RU2533244C1 - Method of high-strength thick-sheet steel production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention can be used for production of 8.0-40.0 mm deep steel sheets for making of platforms for trucks operated in Far North. Slabs are made of steel containing in wt %: 0.13-0.18 C, 0.40-0.60 Si, 0.7-0.9 Mn, 1.3-1.6 Cr, 0.02-0.07 Al, 0.03-0.06 Nb, 0.01-0.06 Ti, 0.002-0.030 Ca, Ni≤0.30, Cu≤0.30, N≤0.010, Fe and impurities making the rest. Cast slabs are annealed at 640-660°C. Thereafter, they are heated and hot rolled at 1200-1260°C and 870-950°C with further quenching in water and tempering.

EFFECT: better mechanical properties and higher yield.

3 tbl

Description

The invention relates to metallurgy and can be used to obtain high-strength sheet steel with a thickness of 8.0-40.0 mm for the manufacture of platforms and other heavily loaded parts of trucks operating in the Far North.

In the manufacture of the aforementioned welded structures of transport and mining machines, thermo-improved hot-rolled sheet metal is used. After thermal improvement, hot rolled sheets must combine high strength, toughness at low temperatures and resistance to abrasion. The required set of properties of hot rolled sheets in the delivery state is given in Table 1.

A known method for the production of high-strength low-alloy steel, including the manufacture of slabs, their heating to a temperature of 1000-1180 ° C, multi-pass hot rolling with a temperature of the end of rolling T _KP = 950 ° C in sheets of finite thickness. Hot rolled sheets are then heated at a rate of at least 25 ° C / min, quenched with water and subjected to tempering [1].

The disadvantages of this method are that the hot-rolled sheets after thermal improvement (tempering with tempering) have low viscosity properties and insufficient strength.

There is also a known method for the production of high-strength sheets of low alloy steel, comprising heating slabs to a temperature not exceeding 1150 ° C and hot rolling in several passes with a total compression of at least 30% and with a temperature of the end of rolling 900-950 ° C. Hot-rolled sheets are heated to a temperature of Ac ₃ ÷ 1000 ° C and quenched, after which they are tempered at a temperature of 200-400 ° C and cooled with water [2].

The disadvantages of this method are that the finished sheets have low viscosity properties. In addition, fluctuations in the content of chemical elements in steel have a significant impact on the level and stability of mechanical properties, which reduces yield.

The closest analogue to the present invention is a method for the production of plate from welded chromium-manganese steel, including casting steel into slabs, heating them to 1230 ° C, multi-pass hot rolling into sheets in a regulated temperature range, water quenching and tempering, according to which hot rolling is carried out with the total relative compression of not less than 50% and complete at a temperature of 830-950 ° C, the hardening of the sheets is carried out at a temperature of 850-940 ° C, and tempering is carried out at a temperature of 600-690 ° C, and s has the following chemical composition, wt.%:

Carbon 0.13-0.18 Silicon 0.4-0.7 Manganese 1.2-1.8 Chromium 0.4-0.8 Copper 0.20-0.45 Vanadium 0.04-0.08 Aluminum 0.02-0.05 Titanium 0.02-0.05 Calcium 0.002-0.030 Niobium no more than 0,06 Cerium no more than 0,05 Sulfur no more than 0,008 Phosphorus no more than 0.015 Iron the rest [3]

The disadvantages of this method are that plate steel has a low and unstable complex of mechanical properties, especially at low temperatures. This, in turn, leads to a decrease in yield.

The technical problem solved by the invention is to increase the complex of mechanical properties and yield.

To solve the technical problem in the known method for the production of high-strength plate steel, which includes continuous casting of steel into slabs, their heating, multi-pass hot rolling of sheets in a regulated temperature range, water quenching and tempering, unlike the closest analogue, steel of the following chemical composition is subjected to continuous casting, wt.%:

Carbon 0.13-0.18 Silicon 0.40-0.60 Manganese 0.70-0.90 Chromium 1.3-1.6 Aluminum 0.02-0.07 Niobium 0.03-0.06 Titanium 0.01-0.06 Calcium 0.002-0.030 Nickel no more than 0.30 Copper no more than 0.30 Nitrogen no more than 0,010 Iron and impurities rest

cast slabs are additionally annealed at a temperature of 640-660 ° C, after which they are heated to a temperature of 1200-1260 ° C and subjected to hot rolling, the temperature of the end of the finish rolling is set to 870-950 ° C.

The invention consists in the following. The final mechanical and functional properties of sheet steel are determined simultaneously by its chemical composition, temperature conditions of rolling, hardening and tempering. In the process of conducting experimental studies, all significant factors were varied, including the chemical composition of steel and temperature conditions of production, achieving the desired and stable mechanical properties, which increases the yield. The following was established.

Carbon reinforces steel. With a carbon content of less than 0.13%, the required strength of the steel is not achieved, and with its content of more than 0.18%, the toughness and wear resistance of the steel deteriorate.

