Non-temper embrittling alloy structural steel and method of making

Abstract

This application provides a temper-free brittle alloy structural steel and its preparation method. The preparation method includes a process flow of blast furnace hot metal smelting – desulfurization – converter smelting – LF refining – RH degassing – continuous casting – slab – slab heating – rough rolling – finish rolling – coiling – air cooling – leveling – heating – quenching – tempering. In the continuous casting process, the molten steel in the ladle comprises the following components by mass percentage: C: 0.03%–0.75%; S≤0.0004%; Ca: 0.0006%–0.0016%; P≤0.0040%; Sb≤0.0005%. Sn ≤ 0.0030%; As ≤ 0.0015%; Pb ≤ 0.0005%; N: 0.0030%~0.0050%; Ti: 0.0108%~0.0200%; H ≤ 0.00015%; Als: 0.04%~0.06%; wherein the mass percentage content of the components satisfies: Ca / S ≥ 2, Ti / N = 3.6~5.0, Si+Mn+Cr+Ni+Mo+V+Nb = 0.5%~12.0%. The method for preparing temper-brittle alloy structural steel provided in this application, by adjusting the chemical composition and content in the steel plate, prevents the alloy structural steel itself from experiencing type I and type II temper brittleness, thus solving the problem of embrittlement of quenched alloy structural steel plates during service within the temper brittleness temperature range.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, specifically to a temper-free brittle alloy structural steel and its preparation method. Background Technology

[0002] The phenomenon that the impact toughness of quenched martensite in steel decreases with increasing tempering temperature is called temper brittleness. Temper brittleness in steel is divided into Type I temper brittleness and Type II temper brittleness. In carbon structural steel, tempering after quenching within the temperature range of 200–400℃ results in a trough in impact toughness, known as Type I temper brittleness, or low-temperature temper brittleness. In alloy steels, the temperature range for Type I temper brittleness is slightly higher than that of carbon steel, generally between 250–450℃. Type I temper brittleness is irreversible; that is, tempering within the non-brittle temperature range (e.g., 150–180℃) after tempering brittleness occurs cannot eliminate it. It requires reheating and quenching followed by tempering within the non-brittle temperature range to eliminate it. The second type of temper brittleness in steel, also known as high-temperature temper brittleness of quenched martensite, occurs at a temper brittleness temperature above that of the first type. Once this type of temper brittleness occurs, it can be eliminated simply by tempering at a temperature above the temper brittleness temperature and by rapid cooling after tempering. Therefore, this type of temper brittleness is also called reversible temper brittleness.

[0003] Alloy structural steels that have undergone quenching and low-temperature tempering outside the first type of temper embrittlement temperature range will still exhibit first-type temper embrittlement if held at the first-type temper embrittlement temperature range, and will still exhibit second-type temper embrittlement if held at the second-type temper embrittlement temperature range. This indicates that high-strength heat-treated steels that have undergone quenching and low-temperature tempering will still experience first-type or second-type temper embrittlement and become brittle if used within the temperature range where temper embrittlement occurs. In recent years, driven by market demand, high-temperature resistant NM400, NM450, and NM500 steels have been under development. The service temperature of high-temperature wear-resistant steel plates is generally required to be in the range of 200℃ to 500℃, which falls within the first and second-type temper embrittlement temperature range. Therefore, the development of high-temperature wear-resistant steel plates needs to address the temper embrittlement problem of steel, and this can be achieved by adding elements such as Mo and V to the composition of ordinary NM400, NM450, and NM500 to improve thermal strength, and adding elements such as Cr to improve oxidation resistance. Summary of the Invention

[0004] To address the aforementioned technical problems, this application provides an alloy structural steel without temper brittleness and its preparation method, aiming to provide a heat-treatable high-strength alloy structural steel that is free from both first-type and second-type temper brittleness, thereby improving production efficiency and broadening the application range.

[0005] In a first aspect, embodiments of this application provide a method for preparing temper-free brittle alloy structural steel, comprising a process flow of blast furnace hot metal smelting-desulfurization-converter smelting-LF refining-RH degassing-continuous casting-slab-slab heating-rough rolling-finish rolling-coiling-air cooling of steel coils-leveling-steel plate heating-quenching-tempering; wherein, in the continuous casting process, the molten steel in the ladle comprises the following components by mass percentage: C: 0.03%~0.75%; S≤0.0004%; Ca: 0.0006%~0.0016%; P≤0.0040%; S b≤0.0005%; Sn≤0.0030%; As≤0.0015%; Pb≤0.0005%; N: 0.0030%~0.0050%; Ti: 0.0108%~0.0200%; H≤0.00015%; Si≤0.70%; Als: 0.04%~0.06%; wherein, the mass percentage content of the components satisfies the following relationships: Ca / S≥2, Ti / N=3.6~5.0, Si+Mn+Cr+Ni+Mo+V+Nb=0.5%~12.0%.

