CN110846564A - Low-cost high-strength beam steel 750L and production method thereof - Google Patents
Low-cost high-strength beam steel 750L and production method thereof Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/04—Making ferrous alloys by melting
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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Abstract
The invention relates to low-cost high-strength beam steel 750L, which comprises the following chemical components in percentage by mass: c: 0.065-0.085%, Si: 0.05-0.15%, Mn: 1.30-1.70%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Nb: 0.035 to 0.050%, Ti: 0.080-0.100%, Als: 0.015-0.035%, V: 0.040-0.050%, 0-0.0020% of B, less than or equal to 0.005% of N, and the balance of Fe and impurities within an allowable range. The production method comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling. The high-strength beam steel 750L produced by the method has low cost and good low-temperature impact toughness.
Description
Technical Field
The invention belongs to the technical field of metallurgical rolling, and particularly relates to low-cost high-strength beam steel 750L and a production method thereof.
Background
750L belongs to high strength automobile girder steel, mainly used commercial car girder processing. At present, under the pressure of energy conservation and environmental protection, the elimination of old commercial vehicles is greatly accelerated by China, and the over-limit overload treatment force is increased. The light-weight steel for the commercial vehicle is selected, so that the weight of the vehicle can be reduced, the effective load is improved, and the total fuel consumption can be effectively reduced. With the increasing market proportion of new energy commercial vehicles, the light weight is also the key for solving the problem of the endurance mileage of the new energy vehicles. The main forming mode of the automobile beam steel plate is as follows: the steel plate of the girder has high requirements on flattening longitudinal shearing, bending forming and welding assembly, and needs to have high toughness, fatigue resistance and cold formability.
The light weight of the commercial vehicle is concerned by the whole industry chain at present, and the great progress is made, and in the aspect of material use, domestic material selection still has a gap with the world advanced level. Advanced high-strength steel, aluminum alloy, magnesium alloy, non-metal composite materials and the like are used on domestic commercial vehicles, so that the light weight, safety and reliability of the commercial vehicles are promoted, but the application proportion and the application maturity of the advanced materials of the commercial vehicles in China are lower than those of the domestic advanced level. In the aspect of chassis and girder steel, at present, 510-610L 650MPa girder steel plates are mainly used in China, and 700-800 MPa girder steel plates are used in a small number of vehicle types. The strength of the common steel for the frames of international advanced commercial vehicles reaches the level of 700 MPa-800 MPa. Some companies have also adopted heat treated vehicle frames above the 1200MPa level, even aluminum alloy vehicle frames. The weight of the frame is obviously lower than that of the domestic similar products, and the domestic development of the ultrahigh-strength girder steel is imperative.
The existing 750L steel factory for mass supply mainly adopts a low C + high Mn and Nb and Ti composite strengthening system, and a small amount of noble metal Mo is added. The component system has high Ti content, generally between 0.110 and 0.130wt%, high Mn content, generally between 1.85 and 2.05wt%, and about 350 yuan increase of cost by adding 0.15 to 0.30wt% of Mo alloy. During production, TiN inclusions are more, the banded structure of a finished product is more serious, although the strength is higher, the fatigue and the formability of the product are deteriorated, and the impact energy performance of the finished product is greatly fluctuated due to the fact that TiC and Ti (NC) precipitates and TiN inclusions are increased due to the fact that the content of Ti and iron is higher.
Disclosure of Invention
The invention aims to solve the technical problem of providing a low-cost high-strength beam steel 750L and a production method thereof, and the high-strength beam steel 750L with low cost and good low-temperature impact toughness is produced by optimizing the component proportion and the production process, so that the defects of the background technology are overcome.
In order to solve the technical problems, the invention adopts the technical scheme that:
the low-cost high-strength beam steel 750L comprises the following chemical components in percentage by mass: c: 0.065-0.085%, Si: 0.05-0.15%, Mn: 1.30-1.70%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Nb: 0.035 to 0.050%, Ti: 0.080-0.100%, Als: 0.015-0.035%, V: 0.040-0.050%, 0-0.0020% of B, less than or equal to 0.005% of N, and the balance of Fe and impurities within an allowable range.
When the thickness of the finished girder steel strip is less than 10.0mm, the content of B in the chemical components of 750L of the low-cost high-strength girder steel is 0; when the thickness of the girder steel strip finished product is more than or equal to 10.0mm, the content of B is 0.0010-0.0020 wt%.
The high-strength beam steel 750L with low cost has the thickness specification of 2.5-16.0 mm, the metallographic structure of quasi-polygonal ferrite, acicular ferrite and TiC precipitate and the grain size of 11.5-12.5 grade.
In the low-cost high-strength beam steel 750L, the yield strength of the high-strength beam steel 750L is 723-764 MPa, the tensile strength is 786-831 MPa, the elongation is 16-24%, and the yield ratio is 0.90-0.94; when the thickness of the high-strength beam steel strip is more than or equal to 10.0mm, the impact energy of a full-size sample at the temperature of minus 20 ℃ is 190-220J.
A production method of low-cost high-strength beam steel 750L comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling; in the continuous casting process, when the thickness of a finished product of the steel strip for producing the girder steel is less than 10.0mm, the continuous casting billet comprises the following chemical components in percentage by mass: c: 0.065-0.085%, Si: 0.05-0.15%, Mn: 1.30-1.70%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Nb: 0.035 to 0.050%, Ti: 0.080-0.100%, Als: 0.015-0.035%, V: 0.040-0.050% and N is less than or equal to 0.005%; the balance of Fe and impurities within an allowable range; when the thickness of the finished product of the steel strip for producing the girder steel is more than or equal to 10.0mm, 0.0010-0.0020wt% of B is also required to be added into the components of the continuous casting billet so as to improve the hardenability of the steel strip.
