Method for producing hot-rolled sheet steel for automobile wheels with increased strength, corrosion resistance and cold resistance

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to methods for producing hot-rolled sheet metal from microalloyed steels of high strength, corrosion resistance and cold resistance, which can be used in the automotive industry, in particular for the manufacture of wheel systems for automobiles. The method for producing hot-rolled sheet metal for automobile wheels includes smelting, ladle treatment, continuous casting, heating the slab for hot rolling, rolling it in roughing and finishing continuous groups of stands of a wide-strip mill to obtain a strip, cooling the strip with water on a discharge roller table, and subsequent winding into a roll. The steel produced comprises, by weight %: carbon 0.05-0.10, silicon 0.03-0.20, manganese 0.35-1.10, phosphorus 0.004-0.016, sulphur 0.001-0.015, aluminium 0.02-0.06, niobium 0.010-0.035, titanium 0.010-0.040, the rest is iron and inevitable impurities. The finishing temperature of rolling in the roughing group of stands is set at 1060-1120 °C, hot rolling is completed at a temperature of 810-860 °C, whereas the temperature of winding hot-rolled strips into rolls is determined depending on the thickness of the rolled product in accordance with the equation: Tcm = (705-10τ)± 20 °C, where Tcm is the temperature of winding hot-rolled strips into rolls,°C, 705 - empirical coefficient,°C, 10 - empirical coefficient,°C / mm, τ- rolled product thickness, mm.EFFECT: increase in the corrosion resistance and cold resistance of hot-rolled sheet metal while maintaining a high level of strength and ductility.1 cl, 2 tbl, 2 ex

Description

[0001] The invention pertains to metallurgy, specifically to methods for producing hot-rolled microalloyed sheet steels that can be used in the automotive industry, particularly for the manufacture of wheel assemblies. Currently, there is a growing trend toward reducing vehicle weight and improving operational safety through the use of high-strength sheet steel. At the same time, it is important to ensure that hot-rolled sheet steel exhibits high ductility, necessary for cold stamping of parts requiring deep expansion and drawing, as well as high cold resistance, allowing finished products to be used under extreme operating conditions at low temperatures. Considering that such steels can also be used without zinc coating, it is essential to ensure a high level of corrosion resistance.

[0002] A method is known for producing high-strength hot-rolled steel, which includes smelting low-alloy steel, casting, hot rolling, cooling with water, and winding the strips into rolls, characterized in that steel is smelted containing components in the following ratio, wt. %:

[0003]

[0004]

[0005] In this case, the end rolling temperature is maintained in the range of 830-880°C, and the coiling temperature is in the range of 510-640°C. In addition, the steel may additionally contain vanadium in an amount of 0.01-0.08 wt.%, and the total content of niobium, titanium and vanadium should not exceed 0.117 wt.%. Steel may also additionally contain calcium in an amount of 0.0005-0.010 wt.%. In addition, the carbon content is related to the required strength class by the dependence: mass %, where 0.0002 and 0.002 are empirical coefficients, %, - a dimensionless indicator numerically equal to the required minimum yield strength; the manganese content in steel is related to the required strength class by the relationship: wt. %, where 0.0022 and 0.15 are empirical coefficients, %, - a dimensionless indicator numerically equal to the required minimum yield strength; the total content of niobium, titanium and vanadium is related to the required strength class by the dependence: wt. %, where 0.0002 and 0.013 are empirical coefficients, %, - a dimensionless indicator numerically equal to the required minimum yield strength.

[0006] (Patent RU2361930, IPC C21D 8 / 04, B21B 1 / 46, C22C 38 / 06, published 07 / 20 / 2009).

[0007] This method produces steel with high strength characteristics and also allows for the production of hot-rolled sheet metal of the required strength class. However, a disadvantage of this known method may be insufficient corrosion resistance and cold resistance of the resulting rolled products.

[0008] A method is known for producing hot-rolled coiled products from low-alloy steel with a thickness of 4.0-9.0 mm, including smelting, ladle treatment, continuous casting, austenitization of the workpiece with heating above Ac3, preliminary deformation of the strip in the roughing group of mill stands and final deformation of the strip, cooling the surface of the strip with water and winding it into a roll, characterized in that steel of the following chemical composition is smelted with a ratio of components, wt. %:

[0009]

