RU2605037C1 - Method for production of high-strength hot-rolled steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, specifically to production of high-strength hot-rolled steel, used for making articles for petrochemistry and high-speed transport operating in extreme conditions, as well as base layer of bimetallic structures. Workpiece is produced from steel containing following component ratio, wt%: carbon 0.16-0.45, silicon 0.05-0.70, manganese 0.50-1.50, sulphur 0.002-0.008, phosphorus not more than 0.015, chromium not more than 0.15, nickel not more than 0.15, copper not more than 0.15, niobium from 0.005 to less than 0.01, acid-soluble aluminium 0.02-0.05, iron and unavoidable impurities - balance, wherein content of manganese and sulphur is linked by relationship [Mn]·[S] < 0.005. Workpiece is heated to temperature within range from more than 1,250 to 1,300 °C and subjected to hot rolling.

EFFECT: obtained rolled product has high strength properties.

1 cl, 2 tbl

Description

The invention relates to the field of metallurgy, to methods for the production of hot rolled steel with high strength properties, intended for use in oil and gas chemistry and high-speed vehicles operating in extreme conditions, as well as as the main layer of bimetallic products for this purpose.

Traditionally, carbon (St. 3, 20K) and low alloyed, including high strength (16GS, 09G2S) structural steels are used for these purposes, which provide the required strength, resistance to shock loads, manufacturability during bending, stamping, and high weldability.

The requirements for the properties of modern steels are constantly growing, in particular, this relates to high strength while maintaining high ductility and viscosity of the material. High strength is achieved through a combination of various hardening mechanisms, the most favorable of which is the grinding of the grain structure, since this simultaneously increases both strength and ductility. In turn, grain grinding can occur by various mechanisms. In particular, this is possible due to the presence of barriers in the initial billet or due to the formation of barriers during high-temperature heating that will inhibit the growth of austenite grain, for example, grain-boundary segregations of the main (manganese, silicon) or impurity (sulfur, phosphorus) elements, or precipitates excessive phases involving these and / or microalloying elements. The second currently most important reason for the formation of a fine-grained structure of finished steel is the inhibition of recrystallization by creating conditions for the release of excess phases during sub-micron precipitation during hot rolling. Traditionally, this is achieved by microalloying steel with carbon-nitride-forming elements - titanium, niobium, vanadium, which form precipitates of excess phases that contribute to grain refinement, in particular, by the formation of submicron particles inhibiting recrystallization during hot rolling. At the same time, microalloying leads to a significant increase in the cost of steels, which limits their use.

For steels that do not contain microalloying elements, the method of ensuring high strength is the use of heat treatment - quenching with tempering, which also leads to an increase in the cost of metal products.

This indicates the feasibility of developing steels with an improved composition and technologies for their production, providing a high set of properties by controlling the structure formation in steel, in particular, during hot rolling, while reducing the content of microalloying elements in steel.

A known method of manufacturing sheet steel with high strength and impact strength, with a yield strength of 900 MPa, which consists in the manufacture of steel of the following chemical composition (in wt.%) With 0.07 ~ 0.14, Si 0.25 ~ 0.50, Μn 1.70 ~ 2.20, Cr 0.05 ~ 0.50, Ni 0.20 ~ 0.50, Nb 0.03 ~ 0.10, V 0.03 ~ 0.10, Ti 0.01 ~ 0.0, Al 0.02 ~ 0.04, V 0.0006 ~ 0.0025, the rest is iron and inevitable impurities (Patent CN 102337482 B, IPC С22С 38/58, B21B 37/74, publ. 11/20/2013). The use of thermomechanical controlled rolling and cooling technology makes it possible to obtain a matrix of ultra-thin bainite plates in steel of a given chemical composition with yield strength ≥900 MPa, tensile strength ≥980 MPa, and Charpy impact strength at (-20 С) more than 150 J. Predisposition index to the formation of welded cracks of such steel Pcm≤0.25%, which indicates good weldability.

The specified invention provides a high combination of strength, toughness, weldability. However, the increased content of alloying and microalloying elements leads to a high cost of steel. The need to use complex thermomechanical processing requires a high production culture.

