RU2633858C1 - Method for producing cold-rolled two-phase ferrite-martensite automobile body sheet steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method includes steel melting containing, wt %: C 0.10-0.15, Si 0.10 -0.40, Mn 1.8-2.4, Cr 0.20-0.40, Mo 0.10-0.40, Al 0.02-0.08, P no more than 0.02, S no more than 0.02, Fe and unavoidable impurities, hot rolling at beginning temperature from 1050 to 1200°C and the ending at 800-890°C, coiling the sheet into a roll at 580-650°C, cold rolling with total reduction of 45-70% for thickness 0.9-1.5 mm and heat treatment in continuous-action unit by heating to annealing temperature 730-790°C, soaking, slow cooling to temperatures below A_r1, accelerated cooling to 250-330°C and ageing at said temperature. The sheet is moved in the unit at a speed provided that: V_smr=[(T_an-680°C/k-10m/min]÷[(T_an-680°C/k+10 m/min], where V_smr- strip movement rate, m/min, k=1×min×°C/m, T_an - annealing temperature, °C.

EFFECT: provision of required level of BH-effect and capacity to crush the hole with preservation of mechanical properties, with strength class of 780 mpa of ferrite-martensite steel.

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Description

The invention relates to the field of metallurgy, and in particular to methods for the production of cold-rolled high-strength sheet two-phase ferritic-martensitic steels that can be used in the automotive industry.

The main requirements for the developed steel include: tensile strength (σ _in ) not less than 780 MPa, yield strength (σ _t ) in the range of 440-550 MPa, elongation (δ) not less than 14%, VN effect (VN) not below 30 MPa. Also important for car manufacturers is such a parameter as the ability to distribute holes (1), which characterizes the ability of the car to conduct cold stamping without defects.

A known method of manufacturing a cold or hot rolled strip of two-phase steel with increased strength and high deformability, intended, in particular, for cars with a lightweight structure containing the following elements, wt.%: Carbon from 0.1 to 0.16, aluminum from 0.02 to 0.05, silicon from 0.40 to 0.60, manganese 1.5 to 2.0, phosphorus less than or equal to 0.020, sulfur less than or equal to 0.003, nitrogen less than or equal to 0.01, niobium 0, 01-0.04, vanadium 0.02-0.08, the rest is iron and the accompanying elements inherent in steel, as well as the optimal addition of titanium to 0, 01, wherein a two-phase structure is formed during continuous annealing, characterized in that the cold or hot rolled steel strip is heated in a continuous furnace in one step to a temperature of from 820 to 1000 ° C, preferably from 840 to 1000 ° C, then the annealed steel strip is cooled from the annealing temperature at a speed of 15 to 30 ° C / s. The method provides uniform mechanical and technological properties in the manufacture of tapes with varying thicknesses along the length and width (Patent RU 2443787, IPC C21D 8/02, C21D 9/46, C22C 38/12, published 02.27.2012.)

The disadvantage of this method is that the heat treatment is carried out to temperatures significantly higher than A _c3 , which can lead to the formation of a heterogeneous structure and, therefore, to anisotropy of properties, there is also no tempering of the rolled products after accelerated cooling, which negatively affects such ductility indicators as relative elongation and ability to distribute holes.

A known method for the production of cold rolled sheets of two-phase steel containing in wt. %: 0,055≤С≤0,095, 2≤Mn≤2,6, 0,005≤Si≤0,35, S≤0,005, Р≤0,050, 0,1≤Al≤0,3, 0,05≤Mo≤0, 2, 0.2≤Cr≤0.5, provided that Cr + 2Mo≤0.6, Ni <0.1, 0.010≤Nb≤0.040, 0.010≤Ti≤0.050, 0.0005≤В≤0, 0025, 0.002≤N≤0.007, the rest is iron and unavoidable impurities, cast the semifinished product, heat it to 1150 ° C≤T _R ≤1250 ° C and hot rolling at the temperature of the end of rolling T _FL ≤Ar3, and then roll it at a temperature within 500 ° C≤T _bob ≤570 ° C. It is cleaned of scale and cold rolled during compression from 30 to 80%. The cold-rolled semi-finished product is heated at a speed of 1 ° C / s≤VC≤5 ° C / s to the annealing temperature Tm, defined as Ac1 + 40 ° C≤Tm≤Ac3-30 ° C / s, at which it is kept for 30 seconds≤ tm≤300 sec for the formation of a structure containing austenite, and then cooled to a temperature below Ms at a speed V high enough to convert the entire amount of austenite to martensite. The resulting sheets have good forming ability, especially good bending ability, while ensuring steel strength from 980 to 1100 MPa and elongation at break above 9%. (Patent RU 2470087, IPC C22C 38/58, C21D 8/02, published December 20, 2012.)

The disadvantage of this method is that the winding during hot rolling in the specified temperature range will lead to the fact that niobium carbonitrides will not be released during winding. Also, tempering after accelerated cooling is not applied, which negatively affects such ductility indicators as elongation and ability to distribute holes.

