CN112522579A

CN112522579A - 30CrMo hot rolled steel plate/strip using scrap steel and production method thereof

Info

Publication number: CN112522579A
Application number: CN201910888783.4A
Authority: CN
Inventors: 吴建春; 方园
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2019-09-19
Filing date: 2019-09-19
Publication date: 2021-03-19
Anticipated expiration: 2039-09-19
Also published as: CN112522579B

Abstract

A30 CrMo hot rolled steel plate/strip using scrap steel and a production method thereof comprise the following components in percentage by weight: 0.24 to 0.34 percent of C, 0.1 to 0.5 percent of Si, 0.6 to 1.5 percent of Mn, less than or equal to 0.03 percent of P, less than or equal to 0.007 percent of S, 0.80 to 1.50 percent of Cr, 0.10 to 0.30 percent of Mo, 0.004 to 0.010 percent of N, Als<0.001% of total oxygen [ O ]]_T: 0.007-0.020% of Cu 0.10-0.60% or Sn 0005-0.04% of one or two elements; Mn/S>250 of (a); the balance of Fe and unavoidable impurities. The method fully utilizes the residual elements such as Sn, Cu and the like in the scrap steel for smelting, and selectively adds the elements such as Mo, Cr, N and the like; controlling 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; and (3) obtaining a cast strip by adopting a twin-roll thin strip continuous casting process, and then carrying out online hot rolling to obtain the strip steel. The invention realizes the 'one-shot forming' of the thin 30CrMo hot rolled steel plate/strip, greatly improves the production efficiency and greatly reduces the production cost.

Description

30CrMo hot rolled steel plate/strip using scrap steel and production method thereof

Technical Field

The invention belongs to a continuous casting process, and particularly relates to a 30CrMo hot rolled steel plate/strip utilizing scrap steel and a production method thereof.

Background

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

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

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

The traditional thin strip steel is produced by casting blanks with the thickness of 200-250mm through multi-pass continuous rolling, and the traditional hot rolling process flow is as follows: continuous casting, casting blank reheating and heat preservation, rough rolling, finish rolling, cooling and coiling, namely firstly, a casting blank with the thickness of about 200mm is obtained through continuous casting, the casting blank is reheated and heat preserved, then rough rolling and finish rolling are carried out to obtain a steel strip with the thickness generally larger than 2mm, and finally laminar cooling and coiling are carried out to the steel strip to complete the whole hot rolling production process. The difficulty is relatively great if steel strip with a thickness of less than 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 50mm) on the casting blank in the traditional process through repeated multi-pass rolling; and the casting blank of the thin slab process directly enters a soaking pit furnace for soaking and heat preservation without cooling or supplementing a small amount of heat, so that the thin slab process greatly shortens the process flow, reduces the energy consumption and the investment, thereby reducing the production cost. However, the higher cooling rate of the continuous casting and rolling of the thin slab leads to the improvement of the strength of the steel and the yield ratio, thereby increasing the rolling load, so that the thickness specification of the hot rolled product which can be economically produced cannot be too thin, generally more than or equal to 1.5mm, see the Chinese patent application numbers CN200610123458.1, CN200610035800.2 and CN 200710031548.2.

The ESP realizes the continuous casting of the slab continuously, cancels the slab flame cutting and the heating furnace with the functions of heat preservation, soaking and slab transition, and greatly shortens the length of the whole production line to about 190 meters. The thickness of the continuously cast plate blank of the continuous casting machine is 90-110mm, the width of the continuously cast plate blank is 1100-1600mm, the continuously cast plate blank plays a role in heat preservation and uniform heating on the plate blank through a section of induction heating roller way, and then the continuously cast plate blank sequentially enters the working procedures of rough rolling, finish rolling, layer cooling and coiling to obtain a hot rolled plate. The process realizes endless rolling, can obtain the hot rolled plate with the thinnest thickness of 0.8mm, expands the specification range of the hot rolled plate, and has the output of a single production line reaching 220 kiloton per year. At present, the process is rapidly developed and popularized, and a plurality of ESP production lines are operated and produced in the world at present.

