Production method for improving z-direction performance of heavy hot-rolled h-section steel, and heavy hot-rolled h-section steel

Abstract

A production method for improving the Z-direction performance of heavy hot-rolled H-section steel, and a heavy hot-rolled H-section steel. The production method comprises the following steps: molten iron pretreatment, converter double-slag smelting, LF furnace refining, VD vacuum degassing, continuous casting, slow cooling, hot rolling, and cooling. In the step of continuous casting, low-superheat, constant-drawing speed casting is used, superheat being 10-35°C, and the continuous casting drawing speed of a profiled billet section being 0.45-0.95 m / min; the secondary cooling water ratio is 0.45-0.77 L / kg, the foot roller area is cooled by water, and the active section, the first section, the second section and the third section are cooled by air-water atomization; the water distribution ratio of the foot roller area, the active section, the first section, the second section and the third section is 25-33%:40-45%:15-20%:7-10%:4.5-6%; the Z-direction performance of the produced heavy hot-rolled H-shaped steel is 35-65%, and the cast billet surface quality is good.

Description

A production method for improving Z-direction performance of heavy hot-rolled H-beam and heavy hot-rolled H-beam

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on November 21, 2023, with application number 202311556112.0 and invention name “A production method for improving the Z-direction performance of heavy hot-rolled H-beam and heavy hot-rolled H-beam”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present invention belongs to the technical field of H-shaped steel, and in particular relates to a production method for improving the Z-direction performance of heavy hot-rolled H-shaped steel and the heavy hot-rolled H-shaped steel. Background Art

[0003] H-shaped steel is a new type of economical construction steel. Its cross-sectional shape is economical and reasonable, and its mechanical properties are excellent. During rolling, its elongation at all points on the cross section is relatively uniform, and its internal stress is low. It is widely used in high-rise buildings, large venues, and major energy projects both domestically and internationally. However, due to the structural design and mechanical requirements of these buildings, high performance requirements are also imposed on steel. Among these, the Z-axis performance of steel is a key property, ensuring the stability and safety of building steel structures.

[0004] To achieve high-strength, Z-direction performance in heavy-duty hot-rolled H-beams, a certain amount of alloying elements, such as Nb, V, Ti, and Al, is typically required. The continuous casting process for these complex, micro-alloyed, high-performance beam blanks differs significantly from that for ordinary carbon steel, making the quality of the resulting continuous-cast blanks even more difficult to guarantee. Domestically, there are no existing special-shaped continuous casting machines capable of producing large-section, high-performance Z-direction beam blanks.

[0005] Chinese patent CN110788293A discloses a continuous casting process for beam blanks for high-weather-resistant hot-rolled H-beams. The secondary cooling mode parameters in this continuous casting process include: a water content of 0.5-0.55 L / kg; a water distribution ratio of 50%-55% for Section A, 25%-27% for Section B, and 20%-25% for Section II. Section A has the same water content in the inner and outer arcs, with the water content on both sides accounting for 50% of the total water content. Section B has the same water content in the inner arc as in Section I, with the water content on both sides accounting for 65%-69% of the total water content. Section II has the same water content in the inner arc as in Section II, with the water content on both sides accounting for 60% of the total water content. This patent successfully developed beam blanks for high-weather-resistant hot-rolled H-beams for heavy-load trains using this continuous casting process. However, the patent does not address the Z-direction properties of the H-beams.

[0006] Chinese patent CN 113564480 A discloses a heavy hot-rolled H-beam with Z-direction properties and its production method. The main elements are C, Si, Mn, Nb, Ti, N, B, and Als, with the balance being iron and unavoidable impurities. The production method comprises the following steps: molten iron pretreatment → converter smelting → argon refining → RH → full-protection casting of profiled blanks → stacking and slow cooling → rolling → post-rolling air cooling. The present invention utilizes reasonable component ratios and process control, through a process of cogging rolling + universal rolling + post-rolling air cooling, and a combined strengthening method of phase transformation + precipitation + grain refinement to control the amount of secondary particle precipitation, achieving a post-rolling granular bainite content between 10% and 20%. This results in heavy hot-rolled H-beams with flange thicknesses under 80 mm exhibiting excellent strength and toughness, as well as Z-direction properties of 65% to 80%. The invention primarily focuses on controlling Z-direction properties from the perspective of the rolling process, without addressing how to control the Z-direction properties of the finished product during the continuous casting process.

