Q690D high-strength steel and a preparation method thereof

Abstract

The application belongs to the technical field of metallurgy, and particularly relates to a Q690D high-strength steel plate and a preparation method thereof. The application controls the LF final slag basicity and the CaO / Al2O3 ratio, formulates a reasonable argon blowing system, improves the adsorption efficiency of LF on inclusions, controls the RH calcium treatment timing and the calcium wire addition amount, considers the castability and the inclusion removal efficiency, improves the cleanliness of molten steel, reduces the total amount of inclusions and large-size inclusions, and makes the D-class and Ds-class inclusions in rolled materials all less than 1.0 grade, so that the flaw detection qualified rate of the steel plate is increased to more than 99.9%.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a Q690D high-strength steel and its preparation method. Background Technology

[0002] Q690D steel is a low-alloy high-strength steel with a yield strength of 690MPa. It is widely used in marine engineering, oil and gas pipelines, construction, bridges, energy, and engineering machinery, among other fields. It is generally required to be delivered in a flaw-tested or flaw-tested + heat-treated condition. However, flaw detection discrepancies frequently occur, affecting steel plate delivery and subsequent product orders. The most typical flaw detection discrepancy is due to abnormal non-metallic inclusions in the steel plate. These inclusions have different properties than the steel and behave inconsistently during processing, leading to stress concentration around the inclusions and making them prone to microcracks, thus causing flaw detection discrepancies.

[0003] Chinese patent application CN117187695 A discloses a Q690D high-strength steel plate and its preparation method. The Q690D high-strength steel plate, by weight percentage, comprises: C: 0.06-0.2%, Si: 0.1-0.6%, Mn: 0.8-2.0%, P≤0.03%, S≤0.02%, Nb: 0.01-0.05%, V: 0.08-0.12%, N: 0.01-0.02%, Ni: 0.2-0.3%, Cr: 0.3-0.5%, with the balance being iron and unavoidable impurities. The production process of this invention includes converter smelting, LF refining, continuous casting, heating, rolling, and cooling. The steelmaking process does not involve vacuum degassing refining; the continuous casting billet is heated using hot charging; and no heat treatment is required after rolling, reducing energy consumption and lowering production costs. However, this patent application is aimed at reducing the production of Q690D high-strength steel through a short process and reducing energy consumption, and it still has the problem of non-compliance in flaw detection. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a Q690D high-strength steel and its preparation method, which reduces the number and size of inclusions, improves product quality, and increases the steel plate flaw detection pass rate to over 99.9%.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides a Q690D high-strength steel, the chemical composition of which, by weight percentage, comprises: C: 0.14-0.2%, Si: 0.2-0.4%, Mn: 1.1-1.6%, P≤0.015%, S≤0.005%, Cr: 0.1-0.4%, Nb: 0.01-0.04%, Ti: 0.01-0.03%, Mo: 0.1-0.4%, Als: 0.01-0.045%, B: 0.001-0.003%, CEV≤0.62, with the balance being iron and unavoidable impurities.

[0007] This invention further provides a method for preparing Q690D high-strength steel, comprising the following steps:

[0008] 1) Converter smelting:

[0009] The smelting process is carried out in a top-and-bottom blown converter, and the ladle is a clean, bottom-blown ladle with a baking temperature of ≥800℃.

[0010] Molybdenum oxide briquettes are placed at the front of the scrap steel hopper and added together with the scrap steel. The amount of molybdenum oxide briquettes added is 4.38-8.77 kg / t of steel.

[0011] The molybdenum oxide briquettes are made by pressing molybdenum oxide powder with 10% reduction accelerator, 7% volatilization inhibitor, and 3% water. The reduction accelerator and volatilization inhibitor used in this invention are commonly used raw materials in the field and are readily available for purchase.

[0012] Molybdenum oxide briquettes: Mo: 48%-55%, Si≤0.1%, P≤0.05%.

[0013] The carbon content at the smelting endpoint is controlled at 0.05-0.09%, the tapping temperature is controlled at 1630-1650℃, and the final slag basicity is controlled within the range of 3.0-4.0.

