A method for producing two kinds of dual-phase steel of 590 MPa grade continuous annealing and galvanizing with same composition

Abstract

The present application relates to the technical field of metallurgy, and particularly relates to a method for producing 590MPa grade continuous annealing and galvanizing two kinds of dual-phase steel with the same composition. The chemical composition of the steel material includes C: 0.08%-0.10%, Si: 0.05%-0.09%, Mn: 1.40%-1.50%, Als: 0.8%-1.0%, Mo: 0.15%-0.20%, P≤0.015%, S≤0.003%, N≤0.005%, and the rest is Fe and inevitable impurities. The production steps include smelting continuous casting, hot rolling and coiling, pickling, continuous annealing or continuous annealing and galvanizing. The present application can reduce the frequency of surface quality problems such as pitting, and improve the surface quality. The present application can improve the adhesion of the zinc coating of the galvanizing dual-phase steel, and further improve the surface quality of the galvanizing dual-phase steel. The prepared continuous annealing dual-phase steel and galvanizing dual-phase steel both meet the mechanical property requirements of 590MPa grade dual-phase steel.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a method for producing two types of duplex steel with the same composition: 590MPa grade continuous annealing and galvanizing. Background Technology

[0002] In the automotive and construction machinery manufacturing sectors, component design is trending towards greater complexity and customization, while safety performance requirements are becoming increasingly stringent. Duplex steel, with its advantages of high strength, low yield strength ratio, good formability, and simple manufacturing process, is widely used in structural and reinforcing components of automobiles and construction machinery, resulting in high demand and large usage.

[0003] 590MPa grade duplex steel is a commonly used grade of duplex steel, and it can be further divided into two products based on the production process: continuously annealed duplex steel and galvanized duplex steel. Continuously annealed duplex steel is produced through a process of smelting-continuous casting-hot rolling-pickling-continuous annealing, while galvanized duplex steel is produced through the same process. The final processes differ between the two. In existing technology, for ease of production organization, molten steel of the same composition can first be processed into cold-hardened steel coils through a process of smelting-continuous casting-hot rolling-pickling, and then, depending on the requirements, continuously annealed or continuously annealed-galvanized steel coils are processed into continuously annealed duplex steel or galvanized duplex steel products.

[0004] However, in the production of both 590MPa grade continuous annealing and galvanized duplex steel using existing processes with the same composition, a significant amount of oxides precipitate on the steel surface. On one hand, these precipitated oxides tend to accumulate on the furnace roll surface, causing nodules, which in turn compress the steel surface during rolling and transportation, leading to a higher frequency of surface defects such as pitting in both continuously annealed and galvanized duplex steels. On the other hand, these oxides can easily reduce the wettability of the zinc bath during the galvanizing process of galvanized duplex steel production, causing coating defects such as incomplete plating, pinholes, and uneven coating adhesion, thus reducing the surface quality of the galvanized duplex steel. Summary of the Invention

[0005] To address the issues of excessive oxide precipitation during the production of 590MPa grade continuous annealing and galvanizing duplex steels using existing processes with the same composition, leading to a higher frequency of surface defects such as pitting in both types of steel, and the potential for reduced zinc bath wetting properties during the galvanizing process, resulting in coating defects, this invention provides a method for producing 590MPa grade continuous annealing and galvanizing duplex steels with the same composition. This method reduces oxide generation during steel production, decreases furnace roll nodule formation, lowers the frequency of surface quality problems such as pitting, and improves surface quality. It also improves the adhesion of the zinc coating in galvanized duplex steel, further enhancing its surface quality. Both the resulting continuous annealing and galvanizing duplex steels meet the mechanical property requirements for 590MPa grade duplex steel.

[0006] The technical solution of this invention is as follows:

[0007] A method for producing two types of duplex steel with the same composition: 590MPa continuous annealing and galvanizing. The chemical composition of the steel by mass percentage includes C: 0.08%-0.10%, Si: 0.05%-0.09%, Mn: 1.40%-1.50%, Als: 0.8%-1.0%, Mo: 0.15%-0.20%, P≤0.015%, S≤0.003%, N≤0.005%, with the remainder being Fe and unavoidable impurities.

[0008] The production steps include:

[0009] (1) Smelting and casting: molten steel is smelted and continuously cast to obtain a continuously cast billet;

[0010] (2) Hot rolling and coiling: The continuously cast billet is hot rolled to obtain a hot-rolled plate, and the hot-rolled plate is coiled.

[0011] (3) Pickling: The hot-rolled sheet is pickled to obtain cold-hardened steel strip;

[0012] (4) Continuous annealing or continuous annealing and galvanizing: continuously annealing cold-hardened steel strip to obtain continuously annealed dual-phase steel, or continuously annealing and galvanizing cold-hardened steel strip to obtain galvanized dual-phase steel.

