KR100282192B1 - Manufacturing method of cold rolled steel sheet for deep processing with excellent fatigue characteristics and secondary workability - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a cold rolled steel sheet having a good press workability while being excellent in fatigue characteristics and secondary workability as a cold rolled steel sheet mainly used in automobiles and home appliances. The method for manufacturing a cold rolled steel sheet for deep working according to the present invention includes carbon (C) of 0.01% by weight or less, sulfur (S) of 0.01% by weight or less, nitrogen (N) of 0.007% by weight or less, and Ti ^* = total Ti − Ordinary Ti formulated so that the atomic equivalent ratio (Ti ^* / 4C wt%) between the amount of effective titanium (Ti ^* ) and the amount of carbon satisfying the conditions of 3.43 (wt% N)-1.5 (wt% S) is 1 or more Reheating the added ultra low carbon IF steel slab to a temperature of about 1200 ° C, inserting it into a hot rolling mill to complete rough rolling at 1050 ° C or higher, and then finish rolling while simultaneously cooling with water for controlling precipitates. It is characterized in that it comprises a step, immediately wound at a relatively low temperature of 560 to 650 ℃ to produce a hot rolled sheet, and the usual pickling, cold rolling and continuous annealing treatment using this hot rolled sheet.

Description

[Name of invention]

Manufacturing method of cold rolled steel sheet for deep processing with excellent fatigue characteristics and secondary workability

Detailed description of the invention

[Purpose of invention]

[Technical field to which the invention belongs and the prior art in that field]

The present invention relates to a method for manufacturing a cold rolled steel sheet for deep processing mainly used in automobiles and home appliances, and more particularly, good press workability with excellent fatigue properties and secondary workability and brittleness, which are emerging as very important cold rolled material properties. It relates to a method for producing a cold rolled steel sheet having a.

In general, in order to manufacture a cold-rolled steel sheet for deep processing, so-called ultra-low carbon IF steel (Interstitial Free Steel) in which carbon (nitride) forming elements such as titanium (Ti) or niobium (Nb) is added to ultra low carbon steel. However, the ultra low carbon steel having a low carbon content in steel has good press formability, but the yield strength and grain boundary bonding strength are inferior. This is because the fatigue strength is generally proportional to the yield strength of the steel sheet, and the secondary work brittleness is proportional to the bonding strength of the grain boundary, and the carbon in the steel acts to increase these strengths. Therefore, in order to solve this problem of cold-rolled ultra-low carbon IF steel, attempts to add a small amount of boron (B), which is a critical strengthening element, have been conducted mainly in blast furnaces in Japan.

However, when boron is added, fatigue properties and secondary processing brittleness are somewhat improved. However, when the boron is added less than an appropriate amount of about 5-10 ppm, the effect is almost insignificant. There is a problem that the sex is sharply lowered. In other words, the addition amount of boron should be precisely controlled to about 5-10 ppm, but it is very difficult to accurately control the boron addition amount in industrial practice. In addition, since boron is an element that adversely affects formability, a certain deterioration in formability is caused. Inevitably, there was a problem that the alloying element addition cost also increased in terms of manufacturing cost.

In addition, an important problem related to the material properties of the ultra low carbon IF steel is related to the hot rolled coiling temperature in copper steel. In general, each manufacturing process for manufacturing deep-rolled cold rolled steel sheet shows that in the steelmaking process, the carbon content in the steel is extremely lowered, and titanium and niobium, etc., are used to fix even a small amount of carbon, such as a few tens of ppm remaining as a decarburization limit. In the hot rolled winding step, it was the basic manufacturing principle of IF steel to obtain excellent processability by developing a {111} texture during cold rolling and annealing by performing high temperature winding to coarsen the precipitate. However, high temperature windings, which are usually carried out at a high temperature of about 700 ° C., improve moldability, not only cause many surface defects such as hot rolled scale, but also the fatigue characteristics and secondary processing brittleness due to critical P segregation. It has been the cause of deterioration.

[Technical problem to be achieved]

Therefore, in the present invention, when manufacturing a cold rolled steel sheet for deep processing based on ultra low carbon IF steel, the fatigue property and the secondary workability are good without addition of an alloying element such as boron, and the surface which is a problem when winding at high temperature The purpose is to provide a manufacturing method that can solve the problem of defects.

