CN112522641A

CN112522641A - High-strength thin-specification high-corrosion-resistance steel and manufacturing method thereof

Info

Publication number: CN112522641A
Application number: CN201910888786.8A
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: 2021-03-19
Anticipated expiration: 2039-09-19
Also published as: CN112522641B

Abstract

The high-strength thin high-corrosion-resistant steel comprises 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: less than or equal to 0.007 percent, N: 0.004-0.010%, Als:<0.001%, B: 0.001-0.006% of total oxygen [ O ]]_T: 0.007 to 0.020%, the balance Fe and other unavoidable impurities, and, including Cu: 0.1-0.6% or Sn: 0.005-0.04% of one or two; contains Nb: 0.01-0.08% or V: 0.01-0.08% of one or two; Mn/S>250. The method utilizes residual Sn, Cu and other elements in the scrap steel to smelt the molten steel, and selectively adds microalloy elements such as Nb/V and B elements; 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; the strip steel is obtained by adopting a double-roller thin-strip continuous casting process and carrying out online hot rolling.

Description

High-strength thin-specification high-corrosion-resistance steel and manufacturing method thereof

Technical Field

The invention belongs to a continuous casting process, and particularly relates to high-strength thin high-corrosion-resistance steel and a manufacturing method thereof.

Background

In the traditional steel production flow, tin (Sn) and copper (Cu) are typical residual elements or harmful elements in steel, the Sn and the Cu are difficult and expensive to remove fully in the steel making process, once the steel contains the Sn and the Cu, the Sn and the Cu cannot be completely eliminated basically, and the contents of the Sn and the Cu can be reduced only by diluting molten steel, which causes the increase of the smelting cost of steel products.

In recent years, as scrap steel is continuously recycled, scrap steel resources are more and more, electricity prices are continuously reduced, domestic scrap-based short-flow electric furnace steel making is increasingly started, so that the content of residual elements such as Sn and Cu in steel is gradually increased, Sn and Cu in steel are easy to segregate and are easy to concentrate in grain boundaries to cause defects such as cracks, and the like, so that the content of Sn and Cu elements in the traditional process is strictly controlled, and in common structural steel, the content of Sn and Cu has clear requirements: sn (wt%) is less than or equal to 0.005%; cu (wt%) is less than or equal to 0.2%.

Therefore, if the residual elements such as Sn, Cu and the like in steel (particularly scrap steel) can be reasonably utilized, the 'harm is turned into good', the positive influence on the whole metallurgical boundary is generated; the method can realize the effective utilization of the existing steel scrap or low-quality inferior ore resources (high tin ore and high copper ore), promote the recycling of steel, reduce the production cost and realize the sustainable development of the steel industry.

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 thin slabs leads to the improvement of the strength of steel and the improvement of the yield ratio, thereby increasing the rolling load, so that the thickness specification of economically producing hot rolled products cannot be too thin, generally being equal to or larger than 1.5mm, see chinese patents CN200610123458.1, CN200610035800.2 and CN200710031548.2, and none of these patents relate to elements Sn and Cu.

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, further a cast strip 11 with the thickness of 1-5mm is formed at the position (nip point) of the two casting 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 fed into a coiling machine 19 to be coiled after being cut by a flying shear device 16.

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 closed chamber means is used from the casting rolls to the entrance of the rolling mill to prevent the oxidation of the strip, and the scale thickness on the surface of the strip can be controlled by adding hydrogen gas in the closed chamber means 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, which is 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 above patents also mention the addition of boron (B) element to steel, such as patents CN200610125125.2 and US6315946, but the disclosure of the invention does not relate to a specific process control method after the addition of boron (B) element, and the amount added is relatively small.

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 trace element B is added in the composition, the content of Cr is obviously increased, the Cu and Sn elements in the scrap steel raw material are fully utilized, the obvious distinguishing characteristics are achieved, and the subsequent treatment processes are completely different.

