RU2303638C1 - Method for producing of chromium-nickel sheet steel - Google Patents

Abstract

FIELD: rolling and thermal processing in metallurgical industry.

SUBSTANCE: method involves heating flat blanks to austenite conditioning temperature; subjecting blanks to multiple-pass reversing hot rolling process with regulated reducing action in passes; finishing rolling process at predetermined final rolling stage temperature; holding sheets in air and quenching in water, with holding duration time and temperature at final rolling stage being set depending on content of chemical elements in steel according to ratios: T_fr≥850 C and ι≤30 s at 2·C+Ni+Cr=0.90-1.80; T_fr≥750 C and ι≤55 s at 2·C+Ni+Cr=1.81-3.40; T_fr≥700 C and ι≤65 s at 2·C+Ni+Cr=3.41-4.80, where T_fr is temperature at final rolling stage; ι is holding duration time before quenching process; C, Ni, Cr is content in steel of carbon, nickel and chromium, correspondingly, wt%. On rolling of sheets having thickness below 7 mm, relative reduction in each pass is kept within the range of 30-50%.

EFFECT: improved quality, stabilized mechanical properties, increased resistance to high-energy impact-pulsed loadings and increased yield of steel sheets.

2 cl, 2 tbl, 3 ex

Description

The invention relates to the field of rolling production and heat treatment and can be used to obtain high-strength sheet steel for special and universal purposes, including for armored structures.

A known method for the production of high-strength steel, including heating the slabs to an austenitizing temperature of 1000-1180 ° C, multi-pass hot rolling with a temperature of the end of rolling 950 ° C to a final thickness. The 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 chromium-nickel steel sheet after rolling and hardening has low and uneven mechanical properties, unsatisfactorily resists high-energy shock-impulse loads (WINE) that occur during test firing of small arms samples.

There is also known a method of manufacturing high-strength steel sheets, including heating slabs to an austenitic temperature, but not more than 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. The hot-rolled sheet is heated to a temperature of Ac ₃ -100 ° C and quenched, and then subjected to tempering at a temperature of 200-400 ° C and cooled with water [2].

The disadvantages of this method are that the finished sheets have insufficient resistance against WINE. In addition, a change in the content of chemical elements in steel has a significant effect on the stability of mechanical properties.

The closest analogue to the present invention is a method for the production of chromium-nickel sheet steel, comprising heating flat billets (slabs) to an austenitizing temperature of 1200-1300 ° C, multi-pass reversible hot rolling on a plate mill 2800 to a given thickness of 20 mm, which is completed at a regulated temperature of the end of rolling not higher than 950 ° С and compression in the last pass of not less than 15%, heating of sheets in a roller hardening furnace to a temperature of not more than 940 ° C and not less than 920 °, hardening of sheets with water and high tempering to at a temperature of 590-640 ° C [3] - the prototype.

The disadvantages of this method are as follows. After hardening and tempering, the sheets have low mechanical properties and do not withstand the testing of WINVE. A change in the content of chemical elements in steel within the same grade leads to instability of mechanical properties. When rolling sheets of small thicknesses (not more than 7.0 mm), due to their accelerated cooling, the required temperature of the end of rolling is not achieved, which reduces the yield. In addition, the known method requires separate heating of the sheets for hardening and increased energy consumption.

The technical problem solved by the invention is to increase the level and stability of the mechanical properties of the sheets, increase their resistance to shock-pulse loads of high energy and yield. In addition, there is a decrease in energy consumption.

To solve the technical problem in the known method for the production of chromium-nickel sheet steel, which includes heating flat billets to an austenitizing temperature, multi-pass reversible hot rolling, which is completed at a regulated temperature at the end of rolling, and the subsequent hardening of the sheets in water, according to the proposal, the sheets are quenched after exposure to air, and the duration of exposure and the temperature of the end of the rolling set depending on the content of chemical elements in st Whether on relations

T _kp ≥850 ° C and τ≤30 s at 2 · C + Ni + Cr = 0.90-1.80;

T _kp ≥750 ° C and τ≤55 s at 2 · C + Ni + Cr = 1.81-3.40;

T _kp ≥700 ° C and τ≤65 s at 2 · C + Ni + Cr = 3.41-4.80,

where T _kn - the temperature of the end of rolling;

τ is the exposure time before quenching;

τ is the exposure time before quenching;

C, Ni, Cr — carbon, nickel and chromium content in steel, respectively, wt.%.