A known method of producing steel with a tensile strength of at least 900 MPa, comprising heating a steel slab to 950-1250 ° C; hot rolling of a slab at a temperature not exceeding 950 ° C with a total degree of compression of at least 25% for the formation of plate steel, completion of hot rolling at a temperature not lower than the temperature of the phase transition Ar3 upon cooling or at 700 ° C, depending on which of these temperatures will be higher; and cooling the steel plate from a temperature not lower than 700 ° C with a cooling rate in the range of 10-45 ° C / s, when measuring it in the central zone of the steel plate until the central zone of the steel plate cools to a temperature not exceeding 450 ° C . Steel contains, by weight. %: carbon 0.02-0.1, silicon no more than 0.6, manganese 0.2-1.7, nickel 0.2-1.2, niobium 0.01-0.1, titanium 0.005-0.03, aluminum no more than 0.1, nitrogen 0.001-0.006, copper 0-0.6, chrome 0-0.8. molybdenum 0-0.6, vanadium 0-0.1. boron 0-0.0025, calcium 0-0.006, phosphorus ≤0.015, sulfur ≤0.003, iron and impurities - the rest. Moreover, steel contains carbides with a particle size of less than 5 μm and has a carbon equivalent value Vs equal to 0.15-0.42 and determined by the formula Vs = С + (Μn / 5) +5 Ρ- (Ni / 10) - (Mo / 15) + (Cu / 10). Steel has a mixed microstructure consisting of martensite and lower bainite, the mixed structure being at least 90 vol.% Of the microstructure of the steel, lower bainite being at least 2 vol.% Of the mixed structure, and previous austenitic grains have a length to width ratio at least 3.

Steel has high performance: toughness over its entire thickness, a high level of properties in welded joints and has a tensile strength of at least 900 MPa.

The technical result of this invention is the high strength and toughness due to the complex alloying system. (Patent RU 2205245 IPC С22С 38/08, С22С 38/50, С22С 38/58, publ. 05.27.2003).

In the said invention, hardening is supposed to be achieved by creating shear structures, as well as by inhibiting the recrystallization of austenite. Grinding of austenitic grain in this method is carried out due to the expensive alloying with carbonitride-forming elements.

The closest analogue of the claimed invention is a method for the production of rolled products from structural low-alloy steel 09G2S. This steel is used for various products and equipment, including the petrochemical industry, operating at temperatures from -70 to + 425 ° C under pressure. Steel contains, by weight. %: carbon up to 0.12, silicon 0.5-0.8, manganese 1.3-1.7. nickel to 0.3, chromium to 0.3, nitrogen to 0.008, copper to 0.3, phosphorus to 0.035, sulfur to 0.04, iron and impurities - the rest. Rolled steel of such steel has the following mechanical properties: in the hot-rolled state, the tensile strength is 490 MPa, the yield strength is 345 MPa, after normalization, the tensile strength is 470 MPa, the yield strength is 265 MPa, and after quenching with tempering, the tensile strength is 510 MPa and the yield strength is 340 MPa. (GOST 19281-89 Rolled steel of increased strength. General specifications - prototype)

The disadvantage of this steel is that the highest mechanical characteristics are achieved only through additional technological operations, such as quenching with tempering. At the same time, the level of strength also remains relatively low.

The problem to which the invention is directed is to optimize the chemical composition and technology of steel production with a technical result in the form of increasing the strength and ductility of steel by grinding the structure during hot rolling, while reducing the cost of steel production by simplifying the process chain.

The technical result is achieved by the fact that in the method of manufacturing high-strength hot-rolled steel, which includes obtaining a billet from steel and hot rolling of a billet, according to the invention, the billet is obtained from steel containing components in the following ratio, wt. %:

Carbon 0.16-0.45 Silicon 0.05-0.70 Manganese 0.50-1.50 Sulfur 0.002-0.008 Phosphorus no more than 0.015 Chromium no more than 0,15 Nickel no more than 0,15 Copper no more than 0,15 Niobium from 0.005 to less than 0.01 Acid-soluble aluminum 0.02-0.05 Iron and inevitable impurities rest,

while the content of manganese and sulfur is related by the dependence [Mn] · [S] <0.005, heating the preform before hot rolling is carried out in the range from more than 1250 to 1300 ° C, and hot rolling is carried out with the provision in the process of rolling submicron particle emissions manganese sulfide MnS with a size of 0.1-0.8 microns.

The essence of the invention lies in the fact that the provision of high strength indicators is achieved by obtaining finer ferrite grains in the finished product, it is justified by the allocation during the hot rolling of submicron particles of excess phases - primarily manganese sulfide, and with this chemical composition of steel - to a lesser extent niobium nitride . The selection of these particles during hot rolling inhibits the recrystallization processes, contributing to the formation after rolling of elongated and small austenitic grains with a large length of grain boundaries. The nucleation of ferrite grains during polymorphic transformation during subsequent cooling begins precisely at the boundaries of such austenitic grains, and with an increase in the length of the boundaries, the number of nucleation centers of ferrite grains increases, which leads to a refinement of the final structure and an increase in strength. In this steel, the precipitates of excess phases formed during rolling are not only niobium carbonitride particles traditionally used for this purpose, but also manganese sulfide. With an increase in the heating temperature for rolling and while ensuring the content of sulfur and manganese in steel, as well as their products in the claimed ranges, conditions are created for dissolving during the high-temperature aging of relatively large particles of manganese sulfide that were present in the metal of the cast billet. Sulfur (as well as manganese) passing into the solid solution can affect the refinement of the final rolled metal structure by participating in the formation of smaller (submicron) precipitates of manganese sulfide during hot rolling.