[0013] The functions of each chemical component in the product of this invention and the reasons for content control are as follows:

[0014] Carbon is the most effective strengthening element. The carbon content determines the hardness, percentage of martensite phase and morphology of duplex steel. In order to control the relative enrichment of carbon in austenite and ferrite, the carbon content range needs to be reasonably controlled. Therefore, this invention requires the carbon content to be controlled at 0.08%-0.10%.

[0015] Silicon (Si) is a solid solution strengthening element in ferrite. It accelerates the segregation of carbon into austenite, removes and purifies dissolved carbon in ferrite, reduces interstitial solid solution strengthening, and inhibits the formation of coarse carbides during cooling, thus improving the ductility of duplex steel. Silicon is also a good deoxidizer. Silicon can have a beneficial effect on the phase transformation behavior of metals. However, during annealing, Si tends to accumulate and oxidize on the surface of the strip steel, forming a SiO2 oxide film that is difficult to reduce with hydrogen. This reduces the wettability of galvanized steel, causing coating defects such as incomplete plating, pinholes, and poor coating adhesion, affecting surface quality. At the same time, Si also has an adverse effect on the spot welding performance of galvanized sheets. To minimize the impact of Si on the surface quality of galvanized steel, this invention requires the Si content to be controlled at 0.05%-0.09%.

[0016] Mn is a typical austenite stabilizing element that significantly improves the hardenability of steel and plays a role in solid solution strengthening and refining ferrite grains. It can significantly delay the transformation of pearlite and bainite. To achieve the above effects, the present invention requires the Mn content to be controlled at 1.40%-1.50%.

[0017] Al (Al) is added to steel as a deoxidizer or alloying element, and its deoxidizing ability is much stronger than that of silicon and manganese. The main role of aluminum in steel is to refine grains and fix nitrogen in the steel, thereby significantly improving the impact toughness of the steel and reducing its tendency for cold brittleness and aging. Aluminum can also improve the corrosion resistance of steel, especially when combined with elements such as molybdenum, copper, silicon, and chromium. This invention requires the Al content to be controlled between 0.8% and 1.0%.

[0018] Mo is a medium-strength carbide element that has a good influence on the hardenability of austenite formed during critical zone heating. It can promote the transformation regions of ferrite, pearlite and bainite to shift to the right and expand the austenite region. However, as the Mo content increases, the yield strength ratio of steel will decrease. Therefore, this invention requires the Mo content to be controlled at 0.15%-0.20%.

[0019] P, S, and N are all harmful elements in steel. P tends to segregate at grain boundaries, which worsens the plasticity of steel plates and increases cold brittleness. S tends to combine with Mn to form coarse MnS inclusions, which worsens the formability and increases hot brittleness. If the N content is too low, the number of TiN particles generated will be small and the size will be large, which will not improve the grain refinement of steel. If the N content is too high, it will damage the toughness of steel. Therefore, it is necessary to strictly control the content of P, S, and N elements in steel. This invention requires that the content of P, S, and N elements be controlled at: P≤0.015%, S≤0.003%, and N≤0.005%, respectively.

[0020] Furthermore, the continuous casting process in step (1) is as follows: the continuous casting machine adopts constant casting speed control, with a casting speed of 1.0-1.2m / min. Argon blowing protection is used throughout the continuous casting process of molten steel. Medium carbon covering agent is used to avoid the molten steel being exposed. The secondary cooling water is distributed according to the model water distribution mode. Alkaline intermediate ladle protective slag is used in the continuous casting process, and high-alumina steel special protective slag is used in the crystallizer. This can avoid problems such as nozzle blockage and excessive inclusions in the continuous casting process, and ensure the surface quality of the billet.

[0021] Furthermore, the hot rolling process in step (2) includes heating, rough rolling, and finish rolling. The heating temperature is 1210-1230℃, and the holding time in the furnace is ≥130min. Austenitization is achieved within this temperature range, casting defects are eliminated, the microstructure is homogenized, deformation resistance is reduced, and abnormal grain growth is prevented due to excessive temperature. At the same time, all alloying elements are uniformly dissolved, and the control of the subsequent finish rolling temperature is ensured.

[0022] The roughing mill exit temperature is 1060-1090℃, and the finishing mill final rolling temperature is 840-890℃. After hot rolling, the hot-rolled plate is slowly cooled for 72 hours after being removed from the line. This ensures that the structure of the hot-rolled plate is uniform and prevents coarse grains caused by excessive finishing mill temperature or mixed grain structure caused by excessive finishing mill temperature.

[0023] Furthermore, the winding method in step (2) is U-shaped winding, and the winding temperature is 600-630℃, which can obtain a fine and uniform structure and improve product performance.

[0024] Furthermore, in step (3), the acid rolling reduction is 60%-70%, which can balance the squeezing capacity of the rolling mill and the accumulation of deformation energy storage.

[0025] Furthermore, in step (4), the continuous annealing process for preparing continuously annealed dual-phase steel includes:

[0026] The preheating temperature is 200-250℃, and the soaking temperature is 810-830℃. This allows for reasonable control of the ferrite and austenite ratio, ensuring a sufficient martensite content and guaranteeing full recrystallization.

