CN112522637B

CN112522637B - B-containing thin 30CrMo hot rolled steel plate/strip and manufacturing method thereof

Info

Publication number: CN112522637B
Application number: CN201910888798.0A
Authority: CN
Inventors: 吴建春; 方园
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2019-09-19
Filing date: 2019-09-19
Publication date: 2022-11-18
Anticipated expiration: 2039-09-19
Also published as: CN112522637A

Abstract

A thin 30CrMo hot rolled steel plate/strip containing B and a manufacturing method thereof comprise the following chemical components in percentage by weight: c:0.24-0.34%, si:0.1-0.5%, mn:0.6-1.5%, P is less than or equal to 0.03%, S is less than or equal to 0.007%, cr:0.80-1.50%, mo:0.10-0.30%, N:0.004-0.010%, als: <0.001%, B:0.001-0.006% and Mn/S >250; the balance being Fe and unavoidable impurities. According to the invention, by adding a proper amount of N and B elements and matching with a reasonable on-line hot rolling process and a cooling measure after rolling, the uniformity of the structure is effectively improved, the problems of internal shrinkage porosity and shrinkage cavity are solved, the quality and performance of the product are improved, and the 'one-shot forming' of the thin 30CrMo hot rolled steel plate/strip is realized. The produced thin 30CrMo hot rolled steel plate/strip is an ideal material for thin steel fields in the industries of mechanical manufacture, petrochemical industry, boiler manufacture and saw blade cutting die.

Description

B-containing thin 30CrMo hot rolled steel plate/strip and manufacturing method thereof

Technical Field

The invention belongs to a continuous casting process, and particularly relates to a B-containing thin 30CrMo hot rolled steel plate/strip and a manufacturing method thereof.

Background

The traditional thin strip steel is produced by continuously rolling a casting blank with the thickness of 200-250mm in multiple passes, 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 2.0mm (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 50-120mm, 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 a small amount of temperature compensation, so the thin slab process greatly shortens the process flow, reduces the energy consumption and the investment, and further reduces the production cost. However, the strength of steel and the yield ratio of the sheet bar are improved due to the faster cooling speed in the continuous casting and rolling of the sheet bar, so that the rolling load is increased, and the thickness specification of the hot rolled product which can be economically produced cannot be too thin, generally more than or equal to 1.5mm, see the Chinese patent application numbers 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 a plate blank continuously cast by the continuous casting machine is 90-110mm, the width of the plate blank is 1100-1600mm, the plate blank continuously cast by the continuous casting machine plays a role in heat preservation and soaking through a section of induction heating roller way, and then the 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 0.8mm thickness, expands the specification range of the hot rolled plate, and has the output of a single production line reaching 220 ten thousand tons 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 technology changes the production process of steel strips in the traditional metallurgy industry, integrates continuous casting, rolling, even heat treatment and the like into a whole, enables the produced thin strip to be subjected to one-time online hot rolling to form a thin steel strip, realizes the 'one-time rolling' of a thin steel strip product, greatly simplifies the production process, shortens the production cycle, and has the process line length of only about 50 m; the equipment investment is correspondingly reduced, the product cost is obviously reduced, and the method is a low-carbon and environment-friendly hot-rolled thin strip production process. The twin-roll thin strip continuous casting process is a main form of the thin strip continuous casting process and is the only thin strip continuous casting process for realizing industrialization in the world.

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

rolls

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

rolls

8a and 8b and gradually grows, a cast strip 11 with the thickness of 2-5mm is formed at the minimum gap (nip point) between the two crystallizing rolls, the steel strip is guided to a pinch roll 12 through a guide plate 9 and is fed into a rolling mill 13 to be rolled into a thin strip with the thickness of 0.7-2.5mm, and then the thin strip is cooled through 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 30CrMo alloy steel has the advantages of higher strength and toughness, good hardenability, excellent comprehensive mechanical properties after quenching low-temperature tempering and quenching high-temperature tempering, and the like. After being quenched and tempered, the steel grade has higher strength below 550 ℃, good low-temperature toughness, no temper brittleness, and good welding performance and machinability. The 30CrMo is widely applied to machinery manufacturing, petrochemical industry, boiler manufacturing industry, saw blade cutter die industry and the like, according to market statistics, the market consumption of the thin 30CrMo is 10-15 ten thousand t/year, and the market consumption and the future potential are huge. The reasonable matching of Cr and Mo elements in the 30CrMo steel ensures that the steel has good normal temperature and high temperature performance, does not remarkably reduce plasticity while obtaining compact structure and high strength, simultaneously improves the hardenability of the steel, increases the stability during tempering, and eliminates the tempering brittleness and the tendency of grain growth at high temperature.