In addition, when rolling sheets with a thickness of not more than 7.0 mm, the relative compression in each pass is maintained within 30-50%.

The invention consists in the following.

In the process of multi-pass reverse rolling and exposure to air before hardening of sheets of chromium-nickel steels with armor-resistant properties (withstanding WINE), it is necessary to form a dislocation and grain microstructure of deformed austenite with subsequent fine-hardening martensite microstructure of the rack morphology and increasing its mechanical properties, which increases the mechanical resistance of the martensite and this increases the mechanical properties . Since hardening is carried out at a temperature close to the temperature of the end of rolling, this temperature should exceed the temperature values of the critical point Ar ₃ steel, which, in turn, decreases with increasing total content of carbon, chromium and nickel. In addition, after rolling is completed, it is necessary to maintain the sheet in air before hardening to start recrystallization and alignment of the shape of the deformed austenitic grains. In this case, the temperature drop can be neglected, but the austenite grain structure after aging becomes more uniform, the elongation of the grains disappears. Moreover, since the recrystallization rate decreases with decreasing temperature of the sheets, the values of τ must be increased with decreasing T _CP . Due to this, the isotropy of the mechanical properties of hardened sheets, their resistance to WINE, is increased, and, as a result, the yield is increased.

Therefore, the temperature of the end of rolling and the exposure time are set by the following ratios:

T _kp ≥850 ° C and τ≤30 s at 2 · C + Ni + Cr = 0.90-1.80;

T _kp ≥750 ° C and τ≤55 s at 2 · C + Ni + Cr = 1.81-3.40;

T _kp ≥700 ° C and τ≤65 s at 2 · C + Ni + Cr = 3.41-4.80,

where T _kn - the temperature of the end of rolling;

τ is the exposure time before quenching;

C, Ni, Cr — carbon, nickel and chromium content in steel, respectively, wt.%.

When these ratios are fulfilled, the need for separate heating for quenching is eliminated, the level of mechanical properties and resistance to WINPE are increased due to the preservation of the deformation component of hardening, and the effect of changes in the chemical composition of steel decreases the stability of the properties of sheets.

Hot rolling on a reversing mill is performed at relatively low speeds and in order to organize deformation cycling with constant grinding of the austenitic structure when rolling sheets with a thickness of not more than 7.0 mm, the compression per pass should be within 30-50%. The heat into which the work of deformation is converted provides a decrease in the temperature drop of a thin sheet, and increased single-time reductions reduce the required number of passes, reduce the total rolling time and heat loss. Due to this, the regulated temperature of the end of the rolling will always be achievable and more stable.

It has been experimentally established that if:

T _kp <850 ° C and τ> 30 s at 2 · C + Ni + Cr = 0.90-1.80%;

T _kp <750 ° C and τ> 55 s at 2 · C + Ni + Cr = 1.81-3.40%;

T _kp <700 ° C and τ> 65 s at 2 · C + Ni + Cr = 3.41-4.80%,

then the temperature T _kn will be lower than the temperature of the critical point Ar ₃ for steel with a specific content of 2 · C + Ni + Cr. In this case, the last pass during rolling and hardening of sheets in water will be carried out in a two-phase region. This will lead to a decrease in the stability and level of mechanical properties of the sheets, as well as to a decrease in their resistance to WINE and yield. At the same time, an increase in the exposure time τ will lead to an enlargement of the microstructure grains and a decrease in mechanical properties, which is unacceptable.

If, when rolling sheets with a thickness of not more than 7.0 mm, the relative compression in each pass will be less than 30%, this will reduce the adiabatic heat generation during rolling and increase the total number of passes. As a result, T _CP will be below the regulated value. This will lead to a decrease in mechanical properties and yield of sheets with a thickness of not more than 7.0 mm. An increase in the relative compression in each pass of more than 50% will lead to a loss of flatness of the sheets, the formation of an anisotropic microstructure. The consequence of this will be a decrease in the stability and level of mechanical properties of the sheets, their resistance to wind power and yield.

Method implementation examples

For the production of sheets using flat billets of medium carbon chromium-nickel steels, the compositions of which are presented in table 1.