[0014] The steel is alloyed using silicon manganese, high-carbon ferromanganese, medium-carbon ferrochrome, and ferroniobium, with the following addition amounts: silicon manganese 14.15-16.18 kg / t steel, high-carbon ferromanganese 2.65-4.65 kg / t steel, medium-carbon ferrochrome 2.6-3.5 kg / t steel, and ferroniobium 0.23-0.31 kg / t steel. The alloys are added when one-fifth to three-quarters of the steel has been poured, and the alloys are added to the impact zone of the steel flow.

[0015] Silicon-manganese: Mn: 60%-70%, Si: 16.5%-20%, C≤1.8%, P≤0.25%, S≤0.04%;

[0016] High-carbon ferromanganese: Mn: 65%-72%, Si≤2.5%, C≤7%, P≤0.25%, S≤0.03%;

[0017] Medium carbon ferrochrome: Cr ≥ 52%, C ≤ 2.0%, Si ≤ 3.0%, P ≤ 0.06%, S ≤ 0.05%;

[0018] Ferroniobium: Nb: 50%-60%, Si≤2.5%, Al≤2.0%, C≤0.05%, P≤0.05%, S≤0.03%;

[0019] Aluminum manganese ferrooxidant is used for deoxidation. The amount of aluminum manganese ferrooxidant added is 2.0-2.5 kg / t of steel. Additional aluminum manganese ferrooxidant is added depending on the degree of over-oxidation of the molten steel.

[0020] When feeding steel, a sliding plate is used to block slag, and top lime is added along with the steel flow.

[0021] The use of molybdenum oxide to provide molybdenum to steel involves the chemical reaction between a reducing agent and molybdenum oxide during the steelmaking process to alloy the molybdenum element. In the early and middle stages of converter blowing, the molten pool temperature is low, the volatilization rate of molybdenum oxide is low, and the molten pool contains high levels of elements such as [C], [Si], and [Mn], which can undergo a reduction reaction with molybdenum oxide. Therefore, molybdenum oxide briquettes are placed at the front of the scrap steel hopper and added together with the scrap steel, and the molybdenum recovery rate can reach 96.7-97%.

[0022] 2) LF Refining:

[0023] The LF inlet temperature is 1540-1560℃.

[0024] LF refining adopts bottom blowing argon stirring throughout the process. In the early stage of strong stirring and shell breaking, the argon flow rate is 500-750L / min and the argon blowing stirring time is 2-3min. In the stage of adding slag with electricity to make white slag, the argon flow rate is 100-200L / min. In the stage of white slag refining and fine-tuning composition, the argon flow rate is 100-180L / min. Before leaving the station, a low-pressure soft blowing is adopted, with an argon flow rate of 50-70L / min and a soft blowing time of 5-7min.

[0025] Based on the viscosity, color, and degree of foaming of the slag, lime and calcium carbide are added in batches to adjust the slag. The addition amounts are 200-500 kg / furnace for lime and 40-60 kg / furnace for calcium carbide. Before leaving the station, the top slag should reach a yellow-white slag level, the final slag basicity should be controlled at 5-6, the CaO / Al2O3 ratio of the refining slag should be controlled at 1.8-2.0, and the slag thickness should be 110-120 mm to improve the refining slag's ability to adsorb inclusions in the steel.

[0026] Aluminum wire is used to increase aluminum content, titanium wire to increase titanium content, and ferroboron wire to increase boron content. The addition amounts are 0.76-2.3 m / t steel for aluminum wire, 0.53-0.84 m / t steel for titanium wire, and 0.08-0.12 kg / t steel for ferroboron wire.

[0027] The TS content in the steel when leaving the LF station is 75-80ppm.

[0028] 3) RH refining:

[0029] The RH process uses this operating mode, with the immersion tube inserted to a depth of 400 mm, a vacuum of 120-133 Pa for 15-20 min, and a pure degassing time of 10-15 min, which can be finely adjusted according to the target composition.

[0030] After RH treatment, feed 70-80m / furnace of nano high-calcium wire, with a soft blowing time of 10-12min, a slag thickness of 100-120mm, and control the T.Ca content in the steel at 8-10ppm.

[0031] 4) Continuous casting

[0032] During continuous casting, full-process protective pouring is adopted, with an argon seal flow rate of 90-100 L / min in the ladle and carbonized rice husks covering the liquid surface in the tundish, with an addition amount of 1-1.5 kg / t of steel.