The process flow shorter than the thin slab continuous casting and rolling is a thin strip continuous casting and rolling process, the thin strip continuous casting technology is a leading-edge technology in the fields of metallurgy and material research, the emergence of the technology brings a revolution to the steel industry, the production process of steel strips in the traditional metallurgy industry is changed, continuous casting, rolling, even heat treatment and the like are integrated into a whole, the produced thin strip is subjected to one-time online hot rolling to form a thin steel strip, the 'one-shot forming' of a thin steel strip product is realized, 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 2-5mm 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 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 cutting 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 the steel is higher, the drawing speed is lower when the thick plate blank is continuously cast, the molten steel is slowly solidified, so that 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) severe cracks on the surface and 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 200-250mm) in order of magnitude and essence, so the problem of element segregation cannot be fundamentally solved; meanwhile, the continuously cast plate blank still passes through a tunnel type 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 gauges 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, for example, in the strip casting process, the closed chamber means 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 can be controlled by adding hydrogen gas in the closed chamber means such as US6920912 and controlling the oxygen content to be less than 5% in US 20060182989. However, there are few patents on how to control the thickness of the scale during the transport from the rolling mill to the coiling, and particularly, in the cooling of the strip by laminar cooling or spray cooling, the strip at high temperature is in contact with cooling water, and the scale thickness on the surface of the cast strip increases rapidly. Meanwhile, the contact between the high-temperature strip steel and the cooling water also brings about a plurality of problems: firstly, water spots (rusts) are formed on the surface of strip steel, which affects the surface quality; secondly, cooling water for laminar cooling or spray cooling easily causes uneven local cooling on the surface of the strip steel and uneven microstructure inside the strip steel, thereby causing uneven performance of the strip steel and influencing the product quality; thirdly, the local cooling of the surface of the strip steel is uneven, which causes the deterioration of the plate shape and influences the quality of the plate shape.

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 30CrMo hot rolled steel plate/strip utilizing scrap steel and a production method thereof, which effectively improve the uniformity of the structure, solve the problem of internal shrinkage porosity and improve the quality and performance of products and realize the 'one-shot forming' of the thin 30CrMo hot rolled steel plate/strip by adding a proper amount of N element and matching with a reasonable on-line hot rolling process and a cooling measure after rolling. 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.

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

the invention fully utilizes the residual elements of Sn, Cu and the like in the scrap steel to smelt the molten steel, and selectively adds the microalloy elements of Mo, Cr, N and the like in the steel; controlling the alkalinity of slag, the type and melting point of inclusions in steel, the content of free oxygen in molten steel and the content of acid-soluble aluminum Als in the smelting process; then, performing twin-roll thin-strip continuous casting to cast a casting strip with the thickness of 2.0-5.0mm, directly entering a lower closed chamber with non-oxidizing atmosphere after the casting strip is taken out of a crystallization roll, and entering an online rolling mill under the closed condition to perform hot rolling to obtain strip steel with the thickness of 1.5-3.0 mm; and cooling the strip steel by adopting an air atomization cooling mode after rolling. 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; the manufacturing method realizes the 'one-shot forming' of the thin 30CrMo hot rolled steel plate/strip, greatly improves the production efficiency and greatly reduces the production cost.

Specifically, the 30CrMo hot rolled steel plate/strip utilizing the scrap steel comprises the following 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% of total oxygen [ O ]]_T: 0.007-0.020%; the balance of Fe and other unavoidable impurities, and must satisfy simultaneously:

contains Cu: 0.10-0.60% or Sn: 0.005-0.04% of one or two elements;

Mn/S>250。

in the composition design of the 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 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 level of the steel is determined to a great extent by the content of C, namely, higher C content corresponds to higher strength level. However, 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 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 0.24-0.34%.