[0010] in this case, the temperature of the rolled product in the last pass of the roughing group of mill stands is maintained in the range of 1010-1050°C, the final deformation of the strip is carried out in a continuous mode with a total degree of deformation of at least 70% and completion of plastic deformation in the temperature range of 790-840°C, after completion of the final deformation on the discharge roller table, differentiated cooling of the upper and lower surfaces of the strip is carried out, and the cooling of the upper surface of the strip is carried out with an intensity determined from the expression: Where - cooling rate of the upper surface of the strip, deg / s, - the final thickness of the strip, mm, and cooling of the lower surface of the strip is performed monotonically uniformly along its entire length, while the temperature of the strip before winding is maintained in the range of 550-600 °C. In this case, for strips with a thickness of 4.0-6.0 mm inclusive, the temperature of completion of plastic deformation is 825±15 °C, and the temperature of winding the strip into a roll is taken equal to 585±15 °C; for strips with a thickness of 6.1-8.0 mm inclusive, the temperature of completion of plastic deformation is 815±15 °C, and the temperature of winding the strip into a roll is taken equal to 575±15 °C; for strips with a thickness of 8.1-9.0 mm inclusive, the temperature of completion of plastic deformation is 805±15 °C, and the temperature of winding the strip into a roll is taken equal to 565±15 °C.

[0011] (Patent RU2450061, IPC C21D 8 / 04, C22C 38 / 20, B21B 1 / 46, published 10.05.2012).

[0012] This method produces hot-rolled steel with high ductility and formability, but with insufficient resistance to atmospheric corrosion. Furthermore, the strength of such rolled steel may be insufficient, leading to reduced consumer properties of finished products.

[0013] The closest analogue of the claimed invention is a method for producing hot-rolled sheet metal for automobile wheels, including smelting, ladle treatment, continuous casting, heating the slab for hot rolling, rolling it in roughing and finishing continuous groups of stands of a wide-strip mill to obtain a strip, cooling the strip with water on a discharge roller table with subsequent winding into a roll, characterized in that steel of the following chemical composition is smelted, wt. %:

[0014]

[0015]

[0016] Rolling in the finishing group of stands is carried out with an acceleration of 0.01-0.05 m / s 2 , the end temperature of hot rolling is set at 860-930°C, after which differentiated cooling of the upper and lower surfaces of the strip is carried out, and during the cooling of a strip with a thickness of more than 5.5-6.0 mm, water is supplied by opening all the valves of the pipes at the top and bottom of the strip immediately after it exits the last stand of the hot rolling mill, and during the cooling of a strip with a thickness of 3.0-5.5 mm by opening every fourth valve of the pipes at the top and bottom of the strip after the strip reaches the coiler, while the coiling temperature is 620-680°C. In addition, the amount of sulfide non-metallic inclusions in the molten steel does not exceed 2 points, the rolled product has a ferrite-pearlite structure with a grain size of 9-12 numbers, and the strip after hot rolling can be additionally subjected to pickling.

[0017] (Patent RU2602206, IPC B21B 1 / 26, published 10.11.2016).

[0018] This method produces high-strength hot-rolled steel while maintaining high ductility and formability (yield strength and tensile strength of at least 360 MPa and 440 MPa, respectively, with a relative elongation of at least 30%). The resulting steel can be used to manufacture automobile wheel rims. However, this method does not produce hot-rolled steel with sufficiently high corrosion and cold resistance.

[0019] The technical result of the present invention is the optimization of a high-performance technology for producing hot-rolled products with increased corrosion resistance and cold resistance while maintaining a high level of strength and ductility.

[0020] The said technical result is achieved by the fact that in the method for producing hot-rolled sheet metal for automobile wheels, which includes smelting, ladle treatment, continuous casting, heating the slab for hot rolling, rolling it in roughing and finishing continuous groups of stands of a wide-strip mill to obtain a strip, cooling the strip with water on a discharge roller table with subsequent winding into a roll, according to the invention, steel is smelted containing, in wt. %:

[0021]

[0022] The finishing temperature of rolling in the roughing group of stands is set at 1060-1120°C, hot rolling is finished at a temperature of 810-860°C, while the temperature of winding hot-rolled strips into rolls is determined depending on the thickness of the rolled product in accordance with equation (1):

[0023]

[0024] where T см - temperature of hot-rolled strips coiling, °C,

[0025] 705 - empirical coefficient, °C,

[0026] 10 - empirical coefficient, °C / mm,

[0027] τ - rolled product thickness, mm.

