RU2209253C1 - Method of finishing low-carbon cold-rolled strip steel - Google Patents

Abstract

FIELD: ferrous metallurgy; plastic metal working; production of low-carbon cold-rolled strip steel for automobile body sheets, for example. SUBSTANCE: proposed method includes temper rolling after annealing of strips 0.6-2.5 mm thick followed by dressing through tension at preset magnitudes of deformation; temper rolling and dressing are performed within range of deformation degrees selected from the following relationship: K₁/K₂ = 2,5, where K₁ is empirical coefficient; K₂ is empirical coefficient. It is good practice to perform temper rolling at relative reduction equal to 0.6-0.8% when no lubricant is used and 1.1- 1.3% when lubricant is used for process. Proposed method ensures obtaining required mechanical properties and enhanced planeness due to optimization of deformation at temper rolling and dressing. EFFECT: enhanced efficiency. 3 cl, 1 ex

Description

 The invention relates to the field of ferrous metallurgy, in particular to the processing of metals by pressure, and can be used in the production of low-carbon cold-rolled strip steel with special properties, for example, a sheet.

 The level of perfection of technological operations in the production of cold-rolled strip steel, in particular, a sheet, is assessed by surface quality. The size and uniformity of the surface roughness of strip steel determine not only the mechanical properties and stampability, but also the strength and appearance of the paintwork and other coatings. The final operations for the production of cold-rolled strip steel are: annealing and tempering, and the latter is usually attributed to the final finishing of sheets. To improve the flatness of the strip steel, especially intended hereinafter for galvanizing, it is subjected to stretching editing. The most important parameters of the finishing operations of training and dressing is the degree of deformation (for training - the value of the relative compression, for dressing - the value of stretching), which largely determine the required properties, including mechanical properties, of the finished autolist.

 A known method of finishing cold-rolled steel strips, in which training is carried out with a given degree of compression, depending on the thickness and grain size in the structure of the strip, while increasing the thickness of the strip from 0.7 to 3.5 mm corresponds to a reduction in the degree of reduction from 0.8 to 0 , 3%, and when the grain size in points of 7-9 or higher, the compression ratio is increased by 0.2 and 0.4%, respectively (see A.S. USSR 1458046, class B 21 V 37/12, publ. 02/15/89).

 The known method provides increased stability of the mechanical properties of the finished product along its length, however, this does not provide the requirements for a sheet used for internal and external parts with deep and very deep drawing. This is especially true of external painted parts, since with a grain value of 7-9 or higher, the compression ratio is increased by 0.2 and 0.4%, respectively, satisfying the requirements for samples according to Olsen, but they do not provide a given surface roughness of due to the unevenness of macro- and microstructure. The smaller the grain size, the greater the elongation, i.e. the longer the yield area and the greater the degree of compression required during training, so this method does not provide sheet metal with a particularly high purity of surface finish.

 There is a known technology of training - straightening by stretching on the combined setup described in the book of Y. S. Finkelshtein. Handbook of rolling and pipe production. - M.: Metallurgy, 1975, p.138-139.

 This technology includes the training of strips after annealing and their straightening by stretching with the specified strain values and is characterized by the fact that the straightening of the strips is carried out by multiple bending of the metal in the vertical plane using roller tensioners both before and after training.

The known method does not provide cold rolled strip, in particular a sheet, with the required standard indicator, mainly due to the fact that each of the trim elements is designed to solve certain local problems, for example, when training to improve the surface, prevent shear lines (Chernov lines Moders), reducing undulation and warping. In this case, the yield strength (σ _t ) of low-carbon steel at relative compressions up to 0.8-1.2% decreases and only with relative compressions greater than the specified range, it begins to increase. Further “easy” dressing leads to a deterioration in mechanical characteristics, since the length of the yield plate increases, which can lead to aging of the trained metal. Insignificant editing has a more harmful effect than intensive, because the length of the yield area after minor editing is greater than after intensive. This is due to the fact that with intensive dressing not only the residual stresses of metal training are reduced, but also significant stresses are again created in the surface layer, and therefore (σ _t ) is slightly reduced. With intense tension, the effect of training decreases due to an increase in the number of dislocations and their accumulation at the grain boundaries, which complicates the occurrence of shear processes leading to hardening and an increase in the yield strength of the metal. With an increase in compression during training, the degree of influence of editing on the aging pattern of trained metal decreases. Tensile strength (σ _in ), elongation (δ,%), hardness (HRC) in all cases of combining these deformation operations are reduced compared with the corresponding values obtained after training, so when training carbon steel with a thickness of 0.5-1.5 mm tension is 0.7-0.8 (σ _t ). When editing by pure stretching, the result of changing the mechanical properties of the trained sheet is similar to that obtained after "easy" editing. However, this method does not provide a stable value of roughness, which can both worsen the stampability of a sheet, and reduce the quality of paints and other coatings of stamped parts.