[0033] The crystallizer uses non-sinusoidal vibration, the liquidus temperature is 1510-1520℃, the superheat of the tundish is controlled at 20-30℃, and the pulling speed is 1.1-1.4m / min.

[0034] The continuously cast billet is slowly cooled after leaving the production line, and then piled up to cool to below 400°C.

[0035] The slab thickness is 200-250mm, and the TO content in the continuously cast slab is 6-7ppm.

[0036] 5) Rolling

[0037] Heating of continuously cast billets: tapping temperature is 1180-1210℃, heating rate is 8-10min / cm, and furnace time is 210-230min.

[0038] The total number of rolling passes is 12-16, the total number of roughing passes is 5-7, the total number of finishing passes is 7-9, and the rolling thickness is 24.7-25.05 mm.

[0039] Rough rolling: The initial rolling temperature for rough rolling is 1140-1170℃, and the final rolling temperature for rough rolling is 1050-1120℃.

[0040] Finishing rolling: The initial rolling temperature for finishing rolling is 890-950℃, and the final rolling temperature for finishing rolling is 810-860℃.

[0041] Cooling: Cooling inlet temperature 770-820℃, cooling outlet temperature 670-700℃, cooling rate controlled at 5-10℃ / s.

[0042] 6) Heat treatment

[0043] Quenching temperature: 900-920℃, holding time: 7-13min; Tempering temperature: 510-530℃, holding time: 15-25min.

[0044] The steel plate obtained by this invention has a yield strength of 720-790MPa, a tensile strength of 770-940MPa, an elongation after fracture of 14.5-19.5%, an impact energy of 114-252J at -20℃, and the D-class and Ds-class inclusions in the rolled material are both <1.0 grade.

[0045] Compared with the prior art, the advantages of the present invention are:

[0046] 1. This invention uses molybdenum oxide briquettes instead of ferromolybdenum. Adding 0.01% Mo can save 1-1.5 yuan. The molybdenum content in Q690D high-strength steel is 0.2-0.4%, which can reduce alloy costs by 20-60 yuan per ton of steel. Calculations show that the most economical and suitable manganese alloy for Q690D is silicon manganese + high-carbon ferromanganese. In manganese alloys, the cost of 0.01% Mn in high-carbon ferromanganese is 0.99 yuan, in medium-carbon ferromanganese it is 1.13 yuan, and in metallic manganese it is 1.25 yuan. Using high-carbon ferromanganese can save 0.14-0.26 yuan. The manganese content in Q690D high-strength steel is 1.1-1.6%, which can reduce alloy costs by 15-30 yuan per ton of steel. In summary, this invention can significantly reduce alloy costs.

[0047] 2. This invention improves the adsorption efficiency of LF for inclusions by controlling the basicity of the final slag and the CaO / Al2O3 ratio, and by formulating a reasonable argon blowing system. At the same time, it controls the timing of RH calcium treatment and the amount of calcium wire added, taking into account both castability and inclusion removal efficiency. This results in the TS content in the steel being controlled at 75-80ppm, the T.Ca content at 8-10ppm, and the TO content at 6-7ppm, thereby improving the cleanliness of the molten steel, reducing the segregation of elements in the center of the billet, reducing the total amount of inclusions and large-sized inclusions, and ensuring that both D-class and Ds-class inclusions in the rolled product are <1.0 grade.

[0048] 3. By controlling the heating temperature and rolling method of the continuously cast billet, this invention reduces internal segregation of the billet, fully breaks up the core grains of the billet, reduces the looseness of the weld center, refines the internal grains of the steel plate, improves the internal density of the structure, and increases the steel plate flaw detection pass rate to over 99.9%. Attached Figure Description

[0049] Figure 1 This is an OTS inclusion analysis diagram of the steel plate produced in Example 1.

[0050] Figure 2 This is an OTS inclusion analysis diagram of the steel plate produced in Example 2.

