CN112522589B

CN112522589B - B-containing hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion and manufacturing method thereof

Info

Publication number: CN112522589B
Application number: CN201910889375.0A
Authority: CN
Inventors: 方园; 吴建春
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2019-09-19
Filing date: 2019-09-19
Publication date: 2022-10-21
Anticipated expiration: 2039-09-19
Also published as: CN112522589A

Abstract

A hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion containing B and a manufacturing method thereof are disclosed, the invention selectively adds micro alloy elements such as Cr, ti, sb, cu and the like 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; casting a casting strip by adopting twin-roll thin strip continuous casting, wherein the casting strip enters a lower closed chamber with non-oxidizing atmosphere after being discharged from a crystallization roll and enters an online rolling mill for hot rolling under the closed condition; cooling the strip steel by adopting an air atomization cooling mode after rolling, and finally coiling the strip steel. The steel belt produced by the invention can be widely applied to the product field with sulfuric acid dew point corrosion resistance requirement in the use environments of tobacco baking equipment, air preheater heat exchange elements in the industries of petroleum, chemical industry, electric power, metallurgy and the like, conveying pipelines, flues, chimney manufacturing structural parts, boiler preheaters, economizer equipment and the like.

Description

B-containing hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion and manufacturing method thereof

Technical Field

The invention belongs to a continuous casting process in the metallurgical industry, and particularly relates to a B-containing hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion and a manufacturing method thereof.

Background

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

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

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.

The thin hot rolled steel plate/strip product for resisting sulfuric acid dew point corrosion is widely applicable to the field of products with sulfuric acid dew point corrosion resistance requirements in use environments such as tobacco baking equipment, air preheater heat exchange elements in the industries of petroleum, chemical engineering, electric power, metallurgy and the like, conveying pipelines, flues, chimney manufacturing structural parts, boiler preheaters, economizer equipment and the like. These fields of application are generally subject to the problem of corrosion of the equipment due to the formation of sulphuric acid at the dew point temperature, known as "sulphuric acid dew point corrosion", which is the higher sulphur content of the fuel. Boiler flue gases using higher sulfur content fuels contain SO2 and SO3, which combine with the water vapor in the flue gas to produce sulfurous and sulfuric acids, which condense on the low temperature parts of the boiler, causing sulfuric acid dew point corrosion. Due to the fact thatThis type of steel requires increasing the amount of SO below the dew point of the steel when it comes into contact with sulfur gases (e.g., steel chimneys, tobacco-baking heat exchangers, etc. that emit sulfur-containing waste gases) ₃ 、SO ₂ And H ₂ Sulfurous acid or sulfuric acid generated by O combination has corrosion resistance. The parts made of plain carbon steel in the application fields have poor sulfuric acid dew point corrosion resistance, serious corrosion and short service life. If the parts are made of acid-resistant stainless steel, the material price is very high, and the engineering construction cost is greatly increased. Therefore, the thin-specification sulfuric acid dew point corrosion resistant hot rolled steel plate/strip product produced by adopting the thin strip continuous casting process has certain manufacturing and cost advantages, and the successful development of the thin-specification sulfuric acid dew point corrosion resistant hot rolled steel plate/strip product provides wide prospects for the industries in the aspects of environmental protection, cost reduction, efficiency improvement and the like.

The hot rolled steel plate/strip product for resisting the sulfuric acid dew point corrosion is produced by adopting the strip continuous casting, and the strip continuous casting process has stronger manufacturing and cost advantages due to the thinner thickness. The thin-specification sulfuric acid dew point corrosion resistant hot rolled steel plate/strip for the product specification has the characteristic thickness range of 1.2-2.0mm, and the product thickness is thin, so if the traditional continuous casting and hot continuous rolling production line is difficult to produce, particularly the hot rolling thickness is less than 1.5mm, the traditional hot rolling production line can not meet the requirement basically, and even if the thin slab continuous casting and rolling is adopted, the roll consumption is high, so the production cost of the thin-specification sulfuric acid dew point corrosion resistant hot rolled steel plate/strip product is increased.

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 typical twin roll strip casting process, a closed chamber device is used from the crystallizing roll to the entrance of the rolling mill to prevent the oxidation of the strip, and the thickness of the scale on the surface of the strip is 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 on the surface of the strip steel is not uniform, which causes the deterioration of the plate shape and influences the quality of the plate shape.

