RU2452778C1 - Method of strip cold-rolling from low-alloy steel of strength class 200 - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel with preset chemical composition and carbon content of 0.02-0.049 wt % is cast. Steel is teemed to produce slab to be hot-rolled and strip surface is water cooled to reel strip. Scale is removed from strip surface by etching to roll strip at continuous mill to produce 0.7-3.0 mm-thick strip to be heat-treated and temper-rolled. Temperature conditions of hot-rolling end and strip reeling are controlled. Hot-rolled strip thickness depending upon final thickness of cold-rolled strip is controlled by the following equation:

where H_hr is thickness of hot-rolled strip, mm; H_CR is final thickness of cold-rolled strip, mm.

EFFECT: mechanical properties complying with strength class 220, higher product yield.

1 ex, 2 tbl

Description

The invention relates to rolling production and can be used in the manufacture of cold-rolled strips of steel of strength class 220, with improved exhaust characteristics and intended for subsequent manufacture of car body parts by stamping.

Known methods for the production of low-carbon cold-rolled strips from hot-rolled coils, including the production of hot-rolled coils, descaling from its surface by etching, subsequent cold rolling on a continuous mill, annealing and training of the annealed strip (RF Patent No. 2307173, RF Patent No. 2312906, RF Patent No. 2312906, 2212468).

The disadvantages of the known methods are the difficulty of providing a required complex of mechanical properties in a thin cold-rolled strip corresponding to strength class 220, as well as the inability to seamlessly process a hot-rolled strip rolling into high-quality cold-rolled metal products. This is due to the fact that the known methods for the production of strips of low-carbon low-alloy steel do not take into account the temperature conditions of hot rolling and winding of the hot-rolled strip into a roll, which complicates the formation of a given microstructure and, therefore, mechanical properties along the entire length of the strip at the stage of hot rolling. In addition, it is not possible to roll out the hot rolled tackle to a predetermined thickness at the stage of cold rolling.

There is also known a method of manufacturing rolled products from low alloy steel containing the following components, wt.%:

Carbon - 0.02-0.08

Manganese - 0.2-0.6

Silicon - 0.005-0.1

Copper - 0.01-0.1

Aluminum - 0.02-0.07

Boron - 0.001-0.05

Calcium - 0.0005-0.01

Nitrogen - 0.001-0.006

Vanadium - 0.0005-0.003

Niobium - 0.0005-0.003

Iron - the rest, with a total content of vanadium and niobium in steel - 0.0055% (RF Patent No. 2154123).

The disadvantage of this method is the presence of carbonitride-forming elements in the chemical composition of the steel, which unnecessarily increases the hardening of the rolled products at the hot rolling stage by inhibiting grain growth during recrystallization, and accordingly there is a technological difficulty in providing the required mechanical properties corresponding to strength class 220 in the cold-rolled strip, intended for subsequent cold stamping.

The closest analogue to the claimed object is a method for the production of cold rolled strips with a thickness of 0.15-3.0 mm, obtained from a hot rolled etched strip of low alloy steel containing the following components, wt.%:

Carbon 0.06-0.10 Manganese 0.20-0.50 Silicon 0.01-0.30 Copper 0.01-0.30 Aluminum 0.02-0.07 Phosphorus 0.07-0.12 Nitrogen 0.003-0.009 Sulfur 0.005-0.025 Calcium 0.0005-0.001 Boron 0.0008-0.005 Chromium 0.01-0.30 Nickel 0.01-0.30 Titanium 0.002-0.02 Iron and inevitable impurities rest

(see RF Patent No. 2362815).

The disadvantage of this method lies in the difficulty of providing in the finished cold-rolled strip corresponding to strength class 220 of the required complex of mechanical properties uniformly distributed along the length of the strip. This is due to the excess in the structure of the known low alloy steel of a sufficient number of hardening phases, the formation of which is due to additional microalloying, as well as temperature conditions during hot rolling and winding of the strip into a roll. The microstructure formed during hot rolling during subsequent cold rolling of a strip with a thickness of 0.7-3.0 mm on a continuous mill in the absence of a regulated range of relative total reductions during cold rolling leads to the formation of cracks, numerous gusts, which, in turn, does not allow to provide the required the quality of the produced cold-rolled strip over its entire length and significantly reduces the yield during rolling, and further processing of such rolled products into cold-pressed duction.

The technical problem solved by the present invention is the provision of strength class 220 of a complex of mechanical properties along the entire length of the cold-rolled strip of low-alloy low-carbon steel with an increased yield (more than 95%).