The closest analogue of the claimed invention is a method for producing high-strength cold-rolled steel sheet with a tensile strength of 780 MPa or more. The method includes obtaining slabs from steel containing, by weight. %: C: 0.05-0.09, Si: 0.4-1.3, Mn: 2.5-3.2, P: 0.001-0.05, N: 0.0005-0.006, Al: 0.005-0.1, Ti: 0.001-0.045, S in the range determined by expression (A), the rest is Fe and inevitable impurities. The steel may further comprise, by weight. %: Nb 0.001-0.04, B 0.0002-0.0015, Mo 0.05-0.50, Ca 0.0003-0.01, Mg 0.0002-0.01, REM 0.0002- 0.01, Cu 0.2-2.0, Ni 0.05-2.0. The slabs are placed in a reheating furnace in a high temperature state or after cooling to room temperature, heated in a temperature range from 1150 to 1250 ° C, then subjected to finish rolling in a temperature range from 800 to 950 ° C and cooled to a temperature of 700 ° C or lower and as a result, hot-rolled steel sheets are obtained which are subjected to pickling, cold rolling and annealing at a temperature of from 700 ° C to less than 900 ° C. The microstructure of the resulting steel sheet consists of 7% or more bainite, and the rest is ferrite, martensite, hardened martensite and residual austenite, or a combination thereof. The sheet has high strength and good weldability. (Patent RU 2312163, IPC C22C 38/04, published December 10, 2007, description, prototype.)

The disadvantage of the prototype method is the lack of such important indicators of mechanical properties as the HV effect and the ability to distribute holes.

The technical result of the present invention is to obtain the necessary level of HV effect and the ability to distribute holes while maintaining a set of mechanical properties inherent in the strength class of 780 MPa of two-phase ferritomartensitic steel.

The specified technical result is achieved by the fact that in the method for the production of cold-rolled two-phase ferritic-martensitic sheet steel, including hot rolling, cold rolling to a thickness of 0.9-1.5 mm and heat treatment in a continuous unit according to the mode, consisting of heating to annealing temperature , aging, delayed cooling to temperatures below A _r1 , accelerated cooling and overcooking, according to the invention, hot rolling begins in the temperature range from 1050 to 1200 ° C and ends in the temperature and in the range of 800-890 ° C, the temperature of the coil in a roll of 580-650 ° C, cold rolling is carried out with a total compression of 45-70%, heat treatment is carried out at an annealing temperature of 730-790 ° C, the end of accelerated cooling and overcooking is carried out at temperatures of 250- 330 ° C, while the steel contains the following components, wt. %:

Carbon 0.10-0.15

Silicon 0.10-0.40

Manganese 1.8-2.4

Chrome 0.20 0.40

Molybdenum 0.10-0.40

Aluminum 0.02-0.08

Phosphorus no more than 0.02

Sulfur no more than 0,02

Iron and inevitable impurities rest.

the speed of the strip in the unit is set depending on the annealing temperature in accordance with the condition

V _{double floor} = [(T _oil -680 ° C) / k-10 m / min] ÷ [(T _oil -680 ° C) / k + 10 m / min],

where V _dv.pol - the speed of the strip in the unit, m / min,

k = 1 × min × ° C / m,

T _anne - annealing temperature, ° C.

The essence of the invention lies in the fact that the provision of the necessary complex of mechanical properties, including tensile strength, yield strength, elongation, VN effect, the ability to distribute holes is achieved using a certain chemical composition and method for producing rolled two-phase ferritomartensitic steel. The magnitude of the VN effect changes significantly with a change in the speed of the strip while maintaining a constant annealing temperature, so when the speed of the strip decreases, the level of the VN effect also decreases. There is also a relationship between the ability to distribute the hole and the speed of the strip, so as the speed of the strip increases, the ability to distribute the hole decreases. Therefore, it is important to ensure a balanced speed of the strip, which will allow the necessary level of mechanical properties to form.

At a hot rolling start temperature below 1050 ° C, the load on the stands of the hot rolling mill increases. If the temperature of the onset of hot rolling is higher than 1200 ° C, then an excessive growth of austenite grain occurs and this reduces the ductility of the final rolling.

Lowering the temperature of the end of rolling below 800 ° C leads to excessive grinding of the grain structure, which leads to increased values of yield strength. If the temperature of the end of rolling is higher than 890 ° C, then due to the high stability of austenite in the hot-rolled strip, bainite instead of perlite is formed, while the processability of cold rolling is reduced, and this also leads to the formation of more stable austenite during heat treatment, which in turn forms a large number of hardening phases and increases the yield strength.

Winding temperatures below 580 ° C also contribute to the formation of bainite, which leads to the formation of more stable austenite during heat treatment, which in turn forms a large amount of hardening phase and increases the yield strength.