Chinese patent CN200610123458.1 discloses a method for producing 700MPa grade high strength corrosion resistant steel by Ti micro alloying process based on thin slab continuous casting and rolling process, the chemical components of the corrosion resistant steel plate manufactured by the method are: c:0.03 to 0.07%, si:0.3-0.5%, mn:1.2-1.5, P: less than or equal to 0.04%, 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 microalloying corrosion resistant steel based on thin slab continuous casting and rolling process, the chemical components of the method for manufacturing the corrosion resistant steel plate are: 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.

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 and tempering at 400-700 ℃ to precipitate or recrystallize copper elements in the strip. 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.

These patents all relate to the corrosion resistance of steel, but do not relate to the effect of resistance to sulfuric acid dew point corrosion, and the present invention is directed to a steel grade particularly against sulfuric acid dew point corrosion.

Disclosure of Invention

The invention aims to provide a hot rolled steel plate/strip containing B for resisting sulfuric acid dew point corrosion and a manufacturing method thereof, wherein a thin hot rolled steel plate/strip product for resisting sulfuric acid dew point corrosion is produced by adopting a thin strip continuous casting process, has certain manufacturing and cost advantages, and can be widely applied to the field of products which have sulfuric acid dew point corrosion resistance requirements in use environments of tobacco baking equipment, air preheater heat exchange elements in the industries of petroleum, chemical engineering, electric power, metallurgy and the like, conveying pipelines, flues, chimney manufacturing structural parts, boiler preheaters, economizer equipment and the like.

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

in the invention, microalloy elements such as Cr, ti, sb, cu and the like and B elements are selectively added into steel; controlling the alkalinity of slag, the type and melting point of inclusions in steel, the content of free oxygen in molten steel and the content of acid-soluble aluminum Als in the smelting process; then, performing double-roller thin-strip continuous casting to cast strip steel with the thickness of 1.5-3mm, directly entering a lower closed chamber with non-oxidizing atmosphere after the strip steel exits from a crystallization roller, and entering an online rolling mill for hot rolling under the closed condition; cooling the rolled strip steel by adopting an air atomization cooling mode, and finally coiling the strip steel.

In particular to a B-containing hot rolled steel plate/strip for resisting dew point corrosion of sulfuric acid, which comprises the chemical componentsThe weight percentage is as follows: c:0.02-0.06%, si:0.10-0.55%, mn: less than or equal to 1.5 percent, less than or equal to 0.03 percent of P, less than or equal to 0.007 percent of S, ti:0.03-0.15%, cr:0.50-1.20%, ni:0.10 to 0.30%, sb:0.04-0.30%, cu:0.20-0.60%, N:0.004-0.010%, als:<0.001％，B：0.001-0.006％；Mn/S>250, total oxygen [ O ]] _T :0.007-0.020%; the balance being Fe and other unavoidable impurities.

The microstructure of the hot rolled steel plate/strip for resisting the sulfuric acid dew point corrosion is a mixed microstructure of acicular ferrite and pearlite.

The yield strength of the hot rolled steel plate/strip for resisting the dew point corrosion of the sulfuric acid is more than 330MPa, the tensile strength reaches more than 440MPa, and the elongation rate reaches more than 22%.

In the chemical composition design of the hot rolled steel sheet/strip for resisting sulfuric acid dew point corrosion according to the invention:

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 grade of the steel is determined to a large extent by the C content, i.e. higher C content corresponds to higher strength grade. 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 0.10-0.55%.

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 a Mn content results in deterioration of weldability and toughness of weld heat-affected zone. Therefore, the Mn content range adopted by the invention is less than or equal to 1.5 percent.

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 0.03 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 range thereof is 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 can obviously improve the oxidation resistance of the steel, increase the corrosion resistance of the steel, improve the strength and the wear resistance of the steel, and also can improve the sulfuric acid resistance at high temperature (> 200 ℃), but the welding performance can be seriously deteriorated if the content is too high, and the Cr content is limited to 0.5-1.2 percent in the invention.

Ni: the alloy can improve hardenability, remarkably improve the low-temperature performance of steel, and is a favorable element for improving corrosion resistance and toughness of steel, meanwhile, ni can adversely affect the welding performance by punching Cr, and Ni can effectively prevent the hot brittleness of Cu. The present invention limits the Ni content to 0.1-0.3%.