[0006] According to an embodiment of the first aspect of this application, after the blast furnace hot metal-desulfurization-converter smelting-LF refining-RH degassing process, the molten steel in the tundish during the continuous casting process comprises the following components by mass percentage: C: 0.03%–0.75%; S ≤ 0.0004%, Ca: 0.0006%–0.0016%; P ≤ 0.0040%; Sb ≤ 0.0005%, Sn ≤ 0.0030%, As ≤ 0.0015%, Pb ≤ 0.0005%. N: 0.0030%~0.0050%, Ti: 0.0108%~0.0200%; H≤0.00015%; Si≤0.70%; Als: 0.04%~0.06%; wherein, the mass percentage content of the components satisfies the following relationships: Ca / S≥2, Ti / N=3.6~5.0, Si+Mn+Cr+Ni+Mo+V+Nb=0.5%~12.0%; the remainder is Fe and other unavoidable residual elements or impurity elements in smelting.

[0007] According to an embodiment of the first aspect of this application, in the continuous casting slab process, the mass percentage content in the tundish during the continuous casting process satisfies the following relationship: Ca / S = 2~4, S ≤ 0.0004%, Ca: 0.0006%~0.0016%.

[0008] According to an embodiment of the first aspect of this application, in the blast furnace hot metal process, the blast furnace hot metal contains Sn ≤ 0.0020%, As ≤ 0.0010%, and P ≤ 0.10%.

[0009] According to an embodiment of the first aspect of this application, after the desulfurization process, the sulfur content of the molten iron satisfies S≤0.0005%; after the LF refining process, the sulfur content of the molten steel satisfies S≤0.0002%.

[0010] According to an embodiment of the first aspect of this application, in the converter smelting process, a dual-slag smelting process is adopted and iron oxide scale is added, with the final P mass percentage content ≤0.0030%.

[0011] Secondly, embodiments of this application provide an alloy structural steel without temper brittleness. The alloy structural steel comprises the following components by mass percentage: C: 0.03%–0.75%; S ≤ 0.0004%; Ca: 0.0006%–0.0016%; P ≤ 0.0040%; Sb ≤ 0.0005%; Sn ≤ 0.0030%; As ≤ 0.0015%; Pb ≤ 0.00%. 0.005%; N: 0.0030%~0.0050%; Ti: 0.0108%~0.0200%; H≤0.00015%; Si≤0.70%; Als: 0.04%~0.06%; wherein, the mass percentage content of the components satisfies the following relationships: Ca / S≥2, Ti / N=3.6~5.0, Si+Mn+Cr+Ni+Mo+V+Nb=0.5%~12.0%.

[0012] According to the embodiments of the second aspect of this application, the tensile strength of the non-tempered brittle alloy structural steel is 975 MPa to 805 MPa, the yield strength is 930 MPa to 755 MPa, the elongation is 27% to 38%, and the impact energy is 198 J to 294 J (at -60°C).

[0013] Compared with the prior art, the present invention has at least the following beneficial effects:

[0014] The method for preparing temper-free brittle alloy structural steel provided in this application, by adjusting the chemical composition and content in the steel plate, prevents the alloy structural steel itself from experiencing first and second types of temper brittleness, thus solving the problem of embrittlement of alloy structural steel during service within the temper brittleness temperature range. Detailed Implementation

[0015] The embodiments of this application will be described in further detail below with reference to the examples. The detailed description of the following embodiments is used to illustrate the principles of this application, but should not be used to limit the scope of this application, that is, this application is not limited to the described embodiments.

[0016] In the description of this application, it should be noted that, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," and "outer," etc., indicating orientation or positional relationships, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. "Vertical" does not require strict verticality, but may include permissible errors. "Parallel" does not require strict parallelism, but may include permissible errors.

[0017] Alloy structural steels commonly exhibit both Type I and Type II temper brittleness. Currently, the common practice is to select a tempering temperature that does not cause temper brittleness during the post-quenching tempering process. For example, the highest tempering temperature that can be used for producing heat-treated high-strength steels with yield strengths of 1100 MPa, 1300 MPa, and 1500 MPa is 220℃. However, if the tempering temperature is too low, the elongation will be too low to meet customer requirements. For instance, if a customer requires an elongation of ≥11% for a proportional gauge length tensile test specimen of 1100QT, using a tempering temperature of 220℃ will result in a failure to meet the elongation requirements.

