Low-yield-ratio high-strength steel plate and manufacturing method thereof

Abstract

The invention relates to a low-yield-ratio high-strength steel plate and a manufacturing method thereof, and belongs to the field of metallurgy. The metallographic structure of the steel plate is bainite, ferrite and nanoscale TiC precipitates, the proportion of the ferrite is 10%-20%, and the grain size is larger than or equal to 7 grades. The yield strength Rp0.2 is greater than or equal to 700MPa, the tensile strength Rm is greater than or equal to 850MPa, the yield ratio is less than or equal to 0.86, the cold bending property d is equal to 2a, and no crack is generated during 90-degree cold bending. The steel plate has high strength, high toughness and excellent cold bending performance, and can be widely applied to manufacturing of large-scale engineering machinery equipment or steel structures with strict requirements on formability and weldability, such as high-rise buildings, bridges, ocean platforms and the like.

Description

Technical Field

[0001] The invention belongs to the field of iron and steel metallurgy, and in particular relates to a low yield ratio high-strength steel plate and a manufacturing method thereof. Background Art

[0002] With the rapid development of the world's steel structure industry, building bridges, engineering machinery, mining machinery and other industries, the demand for high-strength steel plates with a yield strength of 690MPa is becoming increasingly strong. Since high-strength steel plates often need to be processed into various shapes during use to meet the needs of engineering machinery and equipment assembly, the cold bending performance of steel plates is crucial for the application of high-strength steel plates.

[0003] Traditional high-strength steel plates with a yield strength of 690MPa are generally produced by quenching + high-temperature tempering, which can improve strength while also satisfying toughness. However, in order to obtain high strength, this production method often uses precious metal alloys such as Cr, Ni, and Mo, which is costly and has a long production process. At the same time, it also leads to high carbon equivalent, poor welding performance, and decreased cold bending performance. How to use a short process and low cost method to produce easy-to-weld high-strength steel plates with low yield ratio has gradually become one of the important development directions of major steel companies.

[0004] Patent publication number CN104988429A discloses a structural steel plate for bridges with a yield strength of 690MPa. The invention is produced using the traditional quenching + tempering process, and has a low carbon content (C: 0.04~0.08%). In order to improve strength and toughness, the invention adds precious metals such as Cr / Mo / Ni / Cu, which is costly and not conducive to promotion.

[0005] Patent publication number CN101418418A discloses a low crack sensitivity steel plate with a yield strength of 690MPa and a manufacturing method thereof. The invention adopts a low-carbon bainite design (C: 0.03~0.06%) with good crack resistance; the TMCP process is used in the process. In order to increase the hardenability, the invented steel plate is added with an appropriate amount of Cr, Mo, and B alloys, and a higher V content (V: 0.04~0.12%) is added. The invention has a short production process, but the addition of Cr / Mo / V / B alloys leads to higher costs.

[0006] Patent publication number CN104278216A discloses a structural steel plate that combines online quenching technology and medium carbon content design (C: 0.10-0.20%) to develop a structural steel plate for engineering machinery with a yield strength of 690MPa. The steel plate also contains hardenability elements such as Cr, Ni, Mo, and B.

[0007] In summary, the existing production technologies for high-strength steel plates with a yield strength of 690MPa mainly include quenching + tempering or online TMCP; the composition design mostly adopts Cr-Mo-B alloying to obtain a martensite or bainite structure with higher strength. However, martensite or bainite has high strength and low toughness, and the steel plate can generally only meet the cold bending requirements of d≥3a (d is the cold bending radius, a is the thickness of the bent steel plate). Summary of the invention

[0008] The purpose of the present invention is to provide a high-strength steel plate with a yield strength Rp0.2≥700MPa and a tensile strength Rm≥850MPa, wherein the yield strength of the steel plate is ≤0.86, the cold bending performance satisfies: d=2a, no cracks in 90° cold bending, a carbon equivalent CEV of 0.38~0.45%, and good welding performance.

