CN112522597B

CN112522597B - Boron-containing high-reaming-hole steel and production method thereof

Info

Publication number: CN112522597B
Application number: CN201910889408.1A
Authority: CN
Inventors: 吴建春; 方园; 范建勇
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2019-09-19
Filing date: 2019-09-19
Publication date: 2022-10-21
Anticipated expiration: 2039-09-19
Also published as: CN112522597A

Abstract

The boron-containing high reaming steel comprises the following chemical components in percentage by weight: c:0.01-0.05%, si:0.2-0.6%, mn:0.8-1.5%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, N is less than or equal to 0.008%, als:<0.001%, ca is less than or equal to 0.0050%, B:0.001-0.006% of total oxygen [ O ]] _T ：0.007～0.020％，Mn/S>250, balance Fe and other unavoidable impurities. In the invention, microalloy elements such as B and the like are selectively added into steel; in the smelting process, the alkalinity of slag, the type and melting point of inclusions in steel, the free oxygen content in molten steel and the content of acid-soluble aluminum Als are controlled; then, performing twin-roll thin strip continuous casting to cast a casting strip with the thickness of 1.5-3mm, directly entering a lower closed chamber with non-oxidizing atmosphere, and entering an online rolling mill for hot rolling under the closed condition; after rolling, the strip steel is cooled by adopting an air atomization cooling mode, and the produced steel coil can be directly used as a hot rolled plate and also can be used after acid washing and leveling.

Description

Boron-containing high-reaming-hole steel and production method thereof

Technical Field

The invention relates to a manufacturing technology of high-reaming-hole steel, in particular to boron-containing high-reaming-hole steel and a production method thereof.

Background

The traditional thin strip steel is produced by continuously rolling a casting blank with the thickness of 70-200mm for multiple times, and the traditional hot rolling process flow is as follows: continuous casting, casting blank reheating and heat preservation, rough rolling, finish rolling, cooling and coiling, namely firstly, a casting blank with the thickness of about 200mm is obtained through continuous casting, the casting blank is reheated and heat preserved, then rough rolling and finish rolling are carried out to obtain a steel strip with the thickness generally larger than 2mm, and finally laminar cooling and coiling are carried out to the steel strip to complete the whole hot rolling production process. The difficulty is relatively great if steel strip with a thickness of less than 1.5mm inclusive is to be produced, which is usually done by subsequent cold rolling and annealing of the hot rolled strip. And the process flow is long, the energy consumption is high, the number of unit equipment is large, the capital construction cost is high, and the production cost is high.

The thin slab continuous casting and rolling process flow is as follows: continuous casting, heat preservation and soaking of a casting blank, hot continuous rolling, cooling and coiling. The main differences between the process and the traditional process are as follows: the thickness of a casting blank in the thin slab process is greatly reduced to 50-90mm, and because the casting blank is thin, the casting blank can be reduced to the required specification before finish rolling only by carrying out 1-2 times of rough rolling (when the thickness of the casting blank is 70-90 mm) or not carrying out the rough rolling (when the thickness of the casting blank is 50 mm) on the casting blank, while the casting blank in the traditional process can be repeatedly rolled for multiple times; and the casting blank of the thin slab process directly enters a soaking pit furnace for soaking and heat preservation without cooling or supplementing a small amount of heat, so that the thin slab process greatly shortens the process flow, reduces the energy consumption and the investment, thereby reducing the production cost. However, the fast cooling rate of the continuous casting and rolling of the thin slab leads to the improvement of the steel strength and the improvement of the yield ratio, thereby increasing the rolling load, so that the thickness specification of the hot rolled product which can be economically produced is unlikely to be too thin, generally being equal to or larger than 1.5mm, see the Chinese patents CN200610123458.1, CN200610035800.2 and CN200710031548.2.

The ESP realizes the continuous casting of the plate blank, eliminates a heating furnace with the functions of flame cutting, heat preservation, soaking and plate blank transition, greatly shortens the length of the whole production line to about 190 meters, ensures that the plate blank continuously cast by a continuous casting machine has the thickness of 90-110mm and the width of 1100-1600mm, ensures that the continuously cast plate blank plays the role of heat preservation, soaking and heating through a section of induction heating roller table, and then sequentially enters the procedures of rough rolling, finish rolling, layer cooling and coiling to obtain the hot rolled plate. At present, the process is rapidly developed and popularized, and a plurality of ESP production lines are operated and produced in the world at present.

The process flow shorter than the thin slab continuous casting and rolling is a thin strip continuous casting and rolling process, the thin strip continuous casting technology is a leading-edge technology in the fields of metallurgy and material research, the emergence of the technology brings a revolution to the steel industry, the production process of steel strips in the traditional metallurgy industry is changed, continuous casting, rolling, even heat treatment and the like are integrated into a whole, the produced thin slab is subjected to online hot rolling once to form a thin steel strip, the production process is greatly simplified, the production period is shortened, and the length of the process line is only about 50 m; the equipment investment is correspondingly reduced, the product cost is obviously reduced, and the method is a low-carbon and environment-friendly hot-rolled thin strip production process. The twin-roll thin strip continuous casting process is a main form of thin strip continuous casting process and is also a thin strip continuous casting process which only realizes industrialization in the world.

The typical process flow of twin roll strip casting is shown in figure 1-molten steel in a large ladle 1 is directly poured into a molten pool 7 enclosed by two relatively rotating and rapidly cooled crystallizing

rollers

8a, 8b and side sealing devices 6a, 6b through a large ladle long nozzle 2, a tundish 3, a submerged nozzle 4 and a distributor 5, the molten steel is solidified on the circumferential surfaces of the rotating crystallizing

rollers

8a, 8b to form solidified shells and gradually grow, a steel strip 11 with the thickness of 1-5mm is formed at the minimum clearance (nip point) of the two crystallizing rollers, the steel strip is guided by a guide plate 9 to a pinch roller 12 and is fed into a rolling mill 13 to be rolled into 0.7-2.5mm thin strips, then the thin strips are cooled by a cooling device 14 and are cut by a flying shear device 16 and then fed into a coiler 19 to be coiled.