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, too high Mn content increases 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 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 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 the element C, the element N can improve the strength of the steel through interstitial solid solution, and 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, the secondary hardening effect is achieved, the hardness and wear resistance of the steel can be improved, and the steel is not embrittled; the Cr is matched with Cu in the scrap steel to improve the corrosion resistance of the steel. 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 may significantly increase the raw material smelting cost of steel. Therefore, in the present invention, the Mo content is limited to 0.10 to 0.30%.

Cu: the steel mainly plays a role in solid solution and precipitation strengthening, and the corrosion resistance of the 30CrMo steel can be improved under the combined action of Cu and Cr, so that the steel is suitable for some corrosive occasions and is weather-resistant and difficult to rust. It is worth to say that the invention uses scrap steel as raw material, and no additional Cu is needed to be added during smelting. 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 improvement of the Cu content can fully utilize the scrap steel in a certain sense, and screening is not needed when the scrap steel raw material is prepared, so that the smelting operation rate is improved, the cost is reduced, the recycling of steel is promoted, and the purpose of sustainable development is realized; and the effective utilization of copper in poor-quality ore resources (such as high copper ore) can be realized.

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

The invention relates to a method for producing a 30CrMo hot rolled steel plate/strip by utilizing scrap steel, which comprises the following steps:

1) smelting and continuous casting

Smelting according to the components, and the basicity a of slagging in the steelmaking process is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a ═ 0.7-1.0; MnO-SiO in molten steel₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the concentration to be 0.5-2, preferably 1-1.8; free oxygen [ O ] in molten steel]_FreeThe range is as follows: 0.0005-0.005%;

the continuous casting adopts double-roller thin strip continuous casting to form a casting strip with the thickness of 2.0-5.0 mm; the diameter of the crystallization roller is 500-1500mm, preferably the diameter of the roller is 800mm, and water is introduced into the crystallization roller for cooling; the casting speed of the casting machine is 40-100 m/min; controlling the ladle pouring 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

After the casting strip is taken out of the crystallization roller, the temperature of the casting strip is 1360-;

3) in-line hot rolling

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

4) 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 of the air atomization cooling is 10-100 ℃/s;

5) coiling

And cutting the head of the cooled hot-rolled strip steel by using a cutting head to remove the head with poor quality, directly coiling the hot-rolled strip steel, and controlling the coiling temperature of the hot-rolled strip steel to be 700-760 ℃.

Further, step 6) is included, after the steel plate/strip is opened and cut, a user carries out cutting machining and mechanical machining according to the size of a final product, and finally, quenching and tempering heat treatment is carried out: 840 ℃ plus 880 ℃ oil quenching and 400 ℃ plus 440 ℃ tempering ensure that the material obtains uniform sorbite tissues and hardness distribution and can reduce the deformation of the sheet body. The required material hardness can be obtained after quenching and tempering heat treatment: HRC35 + -2.

Preferably, in the step 1), 100% of all-waste steel can be selected as smelting raw materials, pre-screening is not needed, and electric furnace steelmaking is adopted for molten steel smelting; or, the smelting adopts a converter for steel making, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.

Preferably, in step 2), the non-oxidizing gas comprises an inert gas, N₂Or CO obtained by sublimation on dry ice₂Gas, N₂And H₂The mixed gas of (1).

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

Preferably, in step 5), the coiling is performed in a double coiler mode or a carrousel coiling mode.

In the method for producing the steel of the present invention:

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

To improve the castability of thin strip continuous casting molten steel, it is necessary to obtain MnO-SiO of low melting point₂-Al₂O₃Ternary inclusions, e.g. shaded area of FIG. 4, MnO-SiO₂-Al₂O₃MnO/SiO in ternary inclusions₂The concentration is controlled to be 0.5 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 point₂-Al₂O₃The ternary inclusions of (2) require free oxygen [ O ] in molten steel]_FreeThe range is as follows: 0.0005-0.005%.

