RU2796666C1 - Method for production of hot-rolled steel strips - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention can be used in production of hot-rolled steel with a two-phase ferrite-martensite structure, intended for production of machine parts and mechanisms with a guarantee of manufacturing products by cold deformation. Method for production of hot-rolled steel strip includes steel smelting, continuous casting into slabs, heating of slabs, hot rolling, cooling and winding of strips into coils. Steel is smelted containing, wt.%: carbon 0.04-0.09, silicon no more than 0.3, manganese no more than 1.5, the total content of chromium and molybdenum no more than 1.0, iron and inevitable impurities - the rest, moreover, the ratio of silicon to phosphorus in the smelted steel is 3-7. The end temperature of hot rolling is maintained in the range of 800-870°C, cooling is carried out for 5-25 s at a speed of 8-30°C/s up to a temperature of 650-800°C and then at a rate of 40-80°C/s up to a coiling temperature of not more than 300°C, with formation in the strips of a structure containing 70-90% ferrite and 10-30% martensite or martensite and untransformed austenite.

EFFECT: increase of yield of hot-rolled steel with a two-phase ferrite-martensite structure is provided by increasing the complex of mechanical properties: the anisotropy and suitability of the metal for further processing are improved.

2 cl, 2 tbl, 1 ex

Description

The invention relates to the field of metallurgy and can be used in the production of hot-rolled steel strips intended for the production of machine parts and mechanisms with a guarantee of manufacturing products by cold deformation (for example, cold stamping) and coating at the consumer (painting).

A method is known for the production of high-strength multi-phase steel with a minimum tensile strength of 580 MPa, mainly with a two-phase structure for a hot-rolled steel strip, consisting of elements in wt.%: carbon 0.075 to ≤ 0.105; silicon 0.200 to ≤ 0.300; manganese 1,000 to ≤ 2,000; chromium 0.280 to ≤ 0.480; aluminum 0.010 to ≤ 0.060; phosphorus ≤0.020; niobium ≥0.005 to ≤ 0.025; nitrogen ≤0.0100; sulfur ≤0.0050; the rest is iron, including the usual accompanying steels not listed above. The method includes continuous annealing to create a two-phase structure, wherein the hot-rolled steel strip is heated in a continuous annealing furnace to a temperature in the range of 700-950°C and said heated steel strip is cooled from the annealing temperature to an intermediate temperature of 200-250°C with a cooling rate of 15- 100°C/s, and then cooled in air to ambient temperature with a cooling rate of 2-30°C/s (RF Patent 2615957, publ. 10.05.2016, IPC C21D 8/02, C21D 9/46, C22C 38/00, C23C 2/06).

The disadvantage of the known method is the mandatory use of an additional annealing operation, which in turn leads to the economic inexpediency of using this method, as well as the presence of an unacceptable defect on the surface of the rolled product in the form of "tiger" (red) scale.

The closest in technical essence to the proposed invention is a method for the production of hot-rolled high-strength two-phase steel containing, wt.%: 0.06-0.09 carbon, 0.8-2.0 manganese, not less than 0.4 chromium, not more than 0 0.08 silicon, no more than 0.05 phosphorus, 0.005-0.010 nitrogen, the rest of the iron are inevitable impurities, while the Al / N ratio is not more than 10 and the total content of niobium, titanium and molybdenum is not more than 0.015. The slab is heated at a temperature of 1150-1300°C, hot rolling is carried out with a temperature of the end of rolling of 800-850°C, then the strip is cooled at a rate of 40-70°C/sec and to a coiling temperature below 100°C, while the strip has a structure ferrite plus martensite, free of pearlite and bainite, volume fraction of martensite 10-25%. The hot-rolled strip has a tensile strength of at least 590 MPa and an elongation of at least 17.5% (International application WO2014149505, publ. 25.09.2014, IPC C21D 8/02, B21B 1/24).

The disadvantage of this method is the low yield of suitable hot-rolled steel due to the resulting high anisotropy of properties, unsatisfactory quality in terms of flatness of the strip, warping of rolled metal, high springback of the metal during winding and further processing during cold stamping.

The technical result of the invention is to increase the yield of suitable hot-rolled strips by increasing the complex of mechanical properties: improving the anisotropy and suitability of the strips for further processing.

The technical result is achieved by the fact that in the method for the production of hot-rolled steel strips, including steel smelting, continuous casting into slabs, heating of slabs, hot rolling, cooling and winding of strips into coils, according to the invention, steel is smelted containing, wt.%:

carbon 0.04-0.09 silicon no more than 0.3 manganese no more than 1.5 total content of chromium and molybdenum no more than 1.0 iron and inevitable impurities rest,

moreover, the ratio of silicon to phosphorus in the steel being cast is 3-7, the temperature of the end of hot rolling is maintained in the range of 800-870 ° C, cooling is carried out for 5-25 s at a rate of 8-30 ° C / s to a temperature of 650-800 ° C, and then at a speed of 40-80 ° C / s to a coiling temperature of not more than 300 ° C, ensuring the formation in the strips of a structure containing 70-90% ferrite and 10-30% martensite or martensite and untransformed austenite.

