A method for producing a hot-rolled strip steel for high-frequency welded pipe with a yield strength of 700 MPa

Abstract

The application provides a production method of a hot-rolled strip steel for a 700MPa-grade high-frequency welded pipe, which comprises the following steps of steelmaking, continuous casting, heating, rolling, cooling, coiling and flattening. A secondary calcium treatment process is adopted in the steelmaking stage, and the contents of nitrogen, sulfur and calcium in the refined molten steel and the mass ratio of calcium to sulfur are controlled; the subsequent continuous casting process, multi-stage heating process, rolling process and three-stage cooling process are combined, and finally the obtained hot-rolled strip steel has excellent strength and plasticity and good overall strength and plasticity stability. The yield strength of the hot-rolled strip steel is 740-780MPa, the tensile strength is 820-850MPa, the elongation is 22-25%, and the product of strength and plasticity is 18-23GPa·%. Meanwhile, the elongation of the steel pipe prepared by high-frequency welding of the hot-rolled strip steel can also reach more than 20%, the flattening performance and flaring are good without cracking phenomenon, the internal stress of the steel pipe is low, the welding performance is good, and the right angle is not greater than 2mm / 10m.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel processing, in particular to a production method of hot-rolled strip steel for 700MPa grade high-frequency welded pipe. BACKGROUND

[0002] High-frequency welded pipe is a kind of metal pipe material which is formed by heating the edge of the steel strip to a molten state through the skin effect and proximity effect of high-frequency current, and then pressing and connecting it into a pipe by an extrusion roller. It has the advantages of small welding heat affected zone and high production efficiency, and is widely used in the manufacturing of new energy automobile structural parts such as battery frame, frame, and anti-collision beam.

[0003] 700MPa grade high-strength steel is mainly used for the manufacturing of automobile beams, body compartments and other cold-bent structural parts. In recent years, it has been gradually applied to the high-frequency welded pipe industry. However, during the high-frequency welding process, there are problems such as low elongation of the strip steel, high internal stress, large strength fluctuation, poor welding performance, and poor straightness of the welded pipe, which to some extent limit its further promotion in the high-frequency welded pipe industry.

[0004] The influence of strip steel quality on high-frequency welded pipe processing mainly reflects in the following aspects: (1) low elongation of the strip steel will result in poor shaping of the finished steel pipe, which is difficult to meet the use requirements, and even there may be a phenomenon of cracking during pipe bending processing; (2) factors such as strength fluctuation in the width direction and rolling direction of the strip steel, and internal stress distribution state of the strip steel will affect the butt joint accuracy of the weld, and then affect the weld quality and straightness of the welded pipe; (3) the content of impurity elements and the morphology of inclusions in the strip steel are also important aspects affecting the welding quality of the strip steel, which can easily cause problems such as welding cracks and flattening cracks.

[0005] In order to solve the problem of large strength fluctuation of high-strength steel plate for high-frequency welded pipe, some steel plants use Mo micro-alloying method. For example, patent publication No. CN 118531312 A discloses an 800MPa grade nano precipitation steel and its preparation method, which adds 0.10~0.40%Mo, and the tensile strength of the steel plate is greater than 800MPa, which significantly increases the alloy cost. However, the problem of low elongation of high-frequency welded pipe has not been solved. SUMMARY

[0006] In order to solve the technical problems of low elongation and poor straightness of 700MPa grade high-frequency welded pipe in the prior art, the present application provides a production method of hot-rolled strip steel for 700MPa grade high-frequency welded pipe. The hot-rolled strip steel has excellent tensile strength and plasticity, and at the same time has small internal stress, is easy to weld, and the elongation of the steel pipe prepared by high-frequency welding is high and the straightness is good.

[0007] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0008] A production method of a hot-rolled strip steel for a 700MPa grade high-frequency welded pipe with yield strength, comprising the following steps:

[0009] (1) Steelmaking

[0010] After the converter molten steel is subjected to LF refining, primary calcium treatment, RH treatment and secondary calcium treatment, refined molten steel is obtained; the nitrogen content in the refined molten steel is ≤0.0050%, the sulfur content is ≤0.0030%, the calcium content is 0.0015~0.0040%, and the calcium-sulfur mass ratio is 0.8~4.0;

[0011] (2) Continuous casting

[0012] The refined molten steel is subjected to continuous casting to obtain a slab, and in the continuous casting process, the tundish temperature is T L +10~25℃, the casting speed is 1.20±0.1m / min; the water quantity of the crystallizer is 3600~3800L / min on the wide side and 500~600L / min on the narrow side; electromagnetic stirring is used in the secondary cooling zone, and the stirring current is 200~400A and the frequency is 5~7Hz;

[0013] (3) Heating

[0014] The slab is heated to obtain an intermediate billet, and the heating includes a preheating stage, a first heating stage, a second heating stage and a soaking stage; the initial temperature of the preheating stage is 400~700℃, the temperature rising rate from the preheating stage to the second heating stage is 5.0~8.0℃ / min, the temperature of the second heating stage is 1280~1310℃, the temperature of the soaking stage is 1270~1300℃, and the heating time of the soaking stage is 50~700min;