According to the production method of the low-cost high-strength beam steel 750L, the rolling process is controlled, the rough rolling and final rolling temperature of the first stage is 1010-1060 ℃, and the finish rolling temperature of the second stage is finish rolling; when a steel strip with the thickness of less than 10.0mm is rolled, the temperature of a finish rolling inlet is controlled to be 950-1050 ℃, the temperature of the finish rolling inlet is not kept, the finish rolling temperature is controlled to be 880-900 ℃, and the coiling temperature is 590-620 ℃; when the steel strip with the thickness of more than or equal to 10.0mm is rolled, the temperature of a finish rolling inlet is kept, the temperature of the finish rolling inlet is controlled to be between 950-containing-materials and 1000 ℃, the finish rolling temperature is lower and is controlled to be between 840-containing-materials and 860 ℃, the coiling temperature is controlled to be between 580-containing-materials and 590 ℃, the strength of the material is ensured, and meanwhile, the impact toughness is improved.
In the production method of the low-cost high-strength beam steel 750L, the molten iron pretreatment step comprises the following steps of: 0.030-0.050 wt%, and the molten iron S after desulfurization treatment in the desulfurization station is less than or equal to 0.008 wt%;
in the production method of the low-cost high-strength beam steel 750L, the smelting process adopts the processes of converter smelting, LF refining and RH refining; in the RH refining process, a whole-process deep vacuum mode is adopted, the vacuum degree of a vacuum groove is less than or equal to 1.0mbar, alloy adjustment is started after vacuum treatment is carried out for 3-5min, the process is circulated for 3-5min after Ti iron is adjusted to a target value, the vacuum treatment time is 20-25 min, and the pure degassing time is 6-10 min; after vacuum is finished, Ca treatment is carried out when the molten steel reaches a wire feeding position, the Ca content of the molten steel is controlled to be 20-30 ppm, soft blowing time is 6-8 min after the Ca treatment is finished, and standing time of the molten steel is guaranteed to be 20-40min after the soft blowing is finished and continuous casting is started; when the thickness of the finished product of the steel strip for producing the girder steel is more than or equal to 10.0mm, ferroboron is required to be added after the ferrotitanium is adjusted to a target value and circulated for 3-5min, so that the ferroboron is prevented from being combined with C/N in molten steel, effective boron in the molten steel is ensured, and then vacuum treatment and subsequent processes are continuously carried out.
In the continuous casting process, dynamic soft reduction is adopted, the reduction of the solidified tail end is increased, the position of the tail end is determined according to the pulling speed, the reduction of the tail end is manually adjusted to 6.0-8.0mm, and the center quality of a casting blank is improved.
In the casting blank heating process, the temperature of a heating section is 1230-1270 ℃, the total heating time is 200-300 min, and the heating temperature and the heating time are slightly higher than those of common alloy steel, so that the microalloy compound can be fully redissolved.
The invention has good low-temperature impact toughness and high strength beam steel 750L product standard reference GB/T3273-2015; the product performance detection method is in standard reference GB/T228/229/232.
Design principle of strengthening elements in steel:
and the Si is added in a trace amount during designing components, and is mainly used for improving the fluidity and the castability of the molten steel and improving the purity of the molten steel. However, the content of Si is controlled to be less than 0.15wt%, and the coating performance is affected by excessively high Si or red rust on the surface of the finished product.
Mn content has great influence on the obdurability of the steel, if the content is lower, ideal microstructure and target strength cannot be ensured to be obtained, and if the content is too high, the finished product strip-shaped structure is serious, the welding performance and the corrosion resistance of the steel plate are influenced, and the rolling load is increased. In the component design of novel low-carbon microalloy high-strength girder steel, Mn and S in the steel easily form MnS inclusions, and sulfide inclusions along the rolling direction can cause the increase of anisotropy of a steel plate, so that in the production of the girder steel, the content of S should be strictly controlled while the content of high Mn is kept, and the deterioration risk of the generation of MnS on toughness and plasticity is reduced. Therefore, the Mn content is designed to be controlled to 1.30-1.70 wt%.
Nb is a commonly used microalloy element in steel and mainly plays roles in grain refinement and precipitation strengthening. Is dissolved in
Both Nb and Nb-containing carbonitrides (NbC, Nb (C, N)) in the steel exhibit recrystallization behavior of austenite
Strong inhibitory effect. Nb is added into common low alloy steel, so that atmospheric corrosion capability can be improved, and crystallization can be prevented
And (4) corrosion, and welding performance is improved. Therefore, adding a certain amount of Nb in the automobile beam steel can improve the steel
Strength, toughness and improved weldability of the steel are beneficial. However, the Nb content in the microalloy steel needs to be designed in a proper range, too low content can not play a role in refining grains, too high content is easy to cause mixed crystal structure and increase the alloy cost, and the actual effective Nb content in the composition system is 0.035-0.050 wt%.