[0018] The inventors of this application have noted that alloy structural steels that have undergone quenching and low-temperature tempering without first-type temper embrittlement temperatures will still experience first-type temper embrittlement when held at temperatures within the first-type temper embrittlement temperature range, and will still experience second-type temper embrittlement when held at temperatures within the second-type temper embrittlement temperature range. This indicates that high-strength heat-treated steels that have undergone quenching and low-temperature tempering will still experience first-type or second-type temper embrittlement and become brittle if used within the temperature range where temper embrittlement occurs. Existing research and practical results show that when tempered at a tempering temperature without temper embrittlement, the elongation is positively correlated with the tempering temperature; that is, the higher the tempering temperature, the higher the elongation. If a higher tempering temperature can be used without temper embrittlement, the problem of excessively low elongation that easily occurs in quenched steel plates (produced using quenching + low-temperature tempering process) will be readily solved. If a heat-treatable high-strength alloy structural steel that is free from both Type I and Type II temper brittleness can be developed, the properties of the heat-treatable high-strength steel can be adjusted by regulating the tempering temperature. This can significantly reduce the number of steel grades and help steel plants improve production efficiency and product quality control.

[0019] In view of this, the inventors of this application, through extensive experiments, provide a temper-resistant brittle alloy structural steel and its preparation method, which solves the problem that alloy structural steel is prone to embrittlement during service in the temper brittle temperature range.

[0020] Preparation method of temper-free brittle alloy structural steel

[0021] In a first aspect, embodiments of this application provide a method for preparing temper-free brittle alloy structural steel, comprising a process flow of blast furnace hot metal smelting-desulfurization-converter smelting-LF refining-RH degassing-continuous casting-slab-slab heating-rough rolling-finish rolling-coiling-air cooling of steel coils-leveling-steel plate heating-quenching-tempering; wherein the molten steel in the continuous casting ladle comprises the following components by mass percentage: C: 0.03%~0.75%; S≤0.0004%; Ca: 0.0006%~0.0016%. %; P≤0.0040%; Sb≤0.0005%; Sn≤0.0030%; As≤0.0015%; Pb≤0.0005%; N: 0.0030%~0.0050%; Ti: 0.0108%~0.0200%; H≤0.00015%; Si≤0.70%; Als: 0.04%~0.06%; wherein, the mass percentage content of the components satisfies the following relationships: Ca / S≥2, Ti / N=3.6~5.0, Si+Mn+Cr+Ni+Mo+V+Nb=0.5%~12.0%.

[0022] According to the embodiments of this application, the design principle and production process design of the above-mentioned chemical components are as follows:

[0023] For alloy structural steel that has undergone quenching and tempering, carbon (C) is the main element determining the steel's hardenability, strength, and hardness. Alloy structural steel has a wide range of applications, and the performance requirements for different applications vary significantly. Therefore, alloy structural steels for different applications have different C contents. In some Ni alloy structural steels with an alloy content of around 10% and microalloyed high-strength structural steels, the lower limit of C content is approximately 0.03%, while the upper limit of carbon content in carbon structural steel generally does not exceed 0.75%, although the upper limit of C content in alloy structural steel usually does not exceed 0.5%. While the percentage of carbon in the alloy structural steel is 5%, the method for preparing a temper-free brittle alloy structural steel proposed in this application is also applicable to carbon structural steel and high-quality carbon structural steel. Therefore, the mass percentage of carbon in the alloy structural steel in this application is designed to be between 0.03% and 0.75%. For example, the mass percentage of carbon can be 0.03%, 0.06%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, or 0.75%. The specific target carbon content is controlled through a converter-LF refining-RH treatment process.

[0024] Sulfur (S) is a grain boundary segregating element. Existing technology considers S content in steel to be one of the main impurity elements causing type II temper embrittlement. The research results of this application show that the S content also has a significant impact on type I temper embrittlement; the higher the content, the more severe the type I temper embrittlement. Therefore, in this application, the S content in the molten steel of the continuous casting tundish is controlled at 0.0000%-0.0004%. For example, the mass percentage of S can be 0.0001%, 0.0002%, 0.0003%, or 0.0004%, and the S content is controlled through KR desulfurization and LF deep desulfurization refining processes in blast furnace hot metal.

[0025] Ca can react with sulfur in molten steel to form CaS, thus eliminating the segregation of sulfur towards the original austenite grain boundaries during tempering. This is beneficial for eliminating type I and type II temper embrittlement. The research results of this application show that Ca / S ≥ 2 has a significant effect on suppressing temper embrittlement. For the LF+RH duplex refining process, Ca wire is fed at the RH station, and for the LF single refining process, Ca wire is fed at the LF station, achieving the control target of Ca / S ≥ 2 and Ca content of 0.0006% to 0.0016%. For example, the mass percentage of Ca can be 0.0006%, 0.0008%, 0.0010%, 0.0012%, 0.0014%, or 0.0016%, and Ca / S can be 2, 3, 4, 5, or 6.