The manufacturing method of the high-copper low-alloy thin strip mentioned in the patent US2008264525/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 trace element B is added, the content of Cr is obviously increased, the Cu and Sn elements in the scrap steel raw material are fully utilized, and the subsequent treatment method of the strip steel after strip production is 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: 0.01-0.20%, 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 high-strength thin high-corrosion-resistance steel and a production method thereof, which fully utilize scrap steel as a raw material to reduce the cost of molten steel, and can further reduce the cost of production procedures and improve the product performance, particularly the corrosion resistance of the product, by thin-strip continuous casting. 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:

the method utilizes residual Sn, Cu and other elements in the scrap steel to smelt the molten steel, and selectively adds microalloy elements such as Nb/V and B elements in the steel; controlling the alkalinity of slag, the type and melting point of inclusions in steel, the content of free oxygen in molten steel and the content of acid-soluble aluminum Als in the smelting process; then, performing double-roller thin-strip continuous casting to cast strip steel with the thickness of 1.5-3mm, directly entering a lower closed chamber with non-oxidizing atmosphere after the strip steel exits from a crystallization roller, and entering an online rolling mill for hot rolling under the closed condition; the rolled strip steel is cooled by adopting an air atomization cooling mode, 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 high-strength thin high-corrosion-resistance steel comprises 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: less than or equal to 0.007 percent, N: 0.004-0.010%, Als:<0.001%, B: 0.001-0.006% of total oxygen [ O ]]_T: 0.007-0.020%, the balance being Fe and other unavoidable impurities, and, simultaneously:

comprises Cu: 0.1-0.6% or Sn: 0.005-0.04% of one or two;

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

Mn/S>250。

the yield strength of the high-strength high-corrosion-resistance steel strip 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.

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

In the component design of the high-strength thin-specification high-corrosion-resistance steel, the components are 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 0.02-0.06%.

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 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 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%.

Cu: the Cu is an easily segregated element, so that the Cu content is generally controlled strictly in the traditional process flow. The invention improves the upper limit of Cu to 0.60% by applying the rapid solidification effect of thin strip continuous casting. The improvement of the Cu content can realize the effective utilization of copper in scrap steel or inferior ore resources (high copper ores) in a certain sense, promote the recycling of steel, reduce the production cost and realize the purpose of sustainable development.

Sn: the Sn element is also one of main participating elements in scrap steel, and is generally recognized as a harmful element in steel, because Sn is an easily segregated element, a small amount of Sn can be enriched in grain boundaries, and defects such as cracks can be caused, so that the content of the Sn element is strictly controlled in the conventional process. Due to the characteristic of rapid solidification, segregation of elements among dendrites is greatly reduced, and the solid solution amount of the elements can be greatly improved, so that the range of Sn elements can be expanded under the condition of a strip continuous casting process, and the steelmaking cost can be greatly reduced. Fig. 2 is a relationship between Sn element and average heat flux density. As can be seen from fig. 2, when the Sn addition is less than 0.04%, the influence on the heat flux density is not great, i.e., the ribbon solidification process is not affected. FIG. 3 is a relationship between Sn content and surface roughness. Since cracks on the surface of the cast strip usually occur at the uneven folds of the surface of the cast strip, the surface roughness is used to characterize the occurrence of surface cracks. If the roughness is large, the probability of occurrence of cracks is high. As is clear from FIG. 3, the increase in Sn content does not adversely affect the surface quality of the cast strip under the rapid solidification conditions. From the results of fig. 2 and 3, it is clear that Sn does not adversely affect the solidification and surface quality of the cast strip. Therefore, in the present invention, the requirement for Sn content can be further relaxed, and the Sn content is designed to be in the range of 0.005-0.04%.

B: b in steelThe remarkable effects are as follows: the hardenability of the steel can be multiplied by trace boron, and B can preferentially precipitate coarse BN particles in high-temperature austenite so as to inhibit the precipitation of fine AlN, weaken the pinning effect of the fine AlN on grain boundaries, and improve the growth capability of grains, so that austenite grains are coarsened and homogenized, the recrystallization after rolling is facilitated, and after the coarsening homogenization of the austenite grains, the improvement of the yield ratio of a product is facilitated, and the forming performance of the product is improved; in addition, the combination of B and N can effectively prevent the grain boundary low melting point phase B₂O₃Is present.

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

The invention relates to a production method of high-strength thin high-corrosion-resistance steel, which comprises the following steps:

a) smelting

Smelting according to the requirements of the chemical components, wherein the basicity a of slagging in the steelmaking 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 casting roller is between 500mm and 1500mm, and the preferred diameter is 800 mm; water is introduced into the casting rolls for cooling, and 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 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 ℃;

d) in-line hot rolling

The cast strip is sent to a rolling mill 13 in a lower closed chamber through a pinch roll 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) cooling after rolling

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

f) 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 to form a coil, wherein the coiling temperature of the hot rolled strip steel is controlled to be 500-600 ℃.