[0015] (4) Rolling

[0016] The intermediate billet is subjected to rough rolling and finish rolling to obtain a finished plate shape; the thickness of the rough rolling outlet intermediate billet is 35~38mm, and the plate shape target value of the finish rolling outlet is set according to the thickness t of the finished plate shape; when the thickness of the finished plate shape is 1.8≤ t <2.5mm, the plate shape target value is set to 4~6IU; when the thickness of the finished plate shape is 2.5≤ t <4.0mm, the plate shape target value is set to 3~5IU; and when the thickness of the finished plate shape is 4.0≤ t ≤6.0mm, the plate shape target value is set to 1~3IU;

[0017] (5) Cooling and coiling

[0018] The finished plate shape is cooled by adopting a three-stage cooling mode of water cooling-air cooling-water cooling, and then coiled to obtain a coiled strip steel;

[0019] (6) Leveling

[0020] The flattening process adopts a negative convexity of 0.015 mm sine curve roll shape, the rolling force is set to 300-350 tons, and the bending force is set to 30-60 tons.

[0021] Further, the refined molten steel comprises the following components in percentage by mass: C: 0.05-0.10%, Si: 0.10-0.30%, Mn: 1.50-1.70%, Nb: 0.020-0.050%, Ti 0.10-0.13%, Al 0.020-0.040%, Ca 0.0015-0.0040%, P≤0.018%, S≤0.0030%, N:≤0.0040%, and the rest is iron and inevitable impurities.

[0022] The roles of the elements in the above component design of the refined molten steel and the selection of the amount of each element are as follows:

[0023] Carbon (C): Carbon is one of the most important alloying elements in steel, and its content directly affects the performance of the steel. In the present application, the mechanical properties of the steel are mainly ensured through precipitation strengthening and fine-grain strengthening. Carbon and micro-alloying elements such as Nb and Ti in the steel form dispersed precipitates, which can refine the grains and improve the strength of the steel. If the carbon content is too low, the strength of the steel cannot meet the requirements, and if the carbon content is too high, the plasticity and welding performance of the steel will deteriorate. Therefore, the carbon content is selected to be 0.05-0.10%.

[0024] Silicon (Si): Silicon does not form carbides in steel, but exists in the form of a solid solution in ferrite or austenite, which can improve the strength and hardness of the steel. If the silicon content is too high, it will cause red iron oxide defects on the surface of the strip steel. In the present application, the silicon content is controlled to be 0.10-0.30%.

[0025] Manganese (Mn): Manganese has the effects of solid solution strengthening, stabilizing austenite, and refining grains in steel. If the manganese content is too high, it will easily form segregation, which will affect the cold working performance of the steel. Therefore, the manganese content is selected to be 1.50-1.70%.

[0026] Phosphorus (P): Phosphorus is an impurity element in steel. If the phosphorus content in steel is high, it will easily cause the low-temperature toughness of the steel to deteriorate. Therefore, the phosphorus content in steel needs to be controlled, but if the phosphorus content is too low, it will also increase the smelting cost of the steel. Therefore, the phosphorus content is selected to be ≤0.018%.

[0027] Sulfur (S): Sulfur is an impurity element in steel, which easily combines with Mn to form MnS inclusions. After rolling deformation, MnS is in the form of long strips, which destroys the continuity of the matrix. In high-frequency welding, low-melting-point sulfide inclusions are one of the main reasons for causing welding cracks. Therefore, the sulfur content is selected to be ≤0.0030%.

[0028] Aluminum (Al): Aluminum is a deoxidizing element in steel, and excessive aluminum can increase Al2O3 inclusions in steel, affecting the cold workability of the steel. Therefore, the aluminum content is selected to be 0.020-0.050%.

[0029] Calcium (Ca): The main purpose of calcium treatment in the steelmaking process is to improve the castability of molten steel and improve the purity of the steel. After calcium treatment in the steelmaking process described in this application, a certain amount of calcium will remain in the molten steel, and the calcium to sulfur ratio of the molten steel will be controlled. By reacting Ca with S in the steel, spherical CaS is produced, thereby reducing the low-melting, strip-shaped distribution of MnS inclusions and avoiding the formation of high-frequency welding cracks.

[0030] Micro-alloying elements (Nb and Ti): (1) Nb and Ti are added together, supplemented by appropriate rolling processes, to inhibit austenite grain growth during heating, promote the precipitation of fine dispersed carbonitride during rolling, thereby inhibiting recrystallization, and precipitate during cooling and coiling, thereby refining the structure of the steel and improving the strength of the steel. (2) Nb and Ti, as strong carbide-forming elements, are added to the steel, which can reduce the activity of C in austenite, reduce the diffusion rate of C in the steel, and increase the undercooling degree of phase transition. In addition, the dispersed micro-alloyed carbonitride generated during rolling can provide more nucleation sites for phase transition. Through the increase of phase transition undercooling degree and nucleation sites, grain refinement is achieved, which, together with subsequent laminar cooling process, improves the plasticity of the steel and enhances the cold workability of the steel.

[0031] Further, the vacuum degree during the RH treatment is 0-2 mBar, and the treatment time is 10-15 min.

[0032] Further, the first calcium treatment feeds 130-150 m of calcium-silicon wire per furnace, with a soft stirring time of 5-7 min, and the second calcium treatment feeds 100-120 m of calcium-silicon wire per furnace, with a soft stirring time of 10-12 min.

