CN112522629B

CN112522629B - Nb microalloying high-strength high-hole-expansion steel and production method thereof

Info

Publication number: CN112522629B
Application number: CN201910888756.7A
Authority: CN
Inventors: 吴建春; 方园
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: 2022-06-24
Anticipated expiration: 2039-09-19
Also published as: CN112522629A

Abstract

A Nb microalloying high-strength high-hole-expansion steel and a production method thereof are disclosed, which comprises the following chemical components in percentage by weight: 0.01-0.05% of C, 0.2-0.6% of Si, 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≤0.0050％，Nb 0.01‑0.08％，Mn/S>250, total oxygen [ O ]]_T: 0.007-0.020% of Fe and inevitable impurities. In the invention, microalloy elements such as Nb 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, casting a cast strip by twin-roll thin strip continuous casting, wherein the cast strip directly enters a lower closed chamber with non-oxidizing atmosphere and enters 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 finally produced steel coil can be directly used as a hot rolled plate or can be used after acid washing and leveling.

Description

Nb microalloying high-strength high-hole-expansion steel and production method thereof

Technical Field

The invention relates to a manufacturing technology of high-hole-expansion steel, in particular to Nb microalloying high-strength high-hole-expansion steel and a production method thereof.

Background

The traditional thin strip steel is produced by casting blanks with the thickness of 70-200mm through multi-pass continuous rolling, 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 strips with a thickness of less than 1.5mm inclusive are 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 the casting blank is thin, so that 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 in the traditional process through repeated multi-pass rolling; 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 higher cooling rate of the continuous casting and rolling of the sheet bar leads to the improvement of the strength of steel and the yield ratio, thereby increasing the rolling load, so that the thickness specification of the hot rolled product which can be economically produced cannot be too thin, generally more than or equal to 1.5mm, see Chinese patents CN200610123458.1, CN200610035800.2 and CN 200710031548.2.

The ESP realizes the continuous casting and rolling of the slab, cancels the slab flame cutting and heating furnace with the functions of heat preservation, heat equalization and slab transition, greatly shortens the length of the whole production line to about 190 meters, ensures that the slab continuously cast by a continuous casting machine has the thickness of 90-110mm and the width of 1100-1600mm, ensures that the continuously cast slab plays the role of heat preservation and equalization on the slab through a section of induction heating roller way, then sequentially carrying out the procedures of rough rolling, finish rolling, layer cooling and coiling to obtain the hot rolled plate, and the process realizes endless rolling, the thinnest hot rolled plate with the thickness of 0.8mm can be obtained, the specification range of the hot rolled plate is expanded, and the single production line yield can reach the scale of 220 kilotons per year. 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 the thin strip continuous casting process and is the only thin strip continuous casting process for realizing industrialization in the world.

The typical process flow of twin roll strip casting is shown in fig. 1, molten steel in a ladle 1 is directly poured into a molten pool 7 surrounded by two relatively rotating and rapidly cooled crystallizing

rollers

8a, 8b and side sealing devices 6a, 6b through a 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 position (nip point) where the roll gaps of the two crystallizing

rollers

8a, 8b are minimum, the steel strip is fed into a rolling mill 13 through a guide plate 9 to be rolled into a 0.7-2.5mm strip, then is cooled through a cooling device 14, is cut by a flying shear device 16 and is finally 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.

The requirements for the chassis structure are increasingly improved along with the automobile design, the part forming is more complex, the requirements for the flanging and hole expanding performance of the steel plate are further improved, the strength and rigidity of the part can be improved through the flanging and local hole expanding shape design of the automobile part, the aims of thinning and lightening the automobile steel plate can be achieved, the forming requirements for the automobile chassis and the cantilever part can be easily met by the traditional carbon-manganese solid solution strengthening steel and the low alloy precipitation strengthening steel structure steel plate, for example, the hole expanding rate of the traditional 440MPa steel plate carbon-manganese solid solution strengthening steel and the low alloy precipitation strengthening steel is only 50-70%, and therefore high hole expanding steel is produced. In 90 s in 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 with good formability, particularly flanging performance requirements, 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 and Mo, but the cost of the hot rolled steel plates is high.

In the past, two options are generally available to meet the use conditions of a steel plate on a car chassis, wherein one option is to use the steel plate with reduced strength (less than or equal to 300MPa) 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 are 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 is carried out by adopting a hot continuous rolling process and then cold rolling, and the production cost of the thin-specification high-hole-expansion steel is increased by the production process.