The closest analogue of the claimed invention is a method for finishing a cold-rolled carbon strip, for example, from sheet steel (Benyakovsky MA, Bogoyavlensky KN, Vitkin AI, etc. Rolling production technology: Handbook. - M .: Metallurgy, 1991, p. 696-700). The method includes training a strip having a temperature of not more than 40 ^{° C.} with relative compressions of 0.8-2.0%, determined by the grain size, strip surface and roll diameter. To eliminate contamination of the metal surface and prevent surface defects, training is carried out using technological lubricant. Next, the cold-rolled strip is subjected to stretching editing on roller and stretching straightening machines with a relative strain equal to 0.5-2.0%.

 The known method does not provide the desired technical result for the following reasons.

In autosheet grades of steel, for example 08Yu, during cold deformation with a relative compression of 20-25%, ductility is intensively reduced, which further deteriorates. After annealing in a delayed cooling chamber, the strip is cooled to 500 ^o C, which ensures the complete precipitation of finely dispersed carbides dissolved in ferrite, necessary to increase ductility during subsequent training and dressing.

However, the analysis of experimental data shows that, despite the increase in the yield strength of strip steel from 18.5-19 kgf / mm ² (with surface roughness R _a = 1 μm) to 20-22.5 kgf / mm ² (with R _a = 5 microns) and a decrease in the tensile strength by 1.0-1.5 kgf / mm ² , the ultimate degree of drawing in the tests for stampability by the Swift method increases with increasing roughness. At the same time, the surface hardness increases, since a surface with a rough roughness has a higher hardness compared to a smooth one, therefore, increased degrees of relative reduction (more than 0.8% without lubrication and more than 1.3% with lubricant) when stamping strips even with a very smooth finish contribute to the opening of traces of rolling on a multi-roll mill. At the same time, with a too smooth surface, difficulties may arise during stamping: too smooth a surface does not hold grease well, which makes it difficult to extract metal in the dies.

 Subsequent editing by stretching with plastic deformation equal to 0.5-2.0%, increases the strength properties of the strip, reduces the flatness of the finished product due to the creation of significant internal stresses, which also negatively affects the formability.

 The basis of the invention is the task of improving the method of finishing low-carbon cold-rolled strip steel, for example, a sheet, in which, by optimizing the strains during training and dressing, the required mechanical properties and high flatness of the finished product are obtained.

где ε $\genfrac{}{}{0ex}{}{\text{в}}{\text{∂}}$ - верхнее значение относительного обжатия при дрессировке из выбранного диапазона;
ε $\genfrac{}{}{0ex}{}{\text{н}}{\text{∂}}$ - нижнее значение относительного обжатия при дрессировке из выбранного диапазона;
ε $\genfrac{}{}{0ex}{}{\text{н}}{\text{p}}$ - нижнее значение относительной деформации при правке из выбранного диапазона;
К₂ - эмпирический коэффициент, равный

где ε $\genfrac{}{}{0ex}{}{\text{в}}{\text{p}}$ - верхнее значение относительной деформации при правке из выбранного диапазона, при этом величину относительной деформации при правке растяжением принимают равной 0,2-0,5%.The problem is solved in that in the method of finishing low-carbon cold-rolled strip steel, including training after annealing of strips with a thickness of 0.6-2.5 mm and dressing by stretching with the specified values of deformations, according to the invention, training and dressing are carried out in the range of degrees of deformation selected from ratios:
K ₁ / K ₂ = 2.5
where K ₁ is an empirical coefficient equal to

where ε $\genfrac{}{}{0ex}{}{\text{in}}{\text{∂}}$ - the upper value of the relative compression during training from the selected range;
ε $\genfrac{}{}{0ex}{}{\text{n}}{\text{∂}}$ - the lower value of the relative compression during training from the selected range;
ε $\genfrac{}{}{0ex}{}{\text{n}}{\text{p}}$ - the lower value of the relative deformation when editing from the selected range;
K ₂ is an empirical coefficient equal to

where ε $\genfrac{}{}{0ex}{}{\text{in}}{\text{p}}$ - the upper value of the relative deformation during editing from the selected range, while the value of relative deformation during editing by stretching is taken equal to 0.2-0.5%.

 It is advisable to train without lubrication with a relative compression of 0.6-0.8%.

 It is advisable to train with lubricant with a relative compression of 1.1-1.3%.

 The essence of the technical solution found is to optimize the strains during training and dressing, which ensures the required quality of low-carbon strip steel (including a sheet for galvanizing).

 To meet the requirements of deep drawing during training, the largest possible non-deformable part of the metal is obtained along the strip section, and in a thin surface layer, a deformation of such a magnitude that along with minimal hardening eliminates the possibility of shear lines.

