Method for producing hot-rolled sheet steel with high strength, corrosion resistance and cold resistance

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to the production of hot-rolled products from microalloyed steel for the automotive industry. The method includes smelting steel, its secondary processing, continuous casting into slabs, heating the slabs for hot rolling to produce strips, cooling the strips with water on a discharge roller table, and then winding them into a roll. The steel produced comprises, by weight: carbon 0.05-0.12, silicon no more than 0.25, manganese 0.35-1.10, phosphorus no more than 0.020, sulphur 0.001-0.015, aluminium 0.02-0.07, titanium 0.04-0.09, the rest is iron and inevitable impurities. Hot rolling is completed at a temperature of 800-870 °C. The hot-rolled strips are wound into rolls at a temperature determined depending on the thickness of the rolled product in accordance with the equation Tcm = (685-10τ)± 20 °C, where Tcm is the temperature of winding hot-rolled strips into rolls, °C, 705 - empirical coefficient, °C, 10 - empirical coefficient, °S / mm, τ - rolled product thickness, mm.EFFECT: production of steel with a corrosion rate of no more than 0.30 mm / year, the required level of cold resistance, as well as strength and plastic properties.1 cl, 2 tbl

Description

[0001] The invention relates to metallurgy, specifically to hot-rolled sheet metal technology, and can be used to produce hot-rolled microalloyed steel sheets with increased strength, corrosion resistance, and cold resistance. Currently, there is a growing trend toward improving the operational safety of hot-rolled steel products through the use of high-strength sheet metal. At the same time, it is important to ensure high ductility, corrosion resistance, and cold resistance in hot-rolled sheet metal due to the potential use of products made from it in extreme conditions during operation at low temperatures.

[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] carbon 0,045-0,12 silicon no more than 0.50 manganese 0,35-1,15 aluminum 0,01-0,09 nitrogen no more than 0.010 niobium and / or titanium 0.01-0.08 each iron and inevitable impurities rest,

[0004] 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.%. The 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: [C] = (0.0002⋅K пр +0.002)±0.02, wt. %, where 0.0002 and 0.002 are empirical coefficients, %, K пр - 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: [Mn] = (0.0022⋅K пр -0.15)±0.20, wt. %, where 0.0022 and 0.15 are empirical coefficients, %, K пр- 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 relationship: [Nb+Ti+V]=(0.0002⋅K пр -0.013)±0.03, wt. %, where 0.0002 and 0.013 are empirical coefficients, %, K пр - a dimensionless indicator numerically equal to the required minimum yield strength.

[0005] (Patent RU2361930, IPC C21D 8 / 04, B21 B 1 / 46, C22C 38 / 06, published 07 / 20 / 2009).

[0006] 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.

[0007] 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. %:

[0008] carbon 0,10-0,20 silicon 0,10-0,50 manganese 1,15-1,45 sulfur 0.010 max. phosphorus 0.015 max. chromium 0.10 max. nickel 0,15-0,25 copper 0,15-0,25 aluminum 0,020-0,050 niobium 0,05-0,08 vanadium 0,03-0,05 titanium 0,010-0,025 iron rest,

[0009] 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: V вepx =-3.4⋅ln(h ср )+l1.5, where V вepх - cooling rate of the upper surface of the strip, deg / s, h cp- 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.

[0010] (Patent RU 2450061, IPC C21D 8 / 04, C22C 38 / 20, B21B 1 / 46, published 10.05.2012).

[0011] This method produces hot-rolled steel with high ductility and formability, but with insufficient resistance to atmospheric corrosion. The strength of such steel may be insufficient, leading to reduced performance of finished products. Furthermore, the increased niobium content in this steel may increase the cost of the rolled metal.

[0012] The closest analogue of the claimed invention is a method for producing high-strength hot-rolled steel, including smelting steel, casting slabs, hot rolling the slabs into strips, cooling with water, winding the strips into rolls. According to the invention, steel is smelted containing the following components, wt. %:

[0013] carbon 0,04-0,09 silicon no more than 0.30 manganese 0,41-0,70 phosphorus 0,04-0,12 aluminum 0,01-0,08 nitrogen no more than 0.009 iron and inevitable impurities rest

[0014] The steel additionally contains 0.01-0.03 wt.% titanium and / or 0.0008-0.0030 wt.% boron. Hot rolling is carried out with a rolling end temperature of 800-890°C, and hot-rolled strips are coiled at a temperature of 500-610°C.

[0015] (Patent RU2361932, IPC C21D 8 / 04, C21D 9 / 48, C21D 38 / 04, published 07 / 20 / 2009).

[0016] This method produces hot-rolled products with increased strength while maintaining high ductility and formability (yield strength greater than 340 MPa, ultimate tensile strength greater than 440 MPa, and relative elongation of at least 30%). However, this method does not produce hot-rolled products with sufficiently high corrosion and cold resistance.

[0017] The technical result of the present invention is the optimization of a high-performance technology for producing hot-rolled sheet steel with improved corrosion resistance and cold resistance while maintaining a high level of strength and ductility.

