KR950004709B1 - Making method of cold rolling steel plate - Google Patents

Abstract

The steel sheet is manufactured by continuous casting a steel melt including (by wt.) up to 0.01% C, up to 0.15% Mn, up to 0.008% S, up to 0.015 % P, 0.03--0.07% sol. Al, up to 0.003% N, 0.02--0.07% Ti, balance Fe and inevitable impurities, into a slab, holding at slab surface temperature of 900--1150 deg.C for 10--20 mins., hot-rolling at finish temperature of below 860 deg.C, coiling at 560--680 deg.C, cold rolling, and continuous annealing. The steel sheet is used for inner or outer plate of cars and vehicles, and has good deep drawability.

Description

Manufacturing method of cold rolled steel sheet with excellent deep workability

The present invention relates to a method for manufacturing a cold rolled steel sheet used for the interior, exterior plate, etc. of an automobile, and more particularly, to a method for manufacturing a cold rolled steel sheet excellent in deep workability.

Cold-rolled steel sheet for deep processing mainly used as the inner and outer plates of automobiles is manufactured by performing annealing of low carbon aluminum-kilted steel or titanium (Ti) -added aluminum-kilted steel, and adding a small amount of titanium (Ti) and niobium (Nb). There is a method of manufacturing an ultra low carbon steel by continuous annealing, in which case the most important parameter to ensure the workability is the coiling temperature after hot rolling. In general, in case of annealing materials, the control of texture by aluminum nitride (AIN) precipitated during recrystallization annealing is a prerequisite for ensuring workability, so it is wound at a low temperature after hot rolling to maintain aluminum (Al) and nitrogen and then precipitated during annealing. I am trying to.

The continuous annealing method consists of rapid heating, short time annealing, rapid cooling, and overaging, so that the annealing heat treatment is performed in a short time, so process conditions in steelmaking and hot rolling are very important to ensure processability, and deep workability is required for continuous annealing. In order to manufacture excellent cold rolled steel sheets, strong carbonitride-forming elements such as titanium (Ti) and niobium (Nb) are added to the steel during the steelmaking stage, and the finish rolling temperature during hot rolling is set to be higher than Ar ₃ transformation point. It is essential that the nit is completely recrystallized and then wound at a high temperature of 650 ° C. or higher to completely precipitate carbon and nitrogen in the hot rolling step, thereby eliminating the processability inhibitors of carbon and nitrogen in the steel during cold rolling and annealing. The main metallurgical principle to ensure workability in general hot rolling to date is recrystallization rolling over Ar ₃ transformation point to refine the grains, and then through the run-out table to the steel during the winding process. When the remaining solid carbon is combined with titanium (Ti) to form carbide, the (111) texture increases during continuous annealing after cold rolling, thereby improving workability. In addition, the steel in which the solid solution carbon has been removed in advance before cold rolling is improved in workability as the cold reduction rate increases. However, in the case of ultra low carbon added with titanium (Ti) or niobium (Nb), when filament rolling is rolled under the Ar ₃ transformation point, the aggregate structure and the microscopic structure in the thickness direction become nonuniform, resulting in deterioration of workability after cold rolling and annealing. Known. In this respect solutions with the method to relieve the high lubrication rolling to the shear stress component to cause the texture irregularity in the thickness direction of the hot-rolled coil (Coil) of workability deterioration by minimizing acting on the surface layer of the rolling when the steel sheet and A ₁ After finishing rolling at very low temperature (below 780 ℃) below transformation point, a method of securing processability after cold rolling and continuous annealing has been developed by forming the aggregate structure similarly to the cold rolled structure by winding it at high temperature.

However, the effects of high lubrication rolling are still contrary, and the effect is not clear, and there are still problems in the flowability and rolling property in rolling, and in the case of cryogenic rolling in scale removal and rolling in hot rolling. There are difficulties in practical use, in part. Recently, hot rolled coils for cold rolled steel are manufactured by applying the new hot rolling method, continuous casting-hot rolling hot rolled slab direct transfer rolling method or hot rolling method using thin slab, developed to use high heat of slab. In order to prevent the fall of the finishing rolling temperature and to prevent the deterioration of workability, descaling work is insufficiently performed or rolling is performed directly under the Ar ₃ transformation point. In this case, there is a problem that the hardness of the scale of the cold rolled steel sheet and the hot rolled coil (Coil), such as plate shape defects occur.