Compression during cold rolling below 45% does not sufficiently crush the structure due to the formation of an insufficient number of nucleation centers of grains. With reductions above 70%, the dislocation density increases significantly, which leads to lower temperatures of the onset of recrystallization, an increase in grain size due to the development of collective recrystallization. All this greatly reduces the yield strength.

The annealing temperature below 730 ° C does not allow complete recrystallization to occur, resulting in a decrease in ductility and anisotropy of properties may form. At annealing temperatures above 790 ° C, the amount of the hardening phase also increases due to an increase in the volume fraction of austenite, which increases the yield strength.

Reducing the temperature of overcooking below 250 ° C does not allow the release of martensite, as a result of which ductility of rolled products is significantly reduced. If the over-temperature rises above 330 ° C, softening of the steel occurs due to the tempering process.

The content of carbon, silicon, manganese, chromium and molybdenum in predetermined intervals allows you to obtain the desired set of properties, including yield strength, tensile strength, elongation, VN effect, the ability to distribute holes. The decrease in the content of these elements below a predetermined interval significantly reduces the entire range of properties, in particular the tensile strength, forming a lower strength class. An increase in the content of these elements in turn leads to the formation of a higher strength class. If you change the content of one of these elements, then there is an imbalance leading to poorly predicted results on mechanical properties. The content in the specified interval of aluminum, sulfur and phosphorus can minimize the content in the structure of non-metallic inclusions.

Examples of specific performance of the method.

In a vacuum induction furnace obtained 3 heat with the chemical composition shown in table 1.

Hot rolling to a thickness of 3 mm was carried out according to the regime: the temperature of the start of rolling is 1120 ° C, the temperature of the end of rolling is T _cp = 830 ° C. After rolling, the strip was cooled to a temperature of T _cm = 630 ° C.

Cold rolling of strips with a thickness of 3 mm was carried out to a thickness of 1 mm (total reduction of 66%).

The heat treatment consisted of heating to annealing temperature, holding at this temperature, slow cooling to 680 ° C, accelerated cooling to a temperature of overcooking of 250 ° C, holding at this temperature and final cooling. In this case, the speed of the strip (table 2) in the continuous unit was determined based on the annealing temperature calculated by formula 1.

V _d.floor = [(T _oil -680 ° C) / k-10 m / min] ÷ [(T _oil -680 ° C) / k + 10 m / min],

Transverse samples were cut from the rolled products to determine the tensile strength. Tests were also conducted to determine the HV effect. The amount of hardening during drying (BH) was determined by the formula:

where σ _Tmin is the minimum value of tensile strength after stretching after deformation of 2% and holding at a temperature of 170 ° C for 20 minutes; σ ₂ - stress at strain 2%.

The obtained values of the mechanical properties are shown in table 3.

It is seen that the strength of melting 1 fully complies with the requirements for two-phase ferritomartensitic steels of strength class 780 MPa.

In heat 2, with an aluminum content above the declared limits, the tensile strength is much lower than the required level. A similar effect occurs due to the fact that aluminum strongly shifts critical points towards higher temperatures in steel, as a result of which more ferrite remains in the steel, which increases elongation, but reduces the tensile strength.

For melting 3 having a chemical composition corresponding to the claimed composition, the tensile strength was not obtained as a result of the fact that the speed of the strip was very low, as was the annealing temperature, as a result of which recrystallization occurred only partially.

Thus, it is shown that the combination of the claimed features (chemical composition, modes of hot and cold rolling, heat treatment within the limits indicated in the claims) provides for the production of two-phase ferritic martensitic steels with a favorable set of mechanical properties.

Claims (7)

Method for the production of cold-rolled sheet from two-phase ferritic-martensitic steel, including steel smelting, hot rolling, coiling, cold rolling to a thickness of 0.9-1.5 mm and heat treatment in a continuous unit by heating to annealing temperature, holding, slow cooling to temperatures below A _r1 , accelerated cooling and overcooking, characterized in that the smelting of steel containing, wt.%: carbon 0.10-0.15 silicon 0.10-0.40 manganese 1.8-2.4 chromium 0.20-0.40 molybdenum 0.10-0.40 aluminum 0.02-0.08 phosphorus no more than 0,02 sulfur no more than 0,02 iron and inevitable impurities rest, in this case, hot rolling starts in the temperature range from 1050 to 1200 ° C and ends at 800-890 ° C, winding into a roll is carried out at 580-650 ° C, cold rolling is carried out with a total compression of 45-70%, heating under annealing is carried out to temperatures of 730-790 ° C, accelerated cooling is carried out to a temperature of 250-330 ° C and at this temperature overcooking is carried out, and the speed of the strip in the unit is set in accordance with the condition: V _d.floor = [(T _oil -680 ° C) / k-10 m / min] ÷ [(T _oil -680 ° C) / k + 10 m / min], where V _tsv.pol - the speed of the strip in the unit, m / min, k = 1 × min × ° C / m, T _anne - annealing temperature, ° C.