Manufacturing method for hot rolling bar steel and selection method for steel raw material

Abstract

To provide a manufacturing method for hot rolling bar steel and a selection method for a steel raw material that enable the prediction of surface quality of the hot rolling bar steel prior to hot rolling.SOLUTION: In a manufacturing method for hot rolling bar steel containing copper, the magnetic leakage flux inspection pass rate of the hot rolling bar steel subjected to hot rolling is calculated using Equation (1) to predict surface quality in advance, and the hot rolling bar steel is manufactured. A=-a×D-b×[Cu]+c×T+d+e ...(1) Here, A is the pass rate of magnetic leakage flux inspection, D is the product diameter of the hot rolling bar steel [mm], [Cu] is the Cu content of the hot rolling bar steel [mass%], T is the extraction temperature during the manufacturing of the hot rolling bar steel [°C], a, b, and c are coefficients, d is a constant, and e is the magnetic leakage flux inspection coefficient.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a method for manufacturing hot-rolled bar steel and a method for selecting steel materials.

Background Art

[0002] In the case of steel containing Cu, fine surface defects occur in proportion to the Cu concentration, which inhibits productivity. On the other hand, in Patent Document 1, for thin slab hot-rolled coils, after obtaining a correction value A by substituting the Cu equivalent of the molten steel and the thickness of the coil to be produced into the formula: (Cu equivalent × 100) + (1.5 × coil thickness), the correction value A is applied to the formula: 0.0067 × A 2 -0.088 × A to predict the scale defect index.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, the method of the conventional Patent Document 1 has a problem that it is difficult to apply to hot-rolled bar steel because it is premised on thin slab hot-rolled coils. In addition, there is a problem that the rolling temperature and the surface quality requirement level, which are factors of surface quality variation other than Cu, are not taken into account in the calculation, and the variation cannot be predicted.

[0005] Therefore, the present invention has been made paying attention to the above problems, and an object thereof is to provide a method for manufacturing hot-rolled bar steel and a method for selecting steel materials capable of predicting the surface quality in advance in hot-rolled bar steel.

Means for Solving the Problems

[0006] According to one aspect of the present invention, there is provided a method for manufacturing hot-rolled bar steel containing Cu, For the hot-rolled bar steel that has been hot-rolled, a method for manufacturing hot-rolled bar steel is provided, which calculates the qualified rate of magnetic flux leakage inspection using formula (1) and predicts the surface quality in advance. A = -a×D - b×[Cu] + c×T + d + e ···(1) Here, A is the qualified rate of magnetic flux leakage inspection, D is the product diameter [mm] of the hot-rolled bar steel, [Cu] is the Cu content [mass%] of the hot-rolled bar steel, T is the extraction temperature [°C] when manufacturing the hot-rolled bar steel, a, b, and c are coefficients, d is a constant, and e is the magnetic flux leakage inspection coefficient.

[0007] (1) According to one aspect of the present invention, there is provided a method for manufacturing hot-rolled bar steel containing Cu, wherein for the hot-rolled bar steel to be hot-rolled, the qualified rate of magnetic flux leakage inspection is calculated using formula (1), and the surface quality is predicted in advance. A = -a×D - b×[Cu] + c×T + d + e ···(1) Here, A is the qualified rate of magnetic flux leakage inspection, D is the product diameter [mm] of the hot-rolled bar steel, [Cu] is the Cu content [mass%] of the hot-rolled bar steel, T is the extraction temperature [°C] when manufacturing the hot-rolled bar steel, a, b, and c are coefficients, d is a constant, and e is the magnetic flux leakage inspection coefficient.

[0008] (2) In the method for manufacturing hot-rolled bar steel described in (1) above, in formula (1), the coefficient a is 0.002, the coefficient b is 0.157, the coefficient c is 0.00005, and the constant d is 0.818.

