbon steel.

[0014] [7] A method for producing a steel product as described in [6], wherein oxygen is adsorbed by an oxygen adsorbent having a melting point of more than 1600°C during the cooling process. According to the method for producing a steel material described in [7] above, the generation of bubbles during cooling can be suppressed. Effect of the Invention

[0015] According to the present invention, it is possible to provide a steel material which is expected to suppress hot brittleness even though it is a high carbon steel. The present invention also provides a method for producing such a steel material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, the steel material and the manufacturing method thereof according to the present invention will be described in detail with reference to the embodiments.

[0017] <Definition> The compounds described herein may be derived in whole or in part from fossil sources, from biological sources such as plant sources, from recycled sources such as used tires, or from a mixture of two or more of fossil sources, biological sources, and / or renewable sources.

[0018] In this specification, the term "oxygen adsorbent" refers to a substance that has the property of adsorbing oxygen.

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

[0020] In this specification, the C (carbon) element content and S (sulfur) element content of the steel material are values ​​measured by non-dispersive infrared absorption method, and the Cu (copper) element content is a value measured by ICP atomic emission spectrometry.

[0021] <Steel> The steel material of this embodiment is characterized in that the C (carbon) element content is 0.6 mass% or more and 1.2 mass% or less, the Cu (copper) element content is 0.1 mass% or more, and the S (sulfur) element content is 0.01 mass% or more. The steel material of this embodiment has a C (carbon) content of 0.6% by mass to 1.2% by mass, a Cu (copper) content of 0.1% by mass or more, and a S (sulfur) content of 0.01% by mass or more, and can use recycled iron or the like containing a relatively large amount of carbon as a raw material, thereby contributing to the cyclical use of resources.

[0022] In the steel material of this embodiment, the amount of C (carbon) element is 0.6 mass% or more and 1.2 mass% or less. Since the amount of C (carbon) element is in the above range, the steel material of this embodiment is a high carbon steel. In addition, the amount of C (carbon) element is preferably 0.7 mass% or more and 1.0 mass% or less.

[0023] In the steel material of this embodiment, the Cu (copper) element content is 0.1 mass% or more. When the Cu (copper) element content is in the above range, copper sulfides can be formed. In addition, in order to obtain the target copper sulfides, the Cu (copper) element content of the steel material is preferably 3.0 mass% or less, more preferably 2.0 mass% or less.

[0024] In the steel material of this embodiment, the amount of S (sulfur) element is 0.01 mass% or more. When the amount of S (sulfur) element is in the above range, copper sulfide can be formed. In addition, the amount of S (sulfur) element in the steel material affects the properties of the steel material, so that the amount is preferably 0.050 mass% or less, and more preferably 0.030 mass% or less.

[0025] The steel material of the present embodiment is preferably a steel material derived from recycled iron. If the steel material is derived from recycled iron, it can contribute to the cyclical use of resources. The recycled iron that can be the raw material of the steel material of the present embodiment is not particularly limited as long as it is recycled, and may be derived from scrap iron, steel cords taken out from tires, etc. In addition, from the viewpoint of reducing CO2 emissions, the recycled iron is preferably derived from an electric furnace (electric furnace steelmaking method). In the electric furnace steelmaking method, for example, an arc discharge is generated in an electric furnace, the raw materials are melted by the discharge heat, and impurities are removed to obtain iron. On the other hand, in the blast furnace method used in normal steelmaking, for example, iron ore is reduced in a blast furnace using coke derived from coal to obtain iron, so a large amount of CO2 is inevitably generated. In general, the amount of CO2 emitted in an electric furnace is about 1 / 4 of the amount of CO2 emitted in a blast furnace, so that the amount of CO2 emitted can be significantly reduced by using recycled iron derived from an electric furnace as the raw material of the steel material.

[0026] The steel material of this embodiment preferably has an N (nitrogen) element content of 60 mass ppm or more. Generally, new iron such as iron obtained from a blast furnace has a high purity and an N element content of less than 60 mass ppm, whereas general recycled iron (i.e., recycled iron that has not been highly refined) has an N element content of 60 mass ppm or more. The steel material can be produced using such general recycled iron as a raw material. In addition, since no advanced refining is required, the production process is not complicated, energy consumption can be reduced, and CO2 emissions can also be reduced, which is preferable from an environmental perspective. From the viewpoint of durability, the N (nitrogen) element content of the steel material is preferably 200 mass ppm or less.

[0027] In the steel material of this embodiment, the Cr (chromium) element content is preferably 0.3 mass% or less, and more preferably 0.05 mass% or more and 0.3 mass% or less. Generally, new iron such as iron obtained from a blast furnace has a high purity and a Cr (chromium) element content of less than 0.05 mass%, whereas general recycled iron (i.e., recycled iron that is not highly refined) has a Cr (chromium) element content of 0.05 mass% or more. Recycled iron with a Cr (chromium) element content within the above range is easy to obtain because it does not require high-level refining. In addition, steel material with a Cr element content within the above range can be manufactured from general recycled iron.

