RU2547087C1 - Method of production of higher-strength hot-rolled stock - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method involves making of steel containing the following components, wt %: carbon 0.03-0.12, silicon 0.10-0.50, manganese 1.5-2.0, sulphur max. 0.008, phosphorus max. 0.015, chrome 0.01-0.30, nickel 0.01-0.30, copper 0.01-0.30, aluminium 0.01-0.06, niobium 0.001-0.10, nitrogen 0.002-0.010, vanadium 0.001-0.10, titanium 0.001-0.10, molybdenum 0.005-0.30, calcium 0.0003-0.005, boron 0.0001-0.005, iron and inevitable admixtures - rest, including stannum, lead, zinc - each max. 0.010, hydrogen max. 0.001. Hot rolling in clean group of cages is performed at max. 950°C with degree of rerolling at least five rated thicknesses of finished rolled products. At that end of finished rolling is specified within range 750-860°C. Strip is wound at max. 480°C. At that mode of accelerated cooling is specified based on the CCT diagrams of disintegration of the overcooled austenite to ensure the beinite-martensite-ferrite structure with share of the beinit-martensite phase at least 90%.

EFFECT: production of hot-rolled products with required strength class and guaranteed level of impact energy and relative elongation.

3 cl, 3 tbl, 1 dwg

Description

The invention relates to the field of metallurgy, and specifically to a technology for the production of high-strength hot-rolled steel from low-alloy steel, intended for the manufacture of parts of heavy vehicles, hoisting-and-transport mechanisms and agricultural machines by stamping, bending and profiling.

One of the defining qualities of steels for the automotive industry is their ability to stretch when stamping car parts, high strength and toughness. Hot rolled products of increased strength (not less than 700 MPa) must comply with a set of mechanical properties, for example, according to the requirements of the European standard EN 10149 (table 1):

Table 1 Name of mechanical properties Norms of mechanical properties Sample Type Sample axis minimum maximum Temporary resistance (Rm), N / mm ² 750 950 L = 5.65√So Along Yield Strength (ReH), N / mm ² 700 L = 5.65√So Along Elongation A,% 12 L = 5.65√So Along Work shock KV-20 ° C, J 40 - Along 180 ° bend to parallel sides d = 2a Across

A known method for the production of hot-rolled steel, including the smelting of low alloy steel, casting, hot rolling, water cooling, winding strips into coils, the steel is melted with the following chemical composition in the ratio of ingredients, wt.%:

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

at the same time, the temperature of the roll in the last pass of the draft group of mill stands is maintained in the range of 1010 ÷ 1050 ° C, the final deformation of the strip is carried out in continuous mode with a total degree of deformation of at least 70% and the completion of plastic deformation in the temperature range 790 ÷ 840 ° C, after completion the final deformation on the discharge roller table produce differential cooling of the upper and lower surfaces of the strip, and the cooling of the upper surface of the strip is carried out with an intensity determined from the expression:

V _top = -3.4 · ln (h _av ) +11.5,

where V _top is the cooling rate of the upper surface of the strip, deg / s;

h _cf - the final thickness of the strip, mm,

and the cooling of the lower surface of the strip is monotonously uniform over its entire length, while the temperature of the strip before winding is maintained in the range 550 ÷ 600 ° C (RF patent No. 2450061, C21D 8/04, 2011).

The disadvantage of this method is that it does not provide mechanical properties on hot rolled products corresponding to strength class 700 and above.

The closest in technical essence and the achieved result is a method for the production of hot rolled steel of increased strength, including the smelting of steel containing carbon 0.06-0.15%, silicon - 0.1-0.50%, manganese - 1.35-2, 0%, sulfur - not more than 0.012%, phosphorus - not more than 0.020%, chromium - 0.01-0.30%, nickel - 0.01-0.30%, copper - 0.01-0.30%, aluminum - 0.01-0.06%, niobium 0.01-0.10%, nitrogen - 0.002-0.010% and one or more elements from the group: vanadium 0.02-0.15%, titanium - 0.01 -0.15%, molybdenum - 0.003-0.35%, calcium - 0.0003-0.005%, boron - 0.0001-0.005%, iron and unavoidable impurities - the rest, including tin - not more than 0.015%, while the total content of niobium, vanadium and titanium does not exceed 0.22%. The final deformation in the finishing group of a continuous broadband mill is carried out at an inlet temperature of the roll - not more than 1020 ° C and a total degree of deformation of the strip of not less than 78%, the temperature of the end of rolling is maintained in the range of 770-850 ° C, and the winding temperature in the range of 480-560 ° C. The carbon and manganese content in the steel are related to the required strength class of rolled products by the ratios:

where [C] is the carbon content,%;

[Mn] is the manganese content,%;

0.22, 0.0002, 0.0028, 0.05 - empirical coefficients,%;

K _CR - dimensionless indicator, numerically equal to the minimum yield strength.

