Hot-rolled steel having a small loss of compressive strength after pipe processing and method of manufacturing the same

Abstract

This invention provides a hot-rolled steel with minimal loss of compressive strength after pipe processing and its manufacturing method. The hot-rolled steel of this invention, by weight percent, comprises: carbon (C): less than 0.15%, silicon (Si): less than 2.5%, manganese (Mn): less than 2.0%, aluminum (Al): less than 0.05%, and the sum of nitrogen (N) and boron (B): 0.002-0.008%, with the balance being Fe and other unavoidable impurities. The hot-rolled steel satisfies 250 < 450C + 95Si + 70Mn, and its microstructure is a mixture of ferrite and pearlite, with the average size of the ferrite grains being 8-25 μm and less than 20% as defined by Equation 1.

Description

Technical Field

[0001] This invention relates to a hot-rolled steel product with minimal loss of compressive strength after steel pipe processing and its manufacturing method. Background Technology

[0002] In recent years, as a next-generation transportation system, research on ultra-high-speed vacuum tube trains, also known as hyperloops, has been actively conducted both domestically and internationally. Ultra-high-speed vacuum tube trains are essentially a form of transportation that moves trains within a vacuum tube. That is, they are a concept that maintains the inside of the tube in a vacuum state to minimize air resistance, thereby enabling trains to travel at ultra-high speeds.

[0003] Recent trends indicate that using metal steel pipes is more advantageous than concrete for maintaining a vacuum within pipes. Furthermore, considering factors such as pipe material strength and manufacturing costs, steel pipes are the most suitable option.

[0004] As a basic method for manufacturing steel pipes, there is a known method that involves heat-treating hot-rolled material separately as needed, bending it into the form of a steel pipe, and then welding it for use.

[0005] Furthermore, it is well known that in the case of metallic materials, once subjected to tensile stress, the material is affected by the Bauschinger effect, which results in a decrease in yield strength due to compressive stress. According to the manufacturing process of the steel pipe, during bending, the steel is subjected to tensile stress along the circumference of the pipe, thus resulting in a similar loss of compressive strength in the circumferential direction after processing. This loss of compressive strength in the circumferential direction not only leads to shape deformation after processing but also causes serious defects that could lead to pipe collapse in emergency situations. Therefore, the steel material used for pipelines needs to be one that minimizes the loss of compressive strength.

[0006] [Existing Technical Documents]

[0007] [Patent Literature]

[0008] (Patent Document 1) WO2005-080621A1 Summary of the Invention

[0009] Technical problems to be solved

[0010] The purpose of this invention is to provide a hot-rolled steel product with minimal loss of compressive strength during the manufacture of steel pipes and a method thereof.

[0011] Furthermore, the technical problems to be solved by the present invention are not limited to the above-mentioned technical problems, and those skilled in the art can clearly understand other unmentioned technical problems from the following description.

[0012] Technical solution

[0013] One aspect of the present invention relates to a hot-rolled steel with low compressive strength loss after processing, comprising, by weight percent: carbon (C): less than 0.15%, silicon (Si): less than 2.5%, manganese (Mn): less than 2.0%, aluminum (Al): less than 0.05%, nitrogen (N) and boron (B) content summed at 0.002-0.008%, and the balance being Fe and other unavoidable impurities. The hot-rolled steel satisfies 250 < 450C + 95Si + 70Mn, and its microstructure is a mixture of ferrite and pearlite, the average size of the ferrite grains being 8-25 μm, and the tensile stress under 0.3% tensile conditions is set as σ. 拉伸 Let σ be the compressive stress when the stretched material is compressed again by 0.2%. 压缩 At that time, the value defined by the following relation 1 is less than 20%.

[0014] [Relation 1]

[0015]

[0016] The tensile strength σ of the hot-rolled steel 拉伸 It can achieve an impact strength of over 385 MPa, and its impact toughness at 0℃ can be over 50 J.

[0017] Furthermore, another aspect of the present invention relates to a method for manufacturing steel with minimal loss of compressive strength after processing into steel pipes, characterized by comprising the following steps: hot finishing rolling a steel billet having the composition described above at a finishing rolling temperature of 950-1030°C; and cooling the hot-rolled steel sheet to a temperature range of 580-730°C at a cooling rate of 5-50°C and then coiling it up, wherein the temperature change of the steel sheet is controlled within 20°C during the 3 seconds prior to coiling the cooled hot-rolled steel sheet.

[0018] Beneficial effects

[0019] According to the present invention, a steel material with minimal loss of compressive strength after processing can be provided, wherein the tensile stress under a tensile condition of 0.3% is set as σ. 拉伸 Let σ be the compressive stress when the stretched material is compressed again by 0.2%. 压缩 At that time, the value defined by the following relation 1 is less than 20%.