The problem is solved in that in the known method for the production of cold rolled strips of low alloy steel of strength class 220 with a thickness of 0.7-3.0 mm, including melting, casting steel to obtain a crystallized slab, its hot rolling in the stands of a broadband mill with a regulated temperature of the end of rolling, followed by water cooling of the strip surface and winding it into a roll, removing scale from the strip surface by etching, cold rolling on a continuous mill, heat treatment and training, according to the invention steel containing carbon, manganese, silicon, copper, aluminum, phosphorus, nitrogen, sulfur, chromium, nickel, iron is melted in the following ratio of components, wt.%:

Carbon 0.02-0.049 Manganese 0.20-0.35 Silicon less than 0.04 Copper 0.01-0.08 Aluminum 0.03-0.07 Phosphorus 0.04-0.07 Nitrogen 0.003-0.007 Sulfur less than 0.015 Chromium less than 0.06 Nickel less than 0.06 Iron rest,

hot rolling is carried out to obtain a hot-rolled strip with a thickness depending on:

where N _gk - the thickness of the hot rolled strip, mm,

h _xк - final thickness of the cold-rolled strip, mm,

the temperature of the end of hot rolling is taken equal to (840 ÷ 850) ± 15 ° C, the temperature of the winding of the hot-rolled strip in a roll is set to 550 ± 15 ° C.

The essence of the proposed technical solution is to apply a strategy of economical alloying with a minimum content of basic elements (carbon, manganese, silicon, phosphorus, chromium and nickel) in combination with the regulation of temperature conditions of hot rolling and winding of a hot-rolled strip into a roll, as well as the deformation mode during cold rolling . This together makes it possible to increase the yield of cold-rolled metal products with the required level of mechanical properties along the entire length of the strip corresponding to strength class 220, in particular, yield strength σ _in ≥220-260 N / mm ² , temporary tensile strength σ _in ≥340 N / mm ² , elongation δ ₈₀ > 30%, strain hardening index n ₉₀ ≥0.18, normal plastic anisotropy coefficient R ₉₀ ≥1.7.

In the claimed method, the boundaries and the range of contents of the main chemical elements are determined from the considerations of ensuring the maximum possible hardening of the ferrite matrix for the claimed strength class of rolled products, while increasing its plastic properties, which will ensure a given level of rolling and stamping. Therefore, carbonitride-forming elements are excluded from the chemical composition of steel. The claimed range of carbon content (0.02 ÷ 0.049%), in combination with manganese (0.20 ÷ 0.35%) contribute to the formation of the ferritic structure of steel, providing high values of yield strength, tensile strength, elongation. In this case, a further increase in the carbon content worsens the formability of the cold rolled product. The accepted level of silicon content (less than 0.04%) in combination with a low sulfur content (less than 0.015%) provide the necessary purity of steel for non-metallic inclusions. Aluminum is introduced as a deoxidizer, which binds nitrogen to nitrides, in order to strengthen the steel. The limits of aluminum and nitrogen are selected from the need to maintain their ratio 1/10. Deviation from the indicated ranges of aluminum (0.03-0.07) and nitrogen (0.003-0.007) to the lower side can cause a significant enlargement of grain and, as a result, a decrease in the strength characteristics of rolled products; when deflected to the upper side, the mechanism of strain aging can be triggered, which will lead to excessive hardening. The introduction of a minimum amount of nickel and chromium into steel (less than 0.06% of each) eliminates the increased hardening of rolled products while reducing the tendency of steel to possible crack formation during cold deformation processing. The introduction of phosphorus (0.04-0.07%), as in the known solutions, helps to improve the plastic properties of steel during cold rolling and stamping. At the same time, in the absence of additional microalloying with carbonitride-forming elements, phosphorus additionally strengthens the ferrite matrix.

The temperature conditions of hot rolling are selected from the following. To ensure the required set of mechanical properties along the entire length of the hot-rolled strip, intended for the further production of cold-rolled strip corresponding to strength class 220, it is necessary to form a structure with fine grain of polygonized ferrite and dispersed precipitates of nitrides at the stage of hot rolling, which ensures an increase in strength properties (at the optimal ratio yield strength to tensile strength σ _t / σ _in <0.70 ÷ 0.75), on the one hand, and improves the plastic properties (δ ₈₀ ), on the other.