In this case, rolled products of strength class 500-550 have a predominantly ferrite-pearlite structure, and rolled products of strength class 600-650 have a predominantly ferrite-bainite-pearlite structure (RF patent No. 2495942, C21D 8/04, C22C 38/58, 2013) - a prototype .

The disadvantage of this method is that it does not provide the mechanical properties of hot rolled products with a higher level of strength characteristics corresponding to a strength class of 700 and above with a normalized level of impact at -20 ° C, necessary for the manufacture of parts of heavy vehicles, hoisting vehicles mechanisms and agricultural machines by stamping, bending and profiling.

The technical result of the invention is to obtain hot-rolled steel of the required strength class (minimum yield strength of at least 700 MPa) with a guaranteed level of impact at -20 ° C and elongation.

The technical result is achieved by the fact that in the method of manufacturing hot rolled products of increased strength, including smelting low alloy steel, casting, hot rolling, water cooling, winding strips into coils, according to the invention, steel containing carbon 0.03-0.12%, silicon 0 is smelted 10-0.50%, manganese 1.5-2.0%, sulfur not more than 0.008%, phosphorus not more than 0.015%, chromium 0.01-0.30%, nickel 0.01-0.30%, copper 0.01-0.30%, aluminum 0.01-0.06%, niobium 0.001-0.10%, nitrogen 0.002-0.010%, vanadium 0.001-0.10%, titanium 0.001-0.10%, molybdenum 0.005-0.30%, calcium 0.0003-0.005%, boron 0.0001-0.005%, iron and inevitable impurities remain other, including tin, lead, zinc - not more than 0.010% of each, hydrogen not more than 0.001%, carbon equivalent - not more than 0.45%. Hot rolling in the finishing group of stands is carried out at a temperature of not more than 950 ° C with a rolling ratio of at least five nominal thicknesses of finished products, the end of the finish rolling is regulated in the range of 750-860 ° C, the strip winding is controlled at a temperature of not more than 480 ° C, accelerated cooling mode is prescribed on the basis of thermokinetic diagrams of the decomposition of austenite to ensure a bainite-martensite-ferrite structure with a proportion of bainite-martensite phase of at least 90%.

The invention consists in the following. The mechanical properties of hot rolled steel are affected by both the chemical composition, temperature and deformation modes of rolling, and the structure of hot rolled steel.

Carbon is one of the reinforcing elements. When the carbon content is less than 0.03%, the strength characteristics of the steel, especially the temporary resistance, are below the permissible level. An increase in carbon content of more than 0.12% leads to a decrease in the ductility and toughness of steel, especially at low temperatures, which is unacceptable.

Silicon in steel is used as a deoxidizer and an alloying element. When the silicon content in the steel is less than 0.10%, its required strength is not achieved, and when the content is more than 0.50%, the ductility sharply decreases due to the enlargement of the grain size and steel embrittlement takes place.

Manganese provides the desired mechanical properties of the rental. When the manganese content is less than 1.5%, the strength of the steel is below acceptable. An increase in the manganese content of more than 2.0% overly strengthens the steel, worsens its ductility, toughness and cold resistance.

Aluminum is introduced into steel as a deoxidizer. When the aluminum content is less than 0.01%, the ductility of the steel decreases, the steel becomes prone to aging. An increase in aluminum content of more than 0.06% leads to contamination of steel with non-metallic inclusions.

The selected limits of the content of chromium, nickel and copper increase the strength characteristics of the rolled product without significantly reducing its plastic properties and the use of these elements within these limits leads to a saving in alloying materials.

Niobium, vanadium and titanium are used as microalloying elements and provide the necessary strength properties due to grain refinement and dispersion hardening. An increase in the mass fraction of elements of more than 0.10% of each is ineffective. This affects the ductility and toughness of the rolled products due to excessive hardening and increases the cost of alloying.

Nitrogen strengthens steel. With a nitrogen content of more than 0.010%, the steel becomes prone to strain aging due to the formation of iron nitrides, a nitrogen content of more than 0.002% in steels is necessary for the formation of carbonitrides of microalloying elements, strengthening the ferrite matrix.

Molybdenum in an amount of 0.005-0.30% is used as a microalloying element to obtain the necessary strength properties and increase viscosity at low temperatures. When the concentration of molybdenum is less than 0.005% - it is ineffective. An increase in the concentration of molybdenum in excess of 0.30% does not lead to further improvement in mechanical properties, but only increases the cost of alloying materials.

Boron increases the strength of steel, and also grinds the microstructure. With a boron content of less than 0.0001%, its effect is negligible. An increase in boron content of more than 0.005% leads to the appearance of excess phases (borides) at the grain boundaries, which reduces the toughness of steel at low temperatures.