At present, the mainstream production process flow of the 30CrMo steel plate/belt is as follows: converter/electric furnace-refining-conventional thick slab continuous casting-walking beam furnace-rough rolling-multi-frame finish rolling-laminar cooling-coiling. The 30CrMo steel plate/belt produced by the traditional production flow mainly has the following problems: (1) composition segregation and internal porosity: because the alloy content in steel is high, the drawing speed is low when a thick plate blank is continuously cast, and molten steel is slowly solidified, serious component segregation and internal loosening in the plate blank are easily caused, and finally the quality and performance of a product are not uniform; (2) surface decarburization: because the carbon content in steel is higher, the slab is heated by adopting a stepping heating furnace, the heating time is long, the temperature is high, the surface decarburization of the casting blank is serious, and the surface hardness, the wear resistance and the thermal stability of a product are directly influenced; (3) serious cracks on the surface and the edge of the casting blank: the liquidus temperature of 30CrMo is low, the continuous casting speed of a thick plate blank is low, so that the temperature of a casting blank is low, cracks are easily generated on the surface and the edge of the casting blank after the casting blank enters a high-temperature brittle zone, and the product quality is directly influenced.

The specification characteristic thickness of the thin 30CrMo hot rolled steel plate/strip product is 1.5-3.0mm, and the product is relatively thin, so that the production is more difficult if the conventional continuous casting and hot continuous rolling production line is adopted. Even if the thin slab continuous casting and rolling production is adopted, the thickness of the slab also needs to reach 70-120mm, and the size and thickness of the casting blank are not different from those of the traditional thick slab continuous casting (the thickness of the casting blank is 200-250 mm) in order of magnitude and essence, so the problem of element segregation cannot be fundamentally solved; meanwhile, the continuously cast slab still passes through a tunnel heating furnace or an electromagnetic induction heating section, and the problem of decarburization on the surface of the material cannot be completely eliminated; in addition, rolling thinner gauge results in greater roll wear of the rolls. Therefore, the problems of the 30CrMo steel production cannot be completely solved by adopting the traditional thick slab continuous casting or thin slab continuous casting production process flow, and the production cost of the produced thin 30CrMo hot rolled steel plate/strip is very high.

In addition, when the hot-rolled strip steel is used as a thin hot-rolled plate product, 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, such as in the typical 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 scale thickness on the surface of the strip can be controlled by adding hydrogen gas in the closed chamber device such as US6920912 and controlling the oxygen content to be less than 5% in US 20060182989. However, there are few patents on how to control the thickness of the scale during the transport from the rolling mill to the coiling, and particularly, in the cooling of the strip by laminar cooling or spray cooling, the strip at high temperature is in contact with cooling water, and the scale thickness on the surface of the cast strip increases rapidly. Meanwhile, the contact between the high-temperature strip steel and the cooling water also brings about a plurality of problems: firstly, water spots (rusty spots) are formed on the surface of strip steel, and the surface quality is influenced; secondly, cooling water for laminar cooling or spray cooling easily causes uneven local cooling on the surface of the strip steel and uneven microstructure inside the strip steel, thereby causing uneven performance of the strip steel and influencing product quality; 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.

Admittedly, due to the fast solidification process characteristics of the strip continuous casting, the produced product generally has the problems of nonuniform structure, low elongation, nonuniform performance and the like, because austenite grains in the casting strip have obvious nonuniformity, the structure of the final product obtained after austenite phase transformation is nonuniform, and the performance of the product, particularly the elongation and the forming performance are unstable; meanwhile, when the steel grade with higher carbon content is produced by strip continuous casting, because of solidification shrinkage, molten steel has no time to feed, and the problem of internal shrinkage porosity is easy to occur near the central area of the cast strip.

Disclosure of Invention

The invention aims to provide a B-containing thin 30CrMo hot rolled steel plate/strip and a manufacturing method thereof, which effectively improve the uniformity of the structure, solve the problem of internal shrinkage porosity and improve the quality and the performance of products by adding a proper amount of N and B elements and matching with a reasonable on-line hot rolling process and a cooling measure after rolling, realize the 'one-shot forming' of the thin 30CrMo hot rolled steel plate/strip, and can be widely applied to the industries of mechanical manufacturing, petrochemical industry, boiler manufacturing industry, saw blade cutting die and the like.