[0009] The technical solution adopted by the present invention to solve the above problems is: a low yield ratio high strength steel plate, the chemical composition of which is calculated by mass percentage as follows: C: 0.04-0.08%, Si: <0.30%, Mn: 1.50-2.00%, Nb: 0.010%-0.080%, V: ≤0.020%, Ti: 0.01%-0.03%, Al: 0.02-0.07%, Ni: <0.10%, Cu: <0.10%, Cr: <0.01%, Mo: <0.010%, B: <0.003%, Ca: 0.001-0.005%, P: ≤0.012%, S: ≤0.002%, O: ≤0.0025%, N: ≤0.0050%, H: ≤0.0002%, and the balance is Fe and unavoidable impurity elements; carbon equivalent CEV: 0.36-0.45%.

[0010] The thickness of the low yield ratio high strength steel plate of the present invention is ≤30 mm.

[0011] The reasons for limiting the steel composition in the present invention are as follows: C: Carbon as a solid solution element can significantly improve the strength of the steel plate, but it has an adverse effect on the plastic toughness, cold bending performance and welding performance of the steel plate. Based on the strength and toughness matching and cold bending performance requirements of the steel plate, the carbon content in the present invention is controlled to be 0.04-0.08%.

[0012] Si: Si enters ferrite to strengthen solid solution, but when the Si content is too high, oxidation spots are easily formed on the surface of the steel plate during rolling, the surface quality is reduced, and local stress concentration occurs during cold bending, affecting the cold bending performance of the steel plate. The Si content of the present invention is less than 0.30%, the surface quality of the steel plate is excellent, and it is conducive to improving the cold bending performance.

[0013] Mn: reduces the critical transformation temperature Ar3, improves the hardenability of the steel, plays a role in solid solution strengthening in the steel, and significantly improves the strength of the steel. The present invention stipulates that the amount of manganese added is within the range of 1.50-2.00%.

[0014] Nb: It can produce significant grain refinement, precipitation strengthening and precipitation strengthening effects. Nb dissolved in austenite can improve hardenability, and the carbide precipitation phase of Nb has the effect of refining grains and improving matrix strength. In addition, the addition of relatively high Nb can increase the non-recrystallization temperature Tnr of steel plate rolling, so that finishing rolling can be carried out in a higher temperature range. In the present invention, the Nb content is controlled to be 0.010~0.080%. The main purposes are (1) to increase the finishing rolling temperature so that the steel plate can be deformed at high temperature without recrystallization, fully refine the grains, and at the same time be beneficial to the surface quality of the steel plate and the cold bending forming performance of the steel plate; (2) the addition of relatively high Nb can precipitate a large amount of nano-NbC in the subsequent rolling and tempering process, significantly improving the strength of the steel plate.

[0015] Ti: An important alloying element of the present invention, a strong carbonitride-forming element, has the effect of refining grains during high-temperature rolling, improving the toughness and low-temperature toughness of steel; the nano-scale TiC precipitated during the low-temperature process can greatly improve the strength of the steel plate. The present invention eliminates the Cr-Mo-B alloying, and mainly utilizes the low-temperature and large-scale precipitation of nano-scale TiC to ensure that the yield strength of the steel plate is higher than 700MPa. However, when the Ti content is too high, especially when it is greater than 0.20%, it will lead to a decrease in the continuous casting quality, the formation of cracks, and the deterioration of the toughness of the steel plate. Therefore, the present invention controls the Ti content to 0.010%~0.030%.

[0016] V: Grain refinement element; it can play a certain precipitation strengthening role during low temperature tempering. When the V content is too high, the toughness and welding performance of the steel plate will deteriorate. Therefore, the V content of the present invention is controlled to be ≤0.020%.

[0017] Al: deoxidizing and grain refining element. In the present invention, Al also plays the role of fixing N and preserving Ti, avoiding the formation of a large amount of TiN and reducing the strength and cold bending performance of the steel plate. The present invention stipulates that the Al content is 0.02-0.07%.

[0018] Ca: inclusion modifier, can react with long strips of MnS to generate spherical CaS, change the anisotropy of the steel plate, and also deoxidize Al to produce Al 2 O 3 The inclusions are transformed into spherical low-melting-point inclusions, which promote the floating and removal of inclusions and improve the toughness and cold bending performance of the steel plate.

[0019] P: a harmful element in steel, which is easy to segregate and has an adverse effect on the plasticity and toughness of the material. A high P content increases the brittleness of the steel plate and easily causes cold bending cracking of the steel plate. The present invention stipulates that P: ≤0.012%.