The proposed content of sulfur and manganese in steel, while limiting the value of their product and the heating temperature for rolling, ensures the formation of submicron particles of manganese sulfide during hot rolling, which contribute to the inhibition of recrystallization processes and, thus, the formation of elongated and fine austenitic grains with long boundaries after rolling grains.

The limitation of the lower and upper limits of sulfur content (0.002-0.008 wt.%) In steel, as well as the upper allowable value of the product of manganese content and sulfur content (not more than 0.005%) is determined by the need to dissolve in the process of high-temperature aging of large particles of manganese sulfide and the formation of a sufficient amount smaller (submicron) emissions of manganese sulfide during hot rolling. A further decrease in sulfur content does not contribute to increased strength, since this reduces the number of submicron particles of manganese sulfide formed during hot rolling, and reduces the efficiency of the inhibition of recrystallization. In addition, a further decrease in sulfur content leads to a significant increase in the cost of metal products. When the sulfur content is more than 0.008%, the indicated hardening mechanism is not realized, since in this case, manganese sulfide does not dissolve when heated under rolling.

The limitation of the lower limit of manganese content (0.5 wt.%) Is associated with the need to bind sulfur to MnS particles, which prevents the formation of iron sulfide, the presence of which can have a harmful effect on the surface quality of the car. The limitation of the upper limit of the manganese content (1.50 wt.%) Is associated with the need to obtain high ductility and toughness while maintaining high strength.

The carbon content in the stated limits allows you to further influence the strength characteristics of steel, as it affects the amount of carbonitride

secretions, and the formation of reinforcing structural components. Lower carbon content leads to lower strength. Its higher content compared with the stated limits adversely affects ductility.

The content of silicon (0.05-0.70 wt.%) And aluminum (0.02-0.05 wt.%) In the proposed range allows you to provide the required level of deoxidation of steel. In addition, the specified aluminum content leads to the formation of particles of aluminum nitride, inhibiting grain growth, which, in turn, leads to an increase in strength as well as viscosity. An increase in the content of these elements leads to a decrease in the ductility of steel.

The limitation of the content of nickel (<0.15 wt.%), Copper (<0.15 wt.%) And phosphorus (not more than 0.015 wt.%) Avoids cold brittleness of this steel, since it prevents the formation of segregations along grain boundaries.

The limitation of chromium content (<0.15 wt.%) Provides high weldability of steel. An increase in its content negatively affects the weldability and leads to an increase in the cost of production.

The microalloying of steel with niobium (from 0.005 to less than 0.01 wt.%) Within the indicated limits leads to an increase in the strength, toughness and cold resistance of steel due to the release of fine particles of niobium carbonitrides and / or carbides at the final stage of hot rolling and during cooling of the coiled coil , as well as due to the development of dispersion hardening processes that increase strength characteristics.

The temperature of heating the metal for rolling to temperatures from more than 1250 to 1300 ° C ensures the dissolution of manganese sulfide in steel of this chemical composition, and upon subsequent cooling, its release in the form of smaller submicron particles inhibiting the recrystallization of austenite, as well as being the nucleation centers of polygonal ferrite grains in polymorphic transformation process. Heating to a lower temperature of 1250 ° C, despite obtaining finer austenite grain after heating, does not provide the dissolution of manganese sulfide particles, sufficient for its subsequent isolation in the form of submicron particles, affecting the recrystallization of austenite and the formation of ferrite. Heating to a temperature of more than 1300 ° C leads to the formation of a too coarse-grained structure of austenite, which is also inherited in finished products.

Examples of specific performance of the method:

Five carbon steel options (chemical composition is given in table No. 1) were obtained in laboratory conditions. Varying the heating temperatures for rolling (1150 ° С, 1255 ° С, 1300 ° С and 1350 ° С), hot rolling was carried out on a laboratory mill to a thickness of 3.5 mm, followed by simulation of winding into a roll (the obtained strips after rolling were placed in an oven heated to a temperature of 580 ° C and cooled with an oven). In addition to tensile tests with the determination of yield strength, tensile strength and elongation (the results are shown in table No. 2), electron-microscopic studies of manganese sulfide precipitates in finished products were carried out, depending on the temperature of the heating for rolling using a JSM- scanning electron microscope 6610LV of the JEOL company.