[0020] Best practice

[0021] The present invention will now be described.

[0022] This invention relates to a technology for producing steel with minimal compressive strength loss during the manufacture of steel pipes. Specifically, it relates to a hot-rolled steel with minimal compressive strength loss after steel pipe processing. The steel, by weight percent, comprises: carbon (C): less than 0.15%, silicon (Si): less than 2.5%, manganese (Mn): less than 2.0%, aluminum (Al): less than 0.05%, and the sum of nitrogen (N) and boron (B): 0.002-0.008%, with the balance being Fe and other unavoidable impurities. The steel satisfies the condition 250 < 450C + 95Si + 70Mn, and its microstructure is a mixture of ferrite and pearlite, with the average ferrite grain size being 8-25 μm. The tensile stress under 0.3% tensile conditions is set as σ. 拉伸 Let σ be the compressive stress when the stretched material is compressed again by 0.2%. 压缩 At that time, the value defined by the following relation 1 is less than 20%.

[0023] [Steel Composition]

[0024] First, the composition of the steel used to manufacture the steel pipe of the present invention and the reasons for its limitations will be explained. Hereinafter, unless otherwise defined, "%" means "weight %".

[0025] Carbon (C): less than 0.15%

[0026] Carbon (C) is a representative element for improving hardenability and is also an element that effectively helps ensure the strength of steel. Therefore, in this invention, to ensure the strength of the vacuum pipe structure, the carbon (C) content can be in the range of 250 < 450C + 95Si + 70Mn or higher. On the other hand, when the carbon (C) content is too high, it may cause problems such as reduced toughness and weldability of the steel. Therefore, in this invention, the upper limit of the carbon (C) content can be limited to 0.15%. More preferably, the upper limit of the carbon (C) content is limited to 0.12%.

[0027] Silicon (Si): less than 2.5%

[0028] Silicon (Si) is an element that contributes to the deoxidation of steel. Therefore, in this invention, to ensure the cleanliness and strength of the steel, it may contain silicon (Si) of 250 < 450C + 95Si + 70Mn or higher. On the other hand, adding too much silicon (Si) increases the high-temperature strength of the material, which may cause problems in the continuous casting process and hinders the removal of surface oxide scale, thus potentially reducing the surface quality of the product. Therefore, in this 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 is limited to 2.0%.

[0029] Manganese (Mn): below 2.0%

[0030] Manganese (Mn) is an element that helps improve the hardenability of steel; therefore, the present invention may contain more than 250 < 450C + 95Si + 70Mn to ensure the strength of the steel. On the other hand, when too much manganese (Mn) is added, problems such as reduced toughness and weldability of the steel may occur. Therefore, the upper limit of the manganese (Mn) content in the present invention may be limited to 2.0%. More preferably, the upper limit of the manganese (Mn) content is limited to 1.8%.

[0031] Aluminum (Al): less than 0.05%

[0032] Aluminum (Al) is an element that readily reacts with oxygen and is a representative element used in the deoxidation reaction of steelmaking. However, when Al is present in steel, it can form inclusions, so it is preferable to control its presence in the steel to be as minimal as possible. Therefore, in this invention, the upper limit of the aluminum (Al) content can be limited to 0.05%.

[0033] The combined content of nitrogen (N) and boron (B) is 0.002-0.008%.

[0034] Nitrogen (N) and boron (B) are interstitial solid solution elements. Compared to other elements, they are present in relatively small amounts in steel, but their impact on physical properties is relatively large. The inventors have discovered that the sum of the contents of these two elements is related to the loss of compressive strength in the material. Specifically, when the sum of the two elements exceeds 0.008% by weight, it is confirmed that it is difficult to control the value defined by Equation 2 below to be less than 20. Furthermore, controlling the sum of the two elements to be less than 0.002% likely increases the cost of controlling the material composition, and is therefore not preferred.

[0035] Therefore, in order to economically control the value defined by the following relationship 2 to be less than 20, the sum of the contents of the two elements can be limited to the range of 0.002-0.008% by weight. More preferably, the sum of the contents of the two elements can be limited to 0.003-0.007% by weight.

[0036] 250 < 450C + 95Si + 70Mn

[0037] In this invention, the contents of C, Mn, and Si need to be controlled to satisfy the aforementioned inequality. When the calculated value of 450C+95Si+70Mn is below 250, a decrease in material strength may occur.

[0038] [The microstructure of steel]

[0039] The steel of the present invention is composed of a mixed structure of ferrite and pearlite.

