RU2358025C1 - Method of production of cold rolled metal of upgraded strength - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method consists in steel melting, in casting slabs, in slabs hot rolling into bands, in water cooling, in winding bands in coils, in cold rolling, in re-crystallisation annealing in bell-type furnace and in pinch rolling; also there is melted steel containing, wt %: carbon - 0.05-0.10, silicon - not more, than 0.30, manganese - 0.25-1.20, aluminium 0.01-0.07, nitrogen - not more, than 0.009, niobium and /or titanium - 0.01-0.08, iron and unavoidable impurities - the rest; hot rolling is performed with the temperature of rolling finish 820-875°C; winding of hot rolled bands is carried out with the temperature 510-640°C; re-crystallisation annealing is performed at the temperature of 600-700°C; duration of re-crystallisation annealing in the bell-type furnace lasts 9-21 hours, pinch rolling is executed with reducing 0.8-2.1%. According to the invention contents of carbon, manganese and temperature of annealing are connected with minimal yield point (grade of strength) by dependencies [C]=[0.0416·1n(G_str)-0.167]±0.015, %, [Mn]=(0.0035·Gstr-0.46)±0.20, %; T_anneal≥(810-0.5·Gstr), °C, where [C], [Mn] is contents of carbon and manganese in steel, %, G_str - is dimensionless parametre numerically equal to required minimal yield point; 0.0416; 0.167; 0.0035; 0.46; - are empirical coefficients,%, 810; 0.5 are empirical coefficients, °C.

EFFECT: upgraded strength properties of steel at maintaining forming ability, producing steel of required grade of strength.

4 cl, 7 tbl, 3 ex

Description

The invention relates to the field of metallurgy, and specifically to a technology for the production of cold-rolled steel of increased strength from low alloy steel, intended for the manufacture of automobile parts by stamping.

One of the defining qualities of an autosheet is its ability to stretch when stamping car parts. Cold rolled strips with increased strength and high ability to stretch, depending on the strength class, must correspond to a certain set of mechanical properties, for example, according to the requirements of European standards SEW 093-87 and EN 10268-06 (table 1):

Table 1 Standard Strength class
K _ol * Mark Yield strength σ _0.2 (R _el ), H / mm ² Temporary resistance σ _in (R _m ), N / mm ² Relative elongation δ ₈₀ ,%, not less SEW 093-87 260 ZStE260 260-340 350-450 24 300 ZStE300 300-380 380-480 22 340 ZStE340 340-440 410-530 twenty 380 ZStE380 380-500 460-600 eighteen 420 ZStE420 420-540 480-620 16 EN 10268-06 260 HC260LA 260-330 350-430 26 300 HC300LA 300-380 380-480 23 340 HC340LA 340-420 410-510 21 380 HC380LA 380-480 440-560 19 420 HC420LA 420-520 470-590 17 Note: * Strength class is in the brand name according to the specified standards. The numerical value corresponds to the minimum yield strength.

A known method for the production of cold rolled sheets, including smelting and continuous casting into slabs of steel of the following chemical composition, wt.%:

Carbon no more than 0.07

Manganese 0.25-0.35

Silicon 0.01

Phosphorus no more than 0,020

Sulfur no more than 0,025

Nickel no more than 0.06

Copper no more than 0.06

Chrome no more than 0,03

Iron rest

Continuously cast slabs are heated to a temperature of 1300 ° C, rolled into strips with a temperature of the end of rolling of 860-920 ° C, cooled with water to a temperature of 550-650 ° C and above, and then wound into rolls. Hot rolled strips are etched and cold rolled to the required thickness. Then the cold-rolled strips in rolls are annealed at a temperature of 680-690 ° C for 30-40 hours and trained with compression of 1.0-1.5% [S. S. Guseva et al. Continuous heat treatment of steel sheets. - M .: Metallurgy, 1979, S. 9-26].

The disadvantages of this method are that it does not provide the required level of mechanical properties of rolled strength classes from 260 to 420.

A known method for the production of sheet steel for cold drawing, including hot rolling of continuously cast slabs of mild steel, pickling, multi-pass cold rolling with a total compression of 75%, recrystallization annealing of coils in a bell furnace with heating in several stages: heating at an average speed of 70-80 ° C / h to a temperature of 490-510 ° C, re-heating at an average speed of 3-4 ° C / h to an intermediate temperature of 540-560 ° C and final heating at an average speed of 50-55 ° C / h to a temperature of 700-720 ° C, at which the rolls are aged yut for 12-18 hours. Slabs are poured from steel of the following chemical composition, wt.%:

Carbon 0.025-0.050

Silicon 0.003-0.01

Manganese 0.12-0.19

Aluminum 0.02-0.05

Nitrogen no more than 0.011

Iron the rest [RF Patent No. 2255988, IPC C21D 8/04, publ. 07/10/2005].