[0007] Chinese patent CN109897928A discloses a method for producing thick steel plates with core resistance to lamellar tearing by continuous casting. The method for producing thick steel plates of the present invention is as follows: the continuous casting billet shape is selected according to the thickness of the steel plate and the grade of the steel. After the initial refining in the converter, the LF refining and RH vacuum refining processes are strictly controlled to produce high-cleanliness molten steel, which is then controlled by a special continuous steel casting process to obtain a continuous casting slab with central porosity and central segregation that meet the requirements. The continuous casting slab is subjected to controlled rolling and controlled cooling to produce a steel plate with good resistance to lamellar tearing, which is then subjected to heat treatment to achieve the final performance. The steel plate of the present invention has a thickness of >80mm~150mm, excellent resistance to lamellar tearing, and a tensile section shrinkage rate of >35% in the Z direction (thickness direction) at half the thickness. It can be widely used in the construction of steel structures such as large bridges and high-rise buildings. However, this patent is aimed at slabs and is not suitable for special-shaped billets with complex cross-sectional shapes.

[0008] Summary of the Invention

[0009] The object of the present invention is to provide a production method for improving the Z-direction performance of heavy-duty hot-rolled H-beam. By setting the secondary cooling mode of continuous casting of special-shaped billets, defects such as transverse cracks on the surface of the special-shaped billets, white spots in the finished product during tensile testing, and low Z-direction performance are solved. During rolling by the universal unit, the temperature difference between the flange surface and the flange core is controlled within the range of 300 to 500°C. Strain accumulation is used to provide the strain energy storage and nucleation position required for subsequent phase transformation and precipitation, thereby refining the grains and further improving the Z-direction performance.

[0010] The present invention also provides heavy hot-rolled H-shaped steel produced by the production method. The Z-direction performance of the heavy hot-rolled H-shaped steel is 35-65%, and the surface quality of the casting blank is good.

[0011] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0012] A production method for improving the Z-direction performance of heavy hot-rolled H-beam, the production method comprising the following steps: molten iron pretreatment, converter double slag smelting, LF furnace refining, VD vacuum degassing, continuous casting, slow cooling, hot rolling, and cooling;

[0013] In the continuous casting step, low superheat constant pulling speed casting is adopted, the superheat is 10-35°C, and the continuous casting pulling speed of the special-shaped billet section is 0.45-0.95m / min; the secondary cooling water ratio is 0.45-0.77L / kg, the foot roller area is cooled with water, and the movable section, the first section, the second section and the third section are cooled by air-water atomization; the water distribution ratio of the foot roller area, the movable section, the first section, the second section and the third section is 25-33%: 40-45%: 15-20%: 7-10%: 4.5-6%.

[0014] In the continuous casting step, the top of the inner arc between the third sector of the beam blank continuous casting machine and the straightening roller is insulated by a heat insulation cover.

[0015] In the continuous casting step, an immersion nozzle with side holes is used for casting, the pouring point is set at the center of the web, and the part of the lower mouth of the immersion nozzle immersed in the molten steel is flat.

[0016] The specifications of the special-shaped blank obtained by continuous casting are: width 750-1300 mm, height 440-510 mm, and flange top width 80-180 mm.

[0017] The BB5 blank adopts a constant pulling speed of 0.75m / min; the BB6 blank adopts a constant pulling speed of 0.88m / min; and the BB7 blank adopts a constant pulling speed of 0.55m / min.