[0040] In this invention, the fraction of ferrite can be 60-90% by area, and the fraction of pearlite can be 10-40% by area.

[0041] The inventors repeatedly studied methods to reduce the loss of compressive strength in steel after tensile testing. They found that a larger ferrite grain size in the steel is beneficial for reducing compressive strength loss. However, when the average ferrite grain size is 25 μm or larger, the impact toughness is too low to use it as a structural component. Therefore, the upper limit of the average grain size was limited to 25 μm. Furthermore, to achieve the desired reduction in compressive strength loss as described in this invention, an average grain size of 8 μm or larger was determined, and its lower limit was set at 8 μm.

[0042] [Physical properties of steel]

[0043] The steel of the present invention can exhibit the following compressive strength loss: the tensile stress under a 0.3% tensile condition is set as σ. 拉伸 Let σ be the compressive stress when the stretched material is compressed again by 0.2%. 压缩 At that time, the value defined by the following relation 1 is less than 20%.

[0044] [Relation 1]

[0045]

[0046] The inventors, through simulations of the tensile stress experienced by hot-rolled steel during pipe processing and the compressive stress experienced by the corresponding steel pipe in actual use environments, found that the tensile stress was 0.3% and the compressive deformation was 0.2%. Furthermore, it was confirmed that the values ​​defined by Equation 1 need to be controlled to be less than 20% to avoid the risk of shape deformation and sudden collapse of the steel pipe during processing.

[0047] Furthermore, in order to use the steel pipe material processed according to the present invention as a structural material, it is important to ensure a constant level of strength and toughness. Therefore, the tensile strength σ of the steel of the present invention... 拉伸 It can achieve an impact strength of over 385 MPa, and its impact toughness at 0℃ can be over 50 J.

[0048] [Steel Manufacturing Methods]

[0049] Secondly, the method of the present invention for manufacturing hot-rolled steel with minimal loss of compressive strength after steel pipe processing will be described.

[0050] The method for manufacturing steel according to the present invention is characterized by comprising the following steps: hot finishing rolling a steel billet having the composition as described above at a finishing rolling temperature of 950-1030°C; and cooling the hot-rolled steel plate to a temperature range of 580-730°C at a cooling rate of 5-50°C and then coiling it up, wherein the temperature change of the steel plate is controlled within 20°C during the 3 seconds before coiling the cooled hot-rolled steel plate.

[0051] First, in this invention, a steel billet having the composition described above is hot-rolled at a finishing temperature of 950-1030°C.

[0052] Hot finishing temperature is an operational factor that significantly influences the austenite grain size (AGS) of a material. It is generally known that AGS has a very high correlation with the ferrite grain size (FGS), which is the cooling microstructure. That is, when AGS is coarse, FGS is also coarse, and vice versa. The inventors have discovered that to control FGS within the 8-25 μm range, the hot finishing temperature should be above 950°C and below 1030°C. When the hot finishing temperature is below 950°C, AGS becomes very fine, and FGS is less than 8 μm. On the other hand, when the hot finishing temperature exceeds 1030°C, FGS exceeds 25 μm, which may have an adverse effect on impact toughness.

[0053] Next, in this invention, the hot-rolled steel sheet is cooled to a temperature range of 580-730°C at a cooling rate of 5-50°C and then coiled.

[0054] The cooling rate during the cooling of hot-rolled steel sheets affects the surface oxide scale (FGS), surface oxide scale, and internal material variations. When the cooling rate is less than 5°C / second, the FGS increases to over 25 μm, resulting in a thicker surface oxide scale and a reduced yield. Conversely, when the cooling rate exceeds 50°C / second, the FGS decreases to less than 8 μm, leading to a finer FGS.

[0055] Furthermore, during the cooling process, the cooling termination temperature is related to the high-temperature strength of the coil used for winding and the steel's free weight gain (FGS). When the cooling termination temperature is below 580°C, the steel strength at the start of winding increases, leading to equipment load issues during winding and potentially resulting in an FGS less than 8 μm. On the other hand, when the cooling termination temperature exceeds 730°C, the FGS exceeds 25 μm, and the loss in impact toughness and compressive strength according to Equation 1 above can be more than 20%. Therefore, in this invention, it is necessary to cool the hot-rolled steel sheet to a cooling termination temperature of 580-730°C.

