KR950003291B1 - Making method of hot rolling steel plate - Google Patents

Abstract

The method comprises finish-rolling a super low carbon steel slab produced in initial and last stages of continuous casting, at 780-820 deg.C and coiling at 640-680 deg.C. The steel slab is composed of (in wt.%) up to 0.01% C, up to 0.25% Mn, up to 0.1% Si, up to 0.08% Ti or Nb, balance Fe and inevitable impurities.

Description

Hot rolling method of ultra low carbon steels

1 is a stress-strain curve for the present invention and the comparative material.

The present invention relates to a method for manufacturing an ultra low carbon steel slab cast by continuous casting into a hot rolled steel sheet by continuous hot rolling, and more particularly, an ultra cast steel or a horse which is inevitably generated during continuous casting of ultra low carbon steel. A method of hot rolling a cast steel.

Conventionally, the general processing is manufactured by controlling the chemical composition in the case of hot rolled steel sheet and finishing the hot rolling at a temperature above Ar ₃ , which is a temperature at which ferrite transformation starts from austenite. At this time, since the mechanical properties of the steel sheet mainly depends on the chemical composition, the more steel is required, the higher the chemical composition is to be produced by high purity. Therefore, the higher the steel for processing, the lower the carbon and Mn content in the steel and the lower the general strength, the higher the grade of steel that does not require workability, the higher the strength. In addition, once the chemical composition of the steel sheet is determined, since the change in mechanical properties is not large according to the conventional technology, the grade of the product cannot be greatly changed.

However, it is difficult to produce a good product because the ultra-thin or horse cast inevitably occurs during continuous casting because the quality characteristics are inferior to that of the intermediate cast. In particular, the ultra cast or the end cast of the continuous cast slab produced for manufacturing high-grade steel can not satisfy the quality requirements of high-strength steel, it is also difficult to manufacture other grades of steel due to its low strength. For example, according to KS, in the case of SPT1 (SPHC), which is a hot rolled mild steel sheet, the tensile strength must be 28 kg / mm ² or more and the elongation must be 27 to 31% or more depending on the thickness. However, in the case of practical manufacture, the tensile strength needs to be 30kg / mm ² or more and the elongation is 35% or more in order to more stably satisfy the specification. Looking at the mechanical properties of high public gangjaein ultra-low-carbon Steel In this respect, after production with cold series is demonstrated sufficient quality characteristics as deep-drawing steel for but values satisfy the hot rolling condition to be a tensile strength of 27 to stabilize the 32kg / mm ² degree standard can not do it. Therefore, in the case of ultra low carbon steel, which is not suitable for deep drawing steels such as ultra cast or horse cast, it is difficult to commercialize by conventional methods. In order to solve this problem, it is necessary to increase the strength of the slab having the chemical composition of the steel for high processing by hot rolling and to secure a certain elongation.

The related arts related thereto include domestic patent publications 91-1606 (Satoo et al.) And Japanese patent publications No. 59-226149 (senuma et al.). Korean Patent Publication No. 91-1606 discloses a method for manufacturing a hot rolled steel sheet having excellent workability by increasing the strain rate during hot rolling in a ferrite region of 300 / sec or more, but in this case, it is practically difficult. Not only high-speed hot rolling is required, but also the tensile strength of the steel sheet obtained is 28 to 34 kg / mm ² , which does not satisfy the standard stably.

In addition, Japanese Patent Publication No. 59-226149 discloses a method of manufacturing a hot rolled steel sheet for high processing by performing lubrication rolling during hot rolling. In this case, the strength of the hot rolled steel sheet is not considered. There is a problem that lubrication rolling is essential in hot rolling and that a separate recrystallization heat treatment process is required after hot rolling.

Accordingly, the present inventors conducted research and experiments to solve the above problems, and based on the results, the present invention proposes the present invention, and the present invention performs hot rolling after the ferrite transformation occurs in the ultra low carbon steel. Hot-rolled steel sheet with SPT1 (SPHC) grade quality characteristics by increasing the strength by leaving strain in the ferrite and deriving hot-rolling conditions that do not significantly reduce workability and hot-rolling ultra-low carbon cast steels under these conditions. Its purpose is to provide it.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is in weight%, C (carbon): 0.01% or less, MN (manganese): 0.25% or less, Si (silicon): 0.1% or less, Ti or Nb: 0.08% or less, the balance Fe and other unavoidable impurities The present invention relates to a method for hot rolling of ultra-low carbon steel ultra-thin carbon steels, which is hot rolled at a finish rolling temperature of 780 to 820 ° C and a winding temperature of 640 to 680 ° C.

Hereinafter, the reason for limitation of the component, the component range, and the hot rolling conditions will be described.

