CN112522591B

CN112522591B - Method for producing high-strength and high-corrosion-resistance steel by thin-strip continuous casting

Info

Publication number: CN112522591B
Application number: CN201910889378.4A
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: 2022-03-18
Anticipated expiration: 2039-09-19
Also published as: CN112522591A

Abstract

A method for producing high-strength and high-corrosion-resistance steel by thin-strip continuous casting comprises the following steps: a) smelting, namely smelting according to the following chemical components in percentage by weight: 0.02-0.06% of C, 0.1-0.5% of Si, 0.4-1.7% of Mn, less than or equal to 0.02% of P, 4.0-6.0% of Cr, 1.0-3.0% of Ni, less than or equal to 0.007% of S, N: 0.004-0.010% of Als<0.001% of total oxygen [ O ]]_T: 0.007-0.020%, the balance of Fe and inevitable impurities, and simultaneously: contains Nb: 0.01-0.08% or V: 0.01-0.08% of one or two elements; Mn/S>250 of (a); b) continuous casting, namely performing double-roller thin strip continuous casting to obtain a cast strip; c) under closed protection, the casting strip directly enters a lower closed chamber after coming out of the crystallization roller, and non-oxidizing gas is introduced into the lower closed chamber; d) carrying out on-line hot rolling, wherein the cast strip is conveyed to a rolling mill through pinch rolls in a lower closed chamber to be rolled into strip steel; e) cooling the strip steel by adopting an air atomization cooling mode; f) and (5) coiling the strip steel.

Description

Method for producing high-strength and high-corrosion-resistance steel by thin-strip continuous casting

Technical Field

The invention relates to a continuous casting process, in particular to a method for producing high-strength and high-corrosion-resistant steel by continuously casting a thin strip.

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 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 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 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 not possible to be too thin, generally being equal to or larger than 1.5mm, see chinese patents CN200610123458.1, CN200610035800.2 and CN 200710031548.2.

The ESP realizes the continuous casting of the slab continuously, cancels the slab flame cutting and the heating furnace with the functions of heat preservation, soaking and slab transition, and greatly shortens the length of the whole production line to about 190 meters. The thickness of the continuously cast plate blank of the continuous casting machine is 90-110mm, the width of the continuously cast plate blank is 1100-1600mm, the continuously cast plate blank plays a role in heat preservation and uniform heating on the plate blank through a section of induction heating roller way, and then the continuously cast plate blank sequentially enters the working procedures of rough rolling, finish rolling, layer cooling and coiling to obtain a hot rolled plate. The process realizes endless rolling, can obtain the hot rolled plate with the thinnest thickness of 0.8mm, expands the specification range of the hot rolled plate, and has the output of a single production line reaching 220 kiloton 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 large ladle 1 is directly poured into a molten pool 7 surrounded by two relatively rotating and rapidly cooled crystallizing

rolls

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

rolls

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

Thin-gauge high-corrosion-resistance steel is increasingly applied to fields needing high corrosion resistance, such as the transformation of carriages of green cars in the train manufacturing industry, the market quantity of steel for the transformation of the carriages of the green cars is large, and the steel has severe requirements on the corrosion resistance of the steel, and the corrosion resistance of the steel is required to be doubled on the basis of the traditional atmospheric corrosion-resistance steel. And also has certain requirements on cost. Aiming at such a huge market demand, no ready-made steel grade can be directly corresponded and used, and a brand-new steel grade needs to be developed. Stainless steel is not suitable for cost, and the product is required to have good bending and forming properties; the thickness specification of the product is as follows: 1.0-2.0 mm. The invention provides that the thin-strip continuous casting process is adopted to produce the high-strength high-corrosion-resistance steel, has certain advantages, and the successful development of the thin-specification high-strength high-corrosion-resistance steel product provides a wide prospect for the train manufacturing industry on light weight, greenness, energy consumption reduction, high corrosion resistance stainless steel and the like.

Thin-gauge high-strength high-corrosion-resistance steel is produced by adopting thin-strip continuous casting, and the thin-strip continuous casting process has strong manufacturing and cost advantages due to the thin thickness. The specification characteristic thickness of the product supplied by post-processing the high-strength high-corrosion-resistance steel is 1.0, 1.1, 1.2, 1.25, 1.4mm, 1.5mm, 1.6mm, 1.8mm, 2.0mm and the like, and because the product thickness is thin, if the production is difficult by adopting the traditional continuous casting and hot continuous rolling production line, the production is generally carried out by adopting the first hot continuous rolling process and then the cold rolling mode, and the production cost of the thin-specification high-strength high-corrosion-resistance steel is increased by the production flow.