[0033] Further, during the continuous casting process, the refined molten steel is fully protected by using a tundish covering agent, a submerged entry nozzle, and argon sealing, the depth of the submerged entry nozzle is 150-180 mm, and the argon sealing back pressure is 0.05-0.15 Bar.

[0034] Further, the rough rolling adopts 1+5 or 3+3 passes.

[0035] Further, the finish rolling inlet temperature is 1080-1120℃, and the outlet temperature is 900-930℃.

[0036] Further, in the three-stage cooling mode of water cooling-air cooling-water cooling, the first stage of water cooling is cooling to 650-680℃ at a rate of 30-60℃ / s, the second stage is air cooling, and the cooling time is 3-5s, and the third stage of water cooling is cooling to 590-620℃ at a rate of 30-60℃ / s for coiling, and the upper and lower water ratio used in the first stage of water cooling and the third stage of water cooling is 0.8.

[0037] Further, the coiled strip steel is placed in a holding pit for slow cooling after coiling, and the slow cooling time is 48-72h or more.

[0038] Further, the difference between the roll diameters of the upper work roll and the lower work roll of the temper mill in the tempering process is 0-0.5mm.

[0039] The beneficial effects of the present application are:

[0040] The hot-rolled strip steel prepared by the method has excellent strength and plasticity, wherein the yield strength of the hot-rolled strip steel is 740-780MPa, the tensile strength is 800-850MPa, the elongation is 21-25%, and the strength-plasticity product is 16-21.3GPa·%. The overall strength and plasticity stability are good. At the same time, the elongation of the steel pipe prepared by high-frequency welding of the hot-rolled strip steel can also reach more than 20%, the flattening performance and flaring are good without cracking phenomenon, the internal stress of the steel pipe is low, the welding performance is good, and the right angle is not greater than 2mm / 10m.

[0041] In the steelmaking process, S tends to combine with Mn to form manganese sulfide during the solidification process of the molten steel, and in the subsequent hot rolling process, the manganese sulfide inclusions will evolve into slender strip-shaped or spindle-shaped inclusions. In the process of rapid heating and cooling of high-frequency welding, low-melting-point sulfide inclusions are easy to become potential crack sources. Compared with Mn element, Ca has stronger affinity with S and is more likely to react with S to generate spherical CaS or (Ca, Mn) S complex inclusions with higher melting point, which can reduce the influence of S on the quality of high-frequency welding; based on the above factors, the present application adopts a secondary calcium treatment process in the steelmaking stage, and the sulfur content, calcium content and calcium-sulfur mass ratio in the molten steel are controlled to effectively inhibit the generation of slender strip-shaped or spindle-shaped manganese sulfide inclusions, reduce the influence of S on the quality of subsequent high-frequency welding, and combined with subsequent rolling and tempering plate shape strategy, good welding quality is obtained.

[0042] In the high Ti steel, TiN has two precipitation modes of liquid precipitation and solid precipitation, the liquid precipitation TiN has high precipitation temperature and large particle size, which is harmful to the performance of the steel and reduces the effective titanium content in the steel, thereby reducing the strengthening effect of the high Ti steel. The application controls the TiN precipitation behavior by regulating the calcium treatment process, the nitrogen content in the molten steel, the calcium content, and combining the water quantity control process of the continuous casting mold, the lower N content can reduce the formation of coarse liquid precipitated TiN, and the secondary calcium treatment process can effectively spheroidize the inclusions of the steel, promote the floating of large particle inclusions, and the dispersed fine composite inclusions in the steel can act as nucleation sites for fine TiN, under the influence of the water quantity of the continuous casting mold, fine and dispersed precipitation is formed, thereby achieving the purpose of inhibiting the formation of large size TiN and increasing the density of fine TiN. On the one hand, reducing the large particle TiN in the steel can ensure the strength and plasticity of the steel, on the other hand, the precipitation of fine TiN particles in the steel also helps to improve the welding quality. Single LF or RH calcium treatment cannot completely achieve the effect.

[0043] In the continuous casting process, protective casting is adopted during continuous casting, mainly to prevent the molten steel from contacting with air during casting, thereby avoiding the secondary oxidation and nitrogen absorption of the molten steel, improving the purity of the molten steel, and reducing the formation of large particle titanium nitride; appropriate tundish superheat and casting speed are adopted, and electromagnetic stirring process is adopted in the secondary cooling zone, the main purpose is to reduce the center segregation and center crack, avoid the formation of band-shaped distributed manganese sulfide and titanium nitride inclusions at the segregation and crack, thereby reducing the influence of slab defects on the plasticity and strength of the steel.

[0044] In the heating process, the application sets the heating rate of the heating stage to 5.0~8.0℃ / min; sets the temperature of the second adding stage to 1280~1310℃, sets the temperature of the soaking stage to 1270~1300℃, sets the time of the soaking stage to 50~70min, the soaking temperature is lower than the second adding temperature, and the soaking time is ensured, which can effectively improve the temperature uniformity of the slab, thereby ensuring that the micro-alloying elements at different positions in the thickness direction and length direction of the slab are fully solid-solved, providing temperature guarantee for subsequent precipitation strengthening, combined with the cooling process, obtaining uniform precipitation strengthening effect; at the same time, it can also reduce the uneven deformation in the hot rolling process, thereby improving the shape precision of the hot rolled plate and avoiding uneven stress distribution of the cost steel plate.

