JPH0557337B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for manufacturing a cold rolled steel sheet that not only has excellent surface quality but also excellent workability. Conventional technology Cold-rolled steel sheets for press forming with excellent deep drawability are
Traditionally, large quantities have been produced by box annealing, and among box annealing, open coil annealing is particularly preferred over tight coil annealing in terms of uniformity of material in the width direction of the coil and productivity. There is. In this open coil box annealing, usually
After cold rolling, coils that are sent directly with rolling oil on them without being cleaned are recoiled.
The coil is re-wound into a loose coil with a spacer wrapped around it in the unit, and then loaded into an annealing furnace. The rolling oil adhering to the surface of the steel sheet is usually completely evaporated during the annealing process due to the atmosphere in the furnace.
When rolling oil containing special additives is used, it does not completely evaporate and adheres to the surface of the steel plate in the form of carbon, causing stains. Therefore, in such cases, from the viewpoint of preventing contamination, gas cleaning is performed by blowing water vapor into the atmospheric gas when the temperature is raised to completely gasify the carbon. However, in recent years, the requirements for the surface quality of cold-rolled steel sheets have become extremely strict, and in particular, the requirements for the base sheets of surface-treated steel sheets have become stricter.In order to prevent surface quality defects due to contamination, gas cleaning is As the processing temperature range tends to expand, the coil temperature usually starts from 100 to 200℃, and increases to 550℃.
It is finished at ~600℃. Problems to be Solved by the Invention However, when annealing is performed under the gas cleaning conditions as described above, even though the annealing temperature is below _A1 point, the upper side of the coil (furnace When the coil axis is taken as the vertical direction, lump-like or string-like carbides may be formed on the upper side of the coil in the width direction (hereinafter the same shall apply), which may affect the workability of the cold-rolled steel sheet, especially the elongation. Blungability deteriorates and defects such as cracks are likely to occur during press molding. In order to clarify the cause of this, the present inventors conducted detailed research on annealing temperatures and gas cleaning conditions, and found that for cold-rolled steel sheets with a C content of 0.04 to 0.05%, gas cleaning was started at 150°C. ,600℃
When open coil box annealing is performed at 700℃ under the condition that the temperature ends at 0.010~
Decarburization was observed to the extent of 0.015%, and for this reason,
_A: Even at annealing temperatures below the transformation point, a large amount of C
It has been found that during the cooling process, undissolved cementite precipitates as a core, producing coarse carbide. That is, it has been found that decarburization occurs in the high temperature range of gas cleaning during the open coil box annealing process, and this causes the formation of coarse carbides in the coil. Therefore, the present inventors performed open coil box annealing while varying the end temperature of gas cleaning, and found that by lowering the gas cleaning temperature, the lump-like and string-like carbide particles as described above were removed. We found that the formation of carbon stains on the steel plate surface caused by lowering the gas cleaning temperature can be almost completely prevented by using a spacer with a large diameter of 2.5 mm or more to prevent carbon stains between the steel plates. The inventors have discovered that this problem can be solved by widening the interval between them and increasing the flow rate of the atmospheric gas, leading to the present invention. Means for Solving the Problems The method of manufacturing a cold-rolled steel sheet with excellent workability according to the present invention has, in weight percent, C 0.03 to 0.07%, Si 0.20% or less, Mn 0.05 to 0.60%, P 0.10% or less, and Al 0.01. ~0.10%, the balance iron and unavoidable impurities, the steel is hot-rolled and cold-rolled, and the resulting cold-rolled steel plate is re-wound into a loose coil with a spacer wrapped around it in a recoiling unit to obtain ₁ Ac. When annealing an open coil box at the following temperatures, the coil temperature is 350 to 500℃ in a gas cleaning method in which steam is blown into the atmospheric gas when the temperature is raised.