[0051] Figure 3This is an OTS inclusion analysis diagram of the steel plate produced in Comparative Example 1. Detailed Implementation

[0052] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0053] Example 1

[0054] The chemical composition by weight percentage (%) is:

[0055] Brand C Si Mn P S Cr Nb Ti Mo Als B CEV Q690D 0.154 0.25 1.25 0.014 0.004 0.18 0.017 0.018 0.107 0.043 0.0014 0.42

[0056] Main process measures for preparation:

[0057] 1) Converter smelting:

[0058] The smelting process is carried out in a top-and-bottom blown converter, and the ladle is a clean, bottom-blown ladle with a baking temperature of ≥800℃.

[0059] Molybdenum oxide briquettes are placed at the front of the scrap steel hopper and added together with the scrap steel. The amount of molybdenum oxide briquettes added is 5.0 kg / t of steel.

[0060] The carbon content at the smelting endpoint is controlled at 0.06%, the tapping temperature is controlled at 1634℃, and the final slag basicity is controlled within the range of 3.2.

[0061] Ferrosilicon, high-carbon ferromanganese, medium-carbon ferrochrome, and ferroniobium were used for alloying, with addition amounts of 14.3 kg / t steel, 2.95 kg / t steel, 3.0 kg / t steel, and 0.25 kg / t steel, respectively. The alloys were added when one-fifth to three-quarters of the steel was discharged, and the alloys were added to the impact zone of the steel flow.

[0062] Aluminum manganese ferrooxidant is used for deoxidation, with an addition amount of 2.3 kg / t steel.

[0063] When feeding steel, a sliding plate is used to block slag, and top lime is added along with the steel flow.

[0064] 2) LF Refining:

[0065] The LF inlet temperature is 1545℃.

[0066] LF refining adopts bottom blowing argon stirring throughout the process. In the early stage of strong stirring and shell breaking, the argon flow rate is 550L / min and the argon blowing stirring time is 3min. In the stage of adding slag with electricity to make white slag, the argon flow rate is 150L / min. In the stage of white slag refining and fine-tuning composition, the argon flow rate is 150L / min. Before leaving the station, a low-pressure soft blowing is adopted, with an argon flow rate of 60L / min and a soft blowing time of 6min.

[0067] Based on the viscosity, color, and degree of foaming of the slag, lime and calcium carbide are added in batches to adjust the slag, with addition amounts of 300 kg / furnace and 45 kg / furnace, respectively. Before leaving the station, the top slag should reach a yellow-white slag, the final slag basicity should be controlled at 5.5, the CaO / Al2O3 ratio of the refining slag should be controlled at 1.9, and the slag thickness should be 110 mm to improve the refining slag's ability to adsorb inclusions in the steel.

[0068] Aluminum wire is used to increase aluminum content, titanium wire to increase titanium content, and ferroboron wire to increase boron content, with addition amounts of 1.5 m / t steel, 0.6 m / t steel, and 0.1 kg / t steel, respectively.

[0069] 3) RH refining:

[0070] The RH process uses this operating mode, with the immersion tube inserted to a depth of 400 mm, a vacuum of 130 Pa for 15 min, and a pure degassing time of 12 min.

[0071] After RH treatment, feed 70m / furnace of nano high-calcium wire, with a soft blowing time of 11min, a slag thickness of 110mm, and control the T.Ca content in the steel at 8-10ppm.

[0072] 4) Continuous casting

[0073] During continuous casting, full-process protective pouring is adopted, with an argon seal flow rate of 90L / min in the ladle and carbonized rice husks covering the liquid surface in the tundish, with an addition amount of 1.2kg / t steel.

[0074] The crystallizer uses non-sinusoidal vibration, the liquidus temperature is 1510℃, the superheat of the tundish is controlled at 20℃, and the pulling speed is 1.2m / min.

[0075] The slab thickness is 200mm. The continuously cast slab is slowly cooled after leaving the production line and then piled up to cool to below 400℃.

[0076] 5) Rolling

[0077] Heating of continuously cast billets: tapping temperature is 1202℃, heating rate is 9min / cm, and furnace time is 230min.

[0078] The total number of rolling passes is 12, with 5 passes for roughing and 7 passes for finishing, and the rolling thickness is 24.7 mm.

[0079] Rough rolling: The initial rolling temperature for rough rolling is 1163℃, and the final rolling temperature for rough rolling is 1115℃.