[0027] The dew point in the furnace area is controlled at ≤-40℃, which can inhibit the external oxidation of the strip steel and ensure the surface quality of the strip steel.

[0028] The slow cooling outlet temperature is 600-630℃, and the slow cooling rate is 4-6℃ / s. This can transform some austenite into ferrite, control the proportion of ferrite, and increase the hardenability of austenite.

[0029] The outlet temperature of the rapid cooling section is 290-310℃, and the cooling rate is ≥41℃ / s. This can avoid the pearlite and bainite regions, allowing untransformed austenite to be fully transformed into martensite.

[0030] After aging treatment, the strip is cooled to 290-320℃ and then tempered to comprehensively improve its mechanical properties and prevent martensite decomposition caused by excessive temperature.

[0031] The final cooling temperature is 140-150℃, followed by cooling to room temperature, which can improve the strength and ductility of the strip steel.

[0032] The cooled steel strip is finished with a finishing elongation of 0.4%-0.7%, which can control the finished strip shape, surface roughness and yield strength, and ensure surface quality.

[0033] Furthermore, the microstructure of the obtained continuously annealed dual-phase steel consists of ferrite and martensite, with a martensite volume fraction >12%; the yield strength of the continuously annealed dual-phase steel is 370-390 MPa, the tensile strength is 620-630 MPa, the elongation is 25-29%, and the n-value is 0.17-0.2; per 100 dm 2 In continuously annealed dual-phase steel, the number of pits with a diameter ≥ 0.1 mm is less than 5.

[0034] Furthermore, in step (4), the continuous annealing and galvanizing process for preparing galvanized duplex steel includes:

[0035] The preheating temperature is 200-250℃, and the soaking temperature is 800-820℃. This allows for reasonable control of the ferrite and austenite ratio, ensuring a sufficient martensite content and guaranteeing full recrystallization.

[0036] The dew point in the furnace area is controlled at ≤-40℃, which can inhibit the external oxidation of the strip steel and ensure the surface quality of the strip steel after hot-dip galvanizing.

[0037] The slow cooling outlet temperature is 600-630℃, and the slow cooling rate is 4-6℃ / s. This can transform some austenite into ferrite, control the proportion of ferrite, and increase the hardenability of austenite.

[0038] The final temperature of the rapid cooling section is 480-510℃, and the cooling rate is ≥30℃ / s. This can avoid the pearlite and bainite regions, allowing untransformed austenite to fully transform into martensite. At the same time, it controls the surface temperature of the strip when it enters the zinc pot, preventing the strip temperature from being lower than the zinc liquid or the strip temperature from being too high, which would reduce the galvanizing quality.

[0039] Pre-oxidation is carried out simultaneously with continuous annealing. The oxygen content of the pre-oxidation is 1.6%-1.9%, which can ensure the rapid cooling rate of duplex steel, control the phase transformation region, and ensure that easily oxidized elements such as silicon and manganese in duplex steel complete the internal oxidation reaction.

[0040] After rapid cooling, the steel strip is fed into a zinc pot for galvanizing. The temperature of the zinc liquid in the zinc pot is 452-465℃. At this temperature, the zinc liquid has good fluidity, which is conducive to the wetting effect of the zinc liquid on the surface of the steel strip, thereby shortening the galvanizing time, increasing the output, and obtaining a thin and uniform pure zinc coating.

[0041] After the steel strip exits the zinc pot, it is cooled to ≤160℃ by air cooling to ensure that the zinc coating is completely solidified, to prevent the zinc coating from adhering to the top roller, and to avoid affecting the adhesion of the zinc coating.

[0042] The galvanized steel strip is finished with a finishing elongation of 0.4%-0.7%, which can control the shape, surface roughness and yield strength of the finished product and ensure surface quality.

[0043] Furthermore, the Al content of the zinc liquid in the zinc pot is 0.19%-0.23%, and the Fe content is ≤0.012%, which can inhibit the formation of top and bottom slag in the zinc liquid, improve the surface quality, and at the same time ensure that a dense inhibition layer is formed between the aluminum in the zinc liquid and the steel base, thereby improving the adhesion of the coating.

[0044] Furthermore, the matrix microstructure of galvanized duplex steel consists of ferrite and martensite, with a volume fraction of martensite >8%; the yield strength of galvanized duplex steel is 370-390 MPa, the tensile strength is 620-630 MPa, the elongation is 25%-29%, and the n-value is 0.17-0.2; the zinc coating surface is uniform in color and free of defects, the inhibition layer is continuous and dense, the adhesion ball impact test grade of the zinc coating reaches level 1, and the surface quality grade of galvanized duplex steel is ≥ grade C.

[0045] The beneficial effects of this invention are as follows:

[0046] 1. The present invention provides a method for producing two types of duplex steel with the same composition: annealed and galvanized steel. First, cold-hardened steel strips with the same composition are prepared through smelting, continuous casting, hot rolling, and pickling processes. Then, the cold-hardened steel strips are continuously annealed to obtain annealed duplex steel, or the cold-hardened steel strips are continuously annealed and galvanized to obtain galvanized duplex steel. This method achieves the production of both annealed and galvanized duplex steel with the same composition and previous processes, which is convenient for production organization and has high production efficiency.