The high hole expanding steel is an important steel type of advanced high-strength steel (AHSS), has higher strength and elongation, excellent formability and flanging property, can meet the requirements of automobile parts with complex shapes and high forming property requirements, such as automobile chassis rear axle suspension swing arms, and can also be used for other parts needing flange flanging. The flanging capability is expressed by the hole expansion rate, the hole expansion performance is taken as a forming performance index of steel, and the capacity of resisting local cracking of the material in the direction vertical to the hole edge due to overlarge local elongation deformation of the hole edge in the hole expansion process is reflected.

Along with the increasing requirements of automobile design on chassis structures, the forming of parts is more complex, the requirements on flanging and hole expanding performance of steel plates are further improved, the strength and rigidity of the parts can be improved through the flanging and local hole expanding shape design of the automobile parts, the aims of thinning and lightening the automobile steel plates can be achieved, the forming requirements of automobile chassis and cantilever parts can be easily met by the traditional carbon-manganese solid solution strengthened steel and low alloy precipitation strengthened steel structure steel plates, for example, the hole expanding rate of the traditional 440MPa steel plate carbon-manganese solid solution strengthened steel and low alloy precipitation strengthened steel is only 50-70%, and therefore high hole expanding steel is produced. In the 90 s of the 20 th century, hot rolled steel plates with high hole expansion performance of 440-780 MPa level were successively developed in America, japan and the like, the hole expansion rate of the hot rolled steel plates is 70% -131%, the hot rolled steel plates are mainly applied to parts such as automobile chassis, wheels and the like which require good formability, particularly flanging performance, the hole expansion performance of the steel plates is related to components, strength and structural uniformity of the steel plates, and ferrite/bainite dual-phase structures can be obtained under the condition of low cooling speed due to the fact that the hot rolled steel plates contain more precious alloy elements such as Cr, nb, ti, V, mo and the like, but the cost is high.

In the past, two options are generally available for meeting the use conditions of the steel plate on the car chassis, one is to use the steel plate with reduced strength (less than or equal to 300 MPa) to obtain higher hole expansion performance; and the other method is to reduce the flanging amount in the part design so as to reduce the requirement on the hole expanding performance of the steel plate. With the continuous improvement of the strength of the automobile steel, the hole expansion rate of the traditional automobile steel is reduced, and the requirement of the automobile chassis on the hole expansion rate of the steel plate is difficult to meet. With the increasing requirements of automobile design on chassis structures, the shapes of parts are increasingly complex, the strength requirements are continuously improved, the hole expanding rate of steel plates is increased, and high hole expanding steel becomes an important automobile steel variety.

The strength grades of the high-hole-expansion steel which is most commonly used at present are mainly concentrated on the grades of 440 and 590MPa, and the microstructure of the high-hole-expansion steel is mainly ferrite and bainite and sometimes contains a small amount of martensite. The hole expansion performance of steel sheets is related to a number of factors, which mainly include: inclusion level, performance difference of each phase in the structure, structure uniformity, yield ratio, structure type and the like. From the structure type, the ferrite type and bainite type structures have relatively high hole expansion performance, but the strength is relatively low, and the level of 780MPa and above is difficult to reach, which is also the main reason that the high hole expansion steel is mainly concentrated on two strength levels of 440MPa and 590MPa at present. High-bore-expansion steel has become one of the important varieties of automobile steel plates.

Compared with the traditional hot rolling process, the thin strip continuous casting easily generates bainite type microstructures in the cooling process after rolling due to the natural technological process advantages of the thin strip continuous casting, and the produced product has excellent reaming performance. Therefore, the thin-strip continuous casting method for producing the high-hole-expansion steel has natural advantages.

The high-reaming steel is produced by adopting thin-strip continuous casting, and is mainly aimed at the hot-rolled thin-specification automobile steel market with the thickness of less than 1.8mm (inclusive). Due to the thin thickness, the strip continuous casting process has strong manufacturing and cost advantages. The product specification characteristic thickness of the high-reaming steel strip directly supplied in a hot rolling/pickling state is 1.2, 1.25, 1.4, 1.5, 1.6, 1.7mm, 1.8mm and the like, and due to the fact that the product thickness is thin, the traditional thin-gauge high-reaming steel cannot be supplied in full specification due to the capacity limitation of a traditional hot continuous rolling line of many manufacturers; or the production process of firstly adopting the hot continuous rolling process and then carrying out cold rolling increases the production cost of the thin gauge high reaming steel.

When the hot-rolled strip steel is used as a thin hot-rolled product, the surface quality requirement of the strip steel is not the highest. It is generally required that the thinner the scale on the surface of the strip, the better the scale formation, which requires controlling the scale formation in the various stages of the strip casting, as in the typical process shown in fig. 1, a closed chamber device 10 is used from the crystallization roll 8 to the entrance of the rolling mill 13 to prevent the strip from oxidizing, and the thickness of the scale on the surface of the strip casting can be controlled by adding hydrogen in the closed chamber device 10 as in US6920912 and controlling the oxygen content to be less than 5% in US 20060182989. However, there are few patents on how to control the thickness of the scale during the transport from the rolling mill to the coiling, and particularly, in the cooling of the strip by laminar cooling or spray cooling, the strip at high temperature is in contact with cooling water, and the scale thickness on the surface of the cast strip increases rapidly. Meanwhile, the contact between the high-temperature strip steel and the cooling water also brings about a plurality of problems: firstly, water spots (rusts) are formed on the surface of strip steel, which affects the surface quality; secondly, cooling water for laminar cooling or spray cooling easily causes uneven local cooling on the surface of the strip steel and uneven microstructure inside the strip steel, thereby causing uneven performance of the strip steel and influencing product quality and hole expansion performance; thirdly, the local cooling of the surface of the strip steel is uneven, which causes the deterioration of the plate shape and influences the quality of the plate shape.

However, the continuous strip casting has the fast solidification process characteristics, so that the produced steel generally has the problems of uneven structure, low elongation, high yield ratio and poor formability; meanwhile, the austenite grains of the cast strip have obvious unevenness, which can cause uneven texture of a final product obtained after austenite phase transformation, thereby causing unstable performance of the product. Therefore, the thin strip continuous casting production line is adopted to produce products with high hole expanding performance required by the automobile industry and the petrochemical industry, certain difficulty is achieved, certain challenge is achieved, and therefore when the thin strip continuous casting is adopted to produce the high-hole expanding steel, the traditional component process cannot be used for production, and breakthroughs in components and processes are needed.

