RU2031962C1 - Method of production of low-carbon sheet steel - Google Patents

Abstract

FIELD: ferrous metallurgy. SUBSTANCE: in the process of steel smelting the following ratio of components is maintained: Mn = (0.14-0.18) + 1.72 S; C = (0.14-0.16) - 0.5 Mn where S - sulfur content in steel, %; for stretching category 0,006≅ S≅ 0,025, Mn, C - content of manganese and carbon in steel, correspondingly, %. Then hot and cold rolling and annealing of steel were carried out by conventional technology. EFFECT: improved method of steel production. 7 tbl

Description

 The invention relates to the field of ferrous metallurgy, in particular to the production of sheet metal and improves the process of alloying steels of type 08Yu to obtain a sheet of the category of very highly complex hoods (WWS).

 A known method for the production of low-carbon sheet steel, including smelting and alloying of steel, hot and cold rolling, recrystallization annealing (1-4). The method assumes that when alloying, the ratio of Mn to S in steel is maintained in the range of at least 5-7. The main disadvantage of this method is the low yield of the VOSV extract category sheet.

A known method for the production of low carbon sheet steel, selected as a prototype, is as follows. In the converter or open-hearth furnace, steel of type 08Yu is smelted and alloyed. Moreover, with a low Mn content in the range of 0.2-0.24%, the C content is maintained at 0.04%, and the sulfur content is maintained at 0.015-0.016%. Thereafter, hot rolling the slabs produced by the mill in the following example modes: rolling end temperature, 860-920 ^{° C;} coiling temperature of hot-rolled bands of 550-620 ^C. The hot-rolled coils successively subjected to operations of cooling, pickling and cold rolling. The total degree of compression during cold rolling is 70-75%. After cold rolling, the coils are sent to the thermal compartment of bell-type furnaces for recrystallization annealing at temperatures of 650-700 ^о С. The annealed coils are cooled to 40-60 ^о С, they are trained and cut before shipment to consumers.

The main disadvantage of this method is the low stability of the complex of mechanical properties, which results in a correspondingly low yield of a sheet of the VOSV extract category. Translation of the sheet into the lower drawing groups (WWS, CB HF) due to the mismatch of one or more basic mechanical characteristics, such as yield strength (σ _t ), tensile strength (σ _c ), elongation (δ ₄ ) and hardness (HRB), GOST 9045-80 for the production of steel 08Yu. According to GOST 9045-80, for a sheet of VOSV extract category, the main mechanical characteristics must be in the range: σ _t <19 kg / mm ² ; σ _in = 26-33 kg / mm ² ; δ ₄ 40%; HRB≅ 46. The discrepancy between the mechanical characteristics of GOST is a consequence of the fact that when using the known method, the ratio between the upper limits of the content in MnS and C steel is not regulated. As a result, the finished sheet has an increase in strength properties above acceptable GOST values, the sheet is transferred to the lower category hoods. The output of the extract category of WWTP extract does not exceed 20-30%.

 The problem solved by the invention is to stabilize the complex of mechanical properties of the sheet while providing a very particularly complex hood.

The proposed method differs from the prototype in that the alloying of steel is carried out withstanding the following dependence of the content of components:
Mn = 0.14 to 0.18 - 1.72 ˙ S;
C = from 0.14 to 0.16 - 0.5˙Mn.

 The proposed method for the production of low carbon steel is as follows.

Steel 08U is smelted in a converter or open-hearth furnace and its content S is determined in it. Based on the content in steel S, the required content of Mn and C is calculated from the given dependencies:
Mn = 0.14 to 0.18 - 1.72 ˙ S;
C = from 0.14 to 0.16 - 0.5˙Mn.

After that, the steel is alloyed to the required content of Mn and C in it and poured into molds or on continuous casting machines (CCM). The manufactured slabs are sent to a hot rolling mill to obtain a sheet roll for a cold rolling mill. Getting hot-rolled rolled is carried out according to the following modes: temperature of the end of rolling 860-920 ^about ; winding temperature of hot-rolled strips intended for subsequent annealing in bell-type furnaces 550-620 ^о С; the temperature of the winding of hot-rolled strips intended for subsequent annealing in continuous operation units (ANOs) is 750-780 ^о С. Later hot-rolled coils are subjected to cooling, etching, cold rolling with a total degree of reduction of 70-75% and recrystallization annealing in bell furnaces or ANO .

 When using the proposed method, stabilization of the complex of mechanical properties of the sheet, providing it with the BOCB extract category, is achieved by optimizing the content ratio in steel S and Mn, as well as Mn and C.

 It is known that S is a harmful impurity in steel, forming sulfur compounds such as FeS sulfides, which sharply worsen the plastic properties of finished sheet metal. Therefore, in the production of steels intended for deep drawing, they always strive to minimize the S content, up to its complete absence. However, even such expensive sulfur removal measures as iron desulfurization do not always ensure its reduction in steel to minimum values.

Carbon in sheet metal is mainly in the form of cementite - FeC, which on the one hand is a hardening phase, and on the other hand, the beginning of fracture during stamping, as the most fragile component of the microstructure. The increased carbon content leads to an increase in the amount of cementite, with an increase in which the plastic characteristics of the metal are removed - σ _t and δ ₄ . At the same time, at low carbon contents, cold-rolled sheet metal has insufficiently high strength indicators (σ _in ), which ultimately reduces the service life of the finished stamped parts. In addition, a low carbon content is undesirable because of the danger of gas saturation of the steel, because otherwise it is necessary to carry out the operation of evacuation of steel.