[0045] In the rolling process, the rolling process of the application can effectively reduce the rolling load by setting the intermediate billet thickness at the rough rolling outlet, the temperature at the entrance of the finishing rolling and the outlet temperature, thereby improving the temperature uniformity in the width and length directions of the plate, providing a basis for uniform phase change in the width and length directions of the plate in the subsequent cooling process; at the same time, the shape target value of the plate at the outlet of the finishing rolling is set according to the thickness t of the finished plate, which can compensate for the edge wave trend caused by the inconsistent cooling of the edge and the middle part in the laminar cooling process.

[0046] In the cooling process, the cooling stage of the application adopts a three-stage cooling mode. By coordinating the cooling speed, temperature and cooling method, the phase transformation and micro-alloying precipitation process of the steel are precisely controlled. In the first stage, water cooling is fast, the rolling piece is kept in the hardened austenite state, and is cooled to 650-680°C, and then air-cooled, so that the nanocarbide is quickly and uniformly precipitated. Then, in the third stage, fast cooling is continued to 590-620°C, on the one hand to inhibit the growth and coarsening of the precipitates, and on the other hand to obtain fine quasi-polygonal ferrite matrix through phase transformation strengthening. By controlling the upper and lower water ratio in the water cooling section, the upper and lower surfaces of the steel are uniformly cooled, and the cooling and phase transformation stress is reduced.

[0047] In the slow cooling stage, the application places the coiled strip in the holding pit for slow cooling for 48-72h after coiling. On the one hand, the internal stress generated by cooling and phase transformation can be slowly released, and on the other hand, the slow cooling in the holding pit can promote the complete precipitation of the micro-alloyed carbonitride at the head and tail of the strip, thereby improving the performance of the head and tail of the strip.

[0048] In the leveling stage, in view of the characteristics of high strength steel, such as high strength and high internal stress, the application adopts a concave roller shape in the leveling process to avoid uneven deformation on the left and right sides caused by strip deviation; increase the leveling force of the leveling unit to improve the shape of the high-strength strip; reduce the roll diameter difference of the leveling unit to avoid uneven deformation on the upper and lower surfaces caused by the speed difference between the upper and lower rollers. BRIEF DESCRIPTION OF DRAWINGS

[0049] In order to more clearly illustrate the technical solutions in the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiment description. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can also be obtained according to these drawings without creative labor for those skilled in the art.

[0050] Figure 1 Figure 1 is a 500X metallographic morphology diagram of the hot-rolled strip steel obtained in Example 1 of the application;

[0051] Figure 2 Figure 2 is a 5000X scanning morphology diagram of the hot-rolled strip steel obtained in Example 1 of the application;

[0052] Figure 3 Figure 3 is a 20000X scanning morphology diagram of the hot-rolled strip steel obtained in Example 1 of the application. DETAILED DESCRIPTION

[0053] For the purposes of the present application, the technical solutions and advantages will be more apparent, the technical solutions of the embodiments will be combined below to clearly and completely describe the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application. In the description of the present application, the experimental methods are conventional methods unless otherwise specified; the reagents and materials are commercially available unless otherwise specified.

[0054] The following disclosure provides many different embodiments or examples for implementing the present application. In the interest of simplifying the present application, the components and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present application. Those skilled in the art can realize the application of other processes and / or the use of other materials.

[0055] Embodiment 1

[0056] The present embodiment provides a production method of a hot-rolled strip steel for high-frequency welded pipe with a finished plate thickness t of 2.5 mm and a yield strength of 700 MPa, comprising the following steps:

[0057] (1) Steelmaking

[0058] The converter molten steel is subjected to LF refining, first calcium treatment, RH treatment, and second calcium treatment after breaking the vacuum, to obtain refined molten steel, wherein the first calcium treatment feeds 140 m of silicon-calcium wire per furnace, with a soft stirring time of 6 min, and the second calcium treatment feeds 110 m of silicon-calcium wire per furnace, with a soft stirring time of 11 min. The vacuum degree during RH treatment is 1 mBar, and the treatment time is 13 min. After vacuum treatment, the vacuum is broken, and 160 m of seamless silicon-calcium wire is fed for calcium treatment; finally, refined molten steel is obtained. The mass percentage of each chemical component in the refined molten steel is shown in Table 1 below.

[0059] (2) Continuous casting

[0060] The refined molten steel is subjected to full protection casting using a tundish covering agent, a submerged nozzle, and argon sealing to obtain a slab; wherein the submerged nozzle depth is 160 mm, and the argon sealing back pressure is 0.10 Bar; the tundish temperature is T L +20℃, the actual value is 1536℃, and the withdrawal speed is 1.20 m / min; the crystallizer water quantity is 3700 L / min on the wide side and 550 L / min on the narrow side; electromagnetic stirring is used in the secondary cooling zone, with a stirring current of 300 A and a frequency of 6 Hz.