When the hot-rolled strip steel is used as a thin hot-rolled plate or a product which is 'hot to cool', the requirement on the surface quality of the strip steel is high. It is generally required that the thinner the scale on the surface of the strip, the better the scale formation is, and it is required to control the scale formation in the subsequent stages of the strip casting, for example, in the twin roll strip casting process, the oxidation of the strip is prevented by using a closed chamber device in the crystallizing roll up to the entrance of the rolling mill, and the scale formation on the surface of the strip can be controlled by adding hydrogen gas in the closed chamber device such as 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 instability of the performance, particularly the forming performance, of the product. Therefore, the thin strip continuous casting production line for producing high-strength and high-corrosion-resistance steel products has certain difficulties and challenges, and needs to have breakthroughs in components and processes.

At present, a plurality of patents have been applied for corrosion resistant steel and manufacturing methods thereof at home and abroad, wherein the corrosion resistant steel with the strength level of 450MPa or above is mostly prepared by adopting Nb, V, Ti and Mo composite microalloying technology, and the comprehensive mechanical property of the corrosion resistant steel is improved by fine grain strengthening and precipitation strengthening, and the specific patent components and properties are shown in Table 1.

TABLE 1 patented comparison of Corrosion resistant steels (wt%)

The high-strength corrosion-resistant steel adopts a micro-alloying route, all contains Nb, V, Ti, Mo and other alloy elements in a component system, and is produced by adopting the traditional hot rolling process. 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. If a strip having a thickness of less than 2mm is to be produced, the hot rolled strip is generally subjected to further cold rolling and subsequent annealing.

The production of the microalloy high-strength corrosion-resistant steel by using the traditional process has the main problems that:

(1) 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.

(2) The corrosion-resistant steel contains high-content elements which are easy to segregate, such as copper and the like and can improve the corrosion resistance of the steel strip, and the traditional process is low in casting blank solidification and cooling speed, so that macro segregation of the elements, such as copper and the like, can be easily caused, the anisotropy and the macro cracks of the casting blank are caused, and the yield is low.

(3) Because the corrosion-resistant steel has the characteristic of easy segregation in the traditional process, in the component design for producing the high-strength corrosion-resistant steel by utilizing the traditional process, the addition amount of copper is in the range of 0.2-0.55%, the lower limit is usually taken in the actual production, and the addition amount of chromium is in the range of 0.2-1.25, which is generally lower. As a result, the corrosion resistance of the steel strip is not high.

(4) The corrosion resistance of the corrosion-resistant steels cannot meet the corrosion resistance requirement of the steel, and the corrosion resistance requirement of the corrosion-resistant steels is doubled on the basis of the traditional corrosion-resistant steels.

(5) In the traditional process, because microalloy elements cannot be kept as solid solutions in the hot rolling process and are partially precipitated, the steel structure has fine grains, the yield ratio is improved, and the formability is poor, so that the rolling load is obviously increased, the energy consumption and the roller consumption are increased, and the equipment is greatly damaged, thereby limiting the thickness range of economically and practically producing high-strength corrosion-resistant steel hot rolled products, which is usually less than or equal to 2 mm. The steel strip thickness can be further reduced by continuing the cold rolling of the conventional hot rolled product, but the high strength of the hot rolled steel strip also causes difficulty in the cold rolling. Firstly, the requirement of high cold rolling load on equipment is high, and the damage is large; and the second phase precipitated by alloy elements in the hot rolled product obviously increases the recrystallization annealing temperature of the cold rolled steel strip.

If the thin slab continuous casting and rolling process is adopted to produce the microalloy high-strength corrosion-resistant steel, the defects of the traditional process can be overcome to a certain extent. 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; the casting blank of the thin slab process directly enters a soaking pit furnace for soaking and heat preservation without cooling, or a small amount of temperature supplement is carried out, so the thin slab process greatly shortens the process flow, reduces the energy consumption, reduces the investment and further reduces the production cost; in addition, the solidification and cooling speed of the casting blank in the thin slab process is accelerated, and the element macrosegregation can be reduced to a certain extent, so that the product defects are reduced, and the yield is improved.