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

in the invention, micro alloy elements such as Mo, cr, B, N and the like are selectively added into steel; in the smelting process, the alkalinity of slag, the type and melting point of inclusions in steel, the free oxygen content in molten steel and the content of acid-soluble aluminum Als are controlled; then, performing twin-roll thin strip continuous casting to cast strip steel with the thickness of 2.0-5.0mm, directly entering a lower closed chamber with non-oxidizing atmosphere after the strip steel exits from a crystallization roll, and entering an online rolling mill under the closed condition to perform hot rolling to 1.5-3.0mm; 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 performance uniformity and the surface quality of the strip steel. The thin 30CrMo hot rolled steel plate/strip produced by the method has the advantages of uniform structure performance, no shrinkage porosity/shrinkage cavity inside, less decarburized layer, good hardenability, good machinability and machinability, and can be widely applied to the industries of mechanical manufacturing, petrochemical industry, boiler manufacturing industry, saw blade cutting die and the like; the manufacturing method realizes the 'one-shot forming' of the thin 30CrMo hot rolled steel plate/strip, greatly improves the production efficiency, greatly reduces the production cost, and is a low-carbon, green and environment-friendly product.

Specifically, the B-containing thin 30CrMo hot rolled steel plate/strip comprises the following chemical components in percentage by weight: c:0.24-0.34%, si:0.1-0.5%, mn:0.6-1.5%, P is less than or equal to 0.03%, S is less than or equal to 0.007%, cr:0.80-1.50%, mo:0.10-0.30%, N:0.004-0.010%, als: <0.001%, B:0.001-0.006% of Mn/S >250; the balance being Fe and unavoidable impurities.

The thin 30CrMo hot rolled steel plate/strip containing B has the tensile strength of less than 900MPa and the hardness of less than HRC25.

In the composition design of the B-containing thin 30CrMo hot rolled steel plate/strip of the invention:

c: c is the most economical and basic strengthening element in steel, improves the strength of the steel through solid solution strengthening and precipitation strengthening, and is an important element for ensuring quenching hardness and hardenability. 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, the C content is not too high, which results in high deformation resistance during rolling, reduced toughness after quenching and tempering heat treatment, and also affects weldability. Meanwhile, for conventional slab continuous casting, casting in a peritectic reaction region is easy to generate surface cracks of a casting blank, and a steel leakage accident can happen in severe cases. The same applies to the continuous casting of thin strips, and casting of a cast strip blank in the peritectic reaction zone is prone to surface cracks and can cause strip breakage in severe cases. 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 0.24-0.34%.

Si: si plays a role in solid solution strengthening in steel, and Si added in the steel can assist deoxidation to form silicate, thereby being beneficial to improving the machinability and simultaneously improving the purity of the steel, but the excessive Si content can influence the weldability and increase the brittleness of the steel. 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 and quenching hardness of steel, has considerable solid solubility in steel, improves the strength of the steel through solid solution strengthening, basically has no damage to the plasticity and toughness of the steel, is the most main strengthening element for improving the strength of the steel, is matched with Si, and can play a role of deoxidation in the steel. However, the high Mn content increases the thermal stress and structural stress of the cast strip, and the strip is easily broken. Therefore, the Mn content range adopted by the invention is 0.6-1.5%.

P: high content of P is easy to segregate in grain boundary, increases the cold brittleness of steel, deteriorates the welding performance, reduces the plasticity and deteriorates the 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, which causes hot brittleness, decreases ductility and toughness, and causes cracks during rolling. S also reduces weldability and corrosion resistance. Therefore, in the present invention, S is controlled as an impurity element, and the content range thereof is 0.007% or less. And Mn/S >250.

And (3) 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 utilize the action of N and B in the steel to generate a BN precipitated phase, and needs to have certain N content in the steel. Meanwhile, a proper amount of N can promote the precipitation of carbonitride in the quenching and tempering heat treatment process and improve the red hardness of the steel in the cutting and machining processes; however, too high a content of N impairs the toughness of the steel and also increases the brittleness of the cast strip, reducing manufacturability. Therefore, the N content range adopted by the invention is 0.004-0.010%.

Cr: the Cr is added into the thin 30CrMo hot rolled steel, so that the hardenability, high-temperature strength and creep strength of the steel are improved, and the secondary hardening effect is achieved, so that the hardness and wear resistance of the steel can be improved, and the steel is not embrittled; however, the high content of Cr can cause high deformation resistance during rolling, the toughness is reduced after quenching and tempering heat treatment, and cutting processing of users is influenced, and the Cr content is limited to 0.80-1.50 percent in the invention.

Mo: mo can refine grains and improve the strength and the toughness. One part of Mo is dissolved in ferrite to strengthen the ferrite matrix, and the other part of Mo exists in steel in a dispersion mode through carbide, so that the Mo-containing steel has the effects of solid solution strengthening and carbide dispersion strengthening at the same time. Mo has a low diffusion speed in ferrite at high temperature, so that the high-temperature strength and the tempering stability of the steel can be obviously improved. However, too high Mo can significantly increase the raw material smelting cost of the steel. Therefore, in the present invention, the Mo content is limited to 0.10 to 0.30%.