[0026] P is a grain boundary segregating element. When its content exceeds 0.005%, it has a significant adverse effect on both type I and type II temper brittleness. In this application, the P content of molten steel in continuous casting is controlled at ≤0.004%. By using low-P molten iron with a P content ≤0.10% for converter smelting and adopting a double-slag + iron oxide scale smelting process in the converter, the control target of P content of molten steel in continuous casting ≤0.0040% is achieved.

[0027] Nitrogen (N) elements worsen temper brittleness. In this application, the N content in the molten steel of the continuous casting ladle is controlled at 0.0030% to 0.0050% to facilitate the control of the Ti / N ratio. For example, the mass percentage of N elements can be 0.0030%, 0.0035%, 0.0040%, 0.0045%, or 0.0050%. The molten steel after LF refining is treated with RH to achieve the target control of N content.

[0028] Hydrogen (H) promotes temper brittleness, and hydrogen embrittlement will occur when the content exceeds a certain level. In this application, the H content of molten steel in continuous casting is ≤0.00015%; the H content is controlled by RH treatment.

[0029] The role of Al is to remove oxygen from molten steel and slag, facilitating deep desulfurization during LF refining. Simultaneously, Al can combine with nitrogen in steel to form AlN, refining the grain size and improving the steel's toughness. To achieve the sulfur content control target of ≤0.0002% after LF refining, the lower limit of Al content in Al-killed steel needs to be appropriately increased. However, excessively high Al content is detrimental to steel cleanliness. Therefore, in this application, the lower limit of Al content in the ladle steel during continuous casting is increased from the conventional 0.015% to 0.040%, meaning the Al content in the ladle steel during continuous casting is controlled at 0.040%–0.060%. For example, the mass percentage of Al can be 0.040%, 0.045%, 0.050%, 0.055%, or 0.060%.

[0030] Ti is an element that inhibits grain boundary segregation. It combines with nitrogen (N) in steel to form TiN, thus fixing the N and suppressing its segregation towards the original austenite grain boundaries during tempering, which helps improve temper brittleness. The N content of alloy structural steel after RH treatment is controlled at 0.0030%–0.0050%. Insufficient Ti addition is insufficient to fix all the N in the steel, while excessive addition coarsens the TiN particles, which is detrimental to the steel's toughness. The Ti / N ratio is controlled at 3.6–5.0, and the Ti content is controlled at 0.01%. The content of Ti can be controlled from 0.08% to 0.0200%. For example, the mass percentage of Ti can be 0.0108%, 0.0120%, 0.0140%, 0.0160%, 0.0180%, or 0.0200%. In the later stage of RH treatment, ferrotitanium is added at 4.3 times the amount of Ti as N to achieve the control targets of N content 0.0030% to 0.0050%, Ti / N = 3.6 to 5.0, and Ti content 0.0108% to 0.0200%.

[0031] Sb, Sn, As, and Pb are common residual elements in steel, all of which are grain boundary segregating elements. When their content exceeds a certain limit, they promote temper brittleness in steel. In this application, the content of molten steel in the continuous casting ladle is controlled at Sb ≤ 0.0005%, Sn ≤ 0.0030%, As ≤ 0.0015%, and Pb ≤ 0.0005%. Sn and As elements mainly come from blast furnace iron and scrap steel, which are difficult to remove during the steelmaking process. Blast furnace iron is smelted using ores with low Sn and As content, and the Sn content of blast furnace iron is controlled at ≤ 0.0020% and the As content at ≤ 0.0010%. At the same time, clean scrap steel with controlled Sn, Pb, Sb, As, P, and S content is used in the steelmaking process to achieve the control target of Sn, Pb, Sb, and As element content in the continuous casting ladle steel.

[0032] At high temperatures, silicon (Si) forms a dense SiO2 oxide layer on the matrix surface, improving the material's resistance to high-temperature oxidation. However, excessive Si content can lead to surface oxidation and red rust, as well as excessive rolled-in iron oxide scale. Si also has a solid solution strengthening effect, which can inhibit the precipitation of carbides such as cementite in martensite, thus improving the content and stability of retained austenite and enhancing its ductility and toughness. However, a Si content exceeding 0.70% is detrimental to the toughness of the steel. The Si content in alloy structural steel is generally controlled at ≤0.70%. For example, the mass percentage of Si can be 0.20%, 0.30%, 0.40%, 0.50%, 0.60%, or 0.70%.

[0033] Mn has a solid solution strengthening effect and is the most significant element for improving the hardenability of steel. It is the most commonly used alloying element in various alloy structural steels, but the solid solution strengthening effect of Mn is weaker than that of Si.

[0034] Cr significantly improves hardenability, second only to Mn, and also significantly improves the oxidation resistance of steel.

