700MPa-grade high-strength steel with excellent low-temperature toughness and production method

Abstract

The invention discloses 700MPa-grade high-strength steel with excellent low-temperature toughness. The 700MPa-grade high-strength steel comprises the following components in percentage by weight: 0.10-0.18% of C, 0.10-0.25% of Si, 1.2-1.5% of Mn, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.10-0.30% of Cr and the balance of Fe. Ti: 0.01 to 0.03%, B: 0.001 to 0.003%; 0.02-0.05% of Als and less than or equal to 0.006% of N; the production method comprises the following steps: desulfurizing molten iron, and conventionally smelting to set components; heating the plate blank; descaling with high-pressure water; performing finish rolling after conventional rough rolling; cooling to a coiling temperature; performing coiling; quenching is conducted; tempering is conducted; and standing for later use. Under the conditions that the yield strength is larger than or equal to 700 MPa, the tensile strength is larger than or equal to 750 MPa, the percentage elongation after fracture is larger than or equal to 18%, the low-temperature impact energy of the steel plate at-60 DEG C is larger than or equal to 100 J, and the production cost can be reduced by at least 5%.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel and a production method, and in particular to a 700MPa-grade high-strength steel with excellent low-temperature toughness and a production method. Background Art

[0002] With the continuous advancement of technology and scientific and technological progress in various industries, the market demand for high-strength steel applications continues to increase. At the same time, the service environment of materials is becoming increasingly severe, so more stringent requirements are placed on the comprehensive performance of materials, such as high-temperature resistant building structure steel, high-strength steel resistant to various corrosion environments, and low-temperature resistant high-strength steel. However, as the strength of the material continues to increase, the plasticity and toughness of the material will be reduced accordingly. How to balance the strength, plasticity and toughness of the material has become a technical bottleneck for the expansion of high-strength steel in extremely cold areas. High-strength steel with a yield strength of 700MPa has been researched and manufactured in domestic and foreign institutions and enterprises. As searched:

[0003] The Chinese patent publication number is CN96190123.3, which discloses a "high-strength steel with excellent low-temperature toughness and weldability". Its composition and production process are: C: 0.05-0.10%, Si: ≤0.6%, Mn: 1.7-2.5%, P: ≤0.015%, S: ≤0.003%, Ni: 0.1-1.0%, Mo: 0.15-0.60%, Nb: 0.01-0.10%, Ti: 0.005-0.030%, Al: ≤0.06%, N: 0.001-0.006%, and the rest is iron Fe. This design uses more precious alloys, which greatly increases the overall production cost.

[0004] The Chinese patent publication number is CN98812439.4, which discloses an "ultra-high strength steel with excellent low-temperature toughness", whose composition and production process are: C: 0.04-0.12%, Mn: 0.5-2.5%, Ni: ≥1%, Cu: 0.1-1.50%, Mo: 0.10-0.80%, Nb: 0.02-0.10%, Ti: 0.008-0.030%, Al: 0.001-0.050%, N: 0.002-0.005%, and the rest is iron Fe. The process is to produce the steel billet by offline quenching + tempering process after hot rolling. This design uses a large number of precious alloys, and the cost is high.

[0005] The Chinese patent publication number is CN1757783A, which discloses "A method for manufacturing 700MPa grade F / B high-strength strip steel". Its component composition and production process are: C 0.12-0.18, Si≤0.1, Mn 1.60-2.20, Ti 0.15-0.22, P<0.01, S<0.01, and the rest is iron Fe. The continuous casting billet heating temperature is 1200℃, the rough rolling start temperature is 1150℃, the rough rolling final rolling temperature is 1050℃, the intermediate billet thickness is 32-45mm, the finishing rolling start temperature is 1000-950℃, the finishing rolling final rolling temperature is 790-850℃, and the finishing rolling mill is water-cooled; the cooling rate after rolling is 30-50℃ / s, and the coiling is at 450-600℃, and the product thickness is 3-8mm. This design is mainly aimed at CSP production lines. The thickness specifications are limited and only thin-specification products can be produced. At the same time, the products designed do not show excellent low-temperature toughness characteristics. Summary of the invention

[0006] The present invention aims to overcome the shortcomings of the prior art that the process is complicated and leads to high cost, and the low-temperature impact energy of -60°C does not exceed 80J, and provide a 700MPa grade high-strength steel with excellent low-temperature toughness and a production method, which can reduce the overall production cost by at least 5% while ensuring the yield strength ≥700MPa, the tensile strength ≥750MPa, the elongation after fracture ≥18%, and the low-temperature impact energy at -60°C ≥100J.