[0028] In the steel material of this embodiment, it is preferable that at least a part of Cu (copper) is present in the form of copper sulfide. In steel containing a large amount of copper (Cu), the copper (Cu) becomes a fracture nucleus during processing, and problems such as cracks may occur, for example, during rolling processing. However, in the steel material of this embodiment, at least a portion of the Cu (copper) is present in the form of copper sulfide, so it is expected that such problems can be suppressed. Whether or not at least a portion of Cu (copper) is present in the steel material in the form of copper sulfide can be confirmed by observing the prepared steel material with a SEM and TEM (manufactured by JEOL Ltd.), by simple analysis with energy dispersive spectroscopy (EDS), and by observing the segregation of copper and sulfur with an electron probe microanalyzer (EPMA, manufactured by JEOL Ltd.).

[0029] The state of copper sulfide in the steel material of this embodiment is not particularly limited as long as copper sulfide is present. It is also presumed that copper sulfides preferably exist with smaller particle sizes, and it is believed that the particle size of the copper sulfides present can be made smaller by increasing the cooling rate in cooling the molten steel.

[0030] The size of the copper sulfide is preferably in the range of 1.0 nm to 10 μm in diameter.

[0031] The steel material of the present embodiment can be used in applications in which steel materials are usually used, such as for example, making the steel material of the present embodiment into a steel cord by subjecting it to wire drawing or the like. Furthermore, the steel material of this embodiment may contain components other than the above-mentioned components, such as impurities, within a range that does not impair the effects of the present invention. Examples of such components include Si (silicon), Mn (manganese), P (phosphorus), Al (aluminum), Ti (titanium), Sn (tin), Ni (nickel), Mo (molybdenum), and O (oxygen).

[0032] <Steel manufacturing method> In the method for producing a steel material of this embodiment, it is preferable to cool molten steel containing 0.6 mass % or more and 1.2 mass % or less of C (carbon) element at least in the austenite region at a cooling rate of 0.05° C. / sec or more. In the method for manufacturing the steel material of this embodiment, the steel material of this embodiment can be manufactured by cooling molten steel containing 0.6 mass% or more and 1.2 mass% or less of C (carbon) element at the above cooling rate at least in the austenite region. In the method for producing a steel material according to the present embodiment, the temperature can be measured using a thermocouple. The austenite region varies depending on the carbon content of the steel material, but anyone with knowledge in this field can estimate the austenite region of the steel material from the carbon content, etc.

[0033] The amount of Cu (copper) element in the molten steel is preferably 0.1 mass% or more. In order to set the amount of Cu (copper) element in the molten steel within the above range, a substance containing Cu (copper) element may be used as one of the raw materials. An example of the substance containing Cu (copper) element is electrolytic copper. The Cu (copper) element in the molten steel may be derived from the raw materials used. For example, recycled iron can be used as a raw material for steel material containing Cu (copper) element in the raw materials used. The amount of S (sulfur) element in the molten steel is preferably 0.01 mass% or more. In order to set the amount of S (sulfur) element in the molten steel within the above range, a substance containing S (sulfur) element may be used as one of the raw materials. S (sulfur) element may also be contained in the raw material iron and steel, and in such a case, a substance containing S (sulfur) element such as FeS may be added to set the amount of S (sulfur) element within the above range.

[0034] In the method for producing a steel material, the cooling rate is more preferably 3° C. / sec or more, and even more preferably 40° C. / sec or more, at least in the austenite region, from the viewpoint of making the crystals in the steel material finer.

[0035] In the method for producing a steel material according to the present embodiment, the cooling method is not particularly limited as long as the cooling rate is 0.05° C. / sec or more at least in the austenite region. Examples of the cooling method include furnace cooling, air cooling, and water cooling.

[0036] The molten steel can be produced by heating and melting steel containing 0.6% by mass or more and 1.2% by mass or less of C (carbon) element in an electric furnace. For example, the heating in the electric furnace can be set to a heating rate of 500°C / h by program control of the electric furnace. The heating temperature to be reached may be any temperature at which the steel melts, for example, about 1600°C.

[0037] The time for which the steel is heated to form molten steel is not particularly limited, and it is sufficient that the steel is heated until it becomes molten steel.

[0038] The cooling method will be described based on several examples. Note that in this embodiment, the cooling method is not limited to the following, and any cooling method may be used as long as the cooling rate falls within the range of this embodiment.

[0039] (furnace cooling) As one of the cooling methods, furnace cooling may be used. For example, the cooling rate of the molten steel may be set to 500° C. / h by program control of an electric furnace, and the molten steel may be cooled in the furnace. In this case, the cooling rate in the austenite region of the molten steel is about 0.1° C. / sec.

[0040] (Air-cooled) As one of the cooling methods, air cooling may be used. For example, the molten steel can be cooled by exposing a vessel containing the molten steel to the atmosphere after melting and allowing the molten steel to cool in the atmosphere. In this case, the cooling rate of the molten steel in the austenite region is about 5° C. / sec.