[0035] Ni significantly improves the low-temperature toughness and hardenability of steel. For example, the 3Ni, 5Ni, 7Ni, and 9Ni series steels are alloy structural steels with Ni as the main alloying element, and have excellent low-temperature impact toughness.

[0036] However, the synergistic effect of Ni, Cr, and Mn elements with P, Sb, As, and Sn elements in steel at grain boundaries significantly worsens the second type of temper embrittlement.

[0037] Mo improves the hardenability, tempering resistance (hot strength), uniformity of mechanical properties, and weldability of steel; it also significantly inhibits high-temperature temper brittleness. However, excessive Mo2C formation actually increases the tendency for temper brittleness. In the temper-brittle-free alloy structural steel and its preparation method proposed in this application, a reasonable composition design is used to suppress the occurrence of temper brittleness at its source. The purpose of adding Mo is no longer to suppress the second type of temper brittleness, but to determine whether to add Mo or not based on the requirements of mechanical properties and weldability. Preferably, 0.3% to 0.6% Mo is added to the temper-brittle-free alloy structural steel. For example, the mass percentage of Mo can be 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, or 0.60%.

[0038] In alloy structural steel, vanadium (V) plays a role in improving tempering resistance (thermal strength) and enhancing the uniformity of mechanical properties.

[0039] Nitrogen (Nb) has the effect of refining grains and improving toughness; when combined with carbon (C) and nitrogen (N), it forms Nb(N,N), which has a precipitation strengthening effect and is a commonly used microalloying element in the production of microalloyed high-strength structural steel.

[0040] Si, Mn, Cr, Ni, Mo, V, and Nb are the most commonly used alloying elements in alloy structural steel. In fact, carbon structural steel also contains a certain amount of Si and Mn alloying elements. For example, the Si content of high-quality carbon structural steel grade 20 is 0.17%–0.37%, and the Mn content is 0.35%–0.65%. The temper-resistant brittle alloy structural steel and its preparation method proposed in this application are also applicable to high-quality carbon structural steel. Therefore, the lower limit of the content of common alloying elements such as Si+Mn+Cr+Ni+Mo+V+Nb is designed to be 0.5%. The alloy content of alloy structural steel varies significantly depending on its application. Most alloy structures have an alloy content of no more than 5%, but some alloys contain... Alloy structural steels with an alloy content exceeding 10%, such as 9Ni steel with an alloy content of approximately 10.5%, are excluded from the scope of this application. However, stainless steel and other steels with an alloy content exceeding 12% are not included in this application. Martensitic aging steels with an alloy content exceeding 20% ​​but a C content <0.03% are also excluded from the scope of this application. In this application, the mass percentage of Si+Mn+Cr+Ni+Mo+V+Nb is designed to be between 0.5% and 12.0%. For example, the mass percentage of Si+Mn+Cr+Ni+Mo+V+Nb can be 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 9%, 11%, or 12.0%.

[0041] The method for preparing temper-brittle alloy structural steel provided in this application, by adjusting the chemical composition and content in the steel plate, ensures that the alloy structural steel itself does not experience type I or type II temper brittleness, thus solving the problem of embrittlement of alloy structural steel during service within the temper brittleness temperature range.

[0042] In some embodiments, after the blast furnace hot metal smelting-desulfurization-converter smelting-LF refining-RH degassing process, the molten steel in the tundish during the continuous casting process comprises the following components by mass percentage: C: 0.03%–0.75%; S≤0.0004%, Ca: 0.0006%–0.0016%; P≤0.0040%; Sb≤0.0005%, Sn≤0.0030%, As≤0.0015%, Pb≤0.0005%; N Ca: 0.0030%~0.0050%, Ti: 0.0108%~0.0200%; H≤0.00015%; Si≤0.70%; Als: 0.04%~0.06%; wherein, the mass percentage content of the components satisfies the following relationships: Ca / S≥2, Ti / N=3.6~5.0, Si+Mn+Cr+Ni+Mo+V+Nb=0.5%~12.0%; the remainder is Fe and other unavoidable residual elements or impurity elements in smelting.

[0043] By adding Ti to molten steel, TiN is formed, thereby fixing the N element. In the RH degassing process, the N content is controlled at 0.0030% to 0.0050%. In the later stage of the RH degassing process, ferrotitanium is added to make the Ti / N ratio of the molten steel in the continuous casting ladle 3.6 to 5.0, and the Ti content 0.0108% to 0.0200%. This avoids the segregation of N element at the original austenite grain boundaries during tempering after quenching, thereby suppressing temper brittleness.

[0044] In some embodiments, in the continuous casting-slab process, the relationship between the contents of Ca and S in the molten steel ladle during continuous casting satisfies: S: ≤0.0004%, Ca / S = 2~4, Ca: 0.0006%~0.0016%.