[0080] Finishing rolling: The initial rolling temperature for finishing rolling is 899℃, and the final rolling temperature for finishing rolling is 856℃.

[0081] Cooling: Cooling inlet temperature 808℃, cooling outlet temperature 700℃, cooling rate controlled at 7℃ / s.

[0082] 6) Heat treatment

[0083] The quenching temperature is 900℃ and the holding time is 10min. The tempering temperature is 520℃ and the holding time is 20min.

[0084] The present invention has a TS content of 75ppm in the steel when the LF is discharged from the station, a T.Ca content of 8ppm in the steel, a TO content of 7ppm in the continuously cast billet, a maximum size of 15μm for the inclusions in the billet, and D-class and Ds-class inclusions in the rolled material are both <1.0 grade.

[0085] The steel plate obtained by this invention has a yield strength of 726-790MPa, a tensile strength of 770-940MPa, an elongation after fracture of 14.5-19.5%, an impact energy of 114-252J at -20℃, and a flaw detection pass rate of 100%.

[0086] Example 2

[0087] The chemical composition by weight percentage (%) is:

[0088] Brand C Si Mn P S Cr Nb Ti Mo Als B CEV Q690D 0.161 0.25 1.24 0.012 0.003 0.18 0.017 0.017 0.135 0.035 0.0017 0.43

[0089] 1) Converter smelting:

[0090] The smelting process is carried out in a top-and-bottom blown converter, and the ladle is a clean, bottom-blown ladle with a baking temperature of ≥800℃.

[0091] Molybdenum oxide briquettes are placed at the front of the scrap steel hopper and added together with the scrap steel. The amount of molybdenum oxide briquettes added is 7.5 kg / t of steel.

[0092] The carbon content at the smelting endpoint is controlled at 0.06%, the tapping temperature is controlled at 1630℃, and the final slag basicity is controlled within the range of 3.0.

[0093] The alloying process employs silicon manganese, high-carbon ferromanganese, medium-carbon ferrochrome, and ferroniobium, with addition amounts of 14.5 kg / t steel, 4.0 kg / t steel, 3.2 kg / t steel, and 0.25 kg / t steel, respectively. The alloys are added when one-fifth to three-quarters of the steel has been discharged, and the alloys are added precisely to the impact zone of the steel flow.

[0094] Aluminum manganese ferrooxidant is used for deoxidation, with an addition amount of 2.5 kg / t steel.

[0095] When feeding steel, a sliding plate is used to block slag, and top lime is added along with the steel flow.

[0096] 2) LF Refining:

[0097] The LF inlet temperature is 1550℃.

[0098] LF refining adopts bottom blowing argon stirring throughout the process. In the early stage of strong stirring and shell breaking, the argon flow rate is 600L / min and the argon blowing stirring time is 3min. In the stage of adding slag with electricity to make white slag, the argon flow rate is 160L / min. In the stage of white slag refining and fine-tuning composition, the argon flow rate is 150L / min. Before leaving the station, a low-pressure soft blowing is adopted, with an argon flow rate of 70L / min and a soft blowing time of 7min.

[0099] Based on the viscosity, color, and degree of foaming of the slag, lime and calcium carbide are added in batches to adjust the slag, with addition amounts of 400 kg / furnace and 60 kg / furnace, respectively. Before leaving the station, the top slag should reach the yellow-white slag level, the final slag basicity should be controlled at 6, the CaO / Al2O3 ratio of the refining slag should be controlled at 2.0, and the slag thickness should be 110 mm to improve the refining slag's ability to adsorb inclusions in the steel.

[0100] Aluminum wire is used to increase aluminum content, titanium wire to increase titanium content, and ferroboron wire to increase boron content, with addition amounts of 1.5 m / t steel, 0.8 m / t steel, and 0.11 kg / t steel, respectively.

[0101] 3) RH refining:

[0102] The RH process uses this operating mode, with the immersion tube inserted to a depth of 400 mm, a vacuum of 130 Pa for 18 min, and a pure degassing time of 13 min.

[0103] After RH treatment, feed 80m of nano high-calcium wire per furnace, with a soft blowing time of 10min and a slag thickness of 100mm.

