Hot-rolled steel with little loss of compressive strength after steel pipe processing and its manufacturing method

Abstract

The present invention provides a hot-rolled steel product that has little loss of compressive strength after steel pipe processing, and a method for manufacturing the same. The hot-rolled steel material of the present invention contains, by weight, carbon (C): 0.15% or less, silicon (Si): 2.5% or less, manganese (Mn): 2.0% or less, aluminum (Al): 0.05% or less, the sum of the contents of nitrogen (N) and boron (B): 0.002 to 0.008%, the balance being Fe and other unavoidable impurities, and satisfies 250<450C+95Si+70Mn, the microstructure is a mixed structure of ferrite and pearlite, the average grain size of the ferrite is 8 to 25 μm, and the value defined by Relation 1 is less than 20%.

Description

[Technical field]

[0001] The present invention relates to a hot-rolled steel material that has little loss of compressive strength after steel pipe processing, and a method for producing the same. [Background technology]

[0002] In recent years, research into ultra-high-speed vacuum tube trains, known as hyperloops, as a next-generation transportation system, has been actively conducted both domestically and internationally. Ultra-high-speed vacuum tube trains are basically a form of transportation in which trains move inside a vacuum tube. In other words, it is a transportation method based on the concept that trains can operate at ultra-high speeds by minimizing air resistance by maintaining a vacuum state inside the tube.

[0003] According to recent trends, it is known that metal steel pipes are more advantageous than concrete as a tube material from the viewpoint of maintaining a vacuum inside the tube. Also, in consideration of the strength of the tube material and the manufacturing cost, it is true that steel pipes are the most suitable.

[0004] A basic method for manufacturing steel pipes is known in which hot-rolled material is subjected to a separate heat treatment as necessary, then bent into a steel pipe shape and welded for use.

[0005] Meanwhile, it is well known that metal materials are affected by the Bauschinger effect, in which the yield strength of compressive stress decreases once the material has been subjected to tensile stress. According to the above-mentioned steel pipe manufacturing process, the steel material is subjected to tensile stress in the circumferential direction of the steel pipe during bending, and thus a loss of compressive strength occurs in the same circumferential direction after bending. The loss of compressive strength in the circumferential direction of the steel pipe not only causes deformation of the shape after processing the material, but also acts as a cause of serious defects that may cause the tube to collapse in an emergency. Therefore, it is essential to apply a steel material that can minimize the loss of compressive strength in steel pipe materials for tubes. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2005 / 080621 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a hot-rolled steel material that exhibits little loss of compressive strength in the production of steel pipes, and a method for producing the same.

[0008] Furthermore, the technical problems to be achieved in the present invention are not limited to those mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the art to which the present invention pertains from the following description. [Means for solving the problem]

[0009] One aspect of the present invention is a steel sheet having, by weight percent, carbon (C): 0.15% or less, silicon (Si): 2.5% or less, manganese (Mn): 2.0% or less, aluminum (Al): 0.05% or less, the sum of the contents of nitrogen (N) and boron (B): 0.002 to 0.008%, the balance being Fe and other unavoidable impurities, and satisfying 250<450C+95Si+70Mn; The microstructure is a mixed structure of ferrite and pearlite, the average grain size of the ferrite is 8 to 25 μm, and the tensile stress under the elongation condition of 0.3% is

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[0010]

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[0011] Tensile stress of the above hot-rolled steel

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[0012] Another aspect of the present invention is a method for producing a steel slab having the above-mentioned composition, comprising the steps of finish hot rolling the steel slab at a finish temperature of 950 to 1030 ° C.; The finish hot-rolled hot-rolled steel sheet is cooled to a temperature range of 580 to 730 ° C. at a cooling rate of 5 to 50 ° C., and then coiled. This relates to a method for producing hot-rolled steel material with little loss of compressive strength after steel pipe processing, characterized in that the temperature change of the steel plate is controlled to within 20°C for 3 seconds immediately before the cooled hot-rolled steel plate is coiled. Effect of the Invention

[0013] According to the present invention, the tensile stress under the stretching condition of 0.3% is

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[0014] The present invention will now be described.

