RU2393239C1 - Procedure for production of plate iron low-alloyed strip - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: work-piece in strips of 20-40 mm thickness is produced out of steel with following contents of elements, wt %: 0.03-0.06 C; 1.5-1.7 Mn; 0.15-0.35 Si; 0.15-0.3 Ni; 0.04-0.06 Nb; Cr≤0.2; 0.08-0.15 Mo; 0.15-0.3 Cu; 0.02-0.04 V; 0.005-0.02 Ti; 0.02-0.05 Al, iron and impurities, where contents of each element of impurity is less 0.03%, - the rest. Also carbon equivalent is C_equiv≤0.4. Further the work-piece is subject to roughing at temperature 1000-920°C and shrinkage not less, than 9 % per one pass in the first two passes, and not less, than 12 % per pass in the following passes from thickness of a breakdown bar, depending on thickness of a finished strip, determined from the following ratio: H_{breakdown bar}=(161.5+0.0955·h_str ²-4.6191·h_str)±5 mm. The breakdown bar is cooled to 760-800°C and is finish rolled at shrinkage ratio not less, than 12% per pass excluding three last passes. Successively the finished strip is cooled with acceleration to temperature, depending on its thickness, determined from the ratio: T_{coeff. of cooling}=(422-0.1364*h_str ²+3,6273*h_str)±15°C and is cooled with delay.

EFFECT: high durability combined with high processability, plasticity and cold resistance.

1 ex

Description

The invention relates to the field of metallurgy, in particular to the production of sheet metal on a reversible plate mill, and can be used in the manufacture of thick sheets and strips of low alloy steels using controlled rolling.

A known method for the production of thick steel sheets, including heating the slab to an austenitic temperature of 1200 ± 20 ° C and its rough rolling to an intermediate thickness of a roll of 70 mm with a temperature of the end of deformation of 900 ° C. Then, transportation of the roll to the cooling zone outside the rolling line and its cooling in air to a temperature below 800 ° C are provided. After cooling, the roll is finished rolling to a final thickness with a temperature of the end of deformation of 730 ° C and the resulting sheet is cooled to ambient temperature [1].

However, the thick sheet obtained according to the known method is characterized by a relatively low level of mechanical properties, in particular impact strength at low temperatures. This is due to the low cooling rate in vivo of the resulting sheet to ambient temperature.

The closest in technical essence to the proposed invention is a method for the production of cold-resistant sheet metal, comprising obtaining a billet of steel containing, wt.%: C = 0.04-0.1; Mn = 0.60-0.90; Si = 0.15-0.35; Ni = 0.10-0.40; Al = 0.02-0.06; Nb = 0.02-0.06; V = 0.03-0.05; the rest is iron and impurities. The method involves austenization of a workpiece at a temperature of 1100-1150 ° C, preliminary deformation (rough rolling) with a total compression of 35-60% at a temperature of 900-800 ° C, subsequent cooling of the intermediate workpiece (chilling) by 50-70 ° C, final deformation ( fine rolling) with a total degree of compression of 65-75% at a temperature of 830-750 ° C, accelerated cooling of sheet metal to a temperature of 500-260 ° C and slow cooling to a temperature of no higher than 150 ° C [2].

The disadvantages of this method include the fact that the thick sheet of low alloy steel obtained by using it has insufficiently high strength properties. Values of tensile strength and yield strength stated for this method comprise σ _m = 300-320 MPa, σ _in = 400-455 MPa. At the same time, the regulatory requirements for the strip of strength category X70 are σ _t ≥485 MPa, σ _at ≥570 MPa.

The technical result of the invention is to increase the strength properties while maintaining sufficient ductility and increasing the cold resistance of the strip with a thickness of 20-40 mm strength category X70.