[0061] (3) Heating

[0062] The slab is put into a heating furnace to heat the intermediate slab, the heating furnace includes a preheating stage, a first heating stage, a second heating stage and a soaking stage, the entry temperature, i.e. the initial temperature of the preheating stage, is 550°C, the temperature rising rate from the preheating stage to the second heating stage is 6.5°C / min, the temperature of the second heating stage is 1295°C, the temperature of the soaking stage is 1285°C, and the heating time of the soaking stage is 60 min;

[0063] (4) Rolling

[0064] The intermediate slab is roughed and finished to obtain a finished plate shape, the roughing adopts 1+5. The thickness of the roughing exit intermediate slab is 37 mm, the entry temperature of the finishing is 1100°C, and the exit temperature is 915°C. The target value of the finishing exit plate shape is set according to the thickness t of the finished plate shape; in this embodiment, the thickness t of the finished plate shape is 2.5 mm, and the target value of the finishing exit plate shape is 4IU.

[0065] (5) Cooling and coiling

[0066] The finished plate shape is cooled by using a three-stage cooling mode of water cooling-air cooling-water cooling. In the three-stage cooling mode of water cooling-air cooling-water cooling, the first-stage water cooling is cooled to 670°C at a rate of 40-50°C / s, the second-stage air cooling is 4 s, the third-stage water cooling is cooled to 610°C at a rate of 43°C / s, and then the coiled strip is obtained. The coiled strip is placed in a holding pit for slow cooling for 60 h.

[0067] In the first-stage water cooling and the third-stage water cooling, the upper and lower water ratio is 0.8.

[0068] (6) Leveling

[0069] The coiled strip after slow cooling is leveled to obtain a hot-rolled strip for 700 MPa grade high-frequency welded pipe with a thickness of 2.5 mm and a yield strength of 700 MPa. In the leveling process, a negative crown of 0.015 mm sine curve roll shape is used, the rolling force is set to 330 tons, the bending force is set to 45 tons, and the roll diameter difference of the upper and lower work rolls is not more than 0.1 mm.

[0070] Example 2

[0071] This embodiment provides a production method of a hot-rolled strip for 700 MPa grade high-frequency welded pipe with a finished plate shape thickness t of 5.0 mm and a yield strength of 700 MPa, which comprises the following steps:

[0072] (1) Steelmaking

[0073] The converter liquid steel is refined by LF refining, first calcium treatment, RH treatment, and second calcium treatment after breaking the vacuum, the first calcium treatment feeds 130 m of calcium-silicon wire per furnace, the soft stirring time is 5 min, the second calcium treatment feeds 100 m of calcium-silicon wire per furnace, the soft stirring time is 10 min, the vacuum degree of the RH treatment process is 2 mBar, the treatment time is 10 min, and finally the refined liquid steel is obtained. The mass percentage of each chemical component in the refined liquid steel is shown in Table 1 below.

[0074] (2) Continuous casting

[0075] The refined liquid steel is fully protected cast by using a tundish covering agent, a submerged entry nozzle, and argon sealing, and a slab is obtained. The submerged entry nozzle depth is 150 mm, the argon sealing back pressure is 0.05 Bar, and the tundish temperature is T L +25℃, the actual value is 1545℃, and the casting speed is 1.21 m / min. The crystallizer water quantity is 3600 L / min on the wide side and 500 L / min on the narrow side. Electromagnetic stirring is used in the secondary cooling zone, and the stirring current is 200 A and the frequency is 5 Hz.

[0076] (3) Heating

[0077] The slab is put into a heating furnace to heat and obtain an intermediate billet. The heating furnace includes a preheating stage, a first heating stage, a second heating stage, and a soaking stage. The entry temperature, i.e., the initial temperature of the preheating stage, is 400℃, the temperature rising rate from the preheating stage to the second heating stage is 8.0℃ / min, the second heating stage temperature is 1280℃, the soaking stage temperature is 1270℃, and the soaking stage heating time is 50 min.

[0078] (4) Rolling

[0079] The intermediate billet is roughed and finished to obtain a finished plate shape. The roughing adopts 3+3 passes. The roughing outlet intermediate billet thickness is 35 mm, the finishing entry temperature is 1080℃, and the outlet temperature is 900℃. The finishing outlet plate shape target value is set according to the thickness t of the finished plate shape. In this embodiment, the finished plate shape thickness t is 5.0 mm, and the finishing outlet plate shape target value is 2IU.

[0080] (5) Cooling and coiling

[0081] The finished plate shape is cooled by using a three-stage cooling mode of water cooling-air cooling-water cooling. In the three-stage cooling mode of water cooling-air cooling-water cooling, the first stage water cooling is cooled to 680℃ at a rate of 30℃ / s, the second stage air cooling is 3 s, the third stage water cooling is cooled to 620℃ at a rate of 30℃ / s, and then the coiled strip steel is obtained. The coiled strip steel is placed in a holding pit for slow cooling for 48 h.

[0082] The water ratio of the first stage water cooling and the third stage water cooling is 0.9.

[0083] (6) flattening

[0084] The annealed and coiled strip steel is flattened to obtain a hot-rolled strip steel for 700MPa grade high-frequency welded pipe with a thickness of 5.0mm and a yield strength of 700MPa. In the flattening process, a negative crown of 0.015mm sine curve roll shape is used, the rolling force is set to 300-320 tons, the bending force is set to 30 tons, and the difference between the roll diameters of the upper and lower work rolls is not greater than 0.5mm.