[0018] The cross-section of the BB5 billet is 900mm*450mm*130mm; the cross-section of the BB6 billet is 1030mm*440mm*80mm; and the cross-section of the BB7 billet is 1300mm*510mm*180mm.

[0019] In the slow cooling step, the stack is slowly cooled for 35 to 50 hours.

[0020] In the hot rolling step, the rough rolling speed is ≤2m / s, and the temperature drop during the rough rolling process is ≤120°C; after rough rolling, the billet enters the universal rolling unit for rolling, the rolling speed is ≤3.0m / s, and the difference between the flange surface temperature and the flange core temperature is 200-500°C.

[0021] When the universal unit is rolling, open the side nozzles at the swing roller of the rolling mill to cool the flange end of the H-beam to ensure that the water pressure is not less than 1MPa, and reduce the cooling water pressure of the universal horizontal roller to ≤0.5MPa to ensure that the temperature difference between the flange surface and the flange core is between 200 and 500℃.

[0022] In the cooling step, after the billet is rolled into a finished product and comes off the production line, it is first cooled to 500°C at a rate of 5-10°C / s, and then slowly cooled in a holding pit within a temperature range of 500-300°C for more than 24 hours, with a cooling rate of 0.1-1.0°C / s during this process.

[0023] The present invention also provides heavy hot-rolled H-shaped steel produced according to the production method.

[0024] The heavy-duty hot-rolled H-beam comprises the following chemical composition by weight: C 0.06% to 0.20%, Si 0.25% to 0.55%, Mn 1.35% to 1.60%, P ≤ 0.020%, S ≤ 0.005%, Ni ≤ 1.5%, V 0.001% to 0.09%, Nb: 0.005% to 0.040%, Ti: 0.008% to 0.020%, Als 0.006% to 0.035%, N 30 to 60 ppm, H ≤ 5.5 ppm, with the remainder being Fe and other unavoidable impurities. All steel grades with this chemical composition, when produced according to the production method provided by the present invention, can produce heavy-duty hot-rolled H-beams with good casting surface quality and excellent Z-direction properties.

[0025] The Z-direction performance of the heavy hot-rolled H-shaped steel is 35-65%, and the surface quality of the casting blank is good.

[0026] The present invention provides a production method for improving the Z-direction performance of heavy-duty hot-rolled H-beams. The method utilizes a production process involving molten iron pretreatment, converter double-slag smelting, LF furnace refining, VD vacuum degassing, and continuous casting to produce ingots. This streamlined process reduces production costs. The VD or LF process helps reduce the gas content in the steel, minimizing porosity and shrinkage cavities, thereby preventing deformation and penetration during subsequent rolling due to a low compression ratio, which can affect the Z-direction performance of the finished product. Furthermore, the dual LF furnace refining and VD vacuum degassing processes ensure that the sulfur content of the steel is ≤0.005%, thereby reducing the formation of sulfides and the resulting degradation of Z-direction performance.

[0027] During the continuous casting process, the casting speed for beam blank cross-section continuous casting is 0.45-0.95 m / min. In the secondary cooling mode, the foot roll area is cooled with water, while the active section, first section, second section, and third section utilize air-water atomization. The secondary cooling water ratio is 0.45-0.77 L / kg. The water distribution ratios for the foot roll area, active section, first section, second section, and third section are 25-33%: 40-45%: 15-20%: 7-10%: 4.5-6%. By adjusting the water ratio and the proportions of the secondary cooling water in different sections, the formation and proportion of equiaxed grains during the solidification of oversized beam blanks can be increased, the formation of columnar grains can be reduced, and defects such as central porosity and shrinkage cavities can be reduced. Failure to maintain these water ratios and proportions can lead to excessive columnar grain formation, increased central porosity and shrinkage cavities, and deterioration of Z-axis properties.