The technical result is achieved by the fact that in the known method for the production of cold-resistant sheet metal, which includes the production of a billet, its austenization, roughing and finishing rolling with the reinforcing rolls before finishing rolling, accelerated cooling of the finished strip to a predetermined temperature and its subsequent delayed cooling, according to the invention, the billet is made from steel with the following ratio of elements, wt.%: 0.03-0.06% C; 1.5-1.7% Mn; 0.15-0.35% Si; 0.15-0.3% Ni; 0.04-0.06% Nb; Cr≤0.2%; 0.08-0.15% Mo; 0.15-0.3% Cu; 0.02-0.04% V; 0.005-0.02% Ti; 0.02-0.05% Al; iron and impurities, with the content of each impurity element less than 0.03% - the rest, while the carbon equivalent is C _equiv ≤0.4, rough rolling after austenization is carried out at a temperature of 1000-920 ° C, with a degree of reduction in the first two passes not less than 9% per pass, and in subsequent no less than 12% per pass, on the thickness of the roll, determined, depending on the thickness of the finished strip, from the ratio: H _sub = 161.5 + 0.0955 · h _w ² -4.6191 _w · h) ± 5 mm, to produce a peal cooling temperature 760-800 ° C, finish rolling is performed with a reduction of at least 12% of pass, except the last three passes, the accelerated cooling is finished to produce strip temperature determined, depending on its thickness, from the relation _to T = (422-0,1364 · h _w ² + 3,6273 · h _w) ± 15 ° C.

The invention consists in the following.

First, a billet is made of steel with a given chemical composition. In general, the given content of elements provides the necessary phase composition and carbon equivalent value, as well as the mechanical properties of the strip during the implementation of the proposed technological regimes.

The carbon content in the low alloy steel of the proposed composition determines its strength. A decrease in carbon content of less than 0.03% leads to a decrease in its strength below an acceptable level. An increase in carbon content of more than 0.06% worsens the plastic and viscous properties of strips, leading to their unevenness due to segregation.

Additives of manganese and nickel in the claimed range contribute to solid-solution hardening of the metal and, accordingly, increase the cold resistance and corrosion resistance of the finished product. A lower content of these elements does not allow to provide the required cold resistance, a larger one reduces weldability and is not economically feasible.

When the silicon content is less than 0.15%, the deoxidation of steel deteriorates, the strength of the strips decreases. An increase in the silicon content of more than 0.35% leads to an increase in the number of silicate inclusions, and reduces the toughness of the metal.

The niobium and vanadium additives within the specified limits serve the purpose of dispersion hardening, and also inhibit the growth of austenitic grain and contribute to the appearance of a subgrain structure, which is fixed and stabilized by dispersed carbide particles, upon cooling.

Joint alloying with niobium and vanadium within the accepted limits is especially effective for mild steel, since the dissolution temperature of NbC is 50-70 ° C higher than VC, and as a result, dispersed carbides of VC are released upon cooling, and NbC inhibits the growth of austenite grain upon heating. In addition, co-alloying with niobium and vanadium increases the hot ductility of cast billets.

Reducing the content of niobium and vanadium below the specified limits does not provide sufficient dispersion and grain boundary hardening, exceeding the specified level worsens the weldability of steel and is not economically feasible due to the increase in alloying costs.

Chrome increases the strength of steel. At a concentration of up to 0.2%, it does not adversely affect the weldability of strips in the production of pipes, but expands the possibility of using scrap metal for smelting, which reduces the cost of steel.

The addition of molybdenum in the specified range helps to obtain the required strength characteristics of steel, and also improves its hardenability. If the molybdenum content is less than 0.08%, the goals of doping with molybdenum will not be achieved. An increase in the content of molybdenum over 0.15% is not accompanied by a further increase in the quality of strips, but only increases the cost of alloying, which is not economically feasible.

The addition of copper, within the specified limits, increases the strength and corrosion resistance of steel, including in the marine environment. A lower copper content does not allow to achieve the desired effect, a larger one is not economically feasible.

Titanium is a strong carbide forming element that strengthens steel. The finely dispersed titanium carbides precipitated during hot rolling and cooling of the strips with water are highly resistant to overheating. When the titanium content is less than 0.005%, the strength of the hot rolled sheets decreases. An increase in titanium content in excess of 0.02% leads to a decrease in the viscosity properties of the metal (in particular, at a temperature of -60 ° C), which is unacceptable for steels of this assortment.