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 steel, the better the scale formation is, it is necessary to control the scale formation in each subsequent stage of the strip casting, and a closed chamber is used from the crystallization roll to the rolling mill entrance to prevent the strip casting from being oxidized, and the thickness of the scale on the surface of the strip casting can be controlled by adding hydrogen in the closed chamber, for example, 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 the product quality; 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 nonuniformity, which can cause the structure of a final product obtained after austenite phase transformation to be nonuniform, thereby causing the performance of the product to be unstable. 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-hole-expansion steel plate with 540MPa 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 patent CN103602890 discloses a high hole expansion steel plate with 440MPa tensile strength and a manufacturing method thereof. The patent adopts the traditional continuous casting and the traditional hot rolling process for production, and adopts a one-stage conventional laminar cooling mode.

Chinese patents CN105154769 and CN106119702 respectively disclose 780MPa and 980MPa grade hot-rolled high-strength high-hole-expansion steel and manufacturing methods thereof, which are both high-strength steel categories, and the strengthening of steel types is realized by adding Ti, Mo and more microalloy elements such as Ti, Nb, Cr, V and the like, 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 adopts C, Si, Mn to add a small amount of Nb, Ti alloy element, can produce tensile strength at the reaming steel more than 490MPa, the hot rolling must adopt two segmentation laminar cooling modes, can more accurately simulate two segmentation cooling control in the laboratory, obtain fine test result, but in hot rolling production, the steel speed change in the hot rolling is big, can't measure the steel sheet temperature of air cooling section again, adopt two segmentation cooling model control laminar cooling, the actual temperature fluctuation of steel sheet is big, easily lead to the coil of strip head in the tail performance fluctuation big.

Disclosure of Invention

The invention aims to provide Nb microalloyed high-strength high-hole-expansion 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 Nb 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 twin-roll thin strip continuous casting to cast a cast strip with the thickness of 1.5-3mm, directly entering a lower closed chamber with non-oxidizing atmosphere after the cast strip is taken out of a crystallization roll, and entering an online rolling mill for hot rolling under the closed condition; cooling the strip steel by adopting an air atomization cooling mode after rolling; the finally produced steel coil can be directly used as a hot rolled plate and can also be used after acid cleaning and flattening.

Specifically, the Nb microalloyed high-strength 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≤0.0050％，Nb：0.01-0.08％，Mn/S>250, total oxygen [ O ]]_T: 0.007 to 0.020 percent, and the balance of Fe and inevitable impurities.

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

The yield strength of the Nb microalloyed high-strength high-hole-expansion steel is more than or equal to 440MPa, the tensile strength is more than or equal to 590MPa, the elongation is more than or equal to 19 percent, and the hole expansion rate is more than or equal to 100 percent.

In the chemical composition design of the Nb microalloyed high-strength high-hole-expansion steel, the chemical composition of the Nb microalloyed high-strength high-hole-expansion steel is as follows:

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 is added in the invention to play 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, causes hot brittleness of steel, reduces ductility and toughness of steel, 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 thereof is in the range of 0.005% or less. And Mn/S > 250.

And Als: in order to control inclusions in steel, the invention requires that Al cannot be used for deoxidation, and in the use of refractory materials, the additional introduction of Al is avoided as much as possible, and the content of acid-soluble aluminum Als is strictly controlled: < 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.

Nb: in the strip casting process, the added alloying element Nb can be caused to exist mainly in a solid solution state in the steel strip due to its unique rapid solidification and rapid cooling characteristics, and precipitation of Nb is hardly observed even if the steel strip is cooled to room temperature. The Nb element which is dissolved in the steel can play a role in solid solution strengthening. The content range of Nb designed by the invention is 0.01-0.08%.

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.

