RU2159820C1 - Method of production of low-carbon cold-rolled steel for stamping followed by enameling - Google Patents

Abstract

FIELD: ferrous metallurgy; production of low-carbon cold-rolled steels for manufacture of enameled articles. SUBSTANCE: proposed method consists in adjustment of process due to selection of optimal parameters of hot rolling, cold strain and annealing recrystallization. After melting, steel containing, mass-%: carbon, 0.04 to 0.08' silicon, 0.01 to 0.05; manganese, 0.1 to 0.3; sulfur, up to 0.025; phosphorus, up to 0.03; chromium, up to 0.06; boron, up to 0.005; nickel, up to 0.06; copper, up to 0.06; aluminum, 0.01 to 0.04; iron and unavoidable admixtures, remainder is subjected to hot rolling terminating within temperature range of Ac₁+90C - Ac₃-20C; roll is wound at temperature below temperature Ac₁ by at least 30 C. Cold rolling is performed at degree of strain from 50 to 70%. Heating to annealing temperature is performed after cold rolling at rate of 10 to 50 %/h. EFFECT: possibility of obtaining structure of cold-rolled steel ensuring complex of mechanical properties required for stamping and excluding such defects as "fish scale". 3 cl, 3 tbl, 3 dwg, 3 ex

Description

 The invention relates to the field of metallurgy, in particular to the production of low-carbon cold-rolled sheet steel, used for the manufacture of parts of particularly complex shapes.

 A known method of manufacturing sheet steel for deep drawing, including steel smelting, casting, hot rolling at a given temperature of the end of deformation, winding into a roll at a given temperature, cold rolling, annealing and tempering (see RU 2133284 C1, C 21 D 8/02, 07/20/1999).

The most important requirement for low-carbon cold-rolled sheet steel used for the manufacture of parts of particularly complex shape is its high formability. This steel from the batch to the finished sheet undergoes a fairly lengthy redistribution, which includes the following operations: smelting, hot rolling, cold rolling, annealing and tempering. The task of providing the necessary complex of mechanical properties and forming characteristics of the cold-rolled sheet is currently successfully solved by a number of techniques used at various stages of steel production:
optimization of temperatures of hot rolling and strip winding into rolls (usually they suggest using a sufficiently high temperature of the end of rolling and the lowest possible temperature of the winding);
increase in the degree of deformation during cold rolling up to 75%;
optimal temperature conditions for hot rolling of strips from converter steel 08YU of continuous casting (A.F. Pimenov, V.I. Slavov, V.V. Chashchin et al. / / Bulletin of scientific and technical information. Ferrous metallurgy. 1981, V.II ( 895), S. 54 ... 55).

A known method for the production of cold rolled steel, including obtaining a sheet, heating cold rolled steel at a fairly slow speed, calculated by the empirical formula taking into account aluminum and nitrogen in steel and annealing in the temperature range 400 ... 600 ^o C. However, only this parameter, despite its of fundamental importance, cannot be ensured by both the lack of a tendency of steel to form a “fish scale” defect after enameling and the good formability of steel without a sufficient degree of deformation during cold rolling. (RU 2039839 C1, C 21 D 9/46, 01/30/1994).

The closest analogue to the claimed invention is a known method for the production of low carbon cold rolled steel for stamping and subsequent enameling, including the smelting of steel containing carbon, manganese, sulfur, aluminum, nitrogen, titanium, copper, iron and other inevitable impurities, casting, hot rolling with a given the temperature of the end of rolling 800-900 ^o C, winding into a roll at a temperature of 670-690 ^o C, etching of the scale, cold rolling, annealing and training (RU 2101368 C1, C 21 D 8/02, 01/10/1998).

 The complex of mechanical properties determined in accordance with accepted standards does not allow us to unambiguously assess the formability of cold rolled steel. For this reason, the suitability of the metal for stamping is usually evaluated by R (normal plastic anisotropy coefficient), or by the associated texture index N (the ratio of the intensity of the orientation favorable for stamping {III} <UVW> to the unfavorable {100} <UVW>).