[0045] Since sulfur (S) is a grain boundary segregating element, the research results of this application show that the content of S has a significant impact on the first type of temper embrittlement. The higher the content, the more severe the first type of temper embrittlement. Ca can react with S in molten steel to form CaS inclusions, eliminating the segregation of S to the original austenite grain boundaries during tempering. Therefore, in this application, the mass percentage relationship of Ca and S in the ladle during continuous casting is made to satisfy Ca / S = 2 to 4 to eliminate the segregation effect and thus avoid temper embrittlement. For example, the mass percentage ratio of Ca / S can be 2, 2.5, 3, 3.5 or 4.

[0046] Specifically, Ca treatment is performed during the process flow. If the refining process is a single LF stage, Ca treatment is performed at the LF station; if the refining process is a double LF+RH stage, Ca treatment is performed at the RH station to ensure that the Ca / S ratio of the molten steel in the continuous casting ladle is ≥2.0. Ca can react with S in the molten steel to form CaS inclusions, eliminating the segregation of S towards the original austenite grain boundaries during tempering.

[0047] In some implementations, in the blast furnace iron smelting process, the content of Sn in the blast furnace iron is ≤0.002%, As ≤0.001%, and P ≤0.10%.

[0048] Sn, As, and P are grain boundary segregating elements. When their content exceeds a certain limit, they can cause temper brittleness. In order to prevent the content of Sn, As, and P from going uncontrolled and exceeding the upper limit in the subsequent blast furnace hot metal process, Sn ≤ 0.002%, As ≤ 0.001%, and P ≤ 0.10% are controlled.

[0049] In some embodiments, after desulfurization of molten iron, the sulfur content satisfies S≤5ppm; in some embodiments, after LF refining process, the sulfur content of molten steel satisfies S≤2ppm.

[0050] The desulfurization process is KR desulfurization. According to the embodiments of this application, multi-stage desulfurization can reduce the sulfur content in molten steel, avoiding the difficulty in achieving Ca / S ≥ 2.00 when the sulfur content is high, which is detrimental to the suppression of temper brittleness.

[0051] In some implementations, the converter smelting process employs double-slag smelting with the addition of iron oxide scale, achieving a final P content ≤30ppm. This involves two slag removals during the smelting process, with strict control over slag removal during tapping or in the ladle, to reduce P reversion during LF refining.

[0052] In some implementations, the tempering temperature is 200°C to 550°C, and the quenching heating temperature is 900°C.

[0053] In some embodiments, the tempering temperature is 0℃-480℃ and the quenching heating temperature is 810℃. By adjusting the chemical composition and content in the alloy structural steel, the alloy structural steel itself does not experience the first and second types of temper brittleness, thus solving the problem of embrittlement of alloy structural steel during service within the temper brittleness temperature range.

[0054] Secondly, embodiments of this application provide a non-tempering alloy structural steel. The alloy structural steel comprises, by mass percentage: C: 0.03%–0.75%; S ≤ 0.0004%; Ca: 0.0006%–0.0016%; P ≤ 0.0040%; Sb ≤ 0.0005%; Sn ≤ 0.0030%; As ≤ 0.0015%; Pb ≤ 0.0005%; N: 0.0030%–0.0050%; Ti: 0.0108%–0.0200%; H ≤ 0.00015%; Als: 0.04%–0.06%; wherein the mass percentage content of the components satisfies: Ca / S ≥ 2, Ti / N = 3.6–5.0, Si+Mn+Cr+Ni+Mo+V+Nb = 0.5%–12.0%.

[0055] The alloy structural steel without temper brittleness provided in this application solves the problem of embrittlement of alloy structural steel during service in the temper brittleness temperature range by adjusting the chemical composition and content in the steel plate, so that the alloy structural steel itself does not experience the first and second types of temper brittleness.

[0056] According to the embodiments of the second aspect of this application, the mechanical properties of the non-tempered brittle alloy structural steel vary with the tempering temperature. When the tempering temperature is between 180°C and 480°C, the impact toughness and elongation increase with the increase of the tempering temperature, while the yield strength and tensile strength decrease with the increase of the tempering temperature. The impact energy at -60°C is between 198J and 294J, the elongation is between 27% and 38%, the yield strength is between 930MPa and 755MPa, and the tensile strength is between 975MPa and 805MPa.

[0057] The following examples illustrate in more detail the non-temper brittle alloy structural steel of this application, as well as the significant influence of S content and Ca / S ratio in continuous casting on the first type of temper brittleness of alloy structural steel, but this application is not limited to these examples.