After winding, pickling of hot-rolled strips is carried out.

The essence of the invention is as follows.

Carbon is one of the hardening elements. The carbon content of less than 0.04% does not allow to achieve the required strength. At the same time, an increase in the carbon content of more than 0.09% leads to uneven properties across its thickness as a result of zonal segregation and, when the steel is cooled from the γ-region, it shifts the ferrite “nose” to the right, thus reducing the amount of polygonal ferrite and increasing the amount of low-carbon bainite ( or acicular ferrite). At the same time, the proportion of the γ-phase increases by the time of winding, which increases the strength and reduces the ductility of the steel below the norm.

The presence of silicon contributes to the improvement of steel deoxidation and significantly accelerates the precipitation of polygonal ferrite in low-alloy steels. At the same time, an increase in the silicon content of more than 0.3% is accompanied by an increase in coarse "tiger" scale on the surface of the rolled products and contributes to an increase in the amount of silicate inclusions.

Manganese provides solid solution strengthening. A manganese content of more than 1.5% impairs weldability and leads to coarse zonal segregation.

The total content of chromium and molybdenum over 1.0% reduces the ductility of steel, and is also not economically feasible.

The ratio of silicon to phosphorus in the range of 3-7 is necessary to eliminate the "tiger" (red) scale on the surface of the rolled products.

The end of rolling at a temperature of 800-870 °C is necessary for maximum grain refinement.

Cooling for 5-25 sec. at a rate of 8-30°C/s to a temperature of 650-800°C is required for the phase transformation to occur during cooling on the discharge roller table in order to isolate the required amount of ferrite 70-90%.

With further cooling from a temperature of 650-800 °C at a rate of 40-80 °C/s to a coiling temperature of no more than 300 °C, untransformed austenite enriched in carbon turns into martensite or/and m/a-phase 10-30%. When the coiling temperature is above 300 °C and/or the cooling rate drops below 40-80 °C/s, bainite (or perlite) appears in the structure, the strength properties decrease, and the formability deteriorates.

An example of the implementation of the method.

In an oxygen converter, steels were smelted, the chemical composition of which is shown in Table 1. The smelted steel was cast into slabs by a continuous casting machine. The slabs were heated in a walking beam heating furnace and rolled on a 2000 continuous wide strip mill. The cooled rolls were subjected to hydrochloric acid pickling in a continuous pickling unit. Technological parameters and mechanical properties of rolled products are shown in Table 2.

From table 2 it follows that when implementing the claimed production method (option No. 3), an increase in yield is achieved by increasing the complex of mechanical

Table 1
Chemical composition of steel option number The content of chemical elements, wt.% WITH Si Mn Cr+Mo Fe and unavoidable impurities Si/P ratio 1 0.12 0.40 1.72 1.5 rest 26 2 0.11 0.35 1.50 1.0 rest 9 3 0.07 0.25 1.0 0.8 rest 6 4 0.02 0.18 0.8 0.5 rest 3 5 0.02 0.16 0.5 0.4 rest 2

table 2
Technological parameters and mechanical properties of rolled products option number Production modes Structure Mechanical properties Finish rolling end temperature, °C Cooling time, s Cooling rate to a temperature of 650-800 °С, °С/s Cooling rate to coiling temperature not more than 300°С, °С/s Winding temperature, °С Ferrite content, % Content of martensite and/or m/a-phase, % Tensile strength, MPa Relative extension, % 1 900 thirty 4 100 400 65 -
(bainite 35%) 540 18 2 880 25 6 90 350 70 -
(bainite 30%) 570 19 3 820 15 10 70 200 80 20 640 25 4 780 10 5 75 200 97 3 510 27 5 760 8 3 74 200 98 2 490 29

Claims (4)

1. A method for the production of hot-rolled steel strips, including steel smelting, continuous casting into slabs, heating of slabs, hot rolling, cooling and coiling of strips into coils, characterized in that steel is smelted containing, wt.%: carbon 0.04-0.09 silicon no more than 0.3 manganese no more than 1.5 total content of chromium and molybdenum no more than 1.0 iron and the inevitable rest, moreover, the ratio of silicon to phosphorus in the steel being smelted is 3-7, the temperature of the end of hot rolling is maintained in the range of 800-870 ° C, cooling is carried out for 5-25 s at a rate of 8-30 ° C / s to a temperature of 650-800 ° C , and then at a speed of 40-80 °C / s to a coiling temperature of not more than 300 °C, ensuring the formation in the strips of a structure containing 70-90% ferrite and 10-30% martensite or martensite and untransformed austenite. 2. The method according to claim 1, characterized in that after winding, pickling of hot-rolled strips is carried out.