When the content of C is 0.01% by weight (hereinafter referred to as "%") or more, the area of the abnormal region during actual rolling is wide, and the process control is not easy in continuous hot rolling, and the strength is too high. The content is preferably selected at 0.01% or less.

When the content of Mn is 0.25% or more, like C, the temperature of Ar ₃ is too low and the strength is increased. Therefore, the content of Mn is preferably limited to 0.25% or less.

Since Si is an element which stabilizes ferrite as an element added to steel as a deoxidizer or for the purpose of improving workability, there is no big problem in hot rolling, but when the content is 0.1% or more, the strength is excessively increased. Is preferably limited to 0.1% or less.

The Ti or Nb is an element added to reduce solid solution carbon or solid solution nitrogen to improve workability, and when the addition amount is 0.08% or more, the Ti or Nb causes an increase in strength and a decrease in elongation, so that the Ti or Nb is 0.08% or less. It is desirable to limit.

Other inevitably contained impurity elements include elements such as Al, S, P, and N, and these components do not affect the effects of the present invention within a normal range.

The cause of the increase in strength in the present invention can be explained by the work hardening effect by the deformation applied after the ferrite transformation. At this time, if the work hardening is too severe, the elongation is greatly reduced, the workability of the soft steel can not be secured.

Therefore, it is important to secure the strength and elongation simultaneously by appropriately controlling the hot rolling finish temperature and the coiling temperature.

When the hot rolling finish temperature is lower than 780 ℃, the degree of dynamic recovery that occurs during hot rolling is small, the strength is too high and the elongation is lowered, if higher than 820 ℃ the degree of dynamic recovery during hot rolling is too small, the strength is too large Although excellent, the strength is lowered, the hot rolling finish rolling temperature is preferably limited to 780 ~ 820 ℃.

The coiling temperature is a process factor related to the static recovery and shows a tendency similar to the finish rolling temperature. When the coiling temperature is too low, the elongation is lowered. When the coiling temperature is too high, the strength is lowered. Thus, the coiling temperature is 640 to 680 ° C. It is preferable to limit to.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

The ultra low carbon steels prepared as shown in Table 1 were hot-rolled under the hot rolling conditions as shown in Table 2, and then mechanical properties were measured for each specimen, and the measurement results are shown in Table 2 below.

TABLE 1

TABLE 2

From the results of Table 2, it can be seen that the mechanical properties of the hot rolled steel sheet are greatly changed according to the hot rolling conditions rather than the chemical composition.

In this way, the mechanical properties are highly dependent on the hot rolling conditions. In the conventional technique of finishing rolling above the Ar ₃ temperature, all the deformations applied during the hot rolling of the ultra-rolled and cast slabs are eliminated by the transformation process after hot rolling. This is because, in the present invention, in the case of hot rolling of the first and second cast pieces under the Ar ₃ temperature or less, the deformation applied during hot rolling remains without the transformation process after the hot rolling.

As shown in Table 2, the invention material (ad) hot rolled in accordance with the present invention, the initial and end cast of the component range in accordance with the present invention stably satisfies the soft hot-rolled steel sheet specification [SPT1 (SPHC)] of KS While the tensile strength of 30 kg / mm ² or more and the elongation of 35% or more are obtained, which is a criterion, the comparative material hot rolled under conditions outside the scope of the present invention even if the ultra cast or the end cast has a component range consistent with the present invention. In the case of (1 to 4), it can be seen that the tensile strength and the elongation are not satisfied at the same time.

Tensile tests were conducted on the comparative material (1) and the inventive material (b) in Table 2 to investigate the stress-strain curves, and the results are shown in FIG.

As shown in FIG. 1, when the hot rolling is finished in the ferrite region as described in the present invention (invention material (b)), no yield point stretching phenomenon occurs and shows continuous yield. In this case, it can be seen that yield point stretching occurs. Therefore, in the case of the present invention (b), it is possible to prevent the defects caused by the solid solution carbon or the solid solution nitrogen without performing the control rolling, but in the case of the comparative material (1), the adjustment rolling to prevent the defects Should be done.

As described above, the present invention can be used for the appropriate use of the ultra cast or horse cast produced inevitably generated in the continuous casting process, and can also prevent hot-rolled rolling, which is a major surface defect of the soft hot rolled steel sheet, as well as hot rolling. Since the temperature can be lowered compared to the conventional method, the energy saving effect is large, and the effect of reducing oxide loss during heating is reduced.

Claims (1)

Ultra-low carbon steels composed of C: 0.01% or less, Mn: 0.25% or less, Si: 0.1% or less, Ti or Nb: 0.08% or less, residual Fe and other unavoidable impurities are 780 to 820 ° C. Hot rolling method of ultra-low carbon steel ultra-thin cast steel, characterized in that the hot rolling at the finish rolling temperature and the winding temperature conditions of 640 ~ 680 ℃.