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

The liquidus temperature of the 30CrMo steel grade designed by the invention is about 1500 ℃, and the ladle open casting temperature range is controlled to be 1580-1610 ℃.

After the casting belt is taken out of the crystallization roller, the temperature of the casting belt is 1360-₂Ar, or other non-oxidizing gas, such as CO obtained by sublimation of dry ice₂Gas, N₂And H₂The oxygen concentration in the lower sealed chamber is controlled to be<5 percent. The lower closed chamber protects the cast strip from oxidation to the mill inlet. The temperature of the cast strip at the outlet of the lower closed chamber is 1150-1280 ℃.

The cast strip is sent to a rolling mill through pinch rolls in a lower closed chamber and rolled into a thin strip with the thickness of 1.5-3.0 mm. The rolling temperature is 1100-. 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 on-line hot rolling is cooled after rolling, and the cooling adopts an air atomization cooling mode, so that the thickness of the oxide scale on the surface of the strip steel can be effectively reduced, the temperature uniformity of the strip steel is improved, and the performance and the surface quality of the strip steel are improved. The gas-water ratio of gas atomization cooling is 15: 1-10: 1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa. High-pressure water mist is formed after gas atomization and sprayed on the surface of the strip steel, on one hand, the effect of reducing the temperature of the strip steel is achieved, on the other hand, the water mist can form a compact air film to be coated on the surface of the strip steel, the effect of preventing the strip steel from being oxidized is achieved, and therefore the growth of oxide scales on the surface of the hot-rolled strip steel 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. And controlling the coiling temperature of the hot rolled strip to be 700-760 ℃. The high coiling temperature can ensure that the steel of the invention can obtain the tensile strength (less than 900MPa) and the hardness (HRC25 or less) which are as low as possible, thereby being capable of successfully coiling; it is also easier for downstream manufacturing customers 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.

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 distribution mode. The EUROSTRIP technology is characterized in that the crystallization roller with a large roller diameter of 1500mm phi is large, the molten steel capacity of a molten pool is large, the distribution is easy, and the manufacturing cost and the operation cost of the crystallization roller are high. The CASTIP technology is characterized in that a crystallization roller with a small roller diameter of 500mm phi is small, the molten steel capacity of a molten pool is small, the distribution is very difficult, but the manufacturing, operation and maintenance costs of casting machine equipment are low. CASTIP adopts a three-level steel water distribution system (tundish, transition ladle and distributor) to solve the problem of uniform distribution of small molten pools. Because a three-level flow distribution system is adopted, the cost of the refractory material is directly increased; more importantly, the three-stage flow distribution system enables the flowing path of the molten steel to be lengthened, the temperature drop of the molten steel is large, and in order to meet the temperature of molten steel in a molten pool, the tapping temperature needs to be greatly increased. The increase of the tapping temperature causes problems such as increase of steel-making cost, increase of energy consumption, and shortening of the life of refractory.

The 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 reduction of the tapping temperature can effectively reduce the steel-making cost, save the energy consumption and prolong the service life of refractory materials. The invention is matched with the crystallization roller with the optimal roller diameter of phi 800mm, and adopts a two-stage steel water distribution system, thereby not only realizing the requirement of stable distribution of molten steel, but also realizing the aims of simple structure, convenient operation and low processing cost.

Chinese patent publication No. CN101773929B discloses a method for producing a 30CrMo hot-rolled steel plate, which is based on a thin slab continuous casting and rolling process and mainly comprises the following steps: 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: 0.80-1.10, Mo: 0.15-0.25%, the balance being Fe and unavoidable impurities. The 30CrMo hot rolled steel plate produced by the patent 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 publication No. CN107419192A discloses a 30CrMo steel strip and a production method thereof, and the invention is also based on a thin slab continuous casting and rolling process, mainly comprising 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: 0.80-1.10%, 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 publication No. CN100366779C discloses a stone cutting saw blade steel and a manufacturing method thereof, wherein the chemical components of the 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 unavoidable impurities. The method comprises the following steps: 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 the saw blade 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 production process is essentially different, the invention adopts the twin-roll thin strip continuous casting process, thus simplifying the production flow.

