KR100605719B1 - Method for Manufacturing Soft Steel Strip for Deep Drawing and Soft Steel Strip Manufactured by the Method - Google Patents

Abstract

The present invention relates to a method for manufacturing a soft thin steel sheet for deep processing used for automobile interior and exterior, and to a thin steel sheet produced by the method, by performing hot direct rolling with semi-ultra low carbon steel, thereby providing excellent deep workability. In addition, to provide a method for more economically manufacturing a flexible thin steel sheet excellent in secondary processing brittleness, weld fatigue properties and plating characteristics, and a thin steel sheet produced by the method, and its purpose is to provide.

In the present invention, by weight%, C: 0.005 to 0.015%, Si: 0.02% or less, Mn: 0.4% or less, S: 0.015% or less, P: 0.02% or less, acid value Al: 0.03 to 0.4%, N: 0.008 % Or less, total tramp element (Cu + Ni + Cr + Mo + Sn): 0.15% or less, alone or in combination of Ti and Nb: 0.005-0.040%, other steels made of continuous casting of unavoidable impurities and balance Fe After the preparation, the slab in the high temperature state was directly put into the hot rolling process without undergoing a hot rolling reheating process and subjected to rough rolling at 950-1150 ° C., followed by Ar ₃ -30 ° C. to Ar ₃ using a tandem rolling mill. After hot finishing rolling under the conditions of finishing rolling temperature of 700 ~ 940 ℃ excluding temperature range, winding in 600 ~ 750 ℃ temperature, pickling and cold rolling, and continuous annealing at the temperature of 760 ℃ ~ 860 ℃ The manufacturing method of the deep thin steel plate for deep processing, and the thin steel plate manufactured by this method are made into the summary.

Deep processing, hot direct rolling, semi ultra low carbon steel, secondary brittle resistance, welding characteristics, thin steel sheet

Description

Method for manufacturing soft steel strip for deep processing and thin steel sheet manufactured by the method {Method for Manufacturing Soft Steel Strip for Deep Drawing and Soft Steel Strip Manufactured by the Method}

The present invention relates to a method for producing a flexible thin steel sheet for deep processing mainly used for automotive interior and exterior panels, and a thin steel sheet produced by the method, more specifically, ultra-low carbon steel with 50 to 150ppm carbon content in steel Method of manufacturing soft thin steel sheet having excellent deep workability as well as excellent secondary work embrittlement resistance and weld fatigue characteristics by performing hot direct rolling on the material (hereinafter also referred to as “semi-ultra low carbon steel”). It relates to a thin steel sheet produced by.

As a technique for manufacturing a deep steel sheet, a technique of extremely reducing the amount of invasive employment elements such as C and N in the steelmaking process to less than 50 ppm.

However, it is very difficult to manufacture DDQ (Deep Drawing Quality) grade deep workability by simply lowering the amount of invasive solid elements in the steel, so that the deep work steel sheets are separately formed of the carbonitride forming elements Ti, Nb. It is common to manufacture using so-called ultra-low carbon IF (Interstitial Free) steel which is added alone or in combination.

As a parent technology for manufacturing a thin steel sheet for deep processing using the IF steel, a technology using Ti-added steel disclosed in Japanese Patent No. 56385, a technology using Nb-added steel from Armco of USA, and Japanese Patent No. 1278670 The technique using the improved Ti additive steel disclosed in the above, and the technique using the Ti-Nb composite additive steel of the Japanese Kawasaki Steel Mill (KSC), etc. are mentioned, and the technique which improved these mother technologies is many and continues to improve.

The parent technology and improved technology typically add about 0.03% to 0.07% of carbonitride-forming elements such as Ti or Nb to ultra-low carbon steel to secure workability.

As described above, when carbonitride-forming elements such as Ti or Nb are added to the ultra low carbon steel to improve the workability, the cost increases due to the production of the ultra low carbon steel and the addition of Ti and Nb. There is a problem that leads to the generation of differential brittleness and deterioration of plating characteristics.

In order to improve the secondary brittleness problem described above, grain boundary strengthening elements such as B are added (related to Japanese Patent Laid-Open Publication Nos. 193-268314, 1993-247540, 1994-256899), but in this case, workability by addition of B Of course, there is a problem that the operation is difficult to control the amount of B addition to the fall.