The disadvantage of this method is that it does not provide the required level of mechanical properties of strength classes from 260 to 420.

The closest in technical essence to the present invention is a method for the production of cold rolled sheets, including continuous casting of steel slabs, heating slabs to 1150-1240 ° C, hot rolling with a temperature of rolling end not lower than 870 ° C, cooling the strips with water to 550-730 ° C , winding into a roll, cold rolling with a total compression of at least 70%, annealing at 700-750 ° C with holding at this temperature for 11-34 hours. The training of the bands is with compression of 0.4-1.2%. Slabs are poured from steel of the following chemical composition, wt.%:

Carbon 0.002-0.007

Silicon 0.005-0.05

Manganese 0.08-0.16

Aluminum 0.01-0.05

Titanium 0.05-0.12

Phosphorus no more than 0.015

Sulfur no more than 0,010

Chrome no more than 0.04

Nickel no more than 0.04

Copper no more than 0.04

Nitrogen no more than 0,006

Iron the rest [RF Patent No. 2197542, IPC C21D 8/04, publ. 01/27/2003, prototype].

The disadvantage of this method is that it does not provide the required level of mechanical properties of rolled strength classes from 260 to 420.

The problem to which the invention is directed, is to obtain cold-rolled steel of increased strength, intended for cold stamping.

The technical result of the invention is to increase the strength characteristics of steel while maintaining stampability, as well as obtaining steel of the required strength class.

This result is achieved by the fact that in the method of manufacturing a cold rolled strip for cold stamping, including steelmaking, casting slabs, hot rolling slabs into strips, water cooling, winding strips into rolls, cold rolling, recrystallization annealing in a bell furnace and training, according to the invention, is melted steel containing the following components, wt.%:

Carbon 0.05-0.10

Silicon 0.003-0.30

Manganese 0.25-1.20

Aluminum 0.01-0.07

Nitrogen not more than 0.009

Niobium and / or titanium 0.01-0.08 each

Iron and inevitable impurities rest,

hot rolling is carried out with a temperature of rolling end of 820-875 ° С, winding of hot-rolled strips is carried out at a temperature of 510-640 ° С, recrystallization annealing is carried out at a temperature of 600-700 ° С, the duration of recrystallization annealing in a bell furnace is 9-21 hours, the training of strips produced with compression of 0.8-2.1%.

According to the invention, the carbon content, manganese and annealing temperature are associated with a minimum yield strength (strength class) dependencies:

where [C], [Mn] is the carbon and manganese content in steel,%;

0.0416; 0.167; 0.0035; 0.46 - empirical coefficients,%;

810; 0.5 - empirical coefficients, ° С;

K _CR - dimensionless indicator, numerically equal to the required minimum yield strength.

The invention consists in the following.

The mechanical properties of cold rolled sheet steel are affected by both the chemical composition of the steel and the modes of deformation-heat treatment.

Carbon is one of the reinforcing elements. When the carbon content is less than 0.05%, the strength properties of the steel are below an acceptable level. An increase in carbon content of more than 0.10% leads to a decrease in the ductility of steel, which is unacceptable.

Silicon in steel is used as a deoxidizer and an alloying element. When the silicon content is more than 0.30%, ductility decreases sharply, steel embrittlement takes place.

Manganese provides the desired mechanical properties. When the manganese content is less than 0.25%, the strength of the steel is below acceptable. An increase in the manganese content of more than 1.20% overly strengthens the steel, worsens its ductility.

Aluminum is introduced into steel as a deoxidizer. When the aluminum content is less than 0.01%, the ductility of the steel decreases, the steel becomes prone to aging. An increase in aluminum content of more than 0.07% leads to a deterioration in the complex of mechanical properties.

Nitrogen strengthens steel. With a nitrogen content of more than 0.009%, the steel becomes prone to aging.

Niobium and titanium are used as alloying elements and provide the necessary strength properties. When the content of niobium or titanium is less than 0.01%, it is not possible to obtain the required level of strength. An increase in the content of niobium (more than 0.08%) or titanium (more than 0.08%) is impractical due to excessive hardening of steel and deterioration of ductility.

Hot rolling with temperatures of the end of rolling 820-875 ° C and winding 510-640 ° C provides the formation of an optimal metal texture, which after cold rolling and heat treatment according to the proposed modes is transformed into a texture with a predominant crystallographic orientation <111>, as well as microstructure with high stability and uniformity. Below and above the stated temperature limits, the technical result was not achieved, namely, steel acquired a structure with a texture unfavorable for cold stamping and an uneven microstructure of the ferrite matrix.