As is known, the size and shape of austenitic grains depend on the rate of recrystallization during rolling. Therefore, obtaining the required complex of mechanical properties to ensure increased formability of metal from low alloy steel of strength class 220 without the use of microalloying should be achieved by the formation of a fine-grained structure of ferrite, one of the main conditions for which is the presence of a fine-grained structure of austenite. It, in turn, can be obtained at certain temperatures of the rolled metal, which corresponds to the end of hot rolling in the austenitic region at a temperature close to the temperature of the austenitic transformation. For this, the temperature of the end of rolling must be taken equal to or close to point A _{c3 of} the “iron-carbon” diagram, since in the bands of low alloy steels of the claimed chemical composition, intense recrystallization begins at temperatures of 825-865 ° C. It is especially important that these conditions are met at the end of hot rolling of strips <25 mm thick (see Regulated hot rolling of strips in continuous mills. Tomczykiewicz Jan, Wegrzyn Aleksander. Regulowane walcowanie blach w garacej walcowni ciaglej. "Prz. Now. Hutn. Ze-laza" , 1976, 4, No. 2, 63-67).

Of these conditions, the temperature range of the end of the rolling was selected in the claimed method, since it is in the specified range (840 ÷ 850) ± 15 ° C that the required microstructure is obtained. In addition, the temperature limits of the end of rolling, depending on the thickness of the hot-rolled strip, are determined from the condition: the thicker the strip, the greater its heat capacity. Accordingly, in order to equalize the properties and form an equidistant microstructure with a grain of 8-10 points in the finished hot-rolled strip, the temperature interval of the end of hot rolling at smaller thicknesses is shifted to higher temperatures.

These circumstances also determine the claimed temperature interval for winding a hot-rolled strip into a roll in the range of 550 ± 15 ° C. The temperature of the winding for the selected class of steels should be as close as possible to ensure the cooling rate on the discharge roller of the hot rolling mill for more complete stabilization of carbon, which allows obtaining relatively low yield stresses and the absence of a yield point for hot-rolled steel (see Black W. , Bode R., Hahn P. Interstitial-free Steels: Processing, Properties and Application. In: Metallurgy of Vacuum-Degassed Steel Products, 1990, pp. 73-90).

In addition, in the absence of the claimed regulation of the temperature regimes of the end of rolling and winding, depending on the microstructure of the steel, at low temperatures of the end of rolling and winding (less than the lower declared temperature limit for the corresponding thicknesses), significant variability in structure can appear (more than three adjacent values). On the other hand, at the temperatures of the end of rolling and winding higher than that declared in the microstructure, coarse grain is formed (larger than 8 points), the overall strength decreases, while ductility also decreases, and the yield strength remains practically unchanged, which leads to an increase in σ _t / σ _in , i.e. decrease in stampability. This leads to the fact that in the process of further processing of hot-rolled steel into cold-rolled metal products, a problem arises of rolling the strip to the required thickness. In addition, there are technological difficulties in processing the strip due to numerous gusts during the cold rolling process due to the formation of microcracks, including along the edges of the strip, which significantly reduces the yield of metal products.

In the case of applying the stated regulation of the temperature regime of the hot rolling and winding process, a microstructure with a ferrite grain of 8-10 points is formed, which, from the point of view of the ability of the metal to be further processed by cold rolling and subsequent deep drawing, is the most optimal. The yield strength σ _t in this case in low-alloy low-carbon steel without microalloying with carbonitride-forming elements in the cold-rolled state reaches a level of more than 220 N / mm ² , the tensile strength σ _is ≥340 N / mm ² , the relative elongation (δ ₈₀ ) is at least 30%, which corresponds to the strength class of rolled steel 220. This allows the processing of hot-rolled rolled products to cold-rolled and then to cold-stamped products due to the optimal microstructure over the entire cross section and strip length, eliminating Hovhan cracks at the strip edges, and its breakage, significantly increases the yield.

The mathematical relationship that relates the thickness of the hot rolled strip to the final thickness of the cold rolled strip is empirical and obtained by processing the experimental data when rolling the inventive size-gauge assortment on a broadband hot rolling mill 2000 and a continuous four-stand cold rolling mill 2500 of OJSC Magnitogorsk Iron and Steel Works. This dependence makes it possible to ensure high rolling of hot-rolled steel into the cold-rolled strip of a given final thickness without the formation of microcracks and gusts, as well as optimal energy and power parameters of the rolling equipment.

Thus, the presented set of features of the proposed method for the production of cold-rolled strips with a thickness of 0.7-3.0 mm from low-alloy low-carbon steel of strength class 220 allows to produce high-quality metal products with the required mechanical properties equal to the entire length of the finished strip, while increasing the yield of cold-rolled steel .