[0009] (3) In the method for manufacturing hot-rolled bar steel described in (2) above, the hot-rolled bar steel has a product diameter of 20 mm or more and 120 mm or less, a Cu content of 0.01 mass% or more and 0.30 mass% or less, and the extraction temperature from the heating furnace during hot rolling is 950 °C or more and 1200 °C or less.

[0010] (4) According to one aspect of the present invention, there is provided a method for selecting a steel material in manufacturing a hot-rolled bar steel containing Cu, wherein for the hot-rolled bar steel to be hot-rolled, the qualified rate of magnetic flux leakage testing is calculated using formula (1), and a steel material with a Cu content that satisfies the target qualified rate is selected.

Effect of the Invention

[0011] According to one aspect of the present invention, there are provided a manufacturing method of a hot-rolled bar steel and a method for selecting a steel material, which can predict the prior surface quality in the hot-rolled bar steel.

Brief Description of the Drawings

[0012]

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Mode for Carrying Out the Invention

[0013] In the following detailed description, embodiments of the present invention will be described with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals, and duplicate descriptions are omitted. Each drawing is schematic and may include cases that are different from the actual ones. Further, the embodiments shown below are examples of devices and methods for embodying the technical idea of the present invention, and the technical idea of the present invention does not specify the materials, structures, arrangements, etc. of the components as follows. The technical idea of the present invention can be variously modified within the technical scope defined by the claims described in the claims.

[0014] The method for manufacturing a hot-rolled bar steel according to an embodiment of the present invention first calculates the pass rate of magnetic flux leakage inspection for the hot-rolled bar steel to be hot-rolled before performing hot rolling, and predicts the surface quality in advance. The pass rate of magnetic flux leakage inspection is calculated using the following formula (1). Note that the pass rate is shown as a ratio in the range of 0.00 or more and 1.00 or less, with all passing as 1.00 and all failing as 0.00. A = -a×D - b×[Cu] + c×T + d + e ···(1) Here, A is the pass rate of magnetic flux leakage inspection, D is the product diameter [mm] of the hot-rolled bar steel, [Cu] is the Cu content [mass%] of the hot-rolled bar steel, T is the extraction temperature [°C] from the heating furnace when manufacturing the hot-rolled bar steel, a, b, and c are coefficients, d is a constant, and e is the magnetic flux leakage inspection coefficient.

[0015] In conceiving the present invention, the inventors respectively conducted simple regression analyses on the relationship between the pass rate for each leakage magnetic flux flaw detection level and the product diameter, Cu content, and extraction temperature of hot-rolled bar steel. As a result, as shown in FIGS. 1 and 4, it was found that there is a correlation strength between the pass rate and the product diameter for each surface flaw detection level. Also, as shown in FIGS. 2, 3, and 4, it was found that the Cu content and the extraction temperature each have a correlation strength for each surface flaw detection level. From this, the pass rate of the leakage magnetic flux flaw detection was determined to be a composite factor, and a multiple regression analysis of four variables was performed. In FIGS. 1 to 4, the case of three levels of surface flaw detection levels, namely general (flaw depth 0.15 mm to 0.3 mm × flaw length 20 mm), strict (flaw depth 0.1 mm × flaw length 20 mm), and most strict (flaw depth 0.1 mm × flaw length 2.5 mm), is shown. The surface flaw detection level indicates the depth at which surface flaws are detected by leakage magnetic flux flaw detection, and in ascending order of the detection depth, they are the general, strict, and most strict levels.

[0016] As a result of the multiple regression analysis, as a regression equation showing the pass rate of the leakage magnetic flux flaw detection, in Equation (1), the coefficient a is 0.002, the coefficient b is 0.157, the coefficient c is 0.00005, and the constant d is 0.818. The multiple correlation coefficient is 0.449, and the contribution rate R 2 is 0.202. Also, the leakage magnetic flux flaw detection coefficient e is set in the range of 0 or more and 0.199 or less according to the surface flaw detection level. For example, when the surface flaw detection level is a general level, it is 0.199, when the surface flaw detection level is a strict level, it is 0.104, and when it is the most strict level, it is 0. The regression equation in which the coefficients a, b, c, and the constant d in Equation (1) have the above values is shown as the following Equation (2). A = -0.002×D - 0.157×[Cu] + 0.00005×T + 0.818 + e ···(2)

[0017] As conditions for Equation (2), from the range in which the multiple regression analysis was performed, it is preferable that the product diameter of the hot-rolled bar steel is 20 mm or more and 120 mm or less, the Cu content of the hot-rolled bar steel is 0.01 mass% or more and 0.30 mass% or less, and the extraction temperature is 950°C or more and 1200°C or less.

