JPH0217614B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a continuous cast cold rolled steel sheet for processing. Traditionally, rimmed and capped steels with good surface properties have been mainly used as materials for cold-rolled steel sheets.
Advances in continuous casting technology have significantly improved both the surface and internal properties of slabs, leading to a transition to continuous casting materials that are superior in cost and homogeneity. Al-killed and Si-killed steels have been used as cold-rolled plate materials for continuous casting, but they have not been able to replace all of the properties of conventional rimmed and capped steels. In other words, there are problems with workability, hardness, cold-rolled plate temper color, galvanizing adhesion, carburizing hardenability, etc., and the Al and N levels that can satisfy these characteristics are low (Al
0.02%, N40ppm) steel type (hereinafter referred to as low-Al steel) is required. Low-Al steel can be applied to a wide range of applications where rimmed and capped steels were used;
When processing the outer periphery of a coil in a box annealed material, a problem arises in that roughness tends to occur. The reason for this is low
For Al steel, vacuum degassing treatment is generally performed to improve Al yield and accuracy. Therefore, the oxygen level in low-Al steel is much lower than in rimmed capped steel, making it a so-called clean steel with fewer oxide inclusions. In this case, the recrystallization behavior during annealing of low-Al steel is different from normal Al-killed steel, in that it does not produce elongated grains, and Al nitrides exhibit a primary recrystallization grain suppressing effect, resulting in fine grains and insufficient N fixation, resulting in hardness. It's easy. If the annealing temperature is increased to improve these problems, in parts such as the outer periphery of the coil that are exposed to high temperatures for a long period of time, the crystal grains will coarsen considerably with relatively large grains as cores, resulting in rough skin during processing. Therefore, it is unsuitable for processing steel sheets. The inventors researched various methods to prevent coarsening of grains during annealing of low-Al steel boxes, and found that continuous cast slabs were directly rolled from high-temperature conditions, or subjected to heat retention and light heating alone or in combination. It was revealed that when a hot-rolled steel strip that has been subsequently rolled is used as a cold-rolled material, roughness during box annealing that occurs when using a conventional hot-rolled steel strip that has been rolled after reheating may be prevented. Using this knowledge, we invented a low-Al continuous cast cold-rolled steel sheet for machining. In other words, what constitutes the present invention is C0.03~
0.07%, Mn 0.22~0.35%, Al, and N in the range shown in Figure 1 A, that is, coordinate points a, b, c, d, e,
A high-temperature slab obtained by continuous casting of steel containing the range surrounded by f, g, and h with the remainder consisting of iron and unavoidable impurities is directly cast, or the high-temperature slab is subjected to either a heat retention process or a light heating process. Alternatively, after passing through both heat retention and light heating processes, hot rolled steel strip is hot rolled at a temperature of Ar ₃ or higher, descaled by pickling or other means, cold rolled, and box annealed. This is a method for producing continuously cast cold rolled steel sheets for processing. The reasons for limiting the constituent elements of the present invention will be explained below.
In cold-rolled steel sheets for processing, C must be low from the viewpoint of workability, and the upper limit is 0.07%. On the other hand, the lower limit needs to be 0.03% or more because if C is too low, skin roughness is likely to occur significantly. The upper limit of Mn is 0.35% to prevent hardening and deterioration of ductility, and the lower limit is 0.22% from the minimum amount (15 times S) required to prevent deterioration of hot workability due to the normal amount of S (approximately 0.015%). . In the present invention, the appropriate values of the addition amounts of Al and N are determined as a combination effect, and the appropriate ranges are shown in FIG. In the figure, the horizontal axis is the Al content (unit: 10 ^-3 %), and the vertical axis is the N content (unit: ppm), and the appropriate range is the shaded range a (7, 40), b
(15, 40), c (15, 28), d (22, 20), e (30,
20), f (30, 12), g (15, 12), and h (7, 17). In hot-rolled steel strips that are formed by direct rolling of continuous slabs or by light heating or heat retention and rolling after light heating (hereinafter collectively referred to as direct rolling, etc.), crystal grains are formed during box annealing of cold-rolled sheets. Figure 2 shows that the coarsening of the particles is prevented. Figure 2 shows a cold-rolled sheet manufactured from a hot-rolled steel strip under the above conditions and a hot-rolled steel strip subjected to a normal reheating process (cold rolling rate: 70
%) is annealed at 750°C for 6 hours. For these reasons, when performing direct rolling after continuous casting, coarse grains are not generated during annealing when the Al and N amounts are in the above range A, but coarse grains occur in the reheated material.
The upper right range B of the Al and N ranges of the present invention is a range in which the crystals do not become coarse even when reheated after continuous casting, and there is no special need for effects such as direct rolling after continuous casting, which is described in the present invention. , and the lower left range C is a range in which the effect of preventing grain coarsening by direct rolling after continuous casting of the present invention does not reach, and both are the range where Al,
Not included in N range. In the present invention, the steelmaking and hot rolling processes are the main requirements, and in steelmaking that requires an equipment arrangement that allows direct rolling as described in the present invention, the above component steel is melted using an ordinary steelmaking furnace. Although the deoxidizing components are adjusted, a vacuum degassing device may be used in this process. A process in which the molten steel obtained in this way is directly heated by a continuous slab for a short period of time,