An example implementation of the method.

, где Н_гк - толщина горячекатаной полосы, мм, h_xк - конечная толщина холоднокатаной полосы, мм. Затем горячекатаные полосы подвергали солянокислому травлению в линии НТА холодной прокатке на непрерывном прокатном стане с регламентированным суммарным относительным обжатием, рекристаллизационному отжигу в колпаковых печах и дрессировке с обжатием в пределах 1%. Испытанием на растяжение определяли основные механические свойства холоднокатаной полосы по ее длине: предел текучести σ_т, временное сопротивление разрыву σ_в, относительное удлинение δ₈₀, показатель деформационного упрочнения n₉₀, коэффициент нормальной пластической анизотропии R₉₀. Для чего образцы для испытаний отбирались с переднего и заднего концов рулона, а также в зоне сварного шва (серединная часть полосы по ее длине). Выход годного оценивался по отсутствию порывов и микротрещин на поверхности полосы в процессе холодной прокатки.Smelted by oxygen-converter method 2 melts of steel of the declared chemical composition (see table 1). After the out-of-furnace treatment of the metal and the introduction of the required additives, continuous casting of steel was carried out, followed by its crystallization and cutting into slabs. Next, hot rolling of slabs was carried out into strips of the required thickness, which, depending on the final thickness of the cold-rolled strip, was determined from the expression:

where H _gk is the thickness of the hot rolled strip, mm, h _xk is the final thickness of the cold rolled strip, mm. Then, the hot rolled strips were subjected to hydrochloric acid etching in the NTA line by cold rolling in a continuous rolling mill with regulated total relative compression, recrystallization annealing in bell furnaces, and tempering with compression within 1%. The tensile test determined the basic mechanical properties of cold-rolled strip along its length: the yield stress σ _t, tensile strength σ _B, elongation δ _80, the strain hardening index n _90, the coefficient of normal plastic anisotropy of R _90. For this purpose, test samples were taken from the front and rear ends of the roll, as well as in the weld zone (the middle part of the strip along its length). The yield was evaluated by the absence of gusts and microcracks on the surface of the strip during cold rolling.

Variants of technological parameters according to which, according to the claimed method, hot and subsequent cold rolling of strips with a final thickness of 0.7-3.0 mm from steel of strength class 220 was carried out on a broadband hot rolling mill 2000 and a continuous four-stand cold rolling mill 2500 of OJSC Magnitogorsk Iron and Steel Works , as well as research results are presented in table 2.

The inventive roll manufacturing technology to an example of production of cold rolled strips of low alloy steel strength class 220 provides the following mechanical properties :: σ _m = 220-260 N / mm ^2, σ _s = 340-375 N / mm ^2, δ _80> 30% strain hardening index n ₉₀ ≥0.18, normal plastic anisotropy coefficient R ₉₀ = 1.7-2.4, which meets the requirements for steels of strength class 220.

Based on the foregoing, we can conclude that the claimed method is workable and eliminates the disadvantages that occur in the prototype.

The inventive method can be widely used in the production of cold rolled coiled steel products of strength class 220 for subsequent stamping of car body parts.

Claims (1)

A method for the production of cold-rolled strips of low-alloy steel of strength class 220 with a thickness of 0.7-3.0 mm, including melting, casting steel to obtain a crystallized slab, hot rolling it in the stands of a broadband mill, followed by cooling the strip surface with water and winding it into a roll, removing scale from the surface of the strip by etching, cold rolling in a continuous mill, heat treatment and training, characterized in that the steel is smelted containing carbon, manganese, silicon, copper, aluminum, phosphorus, nitrogen, sulfur, chromium, Ichel and iron at the following ratio, wt.%:
carbon 0.02-0.049 manganese 0.20-0.35 silicon less than 0.04 copper 0.01-0.08 aluminum 0.03-0.07 phosphorus 0.04-0.07 nitrogen 0.003-0.007 sulfur less than 0.015 chromium less than 0.06 nickel less than 0.06 iron rest,
moreover, hot rolling is carried out to obtain a hot-rolled strip with a thickness depending on:

,
where N _gk - the thickness of the hot rolled strip, mm,
h _hk is the final thickness of the cold-rolled strip, mm, while the temperature of the end of hot rolling is taken equal to (840 ÷ 850) ± 15 ° С, and the temperature of winding the hot-rolled strip into a roll is set to 550 ± 15 ° С.