The design principle of Ti element is as follows: titanium as a microalloying element can obviously improve the comprehensive performance of steel, but is not widely applied. The reason for this is that the performance of titanium microalloyed steel fluctuates greatly and the production is unstable, and the smelting process and the rolling process can cause that the effective titanium can not be stably controlled. With the popularization of clean steel smelting technology and controlled rolling air cooling technology, Ti alloys have been gradually popularized and applied. The research on the influence of the increase of the Ti content on the performance of the microalloy steel shows that: when the Ti content is less than 0.10wt%, the yield strength of the steel is gradually increased with the increase of the Ti content, and the yield strength is up to 750 MPa. Therefore, the Ti content in the component system is selected to be 0.08-0.10 wt%. Meanwhile, the addition of Ti is considered, N can be effectively fixed, most of the forming processes of the automobile girder steel are cold deformation, and the requirements on the plasticity and the deformation continuity of the matrix are high. The large-particle and angular TiN can reduce the continuity of a steel matrix in a cold deformation process, thereby causing microcracks and reducing the plastic processing performance of the steel, so that the Ti content in the steel should be properly controlled, the alloy cost is controlled, and the Ti alloy effect is exerted to the maximum extent.
B element design principle: the adding and adding time of ferroboron are key innovation points of the invention; in order to improve the hardenability of 700L with the thick specification (more than or equal to 10.0 mm), a trace amount of B is added, but the B is easily combined with N in the steel to form BN, and the content of effective B in the steel is reduced. Therefore, it is necessary to add the Ti-Fe alloy after finishing the addition in the RH process. The addition of Ti can effectively fix N, thereby improving the utilization efficiency of B. The hardenability-improving effect of the alloying elements generally increases with the increase in the steel content, but an optimum content (range) of boron is not preferable, too much or too little, and the content is small, about 0.0010 wt%, and is generally controlled to 0.0005 to 0.0030 wt%. The selected range of the present invention is 0.0010 to 0.0020 wt%.
V is one of the most commonly used, and also most effective, strengthening elements in micro-alloyed steels. The strength can be increased by 200MPa or more by adding 0.10wt% of V to the steel. At present, most of non-quenched and tempered steels adopt V as a microalloy element, but the non-quenched and tempered steels only containing V have poor toughness and sometimes even can not meet the requirement of engineering materials on high toughness. The microalloy non-quenched and tempered steel adopting the Nb, V and Ti composite addition process has good obdurability matching, and the three-point bending fatigue limit of the microalloy non-quenched and tempered steel is far higher than that of microalloy quenched and tempered steel only containing V, because the toughness of the steel can be improved without reducing the strength of the steel by combining the precipitation strengthening of Nb and V and the grain refinement of Nb, and the range of V selected by the invention is 0.040-0.050wt% in consideration of the strength contribution of V iron and the alloy cost.
Adopt the produced beneficial effect of above-mentioned technical scheme to lie in:
1. the 750L component of the invention adopts a low-carbon high-manganese and trace Nb, V and Ti microalloy component system in design; the structure and the performance can meet the requirements by means of a fine-grain strengthening mechanism and a stronger precipitation strengthening mechanism of Nb, V and Ti alloys.
2. The rolling process adopts TMCP thermo-mechanical rolling, and austenite grains with proper size are obtained by controlling tapping temperature, finish rolling inlet temperature, intermediate billet thickness and coiling temperature, so that the comprehensive mechanical property of the steel is improved; by carrying out pilot test, material thermal simulation and other experiments on the component system, the invention sets the finish rolling temperature, the coiling temperature and the cooling mode of the steel according to different thicknesses so as to ensure that the precipitation strengthening of Ti is fully exerted.
3. The invention can ensure the performance of steel products, is more economical in component design system and production process, creates 300-yuan/ton steel for enterprises, reduces the steel consumption of commercial vehicles and improves the effective load.
4. 750L of trial-produced high-titanium-content system girder steel overcomes the problem of low-temperature impact power fluctuation at-20 ℃ when the thickness is larger than or equal to 10mm, the 3/4 size and the full-size impact power control condition are greatly improved, the 3/4 size impact power is improved to 85-100J, and the full-size impact power is controlled to 190-220J.
Drawings
FIG. 1 is a drawing (500X) showing the 1/4 position microstructure of the 750L steel coil of the high-strength beam steel in example 1;
FIG. 2 is a drawing (500X) showing the 1/2 position microstructure of the 750L steel coil of the high-strength beam steel in example 1;
FIG. 3 is a microscopic structural diagram (500X) of the 1/4 position of the 750L steel coil thickness of the high-strength beam steel in example 2;
FIG. 4 is a microscopic structural diagram (500X) of 1/2 positions of the 750L steel coil thickness of the high-strength beam steel in example 2;
FIG. 5 is a graph (500X) showing the 1/4 position microscopic structure of the 750L steel coil thickness of the high-strength beam steel in example 3;
FIG. 6 is a microscopic structural diagram (500X) of the 1/2 position of the 750L steel coil thickness of the high-strength beam steel in example 3;
FIG. 7 is a graph (500X) showing the 1/4 position microscopic structure of the 750L steel coil thickness of the high-strength beam steel in example 4;
FIG. 8 is a graph (500X) showing the 1/2 location microstructure of the 750L steel coil of the high-strength beam steel in example 4;
FIG. 9 is a 1/4 location microscopic structural diagram (500X) of the 750L steel coil thickness of the high-strength beam steel in example 5;
FIG. 10 is a drawing (500X) showing the 1/2 position microstructure of the 750L steel coil of the high-strength beam steel in example 5;
FIG. 11 is a graph showing the 1/2 location microscopic structure (500X) of the 750L steel coil thickness of the high-strength beam steel in example 6;
fig. 12 is a microscopic structure view (500 ×) of the 1/2 thickness of 750L steel coil of high-strength beam steel in example 6.
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples.
Example 1
The thickness of the high-strength beam steel with good low-temperature impact toughness is 750L and 2.5mm, and the high-strength beam steel comprises the following chemical components in percentage by mass: c: 0.075%, Si: 0.05%, Mn: 1.30%, P: 0.015%, S: 0.003%, Nb: 0.035%, Ti: 0.100%, Als: 0.030%, V: 0.040%, N: 0.005%, and the balance of Fe and impurities within an allowable range.