[0028] The present invention adopts an immersion nozzle with side holes for casting in the continuous casting process, and the part of the lower mouth of the immersion nozzle immersed in the molten steel is designed to be flat, which is beneficial to the secondary oxidation of the molten steel, controls the flow state and injection speed of the molten steel, promotes the floating of inclusions, and prevents non-metallic inclusions such as protective slag from being drawn into the molten steel, resulting in large and numerous inclusions, thereby reducing the Z-direction performance.

[0029] During the continuous casting process, a low superheat and constant casting speed are used, with a superheat of 10-35°C. The BB5 billet uses a constant casting speed of 0.75 m / min; the BB6 billet uses a constant casting speed of 0.88 m / min; and the BB7 billet uses a constant casting speed of 0.55 m / min. The lower superheat can reduce the formation of columnar crystals, increase the formation of equiaxed crystals, and reduce central porosity and shrinkage cavities under high compression conditions. Furthermore, the constant casting speed promotes uniformity of the microstructure along the length of the billet, resulting in a relatively uniform microstructure throughout the entire billet.

[0030] In the continuous casting process, an insulation cover is used to insulate the top of the inner arc between the third section of the fan-shaped segment of the beam blank continuous casting machine and the straightening roller. The use of the insulation cover makes the temperature drop slower at the end of the beam blank, avoids the high-temperature second brittle zone during the straightening process, and improves the top crack of the beam blank.

[0031] In the hot rolling process, in order to increase the deformation penetration and microstructure refinement during the rolling process, the rolling speed is controlled to ≤3.0m / s during the rolling process of the universal unit, and the measuring nozzle at the swing roller of the rolling mill is opened to cool the flange end of the H-beam to ensure that the water pressure is not less than 1MPa. The cooling water pressure of the universal horizontal roller is reduced to 0.5MPa to ensure that the temperature difference between the flange surface and the flange core is within the range of 200-500℃. Strain accumulation is used to provide the strain energy storage and nucleation position required for subsequent phase transformation and precipitation, refine the grains, and improve the Z-direction performance.

[0032] After the billet is rolled into finished products, it is cooled to 500℃ at a rate of 5-10℃ / s in the high temperature section, and then slowly cooled in the temperature range of 500-300℃ for more than 24 hours. The purpose is to prevent the overflow of hydrogen and make the hydrogen content of the steel ≤5.5ppm. A high hydrogen content in the steel will cause white spots to appear during the stretching process, increase the brittleness of the steel, and sharply deteriorate the Z-direction performance of the steel.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The production method provided by the present invention for improving the Z-direction performance of heavy-duty hot-rolled H-beams takes into account the impact of the continuous casting process on this performance. By controlling the process parameters during continuous casting, using a submerged nozzle with side holes for casting, and designing the portion of the submerged nozzle immersed in the molten steel to be flat, and using an insulation cover to insulate the top of the inner arc between the third sector of the beam blank continuous casting machine and the straightening rollers, the Z-direction performance of the heavy-duty hot-rolled H-beams is improved. Combined with the subsequent hot rolling process, this further ensures the excellent Z-direction performance of the heavy-duty hot-rolled H-beams. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG1 is a port picture of the heavy hot-rolled H-beam tensile specimens in Examples 1 to 8;

[0036] FIG2 is a schematic diagram of a heavy hot-rolled H-beam profile blank flange without transverse crack defects in Example 1;

[0037] FIG3 is a schematic diagram showing that the flange of the heavy hot-rolled H-beam profile blank in Comparative Example 1 has no transverse crack defects;

[0038] FIG4 is a schematic diagram showing that the flange of the heavy hot-rolled H-beam profile blank in Comparative Example 3 has no transverse crack defects;

[0039] FIG5 is a schematic diagram showing that the flange of the heavy hot-rolled H-shaped steel profile blank in Comparative Example 4 has no transverse crack defects. DETAILED DESCRIPTION

[0040] The present invention provides a production method for improving the Z-direction performance of heavy hot-rolled H-beam, the production method comprising the following steps: molten iron pretreatment, converter double slag smelting, LF furnace refining, VD vacuum degassing, continuous casting, slow cooling, hot rolling, and cooling;

[0041] In the continuous casting step, low superheat constant pulling speed casting is adopted, the superheat is 10-35°C, and the continuous casting pulling speed of the special-shaped billet section is 0.45-0.95m / min; the secondary cooling water ratio is 0.45-0.77L / kg, the foot roller area is cooled with water, and the movable section, the first section, the second section and the third section are cooled by air-water atomization; the water distribution ratio of the foot roller area, the movable section, the first section, the second section and the third section is 25-33%: 40-45%: 15-20%: 7-10%: 4.5-6%.