[0015] The present invention relates to a technique for producing a steel material with little loss of compressive strength in the manufacture of steel pipes. Specifically, the steel material contains, by weight, 0.15% or less carbon (C), 2.5% or less silicon (Si), 2.0% or less manganese (Mn), 0.05% or less aluminum (Al), 0.002-0.008% sum of nitrogen (N) and boron (B), the balance being Fe and other unavoidable impurities, and satisfies 250<450C+95Si+70Mn, the microstructure is a mixed structure of ferrite and pearlite, the average grain size of the ferrite is 8-25μm, and the tensile stress under the elongation condition of 0.3% is 0.05% or less.

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[0016] [Steel composition] First, the compositional components of the steel material for manufacturing the steel pipe of the present invention and the reasons for limiting the components will be described. In the following, "%" means "weight %" unless otherwise specified.

[0017] ·Carbon (C): 0.15% or less Carbon (C) is a typical element that improves hardenability and effectively contributes to ensuring the strength of steel. Therefore, in the present invention, from the viewpoint of ensuring the strength of the vacuum tube structure, carbon (C) may be contained in a range that satisfies 250<450C+95Si+70Mn or more. On the other hand, if the carbon (C) content is excessive, there is a possibility that problems will occur in that the toughness of the steel material is reduced and the weldability is reduced. Therefore, in the present invention, the upper limit of the carbon (C) content may be limited to 0.15%. More preferably, the upper limit of the carbon (C) content may be limited to 0.12%.

[0018] Silicon (Si): 2.5% or less Silicon (Si) is an element that contributes to the deoxidization of steel. Therefore, in the present invention, silicon (Si) of 250<450C+95Si+70Mn or more can be included to ensure the cleanliness and strength of the steel. On the other hand, if silicon (Si) is added in excess, it not only increases the high-temperature strength of the material, which may cause problems in the continuous casting process, but also prevents the surface scale from falling off, which may reduce the surface quality of the product. Therefore, in the present invention, the upper limit of the silicon (Si) content can be limited to 2.5%. More preferably, the upper limit of the silicon (Si) content can be limited to 2.0%.

[0019] Manganese (Mn): 2.0% or less Manganese (Mn) is an element that contributes to improving the hardenability of steel. Therefore, in the present invention, in order to ensure the strength of the steel, manganese (Mn) of 250<450C+95Si+70Mn or more can be contained. On the other hand, if manganese (Mn) is added in excess, there is a possibility that problems will occur in that the toughness of the steel will decrease and the weldability will decrease. Therefore, in the present invention, the upper limit of the manganese (Mn) content can be limited to 2.0%. More preferably, the upper limit of the manganese (Mn) content can be limited to 1.8%.

[0020] Aluminum (Al): 0.05% or less Aluminum (Al) is an element that easily reacts with oxygen and is a typical element used in deoxidation reactions in steelmaking. However, since Al may generate inclusions when present in steel, it is preferable to control the amount of Al remaining in the steel as little as possible. Therefore, in the present invention, the upper limit of the aluminum (Al) content can be limited to 0.05%.

[0021] Sum of nitrogen (N) and boron (B) content: 0.002-0.008% Nitrogen (N) and boron (B) are interstitial solid solution elements, and although their content in steel is small compared to other elements, they have a relatively large effect on physical properties. The inventors have found that the sum of the above two element contents is related to the loss of compressive strength of the material. That is, when the sum of the two elements exceeds 0.008% by weight, it has been confirmed that it is very difficult to control the value defined by the below-mentioned Relational Formula 2 to less than 20. Furthermore, controlling the sum of the two elements to less than 0.002% is not preferable because there is a high possibility of an increase in costs from the viewpoint of controlling the material composition.