Additionally, a carbon equivalent limit is introduced - not more than 0.45%.

C _E = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where

C _E is the carbon equivalent,%;

C is the mass fraction of carbon,%;

Mn — mass fraction of manganese,%;

Cr is the mass fraction of chromium,%;

Mo is the mass fraction of molybdenum,%;

V is the mass fraction of vanadium,%;

Ni — mass fraction of nickel,%;

Cu is the mass fraction of copper,%;

6, 5, 15 - empirical coefficients.

Steel with a carbon equivalent of not more than 0.45% has good weldability. With a carbon equivalent of more than 0.45%, the ability of steel to weld is reduced, because the tendency of the weld metal to hardening when it is cooled increases and provokes the receipt of various properties in the heat-affected zone and the base metal. In addition, before welding a metal with a carbon equivalent of more than 0.45%, heating is required to avoid cracking, which leads to an increase in cost and complexity of the process.

Calcium is used within the range of 0.0003-0.005% as a highly active element for enhancing the deoxidizing effect of aluminum and removing phosphorus, sulfur, oxygen from the melt into slag, which leads to a change in the phase composition and an improvement in the shape (globulization) of oxide inclusions, as well as a decrease in their amount .

Sulfur and phosphorus are constant harmful impurities in steel. They seek to reduce their content. Sulfur practically does not affect the strength, but reduces the ductility and toughness of the metal. Phosphorus adversely affects viscosity and cold resistance due to embrittlement of grain boundaries due to the release of iron phosphide. In addition, during stamping of particularly complex parts, metal rupture can occur at sites of sulfide formation larger than 2 points. For this purpose, the sulfur content is limited to max. 0.008%. The phosphorus content is limited to max. 0.015%. The selected restrictions are due to the fact that the negative effect of these impurities with an increase in their mass fraction increases with an increase in the strength class of hot-rolled products.

Limiting the content of tin, lead and zinc impurities to no more than 0.010% of each contributes to a higher plasticity value by minimizing the number of fusible compounds along grain boundaries.

Hot rolling with a rolling start temperature in the finishing group of stands of not more than 950 ° with a rolling ratio of at least five nominal thicknesses of finished products and the end of finishing rolling in the temperature range of 750-860 ° C provide the necessary study of the structure, grain refinement and, as a result, strength characteristics corresponding to strength class of at least 700, elongation, toughness and cold resistance, meeting the requirements of EN 10149.

One of the significant technological parameters is the temperature of the winding. To determine the mode of accelerated cooling, thermokinetic diagrams of the decomposition of supercooled austenite were used for the chemical composition of the assortment of hot-rolled steel under consideration in the range of possible temperatures and cooling rates (Figure 1), based on the analysis of which the interval of necessary winding temperatures was determined (not more than 480 ° C). Winding rolled products in a temperature range of not more than 480 ° C allows you to get in the structure of the bainitic-martensitic phase, in an amount of not less than 90%.

Above the declared temperature limits of the beginning and end of finish rolling, as well as winding, the technical result was not achieved, since the rolling had a low yield strength (less than 700 MPa) and a ferrite-bainite-pearlite structure with a predominant share of the ferrite-bainitic phase.

Method implementation examples

Low oxygen steels were smelted in an oxygen converter, the chemical composition of which is given in Table 2 (including steel grades S600MC, 20GYUT, S700MC,).

The cast iron used for the production of this steel was pre-treated at a desulfurization unit to ensure that the sulfur content in the steel was not more than 0.008%. The smelted steel was cast on a continuous casting machine into slabs with a cross section of 250 × 1070 - 1540 mm. The slabs were heated in a heating furnace with walking beams to a temperature of 1260-1300 ° C for 2.0-2.5 hours and rolled on a continuous broadband mill. The temperature of the strips at the entrance to the finishing group of the stands and the exit from the last stand of the mill is regulated by the need to obtain rolled products of a certain strength class of at least 700. Hot rolled strips on the discharge roller table were cooled with water to certain temperatures and wound into rolls. The accelerated cooling mode was selected using thermokinetic diagrams of decomposition of supercooled austenite (Figure 1). Then, by calculation, we determined the number and sequence of switching on the collectors and sections of the laminar installation necessary to obtain the winding temperature and the accelerated cooling schedule set above in the conditions of Severstal mill 2000.