[0028] The essence of the invention lies in the fact that ensuring the required level of corrosion resistance and mechanical characteristics of hot-rolled products is achieved by using a certain chemical composition and hot rolling modes. A necessary condition for ensuring the required set of properties is to maintain a certain content of the main elements influencing the properties, wt. %: C - 0.05-0.10, Si - 0.03-0.20, Mn - 0.35-1.10, P - 0.004-0.016, S - 0.001-0.015, Al - 0.02-0.06, Nb - 0.010-0.035, Ti - 0.010-0.040, Fe and inevitable impurities - the rest. The lower limit of the content of elements such as carbon and manganese is determined by the need to ensure the required strength. Exceeding the upper limit of these elements' content leads to a decrease in ductility. Ensuring an aluminum content of at least 0.02% in steel ensures a high degree of deoxidation.Maintaining an aluminum content of no more than 0.06% in steel prevents the formation of excessive non-metallic inclusions with a high corundum content, which reduce the steel's corrosion resistance. Limiting the silicon content in steel to no more than 0.20% is necessary to ensure a high level of corrosion resistance. Exceeding this silicon content limit negatively impacts the steel's resistance to atmospheric corrosion, due to silicon's influence on carbon activity and, consequently, on the steel's susceptibility to aging. Reducing the silicon content below 0.03% does not further improve the steel's corrosion resistance, but requires additional manufacturing techniques and is not economically feasible.

[0029] The upper limit of sulfur content is determined by the need to ensure the required ductility. The lower limit of sulfur content is determined by the need to prevent the steel's tendency to ageing and to ensure its high corrosion resistance, since a sulfur content below 0.001% does not produce sufficient amounts of manganese sulfides, which act as substrates for the precipitation of aluminum nitride and ensure a reduction in the nitrogen content in the solid solution and the steel's tendency to ageing. The lower limit of the content of microalloying elements such as niobium and titanium is determined by the need to achieve the required strength properties, as well as impact toughness and cold resistance characteristics, due to ferrite grain refinement and precipitation hardening. However, niobium is part of nanoscale precipitates of excess phases, the increased content of which reduces corrosion resistance, and therefore it is advisable to limit the upper limit of niobium content to 0.035%.Moreover, titanium is predominantly involved in the formation of submicron-sized, rather than nanoscale, precipitates of its carbonitride, which leads to a reduction in the proportion of nanoscale precipitates that negatively impact corrosion resistance. Therefore, it is sufficient to limit the upper limit of titanium content to 0.040%. Higher titanium content may reduce ductility.

[0030] The end of rolling in the roughing group of stands at a temperature of 1060-1120°C is determined by the need to form a homogeneous structure throughout the hot-rolled product, ensuring high strength, toughness, cold resistance, and ductility. At lower rolling end temperatures in the roughing group (rolling temperature), the mechanical properties and corrosion resistance of the steel decrease due to the precipitation of large quantities of excess carbonitride phases during rolling in the roughing group, primarily in the surface layers of the metal. This promotes grain refinement in these areas, but also leads to a decrease in the carbon and nitrogen content of the solid solution. The resulting concentration gradient of interstitial impurities in the solid solution across the thickness of the rolled product causes its diffusion to the surface from the central zones, where a coarse-grained structure is formed.As the rolling temperature decreases, the period during which, due to the resulting concentration gradient in the solid solution across the thickness of the rolled product, diffusion redistribution of elements from the axial zone to the surface increases (including the time the rolled product remains on the intermediate roller table, as well as the subsequent rolling in the finishing stand). This leads to the formation of heterogeneity in the chemical composition and structure across the thickness of the rolled product, which negatively impacts the steel's properties.

[0031] Increasing the rolling temperature above the specified values ​​also leads to a reduction in mechanical properties. High temperatures at the end of rolling in the roughing group of stands are inherited at the beginning of rolling in the finishing group. This reduces the number and increases the size of the formed carbonitride precipitates, leading to grain coarsening and a reduction in the strength and toughness of the steel. Furthermore, the uneven formation of such precipitates across the strip thickness can lead to structural inhomogeneity, particularly across the thickness of the sheet metal, which reduces ductility and cold resistance.

[0032] Completion of hot rolling at a temperature of 810-860°C and winding of strips into rolls at a temperature determined depending on the thickness of the rolled product in accordance with the equation: T см=(705-10τ) ±20°C, leads to the formation of a homogeneous and dispersed microstructure of hot-rolled sheet metal, which ensures the required level of cold and corrosion resistance. Completing rolling and coiling strips at lower temperatures leads to the formation of a large number of nanoscale particles of excess phases in the metal, which negatively impact impact toughness and reduce the corrosion resistance of the steel. Completing rolling and coiling strips at higher temperatures leads to increased grain size variation and structural inhomogeneity across the thickness of the rolled product, as well as an increased number of interphase particles, which reduces the strength, ductility, and toughness of the steel.

[0033] Examples of the invention implementation

[0034] Steels of two chemical compositions were obtained by laboratory melting in a vacuum induction furnace. Table 1 lists the content of the main chemical elements for steels of each chemical composition.