[0085] Example 3

[0086] The present embodiment provides a production method of a hot-rolled strip steel for 700MPa grade high-frequency welded pipe with a finished plate thickness t of 2.0mm and a yield strength of 700MPa, comprising the following steps:

[0087] (1) steelmaking

[0088] The converter molten steel is subjected to LF refining, first calcium treatment, RH treatment, and second calcium treatment after breaking the vacuum to obtain refined molten steel. The first calcium treatment feeds 150m of silicon-calcium wire per furnace with a soft stirring time of 7min, and the second calcium treatment feeds 120m of silicon-calcium wire per furnace with a soft stirring time of 12min. The vacuum degree during the RH treatment process is 0mBar, and the treatment time is 15min, finally obtaining the refined molten steel. The mass percentages of the chemical components in the refined molten steel are shown in Table 1 below.

[0089] (2) continuous casting

[0090] The refined molten steel is fully protected cast using a tundish covering agent, a submerged nozzle, and argon sealing to obtain a slab. The submerged nozzle depth is 180mm, the argon sealing back pressure is 0.15Bar, and the tundish temperature is T L +15℃, the actual value is 1530℃, and the withdrawal speed is 1.19m / min. The crystallizer water quantity is 3800L / min on the wide side and 600L / min on the narrow side. Electromagnetic stirring is used in the secondary cooling zone with a stirring current of 400A and a frequency of 7Hz.

[0091] (3) heating

[0092] The slab is put into a heating furnace for heating to obtain an intermediate slab, the heating furnace comprises a preheating stage, a first heating stage, a second heating stage and a soaking stage, the entry temperature, i.e. the initial temperature of the preheating stage, is 700°C, the temperature rising rate from the preheating stage to the second heating stage is 5.0°C / min, the temperature of the second heating stage is 1310°C, the temperature of the soaking stage is 1300°C, and the heating time of the soaking stage is 70 min;

[0093] (4) Rolling

[0094] The intermediate slab is subjected to rough rolling and finish rolling to obtain a finished plate shape, the rough rolling adopts 1+5 passes. The thickness of the rough rolling exit intermediate slab is 38 mm, the finish rolling entry temperature is 1120°C, and the exit temperature is 930°C. The finish rolling exit plate shape target value is set according to the thickness t of the finished plate shape; in this embodiment, the thickness t of the finished plate shape is 2.0 mm, and the finish rolling exit plate shape target value is 5IU.

[0095] (5) Cooling and coiling

[0096] The finished plate shape is cooled by using a three-stage cooling mode of water cooling-air cooling-water cooling. In the three-stage cooling mode of water cooling-air cooling-water cooling, the first-stage water cooling is cooling to 680°C at a rate of 60°C / s, the second-stage air cooling is 5 s, and the third-stage water cooling is cooling to 590°C at a rate of 60°C / s, and then coiling is performed to obtain a coiled strip. The coiled strip is placed in a holding pit for slow cooling for 108 h.

[0097] In the first-stage water cooling and the third-stage water cooling, the upper-to-lower water ratio used is 0.7.

[0098] (6) Leveling

[0099] The coiled strip after slow cooling is leveled to obtain a hot-rolled strip for 700 MPa grade high-frequency welded pipe with a thickness of 3.0 mm. In the leveling process, a negative crown of 0.015 mm sine curve roll shape is used, the rolling force is set to 350 tons, the bending force is set to 60 tons, and the roll diameter difference of the upper and lower work rolls is not greater than 0 mm.

[0100] Comparative Example 1

[0101] This embodiment provides a production method of a hot-rolled strip with a finished plate shape thickness t of 2.5 mm, comprising the following steps:

[0102] (1) Steelmaking

[0103] The converter liquid steel is refined by LF refining, calcium treatment and RH treatment, the calcium treatment is fed with calcium-silicon wire 120 m / t, the soft stirring time is 4 min, the vacuum degree of the RH treatment process is 3 mBar, the treatment time is 9, and finally the refined liquid steel is obtained. The mass percentage of each chemical component in the refined liquid steel is shown in Table 1.

[0104] (2) Continuous casting

[0105] The refined liquid steel is fully protected cast by using tundish covering agent, submerged nozzle and argon sealing, and the slab is obtained. The submerged nozzle depth is 160 mm, the argon sealing back pressure is 0.10 Bar, the tundish temperature is T L +30℃, the actual value is 1546℃, the pulling speed is 1.25 m / min, the crystallizer water quantity is 3500 L / min on the wide side and 400 L / min on the narrow side, and no electromagnetic stirring is used in the secondary cooling zone.

[0106] (3) Heating

[0107] The slab is put into the heating furnace for heating to obtain the billet, the heating furnace includes a preheating stage, a first heating stage, a second heating stage and a soaking stage, the initial temperature of the preheating stage is 100℃, the temperature rising rate from the preheating stage to the second heating stage is 10.0℃ / min, the temperature of the second heating stage is 1270℃, the temperature of the soaking stage is 1250℃, and the heating time of the soaking stage is 40 min.

[0108] (4) Rolling

[0109] The billet is roughed and finished to obtain the finished plate shape, the roughing adopts 1+5 passes. The thickness of the roughing outlet billet is 40 mm, the temperature of the finishing entry is 1060℃, and the temperature of the finishing exit is 880℃. The target value of the finishing exit plate shape is 1 IU.