The invention relates to a production method of Nb microalloying high-strength high-hole-expansion steel, which comprises the following steps:

1) smelting, namely smelting the raw materials,

smelting according to the chemical components, wherein in the smelting process, the basicity a of slagging is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a is 0.7 to 1.0; it is necessary to obtain MnO-SiO having a low melting point₂-Al₂O₃Ternary inclusions, MnO-SiO₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the temperature to be 0.5-2, preferably 1-1.8; free oxygen [ O ] in molten steel]_FreeThe 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 between 500 and 1500mm, and the preferred diameter is 800 mm; cooling the crystallization roller by introducing water, wherein the casting speed of the casting machine is 60-150 m/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 temperature of the casting strip is 1420-1480 ℃, the casting strip directly enters a lower closed chamber, non-oxidizing gas is introduced into the lower closed chamber, the oxygen concentration in the lower closed chamber is controlled to be less than 5%, and the temperature of the casting strip at the outlet of the lower closed chamber is 1150-1300 ℃;

4) in-line hot rolling

The cast strip is sent to a rolling mill through a pinch roll in a lower closed chamber to be rolled into strip steel, the rolling temperature is 1100-. The thickness of the hot rolled steel strip is 0.8-2.5mm, preferably 1.0-1.8 mm;

5) cooling after rolling

Cooling the strip steel after the online hot rolling, wherein the cooling adopts an air atomization cooling mode, and the cooling rate is more than 50 ℃/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 470-570 ℃.

Preferably, in the step 1), electric furnace steel making or converter steel making is adopted for smelting, and then refining is carried out in an LF furnace, a VD/VOD furnace or an RH furnace.

Preferably, in step 3), the non-oxidizing gas is N₂CO obtained by sublimation of Ar or dry ice₂A gas.

Preferably, in the step 5), the gas-water ratio of the gas atomization cooling is 15: 1-10: 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.

In the production method of the Nb microalloyed high-strength high-hole-expansion steel, the invention comprises the following steps:

in order to improve the castability of thin-strip continuous casting molten steel, the basicity a of slagging in the steel-making process is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a ═ 0.7 to 1.0.

To improve the castability of thin strip continuous casting molten steel, it is necessary to obtain MnO-SiO of low melting point₂-Al₂O₃Ternary inclusions, e.g. shaded areas of FIG. 2, MnO-SiO₂-Al₂O₃MnO/SiO in ternary inclusions₂The 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 point₂-Al₂O₃The ternary inclusions of (2) require free oxygen [ O ] in molten steel]_FreeThe range is as follows: 0.0005-0.005%.

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

According to the designed chemical composition smelting, the molten steel can be smelted by an electric furnace or a converter and then enters a refining process, such as an LF furnace, a VD/VOD furnace, an RH furnace and the like.

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 the 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 of the strip steel is improved. The gas-water ratio of gas atomization cooling is 15: 1-10: 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 range of the gas atomization cooling is more than 50 ℃/s, the strip steel is cooled to 470 ℃ and 570 ℃, and the rolled high-temperature austenite is converted into a mixed microstructure of ferrite and a small amount of bainite.

The reason for the castability of the thin strip continuous casting according to the present invention 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 and surface slag inclusion frequently occur in the casting process, so that production cannot be carried out normally and stably or the product quality cannot meet the 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, and finally, the productivity cannot be exerted, and the qualification rate of the product is low, so that the product is not suitable for being produced by adopting the strip casting process.

The steel grade related to the invention is controlled from the carbon content (avoids peritectic regions to solve uneven solidification shrinkage); alkalinity control, Als control, free oxygen total oxygen control and low-melting-point 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 why the spray cooling is preferably adopted after the strip continuous casting hot rolled steel coil according to the present invention is rolled:

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 a casting blank in an outlet fan-shaped section area of a 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 why the carrousel co-location coiler is preferably adopted for the thin strip continuous casting hot rolled steel coil related to the invention 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, an ESP production line of Avedi (Avedi) company adopts the underground triple-coiling mode, and an FTSC production line of Danielli (Danieli) introduced by Tang steel 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 9.4m), 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, the temperature deviation between the two recoiling machines can reach 49 ℃, and the performance deviation of the steel coil can be seriously influenced.