[0104] 4) Continuous casting

[0105] During continuous casting, full-process protective pouring is adopted, the ladle argon seal flow rate is 100L / min, and the liquid surface of the tundish is covered with carbonized rice husks, with an addition amount of 1.2kg / t steel.

[0106] The crystallizer uses non-sinusoidal vibration, the liquidus temperature is 1520℃, the superheat of the tundish is controlled at 25℃, and the pulling speed is 1.15m / min.

[0107] The slab thickness is 250mm. The continuously cast slab is slowly cooled after leaving the production line and then piled up to cool to below 400℃.

[0108] 5) Rolling

[0109] Heating of continuously cast billets: tapping temperature is 1193℃, heating rate is 9min / cm, and furnace time is 210min.

[0110] The total number of rolling passes is 14, with 7 passes for roughing and 7 passes for finishing, and the rolling thickness is 25.05 mm.

[0111] Rough rolling: The initial rolling temperature for rough rolling is 1147℃, and the final rolling temperature for rough rolling is 1098℃.

[0112] Finishing rolling: The initial rolling temperature for finishing rolling is 943℃, and the final rolling temperature for finishing rolling is 824℃.

[0113] Cooling: Cooling inlet temperature 793℃, cooling outlet temperature 672℃, cooling rate controlled at 8℃ / s.

[0114] 6) Heat treatment

[0115] The quenching temperature is 910℃ and the holding time is 10min. The tempering temperature is 530℃ and the holding time is 20min.

[0116] The present invention has a TS content of 80ppm in the steel when leaving the LF station, a T.Ca content of 9ppm in the steel, a TO content of 6ppm in the continuously cast billet, a maximum size of 18μm for inclusions in the billet, and D-class and Ds-class inclusions in the rolled material are both <1.0 grade.

[0117] The steel plate obtained by this invention has a yield strength of 720-785 MPa, a tensile strength of 760-920 MPa, an elongation after fracture of 15-17.5%, an impact energy of 125-243 J at -20℃, and a steel plate flaw detection pass rate of 99.9%.

[0118] Comparative Example 1

[0119] The chemical composition by weight percentage (%) is:

[0120] Brand C Si Mn P S Cr Nb Ti Mo Als B CEV Q690D 0.154 0.28 1.22 0.013 0.002 0.2 0.018 0.018 0.16 0.036 0.0016 0.43

[0121] 1) Converter smelting:

[0122] The smelting process is carried out in a top-and-bottom blown converter, and the ladle is a clean, bottom-blown ladle with a baking temperature of ≥800℃.

[0123] Alloying is carried out using silicon manganese, low-carbon ferromanganese, medium-carbon ferrochrome, ferroniobium, and ferromolybdenum.

[0124] Aluminum manganese ferrooxidant is used for deoxidation, with an addition amount of 2.5 kg / t steel.

[0125] When feeding steel, a sliding plate is used to block slag, and top lime is added along with the steel flow.

[0126] 2) LF Refining:

[0127] Based on the viscosity, color, and degree of foaming of the slag, lime and calcium carbide are added in batches to adjust the slag. The top slag should reach the yellow-white slag before leaving the station, the basicity of the final slag should be controlled at 4.91, the CaO / Al2O3 ratio of the refining slag should be controlled at 3.5, and the slag thickness should be 110mm.

[0128] Aluminum wire is used to increase aluminum content, titanium wire to increase titanium content, and boron wire to increase boron content.

[0129] 3) RH refining:

[0130] The RH process adopts this treatment operation mode. After the RH treatment is completed, 100m of nano high-calcium wire is fed per furnace, the soft blowing time is 10min, and the slag thickness is 100mm.

[0131] 4) Continuous casting

[0132] During continuous casting, full-process protective pouring is adopted, the ladle argon seal flow rate is 100L / min, and the liquid surface of the tundish is covered with carbonized rice husks, with an addition amount of 1.5kg / t steel.

[0133] The crystallizer uses non-sinusoidal vibration, the liquidus temperature is 1520℃, the superheat of the tundish is controlled at 25℃, and the pulling speed is 1.15m / min.

[0134] The continuously cast billet is slowly cooled after leaving the production line, and then piled up to cool to below 400°C.