Chinese patent publication No. CN102345071B discloses a structural steel 30CrMo plus B steel plate for alloy and a production method thereof, wherein the plate comprises the following chemical components in percentage by mass: c: 0.28-0.33%, Si: 0.20-0.35%, Mn: 0.60-0.80%, P: less than or equal to 0.018%, S: less than or equal to 0.005 percent, Cr: 0.90-1.10%, Mo: 0.15-0.25%, Als: 0.20-0.40%, B: 0.0008 to 0.0015 percent, 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, successfully develops high-strength 30CrMo and B steel with the thickness less than or equal to 100mm, and has the physical mechanical properties: the yield strength, the tensile strength and the high-temperature tempering Brinell hardness are all higher than those of common 30CrMo steel. The most traditional laggard die casting process adopted by the patent is very slow in solidification speed, easy to segregate B and easy to appear in grain boundariesLow melting point B₂O₃The phases lead to thermal cracking. 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, has more excellent quality performance, and is different from the steel plates/strips in components and production methods.

The invention has the main advantages that:

the invention utilizes the thin strip continuous casting technology to produce thin 30CrMo hot rolled steel containing tin (Sn), copper (Cu)/tin (Sn), copper (Cu) and nitrogen (N), and reports are not found so far, and the advantages are 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 reduced₂Greatly 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 10²-10⁴The thickness of the continuously cast steel is only 2-5mm, the alloy elements in the 30CrMo steel are completely solidified within a short time (0.1-0.2s) without time for segregation, and the segregation problem of the elements 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 decarburizing layer through the whole protective casting and rolling;

due to high drawing speed, even if the phase line temperature of 30CrMo molten steel is lower, the temperature of continuously cast strip steel is higher, and the rolling temperature of the strip steel entering a rolling mill is correspondingly higher, so that 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 carbon nitride 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. When the electric furnace is used for steelmaking, 100 percent of scrap steel is fully utilized on the raw materials, screening is not added, the scrap steel containing Cu and Sn is utilized, the sub-rapid solidification effect of strip continuous casting is combined, the residual element alloying effect is achieved, the smelting cost is greatly reduced, the harm and the benefit are realized for Cu and Sn in the steel, the full utilization of the existing scrap steel or low-quality poor ore resources (high tin ore and high copper ore) is realized, the recycling of the scrap steel is promoted, the production cost is reduced, and the sustainable development of the steel industry is realized.

If the converter is used for smelting steel, the scrap steel is added into the converter according to the proportion of more than 20 percent of smelting raw materials, and pre-screening is not needed, so that the scrap steel ratio of the converter is improved to the maximum extent, and the smelting cost and the energy consumption are greatly reduced.

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

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

Drawings

FIG. 1 is a schematic diagram of a process arrangement for a twin roll strip casting process;

FIG. 2 is a graph of Sn content versus average heat flux density;

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

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

Detailed Description

The invention is further illustrated by the following examples, which are not intended to limit the invention in any way. Any variations in the practice of the invention that may occur to those skilled in the art and which are made in the light of the teachings of this specification are intended to be within the scope of the following claims.

Referring to fig. 1, molten steel designed according to the chemical composition of the present invention is directly poured into a molten pool 7 surrounded by two 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 cast strip 11 comes out of the