[0110] (5) Cooling and coiling

[0111] The finished plate shape is cooled to 570~590℃ by using the conventional front cooling mode, and then coiled to obtain the coiled strip. During the cooling process, the upper and lower water ratio is 1.0, and after coiling, the coiled strip is placed in the air for cooling.

[0112] (6) Leveling

[0113] The coiled strip is leveled to obtain a hot-rolled strip with a thickness of 2.5 mm. In the leveling process, the flat roller shape is used, the rolling force is set to 250 tons, the bending roller force is set to 200 tons, and the difference between the upper and lower work rolls is 0.6 mm.

[0114] Comparative Example 2

[0115] A production method of a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa, the difference between the method of the present comparative example and the method of Example 1 is only that, in the steelmaking process of the present comparative example, the calcium treatment process is not used in the LF refining and RH treatment, so that the contents of Ca and S in the chemical composition of the present comparative example are slightly different from those of Example 1, see Table 1 for details.

[0116] Comparative Example 3

[0117] A production method of a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa, the difference between the method of the present comparative example and the method of Example 1 is only that, in the steelmaking process of the present comparative example, the calcium treatment process is not used in the LF refining and RH treatment, so that the contents of Ca and S in the chemical composition of the present comparative example are slightly different from those of Example 1, see Table 1 for details.

[0118] Comparative Example 4

[0119] A production method of a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa, the chemical composition, process steps and parameters of the present comparative example are basically the same as those of Example 1, the only difference is that the plate shape target value of the present comparative example is not set according to the thickness t of the finished plate shape, and the plate shape target value is 0 IU.

[0120] Comparative Example 5

[0121] A production method of a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa, the chemical composition, process steps and parameters of the present comparative example are basically the same as those of Example 1, the only difference is that the finishing temperature of the present comparative example is 1050℃, and the outlet temperature is 860℃.

[0122] Comparative Example 6

[0123] A production method of a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa, the chemical composition, process steps and parameters of the present comparative example are basically the same as those of Example 1, the only difference is that the cooling process of the present comparative example is the conventional front-end cooling, and the upper and lower water ratio during cooling is 1.0.

[0124] Comparative Example 7

[0125] A production method of a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa, the chemical composition, process steps and parameters of the present comparative example are basically the same as those of Example 1, the only difference is that in the tempering process of the present comparative example, the difference between the upper and lower work rolls of the temper mill is 1.0 mm.

[0126] In order to better compare the refining molten steel components of each embodiment and comparative example, the chemical composition of the refining molten steel of each embodiment and comparative example is summarized as follows:

[0127] Table 1 Comparison of the mass percentage of the chemical composition of the refining molten steel of each embodiment and comparative example

[0128]

[0129] Performance detection test 1

[0130] In order to better compare the properties of the hot-rolled strip steel obtained by each embodiment and comparative example, the samples required for the tensile test of the hot-rolled strip steel obtained by each embodiment and comparative example are sampled by using the method of GB / T 2975-2018, and the tensile properties of each sample are detected according to the method of GB / T 228, and the detection results are shown in Table 2; the hot-rolled strip steel obtained by each embodiment is welded into a steel pipe according to the standard of GB / T 40316-2021, and the steel pipe is detected for tensile, flattening and flaring according to the standard of GB / T 40316-2021, and the detection results are shown in Table 2.

[0131] Table 2 Performance detection results of the hot-rolled strip steel obtained by each embodiment and comparative example and the steel pipe welded by the hot-rolled strip steel

[0132]

[0133] As can be clearly seen from the detection data of Examples 1-3 shown in Table 2, the yield strength of the hot-rolled strip steel prepared by using the embodiments of the present application is 740-780 MPa, the tensile strength is 800-850 MPa, the elongation is 21-25%, and the strength plastic product is 16-21.3 GPa·%. The overall strength stability is good. The elongation of the steel pipe prepared by welding can also reach more than 20%, the flattening performance and flaring are good without cracking phenomenon, the welding performance is good, and the right angle is less than 2 mm / 10 m.

[0134] As can be clearly seen from the performance detection data of Examples 1 and Comparative Examples 2 and 3, compared with not performing calcium treatment or only performing calcium treatment once, the second calcium treatment in the molten steel refining stage can effectively ensure that the strip steel has excellent tensile strength while improving its plastic properties, and the strip steel has excellent welding performance, the elongation of the steel pipe prepared by welding is high, and the quality is good.

[0135] It can be seen from the detection data of the comparative examples 4-7 that the setting of the shape target value of the finishing mill exit, the setting of the finishing mill entry temperature, the cooling mode of the cooling process and the diameter difference between the upper and lower work rolls in the tempering process have great influence on the welding performance of the strip steel. Meanwhile, it can be further seen from the detection data of the comparative example 5 that the finishing mill entry temperature also has a very obvious influence on the strength-plasticity product performance of the strip steel itself.

[0136] Performance detection test 2

[0137] The scanning of the microstructure and the detection of the metallographic morphology of the hot-rolled strip steel obtained in the embodiment 1 are carried out, and the detection results are shown in Figures 1 to 3 , wherein Figure 1 is a 500X metallographic morphology diagram of the hot-rolled strip steel, Figure 2 is a 5000X scanning morphology diagram of the hot-rolled strip steel, Figure 3 is a 20000X scanning morphology diagram of the hot-rolled strip steel. It can be seen from Figure 1 and Figure 2 that the matrix structure of the steel prepared in the application is composed of quasi-polygonal ferrite + granular M / A islands, the crystal grains are fine and uniformly distributed. It can be seen from Figure 3 that a large amount of dispersedly distributed micro-alloy carbon nitrides are dispersedly precipitated in the ferrite crystal grains.