[0020] S: harmful element in steel, easy to segregate. Mn element easily forms MnS segregation inclusions, leading to delamination and cracking of steel plate, significantly reducing the cold bending performance of steel plate. Since the present invention adopts high Mn design, the S content must be controlled at a low level, and S is specified to be ≤0.002%.

[0021] O, H: harmful gas elements, high content, many inclusions, reduce the plastic toughness of the steel plate, and high risk of cracking of the steel plate. The present invention strictly controls the O content to be no higher than 0.0025%; the H content to be no higher than 0.0002%.

[0022] N: harmful gas. The present invention uses nano-TiC to increase the strength of the steel plate. A high N content will form TiN inclusions with sharp corners, affecting the cold bending performance of the steel plate. Therefore, the present invention strictly controls the N content and stipulates that it is not higher than 0.0050%; CEV: The present invention adopts the carbon equivalent formula CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; carbon equivalent has a great influence on the strength and weldability of steel. High CEV has high strength, but reduced weldability; the present invention controls CEV to 0.36~0.45%.

[0023] The present invention further provides a method for preparing the above-mentioned low yield ratio high strength steel plate, the specific process is as follows: Smelting and continuous casting process: Pre-treatment of molten iron, smelting in electric furnace or converter, then sent to LF refining furnace for refining, and VD or RH vacuum treatment. Trace Ca treatment is carried out after degassing of molten steel. Continuous casting adopts low superheat pouring, argon protection pouring throughout the process, and dynamic soft pressure control. The superheat of molten steel is controlled at 5~20℃, the center segregation is not higher than C1.0 level, and the center looseness is not higher than 1.0 level.

[0024] Heating process: The ingot is heated in a walking beam heating furnace to austenitize the structure and fully dissolve the elements. In order to reduce the impact of iron oxide on the surface quality of the steel plate and refine the grains, the present invention adopts a lower heating temperature, the heating temperature of the ingot is controlled at 1100~1180℃, and the heating time is 8~14min / cm.

[0025] Steel rolling process: high-pressure water descaling is carried out before rolling the ingot, and the descaling pressure at the nozzle is controlled to be above 21MPa. The rough rolling temperature is ≥1000℃. High-pressure descaling is carried out in the odd-numbered passes and the last pass of rough rolling to fully remove the primary and secondary iron oxide scales. The temperature of the finishing rolling is controlled at ≥930℃. High-pressure water descaling is used for finishing rolling, and the total number of descaling passes shall not be less than 3 passes. The final rolling temperature is controlled at a lower 780~840℃, so that the grains are fully deformed and refined, and the strength, toughness and cold bending performance of the steel plate are improved. After rolling, the steel plate is accelerated cooled, the cooling rate is ≥15℃ / s, and the final cooling temperature is controlled at 400~550℃ to produce more than 10% ferrite, followed by air cooling.

[0026] Tempering process: To ensure the precipitation of a large amount of nano-TiC, the steel plate needs to be tempered at a lower temperature. The tempering temperature is 450~550℃. After the furnace temperature reaches the temperature, the insulation time is 30~90min, and air cooling is performed to room temperature. To ensure the uniformity of the steel plate, the temperature control accuracy is preferably ±10℃.

[0027] Compared with the prior art, the present invention has the following advantages: The present invention adopts low C and low Si chemical composition design, eliminates the addition of precious metals such as Cr / Ni / Mo / Cu, and adopts economical Nb-V-Ti composite, which greatly reduces the alloy cost. The steel plate carbon equivalent CEV: 0.36~0.45%, with excellent welding performance.

[0028] In terms of technology, the present invention adopts the TMCP short process to replace the traditional quenching process, which saves costs, releases production capacity, and reduces carbon emissions. The present invention performs tempering at a relatively low temperature to disperse and precipitate a large amount of nano-level TiC and NbC, significantly increasing the strength and plastic toughness of the steel plate.

[0029] The matrix structure of the present invention is martensite and fine ferrite, and the proportion of ferrite is more than 10% by area, 10% to 20%, and the grain size is ≥ level 7. Different from the martensite or bainite structure of traditional high-strength steel with a yield strength of 700MPa, the ferrite matrix is ​​softer, which greatly improves the cold bending performance of the steel plate.