crystallization rollers

8a and 8b, the cast strip is directly fed into the lower closed chamber 10 at the temperature of 1360-₂Ar or other non-oxidizing gas, such as N₂And H₂CO obtained by sublimation of the mixed gas and the dry ice₂Gas, etc., the oxygen concentration in the lower sealed chamber 10 is controlled to be<5 percent. The lower enclosed chamber 10 protects the cast strip 11 against oxidation to the rolling mill 13 inlet. The temperature of the casting belt at the outlet of the lower closed chamber 10 is 1150-1280 ℃; then through the swinging guide plate 9And the pinch roll 12 sends the cast strip to a hot rolling mill 13, hot rolling is carried out to form hot rolled strip steel with the thickness of 1.5-3.0mm, and then the 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 carrousel coiler 19 for coiling. And taking the steel coil off 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: 840 and 880 ℃ oil quenching, 400 and 440 ℃ tempering, and the hardness of the material obtained after quenching and tempering heat treatment is in the range of HRC35 +/-2, so that 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 present invention utilizes the strip casting technology, fully uses the scrap steel, and the 30CrMo hot rolled steel plate/strip manufactured according to the design range of the steel type components provided by the present invention has the tensile strength of less than 900MPa, the hardness of HRC25 or less, can be coiled smoothly, and is easy for downstream manufacturing users to perform various cutting and machining.

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 is an ideal material for thin steel rings in the mechanical manufacturing industry, the petrochemical industry, the boiler manufacturing industry and the saw blade cutting die industry.

Claims

1. A30 CrMo hot rolled steel plate/belt utilizing scrap steel comprises the following 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% of total oxygen [ O ]]_T: 0.007-0.020%; the balance of Fe and other unavoidable impurities, and must satisfy simultaneously:

contains Cu: 0.10-0.60% or Sn: 0.005-0.04% of one or two elements;

Mn/S>250。

2. the 30CrMo hot-rolled steel sheet/strip using scrap steel according to claim 1, wherein the hot-rolled steel sheet/strip has a tensile strength of 900MPa or less and a hardness of HRC25 or less.

3. The method for producing a 30CrMo hot rolled steel sheet/strip using scrap steel according to claim 1 or 2, characterized by comprising the steps of:

1) smelting and continuous casting

Smelting according to the composition of claim 1, wherein the basicity a of the slag-forming during the steel-making process is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a ═ 0.7-1.0; MnO-SiO in molten steel₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the concentration to be 0.5-2, preferably 1-1.8; free oxygen [ O ] in molten steel]_FreeThe range is as follows: 0.0005-0.005%;

the continuous casting adopts double-roller thin strip continuous casting to form a casting strip with the thickness of 2.0-5.0 mm; the diameter of the crystallization roller is 500-1500mm, preferably 800mm, and water is introduced into the crystallization roller for cooling; the casting speed of the casting machine is 40-100 m/min; controlling the ladle casting temperature range of 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

4) cooling after rolling

5) coiling

4. The method for manufacturing a 30CrMo hot rolled steel sheet/strip using scrap steel according to claim 3, further comprising step 6), wherein the steel sheet/strip is cut and machined according to the size of the final product after being opened and sheared, and finally subjected to a thermal refining treatment: 840 ℃ plus 880 ℃ oil quenching and 400 ℃ plus 440 ℃ tempering.

5. The method for producing a 30CrMo hot rolled steel sheet/strip using scrap steel as claimed in claim 3, wherein in the step 1), 100% of the whole scrap steel is selected as the raw material for smelting, and pre-screening is not required, and electric furnace steelmaking is adopted for molten steel smelting; or, the smelting adopts a converter for steel making, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.

6. The method for producing a 30CrMo hot rolled steel sheet/strip using scrap steel according to claim 3, wherein in the step 2), the non-oxidizing property is exhibitedThe gas comprises inert gas, N₂Or CO obtained by sublimation on dry ice₂Gas, N₂And H₂The mixed gas of (1).

7. The method for producing a 30CrMo hot-rolled steel sheet/strip using scrap steel according to claim 3, wherein the gas-water ratio of the gas-atomized cooling in the step 4) is 15: 1-10: 1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

8. The method for producing a 30CrMo hot rolled steel sheet/strip using scrap according to claim 3, wherein in the step 5), the coiling is performed in a twin coiler type or a Caroter coiling type.