RU2432404C1 - Procedure for manufacture of cold rolled strips of low alloyed steel of 260 class of strength - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: there is melted steel containing wt %: carbon 0.06÷0.10, manganese - 0.20÷0.50, silicon - 0.01÷0.30, copper - 0.01÷0.30, aluminium - 0.02÷0.07, phosphorus - 0.07÷0.12, nitrogen - 0.003÷0.009, sulphur - 0.005÷0.025, calcium - 0.0005÷0.001, boron - 0008÷0.005, chromium - 0.01÷0.30, nickel - 0.01÷0.30, niobium + molybdenum + vanadium + titanium ≤ 0.032, iron - the rest. Further, steel is teemed producing crystallised slab; hot slab is rolled in stands of a wide rolling mill and there is manufactured hot rolled strip of thickness by dependence:

where: h_hr - thickness of hot rolled strip, mm, h_cr - finish thickness of cold rolled strip, mm. Surface of strip is cooled with water and it is wound in reel. Scale is removed off strip surface by etching. Successively, strip is cold rolled on a continuous mill, heat treated and dressed, also, temperature of rolling finish and strip winding into coil is chosen depending on thickness of hot rolled strip correspondingly: for strips of thickness over 4.01 mm - 800-820°C, 535-565°C, of thickness from 3.01 to 4.00 mm - 820-840°C, 565-595°C, of thickness from 2.00 to 3.00 mm - 840-860°C, 595-625°C.

EFFECT: manufacture of strip with thickness 0,6-3,0 mm with complex of mechanical properties of 260 class of strength; increased output of accepted product due to elimination of micro-cracks and ruptures.

1 ex, 1 tbl

Description

The invention relates to rolling production and can be used in the manufacture of cold-rolled strips of steel of strength class 260, 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 descaling by pickling, 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. 2212469).

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 260, 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 low-carbon steel strips do not take into account the temperature conditions of the end of hot rolling and winding of the hot-rolled strip into a roll, which complicates the formation of a given microstructure and, consequently, mechanical properties along the entire length of the strip at the stage of hot rolling. As a result, it becomes difficult to further process such rolled products into cold-rolled and then into cold-stamped products. 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 to limit the content of carbonitride-forming elements, which does not allow to organize additional hardening of the rolling at the stage of hot rolling by inhibiting grain growth in the process of recrystallization, and accordingly, there is a technological difficulty in providing the required mechanical properties corresponding to strength class 260 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 is the difficulty of providing in the finished cold-rolled strip corresponding to strength class 260 of the required complex of mechanical properties uniformly distributed along the strip length, in particular, tensile strength σ _of > 390 N / mm ² , elongation of δ ₈₀ > 30%. This is due to the lack of a sufficient number of hardening phases in the structure of the known low alloy steel, the formation of which is caused by additional microalloying, as well as by temperature conditions during hot rolling and winding of the strip into a roll. Moreover, in the known low alloy steel, there are no conditions restraining grain growth during recrystallization at the hot rolling stage, and, as a result, a different-grain structure is formed. This structure during subsequent cold rolling of a strip with a thickness of 0.6-3.0 mm on a continuous mill with total relative reductions of up to 60-70% in the absence of a clear regulation of the thickness of the tack leads to cracks, numerous gusts, which, in turn, does not allow provide the required quality of the cold-rolled strip produced along its entire length and significantly reduces the yield.

The technical problem solved by the present invention is to provide strength class 260 of a complex of mechanical properties along the entire length of the cold-rolled strip of low alloy steel and increase 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 260 with a thickness of 0.6-3.0 mm, including smelting, casting steel to obtain a crystallized slab, its hot rolling in stands of a broadband mill with water cooling of the strip surface and winding it into a roll, removing scale from the strip surface by pickling, cold rolling in a continuous mill, heat treatment and subsequent training, according to the invention, steel is melted in the following ratio 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 niobium + molybdenum + vanadium + titanium ≤0.032 iron rest,

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

where h _gk is the thickness of the hot rolled strip, mm;

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

the temperature of the end of rolling and winding of the strip into a roll is selected depending on the thickness of the hot-rolled strip, respectively: for strips more than 4.01 mm thick - 800-820 ° C, 535-565 ° C, from 3.01 to 4.00 mm thick - 820-840 ° C, 565-595 ° C, a thickness of 2.00 to 3.00 mm - 840-860 ° C, 595-625 ° C.

The essence of the proposed technical solution lies in the application of microalloying with carbonitride-forming elements of low alloy steel, in the choice of the thickness of the hot rolled strip depending on the final thickness of the cold rolled strip, as well as the regulation of temperature conditions of hot rolling and winding of the hot rolled strip into a roll depending on its thickness, which together allows increase in yield of cold-rolled metal products with the required level of mechanical properties along the entire length of the strip, corresponding etstvuyuschih strength class 260.

In the claimed method, the boundaries and the range of contents of the main chemical elements: carbon, manganese, silicon, copper, aluminum, phosphorus, calcium, boron, nitrogen, sulfur, chromium, nickel and titanium, are defined, as in the prototype method, in order to maximize possible hardening of the ferrite matrix, while increasing the plastic properties to increase the rollability and stampability. At the same time, for additional hardening of rolled steel from the declared chemical composition, the strategy of additional microalloying with Nb, V and Mo elements was applied. This is explained by 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 260, it is necessary to form a structure with fine grain of polygonized ferrite and dispersed precipitates of carbonitrides 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 plastic properties (δ ₈₀ ), on the other ugo.