[0135] 5) Rolling

[0136] Heating of continuously cast billets: tapping temperature is 1195℃, heating rate is 9min / cm, and furnace time is 214min.

[0137] Rough rolling: The initial rolling temperature of rough rolling is 1160℃, and the final rolling temperature of rough rolling is 1111℃.

[0138] Finishing rolling: The initial rolling temperature for finishing rolling is 896℃, and the final rolling temperature for finishing rolling is 832℃.

[0139] Cooling: Cooling inlet temperature 802℃, cooling outlet temperature 695℃, cooling rate controlled at 8℃ / s.

[0140] 6) Heat treatment

[0141] The quenching temperature is 910℃ and the holding time is 10min. The tempering temperature is 530℃ and the holding time is 20min.

[0142] The comparative example LF had a TS content of 115ppm in the steel leaving the station, a T.Ca content of 12ppm in the steel, a TO content of 12ppm in the continuously cast billet, a maximum size of 35μm for inclusions in the billet, and D-class 1.0 and Ds-class 2.5 inclusions in the rolled product.

[0143] The yield strength of the steel plate obtained in the comparative example is 710-780MPa, the tensile strength is 750-930MPa, the elongation after fracture is 14-17.5%, the impact energy at -20℃ is 90-185J, and the steel plate flaw detection pass rate is 96%.

[0144] Examples 1 and 2 use molybdenum oxide briquettes instead of ferromolybdenum for alloying. Ferromolybdenum costs 216,372 yuan / ton, while molybdenum oxide briquettes cost 164,035 yuan / ton. Each 0.01% Mo reduces costs by 1-1.5 yuan. The comparative example shows that using ferromolybdenum alloying reduces alloying costs by 20-60 yuan / ton of steel. Examples 1 and 2 also use ferrosilicon manganese + high-carbon ferromanganese, while the comparative example uses ferrosilicon manganese + low-carbon ferromanganese, reducing alloying costs by 15-30 yuan / ton of steel.

[0145] Example 1: OTS inclusion analysis diagram of steel plate production. Figure 1 As shown; the OTS inclusion analysis diagram of the steel plate produced in Example 2 is shown below. Figure 2 As shown; the OTS inclusion analysis diagram of the steel plate produced in Comparative Example 1 is shown below. Figure 3 As shown. From Figure 1-3 It can be seen that in Examples 1 and 2, the TS content in the steel leaving the LF station was 75-80 ppm, the T.Ca content was controlled at 8-9 ppm, the TO content in the continuously cast billet was 6-7 ppm, the maximum size of inclusions in the billet was 15-18 μm, and both Class D and Class Ds inclusions in the rolled product were <1.0 grade. In contrast, in the Comparative Example, the TS content in the steel leaving the LF station was 115 ppm, the T.Ca content was controlled at 12 ppm, the TO content in the continuously cast billet was 12 ppm, the maximum size of inclusions in the billet was 35 μm, the Class D inclusions in the rolled product were 1.0 grade, and the Class Ds inclusions were 2.5 grade. The cleanliness of the steel in Examples 1 and 2 was significantly better than that of the Comparative Example, and the flaw detection pass rate of Examples 1 and 2 was higher than that of the Comparative Example.

[0146] The upper and lower limits of the process parameters (such as temperature, time, etc.) and the range values ​​of the present invention can all achieve the method, and examples are not listed here.

[0147] All aspects not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0148] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A Q690D high-strength steel plate, characterized in that, The chemical composition of the Q690D high-strength steel plate, by weight percentage, includes: C: 0.14-0.2%, Si: 0.2-0.4%, Mn: 1.1-1.6%, P≤0.015%, S≤0.005%, Cr: 0.1-0.4%, Nb: 0.01-0.04%, Ti: 0.01-0.03%, Mo: 0.1-0.4%, Als: 0.01-0.045%, B: 0.001-0.003%, CEV≤0.62, with the balance being iron and unavoidable impurities; The method for preparing the Q690D high-strength steel plate includes the following steps: 1) Converter smelting: The carbon content at the smelting endpoint is controlled at 0.05-0.09%, the tapping temperature is controlled at 1630-1650℃, and the final slag basicity is controlled within the range of 3.0-4.

