Method for producing bar steel

Abstract

To provide a method for producing a bar steel with a small energy consumption.SOLUTION: A method for producing a bar steel includes: A: preparing billets comprising austenitic stainless steels having a C content of 0.08 mass% or less; B: heating the billets to a temperature T1 of 1010°C or more to 1300°C or less; C: hot-rolling the billets to obtain intermediate products having a temperature T2 of 850°C or more; D: water-cooling the intermediate products; and E: air-cooling the intermediate products to a temperature T3 of 575°C or less. A required time Nt from the start of the water cooling until the temperature of the intermediate product reaches 575°C is within 45 minutes. The water cooling can be performed by means of a water cooling facility 14.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This specification discloses a method for manufacturing a steel bar whose material is austenitic stainless steel. [Background technology]

[0002] In austenitic stainless steel, solution heat treatment is known as a method for dissolving precipitates into a solid solution. An example of this solution heat treatment is disclosed in Japanese Patent Laid-Open Publication No. 10-110215. In this heat treatment, a steel wire obtained by hot working is cooled in-line to obtain a supersaturated solid solution. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-110215 Summary of the Invention [Problem to be solved by the invention]

[0004] Steel bars are typically obtained by hot rolling. In hot rolling, a billet is heated in a heating furnace. This billet is subjected to continuous rolling using a roughing mill, an intermediate mill, and a finishing mill to obtain an intermediate product. In the case of steel bars made of austenitic stainless steel, this intermediate product is subjected to solution heat treatment. In the solution heat treatment, the intermediate product is heated and cooled. This heating requires energy. From the perspective of carbon neutrality, the use of energy derived from fossil fuels is undesirable.

[0005] The applicant's intention is to provide a method for producing steel bars made of austenitic stainless steel with low energy consumption. [Means for solving the problem]

[0006] This specification discloses a method for manufacturing a steel bar, the method comprising: A: A step of preparing a billet made of austenitic stainless steel having a C content of 0.08% by mass or less; B: a step of heating the billet to a temperature T1 of 1010°C or higher and 1300°C or lower; C: A step of hot rolling the billet to obtain an intermediate product having a temperature T2 of 850°C or higher; D: A process of water cooling the intermediate product. and E: A process of cooling the intermediate product in air to reduce the temperature T3 of the intermediate product to 575°C or less. The time required for the intermediate product temperature to reach 575°C from the start of water cooling (Nt) is within 45 minutes.

[0007] Preferably, in step D, the intermediate product is submerged in water.

[0008] The present specification further discloses a cooling device for steel bars, the device comprising: a plurality of troughs aligned along the length; A roller located between two troughs, and A supply that can inject a cooling medium into the trough It has. [Effects of the Invention]

[0009] In this manufacturing method, the hot intermediate product is cooled directly after hot rolling, which reduces energy consumption. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a perspective view showing a part of a steel bar obtained by a manufacturing method according to one embodiment. [Figure 2] FIG. 2 is a conceptual diagram showing a manufacturing apparatus for the steel bar of FIG. [Figure 3] FIG. 3 is a flowchart showing a method for manufacturing the steel bar of FIG. [Figure 4] FIG. 4 is a perspective view showing a part of the water cooling device of the apparatus of FIG. 2 together with an intermediate product. [Figure 5] FIG. 5 is an enlarged cross-sectional view taken along line VV in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] FIG. 1 shows a steel bar 2. This steel bar 2 is long. In FIG. 1, an arrow A1 indicates the length direction of the steel bar 2. As will be described in detail later, this steel bar 2 can be obtained by rolling. The arrow A1 also indicates the rolling direction. The material of this steel bar 2 is austenitic stainless steel.

[0012] 2 shows a manufacturing apparatus for steel bars 2. The apparatus includes a heating furnace 4, a roughing mill 6, an intermediate mill 8, a finishing mill 10, a water cooler 14, a cooling bed 16, and a cold shear 17.

[0013] 3 shows an example of a method for manufacturing a steel bar 2. In this manufacturing method, a billet is first prepared (STEP 1). This billet can be obtained through processes such as refining, ingot making, and blooming.

