KR20120033009A - Cold rolled soft steel sheet having improved roll fatigue defect and method for making the same - Google Patents

Abstract

PURPOSE: A high-formability soft cold-rolled steel sheet with improved hot-rolling fatigue and a manufacturing method thereof are provided to prevent rolling fatigue without regulation of rolling orders or insertion of a recovery material. CONSTITUTION: A method for manufacturing a high-formability soft cold-rolled steel sheet comprises the steps of: hot-rolling slab(S110), cooling and coiling the hot rolled plate(S120), pickling and cold-rolling the coiled plate(S130), and annealing the cold-rolled plate(S140). The slab comprises carbon of 0.006 weight% or less, silicon of 0.003-0.025 weight%, manganese of 0.05-0.15 weight% or less, phosphorus of 0.025 weight% or less, sulfur of 0.008-0.015 weight%, nitrogen of 0.005 weight% or less, niobium of 0.005-0.015 weight%, titanium of 0.005-0.050 weight%, and iron and inevitable impurities of the remaining amount.

Description

Hot-forming soft cold rolled steel with improved defects in hot rolled roll and its manufacturing method {COLD ROLLED SOFT STEEL SHEET HAVING IMPROVED ROLL FATIGUE DEFECT AND METHOD FOR MAKING THE SAME}

The present invention relates to a technique for preventing a roll fatigue defect by controlling an alloying element and a manufacturing method of ultra-low carbon steel in order to improve the hot-rolled roll fatigue defect.

Recently, research interests in the steel industry and the automotive industry are focused on high strength and light weight, and demands for high strength, excellent workability and formability as consumer demands are diversified.

In particular, in the case of cold rolled steel used as exterior materials of automobiles, it is important to maintain the shape, safety, and energy reduction through the improvement of strength according to the trend of the times. There is a demand for high quality steel sheets with basic surface quality.

An object of the present invention is a high-temperature soft-rolled cold-rolled steel which is capable of improving the roll fatigue defect caused by the thermal fatigue applied to the roll when rolling is performed at a very high temperature in the ultra low carbon steel, which is a low carbon steel. An alloy element component of a steel sheet and a method of manufacturing the same.

In order to achieve the above object, the method for manufacturing a high-forming flexible cold-rolled steel sheet having improved hot-rolled roll fatigue defects is weight%, carbon (C): 0.006% or less, silicon (Si): 0.003%-0.025%, Manganese (Mn): 0.05% ~ 0.15% or less, Phosphorus (P): 0.025% or less, Sulfur (S): 0.008% ~ 0.015%, Nitrogen (N): 0.005% or less, Niobium (Nb): 0.005 ~ 0.015% ; Titanium (Ti): 0.005% to 0.050%, the remainder being hot rolled slab plate made of Fe and other unavoidable impurities; Cooling and winding the hot rolled sheet; Cold rolling the plated and wound plates after pickling; And annealing heat treatment of the cold rolled sheet material.

In order to achieve the above object, a method for manufacturing a high-forming flexible cold rolled steel sheet having improved hot rolled-roll fatigue defects may further include reheating the slab plate before hot rolling. The reheating temperature of the slab plate may further include It is characterized in that the 1150 ~ 1250 ℃.

In order to achieve the above object, a method of manufacturing a high-forming flexible cold-rolled steel sheet having improved hot-rolled roll fatigue defects, wherein the hot rolling step has an entrance temperature of 970 ° C to 1030 ° C during finishing rolling and an exit temperature of 910 ° C or higher. It features.

Hot-rolled rolled cold rolled steel sheet manufacturing method according to the present invention for achieving the above object is characterized in that the winding temperature in the cooling and winding step is 650 ℃ ~ 750 ℃.

Hot-rolled rolled cold rolled steel sheet manufacturing method according to the present invention for achieving the above object is characterized in that the cold rolling rate in the cold rolling step is 65 ~ 80%.

Hot-rolled rolled cold rolled steel sheet manufacturing method according to the present invention for achieving the above object is characterized in that the annealing temperature in the step of annealing heat treatment is 790 ℃ ~ 840 ℃.

The alloying element and the manufacturing method according to the present invention have an effect of improving the roll fatigue defect and ultimately preventing by controlling the components of sulfur (S).

