RU2471876C1 - Method of making low-carbon cold-rolled thin-sheet steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method comprises cold coil rolling, annealing and tempering. Cold-rolled steel contains 0.0.6-0.09 wt % of carbon, 0.40-0.55 wt % of manganese, and not over 0.05 wt % of silicon. Annealing is carried out in bell furnaces with protective atmosphere consisting of 93%-N₂ and 7%-H₂ with heating to temperature of final curing exceeding chilling point of heated steel and interim holding at 580°C and 650°C as shown by bench thermocouple. Total duration of heating makes 36-64 h depending upon strip width and weight of bottom coil in load. Tempering is performed at rolls with surface roughness Ra 4.5-6.0 mcm and relative reduction ε_t 0.9-1.5%. DC01 cold-rolled steel is produced that features thickness of 0.60-1.20 mm, yield point σ_y of 220-260 N/mm², point of maximum load σ_t of 290-360 N/mm², elongation δ₄, smaller than 32%, and surface roughness Ra of 1.4-2.2 mcm.

EFFECT: better steel characteristics.

1 tbl, 3 ex

Description

The invention relates to rolling production and can be used in the manufacture of cold rolled low carbon steel, in particular for stamping.

A known method for the production of cold rolled strips for processing by cold forming from steel containing 0.07% carbon with a ratio of manganese to sulfur of at least 7 and calmed with aluminum, including hot rolling of slabs and hot rolled strips in a cold rolling mill, after which the strips are subjected to recrystallization annealing ( see U.S. Patent No. 3,668,016, CL B22D 5/06, publ., 06/06/72). However, this method is not suitable for the production of mild steel with additional requirements.

The closest analogue to the claimed method is a method for the production of low-carbon thin-sheet cold rolled steel (RF patent No. 2379360 from 01.20.2010).

This method includes cold rolling, heat treatment and tempering of strips with predetermined process modes and for obtaining sheet metal with a thickness of 0.60 ... 1.2 mm of a very particularly complex category of drawing with microroughness of the surface within Ra = 0.7 ... 1.2 μm from steel containing 0.02 ... 0.04 wt.% carbon, 0.15 ... 0.25% manganese, not more than 0.01% silicon, chromium, nickel and copper in a total amount of not more than 0.1%, as well as sulfur , phosphorus, nitrogen and aluminum, cold rolling is carried out with a total compression within 57 ... 70%, annealing of the coils is carried out at a temperature of Flax shutter 710 ° C without its reduction, and temper rolling the annealed strips are in rolls with a roughness of Ra = 2.5 barrels ... 3.0 microns with largest relative reduction ε _d in the range 0.9 ... 1.1%.

However, this technology does not provide the specific required mechanical properties of the DC01 low carbon cold rolled steel according to EN 10130.

An object of the present invention is to increase consumer properties and preparation of low carbon cold-rolled steel DC01 EN 10130 brand of thickness 0.60 ... 1.20 mm with the following mechanical properties: yield strength σ _m of 220-260 N / mm ^2, tensile strength σ _of 290 -360 N / mm ² , the elongation δ _{4 of} at least 32% and the surface roughness of the strip Ra 1.4–2.2 μm.

To solve this problem, in a method for the production of cold rolled sheet steel from mild steel with a thickness of 0.60 ... 1.2 mm, mainly of DC01 grade, which includes cold rolling, annealing and tempering, in contrast to the closest analogue, cold rolled steel from a content of 0.06 ... 0.09 wt.% Carbon, 0.40 ... 0.55% manganese, not more than 0.05% silicon are annealed in bell-type furnaces with a protective atmosphere, consisting of 93% N ₂ and 7% H ₂ , with heating to a temperature final exposure 120 ° C above the temperature of the onset of recrystallization metal and intermediate shutter speeds at temperatures of 580 ° C and 650 ° C according to a bench thermocouple, while the total heating time is 36-64 hours, depending on the strip width and mass of the lower roll in the cage, training is carried out on rolls with a surface roughness of Ra 4,5 -6.0 μm and a relative compression ε _{d of} 0.9-1.5%.

The given process parameters are obtained empirically and are empirical.