Table 1.
The chemical composition of steels room
composition The content of chemical elements, wt.% FROM Mn Si Ni Cr 2 · C + Ni + Cr P S one. 0.13 0.30 0.27 0.20 0.10 0.70 0.017 0.016 2. 0.14 0.50 0.28 0.50 0.12 0.90 0.015 0.014 3. 0.21 0.55 0.32 0.60 0.33 1.35 0.014 0.015 four. 0.28 0.53 0.29 0.74 0.50 1.80 0.016 0.017 5. 0.19 0.51 0.33 1.00 0.43 1.81 0.018 0.013 6. 0.23 0.52 0.32 1.31 0.84 2.61 0.013 0.018 7. 0.42 0.50 0.29 1,50 1.06 3.40 0,022 0.024 8. 0.32 0.55 0.35 1.68 1.09 3.41 0,023 0,026 9. 0.42 0.53 0.22 1.84 1.43 4.11 0.018 0,022 10. 0.55 0.54 0.31 2.00 1.70 4.80 0,022 0,025 eleven. 0.25 0.58 0.27 2.92 1.48 4.90 0.114 0.016 Note: steels of all compositions contain 0.2% molybdenum

Example 1. Flat billets with a thickness of H ₀ = 200 mm from steel of compositions 2, 3, 4 are heated to an austenitization temperature of 1250 ° C and hot rolled on a quarto 2000 plate mill to a final thickness of H ₁ = 24 mm in 9 passes. Since the ratio 2 · C + Ni + Cr = 0.90-1.80% is satisfied for compositions 2, 3, 4, rolling of the sheets is completed at a temperature T _kn = 870 ° C> 850 ° C. The rolled sheets after exposure to air for a time τ = 20 s, during which the sheets are transported to the quenching machine, are quenched with water from the temperature of the end of rolling T _cn = 870 ° C.

Since for the indicated range of contents 2 · C + Ni + Cr, the temperature of the critical point Ar _{3 is} lower than the temperature of the end of rolling — hardening, the sheets hardened from the single-phase region acquire high and stable mechanical properties, high resistance against WINE and maximum yield. Eliminating the need for heating the sheets before hardening provides a reduction in energy consumption.

Example 2. Flat billets with a thickness of H ₀ = 180 mm from steels of compositions 5, 6, 7 are heated to an austenitization temperature of 1250 ° C and hot rolled on a quarto 2000 plate mill to a final thickness of H ₁ = 14 mm in 7 passes.

Since the ratio 2 · C + Ni + Cr = 1.81-3.40% is satisfied for compositions 5, 6, 7, rolling of the sheets is completed at a temperature T _kn = 770 ° C> 750 ° C. After holding the sheets after exposure to air for 45 s, they are quenched with water from the temperature of the end of rolling T _kn = 770 ° C. Increasing the content of 2 · C + Ni + Cr to 1.81-3.40% reduces the temperature of the critical point of Ar ₃ , therefore, despite the fact that the temperature of quenching has decreased to 770 ° C, quenching is performed from the single-phase region of γ-iron. Therefore, the hardened sheets also acquire high and stable mechanical properties, high resistance against WINE and maximum yield. Eliminating the need for heating the sheets before hardening provides a reduction in energy consumption.

Example 3. For rolling sheets with a thickness of H ₁ = 6.5 mm (ie, less than 7.0 mm) using flat billets with a thickness of 140 mm

Rolling is carried out with relative reductions in each pass ε = 40% according to the scheme:

140 mm → 84 mm → 50.4 mm → 30.24 mm → 18.14 mm → 10.88 mm → 6.5 mm.

Since when rolling sheets with a thickness of H ₁ = 6.5 mm on a plate reversing mill, the temperature drop of the sheets occurs intensively due to the heat transfer to the rolls and the cooling roll water, the temperature of the end of rolling spontaneously decreases to T _kn = 720 ° C. For this case, it is necessary to use steel billets with a content of 2 · C + Ni + Cr = 3.41-4.80%, which corresponds to compositions 8, 9, 10 of table 1, because at the indicated content of chemical elements, the permissible temperature T _kp ≥700 ° C.

Laminated with Т _кп = 720 ° C, sheets of thickness H ₁ = 6.5 mm are held in air for a time τ = 70 s, after which they are quenched with water.

Increasing the content of 2 · C + Ni + Cr to 3.41-4.80% reduces the temperature of the critical point Ar ₃ , therefore, despite the fact that the hardening temperature decreased during the exposure time τ to 710 ° C, hardening is performed from the single-phase region -gland. Therefore, the hardened sheets also acquire high and stable mechanical properties, high resistance against WINE and maximum yield. Eliminating the need for heating the sheets before hardening provides a reduction in energy consumption.

Implementation options for the proposed method and indicators of their effectiveness are given in table.2.