MEANS TO SOLVE THE PROBLEM The present inventors carried out research and experiment in order to solve the above-mentioned all the problems of the prior art, and based on the result, the present invention proposes the present invention. In addition to the excellent and excellent secondary secondary embrittlement resistance, weld fatigue properties and plating properties to provide a method for more economically manufacturing a flexible thin steel sheet and a thin steel sheet produced by this method, will be.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, by weight%, C: 0.005 to 0.015%, Si: 0.02% or less, Mn: 0.4% or less, S: 0.015% or less, P: 0.02% or less, acid value Al: 0.03 to 0.4%, N: 0.008 % Or less, total tramp element (Cu + Ni + Cr + Mo + Sn): 0.15% or less, alone or in combination of Ti and Nb: 0.005-0.040%, other steels made of continuous casting of unavoidable impurities and balance Fe After the preparation, the slab in the high temperature state was directly put into the hot rolling process without undergoing a hot rolling reheating process and subjected to rough rolling at 950-1150 ° C., followed by Ar ₃ -30 ° C. to Ar ₃ using a tandem rolling mill. After hot finishing rolling under the conditions of finishing rolling temperature of 700 ~ 940 ℃ excluding temperature range, winding in 600 ~ 750 ℃ temperature, pickling and cold rolling, and continuous annealing at the temperature of 760 ℃ ~ 860 ℃ It relates to a method for producing a soft thin steel sheet for deep processing.

The present invention also relates to a soft thin steel sheet for deep working according to the method of the present invention.

That is, the present invention is in the weight%, C: 0.005 ~ 0.015%, Si: 0.02% or less, Mn: 0.4% or less, S: 0.015% or less, P: 0.02% or less, acid value Al: 0.03 ~ 0.4%, N : 0.008% or less, total tramp element (Cu + Ni + Cr + Mo + Sn): 0.15% or less, Ti or Nb alone or in combination: 0.005-0.040%, composed of other unavoidable impurities and residual Fe, and precipitates These precipitates are related to a soft steel sheet for deep processing, characterized in that the number of precipitates having a size of 5 to 100nm is more than 80% of the total number of precipitates.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the case of manufacturing a thin steel sheet by adding Ti and Nb into steel, the {111} aggregate structure parallel to the rolling direction is developed compared to the case where it is not, and the deep drawing property is greatly improved. Theoretically it is not clear.

Several theories have been suggested for the above phenomena, for example, precipitation theory that carbonitrides formed in steel itself develop aggregates (Steel Subsection of Japan Research Association, 1982, 73, etc.), and Ti, Nb are the employment elements. Employment novels (steel, 82, p.35, iron and steel, 81, p.185, etc.) and reduced employment carbon in steel (140th Nishiyama Technical Course, 1991, p) .35 etc.)

These theories are presented as a mechanism for the phenomenon that occurs in the current main hot rolling method, cold slab rolling (charging the slab cooled after casting into a reheater, extracting and rolling; Cold Charge Rolling (CCR)). None of these accounts for all of the phenomena present in the manufacture of IF steel.

However, it is commonly recognized in the theories that there is a close relationship between precipitation of invasive employment elements (especially employed carbon) and the phenomenon of formation of aggregates and formation of aggregates in the river.

On the other hand, Hot Direct Rolling (HDR), which is a method in which a slab in a high temperature state is directly put into a hot rolling process and rolled after a performance, has been developed, and a mini mill process basically uses a hot direct rolling method for rolling. Doing.

In the case of hot rolling of ultra low carbon steel by using the hot direct rolling method, the heat history received by the slab is very different from that of the conventional cold-loading rolling process. There is a big difference from that of.

MEANS TO SOLVE THE PROBLEM The present inventors study the precipitation, the employment behavior, etc. of the solid-solution element in steel by using the said hot direct rolling method, and completed this invention based on the result.

That is, the present invention is to enable the production of soft thin steel sheet for deep processing in semi-low carbon steel (B not added steel) of about 0.005 ~ 0.015% C content, not the conventional ultra low carbon steel.