 The manufacture of stamped products from enamelled cold-rolled steel imposes a number of specific requirements on the metal, the most important of which is the lack of tendency to form enamel spalls (the so-called fish scale defect). The appearance of these spalls is usually associated with the release of hydrogen from the product that has fallen into steel during enameling. The tendency of steel to form enamel spalls is usually evaluated by the so-called hydrogen index N.

 Ensuring that cold-rolled steel has good stampability and at the same time lack of a tendency to form a fish scale defect after enameling is quite difficult to do, since both requirements are often achieved by mutually exclusive parameters of steel processing conditions.

 The technical result of the invention is to obtain the structure of cold rolled steel, providing the necessary set of mechanical properties for stamping and at the same time guaranteeing the absence of tendency of the enameled product to the occurrence of a fish scale defect.

 Studies have shown that enamel products made of steel, the structure of which before the enameling operation was characterized by the presence of sufficiently large, rounded cementite inclusions collected in conglomerates of a somewhat elongated "worm-like" shape, are the least prone to the formation of a fish scale defect (2..3 points scale B according to GOST 5640). Also favorable from the point of view of enameling is the absence in the subsurface layer (10..50 microns) of a cold-rolled sheet of cementite precipitates of any shape and size, i.e. the presence of a decarburized layer during processing.

 Large precipitates of cementite with an incoherent border with ferrite are (due to a well-developed surface and the presence of a region of micro-distortions near it) the best "traps" for hydrogen ions (atoms) penetrating the steel during enameling and subsequent firing. Accumulating in such "traps", hydrogen ions mobilize, after which the back diffusion of hydrogen to the enameled surface becomes impossible.

The absence of cementite-type carbide particles in the surface layer prevents enamel “boiling” (enamel spikes) in the initial stages of firing, when the main gases released from steel are CO and CO ₂ .

In addition, the presence of a decarburized layer prevents steel from occurring in its surface layer during firing (t> 750 ^o C) areas of austenin, the solubility of hydrogen in which significantly exceeds its solubility in ferrite. The appearance of austenin in the subsurface layer leads to the accumulation of hydrogen in it during firing and the back diffusion of hydrogen to the surface upon cooling during the decomposition of the gamma phase. This leads to the appearance of chips on the enameled surface, immediately after the end of the enameling process.

Despite the fact that the production cycle of manufacturing cold rolled steel for stamping and enameling is quite long and includes several operations, the prerequisites for the formation of the metal structure are laid already at the stage of hot rolling. Obtaining the optimal structure of the tackle is possible if the temperature regime of hot rolling is selected taking into account the temperature regions of phase transformations in steel. If the completion of hot rolling occurs in a two-phase region (below temperature A _c3 ), several elongated grains of ferrite and austenite simultaneously exist in the steel structure, the latter being substantially enriched in carbon. The decay of high-carbon regions of the gamma phase at sufficiently high winding temperatures of the strip occurs in the first degree with the release of large particles of cementite of a somewhat elongated shape. Moreover, high winding temperatures contribute to the partial coagulation of carbides and at the same time decarburization of the surface layers of steel due to the removal of carbon in the scale (a variant of partial "black" annealing).

Lower temperatures of the strip winding do not allow obtaining cementite of optimal shape and size. At winding temperatures exceeding A _s1 (in the temperature region where austenite is still quite stable), the coarse cementite is excessively coarsened, which adversely affects the stampability of products during further processing. To achieve a technical result in the known method for the production of low carbon cold rolled steel for stamping and subsequent enameling, including steel smelting, casting, hot rolling, scale etching, cold rolling, annealing and tempering, steel containing, wt.%: Is smelted:
Carbon - 0.04-0.08
Silicon - 0.01-0.05
Manganese - 0.1-0.3
Sulfur - Up to 0.025
Phosphorus - Up to 0.03
Chrome - Up to 0.06
Boron - Up to 0.005
Nickel - Up to 0.06
Copper - Up to 0.06
Aluminum - 0.01-0.04
Iron and Inevitable Impurities - Else
hot rolling is completed in the temperature range Ac1 + 90 ^o C - Ac3-20 ^o C, and the winding of the hot-rolled coil is carried out at a temperature of not more than 30 ^o C below the temperature Ac1. Cold rolling is carried out with a degree of deformation of 50-70%. Heating to the annealing temperature is carried out after cold rolling at a speed in the range of 10-50 ^o / h