0. Molybdenum oxide briquettes are placed at the front of the scrap steel hopper and added together with the scrap steel. 2) LF Refining: The final slag basicity is controlled at 5-6, the refining slag CaO / Al2O3 ratio is controlled at 1.8-2.0, and the slag thickness is 110-120mm; 3) RH refining: After RH treatment, feed 70-80m of nano high-calcium wire per furnace, with a soft blowing time of 10-12min, a slag thickness of 100-120mm, and control the T.Ca content in the steel at 8-10 ppm; 4) Continuous casting: The crystallizer uses non-sinusoidal vibration, the liquidus temperature is 1510-1520℃, the superheat of the tundish is controlled at 20-30℃, and the pulling speed is 1.1-1.4m / min; 5) Rolling: Heating of continuously cast billets: tapping temperature is 1180-1210℃, heating rate is 8-10min / cm, and time in the furnace is 210-230min; Rough rolling: The initial rolling temperature for rough rolling is 1140-1170℃, and the final rolling temperature for rough rolling is 1050-1120℃; Finishing rolling: The initial rolling temperature for finishing rolling is 890-950℃, and the final rolling temperature for finishing rolling is 810-860℃; Cooling: Cooling inlet temperature 770-820℃, cooling outlet temperature 670-700℃, cooling rate controlled at 5-10℃ / s; 6) Heat treatment: Quenching temperature: 900-920℃, holding time: 7-13min; Tempering temperature: 510-530℃, holding time: 15-25min.

2. The Q690D high-strength steel plate according to claim 1, characterized in that, In step 1), silicon manganese, high-carbon ferromanganese, medium-carbon ferrochrome, and ferroniobium are used for alloying, with addition amounts of 14.15-16.18 kg / t steel, 2.65-4.65 kg / t steel, 2.6-3.5 kg / t steel, and 0.23-0.31 kg / t steel, respectively. The alloys are added when one-fifth to three-quarters of the steel has been discharged, and the alloys are added to the impact zone of the steel flow.

3. The Q690D high-strength steel plate according to claim 1, characterized in that, The chemical composition of the molybdenum oxide briquettes by weight is: Mo: 48%-55%, Si≤0.1%, P≤0.05%.

4. The Q690D high-strength steel plate according to claim 1, characterized in that, In step 2), LF refining adopts bottom blowing argon stirring throughout the process. In the early stage of strong stirring and shell breaking, the argon flow rate is 500-750 L / min and the argon blowing stirring time is 2-3 min. In the stage of adding slag with electricity to make white slag, the argon flow rate is 100-200 L / min. In the stage of white slag refining and fine-tuning composition, the argon flow rate is 100-180 L / min. Before leaving the station, low-pressure soft blowing is adopted, with an argon flow rate of 50-70 L / min and a soft blowing time of 5-7 min.

5. The Q690D high-strength steel plate according to claim 1, characterized in that, In step 3), RH adopts this processing operation mode. During the processing, the immersion tube is inserted to a depth of 400 mm, the vacuum degree is 120-133 Pa and the holding time is 15-20 min, and the pure degassing time is 10-15 min.

6. The Q690D high-strength steel plate according to claim 1, characterized in that, In step 4), during continuous casting, full-process protective pouring is adopted, the ladle argon sealing flow rate is 90-100L / min, and the liquid surface of the tundish is covered with carbonized rice husks, with an addition amount of 1-1.5kg / t steel.

7. The Q690D high-strength steel plate according to claim 1, characterized in that, In step 4), the continuously cast billet is slowly cooled off the production line and piled up to below 400°C.

8. The Q690D high-strength steel plate according to claim 1, characterized in that, In step 5), the total number of rolling passes is 12-16, the total number of roughing rolling passes is 5-7, the total number of finishing rolling passes is 7-9, and the rolling thickness is 24.7-25.05 mm.

9. The Q690D high-strength steel plate according to claim 1, characterized in that, The resulting steel plates have a yield strength of 720-790 MPa, a tensile strength of 770-940 MPa, an elongation after fracture of 14.5-19.5%, an impact energy of 114-252 J at -20℃, and the D-class and Ds-class inclusions in the rolled material are both <1.0 grade.

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