[0018] Furthermore, the inventors of the present invention performed magnetic flux leakage testing on hot-rolled bar steels with a product diameter of 60 mm under conditions where the magnetic flux leakage inspection level, Cu content, and extraction temperature were different, and verified the difference between the calculated qualification rate by formula (2) and the actual qualification rate. Note that there were two levels for the magnetic flux leakage inspection level, four levels for the Cu content, and three levels for the extraction temperature. As shown in FIG. 6, it was confirmed that the calculated qualification rate corresponded to the actual qualification rate, and it was found that the calculated qualification rate was effective. In addition, FIG. 5 shows the relationship between the calculated qualification rate and the actual qualification rate using the regression formula of simple regression with the Cu content as a variable. Furthermore, FIG. 7 shows the standard deviation of the residuals (actual qualification rate - calculated qualification rate) in the most stringent materials of FIGS. 5 and 6. As shown in FIGS. 5 to 7, it can be confirmed that in the most stringent materials, the prediction accuracy is improved in multiple regression, while there was an inverse correlation in simple regression with the Cu content as a variable.

[0019] After estimating the qualification rate of magnetic flux leakage testing, when the qualification rate calculated by formula (1) is equal to or higher than the target qualification rate, hot rolling is performed under the estimated conditions to produce hot-rolled bar steel. On the other hand, when the calculated qualification rate is less than the target qualification rate, at least one of the selection of steel materials such as billets and the setting of the extraction temperature of the heating furnace is adjusted so as to satisfy the target qualification rate. Specifically, at least one of the selection of steel materials with different Cu contents and the change of the extraction temperature of the heating furnace is performed so that the qualification rate calculated by formula (1) is equal to or higher than the target qualification rate. Thereafter, hot rolling is performed under the adjusted conditions to produce hot-rolled bar steel. In hot rolling, the selected steel material with a Cu content is heated in a heating furnace, then the steel material is extracted at the set extraction temperature, and rough rolling, intermediate rolling, and finish rolling are sequentially performed. The method of hot rolling is not particularly limited, and known techniques can be applied.

[0020] According to the method for manufacturing a hot-rolled bar steel according to this embodiment, by using the formula (1) to estimate the qualified rate of magnetic flux leakage flaw detection from the product diameter, extraction temperature, and Cu content, it becomes possible to accurately predict the qualified rate of magnetic flux leakage flaw detection in advance. Therefore, it becomes possible to select a material for the hot-rolled bar steel suitable for quality requirements. Further, even when the Cu content is increased, the qualified rate of magnetic flux leakage flaw detection can be estimated, and depending on the product diameter, extraction temperature, target qualified rate, etc., it is also possible to use a steel material with a high Cu content. From this, it is possible to reduce the product cost and improve the reliability of the surface quality.

[0021] <Modification example> As described above, the present invention has been described with reference to specific embodiments, but it is not intended to limit the invention by these descriptions. By referring to the description of the present invention, those skilled in the art will also understand other embodiments of the present invention including various modification examples together with the disclosed embodiments. Therefore, it should be understood that the embodiments of the invention described in the claims also cover embodiments including these modification examples described herein alone or in combination.