[0047] 2. The steel chemical composition of the present invention has extremely low Si content and high Al content, which reduces the generation of oxides during the steel production process, thereby reducing the occurrence of furnace roll nodules, reducing the frequency of surface quality problems such as pitting, and improving the surface quality of continuously annealed duplex steel and galvanized duplex steel.

[0048] 3. When preparing galvanized duplex steel using the technical solution of this invention, the amount of Si in the steel is extremely low, while the amount of Al is relatively high, resulting in less iron oxide scale formation on the steel strip and a lower negative impact of Si on the surface quality of the zinc coating. Simultaneously, by rationally controlling the aluminum content in the zinc bath during the galvanizing process, the oxides of Mn at the grain boundaries can be replaced by Al elements in the zinc bath, forming a network of Al-rich oxide particles in the inhibition layer. This improves the adhesion of the zinc coating on the galvanized duplex steel, further enhancing its surface quality. The zinc coating surface of the galvanized duplex steel has a uniform color and is free of defects. The inhibition layer is continuous and dense, and the adhesion ball-impact test grade of the zinc coating reaches level 1. The surface quality grade of the galvanized duplex steel is ≥ grade C.

[0049] 4. This invention employs appropriate raw material composition ratios, selects suitable smelting continuous casting, hot rolling coiling, pickling and rolling processes and supporting parameters, and optimizes continuous annealing and galvanizing processes to achieve a reasonable distribution of martensite volume in the product, ensuring the strength of the two types of steel products. The mechanical properties of the resulting continuously annealed duplex steel and galvanized duplex steel can both reach: yield strength 370-390MPa, tensile strength 620-630MPa, elongation 25-29%, n-value 0.17-0.2, meeting the mechanical property requirements of 590MPa grade duplex steel. Detailed Implementation

[0050] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0051] Example 1

[0052] A method for producing two types of duplex steel with the same composition: 590MPa grade continuously annealed and galvanized steel. The production steps include:

[0053] (1) Steel is prepared according to the chemical composition of the product by mass percentage: C: 0.085%, Si: 0.064%, Mn: 1.42%, P: 0.014%, S: 0.0026%, Als: 0.87%, Mo: 0.175%, N: 0.003%, with the remainder being Fe and unavoidable impurities. The steel is smelted and argon is blown throughout the continuous casting process to prevent oxidation. Medium carbon covering agent is used to prevent the steel from being exposed. The secondary cooling water is prepared according to the model water distribution mode to avoid problems such as nozzle blockage and excessive inclusions during continuous casting, thus ensuring the surface quality of the billet. Alkaline intermediate ladle protective slag is used for slag protection during continuous casting, and high-alumina steel special protective slag is used for crystallizer protective slag protection to avoid defects such as billet cracks, thus ensuring the surface quality of the billet.

[0054] (2) The continuous casting billet is hot-rolled to obtain a hot-rolled plate. The heating temperature is 1215℃, the holding time is 140min, the roughing mill exit temperature is 1068℃, the finishing mill final rolling temperature is 850℃, the cooling method of the hot-rolled plate is slow cooling for 72h after leaving the line, and the thickness of the hot-rolled plate is 3mm. The hot-rolled plate is coiled in a U-shaped coiling method at a coiling temperature of 605℃.

[0055] (3) Pickling the hot-rolled plate to obtain cold-hardened steel strip with a pickling reduction of 63% and a thickness of 1.11 mm.

[0056] The specific production process for using the above-mentioned cold-hardened steel strip to produce 590MPa grade continuously annealed duplex steel is as follows:

[0057] The cold-hardened steel strip was continuously annealed. The preheating temperature was 208℃, the soaking temperature was 816℃, the dew point in the furnace area was controlled at -45℃, the slow cooling outlet temperature was 608℃, and the slow cooling rate was 4.8℃ / s. The fast cooling outlet temperature was 298℃, and the cooling rate was 45℃ / s. After aging treatment, it was cooled to 300℃. The final cooling temperature was 140℃, and then cooled to room temperature. After finishing and finished product inspection, 590MPa grade continuously annealed dual-phase steel was obtained with a finishing elongation of 0.46%. It was then packaged and shipped.

[0058] The specific production process for using the above-mentioned cold-hardened steel strip to produce 590MPa grade galvanized duplex steel is as follows:

[0059] For continuous annealing and galvanizing of cold-hardened steel strip, the preheating temperature is 208℃, the soaking temperature is 816℃, the furnace dew point is controlled at -48℃, and the annealing soaking temperature is 810℃; the final temperature of the rapid cooling section is 486℃, and the cooling rate is 36℃ / s; pre-oxidation is carried out simultaneously with continuous annealing, and the oxygen content of the pre-oxidation is 1.68%; after rapid cooling, the steel strip is sent into the zinc pot, and the temperature of the zinc liquid in the zinc pot is 455℃; the Al content in the zinc liquid is controlled at 0.19%, and the Fe content is 0.008%, and galvanizing is carried out. After the steel strip leaves the zinc pot, it is cooled to 155℃ by air cooling, and after finishing and finished product inspection, 590MPa grade galvanized duplex steel is obtained with a finishing elongation of 0.46%, and it is packaged and shipped out.

