RU2593803C1 - Method for production of pipe steel plate, micro alloyed with boron - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly, to rolling, and can be used for production of low alloyed pipe steels. To increase strength properties of steel with K56 strength class at sufficient ductility and impact strength of sheets with thickness of 14-24 mm continuously cast slab is heated and subjected to rough and finish rolling, accelerated cooling of finished rolled to preset temperature, wherein continuously cast slab is made from steel containing, wt%: carbon 0.05-0.07, silicon 0.30-0.55, manganese - 1.40-1.55, sulphur not more than 0.005, phosphorus not more than 0.015, aluminium 0.025-0.045, nitrogen not more than 0.008, titanium 0.015-0.030, niobium - 0.020-0.035, boron 0.0010-0.0025, iron and impurities are rest, rolling end temperature is set to 840±15 °C, and accelerated cooling is performed at rate of 8.0-16.0 °C/sec to temperature defined from relationship: T_f.c. =4·10⁴·B+560±15 °C, where T_f.c. is temperature of accelerated cooling end, °C; B are content of boron in steel, wt%.

EFFECT: production of thick sheets from low alloyed pipe steels.

1 cl, 2 tbl, 1 ex

Description

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

A known method for the production of cold-resistant sheet metal (see RF patent No. 2265067), including 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.

The disadvantages of this method include the fact that obtained when using it, a thick sheet of low alloy steel has insufficiently high strength properties.

There is also known a method (see RF patent No. 2337150), according to which, to ensure an increased level of consumer properties, the pipe billet is obtained from steel containing components, wt.%: Carbon 0.19-0.24, manganese 0.40-0.60 , silicon 0.17-0.30, chromium 0.40-0.60, nitrogen 0.003-0.010, vanadium 0.020-0.040, aluminum 0.020-0.040, titanium 0.020-0.040, boron 0.001-0.003, iron and unavoidable impurities - the rest under the conditions: 500- (Ti / 24-N / 7) + 2.2≥0; 30≥C / 0.01 + V / 0.001≥13. The method provides for obtaining a tube billet with a diameter of 80 to 180 mm with a temporary tensile strength of 480-650 N / mm ² , yield strength of at least 360 N / mm ² , elongation of at least 16%.

The disadvantage of this method is the impossibility of its use for the production of sheet pipe billets.

The closest in technical essence is the method (see RF patent No. 2393239), according to which, to ensure high strength combined with high adaptability, ductility and cold resistance in strips with a thickness of 20-40 mm, the workpiece is obtained from steel with the following content 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 element of the impurity less than 0.03% - the rest, while the carbon equivalent is C _equiv ≤0.4, then the workpiece is subjected to rough rolling at a temperature of 1000-920 ° C with a degree of reduction in the first two passes of not less than 9% per pass, and in subsequent not less than 12% per pass on the thickness of the roll, determined, depending on the thickness of the finished strip, from the ratio: H _cut = (161.5 + 0.0955 · h _w ² -4.6191 · h _w ) ± 5 mm, cool the roll to 760-800 ° C and finish rolling with a reduction of at least 12% per pass, with the exception of the three last of passages, then the finished strip is rapidly cooled to a temperature, determined, depending on its thickness, from the ratio: T _ko = (422-0.1364 · h _w ² + 3.6273-h _w ) ± 15 ° C, and slowly cool.

The disadvantage of this method is the low proportion of the viscous component in the fracture of the steel sheet when tested with a falling load.

The technical result of the invention is to increase the strength properties of economically alloyed tubular steel of strength class K56 while maintaining sufficient ductility and toughness of sheets with a thickness of 14-24 mm.

The specified technical result is achieved by the fact that in the method for the production of plate steel, microalloyed with boron, including the production of continuously cast slab, its heating, roughing and finishing rolling and accelerated cooling of the finished roll to a predetermined temperature, in contrast to the closest analogue of continuously cast slab is obtained from the following chemical steel composition, wt.%:

carbon 0.05-0.07 silicon 0.30-0.55 manganese 1.40-1.55 sulfur no more 0.005 phosphorus no more 0.015 aluminum 0.025-0.045 nitrogen no more 0.008 titanium 0.015-0.030 niobium 0,020-0,035 boron 0.0010-0.0025 iron and impurities rest,

the temperature of the end of the rolling is taken 840 ± 15 ° C, accelerated cooling is carried out at a speed of 8.0-16.0 ° C / s to a temperature determined from the ratio:

where T _to.o is the temperature of the end of accelerated cooling, ° C; In - the content in the steel of boron, wt.%.

The invention consists in the following. First, a continuously cast slab (billet) is made of steel with a given chemical composition. The specified content of the elements provides the required value of the carbon equivalent, as well as the mechanical properties of the finished sheet when implementing the proposed technological processing modes.

The carbon content in the steel of the proposed composition determines its strength. A decrease in carbon content of less than 0.05% leads to a decrease in its strength below an acceptable level. An increase in carbon content of more than 0.07% affects the plastic and viscous properties of plate products and leads to their unevenness due to segregation.

The addition of manganese in the claimed range contributes to solid solution hardening of the metal and, accordingly, to increase the cold resistance and corrosion resistance of the finished product. A lower content of this element 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 0.30%, the deoxidation of steel deteriorates, and the strength of sheet metal decreases. An increase in the silicon content of more than 0.55% leads to an increase in the number of silicate inclusions, reduces the toughness of the metal.