[0056] Furthermore, the surface temperature of the cooled steel sheet rises upon reheating, and the inventors have discovered that this affects the final FGS (Flat Size Gain) during the reheating process. While the exact mechanism is unclear, it is believed that the internal and external surfaces of the steel sheet reach thermal equilibrium during reheating, and when stress is applied during winding, factors such as dislocations affect the FGS. Considering this, in this invention, the temperature change of the steel sheet is limited to within 20°C during the first 3 seconds of winding. This is because if the temperature change of the steel sheet exceeds 20°C during the first 3 seconds of winding, the FGS may exceed 25 μm. Detailed Implementation

[0057] The present invention will now be described in detail through examples.

[0058] (Example)

[0059] Within a temperature range of 1250°C, steel billets composed of the alloy compositions shown in Table 1 were heated. Then, as shown in Table 2, the hot finishing rolling temperature, the cooling rate after hot rolling, the cooling termination temperature, and the temperature change of the steel sheet during the last 3 seconds before coiling were controlled to produce hot-rolled steel sheets with a thickness of 10 mm. Furthermore, for each manufactured steel sheet, the fine-grained glutaraldehyde (FGS) in its microstructure was measured and shown in Table 3 below, and the compressive strength loss was measured and the results are also shown in Table 3 below. Additionally, the impact toughness of each manufactured hot-rolled steel sheet at 0°C was measured, and the results are also shown in Table 3.

[0060] Additionally, in Table 3 below, FGS was measured using a 500x optical microscope after etching each specimen with a nitric acid alcohol solution (Nital) etching method. Tensile and compressive tests were performed primarily according to ASTM Standard E606-04, but σ... 拉伸 and σ 压缩 The compressive strength loss was measured using the values ​​defined in this invention. Additionally, the impact toughness was measured using the V-notch impact test method of KS B 0810. Furthermore, in Tables 2 and 3 below, Examples 1 to 9 of the invention have a mixed microstructure of ferrite and pearlite, and Comparative Examples 1 to 14 also have a mixed microstructure of ferrite and pearlite.

[0061] [Table 1]

[0062]

[0063] *In Table 1, A* is 450C+95Si+70Mn

[0064] [Table 2]

[0065]

[0066] [Table 3]

[0067] Steel plate number Remark FGS(μm) <![CDATA[σ 拉伸 (MPa)]]> <![CDATA[σ 压缩 (MPa)]]> B*(％) Impact toughness (J) 1 Invention Example 1 21 386 320 17 52 2 Invention Example 2 18 387 317 18 53 3 Invention Example 3 15 392 314 20 54 4 Invention Example 4 8 431 362 16 60 5 Invention Example 5 25 385 339 12 50 6 Invention Example 6 10 433 368 15 57 7 Invention Example 7 13 452 371 18 54 8 Invention Example 8 16 453 390 14 53 9 Invention Example 9 19 462 388 16 52 10 Comparative Example 1 15 384 319 17 54 1l Comparative Example 2 17 382 313 18 53 12 Comparative Example 3 20 383 314 18 52 13 Comparative Example 4 7 387 306 21 64 14 Comparative Example 5 6 433 338 22 65 4 Comparative Example 6 7 431 340 21 62 5 Comparative Example 7 26 385 339 12 49 4 Comparative Example 8 27 431 362 16 48 5 Comparative Example 9 7 385 300 22 56 4 Comparative Example 10 6 431 340 21 65 5 Comparative Example 11 26 385 339 12 48 1 Comparative Example 12 26 386 324 16 48 2 Comparative Example 13 26 387 329 15 47

[0068] *In Table 3, B* represents the compressive strength loss (%) defined in Equation 1.

[0069] As shown in Tables 1 to 3, in Invention Examples 1 to 9, where the steel composition and manufacturing conditions meet the scope of this invention, the FGS is 8-25 μm, and the compressive strength loss value defined by Equation 1 is less than 20%, which is excellent. Furthermore, it can be confirmed that the impact toughness value at 0°C is 50 (J) or higher, exhibiting excellent toughness.

[0070] On the other hand, in Comparative Examples 1 to 3, the steel manufacturing conditions were not within the scope of the present invention, but the 450C+95Si+70Mn composition of the steel had a value of 250 or less, therefore the tensile strength σ of the hot-rolled steel sheet manufactured was high. 拉伸 Less than 385 MPa and not excellent.

[0071] Furthermore, Comparative Examples 4 and 5 are cases where the steel manufacturing conditions are within the scope of the present invention, but the N+B content in the steel composition is not within the scope of the present invention. It can be seen that the FGS of the hot-rolled steel sheet manufactured is less than 8 μm and the compressive stress loss value is more than 20%, which is not excellent.