Chinese patent CN103602890 discloses a high-reaming-hole steel plate with tensile strength of 540MPa and a manufacturing method thereof. The patent adopts the traditional continuous casting and the traditional hot rolling process to produce, and adopts a one-section type conventional laminar cooling mode.

Chinese patent CN103602890 discloses a high-reaming-degree steel plate with 440MPa of tensile strength and a manufacturing method thereof. The patent adopts the traditional continuous casting and the traditional hot rolling process to produce, and adopts a one-section type conventional laminar cooling mode.

Chinese patents CN105154769 and CN106119702 respectively disclose 780MPa and 980MPa hot-rolled high-strength high-reaming steel and a manufacturing method thereof, which belong to the category of high-strength steel, and the strengthening of steel grades is realized by adding Ti, mo and more microalloy elements such as Ti, nb, cr, V and the like respectively, so that the alloy cost is high; meanwhile, the production mode adopts the traditional continuous casting and traditional hot rolling process to produce.

International patent WO200928515 adds a small amount of Nb and Ti alloy elements to C, si and Mn, and can produce reaming steel with tensile strength above 490MPa, and hot rolling must employ a two-stage laminar cooling method, and two-stage cooling control can be more accurately simulated in a laboratory, and a good test result is obtained.

Disclosure of Invention

The invention aims to provide boron-containing high-reaming-hole steel and a production method thereof, which fully utilize the short-flow advantage of thin-strip continuous casting, further reduce the production process cost and improve the product performance.

In order to achieve the purpose, the technical scheme of the invention is as follows:

in the invention, microalloy elements such as B and the like are selectively added into steel; controlling the alkalinity of slag, the type and melting point of inclusions in steel, the content of free oxygen in molten steel and the content of acid-soluble aluminum Als in the smelting process; then, performing double-roller thin-strip continuous casting to cast strip steel with the thickness of 1.5-3mm, directly entering a lower closed chamber with non-oxidizing atmosphere after the strip steel exits from a crystallization roller, and entering an online rolling mill for hot rolling under the closed condition; the rolled strip steel is cooled by adopting an air atomization cooling mode, the thickness of oxide scale on the surface of the strip steel can be effectively reduced by adopting the air atomization cooling mode, the temperature uniformity of the strip steel is improved, and the surface quality and the hole expanding performance of the strip steel are greatly improved. The finally produced steel coil can be directly used as a hot rolled plate and can also be used after acid cleaning and flattening.

The boron-containing high hole expansion steel comprises the following chemical components in percentage by weight: c:0.01-0.05%, si:0.2-0.6%, mn:0.8-1.5%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, N is less than or equal to 0.008%, als:<0.001%, ca is less than or equal to 0.0050%, B:0.001-0.006% of total oxygen [ O ]] _T ：0.007～0.020％，Mn/S>250, balance Fe and other unavoidable impurities.

The microstructure of the high-reaming-hole steel is ferrite and bainite, wherein the proportion of a bainite phase is more than or equal to 15%.

The yield strength of the high-hole-expansion steel is more than or equal to 290MPa, the tensile strength is more than or equal to 440MPa, the elongation is more than or equal to 29 percent, and the hole expansion rate is more than or equal to 110 percent.

In the chemical composition design of the high hole expansion steel, the following steps are carried out:

c: c is the most economical and basic strengthening element in steel, and enhances the strength of steel by solid solution strengthening and precipitation strengthening. C is an essential element for precipitating cementite during austenite transformation, so the strength level of the steel is determined to a great extent by the content of C, namely, higher C content corresponds to higher strength level. However, since interstitial solid solution and precipitation of C are harmful to the plasticity and toughness of steel, and an excessively high C content is disadvantageous to welding performance, the C content cannot be excessively high, and the strength of steel is compensated by appropriate addition of alloying elements. Meanwhile, for conventional slab continuous casting, casting in a peritectic reaction region is easy to generate surface cracks of a casting blank, and steel leakage accidents can happen in severe cases. The same is true for thin strip casting, where casting a cast strip in the peritectic reaction zone is prone to surface cracking and, in severe cases, strip breakage. Therefore, strip casting of Fe — C alloys also requires avoidance of the peritectic reaction zone. Therefore, the content range of C adopted by the invention is 0.01-0.05%.

Si: si plays a role in solid solution strengthening in steel, and the Si added in the invention also plays a role in deoxidation, so that the purity of steel can be improved; meanwhile, si can enlarge the ferrite forming range and avoid the appearance of pearlite phase. However, the steel plate surface after rolling is easy to form red iron sheet defect due to the excessively high Si content. Therefore, the Si content range adopted by the invention is 0.2-0.6%.

Mn: mn is one of the cheapest alloy elements, can improve the hardenability of steel, has considerable solid solubility in the steel, improves the strength of the steel through solid solution strengthening, basically has no damage to the plasticity and the toughness of the steel, is the most main strengthening element for improving the strength of the steel, and can play a role in deoxidizing in the steel. However, too high Mn content results in deterioration of weldability and toughness of weld heat-affected zone. Therefore, the Mn content range adopted by the invention is 0.8-1.5%.

P: high content of P is easy to be segregated in grain boundary, increases cold brittleness of steel, deteriorates welding performance, reduces plasticity and deteriorates cold bending performance. In the thin strip continuous casting process, the solidification and cooling rates of the cast strip are extremely high, and the segregation of P can be effectively inhibited, so that the disadvantages of P can be effectively avoided, and the advantages of P can be fully exerted. Therefore, in the invention, the P content is higher than that in the traditional process, the content of the P element is properly relaxed, the dephosphorization procedure is eliminated in the steelmaking procedure, in the actual operation, the dephosphorization procedure is not required to be carried out intentionally, and no extra phosphorus is required to be added, and the range of the P content is less than or equal to 0.02 percent.

S: in general, S is a harmful element in steel, which causes hot brittleness, decreases ductility and toughness, and causes cracks during rolling. S is easy to form MnS in steel, the amount and the form of sulfide in the steel directly influence the hole expansion rate of the steel plate, and the S must be lower than 0.005 percent. The amount and the form of the inclusion elements have great influence on the hole expanding performance of the steel plate, and particularly, the strip-shaped sulfide inclusion is easy to cause cracks in deformation. Therefore, in the present invention, S is controlled as an impurity element, and the content range thereof is 0.005% or less, and Mn/S is more than 250.