[0030] The yield strength of the steel plate is Rp0.2≥700MPa, the tensile strength is Rm≥850MPa, and the elongation is A≥14%; the low-temperature impact of -40℃ is Akv≥100J. The low yield strength ratio of the steel plate is less than or equal to 0.86, d=2a, and there is no crack in 90° cold bending.

[0031] The method of the present invention can be extended and applied to other high-strength steel plates, such as high-strength offshore ship plates, high-rise building plates, bridge plates, engineering machinery plates, pressure vessel plates, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1This is a SEM electron microscope scanning image (1000×) of the typical structure of the test steel in Example 1 of the present invention.

[0033] Figure 2 This is a SEM electron microscope scanning image (30000×) of typical nano-scale precipitates in the test steel of Example 1 of the present invention.

[0034] Figure 3 This is the energy spectrum analysis of nano-scale precipitates in the test steel of Example 2 of the present invention.

[0035] Figure 4 This is a cold bending photo of Example 2 of the present invention. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0037] The production process of the high-strength steel in this embodiment is: converter steelmaking->LF refining->VD or RH vacuum degassing->Ca treatment->continuous casting->heating->rolling cooling->tempering.

[0038] The production method of the high-strength steel plate with a yield strength of 700 MPa of Examples 1 and 2 of the present invention comprises the following steps: (1) Steelmaking and continuous casting: 150-ton converter is used for smelting, then sent to LF furnace for refining and RH vacuum degassing treatment, and Ca treatment is performed by breaking the air. The molten steel is cast into 150 mm thick continuous casting billets. The casting temperature is controlled at 5~20℃ above the liquidus. Dynamic soft reduction is implemented during the casting process. The center segregation is not higher than C1.0 level, and the center porosity is not higher than 1.0 level. The final composition control of the molten steel is shown in Table 1.

[0039] (2) Rolling and cooling: The continuous casting billet obtained in step (1) is placed in a walking beam heating furnace and heated to 1100-1180°C for 8-14 min / cm2 to fully dissolve the alloying elements in the steel to ensure the uniformity of the composition and performance of the final product. After the billet is removed from the furnace, it is descaled by high-pressure water and then subjected to two-stage controlled rolling of rough rolling + finishing rolling. The rolling temperature in the rough rolling recrystallization zone is controlled at 1000-1100°C; the start rolling temperature of the finishing rolling is controlled at ≥930°C, and the final rolling temperature is controlled at 780-840°C to ensure that the steel plate finally obtains fine original austenite grains and improves the strength, toughness and cold bending performance of the steel plate. After rolling, the steel plate is accelerated cooled at a cooling rate of ≥15°C / s, and the final cooling temperature is controlled at 400-550°C, followed by air cooling. The relevant process parameters are shown in Table 2.

[0040] (3) Tempering: The tempering temperature of the steel plate is 450~550℃. After the furnace temperature reaches the temperature, the insulation time is 30~90min.

[0041] The specific composition and process parameters are shown in Tables 1 to 3. The corresponding properties of each example sample are shown in Table 4.

[0042] Figure 1 The microstructure photo of the test steel of Example 1 is given. The steel plate matrix contains fine ferrite structure, and the grain size of the structure is ≥ 7 levels, which ensures that the steel plate has excellent cold bending performance.

[0043] Figure 2 The SEM image of typical nano-scale precipitates of the test steel in Example 1 is given.

[0044] Figure 3 This is the energy spectrum analysis of nano-scale precipitates in the test steel of Example 2. It can be seen that a large amount of nano-scale TiC and NbC are dispersed on the ferrite matrix of the steel plate, ensuring that the steel plate has excellent plasticity and toughness while also having ultra-high strength.