Chinese patent CN200610123458.1 discloses a method for producing 700MPa grade high-strength corrosion-resistant steel by adopting Ti microalloying process based on thin slab continuous casting and rolling process, the chemical components of the corrosion-resistant steel plate manufactured by the method are as follows: c: 0.03 to 0.07%, Si: 0.3-0.5%, Mn: 1.2-1.5, P: less than or equal to 0.04 percent, S: less than or equal to 0.008 percent, Al: 0.025-0.05%, Cr: 0.3-0.7%, Ni: 0.15-0.35%, Cu: 0.2-0.5%, Ti: 0.08-0.14%, N: less than or equal to 0.008 percent, and the balance of Fe and inevitable impurities. The yield strength of the steel plate is more than or equal to 700MPa, the tensile strength is more than or equal to 775MPa, and the elongation is more than or equal to 21 percent. In the patent, the phosphorus is controlled according to impurity elements, the content is less than or equal to 0.04 percent, and is less than or equal to 0.025 percent compared with the traditional process, so that the phosphorus content is relaxed.

Chinese patent CN200610035800.2 discloses a method for producing 700MPa grade V-N microalloyed corrosion-resistant steel based on a thin slab continuous casting and rolling process, and the chemical components of the corrosion-resistant steel plate manufactured by the method are as follows: c: less than or equal to 0.08 percent, Si: 0.25 to 0.75%, Mn: 0.8-2, P: less than or equal to 0.07-0.15 percent, S: less than or equal to 0.04 percent, Cr: 0.3-1.25%, Ni: less than or equal to 0.65%, Cu: 0.25-0.6%, V: 0.05-0.2%, N: 0.015 to 0.03 percent, and the balance of Fe and inevitable impurities. The yield strength of the steel plate is more than or equal to 700MPa, the tensile strength is more than or equal to 785MPa, and the elongation is more than or equal to 21%. In this patent, phosphorus is controlled according to elements that improve corrosion resistance, and the content is 0.07-0.15%; the copper content is 0.25-0.6%, and the lower limit and the upper limit are respectively 0.2% and 0.55% higher than the lower limit and the upper limit of the copper content of the traditional process.

Although the thin slab process has the above advantages in producing microalloy high-strength corrosion-resistant steel, certain problems in the conventional process production still exist in the thin slab process, such as: the microalloy elements can not be kept as solid solutions in the hot rolling process, partial precipitation occurs, and the steel strength is improved, so that the rolling load is increased, the energy consumption and the roller consumption are increased, and the thickness specification of a high-strength corrosion-resistant steel hot-rolled product which can be economically and practically produced is not possible to be too thin and is more than or equal to 1.5mm, see patents CN200610123458.1, CN200610035800.2 and CN 200710031548.2.

Chinese patent CN1633509A discloses a method for producing copper-containing carbon steel products by strip casting, which emphasizes that the strip is subjected to heat treatment processes such as annealing, tempering and the like at the temperature range of 400-700 ℃ to precipitate or recrystallize copper elements in the strip. Compared with the invention, the Cr content in the components of the invention is obviously increased, and the invention has obvious distinguishing characteristics, and simultaneously, the subsequent treatment processes are completely different.

The manufacturing method of the high-copper low-alloy thin strip mentioned in the US 2008264525/Chinese CN200580009354.1 is technically characterized in that the strip steel is cooled to be lower than 1080 ℃ in a non-oxidizing atmosphere before entering a rolling mill so as to prevent the strip steel from generating the phenomenon of hot brittleness. Compared with the invention, the Cr content in the components of the invention is obviously increased, and the subsequent treatment method after the strip steel is taken out is also different.