The niobium 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 during cooling, which is fixed and stabilized by dispersed carbide particles. When the niobium content is less than 0.020%, sufficient dispersion and grain-boundary hardening is not provided. An increase in the niobium content of more than 0.035% leads to a deterioration in the weldability of steel and is not economically feasible in view of the increase in alloying costs.

Titanium is a strong carbide forming element that strengthens steel. Finely dispersed titanium carbides released during hot rolling and cooling of sheets with water are highly resistant to overheating. When the titanium content is less than 0.015%, the strength of the hot rolled sheets decreases. An increase in titanium content in excess of 0.030% 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 0.025%, the complex of mechanical properties of the sheets decreases. An increase in its concentration of more than 0.045% leads to a deterioration in the viscous properties of hot-rolled sheets.

The content of impurity elements of sulfur and phosphorus above the stated limits leads to a deterioration in the plastic and viscous properties of hot-rolled sheets.

Boron, added in the range of 0.0010-0.0025%, sharply increases the hardenability of steel, contributing to the formation of potentially hardening components, bainite or martensite and at the same time slowing down the formation of softer ferrite and pearlite components during cooling of steel from high temperatures to ambient temperatures. Boron in an amount of more than 0.0025 wt.% Can contribute to the formation of embrittlement particles Fe ₂₃ (C, B) ₆ (a form of iron borocarbide). Therefore, an upper limit of about 0.0025% boron is preferred. To obtain the maximum effect on hardenability, a boron concentration of at least 0.0010% is desirable.

The temperature of the end of rolling of 840 ± 15 ° C provides dispersion hardening and grinding of grain.

If the temperature of the end of rolling is more than 840 ± 15 ° C, then the required level of strength properties of sheet metal is not provided.

If the temperature of the end of rolling is less than 840 ± 15 ° C, then the plastic and viscous properties of the rolled plate are deteriorated.

The temperature of the end of accelerated cooling depends on the content of boron in the steel and is determined from the ratio:

where T _to.o is the temperature of the end of accelerated cooling, ° C; In - the content in the steel of boron, wt.%.

If the temperature of the end of accelerated cooling is more than 4 · 10 ⁴ · V + 560 ± 15 ° C, the required level of strength properties is not provided.

If the accelerated cooling end temperature is less than 4 × 10 ⁴ × B + 560 ± 15 ° C, then substantially degraded plastic and viscous properties of the rolled plate.

Accelerated cooling is carried out at a speed of 8.0-16.0 ° C / s to the temperature of the end of accelerated cooling.

If accelerated cooling is carried out at a rate of less than 8.0 ° C / s, then the required level of strength properties is not provided.

If accelerated cooling is carried out at a rate of more than 16.0 ° C / sec, then the plastic and viscous properties of the plate are significantly impaired.

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.

An example of the method

The application of the method is illustrated by an example of its implementation in the production of sheets with a thickness of 16.0 mm from strength class K56. A preform was prepared containing, wt.%: 00.05; Si = 0.36; Mn = 1.55; S = 0.002; P = 0.009; Al = 0.037; N = 0.005; Ti = 0.021; Nb = 0.036, boron = 0.0019, the rest is iron and impurities.

The preform (slab) with a cross section of 300 × 2600 mm was heated in an oven to a temperature of 1170 ° C. Then, a controlled rolling of the metal was carried out with a temperature of the end of rolling of 840 ± 15 ° C. Then produced accelerated cooling of the metal at a rate of 16 ° C / sec.

The temperature of the end of accelerated cooling was determined from the ratio:

Those. accelerated cooling was carried out to a temperature of 636 ± 15 ° C in accordance with the claimed method.

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

The chemical composition of the experimental steels is presented in table. 1. Tests for static tension were carried out on flat specimens in accordance with GOST 1497, and for impact bending on specimens with a V-shaped notch in accordance with GOST 9454 at a temperature of -20 ° C. The following mechanical properties were obtained (Table 2). The specified level of properties fully complies with the requirements for sheets of strength class K56.

Thus, the application of the proposed rolling method provides the desired result — an increase in the strength properties of economically alloyed pipe steel of strength class K56 while maintaining sufficient ductility and toughness of sheets with a thickness of 14-24 mm.

Claims (1)

A method for the production of plate steel, microalloyed with boron, including obtaining continuously cast slab, heating, roughing and finishing rolling and accelerated cooling of the finished roll to a predetermined temperature, characterized in that the continuously cast slab is obtained from steel of the following chemical composition, wt.%:
carbon 0.05-0.07 silicon 0.30-0.55 manganese 1.40-1.55 sulfur no more than 0,005 phosphorus no more than 0.015 aluminum 0.025-0.045 nitrogen no more than 0,008 titanium 0.015-0.030 niobium 0,020-0,035 boron 0.0010-0.0025 iron and impurities rest,
the temperature of the end of the rolling is taken 840 ± 15 ° C, and accelerated cooling is carried out at a speed of 8.0-16.0 ° C / s to a temperature determined from the ratio:
T _c.o. = 4 · 10 ⁴ · V + 560 ± 15 ° C,
where T _k.o. - temperature of the end of accelerated cooling, ° C;
In - the content of boron in steel, wt.%.