[0014] The billet is charged into a heating furnace 4. The billet is heated in the heating furnace 4 (STEP 2). As a result of heating, the temperature of the billet reaches a predetermined temperature. As will be described in detail later, the temperature T1 reached by the billet is preferably 1010°C or higher and 1300°C or lower.

[0015] This billet is subjected to continuous rolling (STEP 3). This continuous rolling is performed in a hot state. This continuous rolling can be performed by a roughing row rolling mill 6, an intermediate row rolling mill 8, and a finishing row rolling mill 10. This continuous rolling reduces the diameter of the billet and lengthens it. This continuous rolling produces an intermediate product. During or after continuous rolling, the temperature of the steel may be adjusted by means of a shower or the like.

[0016] This intermediate product is subjected to water cooling (STEP 4). Water cooling is performed on the line by a water cooling device 14. Details of the water cooling device 14 will be described later. The water cooling device 14 rapidly reduces the temperature of the intermediate product.

[0017] The intermediate product is then transferred to a cooling bed 16 where it is subjected to air cooling (STEP 5). This air cooling further reduces the temperature of the intermediate product. When air cooling is completed, the temperature T3 of the intermediate product is 575°C or lower. This intermediate product is cut by a cold shear 17 to obtain steel bars 2 (STEP 6).

[0018] In this manufacturing method, water cooling (STEP 4) is employed, so the temperature of the intermediate product drops in a short time. The time required for the temperature of the intermediate product to reach 575°C from the start of water cooling (Nt) is within 45 minutes. This cooling is a so-called solution heat treatment. This cooling results in a steel bar 2 whose metal structure is a supersaturated solid solution.

[0019] From the viewpoint of a preferable metal structure, the time Nt required from the start of water cooling until the temperature of the intermediate product reaches 575°C is more preferably 40 minutes or less, and particularly preferably 35 minutes or less.

[0020] This solution heat treatment utilizes the heat given to the billet in the heating step (STEP 2) employed for hot rolling. In other words, heating for solution heat treatment is not required. This manufacturing method can produce high-quality steel bars 2 with low energy consumption. This manufacturing method can contribute to carbon neutrality.

[0021] As described above, the heating (STEP 2) causes the billet temperature T1 to reach 1010°C or higher and 1300°C or lower. In a billet with a temperature T1 of 1010°C or higher, C is sufficiently dissolved in the structure. A high-quality steel bar 2 can be obtained from this billet. From this viewpoint, the temperature T1 is more preferably 1050°C or higher, and particularly preferably 1080°C or higher. In a manufacturing method in which the temperature T1 is 1300°C or lower, overheating of the billet is suppressed. From this viewpoint, the temperature T1 is more preferably 1270°C or lower, and particularly preferably 1250°C or lower. In this specification, the temperature of the billet is measured at the surface of the billet.

[0022] The temperature T2 of the intermediate product immediately before water cooling (STEP 4) is preferably 850°C or higher. An intermediate product with a temperature T2 of 850°C or higher has few precipitated carbides. By cooling this intermediate product (STEP 4 and STEP 5), a steel bar 2 in which C is sufficiently dissolved in the structure can be obtained. From this perspective, the temperature T2 of the intermediate product immediately before water cooling (STEP 4) is more preferably 860°C or higher, and particularly preferably 870°C or higher. In this specification, the temperature of the intermediate product is measured at the surface of this intermediate product.

[0023] Austenitic stainless steel contains C. If the C content is excessive, a large amount of chromium carbide will precipitate during cooling (STEP 4 and STEP 5). In this embodiment, the C content is 0.08 mass% or less. In this steel bar 2, the amount of chromium carbide is small. Therefore, the structure within the crystal grains is a solid solution containing sufficient Cr. This steel bar 2 has excellent corrosion resistance. From the viewpoint of corrosion resistance, the C content is more preferably 0.06 mass% or less. From the viewpoint of hardness and strength of the steel bar 2, this content is preferably 0.005 mass% or more.