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 capacity of grains, so that the austenite grains are coarsened and homogenized, the machinability of a steel coil product is improved, and subsequent users can cut or machine; in addition, the combination of B and N can effectively prevent the grain boundary low melting point phase B ₂ O ₃ Thereby avoiding hot shortness.

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 can obtain better effect when added with Mo compositely, reduce the segregation tendency of C atoms and avoid grain boundary Fe ₂₃ (C,B) ₆ So that more B can be added. Therefore, in the present invention, a higher B content is used than in the conventional process, in the range of 0.001-0.006%.

The invention relates to a method for manufacturing a B-containing thin 30CrMo hot rolled steel plate/strip, which comprises the following steps:

1) Smelting and continuous casting

Smelting according to the components, adopting a twin-roll thin strip to continuously cast a cast strip with the thickness of 1.5-3mm, wherein the casting speed is 60-150m/min, and a fine crystalline layer with the thickness of 100-300nm is formed on the surface of the strip steel; the diameter of the crystallization roller is 500-1500mm, the inner part is cooled by water, and the diameter of the crystallization roller is preferably 800mm; controlling the ladle casting temperature to be 1580-1610 ℃;

2) Lower enclosed chamber protection

After the casting strip is taken out of the crystallization roller, the temperature of the casting strip is 1360-1430 ℃, 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 1150-1280 ℃;

3) In-line hot rolling

Conveying the cast strip to a rolling mill in a lower closed chamber through a pinch roll, and rolling the cast strip into strip steel with the thickness of 1.5-3.0mm, wherein the rolling temperature is 1100-1250 ℃, the hot rolling reduction is 10-50%, and preferably, the hot rolling reduction is 30-50%;

4) Strip steel cooling after rolling

Cooling the strip steel after the online hot rolling after rolling, wherein the cooling adopts an air atomization cooling mode, and the cooling rate range of the air atomization cooling is 10-100 ℃/s;

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.

5) Strip steel coiling

The cooled hot rolled strip steel is cut to remove the head with poor quality through head cutting, and then is directly coiled into coils, and the coiling temperature is controlled to be 700-760 ℃.

Further, after the steel plate/strip produced in the step 6) is opened and cut, a user performs cutting and machining according to the size of a final product, and finally performs quenching and tempering heat treatment: oil quenching at 840-880 deg.C, and tempering at 400-440 deg.C.

Preferably, in step 1), the basicity a = CaO/SiO of the slag making in the steel making process ₂ Control in a<1.5, preferably a<1.2, or a =0.7-1.0.

Preferably, in step 1), the molten continuous casting steel contains MnO-SiO ₂ -Al ₂ O ₃ Ternary inclusions, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The content is controlled to be 0.5-2, preferably 1-1.8.

Preferably, in step 1), the free oxygen [ O ] in the molten steel] _Free Comprises the following steps: 0.0005-0.005%.

Preferably, in the step 1), the molten steel is smelted by electric furnace steelmaking or converter steelmaking and then enters an LF furnace, a VD/VOD furnace or an RH furnace refining process.

Preferably, in the step 1), the continuous casting adopts a two-stage steel water distribution system, namely a tundish and a flow distributor.

Preferably, in step 2), the non-oxidizing gas includes inert gas, N ₂ Or CO obtained by sublimation on dry ice ₂ Gas, N ₂ And H ₂ The mixed gas of (1).

Preferably, in the step 4), the gas-water ratio of the gas atomization cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

Preferably, in step 5), 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:

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. grey shaded area in FIG. 2, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The content 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: 0.0005-0.005%.

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

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 1360-1430 ℃, 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 lower closed chamber protects the cast strip from oxidation to the inlet of a rolling mill. The temperature of the cast strip at the outlet of the lower closed chamber is 1150-1280 ℃.

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

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

BN＝B+N； Log[B][N]＝-13970/T+5.24 (1)

AlN＝Al+N； Log[Al][N]＝-6770/T+1.03 (2)

as shown in FIG. 3, the initial precipitation temperature of BN in the steel is about 1280 ℃ and the precipitation of BN at 980 ℃ is in equilibrium, but the precipitation of AlN is just started (the precipitation temperature of AlN is about 980 ℃), and the precipitation of BN is thermodynamically preferred to AlN. 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, so that the austenite crystal grains are more uniform, and the cutting performance and the machinability of the material are improved; in addition, the combination of B and N can effectively prevent the grain boundary low-melting-point phase B ₂ O ₃ Of the cell.

And conveying the cast strip to a rolling mill through a pinch roll in a lower closed chamber, and rolling the cast strip into strip steel with the thickness of 1.5-3.0 mm. The rolling temperature is 1100-1250 ℃, and the problem of edge crack easily caused by the traditional process can be effectively solved by higher rolling temperature. The hot rolling reduction is 10-50%, preferably, the hot rolling reduction is in the range of 30-50%, and the larger reduction of the single stand can completely solve the problem of internal shrinkage porosity near the central region of the cast strip due to the higher C content.