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. The CASTIP technology is characterized in that a crystallization roller with a small roller diameter of 500mm phi 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 is large, and in order to meet the temperature of molten steel in a molten pool, the tapping temperature needs to be greatly increased. 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 crystallization 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 uses 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 tensile strength of 590MPa level and a manufacturing process thereof, wherein trace amount of Nb is added in the components: 0-0.10% and Ti: 0-0.04 percent by weight, and is produced by adopting the traditional continuous casting and hot rolling process, the steel plate after final rolling is cooled to 600-750 ℃ at the cooling speed of 50-100 ℃/s, then is cooled in the air for 3-10 seconds at the cooling speed of 5-15 ℃/s, then is cooled to 350-500 ℃ at the cooling speed of 70-150 ℃/s, is coiled, and then is air-cooled 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, contains Cr alloy element, 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 obtained to be more than 780MPa, and the hole expansion rate is only more than 68%. The hot-rolled high-hole-expansion steel disclosed in Japanese patent JP2003/016614 has a carbon content of 0.02-0.10%, Si content of 0.5% or less, and a tensile strength of 590MPa or more, but contains many alloying elements such as Nb, Ti, V, Cr, RE and the like, so that the steel-making cost is high, and a good surface coatability is a main objective. 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 U.S. Pat. No. 2006096678 has a strength of 780MPa or more, an elongation of 22% or more, and an expansion ratio of 60% or more. The hot rolled steel sheet disclosed in U.S. Pat. No. 4, 4415376 has a yield strength of 80ksi (550MPa) 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 method for producing Nb microalloying high-strength high-hole-expansion steel by utilizing the thin strip continuous casting technology has no report so far, and has the following summarized 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, and compared with the traditional high-reaming steel, the energy consumption and CO of the production are reduced₂Greatly reduces the emission, 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. 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 performance stability of the strip steel can be improved; effectively reducing the thickness of oxide scale on the surface of the strip steel.

6. In the traditional process, alloy elements are separated out 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.

7. The carrousel reeling machine is selected for reeling, 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-SiO₂-Al₂O₃Ternary phase diagram (shaded area: low melting point region).

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 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 rotation of the

crystallization rollers

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

After the casting strip 11 comes out of the

crystallization rollers

8a and 8b, the casting strip temperature is 1420-₂Ar, or other non-oxidizing gas, such as CO obtained by sublimation of dry ice₂Gas, etc., the oxygen concentration in the lower sealed chamber 10 is controlled to be<5 percent, the lower closed chamber 10 protects the inlet of the rolling mill 13 from oxidation of the cast strip 11; the temperature of the casting belt at the outlet of the lower closed chamber 10 is 1150-1300 ℃; then the cast strip is sent to a rolling mill 13 through a swing guide plate 9 and a pinch roll 12, hot rolling is carried out to form hot rolling strip steel with the thickness of 0.8-2.5mm, and the cooling after rolling adopts an air atomization cooling mode to improve the temperature uniformity of the strip steel. After the head of the hot-rolled strip steel 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 steel after the head is cut enters a coiler19, winding is performed. And 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 manufacturing method of the invention has the process parameters shown in the table 2, and the properties of the finally obtained product are shown in the table 3.

In conclusion, the high-hole-expansion steel manufactured according to the designed steel components by utilizing the thin-strip continuous casting process technology has the yield strength of more than or equal to 440MPa, the tensile strength of more than or equal to 590MPa, the elongation of more than or equal to 19 percent and the hole expansion rate of more than or equal to 100 percent.

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

	C	Si	Mn	P	S	N	O	Als	Nb	Ca
											Example 1	0.04	0.27	1.35	0.008	0.004	0.0064	0.0093	0.0009	0.06	0.003
Example 2	0.05	0.20	0.90	0.013	0.003	0.0068	0.0110	0.0006	0.05	0.004
											Example 3	0.02	0.38	1.28	0.015	0.004	0.0048	0.0150	0.0004	0.03	0.005
Example 4	0.01	0.22	1.26	0.013	0.005	0.0067	0.0130	0.0008	0.03	0.004
											Example 5	0.03	0.41	0.85	0.009	0.002	0.0062	0.0120	0.0007	0.01	0.003
Example 6	0.04	0.45	0.80	0.012	0.002	0.0046	0.0070	0.0008	0.04	0.001
											Example 7	0.02	0.28	0.95	0.015	0.003	0.0040	0.0100	0.0005	0.06	0.002
Example 8	0.05	0.37	1.30	0.014	0.005	0.0100	0.0085	0.0006	0.05	0.004
											Example 9	0.04	0.36	0.84	0.018	0.003	0.0078	0.0200	0.0003	0.04	0.004
Example 10	0.02	0.45	0.90	0.020	0.001	0.0065	0.0125	0.0004	0.07	0.004
											Example 11	0.02	0.60	0.85	0.010	0.002	0.0080	0.0090	0.0009	0.04	0.003
Example 12	0.03	0.59	1.50	0.012	0.005	0.0075	0.0118	0.0003	0.06	0.001
											Example 13	0.05	0.45	1.37	0.018	0.004	0.0045	0.0132	0.0006	0.08	0.002
Example 14	0.02	0.28	1.40	0.017	0.003	0.0064	0.0075	0.0005	0.03	0.003