Further, the method also comprises the step g) of subsequent treatment, and the steel coil can be 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 the step a), 100% of all-waste steel can be selected as smelting raw materials, pre-screening is not needed, and electric furnace steelmaking is adopted for molten steel smelting; or, the smelting adopts a converter for steel making, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.

Preferably, in step c), the non-oxidizing gas comprises N₂CO obtained by sublimation of Ar and dry ice₂A gas.

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 in the form of a double coiler or a carrousel coiler.

In the production method of the present invention:

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 area of FIG. 4, 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 composition should be controlled to satisfy Mn/S > 250.

The raw materials can be 100 percent 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.

In order to save investment cost and production cost, modern iron and steel production enterprises actively carry out technical innovation on the existing production process flow. Aiming at the problems of long process flow, more equipment and complexity of the existing hot strip steel production process, a plurality of manufacturers tightly combine the continuous casting and rolling technology with the traditional process so as to meet the requirements of the continuous casting and rolling process.

The converter steelmaking is adopted to provide molten steel, so that furniture of a production plant needs to be provided with conditions for providing molten iron, and generally, blast furnace ironmaking equipment or non-blast furnace ironmaking equipment is needed, and the converter steelmaking method belongs to the current long-flow steel production mode. However, nowadays, the steel scrap resource is increasingly abundant, the country advocates improving the steel scrap ratio of the converter so as to achieve the purposes of energy saving, consumption reduction and cost reduction, the average level of the steel scrap ratio of the converter is about 8 percent in the past, and the steel scrap ratio of the converter is 15-25 percent in the present and future. The converter scrap ratio of the invention can reach more than 20 percent.

When molten steel is provided by electric steelmaking, the steel scrap is used as a main raw material, and the solidification cooling speed of the traditional process such as die casting or thick plate continuous casting is only 10^-110 ℃/s, the residual elements in the steel scrap can generate grain boundary segregation during the solidification process, deteriorate the performance and quality of the steel, and directly generate cracking and breaking phenomena in severe cases, so that in the traditional process, the harmful elements need to be strictly controlled, in the selection of the steel scrap raw materials, some pre-screening needs to be carried out, and in the steel-making process, some special treatment needs to be carried out, such as adding some fine materials for dilution, and the like, which undoubtedly increases the production operation cost. Because the components of steel need to be controlled, certain quality requirements are required for the adopted steel scrap raw materials, and the steel scrap needs to be pre-screened and classified under general conditions. In order to improve the production efficiency of some domestic electric furnace steel mills, concentrated materials such as outsourced sponge iron, iron carbide and the like are selectively added in the raw material composition to dilute harmful elements which are difficult to remove in the waste steel, so that the quality of the molten steel is improved. Some domestic steel mills with blast furnaces and electric furnaces simultaneously utilize self-produced molten iron to be added into the electric furnaces as raw materials of the electric furnaces to improve the production efficiency of the electric furnaces, thereby greatly shortening the tapping time of the electric furnaces, and the molten iron blending ratio in the electric furnaces can reach 30-50%.

The adoption of twin-roll thin-strip casting technology is a typical sub-rapid solidification process, and the solidification cooling speed is as high as 10²-10⁴The residual harmful elements in the scrap steel, such as Cu, Sn, P and the like, can be dissolved in the matrix of the steel to the maximum extent without generating grain boundary segregation, so that 100 percent of total scrap steel smelting can be realized, pre-screening is not needed, and the raw material cost is greatly reduced. These residual elements also act as solid solution strengthening to produce ultra-thin hot rolled strip with excellent properties. Realizes the comprehensive utilization of the production of inferior steel scrap resources, turns harmful into beneficial to the harmful residual elements in the steel scrap and makes good use of wasteThe effect of "use".

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

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

BN＝B+N；Log[B][N]＝-13970/T+5.24 (1)

AlN＝Al+N；Log[Al][N]＝-6770/T+1.03 (2)

as shown in FIG. 5, the initial precipitation temperature of BN in the steel is about 1280 ℃ and the precipitation of BN at 980 ℃ is in equilibrium, but the precipitation of AlN is just started (the precipitation temperature of AlN is about 980 ℃), and the precipitation of BN is thermodynamically preferred to AlN. According to the invention, the combination of B and N is completed in the lower closed chamber to generate coarse BN particles, so that fine AlN is inhibited from being separated out, the pinning effect of the fine AlN on a crystal boundary is weakened, the growth capability of crystal grains is improved, and austenite crystal grains are coarsened and are more uniform, thereby being beneficial to effectively reducing the yield ratio of a product and improving the product performance; in addition, the combination of B and N can effectively prevent the grain boundary low melting point phase B₂O₃Is present.