From table 2 it follows that when implementing the proposed method (options No. 2, 3, 5, 6, 8, 9, 10) is achieved by increasing the level and stability of the mechanical properties of the sheets, increasing their resistance to shock-pulse loads of high energy and yield .

In the case of transcendental values of the declared parameters (options No. 1, 4, 7, 11), as well as the prototype method (option 12), the level and stability of the mechanical properties of sheets of various thicknesses decrease, the sheets do not withstand tests with high-energy impact pulses, therefore have zero yield.

The technical and economic advantages of the proposed method are that a change in the temperature of the end of rolling and, consequently, the temperature of hardening, depending on the content of carbon, nickel and chromium in the steel, ensures guaranteed hardening of steel from a single-phase state of deformed austenite and the formation of rack martensite as a result of hardening having the highest resistance against WINVE. In addition, in the production of sheets with a thickness of not more than 7.0 mm, which intensively lose temperature, compression in each pass of 30-50% minimizes heat loss, and their exposure to air before quenching for no more than 65 s leads to the formation of equiaxed microstructure grains , which increases the isotropy of mechanical properties. The increase in the total content of 2 · C + Ni + Cr to 2.41-4.80% allows to lower the critical temperature Ar _{3 of} steel as much as possible and harden it at lower temperatures from the single-phase austenic region.

Table 2.
Modes of production of sheets of medium carbon chromium-nickel steels and their effectiveness No. Technological parameters Properties and yield options Composition number 2 · C + Ni + Cr,% H ₁ mm ε per pass,% T _CP , ° C τ, s σ _in , MPa δ ₅ ,% KCU, J / cm ² Test WINWE Yield,% one. one 0.70 24.0 not regl. 845 40 1100 10 85 I can’t stand it. - 2. 2, 3, 4 0.90-1.80 24.0 not regl. 850 thirty 1345 19 97 having stood. 99.8 3. 2, 3, 4 0.90-1.80 24.0 not regl. 870 twenty 1350 eighteen 96 having stood. 99.8 four. 5, 6, 7 1.81-3.40 14.0 not regl. 750 55 990 9 78 I can’t stand it. - 5. 5, 6, 7 1.81-3.40 14.0 not regl. 770 45 1350 17 96 having stood. 99.7 6. 5, 6, 7 1.81-3.40 14.0 not regl 780 40 1350 eighteen 96 having stood. 99.9 7. 8, 9, 10 3.41-4.80 7.0 28 690 65 980 7 62 I can’t stand it. - 8. 8, 9, 10 3.41-4.80 6.5 thirty 700 70 1350 eighteen 96 having stood. 99.7 9. 8, 9, 10 3.41-4.80 5,0 40 710 80 1345 17 95 having stood. 99.5 10. 8, 9, 10 3.41-4.80 4,5 fifty 730 82 1350 eighteen 96 having stood. 99.3 eleven. eleven 2.61-3.55 5.2 52 700 64 990 13 77 I can’t stand it. - 12. prototype 2.1 20,0 twenty 940 - 950 fifteen 75 I can’t stand it. - Note: sheet properties for all production options are given after low-temperature tempering at 200 ° C

As a basic object in calculating the effectiveness of the proposed method, the technology for the production of sheets of medium-carbon chromium-nickel steels at the metallurgical enterprise "Red October" was adopted. The implementation of the proposed method will increase the profitability of the production of sheets of special and universal purposes by 20-25%.

Literary sources used in the preparation of the description of the invention:

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. 2191833, IPC C21D 8/02, 2002 - prototype.

Claims (2)

1. A method of producing chromium-nickel sheet steel, including heating flat billets to an austenitizing temperature, multi-pass reversible hot rolling with a regulated rolling end temperature and hardening of the sheets in water, characterized in that after hot rolling, the sheets are exposed to air and then hardened, and the exposure time of the sheets in air and the temperature of the end of the rolling set depending on the content of chemical elements in steel according to the ratios T _kp ≥850 ° C and τ≤30 s at 2 · C + Ni + Cr = 0.90-1.80; T _kp ≥750 ° C and τ≤55 s at 2 · C + Ni + Cr = 1.81-3.40; T _kp ≥700 ° C and τ≤65 s at 2 · C + Ni + Cr = 3.41-4.80, where T _kn - the temperature of the end of rolling; τ is the exposure time before quenching; C, Ni, Cr — carbon, nickel and chromium content in steel, respectively, wt.%. 2. The method according to claim 1, characterized in that when rolling sheets with a thickness of not more than 7.0 mm, the relative compression in each pass is maintained within 30-50%.