The production method of the 750L high-strength beam steel with good low-temperature impact toughness comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling, and comprises the following specific process steps:
(1) a molten iron pretreatment process: molten iron S before desulfurization: 0.036%, molten iron S after desulfurization treatment in the desulfurization station: 0.007%;
(2) smelting: adopting converter smelting, LF refining and RH refining processes;
the converter smelting process comprises the following steps of controlling the end point C: 0.028%, P: 0.011%, O: 650 ppm; during tapping, a sliding plate is adopted for blocking slag, the slag discharging amount is strictly controlled, and the tapping time is 5.8 min;
an LF refining process, namely adding aluminum particles, lime and fluorite to quickly produce white slag after the steel enters a station, and adjusting the content of molten steel alloy according to components; controlling the refining outlet temperature at 1625 ℃, refining LF for 45min, not performing Ca treatment after refining, and blowing Ar gas for 5 min;
in the RH refining process, a whole-process deep vacuum mode is adopted in the refining process; the circulation vacuum degree is 0.3mbar, alloy adjustment is started after vacuum treatment is carried out for 4min, the vacuum treatment time is 20min, and the pure degassing time is 6 min; after vacuum is finished, Ca treatment is carried out when the molten steel reaches a wire feeding position, the Ca content of the molten steel is controlled to be 28ppm, soft blowing time is 6min after the Ca treatment is finished, and the soft blowing is finished until continuous casting is started to ensure that the molten steel is kept standing for 20 min;
(3) and (3) continuous casting process: the continuous casting process adopts dynamic soft reduction, the reduction of the solidification tail end is increased, and the reduction is increased to 6 mm;
(4) a casting blank heating procedure: the heating section temperature is 1230 ℃, and the total heating time is 200 min;
(5) and (3) controlling a rolling process: the rolling is controlled in two stages, the first stage is rough rolling, the finishing temperature is 1040 ℃, and the thickness of an outlet intermediate billet is 36-38 mm; the second stage is finish rolling, the inlet temperature is controlled to be 1020-1050 ℃, the finish rolling is carried out for 7 times, the descaling water between frames of the finish rolling mill is fully opened, the reduction rate of the last time is 22%, the accumulated reduction rate of the second stage is 94%, the finish rolling temperature is controlled to be 900 ℃, and the coiling temperature is 610 ℃;
(6) laminar cooling: and in the stage of cold lamination, an 1/2 cooling mode is adopted, the cooling water temperature is controlled at 28 ℃, the cooling speed is controlled at 60-70 ℃/s, and the water consumption of each roll is 4300 m.
The embodiment has 750L mechanical properties of good low-temperature impact toughness and high-strength beam steel: the yield strength is 764MPa, the tensile strength is 808MPa, the elongation is 18 percent, the yield ratio is 0.94, and the bending test is qualified; the microstructure of the 1/4 position of the steel coil is shown in figure 1, the microstructure of the 1/2 position of the steel coil is shown in figure 2, the microstructure is quasi-polygonal ferrite, acicular ferrite, free pearlite and TiC precipitate, and the grain size is 12.0 grade according to the figures 1 and 2. The thickness is not subjected to impact test, the alloy composition design is reduced compared with the current compositions such as Mn content, Ti content and Mo content, a small amount of V iron is added, and the comprehensive alloy cost is reduced by about 310 yuan.
Example 2
The thickness of the high-strength beam steel with good low-temperature impact toughness is 5mm, and the high-strength beam steel comprises the following chemical components in percentage by mass: c: 0.085%, Si: 0.08%, Mn: 1.40%, P: 0.014%, S: 0.003%, Nb: 0.040%, Ti: 0.090%, Als: 0.032%, V: 0.040%, N: 0.005%, and the balance of Fe and impurities within an allowable range.
The production method of the 750L high-strength beam steel with good low-temperature impact toughness comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling, and comprises the following specific process steps:
(1) a molten iron pretreatment process: molten iron S before desulfurization: 0.050%, molten iron S after desulfurization treatment in the desulfurization station: 0.007%;
(2) smelting: adopting converter smelting, LF refining and RH refining processes;
converter smelting process, wherein the converter end blowing temperature is 1655 ℃, and the control end C is as follows: 0.035%, P: 0.012%, O: 750 ppm; during tapping, a sliding plate is adopted to block slag, and the slag discharging amount is strictly controlled.
An LF refining process, namely adding aluminum particles, lime and fluorite to quickly produce white slag after the steel enters a station, and adjusting the content of molten steel alloy according to components; controlling the refining outlet temperature at 1628 ℃, refining LF for 45min, not performing Ca treatment after refining, and blowing Ar gas for 6 min;
in the RH refining process, a whole-process deep vacuum mode is adopted in the refining process; the circulation vacuum degree is 0.4mbar, alloy adjustment is started after vacuum treatment is carried out for 3min, the vacuum treatment time is 23min, and the pure degassing time is 8 min; after vacuum is finished, Ca treatment is carried out when the molten steel reaches a wire feeding position, the Ca content of the molten steel is controlled to be 30ppm, soft blowing time is 8min after the Ca treatment is finished, and the soft blowing is finished until continuous casting is started to ensure that the molten steel is kept standing for 30 min;
(3) and (3) continuous casting process: the continuous casting process adopts dynamic soft reduction, the reduction of the solidification tail end is increased, and the reduction is increased to 6 mm;
(4) a casting blank heating procedure: the heating section temperature is 1260 ℃, and the total heating time is 230 min;
(5) and (3) controlling a rolling process: the rolling is controlled in two stages, wherein the first stage is rough rolling and finish rolling at 1060 ℃, and the thickness of an outlet intermediate billet is 40-46 mm; the second stage is finish rolling, the inlet temperature of the finish rolling is controlled to be 950-1030 ℃, after 7-pass rolling, the descaling water between frames of the finish rolling mill is fully opened, the final pass reduction rate is 23 percent, the accumulated reduction rate of the second stage is 88 percent, the finish rolling temperature is controlled to be 880 ℃, and the coiling temperature is 610 ℃;
(6) laminar cooling: and in the stage of cold lamination, an 3/4 cooling mode is adopted, the cooling water temperature is controlled at 28 ℃, the cooling speed is controlled at 35-40 ℃/s, and the water consumption of each roll is 4500 m.