[0060] Example 2

[0061] A method for producing two types of duplex steel with the same composition: 590MPa grade continuously annealed and galvanized steel. The production steps include:

[0062] (1) Steel was prepared according to the chemical composition of the product by mass percentage: C: 0.092%, Si: 0.064%, Mn: 1.48%, P: 0.010%, S: 0.0020%, Als: 0.85%, Mo: 0.165%, N: 0.003%, with the remainder being Fe and unavoidable impurities. The steel was smelted and argon was blown throughout the continuous casting process to prevent oxidation. Medium carbon covering agent was used to prevent the steel from being exposed. The secondary cooling water was prepared according to the model water distribution mode to avoid problems such as nozzle blockage and excessive inclusions during continuous casting, thus ensuring the surface quality of the billet. Alkaline intermediate ladle protective slag was used for slag protection during continuous casting, and high-alumina steel special protective slag was used for crystallizer protection slag protection to avoid defects such as billet cracks, thus ensuring the surface quality of the billet.

[0063] (2) The continuous casting billet is hot-rolled to obtain a hot-rolled plate. The heating temperature is 1220℃, the holding time is 150min, the roughing mill exit temperature is 1070℃, the finishing mill final rolling temperature is 848℃, the cooling method of the hot-rolled plate is slow cooling for 72h after leaving the line, and the thickness of the hot-rolled plate is 3mm. The hot-rolled plate is coiled in a U-shaped coiling manner at a coiling temperature of 610℃.

[0064] (3) For hot-rolled plates, the pickling reduction is 60% and the thickness of cold-hardened steel strip is 1.2mm.

[0065] The specific production process for using the above-mentioned cold-hardened steel strip to produce 590MPa grade continuously annealed duplex steel is as follows:

[0066] The cold-hardened steel strip was continuously annealed. The preheating temperature was 213℃, the soaking temperature was 815℃, the dew point in the furnace area was controlled at -48℃, the slow cooling outlet temperature was 610℃, and the slow cooling rate was 5.2℃ / s. The fast cooling outlet temperature was 300℃, and the cooling rate was 47℃ / s. After aging treatment, it was cooled to 305℃. The final cooling temperature was 146℃, and then cooled to room temperature. After finishing and finished product inspection, 590MPa grade continuously annealed dual-phase steel was obtained with a finishing elongation of 0.50%. It was then packaged and shipped.

[0067] The specific production process for using the above-mentioned cold-hardened steel strip to produce 590MPa grade galvanized duplex steel is as follows:

[0068] For continuous annealing and galvanizing of cold-hardened steel strip, the preheating temperature is 213℃, the soaking temperature is 815℃, the furnace dew point is controlled at -50℃, and the annealing soaking temperature is 816℃; the final temperature of the rapid cooling section is 495℃, and the cooling rate is 40℃ / s; pre-oxidation is carried out simultaneously with continuous annealing, and the oxygen content of the pre-oxidation is 1.65%; after rapid cooling, the steel strip is sent into the zinc pot, and the temperature of the zinc liquid in the zinc pot is 458℃; the Al content in the zinc liquid is controlled at 0.19%, and the Fe content is 0.009% for galvanizing. After the steel strip leaves the zinc pot, it is cooled to 150℃ by air cooling, and after finishing and finished product inspection, 590MPa grade galvanized duplex steel is obtained with a finishing elongation of 0.50%, and it is packaged and shipped out.

[0069] Example 3

[0070] A method for producing two types of duplex steel with the same composition: 590MPa grade continuously annealed and galvanized steel. The production steps include:

[0071] (1) Steel was prepared according to the chemical composition of the product by mass percentage: C: 0.090%, Si: 0.055%, Mn: 1.45%, P: 0.012%, S: 0.0022%, Als: 0.90%, Mo: 0.168%, N: 0.004%, with the remainder being Fe and unavoidable impurities. The steel was smelted and argon was blown throughout the continuous casting process to prevent oxidation. Medium carbon covering agent was used to prevent the steel from being exposed. The secondary cooling water was prepared according to the model water distribution mode to avoid problems such as nozzle blockage and excessive inclusions during continuous casting, thus ensuring the surface quality of the billet. Alkaline intermediate ladle protective slag was used for slag protection during continuous casting, and high-alumina steel special protective slag was used for crystallizer protection slag protection to avoid defects such as billet cracks, thus ensuring the surface quality of the billet.