[0072] Furthermore, Comparative Example 6 involved a steel composition within the scope of the present invention, but the hot-rolling temperature was too low during steel manufacturing. As a result, the hot-rolled steel sheet produced had a free weight distribution (FGS) of less than 8 μm and a compressive stress loss of more than 20%, which was not ideal. Comparative Example 7 involved a hot-rolling temperature that was too high, resulting in a hot-rolled steel sheet with an FGS exceeding 25 μm and an impact toughness value of less than 50 (J), which was also not ideal.

[0073] Furthermore, Comparative Example 8 involved a steel composition within the scope of the present invention, but the cooling rate during steel manufacturing was too low. The resulting hot-rolled steel sheet had a free stress gap (FGS) exceeding 25 μm and an impact toughness value less than 50 (J), which was therefore undesirable. Comparative Example 9 involved a case of excessively fast cooling. The resulting hot-rolled steel sheet had an FGS less than 8 μm and a compressive stress loss value exceeding 20%, which was also undesirable.

[0074] Furthermore, Comparative Example 10 involved a steel composition within the scope of the present invention, but the cooling termination temperature during steel manufacturing was too low. The resulting hot-rolled steel sheet had a free weight distribution (FGS) of less than 8 μm and a compressive stress loss of more than 20%, which was therefore undesirable. Comparative Example 11 involved a cooling termination temperature that was too high. The resulting hot-rolled steel sheet had an FGS exceeding 25 μm and an impact toughness value of less than 50 (J), which was also undesirable.

[0075] Furthermore, Comparative Examples 12 to 13 are cases where the composition of the steel is within the scope of the present invention, but the temperature change of the steel plate exceeds 20°C during the first 3 seconds before coiling in the steel manufacturing conditions. The hot-rolled steel plates manufactured have an FGS of more than 25 μm and an impact toughness value of less than 50 (J), which is not excellent.

[0076] As described above, preferred embodiments of the invention have been illustrated in the detailed description of the invention; however, those skilled in the art can make various modifications without departing from the scope of the invention. Therefore, the scope of the invention is not limited to the illustrated embodiments, but should be determined by the claims and their equivalents.

Claims

1. A hot-rolled steel with low compressive strength loss after processing, comprising, by weight%,: carbon (C): less than 0.15%, silicon (Si): 1.2-2.5%, manganese (Mn): less than 2.0%, aluminum (Al): less than 0.05%, and the sum of nitrogen (N) and boron (B): 0.002-0.008%, with the balance being Fe and other unavoidable impurities. The hot-rolled steel satisfies 250 < 450C + 95Si + 70Mn, and its microstructure is a mixture of ferrite and pearlite, with the average ferrite grain size being 8-25 μm. The tensile stress under 0.3% tensile conditions is set as σ. 拉伸 Let σ be the compressive stress when the stretched material is compressed again by 0.2%. 压缩 When the value defined by the following relation 1 is less than 20%, [Relation 1] 。 2. The hot-rolled steel with minimal loss of compressive strength after processing, as described in claim 1, is characterized in that... The tensile strength σ of the hot-rolled steel 拉伸 It has a strength of 385 MPa or higher and an impact toughness of 50 J or higher at 0℃.

3. A method for manufacturing steel with minimal loss of compressive strength after processing into steel pipes, characterized in that, Includes the following steps: The steel billet is hot-rolled at a finishing temperature of 950-1030℃. The billet, by weight percent, contains: carbon (C): less than 0.15%, silicon (Si): 1.2-2.5%, manganese (Mn): less than 2.0%, aluminum (Al): less than 0.05%, and the sum of nitrogen (N) and boron (B): 0.002-0.008%, with the balance being Fe and other unavoidable impurities. The billet satisfies the following conditions: 250 < 450C + 95Si + 70Mn. The hot-rolled steel sheet is cooled to a temperature range of 580-730°C at a cooling rate of 5-50°C and then coiled. Specifically, the temperature change of the hot-rolled steel sheet is controlled within 20°C during the 3 seconds before the cooled hot-rolled steel sheet is coiled.

4. The method for manufacturing steel with minimal loss of compressive strength after processing, as described in claim 3, is characterized in that... In the coiled hot-rolled steel, the microstructure is a mixture of ferrite and pearlite, with the ferrite grains having an average size of 8-25 μm, and the tensile stress under a 0.3% tensile condition is set as σ. 拉伸 Let σ be the compressive stress when the stretched material is compressed again by 0.2%. 压缩 When the value defined by the following relation 1 is less than 20%, [Relation 1] 。 5. The method for manufacturing steel with minimal loss of compressive strength after processing, as described in claim 4, is characterized in that... The tensile strength σ of the hot-rolled steel 拉伸 It has a strength of 385 MPa or higher and an impact toughness of 50 J or higher at 0℃.

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