[Configuration and Function of Invention]

In order to achieve the above object, the manufacturing method of the cold rolled steel sheet for deep processing excellent in fatigue properties and secondary workability of the present invention according to the present invention, in order to control the size of precipitates due to workability deterioration due to cold winding, very low carbon steel A method for producing a cold rolled steel sheet for deep processing using a metal, comprising: 0.01 wt% or less carbon, 0.01 wt% or less sulfur, 0.007 wt% or less nitrogen, and Ti ^* = total Ti-3.43 (wt% N)-1.5 ( An ordinary Ti-added ultra-low carbon IF steel slab formed so that the atomic equivalent ratio (Ti ^* / 4C weight%) between the amount of effective titanium (Ti ^* ) and the amount of carbon satisfying the conditions of weight% S) is 1 or more. Reheating to a temperature such that only TiN is present as precipitate in the steel; Adding the Ti-added ultra low carbon steel to a hot rolling mill to increase the solid solubility of the solid solution in a rough rolling process at 1050 ° C. or higher; Performing finish rolling in parallel with cooling by water to increase driving force for dynamic precipitation of Ti-based precipitates and to effectively control Ti ₄ C ₂ S _S precipitates during finish rolling; Manufacturing a hot rolled sheet by winding the Ti-added ultra low carbon steel at a relatively low temperature of 560 to 650 ° C .; It is characterized by consisting of a conventional pickling, cold rolling and continuous annealing process using the hot rolled plate.

The present invention relates to a method for manufacturing a cold rolled steel sheet for deep processing, comprising 0.01 wt% or less of carbon (C), 0.01 wt% or less of sulfur (S), and 0.007 wt% or less of nitrogen (N), and Ti ^* = total Ti-3.43 (% by weight N)-1.5 (% by weight) of the effective titanium (Ti ^* ) content satisfying the condition of the atomic equivalent ratio (Ti ^* / 4C weight%) of carbon content is usually formulated to be 1 or more Reheating the ultra-low carbon IF steel slab with Ti to 1200 ℃, finishing the rough rolling at 1050 ℃ by inserting it into a hot rolling mill, and then finishing rolling while simultaneously cooling by water for controlling the precipitate. To perform the step, immediately wound at a relatively low temperature of 560 ~ 650 ℃ to produce a hot rolled sheet, using the hot rolled sheet, characterized by consisting of the usual pickling, cold rolling and continuous annealing step, fatigue characteristics Cold rolled for deep processing with excellent secondary brittleness It relates to a method for producing a plate.

Hereinafter, the conditions of the manufacturing method of the present invention will be described in more detail.

Titanium content added to carbon, sulfur, nitrogen, etc. as carbonitride-forming elements in steel is basically to ensure press workability, and is the core of cold-rolled steel sheet for deep processing manufactured by conventional manufacturing process of performance-hot rolling-pickling-cold rolling-continuous annealing. It is the same as a general component system.

In the present invention, the reason why the reheating temperature and the rough rolling temperature are limited to 1200 ° C. and 1050 ° C. or higher is pretreatment for allowing winding at a low temperature of about 600 ° C. to improve fatigue characteristics and secondary workability. Way. That is, when the slab is heated to a temperature of about 1200 ° C., precipitates in the steel have only TiN, and other precipitates are almost dissolved. In the case of continuous rough rolling, if the rough rolling temperature is limited to 1050 ° C. or higher, the solid solubility of the solid solution element is maintained even after rough rolling, so that Ti ₄ C ₂ S _S can be effectively deposited during the subsequent rolling. Not only that, but also the growth of the fine TiC. In addition, the reason why the cooling by water at the time of finishing rolling is also to increase the driving force for the dynamic precipitation of Ti-based precipitates during rolling.

In the relation between the precipitates and the material in the Ti-added ultra low carbon steel, the influence of TiN on the workability of the cold rolled steel sheet is small, but the precipitation behavior of Ti ₄ C ₂ S _S and TiC has a very large effect. In other words, fine TiC of several tens of Å or less adversely affects the workability. Thus, in order to secure good press workability, precipitates are mainly formed of energy-stable Ti ₄ C ₂ S _S , and TiC has a relatively coarse size of several hundred Å. It needs to be formed.

Rolling conditions in conventional hot rolled ultra-low carbon steel hot rolled manufacturing conditions simply limit the finish rolling temperature to more than Ar ₃ transformation point in order to obtain a fine grain structure, and hot rolled winding conditions reduce the steel element and reduce TiC precipitates. In order to communicate, it was a concept of improving workability by winding at a high temperature of about 700 ° C.