[0024] Preferred austenitic stainless steels are: C: 0.005% by mass or more and 0.08% by mass or less Si: 0.20 mass% or more and 1.00 mass% or less Mn: 0.10 mass% or more and 2.00 mass% or less P: 0.045% by mass or less S: 0.030% by mass or less Ni: 8.00 mass% or more and 15.00 mass% or less Cr: 16.00 mass% or more and 20.00 mass% or less Mo: 0.10 mass% or more and 3.00 mass% or less O: 0.020% by mass or less and N: 0.20% by mass or less Preferably, the balance is Fe and unavoidable impurities. P, S, O, and N are not essential elements. The contents of each of P, S, O, and N may be below the detection limit.

[0025] Si is added for the purpose of deoxidation during the steelmaking stage. From the viewpoint of the deoxidation effect, the Si content is preferably 0.20 mass% or more. Excess Si leads to the formation of δ phase. This δ phase impairs the hot workability of the steel bar 2. From the viewpoint of hot workability, the Si content is preferably 1.00 mass% or less, and particularly preferably 0.90 mass% or less.

[0026] Mn promotes the formation of the γ phase. From this viewpoint, the Mn content is preferably 0.10% by mass or more, more preferably 0.20% by mass or more, and particularly preferably 0.40% by mass or more. Excess Mn combines with S to form inclusions. These inclusions impair the ductility, toughness, and corrosion resistance of the steel bar 2. From the viewpoint of ductility, toughness, and corrosion resistance, the Mn content is preferably 2.00% by mass or less, more preferably 1.90% by mass or less, and particularly preferably 1.85% by mass or less.

[0027] Ni promotes the formation of the γ phase. From this viewpoint, the Ni content is preferably 8.00 mass% or more. Excess Ni increases the cost of the steel bar 2. From the viewpoint of low cost, the Ni content is preferably 15.00 mass% or less, and particularly preferably 14.00 mass% or less.

[0028] Cr can contribute to the corrosion resistance of the steel bar 2. From this viewpoint, the Cr content is preferably 16.00 mass% or more, more preferably 17.00 mass% or more, and particularly preferably 17.50 mass% or more. Excess Cr leads to the formation of a δ phase. This δ phase impairs the hot workability of the steel bar 2. From the viewpoint of hot workability, the Cr content is preferably 20.00 mass% or less.

[0029] Mo can contribute to the corrosion resistance of the steel bar 2. From this viewpoint, the Mo content is preferably 0.10 mass% or more. Excess Mo leads to the formation of a δ phase. This δ phase impairs the hot workability of the steel bar 2. From the viewpoint of hot workability, the Mo content is preferably 3.00 mass% or less, and particularly preferably 2.70 mass% or less.

[0030] This specification is also directed to the water cooling device 14. This water cooling device 14 is shown in Figures 4 and 5. This water cooling device 14 has a plurality of troughs 18. These troughs 18 are lined up in the direction indicated by arrow A2. This direction of arrow A2 is the direction in which the intermediate product 20 moves and also the length direction of each trough 18. The water cooling device 14 has a shaft 22 and rollers 24. The water cooling device 14 further has a pipe 26 as a water supply.

[0031] As shown in Figure 5, the shaft 22 passes through the rollers 24. The rollers 24 are fixed to the shaft 22. When the shaft 22 is rotated by a motor (not shown), the rollers 24 also rotate. The shaft 22 and the rollers 24 are located between two adjacent troughs 18. A portion of the rollers 24 is located above the bottom surface 27 (see Figure 4) of the troughs 18. The rotation of the rollers 24 causes the intermediate product 20 to move forward inside the troughs 18.

[0032] The pipes 26 supply water 28, which serves as a cooling medium, to the troughs 18. There are gaps between adjacent troughs 18. The water 28 flows out from these gaps. The amount of water 28 supplied from the pipes 26 to the troughs 18 is much greater than the amount of water 28 flowing out from the gaps. Therefore, the water 28 is stored in the troughs 18. As shown in FIG. 5, the water level 30 is higher than the intermediate product 20. In other words, the intermediate product 20 is submerged. The water 28 removes a large amount of heat from the intermediate product 20. The water cooling device 14 can rapidly cool the intermediate product 20. The water cooling device 14 can achieve a required time Nt of 45 minutes or less.