[0022] Therefore, in order to economically control the value defined by Relational Formula 2 described below to less than 20, the sum of the contents of the above two elements can be limited to a range of 0.002 to 0.008% by weight. More preferably, the sum of the contents of the above two elements can be limited to a range of 0.003 to 0.007% by weight.

[0023] 250<450C+95Si+70Mn

[0024] In the present invention, it is required to control the contents of C, Mn, and Si so as to satisfy the above inequality. If the calculated value of 450C + 95Si + 70Mn is less than 250, a problem may occur in which the strength of the material decreases.

[0025] [Steel microstructure] The steel material of the present invention has a mixed structure of ferrite and pearlite.

[0026] In the present invention, the area fraction of the ferrite may be 60 to 90% by area, and the area fraction of the pearlite may be 10 to 40% by area.

[0027] The inventors of the present invention have conducted extensive research into methods for reducing the loss of compressive strength after elongation of steel material, and as a result have found that a large grain size of ferrite in the steel material is advantageous in reducing the loss of compressive strength. However, when the average grain size of ferrite is 25 μm or more, the impact toughness is too low to be used as a structural material, so the upper limit of the average grain size is limited to 25 μm. In addition, it has been confirmed that the average grain size needs to be 8 μm or more in order to achieve the reduction in loss of compressive strength targeted in the present invention, and the lower limit has been set to 8 μm.

[0028] [Steel properties] The steel material of the present invention has a tensile stress of 0.3% elongation.

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[0029]

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[0030] The inventors of the present invention simulated the situation in which hot-rolled steel material is subjected to tensile stress during processing of steel pipes and the situation in which the steel pipes are subjected to compressive stress in an actual usage environment, and found that the levels of tensile stress and compressive deformation are 0.3% and 0.2%, respectively. They also confirmed that in order to avoid the risk of shape deformation and sudden collapse of the steel pipes during processing, the value defined by the above Relational Formula 1 needs to be controlled to less than 20%.

[0031] On the other hand, in order to use the steel pipe material of the present invention as a structural material, it is important to ensure that the steel pipe material has a certain level of strength and toughness.

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[0032] [Steel manufacturing method] Next, a method for producing a hot-rolled steel material according to the present invention that reduces loss of compressive strength after processing into a steel pipe will be described.

[0033] The method for producing a steel material of the present invention includes the steps of finish hot rolling a steel slab having the above-mentioned compositional components at a finish temperature of 950 to 1030°C, and cooling the above-mentioned finish hot rolled hot rolled steel sheet to a temperature range of 580 to 730°C at a cooling rate of 5 to 50°C and then coiling the sheet, and is characterized in that the temperature change of the steel sheet is controlled to within 20°C for 3 seconds immediately before coiling the cooled hot rolled steel sheet.

[0034] First, in the present invention, a steel slab having the above-mentioned composition is finish hot rolled at a finish temperature of 950 to 1030°C.

[0035] The finish hot rolling temperature is an operation factor that has a large effect on the austenite grain size (AGS) of the material. It is generally known that the AGS is highly correlated with the ferrite grain size (FGS), which is the cooling structure. That is, if the AGS is coarse, the FGS is also coarse, and vice versa. The inventors have found that in order to control the FGS to 8 to 25 μm, the finish hot rolling temperature needs to be 950 ° C or more and 1030 ° C or less. If the finish hot rolling temperature is less than 950 ° C, the AGS becomes very fine and the FGS is also less than 8 μm. Conversely, if it exceeds 1030 ° C, the FGS may exceed 25 μm, which is disadvantageous in terms of impact toughness.

[0036] Next, in the present invention, the hot-rolled steel sheet subjected to the above finish hot rolling is cooled to a temperature range of 580 to 730° C. at a cooling rate of 5 to 50° C., and then coiled.

[0037] The cooling rate when cooling a steel plate after finish hot rolling affects FGS, surface scale, and material variation inside the steel plate. If the cooling rate is less than 5°C / s, the FGS will exceed 25μm and become large, causing the surface scale to thicken and reducing the yield of the steel plate. On the other hand, if the cooling rate exceeds 50°C / s, the FGS will become less than 8μm and become fine.