TABLE 2 Process parameters of the examples

TABLE 3 Properties of the steels of the examples

Claims

1. The Nb microalloyed high-strength 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≤0.0050％，Nb：0.01-0.08％，Mn/S>250, total oxygen [ O ]]_T: 0.007 to 0.020 percent of Fe and inevitable impurities in balance;

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;

the high hole expansion steel has the pressure of more than or equal to 440MPa, the tensile strength of more than or equal to 590MPa, the elongation of more than or equal to 19 percent and the hole expansion rate of more than or equal to 100 percent;

the production method of the Nb microalloyed high-strength high-hole-expansion steel specifically comprises the following steps:

1) smelting

Smelting according to the chemical components, wherein in the smelting process, the basicity a of slagging is CaO/SiO₂Control in a<1.5; it is necessary to obtain MnO-SiO having a low melting point₂-Al₂O₃Ternary inclusions, MnO-SiO₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the temperature to be 0.5-2 ℃; free oxygen [ O ] in molten steel]_FreeThe 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 between 500 and 1500 mm; cooling the crystallization roller by introducing water, wherein the casting speed of the casting machine is 60-150 m/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

4) in-line hot rolling

Conveying the cast strip to a rolling mill through a pinch roll in a lower closed chamber, and rolling the cast strip into strip steel, wherein the rolling temperature is 1100-;

5) cooling after rolling

6) strip steel coiling

2. A production method of Nb microalloyed high-strength high-hole-expansion steel is characterized by comprising the following steps:

1) smelting

According to claim 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≤0.0050％，Nb：0.01-0.08％，Mn/S>250, total oxygen [ O ]]_T: 0.007 to 0.020 percent of the total weight of the slag, the balance of Fe and inevitable impurities, and smelting the components, wherein in the smelting process, the alkalinity a of the slag forming is CaO/SiO₂Control in a<1.5; it is desirable to obtain low melting point MnO-SiO₂-Al₂O₃Ternary inclusions, MnO-SiO₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the temperature to be 0.5-2 ℃; free oxygen [ O ] in molten steel]_FreeThe 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 between 500-1500 mm; cooling the crystallization roller by introducing water, wherein the casting speed of the casting machine is 60-150 m/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

4) in-line hot rolling

5) cooling after rolling

6) strip steel coiling

3. The method for producing the Nb microalloyed high-strength and high-bore-expansion steel as claimed in claim 2, wherein in the step 1), the steel is smelted by electric furnace steelmaking or converter steelmaking and then refined in an LF furnace, a VD/VOD furnace or an RH furnace.

4. The method for producing the Nb microalloyed high strength and high hole expansion steel as set forth in claim 2, wherein the non-oxidizing gas in the step 3) is N₂CO obtained by sublimation of Ar or dry ice₂A gas.

5. The method for producing the Nb microalloyed high-strength high-bore-expansion steel as set forth in claim 2, wherein in the step 5), the gas-water ratio of the gas atomization cooling is 15: 1-10: 1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

6. The method for producing the Nb microalloyed high strength and high bore expanding steel as set forth in claim 2, wherein the coiling in the step 6) is in a form of a twin coiler or a carrousel coiler.

7. The method of producing Nb microalloyed high strength and high hole expansion steel according to claim 2The production method is characterized in that in the step 1), MnO-SiO₂-Al₂O₃MnO/SiO in ternary inclusions₂The temperature is controlled to be 1-1.8.

8. The method for producing the Nb microalloyed high strength and high bore expanding steel as set forth in claim 2, wherein the diameter of the crystallizing roller in the step 2) is 800 mm.

9. The method for producing the Nb microalloyed high strength and high bore expanding steel as set forth in claim 2, wherein the hot rolling reduction in the step 4) is 30 to 50%.

10. The method for producing the Nb microalloyed high strength and high bore expanding steel as set forth in claim 2 or 9, wherein the thickness of the hot rolled steel strip in the step 4) is 1.0 to 1.8 mm.

11. The method for producing the Nb microalloyed high strength and high bore expanding steel as set forth in claim 2, wherein in the step 1), the basicity a of the slagging is CaO/SiO in the smelting process₂Control a<1.2。

12. The method for producing Nb microalloyed high strength and high hole expansion steel according to claim 2, wherein in the step 1), the basicity a of the slagging is CaO/SiO in the smelting process₂And controlling a to be 0.7-1.0.