The embodiment has 750L mechanical properties of good low-temperature impact toughness and high-strength beam steel: the yield strength is 731MPa, the tensile strength is 786MPa, the elongation is 24 percent, the yield ratio is 0.93, and the bending test is qualified; the microstructure of the steel coil with the thickness of 1/4 is shown in figure 3, the microstructure of the steel coil with the thickness of 1/2 is shown in figure 4, and the microstructure is quasi-polygonal ferrite, acicular ferrite and TiC precipitate, wherein the grain size is 12.5 grade according to figures 3 and 4. Compared with the current components, Mn content, Ti content and Mo content are all reduced, a small amount of V iron is added, and the comprehensive alloy cost is reduced by about 285 yuan.
Example 3
The thickness of the high-strength beam steel with good low-temperature impact toughness is 750L and is 10mm, and the high-strength beam steel comprises the following chemical components in percentage by mass: c: 0.076%, Si: 0.10%, Mn: 1.50%, P: 0.015%, S: 0.003%, Nb: 0.040%, Ti: 0.090%, Als: 0.030%, V: 0.045%, B: 0.0010%, N: 0.005%, and the balance of Fe and impurities within an allowable range.
The production method of the 750L high-strength beam steel with good low-temperature impact toughness comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling, and comprises the following specific process steps:
(1) a molten iron pretreatment process: molten iron S before desulfurization: 0.040%, molten iron S after desulfurization treatment in the desulfurization station: 0.006%;
(2) smelting: adopting converter smelting, LF refining and RH refining processes;
converter smelting process, wherein the converter end blowing temperature is 1655 ℃, and the control end C is as follows: 0.032%, P: 0.008%, O: 700 ppm; during tapping, a sliding plate is adopted for blocking slag, the slag discharging amount is strictly controlled, and the tapping time is 6.0 min;
an LF refining process, namely adding aluminum particles, lime and fluorite to quickly produce white slag after the steel is put into a station, sampling and testing components after the white slag is formed, and adjusting the content of molten steel alloy according to the components; controlling the refining outlet temperature at 1635 ℃, refining LF for 48min, not performing Ca treatment after refining, and blowing Ar gas for 6 min;
in the RH refining process, a whole-process deep vacuum mode is adopted in the refining process; performing vacuum treatment at 0.5mbar for 4min, adjusting alloy, adding ferroboron for 3min after adjusting Ti-Fe alloy, vacuum treating for 23min, and pure degassing for 8 min; and performing Ca treatment when the vacuum is finished to a wire feeding position, controlling the Ca content of the molten steel to be 25ppm, performing soft blowing for 6min after the Ca treatment is finished, and ensuring the standing time of the molten steel for 25min after the soft blowing is finished and continuous casting is started.
(3) And (3) continuous casting process: the continuous casting process adopts dynamic soft reduction, the reduction of the solidification tail end is increased, and the reduction is increased to 8 mm;
(4) a casting blank heating procedure: the heating section temperature is 1260 ℃, and the total heating time is 250 min;
(5) and (3) controlling a rolling process: the rolling is controlled in two stages, the first stage is that the rough rolling finishing temperature is 1050 ℃, and the thickness of an outlet intermediate blank is 48-50 mm; the second stage is finish rolling, the inlet temperature is controlled to be 980-1000 ℃, after 7-pass rolling, the descaling water between the frames of the finish rolling mill is fully opened, the last-pass secondary reduction rate is 21 percent, the cumulative reduction rate of the second stage is 83 percent, the finish rolling temperature is controlled to be 850 ℃, and the coiling temperature is 590 ℃;
(6) laminar cooling: and in the stage of cold lamination, an 3/4 cooling mode is adopted, the cooling water temperature is controlled at 29.5 ℃, the cooling speed is controlled at 35 ℃/s, and the water consumption of each roll is 4700m for carrying out the high-speed thin film high-speed.
The embodiment has 750L mechanical properties of good low-temperature impact toughness and high-strength beam steel: the yield strength is 764MPa, the tensile strength is 831MPa, the elongation is 21%, the yield ratio is 0.92, the 3/4 size-20 ℃ impact energy is 95J, 100J and 80J, which are obviously higher than 35J required by the standard, and the full-size impact energy is tested: 190J, 195J, 192J, which is obviously higher than 47J required by the standard; the bending test is qualified and has no micro-crack visible to naked eyes; the microstructure of the 1/4 position of the steel coil is shown in figure 5, the microstructure of the 1/2 position of the steel coil is shown in figure 6, and the microstructure is quasi-polygonal ferrite, acicular ferrite and TiC precipitate, and the grain size is 12.0 grade according to figures 5 and 6. Compared with the current components, Mn content, Ti content and Mo content of the alloy are all reduced, a small amount of V iron is added, and the comprehensive alloy cost is reduced by about 250 yuan.