[0022] For example, the present invention can also be applied to a method for selecting a steel material when manufacturing a hot-rolled bar steel containing Cu. In this case, for the hot-rolled bar steel to be hot-rolled, the qualified rate of magnetic flux leakage flaw detection is calculated using the formula (1), and a steel material with a Cu content that satisfies the target qualified rate is selected. Further, from the perspective of using more steel materials with a large amount of scrap with a high Cu content in order to reduce the manufacturing cost, it is preferable to use a steel material with as high a Cu content as possible within the range that satisfies the target qualified rate.

Example

[0023] The examples conducted by the inventors will be described. In the examples, for a product with a diameter of 80 mm, an extraction temperature of 1020 °C, and a leakage magnetic flux inspection level for a general hot-rolled bar steel product, the pass rate of the leakage magnetic flux inspection was calculated for the case where the Cu content was 0.14 mass% (test material). As a result of the calculation, the pass rate of the leakage magnetic flux inspection was 0.89. Also, for the steel material (process material) used step by step with an average Cu content of 0.10% (upper limit 0.13%), when calculated in the same way, the pass rate of the leakage magnetic flux inspection was 0.89, and the pass rates were the same.

[0024] Next, for the hot-rolled bar steel under the above conditions, hot rolling was actually carried out and leakage magnetic flux inspection was performed. The results (the relationship between the content of each component of C, Si, Mn, P, S, and Cu and the pass rate of the leakage magnetic flux inspection) are shown in FIGS. 8 to 13. As shown in FIGS. 8 to 13, it was confirmed that at least in the investigated range, the contents of C, Si, Mn, P, S, and Cu did not significantly affect the pass rate. Also, as shown in FIG. 13, for the test material with a Cu content of 0.14 mass%, all 18 samples passed, and for the process material with an average Cu content of 0.10 mass%, the pass rate of the leakage magnetic flux inspection was 0.97. From the above results, it was confirmed that the estimation by equation (1) is effective. Also, since it was confirmed that even when the Cu content was increased to 0.14 mass%, the pass rate did not change from the process conditions, it was also confirmed that the amount of inexpensive scrap used could be increased and the manufacturing cost could be reduced.

Claims

1. A method for manufacturing a hot-rolled bar steel containing Cu, comprising: For the hot-rolled bar steel to be hot-rolled, calculating the qualification rate of magnetic flux leakage testing using formula (1) to predict the surface quality in advance, a method for manufacturing a hot-rolled bar steel. A = -a×D - b×[Cu] + c×T + d + e... (1) Here, A is the qualification rate of magnetic flux leakage testing, D is the product diameter [mm] of the hot-rolled bar steel, [Cu] is the Cu content [mass%] of the hot-rolled bar steel, T is the extraction temperature [°C] when manufacturing the hot-rolled bar steel, a, b, and c are coefficients, d is a constant, and e is the magnetic flux leakage testing coefficient.

2. The method for manufacturing a hot-rolled bar steel according to Claim 1, wherein in formula (1), the coefficient a is 0.002, the coefficient b is 0.157, the coefficient c is 0.00005, and the constant d is 0.

818.

3. The hot-rolled bar steel has a product diameter of 20 mm or more and 120 mm or less, and a Cu content of 0.01 mass% or more and 0.30 mass% or less, The extraction temperature from the heating furnace during hot rolling is 950°C or more and 1200°C or less, the method for manufacturing a hot-rolled bar steel according to Claim 2.

4. A method for selecting a steel material when manufacturing a hot-rolled bar steel containing Cu, comprising: For the hot-rolled bar steel to be hot-rolled, calculating the qualification rate of magnetic flux leakage testing using formula (1) to select a steel material with a Cu content that satisfies the target qualification rate, a method for selecting a steel material. A = -a×D - b×[Cu] + c×T + d + e... (1) Here, A is the qualification rate of magnetic flux leakage testing, D is the product diameter [mm] of the hot-rolled bar steel, [Cu] is the Cu content [mass%] of the hot-rolled bar steel, T is the extraction temperature [°C] when manufacturing the hot-rolled bar steel, a, b, and c are coefficients, d is a constant, and e is the magnetic flux leakage testing coefficient.

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