The essence of the proposed technical solution is to optimize the parameters of the production process of cold-rolled structural steel sheet, providing its high consumer properties: the specified mechanical properties and providing the necessary microgeometry of the strip surface (roughness parameter Ra).

Annealing the coils in furnaces with a protective atmosphere consisting of 93% N ₂ and 7% H ₂ , when the temperature of the final exposure rises above the assigned temperature, does not allow to obtain the required range of plastic characteristics, the yield strength σ _t will be less than 220 N / mm ² .

Annealing the coils in furnaces with a nitrogen protective atmosphere with a final exposure temperature lower than the assigned temperature does not allow a relative elongation of δ _{4 of} at least 32%.

Intermediate exposure at temperatures below 580 ° C affects the quality of the surface of the car.

At an intermediate exposure at a temperature of less than 650 ° C, the roll heats up unevenly, which leads to heterogeneous mechanical properties along the length of the roll.

Intermediate exposures at temperatures above 580 ° C and more than 650 ° C are impractical due to the technical capabilities of controlling the temperature of a bell furnace.

The training of cold-rolled annealed strips with compression ε _d less than 0.9% does not provide a lower level of yield strength σ _t 220 N / mm ² .

Training with compression ε _d more than 1.5% leads to a decrease in the relative elongation δ _{4 of} at least 32%.

The required surface quality of the finished sheets (microgeometry with Ra = 1.4-2.2 μm) was achieved only by training in work rolls with the surface roughness of their barrels Ra = 4.5-6.0 μm.

The decrease in the surface roughness of the barrel of the work rolls Ra less than 4.5 μm and the increase in surface roughness of the barrel of the work rolls Ra more than 6.0 μm does not allow to obtain the required roughness of the strip Ra taking into account the printability coefficient.

Case Studies

An experimental verification of the proposed technical solution was carried out at LPC-5 of OJSC Magnitogorsk Iron and Steel Works.

For this purpose, in the production of strip steel rolls of DC01 grade 1.2 × 1222 mm in size, the parameters of the annealing mode and the reduction value (ε _d ) were varied during the training of annealed steel. The surface roughness parameter of the barrel of the work rolls of the temper mill was in the range Ra = 4.5-6.0 μm.

The results are shown in the table.

Feasibility studies have shown that the introduction of the inventive technology in the production of low-carbon cold rolled sheet steel will increase the yield of sheet metal with desired consumer properties by at least 5% with a corresponding increase in profits from its sale.

Method for the production of low carbon cold rolled
sheet steel Example Number Annealing mode The amount of compression during training ε _d ,% Mechanical properties Note σ _t , N / mm ² σ _in , N / mm ² δ ₄ ,% one t ₁ = 580 ° C t ₂ = 650 ° C t ₃ = 690 ° C 1,0 230 310 33 Meets customer requirements 2 t ₁ = 480 ° C t ₂ = 620 ° C 1 ₃ = 700 ° C 0.6 205 310 32 Does not meet customer requirements 3 t ₁ = 430 ° C t ₂ = 550 ° C t ₃ = 670 ° C 1.8 240 380 thirty Does not meet customer requirements where t ₁ is the temperature of the first intermediate exposure, t ₂ - temperature of the second intermediate exposure, t ₃ is the temperature of the final exposure.

Claims (1)

A method for the production of cold rolled thin-rolled sheet from DC01 low-carbon steel with a thickness of 0.60 ... 1.2 mm, comprising cold rolling into rolls, annealing and tempering, characterized in that cold rolling of steel is carried out with a content of 0.06 ... 0.09 wt.% carbon, 0.40 ... 0.55 wt.% manganese, not more than 0.05 wt.% silicon, annealing is carried out in bell furnaces with a protective atmosphere, consisting of 93% N ₂ and 7% N ₂ , with heating to the final temperature holdings 120 ° C higher than the temperature of the onset of recrystallization of the heated steel and intermediate holdings p At temperatures of 580 ° С and 650 ° С according to a bench thermocouple, the total heating time is 36-64 hours, depending on the strip width and the mass of the lower roll in the cage, training is carried out on rolls with a surface roughness of Ra 4.5-6.0 μm and a relative compression ε _{d of} 0.9-1.5%.