Ti: ti can make the internal structure of steel compact, reduce failure sensitivity and cold brittleness and improve welding performance. Proper amount of Ti is added into the steel, which is favorable for resisting the corrosion of sulfuric acid under the environment of high temperature and high concentration and has good intergranular corrosion resistance. Meanwhile, ti has strong affinity with N and C in steel, and can form carbide and nitride to improve the strength and plasticity of the steel. The content range of Nb designed by the invention is 0.03-0.15%.

Sb: is the main additive element in the sulfuric acid dew point corrosion resistant steel, and can be combined with Cu, cr, ti and other elements, SO that when the steel contacts with sulfur gas, such as steel chimney discharging sulfur-containing waste gas, the dew point is increased and is below the dew point, SO is ₃ And H ₂ And the corrosion resistance of the generated sulfuric acid is combined with O. Because Sb is an element easy to segregate, the content of Sb is strictly controlled when the steel for resisting the sulfuric acid dew point corrosion is produced by the traditional process flow. The invention uses the rapid solidification effect of thin-strip continuous casting to improve the upper limit of Sb to 0.30 percent. The Sb range designed by the invention is 0.04-0.30%.

Cu: the Cu mainly plays a role in solid solution and precipitation strengthening in steel, and meanwhile, in the corrosion process of urban industrial atmosphere and sulfuric acid, the Cu can be mainly concentrated in a rust layer close to the surface of non-corroded steel, and a Cu2S protective film can be generated in the corrosion process of industrial gas and sulfuric acid to retard the cathode and anode reactions, so that the atmospheric and sulfuric acid dew point corrosion resistance of the steel is improved. Because Cu is an easily segregated element, the content of Cu is generally strictly controlled 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 range of Cu contemplated by the present invention is 0.20-0.60%.

B: the significant role of B in steel is: 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 B ₂ O ₃ Is 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 Ti and Cr, produces better effects, reduces the tendency of segregation of C atoms and avoids grain boundary Fe ₂₃ (C,B) ₆ So that more B can be added. Therefore, in the invention, the content of B is higher than that of the traditional process, and the range is 0.001-0.006%.

The invention discloses a method for manufacturing a B-containing hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion, which comprises the following steps:

1) Smelting

Smelting according to the chemical components, wherein the basicity a = CaO/SiO of slagging in the steelmaking process ₂ Is controlled at a<1.5, preferably a<1.2, or a =0.7-1.0; obtaining low melting point MnO-SiO in molten steel ₂ -Al ₂ O ₃ Ternary inclusions of MnO/SiO ₂ Controlled at 0.5-2, preferably 1-1.8; free oxygen [ O ] in molten steel] _Free The range is as follows: 0.0005-0.005%; the control of Mn and S in molten steel must satisfy the following relational expression: mn/S>250；

2) Continuous casting

Adopting twin-roll thin strip continuous casting, forming a casting strip with the thickness of 1.5-3mm at the minimum position of a gap between two crystallizing rolls by molten steel, wherein the diameter of the crystallizing rolls is 500-1500mm, preferably 800mm, and introducing water for cooling the crystallizing rolls; the casting speed of the casting machine is 60-150m/min; a two-stage steel water distribution system is adopted for continuous casting flow distribution, namely a tundish and a flow distributor;

3) Lower sealed chamber protection

After the cast strip exits the crystallization roll, the cast strip is directly fed into a lower closed chamber at 1420-1480 ℃ and a 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%. The temperature of the cast strip at the outlet of the lower closed chamber is 1150-1300 ℃;

4) In-line hot rolling

Conveying the cast strip into a rolling mill through a pinch roll in a lower closed chamber, and rolling the cast strip into strip steel with the thickness of 0.8-2.5mm, wherein the rolling temperature is 1100-1250 ℃, the hot rolling reduction is 10-50%, and preferably, the hot rolling reduction range is 30-50%; the thickness of the hot rolled steel strip is 0.8-2.5mm, preferably 1.2-2.0mm;

5) Cooling after rolling

Cooling the strip steel after the online hot rolling, wherein the cooling adopts an air atomization cooling mode, and the cooling rate is 20-100 ℃/s;

6) Coiling

And directly coiling the cooled hot rolled strip steel after cutting off the head with poor quality, and controlling the coiling temperature of the hot rolled strip to be 600-700 ℃.