The present invention uses a hot direct rolling method to produce a soft thin steel sheet for deep processing of semi-ultra low carbon steel (B not added steel), and appropriately controls the steel components, hot rolling conditions and cold rolled steel annealing conditions.

The C is a component that acts as an invasive solid solution and inhibits the formation of {111} texture structure, which is advantageous for processability during cold rolling and annealing, to lower the deep workability.

Therefore, the content is usually limited to 0.005% or less.

However, in the case of the present invention, it is possible to effectively control the precipitates by hot direct rolling, so that deep workability can be secured even when the content is 0.005% or more.

However, when the C content exceeds 0.015%, the workability is sharply lowered, so the upper limit of the C content is preferably set at 0.015%.

On the other hand, when the C content is less than 0.005%, grain boundary embrittlement occurs, so the lower limit of the C content is preferably set to 0.005%.

As described above, in the case of the present invention, even when the C content in the steel is 0.005 to 0.015%, deep workability can be ensured, so that B, which is a grain boundary strengthening element, is added to prevent brittleness of the ultra low carbon steel as in the conventional method. Component design has the advantage of not being necessary.

Since the Si not only causes red scale but also lowers the plating property, the content thereof is preferably limited to 0.02% or less.

The Mn is a component that is added to form MnS to prevent cracking caused by S, but when the amount exceeds 0.4%, it is economically disadvantageous with a decrease in processability, so the content is limited to 0.4% or less. It is desirable to.

S is an element that forms FeS to cause the edge cracks of the slab, if it is contained in a lot of steel, there is a risk of generating edge cracks during rolling, the content is preferably limited to 0.015% or less.

When the P is contained in a large amount of steel, the grain boundary brittleness occurs and the fatigue property is lowered, so that the content is preferably limited to 0.02% or less in order to avoid the defect.

Sol.Al is a component that acts as a deoxidizer, and in consideration of the aspect of maintaining the dissolved oxygen level in steel sufficiently and economically, in the case of cold-rolled products, the content is generally controlled to produce 0.02 to 0.07%. to be.

On the other hand, in the present invention, Sol.Al not only serves as a deoxidizer, but also plays a role of stably securing deep workability even when the carbon content is relatively high.

That is, in the present invention, Sol.Al not only plays a role of deoxidizer but also clearly shows the effect of inhibiting the recrystallization inhibitory action of solid solution C in the steel, not only promotes recrystallization but also develops the {111} series aggregates. Do it.

If the amount of Sol.Al added is too small, the effect of addition is insignificant. If the amount of addition of Sol.Al exceeds 0.40%, the increase in cost and performance of playability are impaired. Therefore, the amount of addition of Sol.Al is preferably limited to 0.03 to 0.40%. .

When N is in a solid solution state, the content thereof is deteriorated in elongation and drawability, so the content thereof is preferably limited to 0.008% or less.

The main components of the tramp element are Cu, Ni, Cr, Mo, Sn, etc., and are usually mixed from scraps, etc. If the amount exceeds 0.15%, the elongation and drawability are large, so it is difficult to secure a material for deep processing. As well as lowering the surface quality, the content is preferably limited to 0.15% or less.

The Ti and Nb are components added to improve the processability by forming carbonitrides, when the addition amount of Ti and Nb alone or in combination is less than 0.005%, the addition amount is too small, the addition effect is small, the addition amount is 0.040% Since it is uneconomical when exceeding, it is preferable to limit the addition amount of Ti and Nb alone or in combination to 0.005-0.040%.

As described above, in the present invention, even if Ti and Nb are added in much smaller amounts than Ti and Nb in the conventional art of 0.03% to 0.07%, it is possible to secure good drawing property (r value). Since the hot direct rolling method adopted in the present invention is much more advantageous in stabilizing invasive employment elements such as C or N as precipitates, in order to achieve this more effectively, it is necessary to control the rolling temperature at the time of rough rolling and finishing rolling together. More effective.

As mentioned above, since the addition amount of Ti and Nb can be reduced, the outstanding plating characteristic can be ensured.

Hereinafter, a method for producing a thin steel sheet according to the present invention will be described.