[0072] (2) The continuous casting billet is hot-rolled to obtain a hot-rolled plate. The heating temperature is 1218℃, the holding time is 148min, the roughing exit temperature is 1073℃, the finishing rolling temperature is 850℃, the cooling method of the hot-rolled plate is slow cooling for 72h after leaving the line, and the thickness of the hot-rolled plate is 3mm. The hot-rolled plate is coiled in a U-shaped coiling manner at a coiling temperature of 615℃.

[0073] (3) For hot-rolled plates, the pickling reduction is 65% and the thickness of cold-hardened steel strip is 1.05mm.

[0074] The specific production process for using the above-mentioned cold-hardened steel strip to produce 590MPa grade continuously annealed duplex steel is as follows:

[0075] The cold-hardened steel strip was continuously annealed. The preheating temperature was 225℃, the soaking temperature was 825℃, the dew point in the furnace area was controlled at -50℃, the slow cooling outlet temperature was 620℃, and the slow cooling rate was 5.8℃ / s. The fast cooling outlet temperature was 305℃, and the cooling rate was 51℃ / s. After aging treatment, it was cooled to 310℃. The final cooling temperature was 148℃, and then cooled to room temperature. After finishing and finished product inspection, 590MPa grade continuously annealed dual-phase steel was obtained with a finishing elongation of 0.59%. It was then packaged and shipped.

[0076] The specific production process for using the above-mentioned cold-hardened steel strip to produce 590MPa grade galvanized duplex steel is as follows:

[0077] For continuous annealing and galvanizing of cold-hardened steel strip, the preheating temperature is 225℃, the soaking temperature is 825℃, the furnace dew point is controlled at -49℃, and the annealing soaking temperature is 820℃; the final temperature of the rapid cooling section is 500℃, and the cooling rate is 45℃ / s; pre-oxidation is carried out simultaneously with continuous annealing, with a pre-oxidation oxygen content of 1.75%; after rapid cooling, the steel strip is sent to the zinc pot, where the temperature of the zinc liquid is 455℃; the Al content in the zinc liquid is controlled at 0.20%, and the Fe content at 0.008%, and galvanizing is carried out. After exiting the zinc pot, the steel strip is cooled to 152℃ by air cooling, and after finishing and finished product inspection, 590MPa grade galvanized duplex steel is obtained with a finishing elongation of 0.56%, and it is packaged and shipped out.

[0078] Example 4

[0079] A method for producing two types of duplex steel with the same composition: 590MPa grade continuously annealed and galvanized steel. The production steps include:

[0080] (1) Steel was prepared according to the chemical composition of the product by mass percentage: C: 0.086%, Si: 0.065%, Mn: 1.48%, P: 0.014%, S: 0.0024%, Als: 0.96%, Mo: 0.170%, N: 0.004%, with the remainder being Fe and unavoidable impurities. The steel was smelted and argon was blown throughout the continuous casting process to prevent oxidation. Medium carbon covering agent was used to prevent the steel from being exposed. The secondary cooling water was prepared according to the model water distribution mode to avoid problems such as nozzle blockage and excessive inclusions during continuous casting, thus ensuring the surface quality of the billet. Alkaline intermediate ladle protective slag was used for the slag in the continuous casting process, and high-alumina steel special protective slag was used for the crystallizer to avoid defects such as billet cracks and ensure the surface quality of the billet.

[0081] (2) The continuous casting billet is hot-rolled to obtain a hot-rolled plate. The heating temperature is 1225℃, the holding time is 155min, the roughing exit temperature is 1080℃, the finishing rolling temperature is 860℃, the cooling method of the hot-rolled plate is slow cooling for 72h after leaving the line, and the thickness of the hot-rolled plate is 3mm. The hot-rolled plate is coiled in a U-shaped coiling method at a coiling temperature of 625℃.

[0082] (3) For hot-rolled plates, the pickling reduction is 70% and the thickness of cold-hardened steel strip is 0.9mm.

[0083] The specific production process for using the above-mentioned cold-hardened steel strip to produce 590MPa grade continuously annealed duplex steel is as follows:

[0084] The cold-hardened steel strip was continuously annealed. The preheating temperature was 240℃, the soaking temperature was 830℃, the dew point in the furnace area was controlled at -50℃, the slow cooling outlet temperature was 622℃, and the slow cooling rate was 5.5℃ / s. The fast cooling outlet temperature was 306℃, and the cooling rate was 50℃ / s. After aging treatment, it was cooled to 315℃. The final cooling temperature was 148℃, and then cooled to room temperature. After finishing and finished product inspection, 590MPa grade continuously annealed dual-phase steel was obtained with a finishing elongation of 0.65%. It was then packaged and shipped.