The gas cleaning is terminated when a temperature in the range of is reached. First, the chemical composition of the steel used in the method of the present invention will be explained. When the amount of C added is less than 0.03%, it tends to remain as a solid solution in the steel after annealing, significantly deteriorating the aging characteristics, so 0.03% is set as the lower limit amount. However, when added in excess of 0.07%, the steel becomes hard and workability deteriorates, so the upper limit is set at 0.07%. Si deteriorates the surface properties of cold-rolled steel sheets, and
Since it has the effect of increasing solid solution C during annealing,
The lower the content, the better, but an upper limit of 0.20% is permissible. Mn combines with S to prevent hot embrittlement.
To effectively obtain this effect, an addition of at least 0.05% is required. However, if too much is added, the ductility and deep drawability will be significantly deteriorated.
The amount added shall be 0.60% or less. P is the most commonly used reinforcing element in the production of high-strength steel because it can strengthen steel without impairing deep drawability and is inexpensive.
If the addition exceeds %, the weldability deteriorates, so this is set as the upper limit. In addition, in the case of soft steel, usually
It is added in an amount of about 0.01 to 0.02%, but in order to improve ductility, the smaller the amount added, the better. Al is added at least 0.01% for deoxidation and to prevent room temperature aging by fixing N in the steel.
% addition is required. However, when added in excess of 0.10%, processability deteriorates, so the upper limit is set at 0.10%. The present invention involves hot rolling and cold rolling steel having such chemical components according to a conventional method, and recoiling the obtained cold rolled steel sheet with rolling oil still attached.
After rewinding into a loose coil while winding a spacer in the unit, gas cleaning is performed by blowing water vapor into the atmospheric gas when the temperature rises.Ac When annealing the open coil box at a temperature below ₁ point, the coil temperature is 350 to 500℃. The gas cleaning is completed when the temperature within the range of 1 is reached, thereby obtaining a cold-rolled steel sheet with excellent workability as well as surface quality. In hot rolling, from the viewpoint of improving the deep drawability of the final product, it is preferable to finish with A of ₃ or more points.
Further, in order to reduce the amount of scale produced, it is preferable to wind the film at a temperature of _A1 point or lower. In the subsequent cold rolling, in order to develop a recrystallized texture that is advantageous for deep drawability during annealing,
A rolling reduction ratio of 90% is desirable. In open coil box annealing, according to the present invention, the gas cleaning start temperature is 100 to 200°C, and the gas cleaning end temperature is 350 to 200°C.
The temperature shall be in the range of 500℃. When the temperature exceeds 500°C, as mentioned above, decarburization progresses significantly and formation of coarse carbide in the form of lumps or strings cannot be avoided. However, at temperatures lower than 350°C, it is difficult to avoid surface defects. Furthermore, in the present invention, the spacer wound between the strips of the loose coil has a diameter of 2.5 mm.
By using a large diameter one of mm or more, the flow rate of atmospheric gas flowing between the strips is increased, and gas cleaning at low temperatures prevents the decarburization.
Cold-rolled steel sheets with excellent surface quality and no carbon stains can be obtained. Effects of the Invention As described above, according to the method of the present invention, by lowering the gas cleaning temperature during open coil box annealing, the formation of lump-like and string-like carbides caused by decarburization is almost completely prevented. Furthermore, by using a large diameter spacer of 2.5 mm or more to be wound between the strips of the loose coil, it is possible to prevent carbon contamination, thus improving surface quality and workability. It is possible to produce cold-rolled steel sheets. EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way. Example 1 Steel having the chemical composition shown in Table 1 was melted in a converter, and after continuous casting into a slab,
Heated to ℃, hot rolled, finishing temperature 880~930
A hot rolled steel plate with a thickness of 3.2 mm was obtained at a coiling temperature of 450 to 600°C. These hot-rolled steel plates were pickled and then cold-rolled into cold-rolled coils with a thickness of 0.8 mm and a width of 1244 mm. This was rewound to form a loose coil using a spacer with a diameter of 1.6 mm, and then placed in an annealing furnace and annealed at a soaking temperature of 700°C. At this time, gas cleaning started at 150°C and ended at various temperatures between 350 and 600°C. After 1% temper rolling, samples were taken and the occurrence of lump-like and string-like carbides in the width direction of the coil was examined, as well as aging characteristics, stretch flangeability, and surface staining. FIG. 1 shows changes in the location of occurrence of lumpy carbides in the width direction of the coil depending on the end temperature of gas cleaning.