The present invention adopts a hot direct rolling process that performs hot rolling immediately after playing, unlike the conventional manufacturing method of cooling the playing slab and passing it through a reheating furnace. It exists.

Therefore, in the present invention, the precipitation of Ti, Nb-based high-temperature precipitates (mainly nitrides, emulsions and complex precipitates) is actively performed in order to promote the precipitation of solid solution C and N in the supersaturated solid solution state as described above. Rough rolling is carried out at temperature.

In the present invention, after rough rolling as described above, hot finishing rolling is performed by a tandem type rolling mill.

The hot finish rolling during the finish rolling temperature is preferably set to a temperature of 700~Ar ₃ -30 ℃ or Ar ₃ ~940 ℃.

That is, the finish rolling temperature is preferably limited to a temperature range in which the Ar ₃ -30 ° C. to the Ar ₃ temperature range is excluded from the temperature range of 700 to 940 ° C., for the following reason.

That is, when the finishing rolling temperature is 700 ° C. or less, a problem in which the rolling deformation resistance increases greatly occurs, and when the finish rolling temperature is 940 ° C. or more, large grains are formed on the hot rolled sheet, resulting in a decrease in workability. The range is selected to be 700 to 940 ° C.

However, the temperature range of Ar ₃ -30 ℃ ~ Ar ₃ is a metallurgical zone where ferrite and austenite structure coexist, which may cause instability of operation and deterioration of workability in the final product. from Ar ₃ -30 ℃ ~ Ar ₃ hot finish rolling in the temperature range is a limitation to blood.

In addition, if the coiling temperature is less than 600 ℃, deep workability is lowered with the risk of shape defects during winding, and if it exceeds 750 ℃ scale defects are likely to occur, so the winding temperature is selected to 600 ~ 750 ℃ It is preferable.

The hot rolled steel sheet produced as described above is pickled and cold rolled, followed by continuous annealing to produce a soft thin steel sheet for deep processing.

At this time, any of the pickling and cold rolling methods can be applied as long as it is a commonly used method.

The annealing temperature during continuous annealing of the cold rolled steel sheet is preferably limited to 760 ~ 860 ℃, the reason is that when the annealing temperature is less than 760 ℃ difficult to obtain deep workability, in the case of more than 860 ℃ due to high temperature annealing strip This is because there is a high risk of problems with the mail order.

As described above, when the thin steel sheet is manufactured according to the present invention, precipitates are formed in the thin steel sheet, and the precipitates have a size such that the number of precipitates having a size of 5 to 100 nm is 80% or more of the total number of precipitates.

When the thin steel sheet is manufactured by the conventional method, precipitates having a size of 5 nm or less are formed in the thin steel sheet.

In the case of precipitates of 5 nm or less in the IF steel, it is known to degrade workability.

In addition, in the case of manufacturing the thin steel sheet according to the present invention, the precipitates present in the steel sheet are non-equilibrium precipitates, which have good deep workability even when carbonitrides and emulsion forming elements such as Ti and Nb are not added in an amount equal to or more than an atomic equivalent. You can get it.

In addition, as described above, since the addition amount of Ti, Nb and the like is small, the plating characteristics are improved.

On the other hand, the present invention can be equally applied to the production of hot dip galvanized products (GI, GA).

Hereinafter, the present invention will be described in detail through examples.

(Example)

To prepare a steel sheet with a steel composition having the composition of Table 1 below to the manufacturing conditions of Table 2, and then measured the tensile strength, elongation, r value, weld fatigue strength and ductile brittle transition temperature, the results are shown in Table 3 It was.

In Table 3 below, the weld fatigue strength is expressed as the strength of the condition that is not broken even after 10 million repeated loads with the maximum load-to-minimum load ratio as 10, and the ductile brittle transition temperature is formed by forming the cup with the drawing ratio as 1.9. After that, the weight is freely dropped and the minimum temperature at which brittle fracture does not occur is obtained.