And Als: in order to control inclusions in steel, the invention requires that Al cannot be used for deoxidation, al is avoided from being additionally introduced as much as possible in the use of refractory materials, and the content of acid-soluble aluminum Als is strictly controlled as follows: <0.001%.

N: similar to C element, N element can improve the strength of steel through interstitial solid solution, and the invention needs to generate a BN precipitated phase by utilizing the action of N and B in the steel and needs a certain content of N in the steel. However, the interstitial solid solution of N has great harm to the plasticity and toughness of the steel, and the existence of free N can improve the yield ratio of the steel, so that the content of N cannot be too high. The content range of N adopted by the invention is less than or equal to 0.008 percent.

Ca: the form of sulfide in steel can be changed, the long-strip MnS inclusion is converted into spherical CaS inclusion, the plasticity and the toughness of the steel plate are improved, and the hole expansion rate of the steel plate is improved. The invention controls Ca to be less than 0.0050 percent.

B: the significant role of B in steel is: the hardenability of the steel can be multiplied by trace boron, B can preferentially precipitate BN particles in high-temperature austenite so as to inhibit the precipitation of AlN, weaken the pinning effect of AlN on grain boundaries, improve the growth capability of grains, and further homogenize austenite grains; in addition, the combination of B and N can effectively prevent the grain boundary low-melting-point phase B ₂ O ₃ Of the cell.

B is an active segregation-prone element and is easy to segregate in a grain boundary, and the content of B is generally controlled to be very strict and is generally about 0.001-0.003% when B-containing steel is produced by a traditional process; in the thin-strip continuous casting process, the solidification and cooling rates are high, the segregation of B can be effectively inhibited, and more B content is dissolved, so that the content of B can be properly widened; BN particles can be generated through reasonable process control, and the function of fixing nitrogen is achieved. Further studies have shown that B reduces the tendency of C atoms to segregate and avoids precipitation of grain boundary Fe23 (C, B) 6, so that more B can be added. Therefore, in the present invention, a higher B content is used than in the conventional process, in the range of 0.001-0.006%.

The production method of the boron-containing high hole expansion steel comprises the following steps:

1) Smelting

Smelting according to the components, wherein in the smelting process, the slagging alkalinity a = CaO/SiO ₂ Control in a<1.5, preferably a<1.2, or a =0.7 to 1.0; it is necessary to obtain MnO-SiO having a low melting point ₂ -Al ₂ O ₃ Ternary inclusions, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ Controlled at 0.5-2, preferably at 1-1.8; free oxygen [ O ] in molten steel] _Free The range is as follows: 0.0005-0.005% of Mn/S in molten steel>250；

2) Continuous casting

The continuous casting adopts double-roller thin strip continuous casting, and a casting strip with the thickness of 1.5-3mm is formed at the minimum position of a gap between two crystallizing rollers; the diameter of the crystallization roller is 500-1500mm, and the preferred diameter is 800mm; cooling the crystallization roller by introducing water, wherein the casting speed of the casting machine is 60-150m/min; the continuous casting flow distribution adopts a two-stage steel water distribution system, namely a tundish and a flow distributor;

3) Lower sealed chamber protection

After the casting strip is taken out of the crystallization roller, the casting strip is directly fed into a lower closed chamber at the temperature of 1420-1480 ℃, non-oxidizing gas is introduced into the lower closed chamber to realize the anti-oxidation protection of the strip steel, and the oxygen concentration in the lower closed chamber is controlled to be less than 5 percent; the temperature of the cast strip at the outlet of the lower closed chamber is 1150-1300 ℃;

4) In-line hot rolling

Conveying the cast strip to a rolling mill through pinch rolls in a lower closed chamber, and rolling the cast strip into strip steel, wherein the rolling temperature is 1100-1250 ℃, and the hot rolling reduction is controlled to be 10-50%, preferably 30-50%; the thickness of the hot rolled steel strip is 0.8-2.5mm, preferably 1.0-1.8mm;

5) Cooling after rolling

Cooling the rolled strip steel after rolling, wherein the cooling adopts an air atomization cooling mode, and the cooling rate is more than 20 ℃/s;

6) Strip steel coiling

And cutting the head of the cooled hot-rolled strip steel by using a cutting head to remove the head with poor quality, and directly coiling the hot-rolled strip steel into coils at the coiling temperature of 550-650 ℃.

Preferably, in the step 1), the smelting adopts electric furnace steelmaking or converter steelmaking, and then enters an LF furnace, a VD/VOD furnace or an RH furnace for refining.

Preferably, in the step 3) of lower closed chamber protection, the non-oxidizing gas is N ₂ CO obtained by sublimation of Ar or dry ice ₂ 。

Preferably, in the step 5), the gas-water ratio of the gas atomization cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

Preferably, in step 6), the coiling is performed in a double coiler format or a carrousel coiling format.

The method for manufacturing the high hole expansion steel comprises the following steps:

the molten steel can be smelted by an electric furnace or a converter and then enters necessary refining processes, such as an LF furnace, a VD/VOD furnace, an RH furnace and the like.

In order to improve the castability of thin-strip continuous casting molten steel, the alkalinity a = CaO/SiO of slagging in the steel-making process ₂ Control in a<1.5, preferably a<1.2, or a = 0.7-1.0.

To improve the castability of thin strip continuous casting molten steel, it is necessary to obtain MnO-SiO of low melting point ₂ -Al ₂ O ₃ Ternary inclusions, e.g. shaded areas of FIG. 2, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The concentration is controlled to be 0.5-2, preferably 1-1.8.

In order to improve the castability of thin strip continuous casting molten steel in which O is an essential element for forming oxide inclusions, the present invention requires formation of MnO-SiO with a low melting point ₂ -Al ₂ O ₃ The ternary inclusions of (2) require free oxygen [ O ] in molten steel] _Free The range is as follows: 0.0005 to 0.005 percent.

In order to improve the castability of the thin strip continuous casting molten steel, the Mn and S of the above components should satisfy the following relation: mn/S >250.