[0058] Example 1:

[0059] This embodiment provides a non-temper brittle alloy structural steel A, the composition of which, by mass percentage, includes:

[0060] C: 0.045%; S: 0.0003%; Ca: 0.0009%; Ca / S = 3; P: 0.0040%; Sb: 0.0005%, Sn: 0.0030%, As: 0.0015%, Pb: 0.0005%, N: 0.0050%, Als: 0.0442%; Ti: 0.0192%; Ti / N = 3.84; Mn+Ni+Cr+Si = 10.27%; the remainder is Fe and other unavoidable residual or impurity elements during smelting.

[0061] Its preparation method includes the following steps:

[0062] Blast furnace hot metal smelting - desulfurization - converter smelting - LF refining - RH degassing - continuous casting - slab - slab heating - rough rolling - finish rolling - coiling - air cooling - leveling - heating - quenching - tempering process flow;

[0063] Thermal expansion measurements show that the steel's Ac1 point is 530℃ and its Ac3 point is 710℃. Following the basic principles for developing heat treatment processes for alloy structural steel, the maximum acceptable tempering temperature is Ac1 - 50℃ = 480℃. The total alloy content in this steel is 10.27%, and the quenching temperature is Ac3 + 100℃ = 810℃.

[0064] Seven heat-treated samples with a width*length*thickness of 60*120*16.3 mm were cut from a hot-rolled sheet (coil number 1A29769500). After water quenching at 810℃ for 40 min, they were tempered at different tempering temperatures for 60 min each, and then air-cooled after tempering. Each heat-treated impact sample was processed into a transverse impact sample with a specification of 10*10*55 mm. The impact energy at -60℃, yield strength, tensile strength, and elongation after fracture were tested. Water cooling after tempering usually has an inhibitory effect on temper brittleness of type II. Therefore, air cooling was used after tempering in this temper brittleness verification test. However, the impact toughness did not decrease with the increase of tempering temperature within the temper brittleness temperature range (230℃~480℃), i.e., there was no temper brittleness. The test results are shown in Table 1 below.

[0065] Table 1: Test results of tempering brittleness temperature of alloy structural steel A Tempering temperature (°C)

[0066]

[0067] As can be seen from Table 1, within the temperature range of the first and second types of temper brittleness, namely 230℃~480℃, the impact energy did not decrease with the increase of tempering temperature, indicating that the non-temper brittle alloy structural steel A does not have temper brittleness.

[0068] Comparative Example 1:

[0069] This comparative example provides alloy structural steel B, whose S content and Ca / S ratio have a significant impact on the first type of temper brittleness.

[0070] The chemical composition (%) of alloy structural steel B is as follows: C: 0.1668; Si: 0.2513; Mn: 1.2399; P: 0.0089; S: 0.0022; Als: 0.0505; Ti: 0.0152; N: 0.0038; Nb: 0.0133; Ni: 0.4329; Ca: 0.0016; B: 0.0011; Pb: 0.0003; V: 0.0371; H: 0.00011; As: 0.0061; Sn: 0.0044; Sb: 0.0017; Cr: 0.1999; Mo: 0.5859. Its preparation method is the same as in Example 1.

[0071] Twelve heat-treated samples with a width*length*thickness of 60*120*16.3 were cut from hot-rolled plate (coil number 0A14866800). After water quenching at 900℃ for 25 min, they were tempered at different tempering temperatures for 45 min. After tempering, they were air-cooled after being removed from the furnace. Each heat-treated impact sample was processed into a transverse impact sample with a specification of 10*10*55. The impact energy at -40℃ was tested. The test results are shown in Table 2 below.

[0072] Table 2: Temper brittleness test data of alloy structural steel B with S content of 0.0022%, Ca content of 0.0016%, and Ca / S ratio of 0.73.

[0073]

[0074] Comparative Example 2:

[0075] This comparative example provides C for alloy structural steels where S content and Ca / S ratio have a significant impact on the first type of temper brittleness.

[0076] The chemical composition (%) of alloy structural steel C is as follows: C: 0.1718; Si: 0.2337; Mn: 1.2189; P: 0.0097; S: 0.0003; Als: 0.0594; Ti: 0.0178; N: 0.0047; Nb: 0.0151; Ni: 0.4948; Ca: 0.0006; B: 0.0012; Pb: 0.0004; V: 0.0357; H: 0.00009; As: 0.0069; Sn: 0.0052; Sb: 0.0024; Cr: 0.2003; Mo: 0.5818. Its preparation method is the same as in Example 1.

[0077] Eight heat-treated samples with a width*length*thickness of 60*120*16.3 were cut from a hot-rolled plate (coil number 1A01009300). After water quenching at 900℃ for 25 min, they were tempered at different tempering temperatures for 45 min. After tempering, they were air-cooled after being removed from the furnace. Each heat-treated impact sample was processed into a transverse impact sample with a specification of 10*10*55. The impact energy at -40℃ was tested. The test results are shown in Table 3 below.