[0045] Figure 4 This is a cold bending picture of the test steel of Example 2. The cold bending radius d=2a, 90° without cracks, showing excellent cold bending forming performance. Table 1 Chemical composition of steel plates in various examples (wt%) Example C Si Mn P S Al Nb Ti V O N H CEV 1 0.06 0.05 1.95 0.010 0.001 0.04 0.060 0.013 0.015 0.0010 0.0035 0.0001 0.39 2 0.07 0.06 1.90 0.008 0.001 0.05 0.055 0.012 0.010 0.0008 0.0030 0.0001 0.40 Table 2 Rolling process control Example Product thickness specification mm Rough rolling temperature℃ Finish rolling start temperature ℃ Finishing rolling temperature℃ Final cooling temperature℃ 1 8 1010 960 790 470 2 12 1030 940 800 490 Table 3 Heat treatment process control Example Product thickness specification mm Tempering temperature℃ Tempering holding time min 1 8 500 60 2 12 500 60 Table 4 Tensile, impact, bending properties, and grain size of the embodiments of the present invention

[0046] The present invention adopts a high-cleanliness steelmaking continuous casting process, controlled rolling, controlled cooling and tempering process, and controls from the perspectives of chemical composition design, parent material structure, rolling deformation, tempering temperature and time, so as to ensure that the steel plate has high strength, high toughness and excellent cold bending performance. It can be widely used in the manufacture of large-scale engineering machinery equipment or steel structures such as high-rise buildings, bridges, and offshore platforms that have strict requirements on formability and weldability.

Claims

1. A low yield ratio high strength steel plate, characterized in that: The element composition by mass percentage is C: 0.04~0.08%, Si: <0.30%, Mn: 1.50~2.00%, Nb: 0.010%~0.080%, V: ≤0.020%, Ti: 0.01%~0.03%, Al: 0.02~0.07%, Ni: <0.10%, Cu: <0.10%, Cr: <0.01%, Mo: <0.010%, B: <0.003%, Ca: 0.001~0.005%, P: ≤0.012%, S: ≤0.002%, O: ≤0.0025%, N: ≤0.0050%, H: ≤0.0002%, the balance is Fe and unavoidable impurity elements; carbon equivalent CEV: 0.36~0.45%.

2. The steel plate according to claim 1, characterized in that: The production thickness is ≤30mm, the metallographic structure is bainite, ferrite and nano-scale TiC precipitates. Calculated by area percentage, the proportion of ferrite is 10%-20%, and the ferrite grain size is ≥7.

3. The steel plate according to claim 1, characterized in that: Yield strength Rp0.2≥700MPa, tensile strength Rm≥850MPa, yield strength ratio≤0.86, elongation A≥14%; -40℃ low temperature impact Akv≥100J, cold bending performance: d=2a, no cracks in 90° cold bending.

4. A method for manufacturing the steel plate according to claim 1, characterized in that: include, Smelting and continuous casting process: smelting molten steel according to elemental composition, and casting the molten steel into billets using continuous casting process; Heating process: The blank is heated in the furnace, the structure is austenitized, and the elements are fully dissolved; Rolling process: Before rolling, the billet is descaled with high-pressure water. The rolling process is divided into two stages: rough rolling and finishing rolling. The rough rolling temperature is ≥1000℃. The odd-numbered passes and the last pass of rough rolling are descaled under high pressure to remove the primary and secondary iron oxide scales. The start temperature of finishing rolling is controlled at ≥930℃. High-pressure water descaling is used for the start of finishing rolling. The total number of descaling passes in the finishing rolling stage is not less than 3. The final rolling temperature is controlled at 780~840℃ to allow the grains to be fully deformed and refined. After rolling, the steel plate is accelerated cooled at a cooling rate of ≥15℃ / s. The final cooling temperature is controlled at 400~550℃, so that more than 10% ferrite is produced during the cooling period, followed by air cooling. Tempering process: Set the tempering temperature at 450~550℃. After the furnace temperature reaches the temperature, keep it warm for 30~90min, and then air cool it to room temperature to ensure sufficient precipitation of nano-level TiC.

5. The method according to claim 4, characterized in that: Molten steel smelting includes molten iron pretreatment, electric furnace or converter smelting, LF refining furnace, vacuum degassing treatment, and Ca treatment after molten steel degassing; continuous casting adopts low superheat pouring, argon protection throughout the process and dynamic light pressure, the superheat of molten steel is controlled at 5-20℃, the center segregation is not higher than C1.0 level, and the center porosity is not higher than 1.0 level.

6. The method according to claim 4, characterized in that: In the heating step, the heating temperature is set to 1100-1180° C., and the heating time is set to 8-14 min / cm*steel plate thickness.

7. The method according to claim 4, characterized in that: During the tempering step, the temperature was controlled with an accuracy of ±10°C.

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