Aluminum deoxidizes and modifies steel. By binding nitrogen to nitrides, it inhibits its negative effect on the properties of the sheets. When the aluminum content is less than 0.02%, the complex of mechanical properties of the sheets decreases. An increase in its concentration of more than 0.05% leads to a deterioration in the viscosity properties of the bands.

The content of impurity elements (S, P, etc.) of less than 0.03% of each does not have a noticeable negative effect on the quality of the strips.

The limitation of the carbon equivalent value of less than 0.4 ensures high technology welding pipes at low ambient temperatures without preheating. The carbon equivalent value is determined by the formula: Ce = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the values of the elements correspond to their percentage in steel.

Heating the workpieces to austenitization temperature and rough rolling in the temperature range of 1000–920 ° С with the degree of compression in the first two passes of at least 9% per pass, and in the subsequent at least 12% per pass, it is possible to form a finely dispersed carbide phase in the process of static and dynamic recrystallization , preventing the passage of collective recrystallization, and to ensure the grinding of the structure throughout the thickness.

Tinning of the roll with a thickness of H _cut = (161.5 + 0.0955 · h _w ² -4.6191 · h _w ) ± 5 mm (the equation is determined experimentally) to a temperature of 760-800 ° C and subsequent controlled finishing rolling in a two-phase region to The processes of dispersion hardening and grinding of grains are added by the development of texture and the formation of subgrains. Subgrain hardening will be of decisive importance in the formation of the mechanical properties of the finished strip only if finishing rolling is carried out with reductions of at least 12% per pass. The resulting subgrains, in addition to increasing strength, increase the resistance of the metal to brittle fracture and fatigue. In the last three passes, the compression ratio is selected based on the required thickness of the finished strip and can be less than 12% per pass.

Hardening of plate steel during the finish multi-pass rolling in a two-phase region with difficult austenite recrystallization is characterized by the fact that in the first passes the surface layers of the slab are most intensely hardened, where the deformation is maximum. As the surface layers harden, the deformation begins to penetrate deeper and covers the entire thickness. The most plastic deformation penetrates the thickness of the roll when it is rolled in the temperature range from 800 to 760 ° C. Since the indicated degree of reduction during finishing rolling is sufficient for the full development of the structure for the entire thickness of the tackle, grain grinding and increased cold resistance of the finished strip are ensured.

Accelerated cooling of the finished strip _to a temperature T = (422-0,1364 · h _w ² + 3,6273 · h _w) ± 15 ° C (equation determined experimentally) ensures the formation of the desired phase composition ductile metal strip for trunk pipelines. To stabilize the properties of plate steel and relieve residual internal stresses after accelerated cooling is completed, the sheets should be cooled more slowly in order to ensure the removal of residual internal stresses and normal processes occurring in the metal, which increases the level of mechanical properties of thick sheets. This approach contributes to obtaining a fine-grained equilibrium metal structure.

Thus, the full use of the resource of properties corresponding to low-alloy steel of a given chemical composition is ensured by the deformation-thermal regime of strip production. The rolling technology is aimed at obtaining the optimal phase ferrite-pearlite composition and phase morphology, grinding microstructure grains, hardening of solid solution, dispersion hardening, dislocation and texture hardening.

The application of the method is illustrated by an example of its implementation in the production of a strip of strength category X70. A blank was prepared containing, wt.%: C = 0.04; Mn = 1.60; Si = 0.21; Ni = 0.19; Nb = 0.05; Cr = 0.05; Mo = 0.10; Cu = 0.18; V = 0.03; Ti = 0.012; Al = 0.04; the rest is iron and inevitable impurities, the content of each of which is less than 0.03%. In this case, the carbon equivalent is C _equiv = 0.37, i.e. corresponds to the declared range.

When billets with a cross section of 315 × 2000 mm were heated at a temperature of 1190 ° C for 60 minutes, austenization of low alloy steel and dissolution of dispersed carbonitride hardening particles took place. After issuing from the furnace, rough rolling was performed in the temperature range of 980–940 ° C to a thickness of 130 mm (calculated at h _w = 40 mm). The degree of compression in the first two passes was 11%, and in the next 4 - about 15% per pass.