In addition, the alloying element and the manufacturing method according to the present invention can prevent the defects of the roll fatigue with the components of the alloy element without the procedure of adjusting the rolling order or injecting the recovery material during the operation of the surface stiffener to control the roll fatigue defect This has the effect of being improved.

1 illustrates roll fatigue defects observed in a hot rolled sheet.
2 shows a roll fatigue defect remaining in the cold rolled sheet.
Figure 3 is a flow chart schematically showing a method for manufacturing a high-forming flexible cold rolled steel sheet with improved hot-rolled roll fatigue defects according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, only the present embodiments to make the disclosure of the present invention complete, and common knowledge in the art to which the present invention pertains. It is provided to fully inform the person having the scope of the invention, which is defined only by the scope of the claims. Like reference numerals refer to like elements throughout.

Steel sheet according to the present invention is widely applicable to the interior and exterior panels of automobiles and household appliances based on high elongation and r value. The formability of ultra low carbon steel is greatly affected by the texture. Therefore, the ultra low carbon steel texture control technology maximizes the α-fiber (<110> // rolling direction) which develops the γ-fiber ({111} // rolled surface) texture with high r value and degrades the r value. By reducing it, you can improve the drawing.

The manufacturing process of hot rolled coils undergoes a reheating step of heating the slab and a width rolling step of matching the width of the product with the width of the slab, followed by rough rolling, finishing rolling and winding processes to control the thickness of the product.

Hot rolling is advantageously performed at an Ar ₃ temperature or higher in consideration of workability during homogenization and hot rolling of the material. In the case of ultra low carbon steel, the carbon content is very low and the Ar ₃ temperature is very high. In order to minimize in-plane anisotropy, hot rolling is finished at a high temperature of 910 ° C. or higher. After filament rolling, it is desirable to refine the hot rolled structure through rapid cooling of the front end of the cooling zone, and to coarsen the precipitate through high temperature winding, so that the density of the γ-fiber texture is formed after cold rolling annealing.

The structure of the hot rolled roll consists of carbide and roll base. At this time, heating and cooling are repeatedly applied by the contact between the roll and the hot rolled material, and defects are generated on the surface of the roll due to the difference in the coefficient of thermal expansion between the carbide and the matrix. This roll fatigue causes surface defects of the hot rolled material.

The filament rolling temperature of the ultra low carbon steel is 910 ℃ or higher, and the thermal fatigue applied to the roll is a more severe condition than the general low carbon steel grades. It is greatly affected by fatigue defects. 1 illustrates the type of surface defects 100 caused by a roll in a hot rolled sheet.

Such roll fatigue defects are often accompanied by roll components Ni and Cr, and are deep and remain after pickling and cold rolling annealing. 2 illustrates a defect 200 remaining after pickling and cold rolling. Such defects are very fine and difficult to visually identify, but become more prominent in the post-treatment process, which is a problem when applied to surface rigid materials such as exterior materials.

The present invention provides a method for preventing roll fatigue defects of ultra low carbon steel through a method of controlling and manufacturing an alloy element of ultra low carbon steel.

Hereinafter, the content ratio of the steel according to the present invention will be described in detail.

High Forming Flexible Cold Rolled Steel Sheet With Improved Hot Fatigue Defects

The soft cold rolled steel sheet according to the present invention is% by weight, carbon (C): 0.006% or less, silicon (Si): 0.003% to 0.025%, manganese (Mn): 0.05% to 0.15% or less, phosphorus (P): 0.025 % Or less, sulfur (S): 0.008% to 0.015%, nitrogen (N): 0.005% or less, niobium (Nb): 0.005 to 0.015%; Titanium (Ti): 0.005% to 0.050%, the remainder is composed of Fe and other unavoidable impurities.

Hereinafter, the specific role and content of each component included in the steel according to the present invention will be described.

Carbon (C)

Carbon (C) according to the present invention is preferably 0.0005% to 0.006%, more preferably 0.0005% to 0.005%.

When the content of carbon (C) is less than 0.0005% by weight, sufficient tensile strength is not obtained. In the molten steel step, carbon (C) is decarburized by reacting with oxygen coexisting in the molten steel and removing the reactant under reduced pressure. For this reason, when it is less than 0.0005 weight%, the problem of having to perform a vacuum processing for a long time arises.