It is an essential condition of the present invention that the slab be sent to a hot rolling facility and rolled after going through either a light heating process for soaking the slab or a process that combines heat retention and light heating. I explained it earlier. In this case, short-term heat retention is a heating process installed in the slab transfer line, for example, in order to equalize the temperature by using the heat retained in the slab itself within one hour, which is much shorter than the normal heating time, and to prevent a drop in temperature. means heat retention using a protective cover or other heat retention means. Furthermore, light heating for uniform heating refers to heating using a means such as a gas burner or induction heating to locally rapidly heat a portion where the temperature has decreased, such as the surface or end of the slab. The above hot rolling is carried out at a temperature of ₃ or more Ar points for each steel, because if the hot rolling temperature decreases, the material quality of the cold rolled steel sheet deteriorates. The hot rolled steel strip produced by the above method is descaled by ordinary pickling, etc., then cold rolled at a cold rolling ratio of 40 to 90%, preferably 50 to 80%, and further subjected to various cleaning methods as necessary. After cleaning, box annealing is performed. The annealing conditions at this time are determined from the temperature and time necessary to satisfy the material quality level of the cold rolled steel plate for processing. The reason why differences in the process before hot rolling appear during annealing, which determines the material quality of the cold rolled steel sheet, is thought to be as follows, even though the amounts of Al and N are the same. First low Al
The phenomenon in which crystal grains grow abnormally during high-temperature, long-term annealing (corresponding to the outer circumference of a coil) in steel is called secondary recrystallization, and the primary recrystallization is fine, but it contains coarse grains. It can sometimes occur. In cold-rolled sheets obtained from hot-rolled steel strips produced by reheating steel with the same composition as the steel of the present invention, the growth of primary recrystallized grains is suppressed by fine AlN precipitated in the hot-rolled sheets or during annealing. This creates a state in which secondary recrystallization is likely to occur. On the other hand, there is almost no AlN precipitation in hot-rolled sheets manufactured by direct rolling, and precipitation during annealing is delayed, so the growth of primary recrystallized grains is not hindered by AlN, and secondary recrystallization Crystals are less likely to form. In this way, it is understood that the amounts of Al and N and the pre-manufacturing process before hot rolling are interrelated to bring about the effects of the present invention. Next, the effects of the present invention will be explained using examples. Table 1 shows the sample steel composition, process, hot rolling conditions, and length of the part where surface roughness occurs due to annealing (part rejected for processing purposes). In this example, the hot rolled sheet thickness was 2.7 mm, the cold rolling rate was 70% (cold rolled sheet thickness 0.8 mm),
The annealing conditions are box annealing at 700°C for 4 hours. Steel A
The Al and N contents are higher than the range of the present invention, and surface roughness is small even with the conventional reheat-rolled material, but in this example, the material is hot-rolled by retaining and lightly heating a high-temperature cast slab. This is the case when it is used. In contrast to steels C and E, which have the same composition and are reheated and rolled for comparative steels B and D, the occurrence of surface roughness is significant, whereas the occurrence of surface roughness is small due to the effect of lightly heating the continuous cast high-temperature slab of the raw material or rolling it after heat retention. It's summery. Since Steel F has a low Al content, this is an example in which the occurrence of surface roughness could not be prevented even with the material obtained by lightly heating and rolling a continuously cast high-temperature slab. Table 2 shows the mechanical properties (average values within the coil) of the same steel sheets as in Table 1. The steel of the present invention exhibits mechanical properties comparable to those of Comparative Steel A, and exhibits sufficient properties as a cold-rolled steel sheet for processing. As described in detail above, the present invention is a steel that contains relatively low amounts of Al and N and is hot-rolled from a high-temperature slab after continuous casting or after passing through one or both of heat retention and light heating processes. This is a manufacturing method for cold-rolled steel sheets using strips as raw materials.The yield loss due to the generation of abnormally coarse grains during box annealing is small, the material quality is excellent, and fuel costs are reduced by omitting the slab reheating process. It is an economically excellent method that has great cost reduction benefits such as reduction in production costs, labor savings, and process savings.

【table】

【table】

【table】 [Brief explanation of drawings]

Figure 1 shows the range of Al and N in the steel used in the present invention, Figure 2 shows the range of Al and N, the hot rolled steel strip manufacturing process, and abnormal coarse grains in the 750° x 6 hour box annealing conditions. (Rough skin) is a diagram showing the relationship between occurrence conditions.

Claims (1)

[Claims] 1 C0.03~0.07%, Mn0.22~0.35%, Al, N at the coordinate points a, b, c, d, e, f, g, h in Figure 1.
A high-temperature slab obtained by continuous casting of steel containing the range surrounded by , the remainder consisting of iron and unavoidable impurities, is directly or directly subjected to a heat retention process, a light heating process, or a heat retention and Manufacture of continuous casting cold rolled steel sheet for processing, characterized in that the hot rolled steel strip obtained by hot rolling at a temperature of Ar ₃ or higher after passing through both steps of light heating is subjected to cold rolling and box annealing. Method.