As a result of recrystallization annealing at a temperature of 600-700 ° C for 9-21 hours, a homogeneous microstructure is formed with a grain score of 9-10 and a minimum release of structurally free cementite. An increase in the annealing temperature above the declared parameters does not provide the necessary level of mechanical properties. A decrease in the annealing temperature below 600 ° C and a decrease in the holding time of less than 9 hours in bell-type furnaces leads to the appearance of individual intermittent lines of recrystallized grains in the microstructure, which impairs the formability of sheet steel. An increase in the exposure time of more than 21 hours unnecessarily lengthens the annealing.

Finally, the mechanical properties are formed during training with compression of 0.8-2.1%. Compression of less than 0.8% does not provide the necessary level of mechanical properties; it leads to the appearance of a yield site on the tensile diagram during tensile testing, which means aging of the metal. Training with compression of more than 2.1% is limited by the technical capabilities of the training camp.

It was established experimentally that for obtaining the desired strength grade carbon and manganese content should be regulated in accordance with the dependencies [C] = [0,0416 · ln (K _etc.) -0,167] ± 0,015,%; [Mn] = (0.0035 · K _ol - 0.46) ± 0.20,%, and the annealing temperature in accordance with the expression T _anne. ≥ (810-0,5 · K _etc.), ° C.

Method implementation examples

In an oxygen converter smelted 7 steel melts, the chemical composition of which is given in table 2.

Smelted steel was poured on a continuous casting machine into slabs with a cross section of 250 × 1280 mm. The slabs were heated in a heating furnace with walking beams to a temperature of 1250 ° C for 2.5-3.5 hours and rolled on a continuous broadband mill 2000 into strips 2.5-3.5 mm thick. The temperature of the strips at the outlet of the last mill stand is regulated. Hot rolled strips on the discharge roller table were cooled with water to certain temperatures and wound into rolls. Chilled rolls were subjected to hydrochloric acid etching in a continuous etching unit. Then, the etched strips were rolled on a 5-stand mill to a thickness of 1.0-2.0 mm. Cold rolled strips were annealed in bell-type furnaces with a hydrogen protective atmosphere. Annealed strips were trained with a predetermined compression. The technological parameters at the rolling stages are shown in table 3. The mechanical properties of the experimental melts are shown in table 4.

Tables 2-4 show the chemical composition, technological parameters and mechanical properties of the proposed method (smelting 2-6), the method with exorbitant values of the declared parameters (smelting 1 and 7) and the prototype method (smelting 8). Examples of the implementation of dependencies (1) - (3) are given in tables 5-7. From tables 2-7 it is seen that in the case of implementing the proposed method (melting 2-6) on cold-rolled steel, mechanical properties are achieved with a yield strength of 260-540 N / mm ² , a temporary resistance of 350-620 N / mm ² , an elongation of more than 16 %, moreover, according to dependences (1) - (3), the mechanical properties correspond to different strength classes from 260 to 420. With the prohibitive values of the declared parameters (melting 1 and 7) and using the prototype method (melting 8), the mechanical properties of rolled strength classes from 260 up to 420 not achieved: for melting №1 strength class 260 does not meet the yield strength and tensile strength; for melting No. 7, the elongation class 420 does not correspond to elongation; for the prototype method (smelting No. 8), the strength class 260 does not correspond to the yield strength and temporary resistance.

Highly loaded car parts, such as body amplifiers and bearing parts of the car frame, were made from rolled products at the consumer, with good stamping results.

table 2
The chemical composition of the experimental swimming trunks No. of swimming trunks The content of elements, wt.% FROM Si Mn Al N Nb Ti Fe and inevitable impurities one 0,03 0,03 0.20 0.01 0.006 0.008 0.002 Rest 2 0.05 0.11 0.25 0.01 0.005 0.010 0,025 Rest 3 0,07 0.16 0.65 0.04 0.006 0,030 0.003 Rest four 0.08 0.21 0.76 0.04 0.006 0,055 0.010 Rest 5 0.08 0.22 0.85 0.05 0.006 0.002 0,080 Rest 6 0.10 0.30 1.20 0,07 0.009 0,080 0,035 Rest 7 0.11 0.35 1.30 0.08 0.010 0,090 0.002 Rest 8 (prototype) 0.0045 0,028 0.13 0,03 0.003 - 0.09 Rest

Table 3
Technological parameters at the rolling stages No. of swimming trunks The temperature of the end of rolling T _CP , ° C Winding temperature at g / p
T _cm , ° C Recrystallization annealing temperature in bell furnaces, ° С Annealing hold time, h The degree of compression during training,% one 885 650 710 8.5 0.7 2 875 640 700 9 0.8 3 845 570 670 12 1,5 four 845 550 650 eleven 1.8 5 850 530 630 fifteen 1.8 6 820 510 600 21 2.1 7 815 508 590 22 2.2 8 (prototype) 880 640 730 22 0.8 Table 4
Mechanical properties of experimental swimming trunks No. of swimming trunks Yield strength
σ _t , N / mm ² Temporary resistance σ _in , N / mm ² Elongation δ ₈₀ ,% Achieved result one 210 330 37 Strength class 260 does not correspond to yield strength and tensile strength 2 280 360 thirty Strength Class 260 3 340 430 26 Strength Class 300 four 370 465 24 Strength Class 340 5 395 470 twenty Strength Class 380 6 440 495 eighteen Strength Class 420 7 490 530 10 Strength class 420 does not correspond to elongation 8 (prototype) 162-170 300-305 δ ₁₀ 55-56 Strength class 260 does not correspond to yield strength and tensile strength