After the cast strip is taken out of the crystallization roller, the cast strip is directly fed into a lower closed chamber at the temperature of 1420-1480 ℃, non-oxidizing gas is introduced into the lower closed chamber to protect the strip steel, and the oxidation prevention protection of the strip steel is realized in an atmosphere of N ₂ Ar, or other non-oxidizing gas, such as CO obtained by sublimation of dry ice ₂ Gas, etc., the oxygen concentration in the lower sealed chamber is controlled<5 percent. The lower closed chamber protects the cast strip from oxidation to the mill inlet. The temperature of the casting belt at the outlet of the lower closed chamber is 1150-1300 ℃.

The theoretical basis of BN precipitated phase involved in the lower sealing process of the cast strip is as follows:

the thermodynamic equation of boron with nitrogen, aluminum and nitrogen in gamma-Fe in steel is as follows:

BN＝B+N；Log[B][N]＝-13970/T+5.24 (1)

AlN＝Al+N；Log[Al][N]＝-6770/T+1.03 (2)

as shown in FIG. 3, the initial precipitation temperature of BN in the steel is about 1280 ℃ and the precipitation of BN at 980 ℃ is in equilibrium, but the precipitation of AlN is just started (the precipitation temperature of AlN is about 980 ℃), and the precipitation of BN is thermodynamically preferred to AlN. Therefore, the invention completes the combination of B and N in the lower closed chamber by a reasonable process control means, thereby preferentially precipitating BN particles in high-temperature austenite, inhibiting the precipitation of AlN, weakening the pinning effect of AlN on grain boundary, improving the growth capacity of grains and further leading the austenite grains to be more uniform; in addition, the combination of B and N can effectively prevent the grain boundary low melting point phase B ₂ O ₃ Is present.

The strip steel after on-line hot rolling is cooled after rolling, and the cooling adopts an air atomization cooling mode, so that the thickness of oxide scale on the surface of the strip steel can be effectively reduced, the temperature uniformity of the strip steel is improved, and the surface quality and the hole expansion performance of the strip steel are improved. The gas-water ratio of gas atomization cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa. High-pressure water mist is formed after gas atomization and sprayed on the surface of the steel strip, so that on one hand, the temperature of the steel strip is reduced, on the other hand, the water mist can form a compact air film to be coated on the surface of the steel strip, the anti-oxidation effect of the steel strip is achieved, and the growth of oxide skin on the surface of the hot-rolled steel strip is effectively controlled. The cooling mode can avoid the problems caused by the traditional spraying or laminar cooling, so that the surface temperature of the strip steel is uniformly reduced, the temperature uniformity of the strip steel is improved, and the effect of homogenizing the internal microstructure is achieved; meanwhile, the cooling is uniform, so that the shape quality and the performance stability of the strip steel can be improved; effectively reducing the thickness of the oxide scale on the surface of the strip steel. The cooling rate of the gas atomization cooling is in the range of more than 20 ℃/s, the strip steel is cooled to 550-650 ℃, and the high-temperature austenite after rolling is converted into a mixed microstructure of ferrite and a small amount of bainite, as shown in figure 4.

Through the manufacturing process, the final thin strip continuous casting high-hole-expansion steel has the performance yield strength of more than or equal to 290MPa, the tensile strength of more than or equal to 440MPa, the elongation of more than or equal to 29 percent and the hole expansion rate of more than or equal to 110 percent.

The reason why the present invention relates to the castability of the thin strip continuous casting is explained as follows:

the castability is not defined exactly at present, and traditionally it is a frequently used concept closely related to the fluidity of the steel, the chilling tendency, the shrinkage characteristics and the quality of the product, with respect to the metal species and the process factors thereof. The definition "Strip Casting castability (CASC)" refers to the feasibility of twin roll Casting of a steel grade. The castability is good, which means that the restrictive problem that the casting process cannot be carried out or the quality of the cast product cannot meet the requirement cannot be caused in the casting process; poor castability means that problems such as poor molten steel fluidity, molten pool agglomeration and bridging, severe belt breakage, surface cracks, surface slag inclusion and the like often occur in the casting process, so that production cannot be carried out normally and stably or the product quality cannot meet requirements.

The strip continuous casting castability of a steel grade is judged through research and analysis on the strip continuous casting castability, and briefly summarized, the following aspects can be considered: firstly, whether uneven solidification shrinkage can be avoided or not; secondly, the uniformity of interface heat transfer can be improved, so that the solidification uniformity is improved; and thirdly, whether the hot brittleness in the solidification process can be improved or controlled. When the strip casting castability of a steel grade is poor, the production process stability is poor, the quality stability of the produced product is also poor, the productivity cannot be exerted, the qualification rate of the product is low, and the product is not suitable for the strip casting process.

The steel grade of the invention is controlled from the carbon content (avoids a peritectic region to solve uneven solidification shrinkage); alkalinity control, als control, free oxygen total oxygen control and low-melting MnO-SiO2-Al2O3 ternary inclusion control (improving interface heat transfer uniformity and solving solidification uniformity); mn/S control (avoidance of hot shortness) and the like, the castability of strip casting is strictly satisfied.

The reason that the spray cooling is preferably adopted after the thin strip continuous casting hot rolled steel is rolled is as follows:

the traditional continuous casting also adopts spray cooling, but the action area and the temperature are different, the traditional continuous casting carries out spray cooling on the casting blank in the outlet fan-shaped section area of the casting blank outlet crystallizer, the temperature of the casting blank is higher at the moment, and the casting blank is in a high-temperature austenite single-phase area seen on a phase diagram. The main purposes of spray cooling in the area are to control the position of the solidification tail end, accelerate the surface cooling of the casting blank, refine the surface austenite grain structure, improve the surface strength of the casting blank, improve the surface quality of the casting blank and avoid the occurrence of cracks. The invention carries out spray cooling on the ultrathin strip steel after the cast strip is subjected to online hot rolling, the temperature is lower, the ultrathin strip steel is in a solid phase transformation area where high-temperature austenite is transformed into ferrite on a phase diagram, the strip steel is subjected to spray cooling in the area, and the microstructure type after the solid phase transformation can be effectively controlled by adjusting the spray cooling strength, thereby realizing the performance requirement of a final product.