Example 4
The thickness of the high-strength beam steel with good low-temperature impact toughness is 750L and 12.0mm, and the high-strength beam steel comprises the following chemical components in percentage by mass: c: 0.075%, Si: 0.15%, Mn: 1.65%, P: 0.013%, S: 0.001%, Nb: 0.045%, Ti: 0.090%, Als: 0.022%, V: 0.040%, B: 0.0012%, N: 0.0048 percent, and the balance of Fe and impurities within the allowable range.
The production method of the 750L high-strength beam steel with good low-temperature impact toughness comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling, and comprises the following specific process steps:
(1) a molten iron pretreatment process: molten iron S before desulfurization: 0.035%, molten iron S after desulfurization treatment in the desulfurization station: 0.007%;
(2) smelting: adopting converter smelting, LF refining and RH refining processes;
the converter smelting process comprises the following steps of controlling the end point C: 0.038%, P: 0.012%, O: 620 ppm; the sliding plate is adopted to block slag during tapping, the slag discharging amount is strictly controlled,
an LF refining process, namely adding aluminum particles, lime and fluorite to quickly produce white slag after the steel is put into a station, sampling and testing components after the white slag is formed, and adjusting the content of molten steel alloy according to the components; controlling the refining outlet temperature at 1630 ℃, refining LF for 38min, not performing Ca treatment after refining, and blowing Ar gas for 6 min;
in the RH refining process, a whole-process deep vacuum mode is adopted in the refining process; the circulation vacuum degree is 0.3mbar, alloy adjustment is started after vacuum treatment is carried out for 3-5min, ferroboron is added after the Ti-Fe alloy is adjusted for 3min according to the thickness specification, the vacuum treatment time is 22min, and the pure degassing time is 10 min; performing Ca treatment when the vacuum is finished to a wire feeding position, controlling the Ca content of the molten steel to be 20ppm, performing soft blowing for 6min after the Ca treatment is finished, and ensuring the standing time of the molten steel for 40min after the soft blowing is finished and the continuous casting is started;
(3) and (3) continuous casting process: the continuous casting process adopts dynamic soft reduction, the reduction of the solidification tail end is increased, and the reduction is increased to 8 mm;
(4) a casting blank heating procedure: the heating section temperature is 1260 ℃, and the total heating time is 280 min;
(5) and (3) controlling a rolling process: the rolling is controlled in two stages, the first stage is rough rolling for 3+5 times, the finish rolling temperature of the rough rolling is 1050 ℃, and the thickness of a rough rolling outlet intermediate billet is 48-52 mm; the second stage is finish rolling, the inlet temperature is controlled to be 950-980 ℃, after 7-pass rolling, the descaling water between frames of the finish rolling mill is fully opened, the final pass reduction rate is 23 percent, the accumulated reduction rate of the second stage is 67 percent, the finish rolling temperature is controlled to be 840 ℃, and the coiling temperature is 590 ℃;
(6) laminar cooling: and (3) carrying out concentrated cooling in the cold stratification stage, controlling the cooling water temperature to be 28 ℃, controlling the cooling speed to be 25 ℃/s, and carrying out 5600m water consumption per roll.
The embodiment has 750L mechanical properties of good low-temperature impact toughness and high-strength beam steel: the yield strength is 747MPa, the tensile strength is 821MPa, the elongation is 16%, the yield ratio is 0.91, the impact energy at-20 ℃ is 220J, 210J and 215J, which is far higher than 47J required by the standard, and the bending test is qualified; the microstructure of the 1/4 position of the steel coil is shown in figure 7, the microstructure of the 1/2 position of the steel coil is shown in figure 8, and the microstructure is quasi-polygonal ferrite, acicular ferrite and TiC precipitate, and the grain size is 12.0 grade according to figures 7 and 8. Compared with the current components, Mn content, Ti content and Mo content of the alloy are all reduced, a small amount of V iron is added, and the comprehensive alloy cost is reduced by about 220 yuan.
Example 5
The thickness of the high-strength beam steel with good low-temperature impact toughness is 750L and 14mm, and the high-strength beam steel comprises the following chemical components in percentage by mass: c: 0.065%, Si: 0.15%, Mn: 1.60%, P: 0.015%, S: 0.002%, Nb: 0.045%, Ti: 0.090%, Als: 0.025%, V: 0.045%, B: 0.0015%, N: 0.0048 percent, and the balance of Fe and impurities within the allowable range.