Further, the method also comprises a step 7) of subsequent treatment, wherein the steel coil is used as an acid flat coil after acid cleaning, edge cutting and flattening, or used as a finishing coil after edge cutting and flattening.

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

Preferably, in step 3), the non-oxidizing gas comprises N ₂ Ar, or CO obtained by sublimation of dry ice ₂ A gas.

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

Preferably, step 6), the coiling is in the form of a double coiler or in the form of a carrousel coiler.

In the manufacturing method of the present 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.

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

To improve the castability of thin strip continuous casting molten steel, it is necessary to obtain MnO-SiO of low melting point ₂ -Al ₂ O ₃ Ternary inclusions, e.g. shaded areas of FIG. 2, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The concentration is controlled to be 0.5 to 2, preferably 1 to 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 point ₂ -Al ₂ O ₃ The ternary inclusions of (2) require free oxygen [ O ] in molten steel] _Free The range is as follows: 0.0005 to 0.005 percent.

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

After the cast strip is taken out of the crystallization roller, the cast strip is directly fed into a lower closed chamber at the temperature of 1420-1480 ℃, non-oxidizing gas is introduced into the lower closed chamber, and the lower closed chamber protects the anti-oxidation of the cast strip to the inlet of a rolling mill. The temperature of the cast strip at the outlet of the lower closed chamber is 1150-1300 ℃.

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

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

BN＝B+N；Log[B][N]＝-13970/T+5.24 (1)

AlN＝Al+N；Log[Al][N]＝-6770/T+1.03 (2)

as shown in FIG. 3, the precipitation initiation temperature of BN in the steel is about 1280 DEG COn the right hand, precipitation of BN at 980 ℃ is in equilibrium, and precipitation of AlN is just started (the precipitation temperature of AlN is around 980 ℃), and precipitation of BN is thermodynamically preferred to precipitation of 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 B ₂ O ₃ Is 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 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa. High-pressure water mist is formed after gas atomization and sprayed on the surface of a 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 coat 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 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 600-700 ℃, so that the high-temperature austenite structure after rolling is converted into an acicular ferrite + pearlite mixed microstructure. The coiling adopts a double coiler form and a carrousel coiling form, thereby ensuring the continuous production of the strip steel.

Through the manufacturing process, the performance of the final hot rolled steel plate/strip for resisting the dew point corrosion of the sulfuric acid reaches over 330MPa of yield strength, over 440MPa of tensile strength and over 22 percent of elongation. The relative corrosion rate is less than or equal to 10 percent (the test temperature is 30 ℃, the sulfuric acid concentration is 20 percent, and the full immersion test time is 24 hours); the relative corrosion rate is less than or equal to 25 percent (the test temperature is 70 ℃, the sulfuric acid concentration is 50 percent, and the full immersion test time is 24 hours).

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. Due to the adoption of a three-stage flow distribution system, the cost of refractory materials is directly increased; more importantly, the three-stage flow distribution system enables the flowing path of the molten steel to be lengthened, the temperature drop of the molten steel to be larger, and in order to meet the temperature of the molten steel in a molten pool, the tapping temperature needs to be greatly improved. The increase of the tapping temperature causes problems such as increase of steel-making cost, increase of energy consumption, and shortening of the life of refractory.

The crystallization roller with the diameter of 800mm is preferably selected, 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 flow path of the molten steel is shortened, so that the temperature drop of the molten steel is reduced, the tapping temperature can be reduced, and compared with a three-level flow distribution system, the tapping temperature can be reduced by 30-50 ℃. The steel tapping temperature is reduced, so that the steel making cost can be effectively reduced, the energy consumption is saved, and the service life of refractory materials is prolonged. 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.

The invention has the main advantages that:

1. the invention utilizes the strip casting technology to produce the hot rolled steel plate/strip containing boron (B) and used for resisting the sulfuric acid dew point corrosion, and has not been reported so far.

2. 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.

3. The invention omits a plurality of complex intermediate steps in the production of the traditional process, and compared with the traditional production process, the energy consumption and CO of the production are reduced ₂ Greatly reduces the emission, and is a green and environment-friendly product.