International patents WO 2008137898, WO 2008137899, WO 2008137900, and chinese patents CN200880023157.9, CN200880023167.2, CN200880023586.6 disclose a method for producing a microalloyed steel strip with a thickness of 0.3-3mm by using a strip continuous casting and rolling process. The method adopts the following chemical components: less than or equal to 0.25 percent, Mn: 0.20 to 2.0%, Si: 0.05-0.50%, Al: 0.01% or less, and further comprises Nb: 0.01E0.20%, V: 0.01-0.20%, Mo: 0.05 to 0.50% of at least one. Under the technological conditions that the hot rolling reduction is 20-40% and the coiling temperature is less than or equal to 700 ℃, the microstructure of the hot rolled strip is bainite and acicular ferrite. The patent states that the alloying elements mainly present in the solid-solution state in the cast strip inhibit the recrystallization of austenite after hot rolling, and that the recrystallization of austenite is very limited even if the reduction ratio reaches 40%. Since austenite is not recrystallized at the hot rolling reduction of 20 to 40%, hardenability of coarse austenite is maintained after hot rolling, and a room temperature structure of bainite + acicular ferrite is obtained. The temperature ranges used for hot rolling are not given in the patents, but are in articles related to these patents (C.R. Killmore, etc. development of Ultra-Thin case Strip Products by the

Ais Tech, Indianapolis, Indiana, USA, May 7-10,2007), the hot rolling temperature employed was reported to be 950 ℃.

The thin-strip continuous casting low-carbon microalloyed steel product produced by the method has higher strength, the yield strength can reach 650MPa and the tensile strength can reach 750MPa within the range of the component system, but the main problem is that the elongation of the product is not high (less than or equal to 6 percent or less than or equal to 10 percent). The main reasons for the low elongation are: the cast strip obtained by the thin strip continuous casting process has uneven austenite grain size, which is as small as tens of microns and as large as seven-eight-hundred microns. However, the hot rolling reduction rate of the strip casting process is usually hardly more than 50% by only using a 1-2 stand rolling mill, the effect of refining grains by deformation is very small, if austenite grains are not refined by recrystallization, the inhomogeneous austenite structure is hardly effectively improved after hot rolling, and the bainite + acicular ferrite structure generated after transformation of the austenite with inhomogeneous size is also very inhomogeneous, so that the elongation is not high.

In order to improve the strong plasticity matching of the thin strip continuous casting microalloyed steel, Chinese patent No. 02825466.X proposes another method for producing a microalloyed steel thin strip with the thickness of 1-6mm by utilizing a thin strip continuous casting and rolling process. The microalloyed steel component system adopted by the method is C: 0.02 to 0.20%, Mn: 0.1 to 1.6%, Si: 0.02-2.0%, Al: less than or equal to 0.05 percent, S: less than or equal to 0.03%, P: less than or equal to 0.1 percent, Cr: 0.01 to 1.5%, Ni: 0.01-0.5%, Mo: less than or equal to 0.5 percent, N: 0.003-0.012%, and the balance of Fe and inevitable impurities. The hot rolling of the cast strip is carried out at 1150- (Ar1-100) DEG C, and the hot rolling is carried out in the austenite region, austenite ferrite two-phase region, or ferrite region, with a hot rolling reduction of 15-80%. According to the method, an online heating system is designed behind a thin strip continuous casting and rolling unit, the heating temperature range is 670-. When the method is used for production, an online heating system is required to be added during production line design, and the heating uniformity can be ensured only if the heating furnace has enough length due to the length of heating time and the dependence on the belt speed and the length of the heating furnace. The investment cost is increased, the occupied area of the thin strip continuous casting and rolling production line is obviously increased, and the advantages of the production line are reduced.

Disclosure of Invention

The invention aims to provide a method for producing high-strength and high-corrosion-resistance steel by strip continuous casting, which can further reduce the production cost and improve the product performance, particularly the corrosion resistance of the product, by a strip continuous casting process; the yield strength of the high corrosion-resistant steel reaches more than 480MPa, the tensile strength reaches more than 600MPa, and the elongation reaches more than 22%; the relative corrosion rate is less than or equal to 25 percent.

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

in the invention, micro alloy elements such as Nb/V and the like are selectively added into steel, and the alkalinity of slag, the type and melting point of inclusions in the steel, the free oxygen content in molten steel and the content of acid-soluble aluminum Als are controlled 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, and the air atomization cooling mode can effectively reduce the thickness of oxide skins on the surface of the strip steel, improve the temperature uniformity of the strip steel and improve the surface quality of the strip steel. The finally produced steel coil can be used after acid pickling-flattening and can also be used after acid pickling-hot galvanizing.