The production method of the 750L high-strength beam steel with good low-temperature impact toughness comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling, and comprises the following specific process steps:
(1) a molten iron pretreatment process: molten iron S before desulfurization: 0.040%, molten iron S after desulfurization treatment in the desulfurization station: 0.006%;
(2) smelting: adopting converter smelting, LF refining and RH refining processes;
the converter smelting process, the converter end point blowing-off temperature is 1650 ℃, and the control end point C is as follows: 0.025%, P: 0.010%, O: 800 ppm; during tapping, a sliding plate is adopted for blocking slag, the slag discharging amount is strictly controlled, the tapping time is 6min,
an LF refining process, namely adding aluminum particles, lime and fluorite to quickly produce white slag after the steel is put into a station, sampling and testing components after the white slag is formed, and adjusting the content of molten steel alloy according to the components; controlling the refining outlet temperature at 1626 ℃, LF refining time 55min, performing no Ca treatment after refining, and blowing Ar gas for 6 min;
in the RH refining process, a whole-process deep vacuum mode is adopted in the refining process; the circulation vacuum degree is 0.3mbar, alloy adjustment is started after vacuum treatment is carried out for 3.0min, ferroboron is added after the Ti-Fe alloy is adjusted for 3min according to the thickness specification, the vacuum treatment time is 25min, and the pure degassing time is 8 min; after vacuum is finished, Ca treatment is carried out when the wire feeding position is reached, the content of Ca is controlled to be 25ppm, soft blowing time is 8min after the Ca treatment is finished, and the soft blowing is finished until continuous casting is started to ensure that molten steel is kept for 30 min;
(3) and (3) continuous casting process: the continuous casting process adopts dynamic soft reduction, the reduction of the solidification tail end is increased, and the reduction is increased to 8 mm;
(4) a casting blank heating procedure: the heating section temperature is 1260 ℃, and the total heating time is 280 min;
(5) and (3) controlling a rolling process: the rolling is controlled in two stages, the first stage is rough rolling, the finish rolling temperature is 1050-; the second stage is finish rolling, the inlet temperature is controlled to be 950-980 ℃, after 7-pass rolling, the descaling water between the frames of the finish rolling mill is fully opened, the last-pass secondary reduction rate is 20.8 percent, the second-stage accumulated reduction rate is 80 percent, the finish rolling temperature is controlled to be 840 ℃, and the coiling temperature is 590 ℃;
(6) laminar cooling: and (3) adopting a centralized cooling mode in the stage of cold stratification, controlling the temperature of cooling water to be 32 ℃, controlling the cooling speed to be 25 ℃/s, and carrying out 4900m high speed cultivation on each roll of water consumption.
The embodiment has 750L mechanical properties of good low-temperature impact toughness and high-strength beam steel: the yield strength is 723MPa, the tensile strength is 786MPa, the elongation is 18%, the yield ratio is 0.92, the impact energy is 220J, 210J and 196J at the temperature of minus 20 ℃, the impact performance is obviously higher than 47J required by the standard, the bending test is qualified, and no micro-crack can be seen by naked eyes; the microstructure of the 1/4 position of the steel coil is shown in figure 9, the microstructure of the 1/2 position of the steel coil is shown in figure 10, and the microstructure is quasi-polygonal ferrite, acicular ferrite and TiC precipitate, and the grain size is 12.0 grade according to figures 9 and 10. Compared with the current components, Mn content, Ti content and Mo content of the alloy are all reduced, a small amount of V iron is added, and the comprehensive alloy cost is reduced by about 200 yuan.
Example 6
The thickness of the high-strength beam steel with good low-temperature impact toughness is 16mm, and the high-strength beam steel comprises the following chemical components in percentage by mass: c: 0.075%, Si: 0.15%, Mn: 1.70%, P: 0.015%, S: 0.002%, Nb: 0.050%, Ti: 0.080%, Als: 0.015%, V: 0.050%, B: 0.0020%, N: 0.0050%, and the balance of Fe and impurities within the allowable range.
The production method of the 750L high-strength beam steel with good low-temperature impact toughness comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling, and comprises the following specific process steps:
(1) a molten iron pretreatment process: molten iron S before desulfurization: 0.038%, molten iron S after desulfurization treatment in the desulfurization station: 0.008 percent;
(2) smelting: adopting converter smelting, LF refining and RH refining processes;
converter smelting process, wherein the end point blowing-off temperature of a converter is 1660 ℃, and the end point C is controlled: 0.028%, P: 0.013%, O: 760 ppm; during tapping, a sliding plate is adopted for blocking slag, the slag discharging amount is strictly controlled, the tapping time is 6min,
an LF refining process, wherein components are sampled and tested after slagging, and the content of molten steel alloy is adjusted according to the components; controlling the refining outlet temperature at 1628 ℃, refining LF for 52min, not performing Ca treatment after refining, and blowing Ar gas for 6 min;
in the RH refining process, a whole-process deep vacuum mode is adopted in the refining process; performing alloy adjustment after the circulation vacuum degree is 0.4mbar and vacuum treatment is carried out for 4.0min, and adding ferroboron after the Ti-Fe alloy is adjusted for 3min, wherein the vacuum treatment time is 25min, and the pure degassing time is 6 min; after vacuum is finished, Ca treatment is carried out when the wire feeding position is reached, the content of Ca is controlled to be 20ppm, soft blowing time is 8min after the Ca treatment is finished, and the soft blowing is finished until continuous casting is started to ensure that molten steel is kept for 30 min;
(3) and (3) continuous casting process: the continuous casting process adopts dynamic soft reduction, the reduction of the solidification tail end is increased, and the reduction is increased to 8 mm;
(4) a casting blank heating procedure: the temperature of the heating section is 1270 ℃, and the total heating time is 300 min;
(5) and (3) controlling a rolling process: the first stage is rough rolling, the finish rolling temperature is 1050-; the second stage is finish rolling, the inlet temperature of the finish rolling is controlled to be 950-980 ℃, after 7-pass rolling, the descaling water between the frames of the finish rolling mill is fully opened, the last pass secondary reduction rate is 20.8 percent, the cumulative reduction rate of the second stage is 80 percent, the finish rolling temperature is controlled to be 840 ℃, and the coiling temperature is 580 ℃;
(6) laminar cooling: and (3) carrying out cold layer stage by adopting a centralized cooling mode, controlling the temperature of cooling water to be 31 ℃, controlling the cooling speed to be 28 ℃/s, and carrying out 5500m water consumption per roll.