The reason that the strip continuous casting hot rolled steel coil of the invention preferably adopts a carrousel homotopic coiler is as follows:

at present, most production lines of ultrathin hot rolled steel coils adopt an underground double-coiling mode or an underground triple-coiling mode, and the production lines also consider the production of thick hot rolled plates, for example, the coiling of an ESP production line of Avedi (Avedi) company adopts the underground triple-coiling mode, and the coiling of an FTSC production line of Tang Gang introduced into Danieli (Danieli) adopts the underground double-coiling mode. The Castrip strip casting production line of the Umbelliferae, nippon kok (Nucor) adopts the traditional method and also adopts an underground double-coiling mode. The distance interval between the underground recoiling machine and the recoiling machine is generally 8-10 m (typical value is 9.4 m), when the ultrathin hot rolled strip steel is produced by strip continuous casting, the cooling speed of the strip steel in the air is very high, the interval is enough to influence the difference of the recoiling temperature, and the temperature deviation between the two recoiling machines can reach 49 ℃, which can seriously influence the performance deviation of the steel coil.

The invention preferably adopts a carrousel coiling mode, can realize the co-position coiling of the hot rolled steel coil, ensures the same coiling temperature and further greatly improves the stability of the performance of the steel coil product. At present, the carrousel coiling machine is widely used in the field of cold rolling, and has the main advantages of realizing thinner strip steel coiling, occupying small area and greatly shortening the length of a production line, but the carrousel coiling machine is easier to realize in the field of cold rolling due to lower strip steel temperature. The invention provides a carrousel coiling method in the field of coiling ultrathin hot rolled strip steel, which considers the high temperature resistance of equipment and realizes the coiling of the ultrathin hot rolled strip steel. The coiling mode is more advanced than that of a Castrip strip continuous casting production line of Nucor (Nucor) in the United states.

The invention is distinguished and improved from the prior art:

there are many patents on the prior thin strip continuous casting for producing thin strip products and processes, but the production of the high-reaming-hole steel related to the invention by adopting the thin strip continuous casting is not directly reported.

The most obvious characteristic of the invention which is different from the prior thin strip continuous casting technology is the roller diameter of the crystallization roller and the corresponding flow distribution mode. The EUROSTRIP technology is characterized in that the crystallization roller with a large roller diameter of 1500mm phi is large, the molten steel capacity of a molten pool is large, the distribution is easy, and the manufacturing cost and the operation cost of the crystallization roller are high. CASTIP technology is characterized in that the diameter of a crystallization roller is 500mm, the crystallization roller is small, the molten steel capacity of a molten pool is small, the distribution is very difficult, but the manufacturing, operation and maintenance costs of casting machine equipment are low. CASTIP adopts a three-level steel water distribution system (tundish, transition ladle and distributor) to solve the problem of uniform distribution of small molten pools. Because a three-level flow distribution system is adopted, the cost of the refractory material is directly increased; more importantly, the three-stage flow distribution system enables the flowing path of the molten steel to be lengthened, the temperature drop of the molten steel to be larger, and in order to meet the temperature of the molten steel in a molten pool, the tapping temperature needs to be greatly improved. The increase of the tapping temperature causes problems such as increase of steel-making cost, increase of energy consumption, and shortening of the life of refractory.

The diameter of the crystallization roller is 500-1500mm, preferably the diameter of the roller is 800mm, and a two-stage steel water distribution system (a tundish and a flow distributor) is adopted. Molten steel flowing out of the flow distributor forms different flow distribution modes along the roller surface and the two end surfaces, and flows in two paths without mutual interference. Because a two-stage flow distribution system is adopted, compared with a three-stage flow distribution system, the cost of the refractory material is greatly reduced; the reduction of the flow path of the molten steel reduces the temperature drop of the molten steel, and can reduce the tapping temperature by 30-50 ℃ compared with a three-level flow distribution system. The reduction of the tapping temperature can effectively reduce the steel-making cost, save the energy consumption and prolong the service life of refractory materials. The invention is matched with the crystallization roller with the optimal roller diameter of phi 800mm, and adopts a two-stage steel water distribution system, thereby not only realizing the requirement of stable distribution of molten steel, but also realizing the aims of simple structure, convenient operation and low processing cost.

Chinese patent CN101353757 adopts low-carbon microalloy components to produce reaming steel with tensile strength of 440MPa, and trace Nb is added in the components: 0-0.25% and Ti:0-0.03%, and because the coiling temperature of 600 ℃ is adopted, the steel is produced by adopting the traditional continuous casting and traditional hot rolling process, and for a carbon manganese steel hot rolled plate, a banded structure often exists, so that the hole expanding rate of the steel plate is reduced, and meanwhile, a plurality of micro alloys are added, so that the steel making cost is increased. The invention is obviously different from the patent in production process, and the invention adopts the thin strip continuous casting process for production, thereby greatly shortening the production flow, avoiding the banded structure, saving the using amount of the microalloy and achieving the same or more excellent performance by only adding a small amount of the microalloy.

Chinese patent CN101928881 discloses a hot-rolled high-hole-expansion steel plate with 590MPa level of tensile strength and a manufacturing process thereof, wherein trace amounts of Nb:0-0.10% and Ti:0-0.04 percent by adopting the traditional continuous casting and traditional hot rolling process, cooling the steel plate after final rolling to 600-750 ℃ at the cooling speed of 50-100 ℃/s, then cooling in the air for 3-10 seconds at the cooling speed of 5-15 ℃/s, then cooling the steel plate to 350-500 ℃ at the cooling speed of 70-150 ℃/s, coiling, and then air cooling to room temperature. The subsequent cooling adopts complicated three-section type cooling, the coiling temperature fluctuation is large, the performance fluctuation of the head, the middle and the tail of the steel coil is large, and the hole expansion rate fluctuation is also large. The invention adopts the thin strip continuous casting process to produce, greatly simplifies the production process flow, does not need to adopt complex three-section cooling, and has obvious advancement.