As can be seen from table No. 1, the chemical composition of swimming trunks No. 1,2 corresponds to the claims, while for melting No. 3, the sulfur content, as well as the product of sulfur content and manganese content is higher than in the claims, for melting No. 4, the sulfur content lower than stated in the invention, and smelting No. 5 differs from the stated higher content of manganese.

For all smelting options, the heating modes of steel for rolling (table No. 2) with No. 2, 4 correspond to the claims. Mode No. 1 has a lower heating temperature, and mode No. 4 has a higher heating temperature in comparison with the rolling heating temperatures stated in the claims.

As can be seen from the data given in tables No. 1 and No. 2, the highest strength indicators (temporary resistance not lower than 670 MPa) while maintaining ductility (at least 23%) have steel options No. 1 and 2, corresponding to the claims according to the chemical composition , for heating options for rolling to temperatures above 1250 to 1300 ° C, also corresponding to the claims.

In rolled samples with the proposed chemical composition, obtained from billets heated for rolling to temperatures above 1250 to 1300 ° C, submicron particles of manganese sulfide are present, often on oxides, but also of submicron size. In rolled products from billets heated for rolling at lower temperatures (less than 1250 ° C) with the same sulfur content, manganese sulfide precipitates are located on larger complex oxides, often containing calcium and magnesium. At temperatures of 1150 ° C, the molar fraction of manganese sulfide, which dissolves when heated for rolling, which means

released during the rolling process is insufficient for effective inhibition of recrystallization. At 1350 ° C, the growth of austenitic grain during heating under rolling prevails, which is inherited in finished products and leads to a decrease in strength.

As can be seen from the data in tables No. 1 and No. 2, for steel with a high manganese content (more than 1.5%), regardless of the heating temperature for rolling, lower strength characteristics are noted. This is due to insufficient dissolution of manganese sulfide during heating for rolling. However, it is worth noting that with an increase in the heating temperature of steel for rolling, a slight increase in ductility is observed due to a certain fraction of particles. These data confirm the need to limit the manganese content to not more than 1.5%.

If the sulfur content is increased above 0.008% (in particular, to 0.010%), as well as the product of sulfur content and the manganese content (steel of option 3), this mechanism of grinding grain and increasing strength is not implemented, since in this case manganese sulfide does not dissolve when heating for rolling. Regardless of the heating temperature for rolling, a coarse-grained ferrite-pearlite structure and a low level of strength characteristics are formed in such steel. With a low sulfur content, less than 0.002% (for steel of option No. 4 [S] = 0.001%), the total amount of manganese sulfide becomes insufficient for effective inhibition of recrystallization processes. To implement the considered hardening mechanism with the participation of manganese sulfide, steel with a sulfur content from 0.002 to 0.008% is most favorable.

Thus, favorable types of excess phases include precipitates formed during hot rolling and having submicron sizes, mainly 0.1-0.8 microns. Their formation leads to inhibition of the processes of recrystallization of hot-rolled steel, to the corresponding grinding of austenite grain, inherited in the final structure. Such precipitates include not only carbonitride precipitates, but also precipitates of manganese sulfide, the formation of which during hot rolling is possible with a certain content of sulfur and manganese. Obviously, in the case of the implementation of the hardening mechanism associated with the formation of particles of manganese sulfide, the content of niobium in steel can be reduced.

The results are shown in tables No. 1 and No. 2, confirm that the steel options corresponding to the claims demonstrate high strength and ductility after hot rolling, without additional heat treatment.

Claims (1)

A method for the production of high-strength hot-rolled steel, including the preparation of a billet from steel, hot rolling of a billet, characterized in that the billet is obtained from steel containing components in the following ratio, wt.%:
Carbon 0.16-0.45 Silicon 0.05-0.70 Manganese 0.50-1.50 Sulfur 0.002-0.008 Phosphorus no more than 0.015 Chromium no more than 0,15 Nickel no more than 0,15 Copper no more than 0,15 Niobium from 0.005 to less than 0.01 Acid-soluble aluminum 0.02-0.05 Iron and inevitable impurities - the rest,
the content of manganese and sulfur is related by the dependence [Mn] · [S] <0.005, before hot rolling, the workpiece is heated in the range from more than 1250 to 1300 ° C, and during hot rolling, submicron particles of manganese sulfide MnS are separated in size by size 0.1-0.8 microns.