[0042] In the continuous casting step, the top of the inner arc between the third sector of the beam blank continuous casting machine and the straightening roller is insulated by a heat insulation cover.

[0043] In the continuous casting step, an immersion nozzle with side holes is used for casting, the pouring point is set at the center of the web, and the part of the lower mouth of the immersion nozzle immersed in the molten steel is flat.

[0044] The specifications of the special-shaped blank obtained by continuous casting are: width 750-1300 mm, height 440-510 mm, and flange top width 80-180 mm.

[0045] The BB5 blank adopts a constant pulling speed of 0.75m / min; the BB6 blank adopts a constant pulling speed of 0.88m / min; and the BB7 blank adopts a constant pulling speed of 0.55m / min.

[0046] The cross-section of the BB5 billet is 900mm*450mm*130mm; the cross-section of the BB6 billet is 1030mm*440mm*80mm; and the cross-section of the BB7 billet is 1300mm*510mm*180mm.

[0047] In the slow cooling step, the stack is slowly cooled for 35 to 50 hours.

[0048] In the hot rolling step, the rough rolling speed is ≤2m / s, and the temperature drop during the rough rolling process is ≤120°C; after rough rolling, the billet enters the universal rolling unit for rolling, the rolling speed is ≤3.0m / s, and the difference between the flange surface temperature and the flange core temperature is 200-500°C.

[0049] When the universal unit is rolling, open the side nozzles at the swing roller of the rolling mill to cool the flange end of the H-beam to ensure that the water pressure is not less than 1MPa, and reduce the cooling water pressure of the universal horizontal roller to ≤0.5MPa to ensure that the temperature difference between the flange surface and the flange core is between 200 and 500℃.

[0050] In the cooling step, after the billet is rolled into a finished product and comes off the production line, it is first cooled to 500°C at a rate of 5-10°C / s, and then slowly cooled in a holding pit within a temperature range of 500-300°C for more than 24 hours, with a cooling rate of 0.1-1.0°C / s during this process.

[0051] The heavy hot-rolled H-beam includes the following chemical components in weight percentage: C 0.06% to 0.20%, Si 0.25% to 0.55%, Mn 1.35% to 1.60%, P≤0.020%, S≤0.005%, Ni≤1.5%, V 0.001% to 0.09%, Nb: 0.005% to 0.040%, Ti: 0.008% to 0.020%, Als 0.006% to 0.035%, N 30 to 60 ppm, H≤5.5 ppm, and the rest is Fe and other inevitable impurities.

[0052] The present invention is described in detail below with reference to the embodiments.

[0053] The steel types and weight percentages of the components of the heavy hot-rolled H-shaped steel produced in each embodiment and comparative example are shown in Table 1.

[0054] Table 1

[0055] The process parameters of the heavy hot-rolled H-beam continuous casting section produced in each embodiment and comparative example are shown in Table 2.

[0056] Table 2

[0057] The process parameters of the slow cooling, hot rolling and cooling sections of the heavy hot-rolled H-beam produced in each embodiment and comparative example, as well as the Z-direction tensile properties after rolling are shown in Table 3.

[0058] Table 3

[0059] From the above, it can be seen that the production method provided by the present invention can produce heavy-duty hot-rolled H-shaped steel with good surface quality of the ingot and excellent Z-direction performance. However, if the above process is not controlled according to the requirements of the present invention, heavy-duty hot-rolled H-shaped steel with good surface quality of the ingot and excellent Z-direction performance cannot be produced.