Carbon (C) is present as a solid element in the steel sheet and inhibits the formation of the (111) texture, which is advantageous for workability, during the formation of the texture of the steel sheet during cold rolling and annealing, thereby degrading the workability and formability of the steel. In addition, when carbon is present as a solid element in steel, it causes aging problems and causes a defect called a stretcher strain.

Therefore, as the carbon content increases, the amount of titanium and niobium, which are carbide forming elements, must be increased to precipitate carbon. In this case, not only the manufacturing cost of the steel is increased but also the material and the surface properties of the steel may be reduced by the addition of a large amount of niobium and titanium. For this reason, the lower the carbon content, the more advantageous, but the steel plating technology is weak and the strength of the steelmaking technology and the grain boundary embrittlement phenomenon may occur because of the problem is limited to 0.006% by weight or less.

Silicon (Si)

Silicon (Si) according to the present invention is preferably 0.003% to 0.025%.

Silicon (Si) is an inexpensive solid solution strengthening element, and is included for the purpose of improving the strength since the high strength of the thin steel sheet can be achieved at low cost. In addition, the effect of preliminary deoxidation and complex deoxidation can be expected, but it is preferable to limit it to 0.025% by weight or less because a large amount of SiO2 is included in the inclusions, because the original performance of the steel sheet may not be exhibited. In addition, since silicon (Si) is mostly contained in the molten steel stage, a predetermined treatment is required to drastically reduce the amount of silicon, which leads to a decrease in productivity. Therefore, the Si content is preferably 0.003% by weight or more.

Manganese (Mn)

Manganese (Mn) according to the present invention is preferably 0.05% to 0.15%.

Manganese (Mn) with the increase in strength to precipitate sulfur (S) dissolved in the steel to MnS to prevent the occurrence of cracks by sulfur (S). Therefore, it is preferable that content of manganese (Mn) is 0.05 weight% or more. In addition, when the content of manganese (Mn) exceeds 0.15% by weight, workability and moldability are reduced by grain boundary segregation of manganese (Mn).

Phosphorus (P)

It is preferable that phosphorus (P) which concerns on this invention is 0.025 weight% or less.

Phosphorus (P) is an element having a high solid solution strengthening effect and a small decrease in r value (plastic deformation ratio). Phosphorus is added in the IF steel for the purpose of increasing strength. Phosphorus (P) is segregated at the grain boundaries when the content is increased to generate secondary processing brittleness. Induction of secondary processing brittleness by phosphorus (P) is suppressable by addition of boron (B). However, when the phosphorus (P) is excessively contained, it is preferable to limit the content of phosphorus (P) to 0.025% by weight or less, since brittleness due to particle segregation or plating adhesion decreases.

Sulfur (S)

Sulfur (S) according to the present invention is preferably 0.008% to 0.015%, more preferably 0.008% to 0.010%.

When the sulfur (S) is less than 0.008% by weight, the roll fatigue defect is not improved, and when the sulfur (S) is more than 0.015% by weight, the effect of the improvement of the roll fatigue defect is larger than the amount of sulfur (S) increased. it's difficult. This will be described in more detail through Experimental Examples to be described later.

Nitrogen (N)

Nitrogen (N) according to the invention is preferably 0.005% by weight or less.

Nitrogen (N), like carbon (C), exists as a solid solution in steel, reducing elongation and r-value, thereby degrading the workability and formability of the steel sheet. Nitrogen (N) increases the content of titanium and niobium to increase the cost, the smaller the content is less favorable for formability, but difficult to consider the steelmaking level and cost. Therefore, nitrogen (N) is limited to 0.005% by weight or less in consideration of steelmaking level and cost.

Niobium (Nb)

Niobium (Nb) according to the present invention is preferably 0.005 to 0.015% by weight.

Niobium (Nb), like titanium, precipitates carbon and nitrogen as solid solutions in steel in the form of precipitates of NbC and NbN to remove solid solutions in the steel, thereby improving r value and exhibiting non-aging.

Niobium deteriorates formability by lowering the r value and increasing the yield strength by adding more than stoichiometrically precipitated solid solution in steel. In addition, the cost rises to raise manufacturing costs. Therefore, the content of niobium added in the steel is preferably 0.005 to 0.015% by weight. However, niobium is added to the steel to secure excellent plating characteristics.