Specifically, the method for producing the high-strength and high-corrosion-resistant steel by strip continuous casting comprises the following steps:

a) smelting

Smelting according to the following chemical components in percentage by weight: c: 0.02 to 0.06%, Si: 0.1-0.5%, Mn: 0.4-1.7%, P is less than or equal to 0.02%, Cr: 4.0-6.0%, Ni: 1.0-3.0%, S is less than or equal to 0.007%, N: 0.004-0.010% of Als<0.001% of total oxygen [ O ]]_T: 0.007-0.020%, the balance being Fe and unavoidable impurities, and, simultaneously: contains Nb: 0.01-0.08% or V: 0.01-0.08% of one or two elements;

basicity a of slagging in steel-smelting process is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a ═ 0.7-1.0; obtaining low melting point MnO-SiO in molten steel₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the concentration to be 0.5-2, preferably 1-1.8; free oxygen [ O ] in molten steel]_FreeThe contents are as follows: 0.0005-0.005%; in the molten steel composition, Mn/S>250；

b) 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 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;

c) lower seal 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 an outlet of the lower closed chamber is 1150-1300 ℃;

d) in-line hot rolling

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

e) strip steel cooling after rolling

Cooling the rolled strip steel, and cooling the strip steel by adopting an air atomization cooling mode, wherein the cooling rate of air atomization cooling is 20-100 ℃/s;

f) strip steel coiling

And 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 500-600 ℃ so as to convert the high-temperature austenite structure after rolling into bainite or acicular ferrite or a mixed microstructure of bainite and acicular ferrite.

Further, the method also comprises the step g) of subsequent treatment, wherein the steel coil is used as an acid flat coil after acid washing and flattening, or used as a galvanized sheet after acid washing and hot galvanizing.

Preferably, in step a), the basicity a of the slag formation in the steel-making process is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a ═ 0.7 to 1.0.

Preferably, in step a), the continuous casting of the molten steel is required to obtain a low melting point MnO-SiO₂-Al₂O₃Ternary inclusions, MnO-SiO₂-Al₂O₃MnO/SiO in ternary inclusions₂The concentration is controlled to be 0.5-2, preferably 1-1.8.

Preferably, free oxygen [ O ] in the continuous casting molten steel in step a)]_FreeThe range is as follows: 0.0005-0.005% of total oxygen [ O ] in the steel strip]_TThe range is as follows: 0.007-0.020%.

Preferably, in step c), the non-oxidizing gas is N2, Ar or CO2 gas obtained by sublimation on dry ice.

Preferably, in the step e), 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 f), the coiling is carried out in the form of a twin-coiler or in the form of a carrousel coiler.

The microstructure of the high-strength and high-corrosion-resistance steel is bainite or acicular ferrite or a mixed microstructure of bainite and acicular ferrite.

Through the manufacturing process, the performance of the final high-strength high-corrosion-resistance steel strip reaches over 480MPa of yield strength, over 600MPa of tensile strength and over 22 percent of elongation; the relative corrosion rate is less than or equal to 25 percent.

In the method of the invention:

the raw materials of the invention can be from scrap steel, and the molten steel can be smelted by an electric furnace or a converter and then enters necessary refining procedures, such as an LF furnace, a VD/VOD furnace, an RH furnace and the like.

The invention relates to a high-strength high-corrosion-resistance steel, which comprises the following chemical components in the design:

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 0.02-0.06%.

Si: si acts as a solid solution strengthening in steel, and addition of Si to steel improves steel purity and deoxidation, but too high a Si content leads to deterioration of weldability and toughness in weld heat affected zone. Therefore, the Si content range adopted by the invention is 0.1-0.5%.

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 0.4-1.7%.

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 also reduces weldability and corrosion resistance. Therefore, in the present invention, S is also controlled as an impurity element, and the content thereof is in the range of 0.007% 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 the element C, the element N can improve the strength of the steel through interstitial solid solution, but the interstitial solid solution of the element N has great harm to the plasticity and the toughness of the steel, and the yield ratio of the steel can be improved due to the existence of free N, so that the content of the element N cannot be too high. The content range of N adopted by the invention is 0.004-0.010%.

Cr: not only the element for improving the hardenability of the steel, but also Cr is the main alloy element in the stainless steel, the corrosion resistance of the steel can be obviously improved, the welding performance can be seriously deteriorated if the content of Cr is too high, and the content of Cr is limited to 4.0-6.0 percent in the invention.

Ni: the alloy can improve hardenability, remarkably improve the low-temperature toughness of steel, and is a favorable element for improving the corrosion resistance and the toughness of steel, meanwhile, Ni can adversely affect the welding performance of Cr punching, and Ni can effectively prevent the hot brittleness of Cu. The present invention limits the Ni content to 1.0-3.0%.