However, the present invention limits the manufacturing conditions to the concept of continuous operating conditions for the control of precipitates, which are highly related to processability, not to separate operating conditions of slab reheating, rough rolling and finishing rolling, respectively, considering only product dimensions and microstructures. By doing so, even if the cold winding is performed at around 600 ° C. after rolling, excellent deep workability can be ensured. In short, the deep workability is secured by the precipitate controlled rolling so that high temperature winding is not necessary, and the fatigue characteristics and the secondary workability resistance are improved by the low temperature winding.

On the other hand, when the coiling temperature is lowered, fatigue characteristics and secondary workability are improved, which is very related to the behavior of solid carbon in steel. That is, copper is not only a very low carbon steel but also contains Ti, a carbon nitride forming element in the steel, so the yield strength is fundamentally low and the bond strength of grain boundary is weak. However, as the hot rolling is carried out at a lower temperature, the yield strength increases due to an increase in dislocation density and grain refinement in grains, and the P grain boundary segregation, which is a decisive cause of grain boundary brittleness, also decreases as the winding temperature decreases. Material properties are improved. In addition, it is possible to solve the problem of surface defects that have been a problem in the conventional high temperature winding, and also to reduce the material defects by securing the stable material as well as hot rolling, pickling operation.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

EXAMPLE

A conventional titanium-added ultra low carbon steel having a chemical composition of 0.005 wt% carbon, 0.007 wt% sulfur, 0.005 wt% nitrogen, and 0.039 wt% Ti was dissolved and 3.0 mm under the hot rolling conditions of Table 1 below. A hot rolled sheet of thickness was prepared.

The hot rolled plate was pickled with a hydrochloric acid solution, cold rolled at a reduction ratio of 75%, and then subjected to continuous annealing at a temperature of 830 ° C. for 30 seconds. The tensile characteristics of the samples prepared by the hot rolling conditions, Fatigue strength and secondary processing brittleness are shown in Table 2.

Fatigue measurement method of strength is the maximum load and the minimum load ratio of 10: the cyclic loading and to 110 when the ^7th was applied, was the maximum load value measured by the Fatigue fracture does not occur, and the secondary work embrittlement is molded The cup was placed on its side and the weight was dropped to evaluate the Ductile-Brittle Transition Temperature (DBTT).

[Effects of the Invention]

In spite of using the titanium-added ultra low carbon steel of the same component as shown in Table 2, the method of the present invention is much better than the cold rolled steel sheet (comparative material) manufactured by the conventional method. It can be seen that not only the fatigue strength and the secondary workability of brittleness can be exhibited, but also steel sheets having a higher level of deep workability can be produced. In addition, since the low temperature winding of 600 degreeC is possible by the method of this invention, the surface defect which becomes a problem by high temperature winding can be reduced.

The above detailed description and embodiments of the present invention have been disclosed for purposes of illustration, and those skilled in the art will be able to make various modifications, changes, additions, and the like within the spirit and scope of the present invention. It should be seen as belonging to a range.

Claims (1)

In order to control the size of precipitates due to workability deterioration due to cold winding, a method for manufacturing cold rolled steel sheets for deep processing using Ti-added ultra low carbon steel, comprising: 0.01 wt% or less carbon, 0.01 wt% or less sulfur, 0.007 wt% or less Atomic ratio of the amount of effective titanium (Ti ^* ) to the amount of carbon (Ti ^* / 4C weight), which contains nitrogen, and satisfies the conditions of Ti ^* = Total Ti-3.43 (wt% N)-1.5 (wt% S) Reheating the conventional Ti-added ultra-low carbon IF steel slab formulated so that%) is 1 or more at a temperature of about 1200 ° C., so that only TiN is present as a precipitate in the steel; Adding the Ti-added ultra low carbon steel to a hot rolling mill to increase the solid solubility of the solid solution in a rough rolling process at 1050 ° C. or higher; Performing finish rolling in parallel with cooling by water to increase driving force for dynamic precipitation of Ti-based precipitates and to effectively control Ti ₄ C ₂ S _S precipitates during finish rolling; Manufacturing a hot rolled sheet by winding the Ti-added ultra low carbon steel at a relatively low temperature of 560 to 650 ° C .; Method for producing a cold-rolled steel sheet for deep processing, characterized in that the step consisting of the usual pickling, cold rolling and continuous annealing process using the hot rolled sheet.