The present inventors conducted research and experiments to solve the above problems, and based on the results, the present invention proposes the present invention. The present invention is ultra-low carbon aluminum containing 0.01 wt% or less of carbon to which titanium is added. It is an object of the present invention to provide a cold rolled steel sheet having excellent deep workability by carrying out filamentous rolling at a temperature of 860 ° C. or lower after the direct rolling of a slab slab directly at a temperature of 860 ° C.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is wt%, C: 0.01% or less, Mn: 0.15% or less, S: 0.008 * or less, P: 0.015% or less, Sol.Al: 0.03 to 0.07%, N: 0.003% or less, Ti: 0.02 ~ 0.07%, remainder Fe and other steel slabs composed of inevitable impurities are sent directly after the continuous cast structure, and the surface temperature is wound in the temperature range of 900-1500 ℃, and then cold rolled and continuously annealed by the usual method for deep workability. It relates to a method for producing an excellent cold rolled steel sheet.

Hereinafter, the present invention will be described in detail.

It is known that carbon in steel acts as an invasive solid-solution element, which affects the rotation of crystals during the formation of the texture of the steel sheet. Until now, the effects mainly affect the rotation and accumulation energy of crystals during cold rolling. As a result, the workability of the steel sheet is deteriorated, and there is no definite result on the hot rolling or annealing process. In the present invention, the carbon in the steel was fixed in advance during hot rolling with a precipitate (Ti ₄ C ₂ S ₂ ) and then used to form a texture (222), which is advantageous for workability, by removing the influence on the texture development in hot rolling and cold rolling processes. . Therefore, the carbon content in the steel is preferably limited to 0.01wt% or less in order to minimize the Ti addition amount required to form the precipitate and to reduce the effect of the resulting precipitate as possible. The manganese (Mn) combines with sulfur (S) in steel after steelmaking to form manganese sulfide (MnS), in which case depletion of sulfur (S) to bind with titanium (Ti) results in fixing carbon in the steel during hot rolling. Since precipitates (Ti ₄ C ₂ S ₂ ) are not formed, the workability of the cold rolled steel sheet is impaired.

Accordingly, the content of Mn is preferably limited to 0.15 wt% or less so that titanium sulfide (TiS) may be formed prior to manganese sulfide (MnS) in order to promote the formation of precipitates (Ti ₄ C ₂ S ₂ ).

Since the sulfur (S) acts as a factor of hot brittleness during hot rolling, its content is important to exist in the steel in the form of sulfides, and titanium (Ti) and manganese (Mn), which are sulfides in the steel, It is preferable to select 0.008wt% or less in consideration of stability in the range below the maximum amount that can be compounded. The phosphorus (P) segregates at grain boundaries to cause brittleness and increases strength in solid solution. However, in the case of titanium (Ti) -added ultra low carbon steels, the form of titanium iron phosphide (Ti, Fe) P increases to increase the strength. In order to reduce, the content thereof is preferably selected to 0.015 wt% or less.

The aluminum (Al) is essentially a component added to remove oxygen in the steel, it is inevitable to be added in the continuous casting material and is present in the steel in the solid solution state in the titanium-added ultra low carbon steel, aluminum in the solid solution state in the steel There is no clear result of the effect of (Al) on the workability, but aluminum that binds to oxygen because the oxide, which is a combination of oxygen and other components, may adversely affect the workability if oxygen in the steel is not completely removed. Excluding (Al), the excess amount, that is, the content of acid value aluminum (Sol.Al) is preferably limited to 0.03 ~ 0.07wt.

The nitrogen combines with titanium at high temperature to form titanium nitride (TiN), when the amount is high, the strength is increased and the workability is lowered, and also the formation of precipitates (Ti ₄ C ₂ S ₂ ) that serves as carbon removal. It is preferable to limit the content of nitrogen to 0.003wt% or less because it depletes the necessary titanium (Ti). The titanium (Ti) is a component that plays a decisive role in securing the workability of the cold rolled steel sheet in consideration of the addition amount of sulfur (S), carbon, and nitrogen to which titanium is bonded in the steel to minimize the coupling equivalent with these three components and to excess content In order not to exceed 0.05 wt%, the titanium content is preferably limited to 0.02 to 0.07wt%.