[0038] In addition, during the cooling, the cooling end temperature is related to the high temperature strength for coil winding and the FGS of the steel material. If the cooling end temperature is less than 580°C, the strength of the steel material at the time of coiling is high, which not only causes equipment load problems during coiling but also may result in the FGS being less than 8 μm. On the other hand, if it exceeds 730°C, the FGS may exceed 25 μm, and the loss value of the impact toughness and the compressive strength according to the above-mentioned Relational Formula 1 may be 20% or more. Therefore, in the present invention, the finish hot-rolled steel sheet needs to be cooled at a cooling end temperature of 580 to 730°C.

[0039] On the other hand, the surface temperature of the cooled steel sheet increases due to reheating, and the inventors have found that if the steel sheet is coiled while reheating is in progress, this affects the final FGS. Although the exact principle is not clear, the inside and outside of the steel sheet reach thermal equilibrium during the reheating process. In this case, it is determined that if stress is applied during coiling, it will affect the FGS due to factors such as dislocations. In consideration of this, in the present invention, the temperature change of the steel sheet is limited to within 20°C for 3 seconds immediately before coiling. This is because if the temperature change of the steel sheet exceeds 20°C for 3 seconds immediately before coiling, there is a possibility that the FGS will exceed 25 μm. EXAMPLES

[0040] The present invention will be described in detail below with reference to examples.

[0041] (Example) Each steel slab having the alloy composition shown in Table 1 was heated in a temperature range of 1250°C. Then, hot-rolled steel sheets having a thickness of 10 mm were produced while controlling the finish hot rolling temperature, the cooling rate after hot rolling, the cooling end temperature, and the temperature change of the steel sheet for 3 seconds immediately before coiling as shown in Table 2. Then, for each of the produced steel sheets, FGS was measured in the microstructure and the results are shown in Table 3 below, and the loss value of compressive strength was measured and the results are shown in Table 3 below. Furthermore, the 0°C impact toughness of each of the produced hot-rolled steel sheets was measured and the results are also shown in Table 3 below.

[0042] Meanwhile, in Table 3 below, the FGS was measured using an optical microscope with a magnification of 500 after etching each test piece by the Nital etching method, and the tensile and compressive tests were basically performed according to ASTM Standard E606-04, and

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[0043] [Table 1] *In Table 1, A* is 450C+95Si+70Mn

[0044] [Table 2]

[0045] [Table 3] *In Table 3, B* indicates the loss value (%) of compressive strength defined by Relation 1.

[0046] As shown in Tables 1 to 3 above, in the case of Examples 1 to 9 in which not only the compositional components of the steel but also the manufacturing conditions of the steel satisfy the ranges of the present invention, the FGS is 8 to 25 μm in all cases, and the loss value of the compressive strength defined by the relational formula 1 is less than 20%, which is excellent. In addition, it can be confirmed that the 0°C impact toughness value is 50 (J) or more, and the toughness value is also excellent.

[0047] In contrast, in Comparative Example 1-3, although the manufacturing conditions for the steel are within the range of the present invention, the value of 450C+95Si+70Mn in the compositional components of the steel is 250 or less, so the tensile strength of the manufactured hot-rolled steel sheet is

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[0048] In addition, in Comparative Example 4-5, the steel manufacturing conditions were within the range of the present invention, but the N+B content in the steel composition was outside the range of the present invention. The FGS of the produced hot-rolled steel sheet was less than 8 μm, and the loss value of the compressive stress was 20% or more, which was not good.

[0049] In Comparative Example 6, the steel composition is within the range of the present invention, but the finish hot rolling temperature is too low in the steel manufacturing conditions, and the FGS of the hot-rolled steel sheet produced is less than 8 μm and the loss value of compressive stress is 20% or more, which is not good.In Comparative Example 7, the finish hot rolling temperature is too high, and the FGS of the hot-rolled steel sheet produced is more than 25 μm and the impact toughness value is less than 50 (J), which is not good.