[0078] Table 3: Temper brittleness test data of C in alloy structural steel with S content of 0.0003%, Ca content of 0.0006%, and Ca / S = 2.00

[0079]

[0080] Comparing Tables 2 and 3, it can be seen that when the S content of alloy structural steel B decreases from 0.0022% to 0.0003% in alloy structural steel C, and the Ca / S ratio increases from 0.73 to 2.00, the degree of first-type temper brittleness, i.e., the decrease in impact toughness compared to the tempering temperature of 200℃, is significantly reduced. However, due to the excessively high contents of P, As, Sn, and Sb elements, a certain degree of first-type temper brittleness is still observed. Since 0.58% Mo element is added to alloy structural steels B and C, it has a certain inhibitory effect on second-type temper brittleness. The impact toughness of steel tempered at 450℃ decreases by only 11.19% and 9.02% respectively compared to that of steel tempered at 200℃.

[0081] In summary, this application solves the problem of embrittlement of alloy structural steel during service by adjusting the chemical composition and content of the non-temper brittle alloy structural steel, so that the non-temper brittle alloy structural steel itself does not experience type I and type II temper brittleness.

[0082] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing temper-free brittle alloy structural steel, characterized in that, The process flow includes blast furnace iron smelting - desulfurization - converter smelting - LF refining - RH degassing - continuous casting - slab - slab heating - rough rolling - finish rolling - coiling - air cooling - leveling - heating - quenching - tempering. In the continuous casting process, the molten steel in the ladle comprises the following components by mass percentage: C: 0.03%~0.75%; S≤0.0004%; Ca: 0.0006%~0.0016%; P≤0.0040%; Sb≤0.0005%; Sn≤0.0030%; As≤0.0015%; Pb≤0.0005%; N: 0.0030%~0.0050%; Ti: 0.0108%~0.0200%; H≤0.00015%; Si≤0.70%; Als: 0.04%~0.06%; wherein the mass percentage relationship of the components satisfies: Ca / S≥2, Ti / N=3.6~5.0, Si+Mn+Cr+Ni+Mo+V+Nb=0.5%~12.0%; the remainder is Fe and other unavoidable residual elements or impurities in smelting.

2. The method for preparing alloy structural steel according to claim 1, characterized in that, In the continuous casting-slab process, the mass percentage relationship of Ca and S in the molten steel in the tundish satisfies: Ca / S=2~4, S≤0.0004%, Ca:0.0006%~0.0016%.

3. The method for preparing alloy structural steel according to claim 1, characterized in that, In the blast furnace iron smelting process, the blast furnace iron contains Sn ≤ 0.0020%, As ≤ 0.0010%, and P ≤ 0.10%.

4. The method for preparing alloy structural steel according to claim 1, characterized in that, After the desulfurization process, the sulfur content in the molten iron satisfies S≤0.0005%.

5. The method for preparing alloy structural steel according to claim 1, characterized in that, The S content in the molten steel after the LF refining process satisfies S≤0.0002%.

6. The method for preparing alloy structural steel according to claim 1, wherein in the converter smelting process, a double slag smelting process is adopted and iron oxide scale is added, and the final P mass percentage content is ≤0.0030%.

7. The method for preparing alloy structural steel according to claim 1, characterized in that, After the RH degassing process, the composition of the molten steel in the continuous casting ladle meets the following requirements: N: 0.0030%~0.0050%; Ti: 0.0120%~0.0200%; H≤0.00015%; S≤0.0004%; Ca: 0.0006%~0.0016%; wherein, the mass percentage content of the components satisfies: Ca / S≥2.

8. A temper-free brittle alloy structural steel, characterized in that, The component percentages of the non-tempering brittle alloy structural steel, by mass percentage, include: C: 0.03%~0.75%; S≤0.0004%; Ca: 0.0006%~0.0016%; P≤0.0040%; Sb≤0.0005%; Sn≤0.0030%; As≤0.0015%; Pb≤0.0005%; N: 0.0030%~0.0050%; Ti: 0.0108%~0.0200%; H≤0.00015%; Si≤0.70%; Als: 0.04%~0.06%; where the mass percentage content of the components satisfies the following relationships: Ca / S≥2, Ti / N=3.6~5.0, Si+Mn+Cr+Ni+Mo+V+Nb=0.5%~12.0%; the remainder is Fe and other unavoidable residual elements or impurities during smelting.

9. The temper-free brittle alloy structural steel according to claim 8, characterized in that, The non-tempering brittle alloy structural steel has a tensile strength of 975MPa~805MPa, a yield strength of 930MPa~755MPa, an elongation of 27%~38%, and an impact energy of 198J~294J at -60℃.

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