[0007] Measures to achieve the above objectives:

[0008] A 700MPa grade high-strength steel with excellent low-temperature toughness, the components and weight percentage contents of which are: C: 0.10-0.18%, Si: 0.10-0.25%, Mn: 1.2-1.5%, P: ≤0.015%, S: ≤0.005%, Cr: 0.10-0.30%; Ti: 0.01-0.03%, B: 0.001-0.003%; Als: 0.02-0.05%, N≤0.006%, and the balance is Fe and unavoidable impurities.

[0009] Preferably, the weight percentage of Mn is 1.26-1.45%.

[0010] Preferably, the weight percentage of Cr is 0.15-0.26%.

[0011] Preferably: the weight percentage content of B is 0.0018-0.0025%.

[0012] A method for producing a 700MPa grade high strength steel with excellent low temperature toughness, comprising the following steps:

[0013] 1) After desulfurization of molten iron, conventional smelting is performed to the set composition;

[0014] 2) After being cast into a slab, the slab is heated, and the heating temperature of the slab is controlled at 1210-1260°C, and the insulation time is 140-170 minutes;

[0015] 3) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 280-420 bar;

[0016] 4) After conventional rough rolling, finish rolling is carried out, and the reduction ratio of the first two passes is controlled to be greater than 20%, and the cumulative reduction ratio of the first three passes is ≥50%. The final rolling temperature is controlled at 810-870℃; the final thickness of the steel plate is 8-

[0017] 20mm;

[0018] 5) Cool to the coiling temperature at a cooling rate of 10-30°C / s without water spraying in the latter stage;

[0019] 6) Coil the steel sheet and control the coiling temperature at 556-564°C;

[0020] 7) Perform quenching, control the quenching temperature at 880-930°C, and keep warm at this temperature. The holding time is as follows:

[0021] Steel plate thickness t+10min;

[0022] Where: t is the thickness of the steel plate after finish rolling in mm;

[0023] 8) Tempering, control the tempering temperature at 500-580°C, and keep warm at this temperature. The holding time is as follows:

[0024] Steel plate thickness t+10min;

[0025] Where: t is the thickness of the steel plate after finish rolling in mm;

[0026] 9) Set aside.

[0027] Preferably, the quenching temperature is between 885°C and 923°C.

[0028] Preferably, the tempering temperature is between 525 and 568°C.

[0029] Functions and mechanisms of the components and main processes in the present invention

[0030] C: Carbon is a cheap solid solution strengthening element. If its content is less than 0.10%, it cannot meet the hardenability requirements well; if its content is greater than 0.18%, it is easy to precipitate too many carbides during the subsequent tempering process, affecting the toughness of the material. At the same time, too high C content will also affect the welding performance of the material. Therefore, its content is limited to 0.10-0.18%.

[0031] Si: Silicon can purify ferrite and avoid the formation of coarse carbides during cooling, but it is also easy to form Fe2SiO4 and form a eutectic product with FeO on the surface of the steel billet, and solidify into an anchor-like structure, making it difficult to remove FeO and affecting the final surface quality. Therefore, the silicon content is limited to ≤0.25%.

[0032] Mn: Manganese is the most effective element for improving strength and toughness, and can effectively delay pearlite transformation. If its content is less than 1.2%, the material's hardenability and strength requirements cannot be met; manganese also affects the material's welding performance; on the basis of meeting the strength requirements and having good weldability, the manganese content is limited to 1.2-1.5%.