Manganese in steel is a reinforcing element that reduces its plastic properties. In the practice of producing low carbon steels, it is usually introduced into steel during the alloying process to bind sulfur. Manganese with sulfur forms a plastically deformable compound of the MnS type. With large amounts of manganese, in addition to sulfur binding, it forms Mn ₃ C carbide with carbon, which equally increases the strength properties, as does cementite Fe ₃ C.

Consequently, the Mn content in steel should be such that its amount is sufficient to bind sulfur to plastically deformable compounds, but not enough to form Mn ₃ C.

 The optimal ratio between the content of sulfur and manganese, manganese and carbon in the steel was found experimentally, based on the conditions for obtaining mechanical properties in finished sheet metal corresponding to the VOSV drawing category according to GOST 9045-80.

In the table. Figures 1 and 2 show the averaged mechanical characteristics of finished sheet metal at various contents of sulfur and manganese in steel. The carbon content in the steel ranged from 0.03-0.09. When determining the optimal amount of manganese in steel necessary for sulfur binding, we proceeded from the conditions that the relative elongation (δ ₄ ), tensile strength (σ _c ) and hardness (HRB) corresponded to GOST 9045-80 for the VOSV drawing category. From the data table. 1 it follows that there is a definite relationship between sulfur and manganese in steel. The more sulfur in steel, the greater should be the amount of manganese introduced and vice versa. Moreover, if the manganese content is less than it is spent on sulfur binding or equal, then the tensile strength is lower than the permissible state standard. If, however, the amount of manganese is much greater than the required value for sulfur binding, then the plastic properties deteriorate and the elongation becomes less than the allowable value, the hardness increases above the allowable values according to GOST. Applying the methods of mathematical processing to the obtained results, we obtained an experimental dependence on determining the optimal amount of manganese in steel depending on the sulfur content
Mn = 0.14 to 0.18 - 1.723˙S
In the table. 3, 4 and 5 are experimental data characterizing the effect of manganese and carbon on the mechanical properties of finished sheet metal. The results of mechanical tests of the deposited sheet indicate that the lower, ceteris paribus, the manganese content in steel, the higher the carbon content and vice versa, which provide mechanical properties corresponding to the VOSO extraction category.

If the ratio between carbon and manganese in steel is less than the optimal value, the strength characteristics (σ _in ) of the finished product do not comply with GOST. If the ratio is greater than the optimum value, the hardness (HRB) sharply increases and the plastic properties (δ ₄ ) decrease above the permissible GOST values. Similarly, using methods of mathematical data processing, an expression was found to determine the optimal amount of carbon in steel depending on the manganese content
C = from 0.14 to 0.16 - 0.5 Mn.

 The expressions obtained on the basis of experimental studies for the determination of manganese and carbon in steel show that there is a certain interval with upper and lower boundaries, within which stability of the complex of mechanical properties is ensured at the level of the WWTP extract category. The method assumes that the choice of alloying steel with manganese and carbon at the upper or lower limits in each case depends on the technical and economic efficiency of the process. So, for example, it is more profitable to alloy steel with manganese at the upper limit, but it is necessary to reduce carbon to the lower limit, or incur additional costs to reduce sulfur in steel, etc.

The proposed method for the production of low-carbon sheet steel was tested under industrial conditions with a sulfur content of 0.02%. Based on the obtained dependencies, it was found that alloying should ensure that the steel has a manganese content of 0.195% and a carbon content of 0.05%. One melting of steel 08Y was smelted by a known method. The chemical composition of the heats is given in table. 6
After receiving the slabs at the continuous casting machine, they were sent to a hot rolling mill, where they received a sheet tack of 3.2 x 1350 mm in size. Temperature settings for rolling both batches were identical: the rolling end temperature of 890 ^{° C,} coiling temperature of 760 ^C. Hot-rolled coils for cold rolling mill to a thickness of 0.9 mm, i.e. the total degree of compression was within 72%. After that, the ANO carried out heat treatment of the heats according to the WWTP regime. The data of mechanical tests and sorting of the annealed sheet are given in table. 7.

The data table. 2 indicate that the use of the proposed method allows you to stabilize the set of mechanical properties of the sheet, providing it with the category of extracts WWW. So, if when using the proposed method there is some discrepancy between the yield strength σ _{t and} GOST values, then when using the known method, the discrepancy with the requirements of GOST is observed for two indicators - σ _t and HRB. As a result, the use of the proposed method, the output of the sheet category of the WWTP extract increases from 26.4% to 48.8% of the specified.

 Thus, the use of the proposed method for the production of low-carbon sheet steel in comparison with the prototype, allows to increase the stability of the complex of mechanical properties of the sheet, as a result of which the yield of finished products of the BOCB extract category is increased by an average of 1.8 times.

Claims (1)

METHOD FOR PRODUCING LOW-CARBON SHEET STEEL, including steel smelting, hot and cold rolling of the sheet, annealing, characterized in that, in order to stabilize the complex of mechanical properties of the sheet while ensuring a very particularly complex drawing, the content in it of manganese and carbon C is determined by the following formulas:
Mn = (0.14 - 0.18) + 1.72 S,%,
C = (0.14 - 0.16) - 0.5 Mn,%,
where S is the sulfur content in steel,%, for the WWTP extract category - 0.006 ≅ S ≅ 0.025.

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