Silicon deoxidizes steel, increases its strength. At a silicon concentration of less than 0.40%, the strength of the steel is lower than permissible, and at a concentration of more than 0.60%, ductility decreases, the steel does not stand the test for cold bending.

Manganese deoxidizes and strengthens steel, binds sulfur. When the manganese content is less than 0.70%, the wear resistance of sheet steel is sharply reduced. An increase in manganese content of more than 0.90% leads to a decrease in viscosity at low temperatures, a decrease in ductility, and a decrease in yield.

Chrome increases the strength, toughness and wear resistance of steel. At a concentration of less than 1.3%, strength, toughness and wear resistance are below acceptable values. An increase in the chromium content of more than 1.6% leads to a loss of ductility due to the growth of carbides, and a decrease in the yield of thermally improved rolled sheets.

Aluminum preoxidizes steel and grinds grain. When the aluminum content is less than 0.02%, its effect is small, the viscosity properties of steel deteriorate. An increase in the content of this element of more than 0.07% leads to instability of the viscosity properties and a decrease in the yield of sheet metal.

Niobium contributes to the grinding of the microstructure of steel by sheet thickness, increasing cold resistance and strength. Small niobium carbides are located along the boundaries of grains and subgrains, inhibit the movement of dislocations and thereby strengthen the steel. However, if the niobium content is more than 0.06%, the yield will decrease, and the weldability of steel will deteriorate. With a decrease in niobium content of less than 0.03%, a high impact strength at low temperatures is not achieved and the wear resistance of sheet steel is deteriorated.

Titanium, being a strong carbide-forming element, helps to increase the strength properties of the strips while increasing the toughness at low temperatures. A decrease in the titanium content of less than 0.01% leads to a decrease in the strength and viscosity properties of the sheets. An increase in titanium content of more than 0.06% leads to a decrease in mechanical properties and yield of sheet steel.

Calcium is a modifying element. In addition, it binds sulfur to globular sulfides, increasing the viscosity properties of steel. At a calcium concentration of less than 0.002%, its modifying effect is not sufficiently manifested. An increase in calcium concentration of more than 0.030% increases the number and size of non-metallic inclusions, degrades toughness at low temperatures and reduces the yield of sheet metal.

Nickel and copper enhance the plastic and toughness properties of plate steel. However, an increase in the nickel content of more than 0.30% or copper more than 0.30% leads to an increase in the phase composition of the sheet steel after quenching of the residual austenite, which causes a deterioration in mechanical properties.

Nitrogen in chemical compounds with titanium, vanadium and other alloying elements strengthens steel by the dispersion hardening mechanism. However, an increase in nitrogen content of more than 0.010% reduces the ductility of the steel and its toughness.

To equalize the mechanical properties and eliminate crack formation, the cast slabs are annealed at 640-660 ° C. Lowering the annealing temperature below 640 ° C does not exclude the presence of cracks in the slabs, which reduces the yield. Raising the annealing temperature above 660 ° C does not lead to a further increase in the quality and yield of metal products, but only increases production costs.

It was also found that at the temperature of the onset of rolling slabs of steel of the proposed chemical composition 1200-1260 ° C, its austenitization, complete dissolution of sulfides, phosphides, alloying and impurity compounds, carbide reinforcing particles in the austenitic matrix is ensured. At a temperature of rolling onset above 1260 ° C, intensive oxidation of crystallite boundaries takes place, cracking and yield reduction occurs, and at a temperature of rolling onset below 1200 ° C, the set of mechanical properties of the sheets deteriorates.

The rolling process occurs with a continuous decrease in the temperature of the metal, which, by the time the sheets are rolled, decreases to T _kp = 870-950 ° C, which helps to intensify the release of hardening carbide and carbonitride particles, grinding the grain microstructure of steel. At T _cp below 870 ° C, the proposed steel acquires a two-phase composition, uneven grain size microstructure, plate steel retains anisotropy of properties, there is a deterioration in the complex of mechanical properties and a decrease in yield. At Т _кп above 950 ° C, an intensive and uneven growth of microstructure grains takes place, a decrease in the viscosity properties at low temperatures, and a decrease in the yield.

Method implementation examples

Steel of various chemical composition is smelted in an oxygen converter using scrap metal. In the ladle, steel is deoxidized with ferrosilicon, ferromanganese, alloyed with ferrochrome, ferrotitanium, metallic aluminum and niobium are introduced. Calcium is introduced into the melt in the form of silicocalcium, nickel and copper enter the melt from scrap metal. The smelted steels are degassed by evacuation to remove hydrogen and reduce the nitrogen content to concentrations of N≤0.010%. The chemical composition of the smelted steels is given in table.2.

Steel with composition No. 3 is subjected to continuous casting into slabs with a thickness of 250 mm. After casting, slabs are placed in thermostatic pits, where they are slowly cooled to a temperature of ~ 90 ° C. The cooled slabs are reheated to a temperature of 300 ° C and cleaned to remove defects. The peeled slabs are placed in a chamber furnace and annealed at a temperature of T _o = 650 ° C.