[0033] P: Phosphorus is prone to central segregation, affecting the formability; an appropriate P content can reduce steelmaking costs; after comprehensive consideration, the phosphorus content is controlled to ≤0.015% in the present invention.

[0034] S, N: They are easy to combine with Ti in steel, and high content will lead to sulfide inclusions, affecting the toughness and plasticity of steel; stricter sulfur and nitrogen requirements will increase steelmaking costs. In the present invention, sulfur is controlled at ≤0.005% and nitrogen is controlled at ≤0.006%.

[0035] Cr: Chromium is a carbide-forming element with a strong affinity for carbon, which can hinder the diffusion of carbon atoms. Together with the combined effect of manganese, it significantly delays the transformation of pearlite and bainite, thereby improving the hardenability of the material. In the present invention, the chromium content is controlled to 0.10-0.30%.

[0036] Ti: Titanium is a relatively economical strengthening element. By adding a small amount of Ti, the grains can be refined and the strength and toughness of the material can be improved. In the present invention, the chromium content is controlled to be 0.01-0.03%.

[0037] B: Boron is a strong hardenability element. The addition of B element can greatly improve the hardenability of the material, so that the material has a uniform and refined structure. In the present invention, the chromium content is controlled to 0.001-0.003%.

[0038] Als: Aluminum is an effective deoxidizer in the smelting process and has a certain grain refinement effect, which improves the strength of steel, but it is also easy to form Al2O3 inclusions. In the present invention, the chromium content is controlled at 0.02-0.05%.

[0039] The reason why the reduction rates of the first two passes of the finishing rolling are controlled to be greater than 20%, the cumulative reduction rate of the first three passes is greater than or equal to 50%, and the final rolling temperature is controlled at 810-870°C, is that the austenite grains are fully refined by large reduction in the first three passes, and then the material is rolled at a low final rolling temperature so that a large deformation energy is accumulated in the austenite non-recrystallization zone, which can provide more nucleation points in the subsequent phase transformation process and fully refine the grain size.

[0040] The reason why the coiling temperature is controlled within the range of 556-564° C. in the present invention is that relatively fine original structures and moderate mechanical properties can be obtained within this temperature range.

[0041] The present invention controls the quenching temperature to 880-930°C and performs heat preservation at this temperature. The heat preservation time is as follows:

[0042] The steel plate thickness is t+10min. The quenching temperature and time are adopted for two considerations: first, on the basis of ensuring that the material is fully austenitized, the austenite grains will not be significantly coarsened; second, for energy saving considerations.

[0043] The present invention controls the tempering temperature at 500-580°C and performs heat preservation at this temperature. The heat preservation time is as follows:

[0044] The thickness of the steel plate is t+10min. Tempering in this temperature range can obtain the best matching performance of strength and toughness under this composition system.

[0045] Compared with the prior art, the present invention has the advantages of ensuring yield strength ≥ 700MPa and tensile strength

[0046] ≥750MPa, elongation after fracture ≥18%, and low-temperature impact energy of the steel plate at -60℃ ≥100J, the comprehensive production cost can be reduced by at least 5%. DETAILED DESCRIPTION

[0047] The present invention is described in detail below:

[0048] Table 1 is a list of chemical components of various embodiments and comparative examples of the present invention;

[0049] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0050] Table 3 is a table of performance test results of various embodiments and comparative examples of the present invention.

[0051] Each embodiment of the present invention is produced according to the following steps

[0052] 1) After desulfurization of molten iron, conventional smelting is performed to the set composition;

[0053] 2) After being cast into a slab, the slab is heated, and the heating temperature of the slab is controlled at 1210-1260°C, and the insulation time is 140-170 minutes;

[0054] 3) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 280-420 bar;

[0055] 4) After conventional rough rolling, finishing rolling is carried out, and the reduction ratio of the first two passes is controlled to be greater than 20%, and the cumulative reduction ratio of the first three passes is ≥50%, and the final rolling temperature is controlled at 810-870°C; the final thickness of the steel plate is 8-20mm;