As is known, with the simultaneous microalloying of α-ferrite with atoms of several (4-5) alloying (microalloying) elements, their effect on hardening can be summed up (see Special steels. M.I. Goldstein, S.V. Grachev, Yu.G. Veksler. - 2nd ed., Revised and enlarged. - M.: MISIS, 1999. - 408 p.). In small amounts, the strong carbonitride-forming elements Nb, Ti, V, and Mo significantly strengthen the steel as a result of the formation of fine particles and the grinding of ferrite grains. With a certain ratio in steel, the strength parameters of the latter change sharply. The physical nature of this phenomenon lies in the fact that in low alloy steels Ti, V, and Nb are completely included in the carbonitride phase, and Mo is distributed between ferrite and carbides. At the same time, microadditives (for example, V and Ti) practically do not affect the recrystallization temperature, but only slightly restrain grain growth after recrystallization. The presence of Nb noticeably delays the onset of recrystallization and grain growth after its completion, which contributes to the formation of fine grains in the steel structure. The choice of the boundary of the content of Nb, V, Ti, and Mo is associated with the need to form grains no larger than 8-10 points to ensure the rolling of a predetermined thickness into a cold-rolled strip of finite thickness. When the total content of additionally introduced microalloying elements (Nb, V, and Mo), taking into account the Ti content of more than 0.032%, a significant hardening of the ferrite matrix occurs, which does not allow for the efficient processing of hot rolled rolled products into cold rolled and cold stamped products due to numerous gusts and cracks formed during plastic deformations with total reductions in the range of 60-70%.

As is known, the size and shape of austenitic grains depend on the rate of recrystallization during rolling. Therefore, obtaining the required set of mechanical properties to ensure increased formability of metal products from low alloy steel of strength class 260 using microalloying should be achieved by the formation of a fine-grained ferrite structure, one of the main conditions for which is the presence of a fine-grained austenite structure. 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 805-855 ° 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 inventive method, since it is in the specified range (800-860 ° 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 equilateral microstructure with 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 range for winding a hot-rolled strip into a roll in the range of 545-615 ° C, depending on its final thickness. The temperature of the winding for the selected class of steels should be as close as possible to ensure the optimum cooling rate on the discharge roller of the hot rolling mill for more complete stabilization of carbon by isolating or adding titanium carbides (carbosulphides), which allows obtaining relatively low yield strengths and the absence of yield points 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 final thickness of the hot-rolled strip in the microstructure of 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 (more than three adjacent values). On the other hand, at temperatures the ends of rolling and winding higher than those 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 is the most optimal from the point of view of the ability of the metal to be further processed by cold rolling and subsequent deep drawing. The yield strength σ _t in this case in low-alloy steel with microalloying with carbonitride-forming elements in the cold-rolled state reaches 270-285 N / mm ² , the tensile strength σ _in up to 420 N / mm ² (the ratio σ _t / σ _in is in the range of 0.65 -0.70), the relative elongation (δ ₈₀ ) is not less than 30%, which corresponds to the strength class of rolled steel 260. Moreover, in the process of processing hot rolled rolled products into cold rolled products and then into cold-pressed products due to the optimal microstructure over the entire cross section and strip length , is for prison cracking along the edges of the strip, 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 power parameters of the rolling equipment.

Thus, the presented set of features of the proposed method for the production of cold-rolled strips of low-alloy steel of strength class 260 with a thickness of 0.6-3.0 mm from low-alloy steel, microalloyed with carbon-nitride-forming elements, allows to produce high-quality metal products with the required mechanical properties equal along the entire length of the finished strip, while provides improved yield of cold-rolled steel.

An example of the method

Smelted using the oxygen-converter method 3 melts of steel of the claimed 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 was previously determined by the claimed empirical dependence, based on a given final thickness of the cold-rolled strip. Then, the hot rolled strips were subjected to hydrochloric etching, cold rolling, recrystallization annealing, and tempering. 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. 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, by the present method, hot and cold rolling of strips 0.6-3.0 mm thick was carried out from steel of strength class 260, microalloyed with carbonitride-forming elements on a broadband hot rolling mill 2000 and a continuous four-stand cold rolling mill 2500 OJSC Magnitogorsk Metallurgical combine ", as well as the results of the research are presented in table 2.

The inventive technology for the production of coils on the example of the production of cold rolled strips of low alloy steel of strength class 260 provides the following mechanical properties: σ _t ≥270 N / mm ² ,

σ _in ≥380 N / mm ² , δ ₈₀ > 28%, which meets the requirements for steels of strength class 260.

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 260 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 260 with a thickness of 0.6-3.0 mm, including smelting, casting of steel to obtain a crystallized slab, its hot rolling in stands of a broadband mill with water cooling of the strip surface and its winding into a roll, removing scale strip surfaces by etching, cold rolling in a continuous mill, heat treatment and subsequent training, characterized in that the steel is smelted in the following ratio of 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 niobium + molybdenum + vanadium + titanium ≤0.032 iron rest
hot rolling is carried out to obtain a hot-rolled strip, depending on the thickness

where h _gk is the thickness of the hot rolled strip, mm;
h _hk - the final thickness of the cold-rolled strip, mm,
the temperature of the end of rolling and winding of the strip into a roll is selected depending on the thickness of the hot-rolled strip, respectively: for strips with a thickness of more than 4.01 mm - 800-820 ° C, 535-565 ° C, a thickness of 3.01 to 4.00 mm - 820-840 ° C, 565-595 ° C, a thickness of 2.00 to 3.00 mm - 840-860 ° C, 595-625 ° C.