The strip steel after the online hot rolling is cooled after being rolled, and the strip steel is cooled by adopting an air atomization cooling mode, so that the thickness of oxide scale on the surface of the strip steel can be effectively reduced, the temperature uniformity of the strip steel is improved, and the performance and the surface quality of the strip steel are improved. The gas-water ratio of gas atomization cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa. High-pressure water mist is formed after gas atomization and sprayed on the surface of the steel strip, so that on one hand, the temperature of the steel strip is reduced, on the other hand, the water mist can form a compact air film to be coated on the surface of the steel strip, the anti-oxidation effect of the steel strip is achieved, and the growth of oxide skin on the surface of the hot-rolled steel strip is effectively controlled. The cooling mode can avoid the problems caused by the traditional spraying or laminar cooling, so that the surface temperature of the strip steel is uniformly reduced, the temperature uniformity of the strip steel is improved, and the effect of homogenizing the internal microstructure is achieved; meanwhile, the cooling is uniform, so that the shape quality and the performance stability of the strip steel can be improved; effectively reducing the thickness of the oxide scale on the surface of the strip steel. The cooling rate of the gas atomization cooling is in the range of 10-100 ℃/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 700-760 ℃. The high coiling temperature ensures that the inventive steel obtains as low a tensile strength as possible (tensile strength less than 900MPa and hardness (HRC 25 or less) to enable smooth coiling, and also makes it easier for downstream manufacturing users to perform various cutting and machining operations.

The coiling adopts a double coiling machine mode and a carrousel coiling mode, so that the continuous production of the strip steel is ensured. Preferably, the reeling is in the form of carrousel reeling, which allows for shorter and more compact production line lengths.

Further, after the produced steel plate/strip is opened and cut, a user performs cutting and machining according to the size of a final product, and finally performs quenching and tempering heat treatment: oil quenching at 840-880 deg.C, tempering at 400-440 deg.C to ensure uniform sorbite structure and hardness distribution of the material, and reduce deformation of the sheet body. The required material hardness can be obtained after quenching and tempering heat treatment: HRC35 + -2.

Compared with the prior art, the invention has the following advantages:

the most obvious characteristic of the invention which is different from the prior thin strip continuous casting technology is the roller diameter of the crystallization roller and the corresponding flow distribution mode. The EUROSTRIP technology is characterized in that the diameter of the crystallization roller is 1500mm, the crystallization roller is large, the molten steel capacity of a molten pool is large, the distribution is easy, and the manufacturing, operation and maintenance costs 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 in tapping temperature leads to an increase in steel-making cost, an increase in energy consumption, a reduction in the life of refractory, and the like.

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 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 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 diameter of 800mm, and adopts a two-stage molten steel distribution system, thereby not only realizing the requirement of stable molten steel distribution, but also realizing the aims of simple structure, convenient operation and low processing cost.

Chinese patent application No. CN101773929B discloses a method for producing 30CrMo hot rolled steel plates, which is based on a thin slab continuous casting and rolling process and mainly comprises: smelting, refining, thin slab continuous casting, soaking, high-pressure water descaling, hot continuous rolling, cooling, coiling and the like. The components are as follows: 0.26-0.34%, si:0.17-0.37%, mn:0.40-0.70%, P: less than or equal to 0.035%, S less than or equal to 0.035%, cr:0.80 to 1.10, mo:0.15-0.25%, the balance being Fe and unavoidable impurities. The 30CrMo hot rolled steel plate produced by the method has the characteristics of uniform structure property, high thermal stability and high strength, and the quality is also obviously improved. The essential difference between the 30CrMo steel and the production method is that the adopted production process is different, the invention adopts a double-roller thin-strip continuous casting process, and the designed components are different, so that a product with no segregation, no surface decarburized layer and better quality performance can be obtained.

Chinese patent application No. CN107419192A discloses a 30CrMo steel strip and a production method thereof, the invention is also based on a thin slab continuous casting and rolling process, and the invention mainly comprises the following steps: converter smelting, LF refining, sheet billet continuous casting, heating, hot continuous rolling and coiling. The components are as follows: 0.26-0.34%, si:0.17-0.30%, mn:0.40-0.70%, P: not more than 0.025%, S not more than 0.025%, als not less than 0.010%, cr:0.80 to 1.10, mo:0.15 to 0.25 percent of Ni, less than or equal to 0.30 percent of Cu, and the balance of Fe and inevitable impurities. The invention starts from the angle of the whole process, eliminates the center segregation and the center shrinkage cavity of the continuous casting billet, reduces the surface decarburization, can produce the hot-rolled thin steel strip with the thickness of 2.5-4.0mm, and realizes the low-cost production of the 30CrMo steel. The 30CrMo steel and the production method thereof are also essentially different in production process, the production process is simplified by adopting a twin-roll thin-strip continuous casting process, the designed components are different, and the product with no segregation, no surface decarburized layer and better quality performance can be obtained.