[0050] In Comparative Example 8, the steel composition is within the range of the present invention, but the cooling rate in the steel manufacturing conditions is too slow, and the FGS of the hot-rolled steel sheet produced is more than 25 μm and the impact toughness is less than 50 (J), which is not good. In Comparative Example 9, the cooling rate is too fast, and the FGS of the hot-rolled steel sheet produced is less than 8 μm and the loss value of compressive stress is 20% or more, which is not good.

[0051] In Comparative Example 10, the composition of the steel was within the range of the present invention, but the cooling end temperature in the steel production conditions was too low, and the FGS of the produced hot-rolled steel sheet was less than 8 μm and the loss value of the compressive stress was 20% or more, which was not good.In Comparative Example 11, the cooling end temperature was too high, and the FGS of the produced hot-rolled steel sheet exceeded 25 μm and the impact toughness value was less than 50 (J), which was not good.

[0052] Furthermore, in Comparative Examples 12-13, although the compositional components of the steel were within the range of the present invention, the steel production conditions were such that the temperature change of the steel sheet for 3 seconds immediately before coiling exceeded 20°C, and the FGS of the produced hot-rolled steel sheet exceeded 25 μm and the impact toughness value was less than 50 (J), which was not good.

[0053] As described above, the detailed description of the present invention has been given of the preferred embodiment of the present invention, but it is obvious that those skilled in the art can make various modifications without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiment, but should be determined by the claims below as well as equivalents thereto.

Claims

1. In weight percent, carbon (C): 0.15% or less, silicon (Si): 2.5% or less, manganese (Mn): 2.0% or less, aluminum (Al): 0.05% or less, and both nitrogen (N) and boron (B), the sum of which is 0.002 to 0.008%, with the balance being Fe and other unavoidable impurities, satisfying 250<450C+95Si+70Mn; The microstructure is a mixed structure of ferrite and pearlite, and the average grain size of the ferrite is 8 to 25 μm; and The tensile stress under the 0.3% stretch condition is [0010] The compressive stress when the stretched and deformed material is compressed again by 0.2% is [0025] The hot-rolled steel material has a small loss in compressive strength after steel pipe processing, in which the value defined by the following Relational Formula 1 is less than 20% when [0030]

2. Tensile strength of the hot-rolled steel [0045] 2. The hot-rolled steel material having little loss of compressive strength after steel pipe working according to claim 1, wherein the compressive strength at 0° C. is 385 MPa or more and the 0° C. impact toughness is 50 J or more.

3. a step of finish hot rolling a steel slab, which contains, by weight percent, carbon (C): 0.15% or less, silicon (Si): 2.5% or less, manganese (Mn): 2.0% or less, aluminum (Al): 0.05% or less, both nitrogen (N) and boron (B), the sum of which is 0.002-0.008%, with the balance being Fe and other unavoidable impurities, and satisfies 250<450C+95Si+70Mn, at a finish temperature of 950-1030°C; The finish hot-rolled hot-rolled steel sheet is cooled to a temperature range of 580 to 730 ° C. at a cooling rate of 5 to 50 ° C. / s, and then coiled; Controlling the temperature change of the steel sheet to within 20° C. for 3 seconds immediately before coiling the cooled hot-rolled steel sheet; and The coiled hot-rolled steel material has a microstructure of a mixed structure of ferrite and pearlite, and the average grain size of the ferrite is 8 to 25 μm; and The tensile stress under the 0.3% stretch condition is [0050] The compressive stress when the stretched and deformed material is compressed again by 0.2% is [006] The method for producing a steel material having little loss in compressive strength after steel pipe processing, wherein the value defined by the following relational expression 1 is less than 20% when [0070]

4. Tensile strength of the hot-rolled steel [0080] 4. The method for producing a hot-rolled steel material having little loss of compressive strength after steel pipe working according to claim 3, wherein the compressive strength at 0° C. is 385 MPa or more and the 0° C. impact toughness is 50 J or more.

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