The embodiment has 750L mechanical properties of good low-temperature impact toughness and high-strength beam steel: the yield strength is 720MPa, the tensile strength is 800MPa, the elongation is 20%, the yield ratio is 0.90, the impact energy at minus 20 ℃ is 190J, 199J and 208J which are far higher than 47J required by the standard, and the bending test is qualified; the microstructure of the steel coil with the thickness of 1/4 is shown in figure 11, the microstructure of the steel coil with the thickness of 1/2 is shown in figure 12, and the microstructure is quasi-polygonal ferrite, acicular ferrite and TiC precipitate, wherein the grain size is 11.5 grade as shown in figures 11 and 12. Compared with the current components, Mn content, Ti content and Mo content of the alloy are all reduced, a small amount of V iron is added, and the comprehensive alloy cost is reduced by about 170 yuan.
Claims (9)
1. The utility model provides a low-cost powerful roof beam steel 750L which characterized in that: the chemical components and the mass percentage content are as follows: c: 0.065-0.085%, Si: 0.05-0.15%, Mn: 1.30-1.70%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Nb: 0.035 to 0.050%, Ti: 0.080-0.100%, Als: 0.015-0.035%, V: 0.040-0.050%, 0-0.0020% of B, less than or equal to 0.005% of N, and the balance of Fe and impurities within an allowable range.
2. The low-cost high-strength beam steel 750L of claim 1, wherein: when the thickness of the finished girder steel strip is less than 10.0mm, the content of B in the chemical components is 0; when the thickness of the girder steel strip finished product is more than or equal to 10.0mm, the content of B is 0.0010-0.0020 wt%.
3. The low-cost high-strength beam steel 750L of claim 1 or 2, wherein: the 750L high-strength beam steel has the thickness specification of 2.5-16.0 mm, the metallographic structure of quasi-polygonal ferrite, acicular ferrite and TiC educt and the grain size of 11.5-12.5 grade.
4. The low-cost high-strength beam steel 750L of claim 1 or 2, wherein: the yield strength of 750L of the high-strength beam steel is 723-764 MPa, the tensile strength is 786-831 MPa, the elongation is 16-24%, and the yield ratio is 0.90-0.94; when the thickness of the high-strength beam steel strip is more than or equal to 10.0mm, the impact energy of a full-size sample at the temperature of minus 20 ℃ is 190-220J.
5. A production method of low-cost high-strength beam steel 750L comprises the working procedures of molten iron pretreatment, smelting, continuous casting, casting blank heating, controlled rolling and laminar cooling; the method is characterized in that: in the continuous casting process, when the thickness of a finished product of the steel strip for producing the girder steel is less than 10.0mm, the continuous casting billet comprises the following chemical components in percentage by mass: c: 0.065-0.085%, Si: 0.05-0.15%, Mn: 1.30-1.70%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Nb: 0.035 to 0.050%, Ti: 0.080-0.100%, Als: 0.015-0.035%, V: 0.040-0.050% and N is less than or equal to 0.005%; the balance of Fe and impurities within an allowable range; when the thickness of the finished product of the steel strip for producing the girder steel is more than or equal to 10.0mm, 0.0010-0.0020wt% of B is also required to be added into the components of the continuous casting billet.
6. The method of claim 5 for producing low cost high strength beam steel 750L, wherein: in the rolling procedure, the temperature of rough rolling and finish rolling at the first stage is 1010-1060 ℃, and the temperature of finish rolling at the second stage is finish rolling; when a steel strip with the thickness of less than 10.0mm is rolled, the temperature of a finish rolling inlet is controlled to be 950-1050 ℃, the temperature of the finish rolling inlet is not kept, the finish rolling temperature is controlled to be 880-900 ℃, and the coiling temperature is 590-620 ℃; when the steel strip with the thickness of more than or equal to 10.0mm is rolled, the temperature of a finish rolling inlet is kept at the temperature, the temperature of the finish rolling inlet is controlled to be between 950-class and 1000-class temperatures, the temperature of a final rolling is controlled to be between 840-class and 860-class temperatures, and the coiling temperature is controlled to be between 580-class and 590-class temperatures.
7. The method of claim 5 for producing low cost high strength beam steel 750L, wherein: the molten iron pretreatment process, molten iron S before desulfurization: 0.030-0.050 wt%, and the molten iron S after desulfurization treatment in the desulfurization station is less than or equal to 0.008 wt%.
8. The method of claim 5 for producing low cost high strength beam steel 750L, wherein: the smelting process adopts converter smelting, LF refining and RH refining processes; in the RH refining process, a whole-process deep vacuum mode is adopted, the vacuum degree of a vacuum groove is less than or equal to 1.0mbar, alloy adjustment is started after vacuum treatment is carried out for 3-5min, the process is circulated for 3-5min after Ti iron is adjusted to a target value, the vacuum treatment time is 20-25 min, and the pure degassing time is 6-10 min; after vacuum is finished, Ca treatment is carried out when the molten steel reaches a wire feeding position, the Ca content of the molten steel is controlled to be 20-30 ppm, soft blowing time is 6-8 min after the Ca treatment is finished, and standing time of the molten steel is guaranteed to be 20-40min after the soft blowing is finished and continuous casting is started; when the thickness of the finished product of the steel strip for producing the girder steel is more than or equal to 10.0mm, ferroboron is added after the ferrotitanium is adjusted to a target value and circulated for 3-5min, and then vacuum treatment and subsequent processes are continuously carried out.
9. The method of claim 5 for producing low cost high strength beam steel 750L, wherein: in the continuous casting process, dynamic soft reduction is adopted, the tail end position is determined according to the pulling speed, the tail end reduction is manually adjusted, and the reduction is controlled to be 6.0-8.0 mm; in the casting blank heating procedure, the temperature of a heating section is 1230-1270 ℃, and the total heating time is 200-300 min.
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