[0041] (water cooling) As one of the cooling methods, water cooling may be used. For example, the molten steel may be cooled by removing the container containing the molten steel from the furnace after melting and immersing the container in water to cool the molten steel. In this case, the cooling rate in the austenite region of the molten steel is about 50° C. / sec.

[0042] In the steel manufacturing method of this embodiment, it is preferable to adsorb oxygen with an oxygen adsorbent having a melting point of more than 1600°C when cooling the molten steel. In this state, the generation of bubbles can be suppressed even when the cooling rate is increased in high carbon steel. Unlike low carbon steel, in steels such as high carbon steels that have a relatively large amount of carbon elements, bubbles are thought to be 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 of more than 1600° C. and can adsorb oxygen. Examples of the oxygen adsorbent include Ti (titanium), Zr (zirconium), and Ta (tantalum).

[0043] The method for producing a steel material according to the present embodiment may include the step of adding S (sulfur). S (sulfur) is an element that exists as an impurity in steel, but the inclusion of this step makes it easy to adjust the amount of S (sulfur) to a desired range. The method for adding S (sulfur) is not particularly limited, but for example, S (sulfur) can be added by dissolving FeS in steel.

[0044] Furthermore, the method for producing a steel material according to the present embodiment may include any step of adding other components in addition to the above-mentioned melting, cooling, and addition. EXAMPLES

[0045] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0046] The Fe-C alloy, which had been previously arc melted, as well as Fe-S powder, electrolytic iron, and electrolytic copper were weighed out to obtain the desired composition and used as the raw materials for the molten steel. The raw materials were melted to obtain molten steel, and the molten steel was then cooled (furnace cooling, air cooling, or water cooling) to obtain a sample. The melting and cooling methods (furnace cooling, air cooling, or water cooling) were as follows. Table 1 below shows the compositions and cooling methods of the carbon steels in Examples 1 to 3. The vessel in which the melting and cooling were carried out was equipped with a pure titanium (Ti) tube as an oxygen adsorbent.

[0047] (Melted) The temperature rise rate was set to 500°C / h by program control of the electric furnace, and the container containing the raw material was heated to 1600°C. In Examples 1 and 2, the raw material was heated to 1200°C, and then heated at 1200°C for 1 hour, and then heated to 1600°C at a temperature rise rate of 500°C / h and held at 1600°C for 1 hour. In Example 3, the raw material was heated to 1600°C, and then held at 1600°C for 30 minutes, and then the temperature was lowered to 1200°C and held for 30 minutes, and then the temperature was raised again to 1600°C and held for 30 minutes. The temperature was measured using a thermocouple (manufactured by Furuuchi Chemical Co., Ltd.).

[0048] (Cooling of molten steel) The temperature was measured using a thermocouple (Furuuchi Chemical Co., Ltd.) in the same manner as during melting. -Furnace cooling- After melting, the molten steel was cooled in the electric furnace at a cooling rate of 500° C. / h by the electric program control of the electric furnace. The cooling rate of the molten steel at this time is shown in Table 1. -Air-cooled- After melting, the vessel containing the molten steel was removed from the furnace and the molten steel was allowed to cool in the air (temperature: approximately 20-25°C). The cooling rate of the molten steel at this time is shown in Table 1. -Water cooling- After melting, the vessel containing the molten steel was taken out of the furnace and immersed in water to cool it. The cooling rate of the molten steel at this time is shown in Table 1.

[0049] (Method of observing copper sulfides) The precipitates in the prepared samples were observed using SEM and TEM (manufactured by JEOL Ltd.). In addition, the presence of copper and sulfur was confirmed by simple analysis using energy dispersive spectroscopy (EDS). The presence or absence of copper sulfide is shown in Table 1 below. The segregation of copper and sulfur was observed using an electron probe microanalyzer (EPMA, manufactured by JEOL Ltd.).

[0050] [Table 1]

[0051] High carbon steel was produced at any of the cooling rates in Examples 1 to 3. Furthermore, copper sulfides were confirmed in all of the Examples, and it was confirmed that the higher the cooling rate, the more finely the copper sulfides were distributed. [Industrial Applicability]

[0052] The steel material of the present invention can be used for steel filaments and the like.

Claims

1. A steel material having a C element content of 0.6 mass% or more and 1.2 mass% or less, a Cu element content of 0.1 mass% or more, and a S element content of 0.01 mass% or more.

2. The steel material according to claim 1 , wherein the steel material is a steel material derived from recycled iron.

3. The steel material according to claim 1, wherein the N element content is 60 ppm by mass or more.

4. The steel material according to claim 1, wherein the amount of Cr element is 0.3 mass % or less.

5. The steel material according to claim 1, wherein at least a portion of Cu (copper) is present in the form of copper sulfide.

6. A method for producing the steel material according to claim 1, comprising the steps of: A method for producing a steel product, comprising: cooling molten steel containing 0.6 mass% or more and 1.2 mass% or less of a C element at a cooling rate of 0.05°C / sec or more at least in an austenite region.

7. The method for producing a steel material according to claim 6, wherein oxygen is adsorbed by an oxygen adsorbent having a melting point of more than 1600°C during the cooling.

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