division Composition (% by weight) C Mn Si P S N Ti Nb S.Al Total Tremble Element B (ppm) Invention steel One 0.007 0.09 0.01 0.012 0.006 0.0065 0.010 0.009 0.05 0.08 - 2 0.012 0.12 0.01 0.013 0.005 0.0062 0.021 - 0.12 0.11 - 3 0.011 0.13 0.01 0.008 0.011 0.0053 0.012 0.009 0.32 0.12 - 4 0.009 0.08 0.01 0.016 0.008 0.0072 - 0.021 0.04 0.10 - Comparative steel 5 0.0038 0.12 0.01 0.009 0.012 0.0021 0.049 - 0.041 0.05 8 6 0.0031 0.08 0.01 0.008 0.011 0.0029 0.0528 - 0.039 0.04 5

division                                              Steel grade Hot process Rough rolling temperature (℃) Finish rolling temperature (℃) Winding temperature (℃) Cold Rolling Rate (%) Continuous Annealing Temperature (℃) Inventive Example One Inventive Steel 1 Hot direct rolling 1020 833 723 75 820 2 Inventive Steel 2 Hot direct rolling 980 832 632 75 835 3 Inventive Steel 3 Hot direct rolling 1060 892 672 75 798 4 Inventive Steel 4 Hot direct rolling 1130 910 615 75 815 Comparative example 5 Comparative Steel 5 Conventional Rolling 1006 916 632 75 830 6 Comparative Steel 6 Conventional Rolling 1012 915 638 75 830

division The ratio of the number of precipitates having a size of 5 to 100 nm to the total number of precipitates Tensile Strength (kg / mm ² )  Elongation (%)   r value  Weld Fatigue Strength (kgf)  Ductile Brittle Transition Temperature (℃) Inventive Example One 92 31.4 46.2 1.92 95 -65 2 83 32.6 47.5 1.86 90 -70 3 88 30.9 49.8 1.82 95 -70 4 90 31.2 46.4 1.91 90 -65 Comparative example 5 52 35.8 48.2 1.89 85 -50 6 68 35.4 47.3 1.85 80 -45

As shown in Table 3, in the case of Inventive Example (1-4) according to the present invention, compared to the comparative example (1-2) manufactured using the conventional ultra-low carbon steel, the weld fatigue strength and secondary processing It can be seen that the brittle characteristics (ductility brittle transition temperature) is excellent.

Therefore, according to the present invention, it can be seen that it is possible to economically manufacture high-strength steel for deep processing, which is excellent in weld fatigue strength and secondary processing brittleness characteristic (ductile brittle transition temperature).

As described above, according to the present invention, in the case of manufacturing soft thin steel sheet for deep processing using the semi-low carbon steel component system, not only an increase in equipment investment and operating cost for manufacturing ultra-low carbon steel can be avoided. Secondary brittle brittleness, weld fatigue properties and plating properties are deteriorated to eliminate the problems.

Claims (2)

By weight%, C: 0.005 to 0.015%, Si: 0.02% or less, Mn: 0.4% or less, S: 0.015% or less, P: 0.02% or less, acid value Al: 0.03 to 0.4%, N: 0.008% or less, Total tramp elements (Cu + Ni + Cr + Mo + Sn): 0.15% or less, Ti or Nb alone or in combination: 0.005-0.040%, other unavoidable impurities and balance Fe After slab is manufactured by continuous casting of steel, the slab of high temperature is directly put into hot rolling process without undergoing hot rolling reheating process, and rough rolling is carried out at 950 ~ 1150 ℃, then using tandem rolling mill After hot finishing rolling under the conditions of finishing rolling temperature of 700 to Ar ₃ -30 ° C or Ar _{3 to} 940 ° C, winding in a temperature range of 600 to 750 ° C, followed by pickling and cold rolling, followed by 760 ° C to 860 ° C Method for producing a soft thin steel sheet for deep processing, characterized in that the continuous annealing at temperature By weight%, C: 0.005 to 0.015%, Si: 0.02% or less, Mn: 0.4% or less, S: 0.015% or less, P: 0.02% or less, acid value Al: 0.03 to 0.4%, N: 0.008% or less, Total tramp elements (Cu + Ni + Cr + Mo + Sn): 0.15% or less, Ti or Nb alone or in combination: 0.005-0.040%, other unavoidable impurities and balance Fe And precipitates, the precipitates having a size of 5 to 100 nm, wherein the number of precipitates is about 80% or more of the total number of precipitates.