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 recoiling machine adopts a double-coiling mode and can also adopt a carrousel coiling mode to ensure the continuous production of the strip steel.

The invention has the main advantages that:

the high-strength corrosion-resistant steel containing tin (Sn), copper (Cu)/tin (Sn), copper (Cu) and boron (B) is produced by using a thin-strip continuous casting technology, and reports are not found so far, and the advantages are summarized as follows:

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. According to the invention, trace boron is added, coarse BN particles are preferentially precipitated in high-temperature austenite, so that the precipitation of fine AlN is inhibited, the pinning effect of fine AlN on grain boundaries is weakened, the growth capability of grains is improved, and therefore, austenite grains are coarsened and homogenized, and the forming performance of products is favorably improved.

5. The smelting adopts the electric furnace for steelmaking, the smelting raw materials can realize 100 percent of all scrap steel smelting in the true sense, pre-screening is not needed, and the raw material cost is greatly reduced; if the steel is smelted by the converter, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials without pre-screening, so that the scrap steel ratio of the converter is improved to the maximum extent, and the smelting cost and the energy consumption are greatly reduced.

6. The invention utilizes the scrap steel containing Cu and Sn to turn the Cu and Sn in the steel into harmful and beneficial, realizes the full utilization of the existing scrap steel or low-quality inferior ore resources (high-tin ore and high-copper ore), promotes the recycling of the scrap steel, reduces the production cost and realizes the sustainable development of the steel industry.

7. 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.

8. 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.

9. 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.

10. 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.

11. The diameter of the casting roll is 500-1500mm, preferably the diameter of the crystallization roll 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 layout of a twin roll strip casting process;

FIG. 2 is a diagram of: a diagram of the relationship between the Sn content and the average heat flux density;

FIG. 3 is a schematic representation of the relationship between Sn content and surface roughness of a cast strip;

FIG. 4 shows MnO-SiO₂-Al₂O₃Ternary phase diagram (shaded area: low melting point region);

FIG. 5 is a schematic view showing the thermodynamic curves of BN and AlN precipitates.

Detailed Description

The invention is further illustrated below by means of examples and figures, 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 through a ladle 1 through a ladle shroud 2, a tundish 3, a submerged nozzle 4 and a distributor 5 into a molten pool 7 surrounded by two counter-rotating and rapidly-cooled

crystallization rolls

8a, 8b and side closing plate devices 6a, 6b, and the molten steel is solidified on the circumferential surfaces of the rotation of the crystallization rolls 8a, 8b to form solidified shells and gradually grow, and then a cast strip 11 with a thickness of 1.5-3mm is formed at the minimum clearance (nip point) between the two crystallization rolls.

The diameter of the crystallization roller is between 500 and 1500mm, and water is introduced for cooling. The casting speed of the casting machine is 60-150m/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 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, a hot rolled strip with the thickness of 0.8-2.5mm is formed after hot rolling, and the rolled strip steel is cooled by adopting an air atomization cooling mode, so that the temperature uniformity of the strip steel is improved. 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 process of the invention is further illustrated by the examples. 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 manufacturing method of the invention has the process parameters shown in the table 3, and the mechanical properties of the finally obtained hot rolled strip are shown in the 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 thin-gauge high-corrosion-resistance steel produced by the thin-strip continuous casting process according to the designed steel components 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.