Titanium (Ti)

Titanium (Ti) according to the present invention is preferably 0.005 ~ 0.05% by weight.

Titanium (Ti) increases the r value as the amount added, and exhibits unaging. Titanium improves the r value by depositing carbon and nitrogen present as solid solutions in steel in the form of precipitates such as TiN and TiC. In addition, when carbon remains in steel, it exhibits an aging phenomenon that moves as time cures and confines dislocations.

The lower limit was set to the amount of stoichiometric precipitation of the employment element. Typically, the r value increases as the content of titanium (Ti) increases. However, if the titanium (Ti) content increases, the cost will rise as much as the upper limit. Therefore, the preferred content of titanium (Ti) is limited to 0.005 ~ 0.05 wt%.

Process conditions according to the present invention are as follows.

[Slab Reheating Process]

The process of reheating the slab is to reclaim segregated components during casting. Reheat is heated to a temperature range of 1150 ~ 1250 ℃. When the reheating temperature is low, the grain size decreases, which improves many properties such as hardening hardening ability and moldability. However, when the reheating temperature is excessively low, uniform austenite grains cannot be formed, and agglomeration occurs. Cause.

[Hot Rolling Process]-S110

The reheated slabs are produced after the cold rolling and rough rolling with the finishing rolling entrance temperature of 970 ° C to 1030 ° C and the exit temperature of 910 ° C or higher, more preferably 910 ° C to 1000 ° C. The upper and lower portions of the rolled material which do not cause phase change remain in the aggregate during hot rolling, which causes the aggregate retention and microstructure variation according to the thickness of the final material.

In hot rolling, shear deformation acts on the surface layer of the rolling material and plane deformation acts on the center layer due to the friction coefficient between the rolling roll and the material and the geometrical shape of the roll. If the friction coefficient between the rolling roll and the material is low, this deformation state is somewhat uniform to reduce the non-uniform deformation of the rolling material.However, if the friction coefficient between the rolling roll and the material is high, the shear deformation texture generated in the surface layer of the rolling material This results in a variation in the texture and microstructure according to the thickness of the final material.

[Cooling and winding process]-S120

Winding is performed at 650-750 degreeC. Winding is subjected to high temperature winding in order to obtain high formability. Coke of the precipitate is promoted through high temperature winding. When the temperature is excessively high, abnormal grain growth occurs, so it is carried out in the range of 650 ~ 750 ℃.

[Cold Rolling Process]-S130

The rolled and pickled hot rolled steel sheet is cold rolled to obtain the final desired thickness and to secure the desired material. Cold rolling is carried out at a reduction ratio of 65 to 80%, more preferably 65 to 77%.

There is no phase change during cold rolling, so the development of aggregates is stronger. As the reduction rate during cold rolling increases, the grain size of the final material decreases and the <110> // RD rolling texture develops, and the {111} // ND recrystallization texture is developed to optimize formability after annealing. However, in order to develop a uniform microstructure and texture in recrystallization, the friction coefficient is controlled for the same reasons as hot rolling.

In order to form a relatively uniform driving force on the entire layer of the rolled material, a sufficient reduction ratio must be applied. When recrystallization, mixing occurs in the surface layer, so it is controlled to 65 ~ 80%.

[Annealing Process] -S140

Annealing is performed at 790-840 degreeC. If the annealing temperature is excessively low, the moldability deteriorates due to insufficient development of recrystallized texture, and if the annealing temperature is too high, abnormal grain growth occurs, so the temperature is limited to 790 ~ 840 ° C.

Hereinafter, examples and comparative examples will be described in detail through experimental examples of the present invention.

After reheating the slab having a thickness of 220 mm having the components shown in Table 1 at 1230 ° C., after the rolling and rough rolling, the exit rolling temperature was controlled to 970 to 1030 ° C. to produce an exit temperature of 910 ° C. or more. It wound up at 700 degreeC after finishing rolling. After the roll replacement during finishing rolling, the slabs of each component were continuously operated in the order of 10 to 40 times, and the thermal connection due to the roll fatigue was observed.

At this time, the thickness of hot rolled steel was changed from 2.55t to 4t, and no recovery material was applied. After the hot rolled coil, which was subjected to the hot rolled manufacturing process as described above, was cold rolled at a reduction ratio of 65 to 77% and annealed at a temperature of 790 to 840 ° C. The produced cold rolled coil was checked for the occurrence of fatigue defects through the cold roll correction line.