Chinese patent application No. CN100366779C discloses a stone cutting saw blade steel and a manufacturing method thereof, the chemical components of the stone cutting saw blade steel comprise, by weight, 0.45-0.60% of C, 0.1-0.6% of Si, 1.3-1.8% of Mn, less than or equal to 0.02% of P, less than or equal to 0.01% of S, 0.05-0.20% of V, 0.15-0.30% of Cr, 0.005-0.020% of N, 0-0.0050% of Ca, 0.005-0.040% of Al, and the balance Fe and inevitable impurities. The method comprises the following steps: the smelting, casting and continuous casting billet adopt a hot charging and hot conveying process, the temperature of the slab before heating is ensured to be above 300 ℃, the heating temperature of the slab is above 1150 ℃, the finishing temperature is controlled to be above 900 ℃ during hot rolling, air cooling coiling is carried out after rolling, and the coiling temperature is above 700 ℃. The steel has high hardenability, and is particularly suitable for manufacturing saw blades for cutting stone with the diameter of more than 1000 mm. The steel of the invention is different from the steel of the invention, the carbon content of the steel is 0.45-0.60, the carbon content of the invention is 0.24-0.34, and the invention has essential difference in production process, the invention adopts the twin-roll thin strip continuous casting process, thus simplifying the production flow.

Chinese patent application No. CN102345071B discloses a structural steel 30CrMo plus B steel plate for alloy and a production method thereof, and the plate comprises the following chemical components in percentage by mass: c:0.28-0.33, si:0.20-035, mn:0.60-0.80, P: less than or equal to 0.018, S: less than or equal to 0.005, cr:0.90-1.10, mo:0.15-0.25, als:0.20-0.40, B:0.0008-0.0015, and the balance of Fe and residual elements. The adopted production method comprises the following steps: converter steelmaking, LF refining, vacuum refining, pouring, casting blank/steel ingot heating, rolling, slow cooling and heat treatment, the high-strength 30CrMo steel with the weight of less than or equal to 100mm and the physical mechanical properties are successfully developed: the yield strength, the tensile strength and the high-temperature tempering Brinell hardness are all higher than those of common 30CrMo steel. Although trace amount of B is also added, the invention adopts the most traditional laggard die casting process, so the solidification speed is very slow, B is easy to segregate, and B with low melting point is easy to appear in grain boundaries ₂ O ₃ The phases cause thermal cracking, so the addition of B is very low, only 0.0008-0.0015%, with little effect. In addition, it is seen in the examples that the thickness specifications of the products produced are also thicker, at a thickness of 50mm, 100 mm. The invention adopts an advanced twin-roll thin strip continuous casting process to directly produce steel plates/strips with the thickness of 1.5-3.0mm, the addition of B can reach 0.006 percent, the quality performance is more excellent, and the invention is different from the patent in terms of components and production methods.

The invention has the main advantages that:

the thin-gauge 30CrMo hot rolled steel containing boron (B) and nitrogen (N) is produced by using a thin strip continuous casting technology, so far, reports are not found, 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 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. 3. The invention adopts the thin strip continuous casting process to produce the thin 30CrMo hot rolled steel, and can completely solve the problems of surface decarburization, element segregation, edge cracking and the like in the traditional process due to the advancement of the process.

The speed of strip continuous casting reaches 80-150m/min, and the solidification speed of molten steel reaches 10 ² -10 ⁴ The thickness of the alloy element in the continuously cast 30CrMo steel is only 2-5 mm/s, and the alloy element is completely solidified within a short time (0.1-0.2 s) without segregation, and the segregation problem of the element is effectively eliminated by the rapid solidification effect.

Because the drawing speed is high, the continuously cast strip steel quickly enters a lower closed chamber with protective atmosphere and then directly enters an online hot rolling mill for hot rolling, and the surface of the strip steel is almost free of decarburized layer through the whole protective casting and rolling.

Due to the high drawing speed, even if the phase line temperature of 30CrMo molten steel is low, the temperature of continuously cast strip steel is also high, the rolling temperature of the strip steel entering a rolling mill is also correspondingly high, the strip steel is effectively prevented from entering a high-temperature brittle zone for rolling, and the surface and edge cracks of the strip steel are effectively avoided.