Table 2 chemical composition (wt.%) of the example steels

Example (b)	C	Si	Mn	P	S	N	O	Als	Cr	Ni	Nb	V	Cu	Sn	B
																Example 1	0.026	0.25	1.35	0.008	0.005	0.0074	0.0093	0.0009	4.7	1.8	0.01	0.05	0.38		0.003
Example 2	0.046	0.10	0.90	0.013	0.003	0.0061	0.0110	0.0006	5.0	2.3	0.02		0.25	0.005	0.001
																Example 3	0.058	0.38	1.28	0.015	0.004	0.0058	0.0150	0.0004	5.3	3.0	0.05		0.10	0.033	0.004
Example 4	0.020	0.26	1.10	0.013	0.004	0.0087	0.0130	0.0008	4.3	2.6		0.03		0.040	0.006
																Example 5	0.022	0.47	0.65	0.009	0.002	0.0052	0.0120	0.0007	4.0	1.9	0.04		0.44	0.014	0.003
Example 6	0.037	0.43	0.67	0.012	0.002	0.0046	0.0070	0.0008	5.2	1.0		0.08	0.53		0.005
																Example 7	0.045	0.19	0.85	0.015	0.003	0.0040	0.0100	0.0005	6.0	1.2	0.06		0.17	0.035	0.003
Example 8	0.048	0.35	1.00	0.014	0.004	0.0100	0.0085	0.0006	4.2	1.4		0.04	0.60	0.015	0.002
																Example 9	0.036	0.35	0.84	0.018	0.004	0.0078	0.0200	0.0003	5.6	2.4	0.04			0.027	0.004
Example 10	0.060	0.45	0.40	0.020	0.001	0.0055	0.0125	0.0004	5.5	2.2	0.05	0.05	0.52	0.016	0.006
																Example 11	0.057	0.50	0.65	0.010	0.002	0.0090	0.0090	0.0005	4.5	2.7	0.04		0.48		0.003
Example 12	0.033	0.27	1.70	0.012	0.007	0.0085	0.0118	0.0003	5.7	1.7	0.08	0.02	0.35	0.012	0.002
																Example 13	0.049	0.48	1.37	0.008	0.004	0.0045	0.0132	0.0006	4.4	2.9		0.06		0.038	0.005
Example 14	0.025	0.23	1.40	0.017	0.003	0.0064	0.0075	0.0005	5.8	1.8	0.04	0.07	0.27	0.027	0.004

TABLE 5 test results of atmospheric corrosion resistance of the steels of examples

	Average corrosion rate, mg/cm².h	Relative corrosion rate%
			Q345B	0.4902	100
SPA-H	0.2148	43.82
			Example 1	0.1206	24.60
Example 2	0.1158	23.62
			Example 3	0.1048	21.38
Example 4	0.1166	23.79
			Example 5	0.1168	23.83
Example 6	0.1024	20.89
			Example 7	0.1155	23.56
Example 8	0.1106	22.56
			Example 9	0.1202	24.52
Example 10	0.1137	23.19
			Example 11	0.1157	23.60
Example 12	0.1148	23.42
			Example 13	0.1158	23.62
Example 14	0.1219	24.87

Claims

1. The high-strength thin-specification high-corrosion-resistance steel comprises 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: less than or equal to 0.007 percent, N: 0.004-0.010%, Als:<0.001%, B: 0.001-0.006% of total oxygen [ O ]]_T: 0.007-0.020%, the balance being Fe and other unavoidable impurities, and, simultaneously:

comprises Cu: 0.1-0.6% or Sn: 0.005-0.04% of one or two;

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

Mn/S>250。

2. the high strength, thin gauge, high corrosion resistant steel strip of claim 1, wherein the microstructure of the high strength, high corrosion resistant steel strip is bainite, or acicular ferrite, or a mixed microstructure of bainite + acicular ferrite.

3. The high-strength thin gauge high corrosion resistant steel as claimed in claim 1, wherein the yield strength of said high-strength high corrosion resistant steel strip is above 480MPa, the tensile strength is above 600MPa, and the elongation is above 22%; the relative corrosion rate is less than or equal to 25 percent.

4. The method for producing high-strength thin gauge corrosion-resistant steel according to claim 1, comprising the steps of:

a) smelting

Smelting according to the chemical composition requirement of claim 1, wherein the basicity a of slagging in the steelmaking 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

c) Lower sealed chamber protection

d) in-line hot rolling

e) cooling after rolling

f) strip steel coiling

5. The method for producing high strength thin gauge corrosion resistant steel as claimed in claim 4, further comprising step g) of subsequent treatment, wherein the steel coil is used as acid flat coil after acid pickling-flattening, or as galvanized sheet after acid pickling-hot galvanizing.

6. The method for producing high-strength thin-gauge high-corrosion-resistance steel according to claim 4, wherein in the step a), 100% of all scrap steel can be selected as a smelting raw material, pre-screening is not required, and electric furnace steelmaking is adopted for molten steel smelting; or, the smelting adopts a converter for steel making, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.

7. The method for producing a high strength thin gauge corrosion resistant steel as claimed in claim 4, wherein in step c), said non-oxidizing gas comprises N₂CO obtained by sublimation of Ar and dry ice₂A gas.

8. The method for producing high-strength thin gauge corrosion-resistant steel according to claim 4, wherein in 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.

9. The method for producing a high strength thin gauge corrosion resistant steel according to claim 4, wherein in step f), the coiling is in a twin coiler form or a carrousel coiling form.