[0035]

[0036] A total of 7 ingots of each steel with chemical composition A and B were obtained. Steel B's chemical composition corresponded to the invention formula. Steel A had a higher niobium content.

[0037] Hot rolling of the obtained ingots to a thickness of 3.5 mm was carried out according to the following regime: heating temperature of 1200°C, the temperature of the end of rolling in the roughing and finishing groups of stands is presented in Table 2. After the end of rolling, the strip was cooled to a temperature of T см and then kept in an oven heated to the same temperature for 1 hour, followed by cooling in the oven (simulating the cooling of a rolled roll).

[0038] To determine the corrosion resistance of steel, the alternating immersion method was used. This method involves cyclically immersing metal samples in a 2% NaCl aqueous solution, soaking them in the solution for 10 minutes, and then exposing them to air for 50 minutes. The change in sample mass per unit area of ​​the working surface is then assessed. The corrosion resistance of steel is assessed by the specific weight gain (weight increase) of the samples during testing. This value characterizes the amount of corrosion products formed during testing: higher specific weight gain values ​​correspond to lower corrosion resistance of the steel.

[0039] The results of mechanical and corrosion tests of steel after hot rolling under various conditions, corresponding and not corresponding to the formula of the invention, in order to verify the possibility of achieving the declared technical result are given in Table 2. Table 2 also shows the ranges of values ​​of the temperature of the end of rolling in the roughing group of stands (rolling temperature) T раската , temperature of the end of rolling T кп and the temperature of coiling hot-rolled strips into rolls T см , corresponding to the formula of the invention, and the prospective requirements of automobile manufacturing enterprises for the mechanical properties and corrosion resistance of automotive sheet steels, indicated in Table PT.

[0040] The table highlights the values ​​of process parameters that do not correspond to the claims. In addition, the table highlights the values ​​of mechanical properties that do not correspond to the PT - relative elongation less than 30%, yield strength and tensile strength values ​​less than 360 MPa and 440 MPa, respectively, impact toughness values ​​at a test temperature of -60 °C less than 200 J / cm 2 A corrosion rate of more than 0.30 mm / year was considered unsatisfactory for corrosion resistance.

[0041] For steel of composition A, which has an increased niobium content, despite the lower content of manganese, aluminum and titanium and, under other equal conditions (similar processing temperature parameters), higher corrosion rate values ​​were obtained that do not correspond to the presented PT (modes A1-A7).

[0042] The strength, ductility, cold resistance and corrosion resistance indicators corresponding to the properties of the prototype and the claimed technical result are achieved by processing steel samples of variant B according to the mode corresponding to the invention formula (mode B1).

[0043] Decrease as well as increase in temperature T раската (modes B2, B3), leads to a decrease in the plasticity index below the properties of the PT.

[0044] Temperature decrease T кп (mode B4) leads to a decrease in corrosion resistance below the properties of the PT. An increase in temperature T кп (mode B5) leads to a decrease in the strength indicator below the properties of the prototype, as well as a decrease in the cold resistance value below the properties of the PT.

[0045] For the steel under study, with decreasing temperature T см , in relation to the intervals calculated using equation (1), plasticity and impact toughness decrease (mode B6). With increasing temperature T см, in relation to the intervals calculated using equation (1), the plasticity and corrosion resistance indicators corresponding to the properties of PT (mode B7) are achieved, but the strength properties are not achieved.

[0046] Thus, on samples of hot-rolled steel of the stated composition, the required ND set of properties is ensured when the requirements for the rolling production mode set out in the invention formula are met.

[0047]

Claims

A method for producing hot-rolled sheet metal for automobile wheels, including smelting, ladle treatment, continuous casting, heating the slab for hot rolling, rolling it in roughing and finishing continuous groups of stands of a wide-strip mill to obtain a strip, cooling the strip with water on a discharge roller table, followed by winding it into a roll, characterized in that steel is smelted containing, by weight %: carbon 0,05-0,10 silicon 0,03-0,20 manganese 0,35-1,10 phosphorus 0,004-0,016 sulfur 0,001-0,015 aluminum 0,02-0,06 niobium 0,010-0,035 titanium 0,010-0,040 iron and inevitable impurities rest, the finishing temperature of rolling in the roughing group of stands is set at 1060-1120°C, hot rolling is finished at a temperature of 810-860°C, while the temperature of winding hot-rolled strips into rolls is determined depending on the thickness of the rolled product in accordance with equation (1): T см = (705-10τ)±20°С, (1) where T см - temperature of hot-rolled strips coiling, °C, 705 - empirical coefficient, °C, 10 - empirical coefficient, °C / mm, τ – thickness of rolled product, mm.

Images