4. The thin hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion, which is produced by adopting a thin strip continuous casting process, has greatly improved sulfuric acid dew point corrosion resistance; meanwhile, the thickness of the cast strip is thin, the thin product can be directly supplied to the market for use through online hot rolling to the thickness of the expected product, the purpose of supplying the thin hot rolled plate is achieved, and the cost performance of the plate strip can be obviously improved.

5. According to the invention, trace boron is added, and 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 product performance is favorably improved.

6. The invention adopts the air atomization cooling mode of the rolled strip steel, can avoid the problems brought by the traditional spray or laminar cooling, uniformly reduces the surface temperature of the strip steel, improves the temperature uniformity of the strip steel, thereby achieving 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.

7. 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.

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

Drawings

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

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

FIG. 3 is a schematic view of the thermodynamic curves for the precipitation of BN and AlN.

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 which may occur to those skilled in the art and which fall within the scope of the appended claims will be deemed to fall within the scope of the invention.

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 relatively rotating and rapidly-cooled

crystallizing rollers

8a and 8b and side sealing plate devices 6a and 6b through a ladle 1, a tundish nozzle 2, a tundish 3, a submerged nozzle 4 and a distributor 5, and the molten steel is solidified on the circumferential surfaces of the

rotating crystallizing rollers

8a and 8b to form a solidified shell and gradually grow to form a casting band 11 having a thickness of 1.5 to 3mm at the minimum gap (nip point) between the two crystallizing rollers. After the casting strip 11 comes out of the

crystallization rollers

8a and 8b, the casting strip is at 1420-1480 ℃ and directly enters the lower closed chamber 10, the lower closed chamber 10 is filled with inert gas to protect the strip steel, so as to realize the anti-oxidation protection of the strip steel, the atmosphere of the anti-oxidation protection can be N2, ar or other non-oxidizing gases, such as CO2 gas obtained by sublimation of dry ice, and the oxygen concentration in the lower closed chamber 10 is controlled to be less than 5%. 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 as an acid flat coil after acid washing, edge cutting and flattening, and can also be used as a finishing coil after edge cutting and flattening.

The chemical compositions of the examples of the invention are shown in table 1, and the balance of the compositions is Fe and other unavoidable impurities. The technological parameters of the manufacturing method of the invention are shown in the table 2, and the mechanical properties of the finally obtained hot rolled strip are shown in the table 2.

The test of the sulfuric acid corrosion resistance of the steel of the example is carried out: reference standard JB/T7901-1999: the method for the uniform corrosion full-immersion test in the metal material laboratory is used for carrying out a sulfuric acid corrosion resistance test, measuring the relative corrosion rate of the material, and the test results are shown in table 4. Test conditions 1: under the test conditions of the test temperature of 30 ℃ and the sulfuric acid concentration of 20 percent, the full immersion test time is 24 hours; test conditions 2: the test time of the full immersion is 24 hours under the test conditions that the test temperature is 70 ℃ and the sulfuric acid concentration is 50 percent. The comparative sample was designated Q235B.

In conclusion, the hot rolled steel plate/strip containing B and used for resisting sulfuric acid dew point corrosion, which is manufactured according to the design range of the steel components provided by the invention by utilizing the strip continuous casting process technology, has the yield strength of more than or equal to 330MPa, the tensile strength of more than or equal to 440MPa, the elongation of more than or equal to 22 percent and qualified cold-working bending performance. The corrosion resistance comparison result also shows that: the relative corrosion rate of the steel grade of the invention is less than or equal to 10 percent (the test temperature is 30 ℃, the sulfuric acid concentration is 20 percent, and the full immersion test time is 24 hours); the relative corrosion rate is less than or equal to 25 percent (the test temperature is 70 ℃, the sulfuric acid concentration is 50 percent, and the full-immersion test time is 24 hours). The steel can be widely applied to the product field with sulfuric acid dew point corrosion resistance requirement in the use environments of tobacco baking equipment, air preheater heat exchange elements in the industries of petroleum, chemical engineering, electric power, metallurgy and the like, conveying pipelines, flues, chimney manufacturing structural parts, boiler preheaters, economizer equipment and the like.

