RU2164248C2 - Method of producing hot-rolled sheet steel - Google Patents

Abstract

FIELD: plastic metal working. SUBSTANCE: method includes hot rolling of strips, their cooling and pickling with temper rolling. The method is distinguished by the fact that with carbon content in steel within 0.01-0.10%, temperature of rolling at the end of operation is taken equal to 780-800 C, cooling is carried out at rate of 9-13 C/s, and strips are subjected to pickling at temperature of 60-80 C with value of relative reduction in temper rolling ranging within 0.5-1.0%. The proposed method allows to extend technological potentialities of mills of hot rolling of sheets not above 3.9 mm thick. EFFECT: reduced labor input in production of sheet steel with improved pressworkabilty. 1 ex

Description

 The invention relates to rolling production and can be used in the manufacture of hot rolled steel.

Sheet steel (with a thickness of not more than 3.9 mm) is usually rolled on continuous broadband mills (similar to the 2500 hot rolling mill of OJSC Magnitogorsk Iron and Steel Works) at specified parameters (temperature of the beginning, end of rolling and winding, cooling speed of the strips after rolling, etc. .). The strips wound into rolls are transferred to the cold rolling workshop, where they are preliminarily etched (in HCl or H ₂ SO ₄ solutions, moreover, they are often trained for better descaling from the metal surface (i.e. rolling with reductions of no more than 3%) in a special stand installed in the line of a continuous pickling unit (NTA). The technology for the production of hot-rolled thin-sheet steel is described in sufficient detail, for example, in the book of P.I. Polukhin and other Rolling production, M .: Metallurgy, 1982, S. 374-453 .

A known method for the production of high-strength hot-rolled steel sheets, in which steel containing 0.02-0.15% carbon and 1-2% manganese is rolled to a temperature in the range Ar = ± 50 ^o C and wound at a temperature below 250 ^o C, and cooling is carried out in two stages (see Japanese application N 55-91934, class C 21 D 8/00, decl. 30.12.78, N 53-162542, publ. 11.07.80). There is also known a method of manufacturing hot rolled sheet steel with high adhesive strength, containing 0.03-0.30% C, 1.0-2.5% Si and 0.6-3.0 Mn, characterized in that the rolling end temperature is ≥800 ^o C and the metal is cooled before winding at a speed of ≅ 50 ^o C / s, wound at a temperature of 500-650 ^o C, and then subjected to etching in HCl at 65-95 ^o C (see Japanese application N 2138489, class. C 21 D 8/02, 05/28/90).

 A disadvantage of the known methods is the impossibility of producing hot-rolled sheet steel with low strength characteristics and high formability, that is, similar in properties to low-carbon cold-rolled steel of cooker categories “G” and “H” (according to GOST 16523).

 It is known the production of hot-rolled sheet steel, described in the book under the editorship of P.I. Polukhina. Technology of metal forming processes, M .: Metallurgy, 1988, p. 178-179 and 215-217.

 This technology includes the hot rolling of strips, their cooling and subsequent etching, and is characterized by the fact that before the strips are fed into the pickling baths, the metal is passed through a training stand installed in the NTA line, where they are trained, which contributes to faster removal of scale from the metal surface.

 The disadvantage of this technology is the inability to obtain hot rolled steel with low strength characteristics and high punchability.

 The closest analogue to the claimed invention is a method for the production of hot-rolled sheet steel, which includes hot rolling of strips, cooling to a winding temperature, winding, pickling with tempering (see SU 1342930 A, C 21 D 8/02, 10/07/1981).

 An object of the invention is to expand the technological capabilities of the method of hot rolling of thin steel strips and reduce labor costs in the production of sheet steel with improved stampability.

To solve this problem in a method that includes hot rolling of strips, cooling to a winding temperature, winding and pickling with training, when the content in steel is 0.01-0.10% carbon, the temperature of the end of rolling is taken equal to 780-800 ^o C, cooling is carried out with a speed of 9-13 degrees / s, etching is carried out at a temperature of 60-80 ^o C, and training is carried out with a relative compression of 0.5-1.0%.

 The essence of the technical solution lies in the combination of certain parameters of hot rolling (temperature of its end and cooling rate of strips) with some parameters of the etching process of hot-rolled metal (the temperature of its task on the NTA and the value of the relative compression during training). This ensures the production of sheet steel with the required mechanical properties necessary for its high formability.

This property is characteristic of low-alloy steel having a yield stress σ _T = 160 ... 180 MPa, tensile strength σ _B = 290 ... 310 (with the value σ _m / σ _in = 0.55 ... 0.58) , elongation δ ₄ = 38 ... 40% and hardness HR _B = 30 ... 35. Typically, such mechanical characteristics are inherent in cold-rolled steel, which requires an additional (after pickling hot-rolled metal) operation cycle: cold rolling, recrystallization annealing and tempering, which significantly increases the cost of rolling.