Then undermine the roll to a temperature of 780 ° C.

Finishing rolling was performed with reductions, the value of which per pass was 13-16%, and in the last two passes, the compression was 2%. Accelerated cooling of the finished strip with a thickness h _w = 40 mm, after leaving the stand of the plate mill, was carried out to a temperature of 350 ° C (calculated).

Then the sheets were edited with their slow cooling in air.

Mechanical properties were determined on transverse samples. The temperature-strain mode of rolling provided a fine-grained ferrite-bainitic structure with a noticeable transverse and longitudinal grain anisotropy. Static tensile tests were carried out on flat specimens in accordance with GOST 1497, and in impact bending on specimens with a V-shaped notch in accordance with GOST 9454 at a temperature of -40 ° C. The following mechanical properties were obtained for transverse samples: temporary resistance σ _in = 650 N / mm ² ; yield strength σ _t = 540 N / mm ² ; elongation δ = 23.7%. The specified level of properties fully complies with the requirements for a strip of strength category X70.

The technical and economic advantages of the considered invention consist in the fact that the proposed temperature-deformation modes of production allow to use to the greatest extent all the hardening mechanisms of low alloy steel of a given chemical composition: grinding of microstructure grains, dislocation hardening, dispersion hardening, anisotropy of structure and properties. Using the proposed method for the production of strips of strength category X70, a thickness of 20-40 mm from low alloy steel, can increase the yield on this range by 3-5%.

Thus, the application of the proposed rolling method ensures the achievement of the desired result — obtaining a strip for large diameter pipes with a plate of large diameter with a level of mechanical properties corresponding to strength category X70 on a plate reversing mill.

Information sources

1. Japanese application No. 59-61504, IPC B21B 1/38; B21B 1/22, 1984.

2. RF patent No. 2265067, IPC C21D 8/02, 2004.

Claims (1)

A method of producing a plate of low alloy strip, including obtaining a workpiece, its austenization, rough rolling, coiling the roll before finishing rolling, finishing rolling, accelerated cooling of the finished strip to a predetermined temperature and subsequent delayed cooling, characterized in that the workpiece is obtained from steel with the following elements, wt.%:
carbon 0.03-0.06 manganese 1.5-1.7 silicon 0.15-0.35 nickel 0.15-0.3 niobium 0.04-0.06 chrome no more 0.2 molybdenum 0.08-0.15 copper 0.15-0.3 vanadium 0.02-0.04 titanium 0.005-0.02 aluminum 0.02-0.05 iron and impurities, with the content of each impurity element less than 0.03% rest
moreover, the carbon equivalent is C _equiv ≤0.4, while rough rolling after austenization is carried out at a temperature of 1000-920 ° C, with a reduction ratio of at least 9% per pass in the first two passes, and at least 12% per pass in the following, on the thickness of the roll, determined depending on the thickness of the finished strip from the ratio
H _rask = (161.5 + 0.0955 · h _w ² -4.6191 · h _w ) ± 5 mm,
where H _rask - the thickness of the roll, mm;
161.5 - empirical coefficient, mm;
0.0955 - empirical coefficient, 1 / mm;
h _W - the thickness of the finished strip, mm;
4.6191 is a dimensionless empirical coefficient,
cooling of the roll is carried out to a temperature of 760-800 ° C, finishing rolling is carried out with reductions of at least 12% per pass, with the exception of the last three passes, and accelerated cooling of the finished strip is carried out to a temperature determined depending on its thickness from the ratio
_To T = (422-0,1364 · h _w ² + 3,6273 · h _w) ± 15 ° C,
where T _ko is the temperature of the end of cooling, ° C;
422 - empirical coefficient, ° С;
0.1364 - empirical coefficient, ° C / mm ² ;
h _W - the thickness of the finished strip, mm;
3.6273 - empirical coefficient, ° С / mm.