Japanese patent JP2006063394 discloses a hot-rolled high-hole-expansion steel, which contains 0.20-0.48% of carbon, has a tensile strength of 440MPa or more, is added with Cr alloy elements, but has a hole expansion ratio of 70% or more, and is subjected to annealing treatment at 640 ℃ after hot rolling. The carbon content design of the invention reaches the range of medium-high carbon steel, and is obviously higher than the low-carbon design of the invention. The hot-rolled high-strength steel plate disclosed in Japanese patent JP2006305700 adopts a component design of C-Si-Mn + Ti, the tensile strength is more than 780MPa, and the hole expanding rate is only more than 68%. The hot-rolled high-hole-expansion steel disclosed in JP2003/016614 has a carbon content of 0.02-0.10%, si content of 0.5% or less, and tensile strength of 590MPa or more, but contains many alloying elements such as Nb, ti, V, cr, RE, etc., and thus has a high steel-making cost and a main objective of good surface coatability. Compared with the patent, the invention adopts a simple alloy component system, adopts a thin strip continuous casting process to realize the performance of the high-hole-expansion steel, and has the characteristics of simplicity and high efficiency.

The hot rolled steel sheet disclosed in US2006096678 has a strength of 780MPa or more, an elongation of 22% or more, and a hole expansion of 60% or more. The hot rolled steel plate disclosed in US 4415376 has a yield strength of 80ksi (550 MPa) or more and a hole expansion ratio of 58% or more, and is reinforced with Nb and V. The production processes adopted by the patents are the production of the traditional continuous casting and the traditional hot rolling process, are different from the production process of the invention, and have lower hole expansion rate of the product.

The invention has the main advantages that:

the invention utilizes the thin strip continuous casting technology to produce the high hole expansion steel containing boron (B), has no report so far, and has the following advantages:

1. the invention omits the complex processes of slab heating, multi-pass repeated hot rolling and the like, and has the advantages of shorter production flow, higher efficiency and greatly reduced production line investment cost and production cost by adopting the working procedures of double-roller thin-strip continuous casting and one-pass online hot rolling.

2. The invention omits a plurality of complex intermediate steps in the traditional high-reaming steel production, greatly reduces the production energy consumption and CO2 emission compared with the traditional high-reaming steel, and is a green and environment-friendly product.

3. Compared with the traditional hot rolling process, the thin strip continuous casting easily generates bainite type microstructures in the cooling process after rolling due to the natural technological process advantages of the thin strip continuous casting, and the produced product has excellent reaming performance.

4. The invention adopts the thin strip continuous casting process to produce the high-reaming-hole steel, the thickness of the cast strip is thinner, the thin-specification product is produced to the desired product thickness through online hot rolling, the thin-specification product is directly supplied to the market for use without cold rolling, the purpose of supplying the thin-specification hot rolled plate is achieved, and the cost performance of the plate and the strip can be obviously improved.

5. According to the invention, a trace amount of boron element is added, BN particles are preferentially precipitated in high-temperature austenite, so that the precipitation of AlN is inhibited, the pinning effect of fine AlN on grain boundaries is weakened, the growth capability of grains is improved, austenite grains are homogenized, and the high hole expansion performance of the strip steel is favorably exerted.

6. The invention adopts the air atomization cooling mode of the rolled strip steel, can avoid the problems brought by the traditional spray or laminar cooling, uniformly reduces the surface temperature of the strip steel, improves the temperature uniformity of the strip steel, and achieves the effect of homogenizing the internal microstructure; meanwhile, the cooling is uniform, so that the shape quality and the hole expansion performance stability of the strip steel can be improved; effectively reducing the thickness of the oxide scale on the surface of the strip steel.

7. In the traditional process, the alloy elements are precipitated in the cooling process of the plate blank, and the utilization rate of the alloy elements is reduced because the re-dissolution of the alloy elements is insufficient when the plate blank is reheated. In the thin strip continuous casting process, the high-temperature cast strip is directly hot-rolled, and the added alloy elements mainly exist in a solid solution state, so that the alloy utilization rate can be improved.

8. According to the invention, the hot rolled steel strip carrousel coiling machine is selected, so that the length of a production line is effectively shortened; meanwhile, the control precision of the coiling temperature can be greatly improved by the co-position coiling, and the stability of the product performance is improved.

Drawings

FIG. 1 is a schematic diagram of a process arrangement for a twin roll strip casting process;

FIG. 2 shows MnO-SiO ₂ -Al ₂ O ₃ Ternary phase diagram (shaded area: low melting point region);

FIG. 3 is a thermodynamic curve of BN, alN precipitation;

FIG. 4 is a photograph of the microstructure of the steel according to the example of the present invention.

Detailed Description

The invention will be further elucidated by the following examples and figures, which are by no means limitative in any way. Any variations in the practice of the invention that may occur to those skilled in the art and which are made in the light of the teachings of this specification are intended to be within the scope of the following claims.

Referring to fig. 1, molten steel designed according to the chemical composition of the present invention is directly poured into a molten pool 7 surrounded by two

crystallization rollers

8a, 8b which rotate relatively and can be rapidly cooled and side closing plate devices 6a, 6b through a ladle 1, a tundish 2, a tundish 3, a submerged nozzle 4 and a distributor 5, the molten steel is solidified on the circumferential surfaces of the rotation of the

crystallization rollers

8a, 8b, thereby forming solidified shells and gradually growing, and then a casting strip 11 with the thickness of 1.5-3mm is formed at the minimum clearance (nip point) between the two crystallization rollers; the diameter of the crystal is 500-1500mm, and water is introduced for cooling; the casting speed of the casting machine ranges from 60 to 150m/min according to the thickness of the cast strip.

After the cast strip 11 comes out of the crystallization rollers 8a and 8b, the cast strip is directly fed into the lower closed chamber 10 at 1420-1480 ℃, the non-oxidizing gas is introduced into the lower closed chamber 10 to protect the strip steel, so as to realize the anti-oxidation protection of the strip steel, and the atmosphere of the anti-oxidation protection can be N ₂ Also, it isMay be Ar or other non-oxidizing gas, such as CO obtained by sublimation of dry ice ₂ Gas, etc., the oxygen concentration in the lower sealed chamber 10 is controlled to be<5 percent, the lower closed chamber 10 protects the casting strip 11 from oxidation to the inlet of a rolling mill 13, and the temperature of the casting strip at the outlet of the lower closed chamber 10 is 1150-1300 ℃; then the cast strip is sent to a hot rolling mill 13 through a swinging guide plate 9 and a pinch roll 12, a hot rolled strip with the thickness of 0.8-2.5mm is formed after hot rolling, and the temperature uniformity of the strip steel is improved by adopting an air atomization cooling mode for cooling after rolling; after the head of the hot rolled strip is cut by the flying shear device 16, the cut head falls into a flying shear pit 18 along a flying shear guide plate 17, and the hot rolled strip after the head is cut enters a coiler 19 for coiling; taking the steel coil down from the coiling machine, and naturally cooling to room temperature; the finally produced steel coil can be directly used as a hot rolled plate and can also be used after acid cleaning and flattening.