[0033] By submerging in water, the intermediate product 20 can be uniformly cooled. This water cooling device 14 can suppress bending of the steel bar 2 caused by non-uniform cooling.

[0034] 4, arrow G indicates the size of the gap between the trough 18 and the adjacent trough 18. From the viewpoint that the intermediate product 20 may be submerged in water, size G is preferably 30 mm or less, more preferably 20 mm or less, and particularly preferably 10 mm or less.

[0035] The water cooling device 14 may have a cover that covers the gap of the trough 18. In the water cooling device 14 with a cover, the intermediate product 20 can be submerged even if the amount of water 28 supplied from the pipe 26 to the trough 18 is small. [Example]

[0036] The effects of the manufacturing method according to the examples will be explained below, but the scope of the disclosure in this specification should not be construed as being limited based on the description of these examples.

[0037] [Example 1] The billet was heated in a heating furnace. The billet reached a temperature T1 of 1210°C. This billet was subjected to continuous rolling to obtain an intermediate product. The temperature T2 of this intermediate product was 940°C. This intermediate product was cooled in the water cooling device shown in Figures 4 and 5. This intermediate product was further cooled on a cooling bed. The temperature T3 of the intermediate product at the end of cooling was 521°C. This intermediate product was cut with a cold shear to obtain a steel bar. The time from the start of water cooling to the end of cooling was 18 minutes. In this example, the time Nt required from the start of water cooling until the temperature of the intermediate product reached 575°C was within 45 minutes. The composition of this steel bar is shown in Table 1 below.

[0038] [Examples 2-10 and Comparative Examples 1-6] Steel bars were obtained in the same manner as in Example 1, except that the composition and manufacturing conditions were as shown in Table 1 below. In the manufacturing method of Comparative Example 1, cooling by shower was performed instead of cooling by a water cooling device.

[0039] [Conventional example] The billet was heated in a heating furnace. The temperature T1 of the billet was 1200°C. The billet was subjected to continuous rolling to obtain an intermediate product. The intermediate product was cut to a predetermined size. The intermediate product was air-cooled to room temperature. The intermediate product was subjected to solution heat treatment at 1050°C to obtain a steel bar. The composition of the steel bar is shown in Table 1 below.

[0040] Curved The steel bars were visually inspected to determine the degree of curvature, the results of which are shown in Table 1 below.

[0041] [Corrosion resistance] The steel bars were subjected to a corrosion resistance test, after which the metal structure was observed and ranked according to the following criteria: A: It is a stepped structure. B: A mixed structure of stepped and grooved structures. C: Groove-shaped tissue. The results are shown in Table 1 below.

[0042] [Table 1]

[0043] The balance of each composition listed in Table 1 is Fe and unavoidable impurities.

[0044] As shown in Table 1, the steel bars obtained by the manufacturing methods of each Example are of excellent quality. The manufacturing methods of each Example consume less energy than the conventional manufacturing method. The manufacturing methods of each Example emit less carbon dioxide. From these evaluation results, the superiority of the manufacturing methods of each Example is clear. [Industrial Applicability]

[0045] The steel bars described above are suitable for parts of various machines. [Explanation of symbols]

[0046] 2...steel bar 14...Water cooling device 18 Trough 20 Intermediate products 22 shaft 24. Laura 26 Pipe 28...Wednesday 30...water surface

Claims

1. A: A step of preparing a billet made of austenitic stainless steel having a C content of 0.08% by mass or less; B: A step of heating the billet to a temperature T1 of 1010°C or higher and 1300°C or lower; C: A step of hot rolling the billet to obtain an intermediate product having a temperature T2 of 850°C or higher; D: A step of water-cooling the intermediate product; and E: A step of cooling the intermediate product by air cooling to reduce the temperature T3 of the intermediate product to 575°C or less. It contains The method for producing a steel bar, wherein the time Nt required from the start of the water cooling until the temperature of the intermediate product reaches 575 ° C. is 45 minutes or less.

2. The manufacturing method according to claim 1 , wherein in step D, the intermediate product is submerged in water.

3. a plurality of troughs aligned along the length; a roller located between the two troughs; and A supply that can inject a cooling medium into the trough Water cooling device for steel bars, comprising:

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