[0018] The said technical result is achieved by the fact that in the method for producing hot-rolled sheet steel of increased strength, corrosion resistance and cold resistance, including steel smelting, casting, hot rolling to obtain hot-rolled strips, cooling with water and winding the strips into rolls, according to the invention, steel is smelted containing, by weight %:

[0019] carbon 0,05-0,12 silicon no more than 0.25 manganese 0,35-1,10 phosphorus no more than 0.020 sulfur 0,001-0,015 aluminum й0.02-0.07 titanium 0,04-0,09 iron and inevitable impurities rest,

[0020] Hot rolling is completed at a temperature of 800-870°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)

[0021]

[0022] where T см - temperature of hot-rolled strips coiling, °C,

[0023] 685 - empirical coefficient, °C,

[0024] 10 - empirical coefficient, °C / mm,

[0025] τ - rolled product thickness, mm.

[0026] 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 that affect the properties, wt. %: C - 0.05-0.12, Si - no more than 0.25, Mn - 0.35-1.10, P no more than 0.020, S - 0.001-0.015, Al - 0.02-0.07, Ti - 0.04-0.09, Fe and inevitable impurities - the rest. The lower limit of the content of such elements as carbon, manganese and titanium is determined by the need to ensure the required strength. Exceeding the upper limit of the content of these elements leads to a decrease in ductility. Ensuring an aluminum content of at least 0.02% in steel guarantees a high degree of steel deoxidation.Maintaining an aluminum content of no more than 0.07% in steel prevents the formation of excessive amounts of 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 0.25% is determined by the need 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. The upper limit for sulfur content is determined by the need to ensure the required ductility. The lower limit for sulfur content is determined by the need to prevent the steel's susceptibility to aging and to ensure its high corrosion resistance, asSulfur contents below 0.001% do not produce sufficient manganese sulfides, which act as substrates for aluminum nitride deposition and reduce the nitrogen content of steel. The lower limit for microalloying elements such as titanium is determined by the need to achieve the required strength properties through ferrite grain refinement and precipitation hardening. Titanium primarily forms submicron-sized, rather than nanoscale, precipitates of its carbonitride, which reduces the proportion of nanoscale precipitates that negatively impact corrosion resistance. Therefore, limiting the upper limit of titanium content to 0.09% is sufficient.

[0027] Finishing hot rolling at 800-870°C ensures the required level of cold resistance, corrosion resistance, and strength characteristics by forming a homogeneous and dispersed microstructure in hot-rolled sheet steel. Exceeding the recommended finishing temperature range results in grain growth, which negatively impacts strength and cold resistance. Finishing hot rolling at lower temperatures results in the formation of large quantities of nanoscale particles of excess phases, which negatively impact corrosion resistance.

[0028] Temperature of winding strips into rolls at a temperature determined depending on the thickness of the rolled product in accordance with the equation: T см=(685-10τ)±20°C ensures the required level of cold and corrosion resistance. Coiling strips at higher temperatures leads to the formation of a large number of interphase particles, which negatively impact the cold and corrosion resistance of hot-rolled sheet steel. Coiling strips at lower temperatures leads to the formation of a large number of excess phase particles in the metal, which negatively impact impact toughness and reduce the steel's corrosion resistance.

[0029] Examples of the invention implementation

[0030] Steels of two chemical compositions were obtained by laboratory melting in a vacuum induction furnace. Table 1 shows the content of the main chemical elements for steels of each chemical composition.

[0031]

[0032] A total of 7 ingots of each steel with chemical composition A and B were obtained. Steel B fully complied with the invention formula in terms of chemical composition. Steel A had a lower titanium content.

[0033] 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).

[0034] 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.

[0035] 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 end rolling temperature 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.

[0036] 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.

[0037] For steel of composition A, which has a reduced content of titanium, manganese and aluminum 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).

[0038] The strength, plasticity, 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 formula of the invention (modes B1 and B7).

[0039] Lowering temperature T кп (mode B5) leads to a decrease in corrosion resistance and a decrease in the cold resistance index below the properties of PT. An increase in temperature T кп (mode B4) leads to a decrease in the strength indicator below the properties of PT.

[0040] When the temperature T decreases см , compared to the declared values, plasticity and cold resistance index decrease (mode B2). With increasing temperature T см , in comparison with the declared values, plasticity and corrosion resistance indicators corresponding to the properties of PT (mode B3) are achieved, but strength properties are not achieved.

[0041] 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.

[0042]

Claims

A method for producing hot-rolled sheet steel of increased strength, corrosion resistance and cold resistance, including smelting steel, casting, hot rolling to produce hot-rolled strips, cooling with water, and winding the strips into rolls, characterized in that steel is smelted containing, by weight %: carbon 0,05-0,12 silicon no more than 0.25 manganese 0,35-1,10 phosphorus no more than 0.020 sulfur 0,001-0,015 aluminum 0,02-0,07 titanium 0,04-0,09 iron and inevitable impurities rest, Hot rolling is completed at a temperature of 800-870°C, while the temperature for winding hot-rolled strips into rolls is determined depending on the thickness of the rolled product in accordance with equation (1) where T см - temperature of hot-rolled strips coiling, °C, 685 - empirical coefficient, °C, 10 - empirical coefficient, °C / mm, τ – thickness of rolled product, mm.

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