 As shown by metallographic studies conducted in the Central Control Laboratory (CLC) of OJSC Magnitogorsk Iron and Steel Works after experimental rolling and etching with the above parameters, their combination gives for Art. 08kp, ps 1.8 thickness. . . 3.9 mm guarantee the predominant occurrence of an “oladeobraznaya” microstructure of grains with orientation <111>, most favorable for steel, subjected to further deep stamping.

 Experimental rolling and etching of strip steel was carried out on the broadband mill 2500 hot rolling of OJSC MMK and on the NTA sheet rolling shop N 5 of the plant.

To this end, during hot rolling on a mill of strip steel, thickness 1.8. . . 3.9 mm with a carbon content in the range of 0.01 ... 0.10% varied the temperature of the end of rolling (t _k ), the cooling rate of the strips (V _o ) before winding, and on the NTA - the temperature of the task of the rolled strips for etching (t _r ) and the value of their relative compression ε when passing through a training stand. The experimental results were evaluated by the mechanical properties of σ _t, σ _in, δ ₄ and HR _B ) of samples taken from the bands after etching.

The best results are obtained for the steel treated by the above parameters rolling and pickling: up to 98% of the samples had σ _r = 160 ... 180 MPa, σ _in = 290 ... 310 MPa, δ ₄ = 38 ... 40% and HR _B = 30 ... 35. Extraction tests of the samples showed that about 99% of them correspond to category "G" (according to GOST 16523), and the rest - to category "H". Deviations from the optimal process parameters in either direction caused a deterioration in the formability of steel.

For example, at t _k <780 ^{o, the} plastic properties of the metal deteriorated due to an increase in the grain score (up to 10-11), and at t _k > 800, too large a grain was obtained (5th point), which worsened the stampability of the sheets (appearance of shear lines ) The decrease in the cooling rate of the bands V _o <9 deg / s was similar to an increase in t _k , and at V _o > 13 deg / s, grain refinement was observed, which increased the value of σ _t with negative consequences.

At t _t <60 ^o C, the etching process slowed down, which worsened the surface of the strips and adversely affected the subsequent cold rolling. At t _t > 80 ^° C, "hydrogen brittleness" (the appearance of cracks on the metal) was observed due to an increase in the saturation of the steel with hydrogen and an increase in hardness. A decrease in ε (to 0.2 ... 0.4%) led to the appearance of shear lines during stamping, and for ε> 1.0%, ductility worsened due to an increase in σ _t / σ _in and a decrease in δ _4.
It was noted that the value of ε during training should be inversely proportional to the carbon content in steel, i.e. the higher this content, the lower ε and vice versa.

 Experiments with process parameters other than optimal (and claimed) indirectly confirmed the unsuitability of the known technology, selected as the closest analogue (see above), for producing hot-rolled thin-sheet steel with properties similar to those of cold-rolled steel.

 Thus, the experimental verification confirmed the acceptability of the proposed method for solving the problem and its advantages over the known object.

 Technical and economic studies carried out at the Central Metallurgical Complex of OJSC MMK showed that the implementation of the technical solution found significantly expands the technological capabilities of the 2500 hot rolling mill (and similar ones), making its products more competitive. At the same time, labor costs for the production of hot rolled steel with a thickness of 3.9 mm, equivalent to cold rolled sheet steel of the same thickness, are reduced by approximately 45%, with a corresponding reduction in cost and higher profits from the sale of rolled products.

Concrete example
The low-carbon steel grade 08 ps with a thickness of 2.5 mm and a carbon content of 0.05% is obtained after hot rolling with a temperature of its end of 790 ^o C and cooling the strips before winding at a speed of 11 degrees to 700 ^o C. Roll strips set on the NTA at 70 ^o C and before passing through the pickling baths are subjected to compression in the training stand of the unit with ε = 0.75%.

After pickling, hot rolled steel has a σ _m = 170 MPa, σ _in = 300 MPa, σ _m / σ _in = 0,565), δ ₄ = 39% and a hardness HR _B = 32 units.

Claims (1)

A method for the production of hot-rolled thin-sheet steel, including hot rolling of strips, cooling to a winding temperature, winding, pickling with tempering, characterized in that when the carbon content in the steel is between 0.01 and 0.1%, the temperature of the end of rolling is taken to be 780 - 800 ^o C, cooling to the winding temperature is carried out at a speed of 9 - 13 deg / s, etching is carried out at 60 - 80 ^o C, and training is carried out with a relative compression of 0.5 - 1.0%.