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%.

V: in the strip casting process, V and Nb are similar but weaker than Nb and mainly exist in a solid solution state in the steel strip, and even if the steel strip is cooled to room temperature, precipitation of V is hardly observed, and V element dissolved in the steel can play a role of solid solution strengthening. The content range of V adopted by the invention is 0.01-0.08%.

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, oxygen (O) in the steel is an essential element for forming oxide inclusions, and 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, the Mn and S of the above components should satisfy the following relation: Mn/S > 250.

The strip steel after on-line hot rolling is cooled after rolling, the rolled strip steel is cooled by adopting an air atomization cooling mode, and the air atomization cooling mode can effectively reduce the thickness of oxide skins on the surface of the strip steel, improve the temperature uniformity of the strip steel and improve the surface quality of the strip steel. 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 of the gas atomization cooling is in the range of 20-100 ℃/s.

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. The coiling temperature of the hot rolled strip is controlled to be 500-600 ℃, so that the rolled high-temperature austenite structure is converted into bainite, acicular ferrite or a mixed microstructure of bainite and acicular ferrite. The coiling adopts a double coiling machine mode and a carrousel coiling mode, so that the continuous production of the strip steel is ensured.

The invention has the main advantages that:

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 corrosion-resistant steel production by the traditional process, and compared with the traditional production process, the energy consumption and CO2 emission of the production are greatly reduced, thus the invention is a green and environment-friendly product.

3. According to the invention, the hot-rolled thin corrosion-resistant steel is produced by adopting a thin strip continuous casting process, the corrosion resistance is greatly improved by improving the content of Cr without the segregation problem of Cr, the corrosion resistance can be comparable to that of stainless steel, and the corrosion resistance can be doubled on the basis of the traditional corrosion-resistant steel; meanwhile, the thickness of the cast strip is thin, and the thin-specification product is produced without cold rolling by hot rolling on line to the thickness of the expected product and is directly supplied to the market for use, so that the purposes of supplying thin-specification hot rolled plates and cooling in hot bands are achieved, and the cost performance of the plates and strips can be obviously improved.

4. 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 the oxide scale on the surface of the strip steel.

5. 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.

6. 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.

7. 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 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.

8. 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.

Drawings

FIG. 1 is a schematic view of a production process arrangement according to the present invention;

FIG. 2 shows MnO-SiO₂-Al₂O₃Ternary phase diagram (shaded area: low melting point region).

Detailed Description

The invention is further illustrated by the following examples, which are not intended to limit the invention 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 crystallization roller is between 500 and 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 casting strip 11 comes out of the

crystallization rollers

8a and 8b, the casting strip temperature is 1420-₂Gas, etc., the oxygen concentration in the lower sealed chamber 10 is controlled to be<5 percent. The lower enclosed chamber 10 protects the cast strip 11 against oxidation to the rolling mill 13 inlet. The temperature of the cast 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, hot rolling is carried out to form hot rolled strip steel with the thickness of 0.8-2.5mm, and the rolled strip steel is cooled by adopting an air atomization cooling mode to improve the temperature uniformity of the strip steel. After the head is cut by the flying shear device 16, the head falls into the flying shear pit 18 along the flying shear guide plate 17, and the hot rolled strip after the head is cut enters the coiler 19 for coiling. And taking the steel coil off the coiling machine, and naturally cooling to room temperature. The finally produced steel coil can be used after acid pickling-flattening and can also be used after acid pickling-hot galvanizing.

The chemical compositions of the examples of the invention are shown in table 2, and the balance of the compositions is Fe and other unavoidable impurities. The hot rolled strip steel obtained by the method has a microstructure of bainite, acicular ferrite or a mixed microstructure of bainite and acicular ferrite, wherein the microstructure of the high-strength and high-corrosion-resistance steel is shown in a table 3.

The mechanical properties are shown in Table 4.

The corrosion resistance of the example steels was tested: ordinary carbon steel Q345B and traditional atmospheric corrosion resistant steel SPA-H are used as comparison samples, and a periodic infiltration cyclic corrosion experiment for 72 hours is carried out according to a periodic infiltration corrosion test method (TB/T2375-93) of corrosion resistant steel. And calculating the corrosion weight loss of the sample in unit area to obtain the average corrosion rate and further obtain the relative corrosion rate of the steel grade. The test results are shown in Table 5.