Training with the selected values of the relative compression provides hardened surface layers in the finished product while reducing its “soft” core and leads to an increase in the ductility of strip steel by reducing the yield strength (σ _t ) while maintaining a high tensile strength (σ _c ), relative elongation (δ,%) and hardness (HRC). In addition, the proposed finish by training and dressing by stretching while optimizing the deformation values of each of the trim elements eliminates the yield point, which reduces the degree of influence of dressing on the nature of aging of the metal. The proposed finish allows you to save the "soft" inner layer of strip steel and harden its surface layers.

The proposed values of the relative reductions both during training without lubrication and during training with lubrication are assumed to be invariant to the thickness of the trimmed strips. The experimentally established invariance of the value of the relative compression ε _∂ when combining the processes of tempering and dressing by stretching for thin-sheet low-carbon steel is explained by the fact that tensile deformation, firstly, reduces the residual stresses in steel that arise during its compression during tempering, and, secondly , some alignment of the microstructure of the metal, which makes it possible to use the same ε _∂ for different thicknesses of strips, which simplifies the technology of their finishing.

 Example. Experimental verification of the proposed method for finishing low-carbon cold-rolled strip steel was carried out in a mill 2500.

 After annealing, cold-rolled steel coils 08Yu and 08YuR with strip parameters of 1.5x1500 mm were cooled in the final cooling section and fed to the unwinder of the temper mill. The coils subjected to training were installed on the unwinder and fed the front end of the strip to the cage of the temper mill 2500. The training was carried out in two ways: without strip lubrication and strip lubrication (30% emulsion based on emulsion). The value of the relative reduction during training without lubrication ranged from 0.6 to 0.8%, and during training with lubrication from 1.1 to 1.3%. To maintain a given value of the relative compression along the entire length of the strip, a system for automatically controlling the degree of deformation was used.

 After training, the strips were subjected to stretching editing on a straight-stretching machine of the cutting unit with a relative strain in the range of 0.2-0.5%.

 The selected samples were subjected to tests to determine the mechanical properties and surface geometry of the finished sheets.

The highest results (about 92% of strip hire): non-flatness of strips within 1-2 mm / m (with an average non-flatness to finish 8-10 mm / m), σ _t = 185-190 N / mm ² and δ ₄ = 36-39% - obtained using the parameters of the proposed method.

During training with compression values ε _∂ <0.6% (without lubricant) and ε _∂ <1.1% (with lubricant), first of all, the sheet microgeometry deteriorated, which negatively affected the quality of the galvanized strip, and when testing the strip for stretching characteristic yield lines (Luders lines) were observed, which led to rejection of rolled products in 9-16% of cases. When ε _∂ > 0.8% (without lubricant) and ε _∂ > 1.3% (with lubricant), the value of σ _t increased to 196-200 N / mm ² and δ ₄ decreased to 36-35%, which does not correspond requirements for this steel category.

The proposed ε _∂ values (0.6–0.8% and 1.1–1.3%) were found to be acceptable for all investigated strip steel thicknesses.

If in the process of straightening strips by stretching, the relative strain ε _p was less than 0.2%, then the required flatness (1-2 mm / m) of the rolled product could not be achieved, and at ε _p > 0.5%, the mechanical properties of steel deteriorated, so as the values of σ _t and δ ₄ did not meet the required standards.

 The experiments also showed that, subject to the claimed training and dressing parameters, a significant part (87-92%) of them corresponded to the requirements of group I of the surface finish (i.e., a particularly high finish according to GOST 16523), which is an obligatory norm for an autosheet.

 Thus, an experimental verification confirmed the acceptability of the technical solution found and its advantages over a well-known object - the closest analogue.

 Using the proposed method in the production of low-carbon cold-rolled strip steel will increase the output of the sheet by an average of 15%, which will increase profits from the sale of better and more expensive products.

Claims (3)

1. The method of finishing low-carbon cold-rolled strip steel, including the training of strips with a thickness of 0.6-2.5 mm after annealing and dressing by stretching with the specified strain values, characterized in that the training and dressing are in the range of degrees of deformation selected from the ratio
K ₁ / K ₂ = 2.5,
where K ₁ is an empirical coefficient equal to

where ε $\genfrac{}{}{0ex}{}{\text{in}}{\text{d}}$ - the upper value of the relative compression during training from the selected range;
ε $\genfrac{}{}{0ex}{}{\text{n}}{\text{d}}$ - the lower value of the relative compression during training from the selected range;
ε $\genfrac{}{}{0ex}{}{\text{n}}{\text{p}}$ - the lower value of the relative deformation when editing from the selected range;
K ₂ is an empirical coefficient equal to

where ε $\genfrac{}{}{0ex}{}{\text{in}}{\text{p}}$ - the upper value of the relative deformation when editing from the selected range,
while the value of the relative deformation when editing by stretching is taken equal to 0.2-0.5%.  2. The method according to claim 1, characterized in that the training without lubrication is carried out with a relative compression of 0.6-0.8%.  3. The method according to claim 2, characterized in that the training with lubricant is carried out with a relative compression of 1.1-1.3%.