If the amount of titanium (Ti) to be combined with the three components is less than the bonding equivalent, the bond with the carbon having the latest bonding order is incompletely formed and accordingly, the heavy carbon is in solid solution and thus, It affects and further reduces the workability.

In addition, when the surplus content is more than 0.05wt%, the effect of improving workability is no longer improved, but an adverse effect of only raising the recrystallization temperature occurs, thereby setting an upper limit.

The steel slab formed as described above is put into a hot rolling process in the range of 900-1150 ° C. to utilize high temperature heat energy immediately after continuous casting.

At this time, the titanium compound (TiS) is precipitated in the steel, and the precipitate is formed as a matrix (Ti ₄ C ₂ S ₂ ) in which carbon is further bonded in the rolling process. Hot rolling consists of rough rolling and filament rolling, and the temperature of the steel sheet decreases because high pressure water is injected in order to remove the scale present on the surface of the steel sheet. Moreover, the combination of titanium sulfide (TiS) and heavy carbon is reduced. After sufficient occurrence, in order to perform finishing rolling, it is preferable to perform finishing rolling in a ferrite high temperature range, and to set finishing rolling temperature which is a finishing rolling temperature below 860 degreeC.

After hot rolling, the coiling temperature affects the grain size of the hot rolled sheet and the scale formation thickness of the precipitates and the steel sheet surface, and the grain size becomes coarse as the work is performed at high temperature, and the steel sheet surface scale thickness increases.

However, in the present invention, since most of it is already precipitated in the hot rolling region, there is almost no change depending on the winding temperature.

In general, the workability of the cold rolled steel sheet is improved as the structure of the hot rolled sheet is finer, so that the precipitation change is small and the amount of surface scale is reduced, so that the coil is wound at a low temperature of 560-680 ° C to facilitate scale removal during the pickling process. After removing the surface scale by rolling to a 65-75% reduction rate to form a cold rolled sheet of 0.6-1.4mm thickness, the cold rolled sheet is annealed in a continuous annealing apparatus, the continuous annealing heat treatment cycle is heat-cracking-first cooling-showing It consists of effect-final cooling process and the annealing temperature is preferably set in the range of 800-850 ℃ based on the temperature of the cracking process.

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

Example

To prepare a hot rolled sheet under the hot rolling and winding conditions of Table 2 by dissolving the titanium (Ti) -added ultra low-carbon aluminum-kilted steel having the chemical composition shown in Table 1 below after the pickling of the hot rolled sheet 70-73% Cold rolling was carried out to a thickness of about 0.8 mm at a reduction ratio to carry out continuous annealing. Continuous annealing was cracked for about 30 seconds at 800-830 ℃ and then cooled in two stages at a rate of 1-40 / sec and then overaging for 400 seconds at 400 ℃.

Mechanical properties of the provisional specimens prepared as described above were measured, and the measurement results are shown in Table 3 below.

TABLE 1

TABLE 2

TABLE 3

As shown in Table 3, it can be seen that the inventive material (1-5) prepared according to the present invention is excellent in deep workability compared to the comparative material (a-b) outside the scope of the present invention.

As described above, the present invention can reduce the thermal energy required for slab reheating by performing direct rolling of the slab slab directly to ultra-low carbon aluminum-kilted steel containing less than 0.01wt% carbon added with titanium (Ti). In addition to shortening the manufacturing time according to the sequencing of the process, the low temperature winding is performed, and thus, the scale of the hot rolled coil can be easily removed, thereby reducing the scale defect of the cold rolled steel sheet.

Claims (1)

In wt%, C: 0.01% or less, Mn: 0.15% or less, S: 0.008% or less, P: 0.015% or less, Sol.Al: .03-0.07%, N: 0.003% or less, Ti: 0.02-0.07% Steel slab composed of residual Fe and other unavoidable impurities, sent directly after continuous main structure, and the surface temperature is maintained for 10-20 minutes in the temperature range of 900-1150 ℃, and then hot-pressed to finish rolling temperature condition of 860 ℃ or less. Rolling, winding in a temperature range of 560-680 ℃, and then cold rolling and continuous annealing in a conventional method for producing a cold rolled steel sheet excellent in deep workability.