When the gas cleaning end temperature is 500℃ or higher,
The area where lumpy carbide occurs rapidly expands in the width direction of the plate. However, no particular difference is observed depending on the steel type. FIG. 2 shows the relationship between the gas cleaning end temperature and the failure rate due to carbon contamination. It is observed that when the gas cleaning temperature is below 500°C, the carbon contamination defect rate increases rapidly. Figure 3 shows the gas cleaning temperature end temperature.
The relationship between the location where lumpy carbide occurs and the material quality is shown for Steel A and Steel B heated to 600°C, and the aging properties and stretch flangeability are both significantly inferior at the location where lumpy carbide occurs. Example 2 Steel having the chemical composition shown in Table 1 was melted in a converter, and after continuous casting into a slab,
Heated to ℃, hot rolled, finishing temperature 880~920
A hot rolled steel plate with a thickness of 3.2 mm was obtained at a coiling temperature of 450 to 560°C. These hot rolled steel plates

[Table] After pickling, it was cold-rolled to produce a cold-rolled coil with a thickness of 0.8 mm. This was unwound and made into a loose coil using spacers of various diameters, then charged into an annealing furnace and annealed at a soaking temperature of 700°C. At this time, gas cleaning started at 150°C and ended at a temperature of 500°C or lower. After annealing, the surface stains were examined. Also, for comparison, in annealing, the diameter is 1.6
Using a spacer with a diameter of mm, the gas cleaning end temperature was set at 600°C, and annealing was similarly carried out at a soaking temperature of 700°C, and the failure rate due to carbon stains on the surface was investigated. FIG. 4 shows the relationship between the spacer diameter and the carbon contamination failure rate when the gas cleaning end temperature is 450°C. It is shown that by using a large diameter spacer with a diameter of 2.5 mm or more, it is possible to suppress the carbon contamination failure rate to the same level as the failure rate in conventional annealing. Figure 5 uses a large spacer with a diameter of 2.5 mm.
The defect occurrence rate is shown when the end temperature of gas cleaning is varied. When the gas cleaning end temperature is set to 350° C. or higher, by using a large-diameter spacer, the failure rate can be suppressed to a lower level than in the past. However, in this example, no formation of lumpy carbide was observed in the coil.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the gas cleaning end temperature and the location where lumpy carbide occurs in the coil, and FIG. 2 is a graph showing the relationship between the gas cleaning end temperature and the failure rate due to carbon contamination.
Figure 3 shows the gas cleaning temperature end temperature at 600.
Figure 4 is a graph showing the relationship between the location where massive carbides occur and the material quality for Steel A and Steel B at ℃.
Figure 5 is a graph showing the relationship between spacer diameter and carbon contamination failure rate when gas cleaning end temperature is 450℃. It is a graph showing the defective occurrence rate when changing.

Claims (1)

[Claims] 1 Steel consisting of C 0.03 to 0.07%, Si 0.20% or less, Mn 0.05 to 0.60%, P 0.10% or less, Al 0.01 to 0.10%, the balance iron and unavoidable impurities is heated. The cold-rolled steel sheet obtained by inter-rolling and cold-rolling is re-wound into a loose coil while winding a spacer in a recoiling unit, and then annealed in an open coil box at a temperature below Ac ₁ point. In the gas cleaning method that blows water vapor into the atmospheric gas, the coil temperature is 350 to 500℃.
A method for producing a cold-rolled steel sheet with excellent workability, characterized in that gas cleaning is terminated when a temperature in the range of is reached. 2. The method for manufacturing a cold rolled steel sheet according to claim 1, wherein the spacer is a large diameter spacer of 2.5 mm or more.