The chemical compositions of the examples of the invention are shown in table 1, and the balance of the compositions is Fe and other unavoidable impurities. The process parameters of the manufacturing method of the invention are shown in Table 2, and the properties of the final product are shown in Table 3.

In summary, the high hole expansion steel manufactured by the thin strip continuous casting process technology according to the design range of the steel grade components provided by the invention has the yield strength of more than or equal to 290MPa, the tensile strength of more than or equal to 440MPa, the elongation of more than or equal to 29 percent and the hole expansion rate of more than or equal to 110 percent.

Table 1 chemical composition (wt.%) of the example steels

Example (b)	C	Si	Mn	P	S	N	O	Als	Ca	B
											Example 1	0.03	0.26	1.46	0.008	0.004	0.0074	0.0093	0.0009	0.003	0.004
Example 2	0.02	0.22	0.92	0.013	0.003	0.0061	0.0110	0.0006	0.002	0.001
											Example 3	0.03	0.35	1.27	0.015	0.004	0.0058	0.0150	0.0004	0.003	0.003
Example 4	0.03	0.29	1.12	0.013	0.003	0.0077	0.0130	0.0008	0.004	0.006
											Example 5	0.05	0.33	0.86	0.009	0.002	0.0052	0.0120	0.0007	0.005	0.003
Example 6	0.03	0.44	0.80	0.012	0.002	0.0046	0.0070	0.0008	0.001	0.005
											Example 7	0.04	0.20	0.96	0.015	0.003	0.0040	0.0100	0.0005	0.002	0.003
Example 8	0.01	0.37	1.32	0.014	0.005	0.0100	0.0085	0.0006	0.005	0.002
											Example 9	0.04	0.36	0.86	0.018	0.003	0.0078	0.0200	0.0003	0.004	0.004
Example 10	0.03	0.43	0.92	0.020	0.001	0.0055	0.0125	0.0004	0.004	0.006
											Example 11	0.02	0.60	0.85	0.010	0.002	0.0080	0.0090	0.0009	0.005	0.003
Example 12	0.05	0.48	1.50	0.012	0.005	0.0075	0.0118	0.0003	0.001	0.002
											Example 13	0.04	0.56	1.37	0.018	0.004	0.0045	0.0132	0.0006	0.002	0.005
Example 14	0.02	0.37	1.40	0.017	0.003	0.0064	0.0075	0.0005	0.003	0.004

TABLE 2 Process parameters of the examples

TABLE 3 Properties of the steels of the examples

Claims

1. A production method of boron-containing high hole expansion steel comprises the following chemical components in percentage by weight: c:0.01-0.05%, si:0.2-0.6%, mn:0.8-1.5%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, N: less than or equal to 0.008 percent, als:<0.001%, ca is less than or equal to 0.0050%, B:0.001-0.006% of total oxygen [ O ]] _T ：0.007～0.020％，Mn/S>250, balance Fe and other unavoidable impurities; the method specifically comprises the following steps:

1) Smelting

Smelting according to the components, wherein in the smelting process, the basicity of slagging a = CaO/SiO ₂ Controlling a = 0.7-1.0; it is necessary to obtain MnO-SiO having a low melting point ₂ -Al ₂ O ₃ Ternary inclusions, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ Controlling the temperature to be 0.5-2; free oxygen [ O ] in molten steel] _Free The range is as follows: 0.0005-0.005% of Mn/S in molten steel>250；

2) Continuous casting

The continuous casting adopts double-roller thin strip continuous casting, and a casting strip with the thickness of 1.5-3mm is formed at the position with the minimum gap between two crystallizing rollers; the diameter of the crystallization roller is 500-1500 mm; cooling the crystallization roller by introducing water, wherein the casting speed of the casting machine is 60-150m/min; the continuous casting flow distribution adopts a two-stage steel water distribution system, namely a tundish and a flow distributor;

3) Lower enclosed chamber protection

4) In-line hot rolling

Conveying the cast strip to a rolling mill in a lower closed chamber through a pinch roll, and rolling the cast strip into strip steel, wherein the rolling temperature is 1100-1250 ℃, and the hot rolling reduction rate is controlled to be 10-50%; the thickness of the hot rolled steel strip is 0.8-2.5mm;

5) Cooling after rolling

6) Strip steel coiling

Cutting the head of the cooled hot-rolled strip steel to remove the head with poor quality, and directly coiling the hot-rolled strip steel into coils at the coiling temperature of 550-650 ℃;

the microstructure of the high reaming steel is ferrite and bainite, wherein the proportion of a bainite phase is more than or equal to 15 percent;

2. The method for producing boron-containing high-hole-expansion steel according to claim 1, wherein in the step 1), the steel is made by electric furnace steel making or converter steel making, and then is refined in an LF furnace, a VD/VOD furnace or an RH furnace.

3. The method for producing boron-containing high bore-expanding steel according to claim 1, wherein in the step 3) of protecting the lower closed chamber, the non-oxidizing gas is N ₂ CO obtained by sublimation of Ar or dry ice ₂ 。

4. The method for producing the boron-containing high hole expanding steel as claimed in claim 1, wherein in the step 5), the gas-water ratio of the gas atomization cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

5. The method for producing a boron-containing high bore expanding steel according to claim 1, wherein in the step 6), the coiling is performed in a double coiler type or a carrousel coiling type.

6. The method for producing a boron-containing high hole expansion steel according to claim 1, wherein in the step 1), said MnO-SiO is ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The temperature is controlled to be 1-1.8.

7. The method for producing a boron-containing high hole expansion steel according to claim 1, wherein in the step 2), the diameter of the crystallizing roller is 800mm.

8. The method for producing a boron-containing high bore expanding steel according to claim 1, wherein the hot rolling reduction in the step 4) is 30 to 50%.

9. The method for producing a boron-containing high bore expanding steel according to claim 1, wherein in the step 4), the thickness of the hot rolled steel strip is 1.0 to 1.8mm.