Annealed slabs are heated in a methodical furnace to the temperature of rolling start T _np = 1200 ° C and rolled in 12 passes on a plate reversing mill 5000 in sheets with a thickness of H = 20 mm During rolling (in the aisles and pauses between aisles), a decrease in temperature (cooling) of the sheets occurs. Rolling in the last pass is carried out at a temperature T _kn = 910 ° C.

Laminated sheets are stacked, where they are slowly cooled in air.

The rolled sheets are then heated to hardening to a temperature T _c = 915 ° C and subjected to hardening with water in a roller hardening machine. The hardened sheets are subsequently tempered by heating and holding at a temperature T _Otp = 650 ° C.

After thermal improvement, samples are taken from the sheets, mechanical properties are tested, and the yield is determined after grading Q.

Implementation options for the production of high-strength welded sheet steel and indicators of their effectiveness are given in table 3.

From tables 2 and 3 it follows that the proposed production modes of high-strength plate steel (options No. 2-4) provide an increase in the complex of mechanical properties, thereby achieving a maximum yield: Q = 98.3-98.8%.

With exorbitant values of the concentrations of chemical elements in steel, temperature conditions of hot rolling, quenching and tempering (options No. 1 and No. 5) as well as using the closest analogue method [3], there is a decrease in the set of mechanical properties of the finished sheets and yield Q. In these In cases, sheet steel is used for less critical purposes.

The technical and economic advantages of the proposed method consist in the fact that the additional homogenizing annealing of continuously cast slabs of the proposed chemical composition at a temperature of 640-660 ° C, as well as the simultaneous optimization of the chemical composition of steel, temperature conditions of hot rolling, and subsequent quenching with tempering, make it possible to increase the complex of mechanical properties of high-strength plate steel. Due to this, the yield is increased.

Information sources

1. Application No. 61-163210, Japan. IPC C21D 8/00, 1986

2. Application No. 61-223125, Japan. IPC C21D 8/02, C22C 38/54, 1986

3. Patent of the Russian Federation No. 2455105, IPC B22D 11/00, 2012

Table 1 Mechanical properties of sheet steel σ _in , N / mm ² σ _t , N / mm ² δ ₅ ,% KCU ^-40 , J / cm ² KCV ^-40 , J / cm ² HB, units Bending angle, degrees no less 690 590 fourteen 40 thirty 340-400 120

table 2 The chemical composition of high-strength plate steels Composition number The content of chemical elements, wt.% C Si Mn Cr Al Nb Ti Ca Ni Cr N Fe one 0.12 0.3 0.6 1,2 0.01 0.02 0.009 0.001 0.10 0.10 0.008 Rest 2 0.13 0.4 0.7 1.3 002 0,03 0.010 0.002 0.20 0.10 0.007 -: - 3 0.15 0.5 0.8 1,5 0.05 0.04 0,035 0.016 0.20 0.20 0.006 -: - four 0.18 0.6 0.9 1,6 0,07 0.06 0,060 0,030 0.30 0.30 0.010 -: - 5 0.19 0.7 1,0 1.7 0.08 0,07 0,070 0,032 0.40 0.40 0.012 -: -

Table 3 Production modes of high-strength plate steel and indicators of their effectiveness No. p / p Composition number T _o , ° C T _np , ° C T _kp , ° C T _s , ° C T _otp , ° C σ _in , N / mm ² σ _t , N / mm ² δ ₅ ,% KSU ^-40 , J / cm ² KCV ^-40 , J / cm ² Bending angle, degrees Q% one 5 630 900 860 890 620 650 580 19 33 26 89 - 2 2 640 1200 870 900 630 690 590 fifteen 42 31 120 98.7 3 3 650 1230 910 915 650 700 600 eighteen 44 32 130 98.8 four four 660 1260 950 940 670 710 610 17 42 31 120 98.3 5 one 670 1270 960 950 680 680 585 13 32 29th BY -

Claims (1)

Method for the production of high-strength plate steel, including continuous casting of steel into slabs, their heating, multi-pass hot rolling of sheets in a regulated temperature range, water quenching and tempering, characterized in that the steel is subjected to continuous casting of the following chemical composition, wt.%:
carbon 0.13-0.18 silicon 0.40-0.60 manganese 0.70-0.90 chromium 1.3-1.6 aluminum 0.02-0.07 niobium 0.03-0.06 titanium 0.01-0.06 calcium 0.002-0.030 nickel no more than 0.30 copper no more than 0.30 nitrogen no more than 0,010 iron and impurities rest
in this case, the cast slabs are annealed at a temperature of 640-660 ° C before heating, the slabs are heated to a temperature of 1200-1260 ° C and subjected to hot rolling in the temperature range up to 870-950 ° C.