Thus, the best temperature regime for hot rolling from the point of view of enameling products after further processing of steel is hot deformation with a temperature of the end of rolling Ac1 + 90 ^o C ... Ac3-20 ^o C (830 ... 880 ^o C), and the temperature of the winding hot rolled coil no more than 10 ... 30 ^o C below the temperature Ac1 (690 ... 720 ^o C). These results are clearly confirmed by the experimentally obtained dependences of the hydrogen index of the cold-rolled metal on the temperatures of the end of hot rolling (Fig. 1) and winding (Fig. 2).

 However, the use of such hot rolling conditions without taking into account certain requirements for subsequent processing steps does not provide good formability of the steel.

It is known that the best formability of cold rolled steel is ensured by the presence of sufficiently perfect {III} <UVW> texture in crystallites, which is formed as a result of rolling and subsequent annealing for primary recrystallization. The analysis of literature data shows that the best parameters for processing steel to obtain a perfect {III} <UVW> texture in it are: the end of hot rolling at a temperature above the onset of austenite decomposition; reduced to 540 ... 570 ^o C temperature of the strip winding; cold deformation with compression of about 75%; slow heating for primary recrystallization. Our studies show that of the above parameters, the most important is the heating rate during annealing for primary recrystallization. A sufficiently slow heating rate (10 ... 50 ^o C / h) ensures good formability of steel at temperatures from the end of rolling, winding and cold deformation, optimal from the point of view of enameling, with a compression of at least 50%. This is due to the formation of a rather large fraction of the {III} <UVW> texture component in the steel structure during primary recrystallization (Fig. 3).

The effectiveness of the application of this invention can be demonstrated by the following examples:
Example 1
Melting mild steel with a chemical composition:
0.046 wt. % carbon, 0.03 wt.% silicon, 0.25 wt.% manganese, 0.021 wt. % sulfur, 0.018 wt.% phosphorus, 0.05 wt.% chromium, 0.04 wt.% nickel, 0.06 wt. % copper, 0.001 wt.% boron, 0.036 wt.% aluminum, (the rest is iron and inevitable impurities), smelted in the conditions of the Magnitogorsk Iron and Steel Works in a converter way, poured into slabs, and then processed according to the scheme, including hot rolling, etching of scale, cold rolling, annealing and training. When hot rolling in one half of the strips, the temperature of the end of rolling was 860 ^o C, and in the other 940 ^o C. The winding temperature in one half of the strips was 650 ^o C and in the other 700 ^o C. Thus, four modes of hot rolling were carried out. (Table 1). During cold rolling, the strain amounted to 53%. The heating rate during annealing for primary recrystallization was 20 ^o C / h (heating in a bell furnace). After training, samples were taken from the sheets for X-ray texture determination. The parameters of the processing modes and test results are shown in table 1.

 Products with relative deformation of up to 40% (baths) were manufactured from cold-pressed sheet metal by blanking, after which they were subjected to two-layer enameling. The percentage of defects during stamping and after enameling is also shown in table 1.

From the data of the table it follows that both from the point of view of stampability of steel and from the point of view of its enameling, the temperature of the end of hot 850 ^o C and the winding temperature of 700 ^o C are the best processing parameters proposed in the present invention.