Temperature conditions and mechanical properties of rolled products obtained from steel of experimental melts are given in tables 2 and 3.

table 2 The chemical composition of the experimental swimming trunks C _e ,% Option No. Mass fraction of chemical elements,% C Si Mn S P Cr + Ni + Cu Mo Nb + V + Ti Al N AT Sn Pb Zn H one 0.08 0.16 1.86 0.002 0.008 less than 0.90 0.14 less than 0.22 0,044 0.007 0.0024 0.005 0.003 0.001 0,0005 0.43 2 0,07 0.24 1.93 0.005 0.006 less than 0.90 0.11 less than 0.22 0,036 0.010 0,0003 0.007 0.002 0.003 0,0004 0.50 3 0.08 0.16 1.86 0.002 0.010 less than 0.90 0.14 less than 0.22 0,042 0.007 0.0024 0.004 0.006 0.002 0,0006 0.43 4 prototype 0.11 0.40 1,66 0.010 0.015 less than 0.90 0,03 less than 0.22 0,031 0.011 0 0.008 nd nd nd 0.44

Table 3 Technological parameters of production, the results of mechanical tests Option No. Thickness of rolled H (not less) T _beg . _clean , ° C T _kp , ° C _{I eat} T, ° C σ _t , MPa σ _in , MPa δ,% Impact strength, J / cm ² Work Impact, J Microstructure Carbon equivalent one 5 × h 925 830-840 320-360 745 857 fifteen KCV-50 ° C KV-20 ° C 55-60 Beinite Martensite Ferrite 0.43 730 820 fifteen 135-168 On longitudinal samples (transverse) 2 5 × h 846 765-795 543-555 680 865 17 KCV-40 ° C nd Ferrite-Bainite-Perlite 0.50 690 870 16.5 95-98 On longitudinal samples KCU-40 ° C 130-190 (transverse) 3 5 × h 929 841-851 500-517 597 807 16.5 KCV-50 ° C KV-20 ° C 40-61 Ferrite-Bainite-Perlite 0.43 624 800 eighteen 132-163 603 857 17.5 (transverse) On longitudinal samples 4 prototype He was regulated 950 790-869 509-558 630 735 16 KCU-40 ° C nd Ferrite-Bainite-Perlite - 0.44 989 790-869 509-558 620 720 21 127-173 952 789-853 518-558 655 735 23 (transverse) On transverse samples where H is the thickness of the tackle for the finishing group of stands; h is the thickness of the finished product; nd - no data

From tables 2 and 3 it can be seen that in the case of the implementation of the proposed method (option No. 1) and the fulfillment of all the parameters stated in the formula, the mechanical properties of the steel are achieved, corresponding to a strength class of at least 700. The selected combination of technological parameters and chemical composition ensures a good workout in the rental structures and weldability, ductility, toughness and cold resistance, while the structure consists mainly of a bainitic-martensitic phase.

When implementing option No. 2, yield is not guaranteed for yield strength, since The properties for this characteristic are below the limit of the regulated range. In addition, the carbon equivalent of more than 0.45% will require additional costs for the organization of heating of the rolled metal before welding. The ferritic-bainitic structure of rolled metal produced according to options No. 2 and 3 also does not provide the stated requirements for rolling.

When using the prototype method (option No. 4), the strength class 700 is also not achieved.

The proposed technology for the production of hot-rolled steel ensures the satisfaction of non-standard customer requirements: impact strength at test temperatures up to -50 ° C - min. 60 J / cm ² , the purity of the metal, ensured by the absence of coarse non-metallic inclusions - max. 3 points

Claims (3)

1. Method for the production of hot-rolled steel of increased strength, including steel smelting, casting, hot rolling, water cooling and winding the strip into coils, characterized in that the steel is smelted containing the components in the following ratio, wt.%:
carbon 0.03-0.12 silicon 0.10-0.50 manganese 1.5-2.0 sulfur no more than 0,008 phosphorus no more than 0.015 chromium 0.01-0.30 nickel 0.01-0.30 copper 0.01-0.30 aluminum 0.01-0.06 niobium 0.001-0.10 nitrogen 0.002-0.010
one or more elements from the group:
vanadium 0.001-0.10 titanium 0.001-0.10 molybdenum 0.005-0.30 calcium 0.0003-0.005 boron 0.0001-0.005 iron rest
and inevitable impurities, including:
tin no more than 0,010 lead no more than 0,010 zinc no more than 0,010 hydrogen no more than 0,001
at the same time, the thickness of the rolled for finish rolling is at least five nominal thicknesses of the finished rolled, and they begin finishing rolling at a temperature of not more than 950 ° C, finish finishing rolling in the temperature range 750-860 ° C, accelerated cooling is set based on the analysis of thermokinetic diagrams of austenite decomposition to ensure a bainitic-martensitic-ferritic structure with a mass fraction of bainitic-martensitic phase of at least 90%. 2. The method according to claim 1, characterized in that the carbon equivalent of steel is not more than 0.45%. 3. The method according to claim 1, characterized in that the rolling of the coil is carried out at a temperature of not more than 480 ° C.