In conclusion, the high-strength and high-corrosion-resistant steel manufactured by the 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 480MPa, the tensile strength of more than or equal to 600MPa, the elongation of more than or equal to 22 percent, the yield ratio of less than 0.8 and qualified cold-working bending performance. The comparison result of the corrosion resistance also shows that the relative corrosion rate of the steel grade is less than or equal to 25 percent.

Claims

1. A method for producing high-strength and high-corrosion-resistance steel by thin-strip continuous casting is characterized by comprising the following steps:

a) smelting

Smelting according to the following chemical components in percentage by weight: c: 0.02 to 0.06%, Si: 0.1-0.5%, Mn: 0.4-1.7%, P is less than or equal to 0.02%, Cr: 4.0-6.0%, Ni: 1.0-3.0%, S is less than or equal to 0.007%, N: 0.004-0.010% of Als<0.001% of total oxygen [ O ]]_T: 0.007-0.020%, the balance being Fe and unavoidable impurities, and, simultaneously:

contains Nb: 0.01-0.08% or V: 0.01-0.08% of one or two elements;

Mn/S>250；

basicity a of slagging in steel-smelting process is CaO/SiO₂Control in a<1.5; obtaining low melting point MnO-SiO in molten steel₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the temperature to be 0.5-2 ℃; steelFree oxygen [ O ] in water]_FreeThe contents are as follows: 0.0005-0.005%; among the components of the molten steel, the molten steel has the characteristics of high temperature,

Mn/S>250；

b) 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;

c) lower seal protection

d) 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 with the thickness of 0.8-2.5mm, the rolling temperature is 1100-1250 ℃, the hot rolling reduction rate is controlled to be 10-50%, and the thickness of the rolled strip steel is 0.8-2.5 mm;

e) strip steel cooling after rolling

f) strip steel coiling

2. The method for producing high-strength and high-corrosion-resistance steel by strip continuous casting according to claim 1, further comprising the step g) of subsequent treatment, wherein the steel coil is used as an acid flat coil after acid pickling-flattening, or used as a galvanized sheet after acid pickling-hot galvanizing.

3. The thin strip casting production of claim 1Method for producing high-strength, high-corrosion-resistant steel, characterized in that in step c), the non-oxidizing gas is N₂CO obtained by sublimation of Ar or dry ice₂A gas.

4. The method for producing high-strength and high-corrosion-resistant steel by strip continuous casting according to claim 1, wherein in the step e), 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.

5. The method for producing high-strength high-corrosion-resistant steel by strip casting according to claim 1, wherein in step f), the coiling is performed in a twin-coiler type or a carrousel coiling type.

6. The method for producing high-strength and high-corrosion-resistant steel by thin-strip continuous casting according to claim 1, wherein the yield strength of the high-strength and high-corrosion-resistant steel is more than 480MPa, the tensile strength is more than 600MPa, the elongation is more than 22%, and the relative corrosion rate is less than or equal to 25%.

7. The method for producing high-strength high-corrosion-resistance steel by strip casting according to claim 1, wherein in step a), MnO having a low melting point MnO-SiO is obtained in the molten steel₂-Al₂O₃MnO/SiO in ternary inclusions₂And controlling the temperature to be 1-1.8.

8. The method for producing high-strength high-corrosion-resistant steel by strip casting according to claim 1, wherein the diameter of the crystallization roll in step b) is 800 mm.

9. The method for producing high-strength high-corrosion-resistant steel by strip casting according to claim 1, wherein the hot rolling reduction in the step d) is 30 to 50%.

10. The thin strip continuous casting method for manufacturing high-strength high-corrosion-resistant steel according to claim 1 or 9, wherein the thickness of the rolled steel strip in step d) is 1.0 to 1.8 mm.

11. The method for producing high-strength and high-corrosion-resistance steel by strip casting according to claim 1, wherein in step a), the basicity a ═ CaO/SiO of the slag formation is set as in step a)₂Control in a<1.2。

12. The method for producing high-strength high-corrosion-resistance steel by strip casting according to claim 1 or 11, wherein in step a), the basicity a ═ CaO/SiO of the slag formation is set in advance₂And controlling the a to be 0.7-1.0.