Example 2
Melting low carbon steel with a chemical composition: 0.062 wt.% Carbon, 0.02 wt.% Silicon, 0.10 wt.% Manganese, 0.015 wt.% Sulfur, 0.008 wt.% Phosphorus, 0.02 wt.% Chromium, 0 , 02 wt.% Nickel, 0.03 wt.% Copper, 0.003 wt.% Boron, 0.024 wt.% Aluminum, (the rest is iron and inevitable impurities), smelted in the conditions of the Magnitogorsk Iron and Steel Works in a converter method, poured into slabs, and then processed according to the above scheme. The temperature of the end of the hot rolling of the strips was 860 ^° C, the temperature of the winding was 690 ^° C. During hot deformation, half the metal was rolled to a thickness of 3.6 mm, and the rest of the heat was rolled to a thickness of 2.6 mm. The thickness of the steel after cold rolling was 1.5 mm. Thus, the value of cold deformation on one batch of metal was 58.3%, and the other 42.3%. Half of each batch was annealed in a bell furnace with a heating rate of 30 ^o C / h, the remaining half of the batches in a horizontal furnace through passage (heating rate - 400 ^o C / min). This was followed by metal training. After training, samples were taken from the sheets to determine the hydrogen index and samples for radiographic determination of the texture. The parameters of the processing modes and test results are shown in table 2.

 Products with relative deformation of up to 40% (baths) were manufactured from cold-pressed sheet metal by blanking, after which they were subjected to two-layer enameling. The percentage of marriage during stamping and after enameling are also shown in table 2.

From the table it follows that both from the point of view of stampability of steel and from the point of view of its enameling, the best deformations are 58.3% in cold rolling, and the heating rate during annealing for primary recrystallization is 30 ^o C / h, i.e., the parameters processing proposed in the present invention.

Example 3
Table 3 summarizes data on processing according to the above scheme of a representative batch of low-carbon steel melts. The chemical composition of the metal varied in the following ranges: carbon - 0.04 ... 0.08 wt.%, Silicon - 0.01. . .0.05 wt.%, Manganese - 0.1 ... 0.3 wt.%, Sulfur - 0.008 ... 0.025 wt.%, Phosphorus - 0.009 ... 0.03 wt.%, Chromium - 0 , 02 ... 0.06 wt.%, Nickel - 0.02 ... 0.06 wt.%, Copper - 0.3 ... 0.8 wt.%, Boron - 0.0004 ... 0.005 wt.%, Aluminum - 0.01 .. .0.04 wt.%. (the rest is iron and inevitable impurities). Swimming trunks 1 ... 4 were processed with parameters in accordance with the proposed in this invention. Processing of the remaining heats at least in one parameter differed from the optimal one. It can be seen that either in stampability or in yield for enameling, this metal is significantly inferior to steel treated in accordance with the claimed production method.

Claims (4)

1. Method for the production of low-carbon cold-rolled steel for stamping and enameling, including steel smelting, casting, hot rolling, winding, etching of scale, cold rolling, annealing and tempering, characterized in that steel containing wt.% Is smelted:
Carbon - 0.04 - 0.08
Silicon - 0.01 - 0.05
Manganese - 0.1 - 0.3
Sulfur - Up to 0.025
Phosphorus - Up to 0.03
Chrome - Up to 0.06
Nickel - Up to 0.06
Copper - Up to 0.06
Boron - Up to 0.005
Aluminum - 0.01 - 0.04
Iron and Inevitable Impurities - Else
hot rolling is completed in the temperature range A _c1 + 90 ^o C - A _c3 - 20 ^o C, and the winding of the hot-rolled strip is carried out at a temperature of not more than 10 - 30 ^o C below A _c1 .  2. The method according to claim 1, characterized in that the cold rolling is carried out with a degree of deformation of 50 to 75%. 3. The method according to any one of claim 1 or 2, characterized in that after cold rolling, heating is carried out to the annealing temperature at a rate in the range of 10-50 ^° C / h. 4. The method according to any one of claims 1 to 3, characterized in that the annealing is carried out at a temperature below A _c1 at 10 - 30 ^o C with exposure.

Images