[0060] The above-mentioned detailed description of a production method for improving the Z-direction performance of heavy-duty hot-rolled H-shaped steel and heavy-duty hot-rolled H-shaped steel with reference to the embodiments is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A production method for improving the Z-direction performance of heavy hot-rolled H-beam, characterized in that: The production method comprises the following steps: molten iron pretreatment, converter double slag smelting, LF furnace refining, VD vacuum degassing, continuous casting, slow cooling, hot rolling, and cooling; In the continuous casting step, low superheat constant pulling speed casting is adopted, the superheat is 10-35°C, and the continuous casting pulling speed of the special-shaped billet section is 0.45-0.95m / min; the secondary cooling water ratio is 0.45-0.77L / kg, the foot roll area is cooled with water, and the active section, the first section, the second section and the third section are cooled by air-water atomization; the water distribution ratio of the foot roll area, the active section, the first section, the second section and the third section is 25-33%: 40-45%: 15-20%: 7-10%: 4.5-6%.

2. The production method according to claim 1, characterized in that In the continuous casting step, the top of the inner arc between the third sector of the profiled blank continuous casting machine and the straightening roller is insulated by a heat insulation cover.

3. The production method according to claim 1, characterized in that In the continuous casting step, an immersion nozzle with side holes is used for casting, the pouring point is set at the center of the web, and the part of the lower mouth of the immersion nozzle immersed in the molten steel is flat.

4. The production method according to claim 1, characterized in that The specifications of the special-shaped blank obtained by continuous casting are: width 750-1300mm, height 440-510mm, and flange top width 80-180mm.

5. The production method according to claim 1, characterized in that: The BB5 billet adopts a constant pulling speed of 0.75m / min; the BB6 billet adopts a constant pulling speed of 0.88m / min; and the BB7 billet adopts a constant pulling speed of 0.55m / min.

6. The production method according to claim 1, characterized in that: In the slow cooling step, the stack is slowly cooled for 35 to 50 hours.

7. The production method according to any one of claims 1 to 6, characterized in that: In the hot rolling step, the rough rolling speed is ≤2m / s, and the temperature drop during the rough rolling process is ≤120°C; after the rough rolling, the billet enters the universal unit for rolling, the rolling speed is ≤3.0m / s, and the temperature difference between the flange surface temperature and the flange core temperature is 200-500°C.

8. The production method according to claim 7, characterized in that: When the universal unit is rolling, open the side nozzles at the swing roller of the rolling mill to cool the flange end of the H-beam to ensure that the water pressure is not less than 1MPa, and reduce the cooling water pressure of the universal horizontal roller to ≤0.5MPa to ensure that the temperature difference between the flange surface and the flange core is 200-500℃.

9. The production method according to any one of claims 1 to 6, characterized in that: In the cooling step, after the billet is rolled into a finished product off the production line, it is first cooled to 500° C. at a rate of 5 to 10° C. / s, and then slowly cooled in a holding pit within a temperature range of 500 to 300° C. for more than 24 hours.

10. Heavy hot-rolled H-beam produced by the production method according to any one of claims 1 to 9.

11. The heavy hot-rolled H-beam according to claim 10, characterized in that: The heavy hot-rolled H-beam includes the following chemical components in weight percentage: C 0.06%-0.20%, Si 0.25%-0.55%, Mn 1.35%-1.60%, P≤0.020%, S≤0.005%, Ni≤1.5%, V 0.001%-0.09%, Nb: 0.005-0.040%, Ti: 0.008-0.020%, Als 0.006%-0.035%, N 30-60ppm, H≤5.5ppm, and the rest is Fe and other inevitable impurities.

12. The heavy hot-rolled H-beam according to claim 10, characterized in that: The Z-direction performance of the heavy hot-rolled H-shaped steel is 35-65%, and the surface quality of the casting is good.

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