[0138] In the above embodiments, the description of each embodiment has its own emphasis, and the parts not described in detail in a certain embodiment can be referred to the relevant description of other embodiments.

[0139] The production method of the hot-rolled strip steel for 700MPa grade high-frequency welded pipe provided by the embodiments of the application is described in detail above, and the principles and implementation manners of the application are described by applying specific examples in this paper, and the above description of the embodiments is only used to help understand the technical solutions and core ideas of the application; those skilled in the art should understand that the technical solutions recorded in the above embodiments can still be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the application.

Claims

1. A method of producing a hot-rolled strip steel for high-frequency welded pipe of a yield strength of 700 MPa grade, characterized by, It comprises the following steps: (1) Steelmaking The converter liquid steel is subjected to LF refining, primary calcium treatment, RH treatment and secondary calcium treatment to obtain refined liquid steel; the nitrogen content of the refined liquid steel is ≤0.0050%, the sulfur content is ≤0.0030%, the calcium content is 0.0015~0.0040%, and the mass ratio of calcium to sulfur is 0.8~4.0; (2) Continuous casting The refined molten steel is cast into a slab by continuous casting, and the continuous casting process has a tundish temperature of T L +10~25℃, a pulling speed of 1.20±0.1 m / min; a crystallizer water amount of 3600~3800 L / min on the wide surface and 500~600 L / min on the narrow surface; and an electromagnetic stirring in the secondary cooling zone with a stirring current of 200~400 A and a frequency of 5~7 Hz. (3) Heating The slab is heated to obtain an intermediate billet, and the heating comprises a preheating stage, a first heating stage, a second heating stage and a soaking stage; the initial temperature of the preheating stage is 400~700℃, the temperature rising rate from the preheating stage to the second heating stage is 5.0~8.0℃ / min, the temperature of the second heating stage is 1280~1310℃, the temperature of the soaking stage is 1270~1300℃, and the heating time of the soaking stage is 50~700min; (4) Rolling The intermediate billet is subjected to rough rolling and finish rolling to obtain a finished plate shape; the thickness of the rough rolling outlet intermediate billet is 35~38mm, and the target value of the plate shape at the finish rolling outlet is set according to the thickness t of the finished plate shape; when the thickness of the finished plate shape is 1.8≤t<2.5mm, the plate shape target value is set to 4~6IU; when the thickness of the finished plate shape is 2.5≤t<4.0mm, the plate shape target value is set to 3~5IU; and when the thickness of the finished plate shape is 4.0≤t≤6.0mm, the plate shape target value is set to 1~3IU; (5) Cooling and coiling The finished plate shape is cooled by a three-stage cooling mode of water cooling-air cooling-water cooling and then coiled to obtain a coiled strip; In the three-stage cooling mode of water cooling-air cooling-water cooling, the first-stage water cooling is cooled to 650~680℃ at a rate of 30~60℃ / s, the second-stage air cooling has a cooling time of 3~5s, and the third-stage water cooling is cooled to 590~620℃ for coiling at a rate of 30~60℃ / s; the upper and lower water ratios used in the first-stage water cooling and the third-stage water cooling are 0.8; (6) Leveling The leveling process adopts a negative crown of 0.015mm sine curve roll shape, the rolling force is set to 300~350tons, and the bending force is set to 30~60tons; the roll diameter difference of the upper and lower work rolls in the leveling process is 0~0.5mm; The refined liquid steel comprises the following components in terms of mass percentage: C: 0.05~0.10%, Si: 0.10~0.30%, Mn: 1.50~1.70%, Nb: 0.020~0.050%, Ti 0.10~0.13%, Al 0.020~0.040%, Ca 0.0015~0.0040%, P≤0.018%, S≤0.0030%, N:≤0.0040%, and the rest is iron and unavoidable impurities; The finish rolling inlet temperature is 1080~1120℃, and the outlet temperature is 900~930℃; After coiling, the coiled strip is placed in a holding pit for slow cooling, and the slow cooling time is 48~72h or more.

2. The method of producing a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa class according to claim 1, characterized in that: The vacuum degree of the RH treatment is 0-2 mBar, and the treatment time is 10-15 min.

3. The method of producing a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa class according to claim 1, characterized by, The first calcium treatment feeds in 130-150 m of silicon-calcium wire per furnace, and the soft stirring time is 5-7 min; the second calcium treatment feeds in 100-120 m of silicon-calcium wire per furnace, and the soft stirring time is 10-12 min.

4. The method of producing a hot-rolled strip steel for a high-frequency welded pipe with a yield strength of 700 MPa class according to claim 1, characterized by, In the continuous casting process, the refined molten steel is fully protected cast by using a tundish covering agent, a submerged nozzle and argon sealing, the submerged nozzle depth is 150-180 mm, and the argon sealing back pressure is 0.05-0.15 Bar.

5. The method of producing a hot-rolled strip steel for high-frequency welded pipe with a yield strength of 700 MPa class according to claim 1, characterized in that: The rough rolling adopts 1+5 or 3+3 passes.

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