Table 5
Carbon content depending on the desired minimum yield stress according to [C] = [0,0416 · ln (K _etc.) -0,167] ± 0,015,% No. of swimming trunks Content C
(wt.%) Required strength class
K _ol The content of C (wt.%) According to the dependence
[C] = [0,0416 · ln (K _etc.) -0,167] ± 0,015,% Compliance with the claims C _min C _max one 0,03 260 0,049 0,080 Does not match 2 0.05 260 0,049 0,080 Compliant 3 0,07 300 0,055 0,085 Compliant four 0.08 340 0,061 0.09 Compliant 5 0.08 380 0,065 0,095 Compliant 6 0.10 420 0,069 0.010 Compliant 7 0.11 420 0,069 0.010 Does not match Table 6
The manganese content depends on the desired minimum yield stress according to [Mn] = (0,0035 · K _pr -0,46) ± 0,20,% No. of swimming trunks The content of Mn (wt.%) The required strength class K _pr The content of Mn (wt.%) According to the dependence
[Mn] = (0,0035 · K _pr -0,46) ± 0,20,% Compliance with the claims Mn _min Mn _max one 0.20 260 0.25 0.65 Does not match 2 0.25 260 0.25 0.65 Compliant 3 0.65 300 0.39 0.79 Compliant four 0.76 340 0.53 0.93 Compliant 5 0.85 380 0.67 1,07 Compliant 6 1.20 420 0.81 1.21 Compliant 7 1.30 420 0.81 1.21 Does not match

Table 7
The temperature of recrystallization annealing depending on the required minimum yield strength according to
T _Otch. ≥ (810-0,5 · K _etc.), ° C No. of swimming trunks Recrystallization annealing temperature in bell furnaces, ° С Required strength class
K _ol The temperature of recrystallization annealing (° С) according to the dependence of T _anneal. ≥ (810-0,5 · K _etc.), ° C Compliance with the claims Min one 710 260 680 Does not match 2 700 260 680 Compliant 3 670 300 660 Compliant four 650 340 640 Compliant 5 630 380 620 Compliant 6 600 420 600 Compliant 7 590 420 600 Does not match

Claims (4)

1. Method for the production of high-strength cold rolled steel from low alloy steel for cold stamping, including steel smelting, slab casting, hot rolling of slabs into strips, water cooling, winding of strips into coils, cold rolling, recrystallization annealing in a bell furnace and tempering, characterized in that smelted steel containing the following components, wt.%:
carbon 0.05-0.10 silicon no more than 0.30 manganese 0.25-1.20 aluminum 0.01-0.07 nitrogen no more than 0,009 niobium and / or titanium 0.01-0.08 each iron and inevitable impurities rest,
while hot rolling is carried out with a temperature of the end of rolling of 820-875 ° C, the winding of hot-rolled strips is carried out at a temperature of 510-640 ° C, recrystallization annealing is carried out at a temperature of 600-700 ° C, the duration of recrystallization annealing is 9-21 h, the bands are trained with compression of 0.8-2.1%. 2. The method according to claim 1, characterized in that the carbon content is associated with the required strength class by the following relationship:
[C] = [0,0416 · ln (K _etc.) -0,167] ± 0,015,%,
where [C] is the carbon content in steel,%;
0,0416 - empirical coefficient,%;
To _ol - dimensionless indicator, numerically equal to the required minimum yield strength;
0,167 - empirical coefficient,%. 3. The method according to claim 1, characterized in that the manganese content is associated with the required strength class by the following relationship:
[Mn] = (0,0035 · K _pr -0,46) ± 0,20,%,
where [Mn] is the manganese content in steel,%;
0.0035 - empirical coefficient,%;
To _ol - dimensionless indicator, numerically equal to the required minimum yield strength;
0.46 - empirical coefficient,%. 4. The method according to claim 1, characterized in that the recrystallization annealing is carried out at a temperature assigned depending on the required strength class in accordance with the expression:
T _otzh. ≥ (810-0,5 · K _etc.), ° C
where T _anne - annealing temperature, ° C;
810 - empirical coefficient, ° С;
To _ol - dimensionless indicator, numerically equal to the required minimum yield strength;
0.5 - empirical coefficient, ° С.