[0085] The specific production process for using the above-mentioned cold-hardened steel strip to produce 590MPa grade galvanized duplex steel is as follows:

[0086] For continuous annealing and galvanizing of cold-hardened steel strip, the preheating temperature is 240℃, the soaking temperature is 830℃, the furnace dew point is controlled at -50℃, and the annealing soaking temperature is 817℃; the final temperature of the rapid cooling section is 508℃, and the cooling rate is 48℃ / s; pre-oxidation is carried out simultaneously with continuous annealing, and the oxygen content of the pre-oxidation is 1.85%; after rapid cooling, the steel strip is sent into the zinc pot, and the temperature of the zinc liquid in the zinc pot is 455℃; the Al content in the zinc liquid is controlled at 0.20%, and the Fe content is 0.008%, and galvanizing is carried out. After the steel strip leaves the zinc pot, it is cooled to 150℃ by air cooling, and after finishing and finished product inspection, 590MPa grade galvanized duplex steel is obtained with a finishing elongation of 0.65%, and it is packaged and shipped out.

[0087] Comparative Example 1

[0088] A method for producing two types of duplex steel with the same composition: annealed and galvanized steel.

[0089] The production steps include:

[0090] (1) Steel was prepared according to the chemical composition of the product by mass percentage: C: 0.122%, Si: 0.40%, Mn: 1.47%, P: 0.008%, S: 0.001%, Als: 0.51%, Nb: 0.014%, N: 0.004%, with the remainder being Fe and unavoidable impurities. The steel was smelted and argon was blown throughout the continuous casting process to prevent oxidation. Medium carbon covering agent was used to prevent the steel from being exposed. The secondary cooling water was prepared according to the model water distribution mode to avoid problems such as nozzle blockage and excessive inclusions during continuous casting, thus ensuring the surface quality of the billet. Alkaline intermediate ladle protective slag was used for slag protection during continuous casting, and high-alumina steel special protective slag was used for crystallizer protection slag to avoid defects such as billet cracks, thus ensuring the surface quality of the billet.

[0091] (2) The continuous casting billet is hot-rolled to obtain a hot-rolled plate. The heating temperature is 1230℃, the holding time is 200min, the roughing mill exit temperature is 1100℃, the finishing mill final rolling temperature is 880℃, the cooling method of the hot-rolled plate is slow cooling for 72h after leaving the line, and the thickness of the hot-rolled plate is 3mm. The hot-rolled plate is coiled in a U-shaped coiling manner at a coiling temperature of 595℃.

[0092] (3) For hot-rolled plates, the pickling reduction is 60% and the thickness of cold-hardened steel strip is 1.2mm.

[0093] The specific production process for using the above-mentioned cold-hardened steel strip to produce continuously annealed dual-phase steel is as follows:

[0094] The cold-hardened steel strip is continuously annealed. The preheating temperature is 260℃, the soaking temperature is 810℃, the furnace dew point is controlled at -48℃, the slow cooling outlet temperature is 629℃, and the slow cooling rate is 6℃ / s. The fast cooling outlet temperature is 310℃, and the cooling rate is 48℃ / s. After aging treatment, it is cooled to 309℃. The final cooling temperature is 150℃, and then it is cooled to room temperature. After finishing and finished product inspection, the continuously annealed dual-phase steel is obtained with a finishing elongation of 0.50%. It is then packaged and shipped.

[0095] The specific production process for using the above-mentioned cold-hardened steel strip to produce galvanized duplex steel is as follows:

[0096] For continuous annealing and galvanizing of cold-hardened steel strip, the preheating temperature is 260℃, the soaking temperature is 810℃, the furnace dew point is controlled at -48℃, and the annealing soaking temperature is 812℃; the final temperature of the rapid cooling section is 488℃, and the cooling rate is 48℃ / s; pre-oxidation is carried out simultaneously with continuous annealing, and the oxygen content of the pre-oxidation is 1.79%; after rapid cooling, the steel strip is sent into the zinc pot, where the temperature of the zinc liquid is 458℃, for galvanizing. After exiting the zinc pot, the steel strip is cooled to 150℃ by air cooling, and after finishing and finished product inspection, galvanized duplex steel is obtained with a finishing elongation of 0.50%, and it is packaged and shipped out.

[0097] Mechanical properties and surface quality were tested on the continuously annealed dual-phase steels produced in Examples 1-4 and Comparative Example 1. The results are shown in Table 1.

[0098] Table 1. Test results of mechanical properties and surface quality of continuously annealed dual-phase steel.

[0099]

[0100] It can be seen that the mechanical properties of the continuously annealed duplex steels obtained in Examples 1-4 are comparable to those of Comparative Example 1, and both meet the requirements of 590MPa grade duplex steel. However, the frequency of surface pitting of the continuously annealed duplex steels in Examples 1-4 is significantly lower than that in Comparative Example 1, and the surface quality is better.

[0101] Observation of the galvanized duplex steel produced in Examples 1-4 and Comparative Example 1 revealed that the galvanized duplex steel products of Examples 1-4 had uniform color and no defects on the surface, and the inhibition layer was continuous and dense; while the galvanized duplex steel of Comparative Example 1 had uneven zinc coating defects on the surface.

[0102] Mechanical properties and surface quality were tested on the galvanized duplex steel produced in Examples 1-4 and Comparative Example 1. The results are shown in Table 2.