[Table 1]

Roll fatigue defects occur more easily in a soft, high-forming, ultra-low carbon steel, and it was confirmed that fatigue defects were improved by adding 0.010% by weight of sulfur (S). That is, in case of Comparative Example 1, the occurrence of the defect occurred at the ninth time based on the 30th time, but in Examples 1 to 4, it can be seen that the occurrence occurred after the 15th. In addition, even when compared with Comparative Examples 2 and 3 it can be seen that there is no significant difference even when compared with the examples of the occurrence point of the ultra low carbon steel.

However, sulfur (S) forms a TiS precipitate when a large amount is added to reduce the amount of effective Ti that can react with the solid solution carbon (C) and nitrogen (N), so an appropriate amount of addition is necessary.

As seen above, the addition of sulfur (S) after 0.010% by weight is effective in improving the surface defects, but it can be seen that there is no significant change compared to the amount of sulfur (S) increased.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications may belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention will be determined by the claims described below.

100: Roll fatigue defect observed in hot rolled sheet
200: Fatigue of roll fatigue remaining on cold rolled sheet

Claims (10)

By weight%, carbon (C): 0.006% or less, silicon (Si): 0.003% to 0.025%, manganese (Mn): 0.05% to 0.15% or less, phosphorus (P): 0.025% or less, sulfur (S): 0.008% to 0.015%, nitrogen (N): 0.005% or less, niobium (Nb): 0.005 to 0.015%; Titanium (Ti): 0.005% to 0.050%, the remainder being hot rolled slab plate made of Fe and other unavoidable impurities;
Cooling and winding the hot rolled sheet;
Cold rolling the plated and wound plates after pickling; And
Annealing heat treatment of the cold-rolled sheet material; characterized in that it comprises a hot rolled rolled fatigue characterized in that it comprises a.
The method of claim 1,
The sulfur (S) is 0.008% to 0.010% hot rolled roll, characterized in that the fatigue fatigue defect improvement method of manufacturing a high-forming soft-rolled steel sheet.
The method of claim 1,
Reheating the slab plate prior to the step of hot rolling; Hot rolled rolled fatigue, characterized in that it further comprises a;
The method of claim 3, wherein
The reheating temperature of the slab plate in the reheating step is a method of manufacturing a high-strength flexible cold-rolled steel sheet, the hot-rolled roll fatigue defect is improved, characterized in that 1150 ~ 1250 ℃.
The method of claim 1,
The hot rolling step is a method of manufacturing a high-strength soft-rolled cold-rolled steel sheet, the defect of the hot rolled roll, characterized in that the exit temperature is 970 ℃ ~ 1030 ℃, exit temperature is 910 ℃ or more during finishing rolling.
The method of claim 1,
In the cooling and winding step, the coiling temperature is 650 ℃ ~ 750 ℃ manufacturing method of the high-forming flexible cold-rolled steel sheet with improved defects of the hot rolled roll.
The method of claim 1,
The cold rolling in the cold rolling step is a method of manufacturing a high-forming flexible cold-rolled steel sheet, the hot-rolled roll fatigue defect is improved, characterized in that 65 to 80%.
The method of claim 1,
In the annealing heat treatment step, the annealing temperature is 790 ℃ ~ 840 ℃ hot rolled roll, characterized in that the fatigue failure defect manufacturing method of the high-forming flexible cold-rolled steel sheet.
By weight%, carbon (C): 0.006% or less, silicon (Si): 0.003% to 0.025%, manganese (Mn): 0.05% to 0.15% or less, phosphorus (P): 0.025% or less, sulfur (S): 0.008% to 0.015%, nitrogen (N): 0.005% or less, niobium (Nb): 0.005 to 0.015%; Titanium (Ti): 0.005% to 0.050%, and the remainder is a high-forming soft-rolled steel sheet with improved defects of hot rolled roll made of Fe and other unavoidable impurities.
The method of claim 9,
The sulfur (S) is a high-forming soft-rolled cold-rolled steel sheet, characterized in that the hot-rolled roll fatigue defect is characterized in that 0.008% ~ 0.010%.

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