[0056] 5) Cool to the coiling temperature at a cooling rate of 10-30°C / s without water spraying in the latter stage;

[0057] 6) Coil the steel sheet and control the coiling temperature at 556-564°C;

[0058] 7) Perform quenching, control the quenching temperature at 880-930°C, and keep warm at this temperature. The holding time is as follows:

[0059] Steel plate thickness t+10min;

[0060] Where: t is the thickness of the steel plate after finish rolling in mm;

[0061] 8) Tempering, control the tempering temperature at 500-580°C, and keep warm at this temperature. The holding time is as follows:

[0062] Steel plate thickness t+10min;

[0063] Where: t is the thickness of the steel plate after finish rolling in mm;

[0064] 9) Set aside.

[0065] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt%)

[0066]

[0067] Table 1

[0068]

[0069]

[0070] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0071]

[0072] Table 2

[0073]

[0074]

[0075] Table 3 List of mechanical properties test results of various embodiments of the present invention and comparative examples

[0076]

[0077] As can be seen from Table 3, compared with the comparative example, the present design can obtain relatively excellent strength and low-temperature toughness based on a relatively economical composition design, with a yield strength of more than 750MPa, a tensile strength of more than 850MPa, an elongation after fracture of more than 18%, and a low-temperature impact energy of -60°C of more than 130J. Under the condition of achieving the above excellent performance, the present invention has been tested to reduce its comprehensive cost by more than 5% compared with the prior art.

[0078] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A 700MPa grade high-strength steel with excellent low-temperature toughness, the components and weight percentages of which are: C: 0.10-0.18%, Si: 0.10-0.25%, Mn: 1.2-1.5%, P: ≤0.015%, S: ≤0.005%, Cr: 0.10-0.30%; Ti: 0.01-0.03%, B: 0.001-0.003%; Als: 0.02-0.05%, N≤0.006%, and the balance is Fe and unavoidable impurities.

2. The 700MPa grade high-strength steel with excellent low-temperature toughness according to claim 1, characterized in that: The weight percentage of Mn is 1.26-1.45%.

3. The 700 MPa grade high strength steel with excellent low temperature toughness as claimed in claim 1, characterized in that: The weight percentage content of Cr is 0.15-0.26%.

4. The 700 MPa grade high strength steel with excellent low temperature toughness as claimed in claim 1, characterized in that: The weight percentage content of B is 0.0018-0.0025%.

5. A method for producing a 700 MPa grade high-strength steel with excellent low-temperature toughness as claimed in claim 1, comprising the steps of: 1) After desulfurization of molten iron, conventional smelting is performed to the set composition; 2) After being cast into a slab, the slab is heated, and the heating temperature of the slab is controlled at 1210-1260°C, and the insulation time is 140-170 minutes; 3) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 280-420 bar; 4) After conventional rough rolling, finishing rolling is carried out, and the reduction ratio of the first two passes is controlled to be greater than 20%, and the cumulative reduction ratio of the first three passes is not less than 50%, and the final rolling temperature is controlled at 810-870°C; the final thickness of the steel plate is 8-20mm; 5) Cool to the coiling temperature at a cooling rate of 10-30°C / s without water spraying in the latter stage; 6) Coil the steel sheet and control the coiling temperature at 556-564°C; 7) Perform quenching, control the quenching temperature at 880-930°C, and keep warm at this temperature. The holding time is as follows: Steel plate thickness t+10min; Where: t is the thickness of the steel plate after finish rolling in mm; 8) Tempering, control the tempering temperature at 500-580°C, and keep warm at this temperature. The holding time is as follows: Steel plate thickness t+10min; Where: t is the thickness of the steel plate after finish rolling in mm; 9) Set aside.

6. The method for producing a 700 MPa grade high-strength steel with excellent low-temperature toughness according to claim 5, characterized in that: The quenching temperature is between 885 and 923°C.

7. The method for producing a 700 MPa grade high-strength steel with excellent low-temperature toughness according to claim 5, characterized in that: The tempering temperature is between 525 and 568°C.

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