[0103] Table 2. Test results of mechanical properties and surface quality of galvanized duplex steel.

[0104]

[0105] It can be seen that the mechanical properties of the galvanized duplex steels prepared in Examples 1-4 are comparable to those of Comparative Example 1, and both meet the requirements of 590MPa grade duplex steel. However, the ball punching grade and surface quality grade of the galvanized duplex steels in Examples 1-4 are higher than those in Comparative Example 1, and the surface quality is better.

[0106] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for producing two types of duplex steel with the same composition: 590MPa continuous annealing and galvanizing, characterized in that... The chemical composition of the steel, by mass percentage, includes C: 0.08%-0.10%, Si: 0.05%-0.09%, Mn: 1.40%-1.50%, Als: 0.8%-1.0%, Mo: 0.15%-0.20%, P≤0.015%, S≤0.003%, N≤0.005%, with the remainder being Fe and unavoidable impurities. The production steps include: (1) Smelting and casting: molten steel is smelted and continuously cast to obtain a continuously cast billet; The continuous casting process is as follows: the continuous casting machine adopts constant casting speed control, with a casting speed of 1.0-1.2m / min. Argon blowing protection is used throughout the continuous casting process of molten steel. Medium carbon covering agent is used to avoid the molten steel being exposed. The secondary cooling water is distributed according to the model water distribution mode. Alkaline intermediate ladle protective slag is used in the continuous casting process, and high-alumina steel special protective slag is used in the crystallizer. (2) Hot rolling and coiling: The continuously cast billet is hot rolled to obtain a hot-rolled plate, and the hot-rolled plate is coiled. The hot rolling process includes heating, roughing, and finishing. The heating temperature is 1210-1230℃, and the holding time in the furnace is ≥130min. The roughing exit temperature is 1060-1090℃, and the finishing temperature is 840-890℃. After hot rolling, the hot-rolled plate is removed from the line and slowly cooled for 72 hours. The winding method is U-shaped winding, and the winding temperature is 600-630℃; (3) Pickling: The hot-rolled sheet is pickled to obtain chilled steel strip. The pickling reduction is 60%-70%. (4) Continuous annealing or continuous annealing and galvanizing: continuously annealing cold-hardened steel strip to obtain continuously annealed dual-phase steel, or continuously annealing and galvanizing cold-hardened steel strip to obtain galvanized dual-phase steel. The continuous annealing process for preparing continuously annealed dual-phase steel includes: a preheating zone temperature of 200-250℃ and a soaking temperature of 810-830℃; a furnace dew point control of ≤-40℃; a slow cooling outlet temperature of 600-630℃ and a slow cooling rate of 4-6℃ / s; a rapid cooling zone outlet temperature of 290-310℃ and a cooling rate of ≥41℃ / s; cooling to 290-320℃ after aging treatment; a final cooling temperature of 140-150℃, followed by cooling to room temperature; and finishing of the cooled steel strip with a finishing elongation of 0.4%-0.7%. The continuous annealing and galvanizing process for preparing galvanized duplex steel includes: a preheating zone temperature of 200-250℃ and a soaking temperature of 800-820℃; a furnace dew point control of ≤-40℃; a slow cooling outlet temperature of 600-630℃ and a slow cooling rate of 4-6℃ / s; a rapid cooling zone end temperature of 480-510℃ and a cooling rate ≥30℃ / s; simultaneous pre-oxidation during continuous annealing, with an oxygen content of 1.6%-1.9%; after rapid cooling, the steel strip is fed into a zinc pot for galvanizing, with the zinc liquid temperature in the zinc pot at 452-465℃; after exiting the zinc pot, the steel strip is air-cooled to ≤160℃; and the galvanized steel strip is finished with a finishing elongation of 0.4%-0.7%.

2. The method as described in claim 1, characterized in that, The microstructure of the obtained continuously annealed dual-phase steel consists of ferrite and martensite, with a martensite volume fraction >12%. The yield strength of the continuously annealed dual-phase steel is 370-390 MPa, the tensile strength is 620-630 MPa, the elongation is 25-29%, and the n-value is 0.17-0.

2. (The last part about 100 dm is incomplete and likely refers to a specific strength or value.) 2 In continuously annealed dual-phase steel, the number of pits with a diameter ≥ 0.1 mm is less than 5.

3. The method as described in claim 1, characterized in that, The Al content of the zinc liquid in the zinc pot is 0.19%-0.23%, and the Fe content is ≤0.012%.

4. The method as described in claim 3, characterized in that, The microstructure of galvanized duplex steel consists of ferrite and martensite, with a volume fraction of martensite >8%. The yield strength of galvanized duplex steel is 370-390 MPa, the tensile strength is 620-630 MPa, the elongation is 25%-29%, and the n-value is 0.17-0.

2. The zinc coating has a uniform color and no defects, the inhibition layer is continuous and dense, the adhesion ball impact test grade of the zinc coating reaches level 1, and the surface quality grade of galvanized duplex steel is ≥ grade C.

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