Claims

1. A manufacturing method of a B-containing hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion comprises the following chemical components in percentage by weight: c:0.02 to 0.06%, si:0.10 to 0.55%, mn: less than or equal to 1.5 percent, less than or equal to 0.03 percent of P, less than or equal to 0.007 percent of S, ti:0.06-0.15%, cr:0.50-1.20%, ni:0.10-0.30%, sb:0.04-0.30%, cu:0.20-0.60%, N:0.004-0.010%, als:<0.001％，B：0.001-0.006％；Mn/S>250, total oxygen [ O ]] _T The contents are as follows: 0.007 to 0.020 percent, and the balance of Fe and inevitable impurities;

the microstructure of the hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion is a mixed microstructure of acicular ferrite and pearlite;

the yield strength of the hot rolled steel plate/strip for resisting sulfuric acid dew point corrosion is more than 330MPa, the tensile strength reaches more than 440MPa, and the elongation reaches more than 22%;

the method specifically comprises the following steps:

1) Smelting of

Smelting according to the chemical components, wherein the alkalinity a = CaO/SiO of slagging in the steelmaking process ₂ Control at a =0.7-1.0; obtaining low melting point MnO-SiO in molten steel ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ Controlling the temperature to be 0.5-2; free oxygen [ O ] in molten steel] _Free The range is as follows: 0.0005-0.005%; the control of Mn and S in molten steel must satisfy the following relational expression: mn/S>250；

2) Continuous casting

Adopting twin-roll thin strip continuous casting, forming a casting strip with the thickness of 1.5-3mm at the minimum position of a gap between two crystallizing rolls by molten steel, wherein the diameter of the crystallizing rolls is 500-1500mm, and introducing water for cooling the crystallizing rolls; the casting speed of the casting machine is 60-150m/min; the continuous casting flow distribution adopts a two-stage steel water distribution system, namely a tundish and a flow distributor;

3) Lower sealed chamber protection

After the cast strip is taken out of the crystallization roller, the temperature of the cast strip is 1420-1480 ℃, the cast 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 cast strip at an outlet of the lower closed chamber is 1150-1300 ℃;

4) In-line hot rolling

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

5) Cooling after rolling

6) Coiling

2. The method for producing a B-containing hot rolled steel sheet/strip for dew point corrosion resistance by sulfuric acid according to claim 1, further comprising the step 7) of post-treatment, wherein the steel coil is subjected to pickling-edge cutting-flattening and then used as an acid flat coil, or subjected to edge cutting-flattening and then used as a finishing coil.

3. The method of producing a B-containing hot rolled steel sheet/strip for dew point corrosion resistance by sulfuric acid according to claim 1, wherein the molten steel is smelted in the step 1) by electric furnace steelmaking or converter steelmaking and then refined in an LF furnace, a VD/VOD furnace or an RH furnace.

4. The method for producing a B-containing hot-rolled steel sheet/strip for resisting dew point corrosion by sulfuric acid according to claim 1, whereinThat is, in the step 3), the non-oxidizing gas includes N ₂ CO obtained by sublimation of Ar or dry ice ₂ A gas.

5. The method for producing a hot-rolled steel sheet/strip containing B and resistant to dew point corrosion by sulfuric acid according to claim 1, wherein the gas-water ratio of the gas-atomized cooling in the step 5) is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

6. The method for producing a B-containing hot rolled steel sheet/strip for resisting dew point corrosion by sulfuric acid according to claim 1, wherein the coiling in the step 6) is in a twin coiler type or a Carrosael coiling type.

7. The method for producing the B-containing hot rolled steel sheet/strip for resisting dew point corrosion of sulfuric acid according to claim 1 or 3, wherein in the step 1), mnO having a low melting point is obtained from the molten steel ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The temperature is controlled to be 1-1.8.

8. The method for producing a B-containing hot rolled steel sheet/strip for dew point corrosion resistance against sulfuric acid according to claim 1, wherein in the step 2), the diameter of the crystallizing roller is 800mm.

9. The method for producing a B-containing hot rolled steel sheet/strip for resisting dew point corrosion by sulfuric acid according to claim 1, wherein the hot rolling reduction in the step 4) is 30 to 50%.

10. The method of producing a B-containing hot rolled steel sheet/strip for dew point corrosion resistance against sulfuric acid according to claim 1, wherein the thickness of the steel strip after hot rolling in the step 4) is 1.2 to 2.0mm.