The thin strip continuous casting and rolling process organically integrates the traditional continuous casting, heating, hot continuous rolling and other independent processes, greatly shortens the production period, greatly improves the production efficiency, greatly reduces the energy consumption, and greatly improves the product quality and performance.

4. According to the invention, the trace N element is added, so that the precipitation of carbonitride in the quenching and tempering heat treatment process of the steel can be promoted, the red hardness of the 30CrMo steel in the cutting and machining processes can be effectively improved, and the service life of a final product can be prolonged.

5. According to the invention, trace B element 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, austenite grains are coarsened and homogenized, the processability of a steel coil product is improved, and the subsequent cutting or machining by a user is facilitated.

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

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

FIG. 3 is a thermodynamic curve of BN and AlN precipitation.

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

crystallization rolls

8a, 8b and side closing plate devices 6a, 6b through a ladle 1, a tundish 2, a tundish 3, a submerged nozzle 4 and a distributor 5, the molten steel is solidified on the circumferential surfaces of the rotation of the crystallization rolls 8a, 8b, thereby forming solidified shells and gradually growing, and then a casting strip 11 with a thickness of 2.0-5.0mm is formed at the minimum clearance (nip point) between the two crystallization rolls; after the casting strip 11 comes out of the

crystallization rollers

8a and 8b, the casting strip temperature is 1360-1430 ℃, the casting strip directly enters the lower closed chamber 10, and the lower closed chamber 10 is filled with inert gas to protect the casting strip, so that the anti-oxidation protection of the casting strip is realized; the lower closed chamber 10 protects the casting strip 11 from oxidation to the inlet of the rolling mill 13, and the temperature of the casting strip at the outlet of the lower closed chamber 10 is 1150-1280 ℃. Then the cast strip is sent to a hot rolling mill 13 through a swing guide plate 9 and a pinch roll 12, hot rolled strip steel with the thickness of 1.5-3.0mm is formed, the rolled strip steel is cooled by adopting an air atomization cooling mode, the temperature uniformity of the strip steel is improved, after the strip steel is cut by a flying shear device 16, the cut head falls into a flying shear pit 18 along a flying shear guide plate 17, and the hot rolled strip steel after the cut head enters a carrousel coiler 19 for coiling; and taking the steel coil down from the coiling machine, and naturally cooling to room temperature.

Further, after the produced steel plate/strip is opened and cut, a user performs cutting and machining according to the size of a final product, and finally performs quenching and tempering heat treatment: the hardness of the material obtained after 840-880 ℃ oil quenching, 400-440 ℃ tempering and quenching and tempering heat treatment is in the range of HRC35 +/-2, the required hardness distribution of the material is ensured, the deformation of the sheet body can be reduced, and the performance requirement of the thin 30CrMo traditional hot rolled steel is met and exceeded.

The chemical compositions of the examples of the invention are shown in table 1, and the balance of the compositions is Fe and other unavoidable impurities. The manufacturing method of the invention has the process parameters shown in the table 2, and the mechanical properties of the finally obtained hot rolled strip are shown in the table 3.

In summary, the thin gauge 30CrMo hot rolled steel sheet/strip containing B produced by the strip casting process technology according to the design range of steel grade components provided by the invention has the tensile strength of less than 900MPa, the hardness of HRC25 or less, can be coiled smoothly, and is easy to cut and machine for downstream manufacturing users.

The B-containing thin 30CrMo hot rolled steel plate/strip produced by the method has the advantages of uniform structure property, no shrinkage porosity/shrinkage cavity inside, less decarburized layer, good hardenability, good machinability and machinability, and is an ideal material for thin steel fields in the industries of mechanical manufacturing, petrochemical engineering, boiler manufacturing and saw blade cutting dies. The invention can also realize the 'one-shot forming' of the thin 30CrMo hot rolled steel plate/strip, greatly reduces the production cost and is a low-carbon, green and environment-friendly product.

Claims

1. A B-containing thin 30CrMo hot rolled steel plate/strip comprises the following chemical components in percentage by weight: c:0.24-0.34%, si:0.1-0.5%, mn:0.6-1.5%, P is less than or equal to 0.03%, S is less than or equal to 0.007%, cr:0.80-1.50%, mo:0.10-0.30%, N:0.004-0.010%, als <0.001%, B:0.001-0.006% of Mn/S >250; the balance of Fe and inevitable impurities; and is obtained by a process comprising the steps of:

1) Smelting and continuous casting

Smelting according to the components, wherein the slagging alkalinity a = CaO/SiO in the steelmaking process ₂ Controlling at a =0.7-1.0; continuous casting molten steel containing MnO-SiO ₂ -Al ₂ O ₃ Ternary inclusions, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ Controlling the temperature to be 0.5-2; free oxygen [ O ] in molten steel] _Free Comprises the following steps: 0.0005-0.005%;

the continuous casting adopts double-roller thin-strip continuous casting to form a casting strip with the thickness of 2.0-5.0mm, the casting speed is 60-150m/min, and a fine crystal layer with the thickness of 100-300nm is formed on the surface of the strip steel; the diameter of the crystallization roller is 500-1500mm, and water is introduced into the crystallization roller for cooling; controlling the ladle casting temperature to 1580-1610 ℃;

the continuous casting flow distribution adopts a two-stage steel water distribution system, namely a tundish and a flow distributor;

2) Lower sealed chamber protection

3) In-line hot rolling

Conveying the cast strip to a rolling mill in a lower closed chamber through a pinch roll, and rolling the cast strip into strip steel with the thickness of 1.5-3.0mm, wherein the rolling temperature is 1100-1250 ℃, and the hot rolling reduction rate is 10-50%;

4) Cooling after rolling

5) Coiling

And directly coiling the cooled hot-rolled strip steel into a coil after cutting off the head with poor quality, and controlling the coiling temperature to be 700-760 ℃.

2. The B-containing thin gauge 30CrMo hot rolled steel sheet/strip as claimed in claim 1, wherein said B-containing thin gauge 30CrMo hot rolled steel sheet/strip has a tensile strength of less than 900MPa and a hardness of less than HRC25.

3. A manufacturing method of a B-containing thin 30CrMo hot rolled steel plate/strip is characterized by comprising the following steps:

1) Smelting and continuous casting

The hot rolled steel plate/strip comprises the following chemical components in percentage by weight: c:0.24-0.34%, si:0.1-0.5%, mn:0.6-1.5%, P is less than or equal to 0.03%, S is less than or equal to 0.007%, cr:0.80-1.50%, mo:0.10-0.30%, N:0.004-0.010%, als <0.001%, B:0.001-0.006% of Mn/S >250; the balance of Fe and inevitable impurities;

the continuous casting adopts double-roller thin-strip continuous casting to form a casting strip with the thickness of 2.0-5.0mm, the casting speed is 60-150m/min, and a fine crystal layer with the thickness of 100-300nm is formed on the surface of the strip steel; the diameter of the crystallization roller is 500-1500mm, and water is introduced into the crystallization roller for cooling; controlling the ladle casting temperature to be 1580-1610 ℃;

2) Lower sealed chamber protection

3) In-line hot rolling

4) Cooling after rolling

5) Coiling

And directly coiling the cooled hot-rolled strip steel into coils after cutting off the head with poor quality, and controlling the coiling temperature to be 700-760 ℃.

4. The method for manufacturing a B-containing thin gauge 30CrMo hot rolled steel sheet/strip as claimed in claim 3, further comprising step 6), wherein the produced steel sheet/strip is subjected to flattening and shearing, then cutting and machining by a user according to the size of the final product, and finally, subjected to thermal refining: oil quenching at 840-880 deg.C, and tempering at 400-440 deg.C.

5. The method according to claim 4, wherein the microstructure of the B-containing thin 30CrMo hot rolled steel sheet/strip obtained after the thermal refining treatment is a sorbite, and the B-containing thin 30CrMo hot rolled steel sheet/strip has a tensile strength of less than 900MPa and a hardness of HRC35 + -2.

6. The method of manufacturing a thin 30CrMo hot rolled steel sheet/strip containing B according to claim 3, wherein in the step 1), the molten steel is smelted by electric furnace steelmaking or converter steelmaking and then enters an LF furnace, a VD/VOD furnace or an RH furnace refining process.

7. The method for producing a B-containing thin gauge 30CrMo hot rolled steel sheet/strip as claimed in claim 3, wherein in the step 2), the non-oxidizing gas includes an inert gas, N ₂ Ar, or CO obtained by sublimation of dry ice ₂ Gas, N ₂ And H ₂ The mixed gas of (2).

8. The method for producing a B-containing thin 30CrMo hot rolled steel sheet/strip according to claim 3, wherein in the step 4), the gas-water ratio of the gas-atomized 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.

9. The method for manufacturing a thin 30CrMo hot rolled steel sheet/strip containing B as claimed in claim 3, wherein in step 5), the coiling is performed in a twin coiler type or a Caroter coiling type.

10. The method for producing a B-containing thin 30CrMo hot rolled steel sheet/strip according to claim 3, wherein in the step 1), mnO-SiO is added ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The temperature is controlled to be 1-1.8.

11. The method for producing a B-containing thin 30CrMo hot rolled steel sheet/strip according to claim 3, wherein in the step 1), the diameter of the crystallizing roller is 800mm.

12. The method for producing a B-containing thin 30CrMo hot rolled steel sheet/strip as claimed in claim 3, wherein the hot rolling reduction in the step 3) is 30 to 50%.