KR101055818B1 - Manufacturing method of high strength steel sheet for deep processing by mini mill process - Google Patents

Abstract

The present invention relates to a method for manufacturing a high-strength steel sheet for deep processing has a strength of at least 35kg / mm ² by a mini-mill process and at the same time exhibits superior secondary secondary brittleness and weld fatigue characteristics than conventional high-strength steel sheet for deep processing. .

       According to the present invention, C: 0.006 to 0.020%, Si: 0.02% or less, Mn: 0.8% or less, P: 0.04% or less, S: 0.015% or less, acid solubility Al: 0.03 to 0.40%, N: 0.007% or less, total tramp elements (Cu + Ni + Cr + Mo + Sn, etc.): 0.12% or less, carbonitride-forming elements: 0.005 ~ 0.040%, and the rest is made of a slab having a thickness of 50-100 mm Casting and performing rough rolling at a temperature of 950 to 1100 ° C., heating and heat-retaining the rough rolled steel sheet, followed by finishing rolling at a finish rolling temperature between 780 to 930 ° C., 560 of the hot rolled sheet. Mini mill, characterized in that it comprises a step of winding in the range of -700 ℃, pickling the wound steel sheet, cold rolling at a reduction ratio of 65 ~ 80%, and continuous annealing in the temperature range of 780 ℃ ~ 860 ℃ Provided is a method for producing a high strength steel sheet for deep working.

The present invention is very innovative in that it is possible to manufacture a high-strength steel sheet for deep processing with a tensile strength of 35kg / mm ² or more by using a component system of medium and low carbon steel in the mini mill process.

Mini mill, high strength steel sheet for deep processing, heating and heat keeping, low and medium carbon steel component system

Description

Manufacturing Method Of Steel Sheet Having High Stength And Deep Drawability By Minimill Process

The present invention relates to a method for manufacturing a high-strength steel sheet for deep processing, and more specifically, has a strength of at least 35kg / mm ² by the mini-mill process, and at the same time secondary secondary brittleness and weld fatigue compared to the conventional high-strength steel sheet for deep processing It is related with the manufacturing method of the high strength steel plate for deep processing which shows the outstanding characteristic.

Conventional methods for manufacturing high-strength steel sheets for deep processing are as follows in Japanese Patent Nos. 86-15948, 92-247828, 92-28094, 93-70836, 98-96051, 02-363685, etc. Carbon nitride forming elements such as Ti or Nb are added in an amount of about 0.03 to 0.07% and substituted solid solution strengthening elements such as Mn, P, and Si for securing strength, but in this case, ultra low carbon steel and Ti Increasing cost due to Nb addition, secondary brittleness in product characteristics, and deterioration of plating characteristics are difficult to avoid. In order to improve secondary processing brittleness, grain boundary strengthening elements such as B are added (Japanese Patent Nos. 93-268314, 93-247540, 94-256899). Operational difficulties for this remain an inevitable problem.

In addition, in order to manufacture the deep-processed steel sheet, it is necessary to control the aggregate structure having a great influence on the drawing property, which is greatly affected by the distribution and size of precipitates in the steel during the hot rolling process.

Conventionally, steel sheets were manufactured by reheating them after making thick slabs of 200 mm or more (hereinafter referred to as conventional mills), but in mini mills, thin slabs having a thickness of 50 to 90 mm were formed by playing.

It manufactures products by hot direct rolling and manufactures the mini mill. Because of its low investment cost, construction is actively underway around the world, and its production capacity is already 65 million tons per year. Initially, the mini mill process focused on the production of relatively low grade materials by using scrap as an iron source, but recently, various technologies such as general high strength steel, weathering steel and oil well high strength steel have been produced due to the development of various technologies. Deep processing products with a wide variety of uses have not yet been produced. The reason is that in order to manufacture high-strength steel for deep processing, very low carbon steel containing less than 50ppm carbon and less than 30ppm nitrogen should be used. In general, the mini-mill process adopts electric steelmaking process and also ultra-low carbon This is because it does not have secondary refining facilities (eg RH-OB) for steel production, making it difficult to manufacture ultra low carbon steel.

On the other hand, unlike the existing mill, the mini mill process is directly connected to the performance (continuous casting) and hot rolling process, so hot direct rolling is applied during hot rolling. The precipitation behavior of cargoes is expected to change significantly. The present inventors have devised a way to produce a deep-processed steel sheet with a low to medium carbon steel of about 0.006 ~ 0.02wt% carbon content in the mini mill using this difference in precipitation behavior in the mini mill.

An object of the present invention is to produce a high-strength steel sheet for deep processing of a deep milling of 35kg / mm ² grade with a very economical and tensile strength of medium and low carbon steel, rather than conventional ultra-low carbon steel by using the mini-mill process characteristics.

In order to achieve the above object, the present invention is a method for producing a steel sheet by a mini-mill process, by weight% C: 0.006 ~ 0.020%, Si: 0.02% or less, Mn: 0.8% or less, P: 0.04% or less, S : 0.015% or less, acid soluble Al: 0.03 ~ 0.40%, N: 0.007% or less, total tramp element (Cu + Ni + Cr + Mo + Sn, etc.): 0.12% or less, carbonitride-forming element: 0.005 ~ 0.040%, And the remainder is a continuous casting of a steel made of Fe 50 ~ 100mm thick slab to perform the rough rolling at a temperature of 950 ~ 1100 ℃, the step of heating and heating the rough rolled steel sheet, followed by 780 ~ 930 ℃ Finishing rolling at the finish rolling temperature in between, winding the hot rolled sheet in the range of 560-700 ° C., pickling the wound steel sheet, and cold rolling at a reduction ratio of 65 to 80%, and 780 ° C. to 860 It provides a method for producing a high strength steel sheet for deep processing by a mini-mill process, characterized in that it consists of a step of continuous annealing at a temperature section of ℃.

The heating and heat holding are preferably carried out for 10 to 90 minutes in the temperature range of 1030 to 1150 ° C. The finishing rolling is preferably performed by a tandem rolling mill. It is preferable to add Nb alone or in combination.

Hereinafter, the present invention will be described in detail.

Carbon in steel acts as an invasive solid solution and inhibits the formation of {111} texture, which is advantageous for processability during the formation of the texture of the steel sheet during cold rolling and annealing. In the case of the present invention, the lower limit of the present invention is limited to 0.006 wt% because it is possible to effectively control the precipitates by hot direct rolling of the mini-mill process and ensure deep workability even at 0.006 wt% or more. On the other hand, when the content in steel exceeds 0.02 wt%, the workability is sharply lowered, so the amount is limited to 0.006 to 0.020 wt% or less. That is, in the present invention, it is not necessary to add P to enhance the strength by using the steel in the above range as a basic component system, or to add B, which is a grain boundary strengthening element, to prevent brittleness of the ultra low carbon steel as in the conventional method. There is an advantage.                     

Si in the steel not only causes surface scale defects, but also tempered color during annealing and unplated portions during plating, so its content was limited to 0.02 wt% or less.

Mn in steel is added as a substituted solid solution strengthening element to secure the strength, but when the content is 0.8 wt% or more, the r value (Lankford value) decreases with the elongation, so it is limited to 0.8 wt% or less.

Since P in steel is an element that plays a role of lowering grain boundary brittleness and fatigue characteristics as the amount added, it is desirable to add as little as possible in steel, but according to the conventional manufacturing method, it is indispensable for effective strength increase while minimizing the deterioration of drawing property. 0.03-0.06wt% was the only element to be added. However, in the present invention, since the basic component system uses a medium-low carbon steel rather than an ultra-low carbon steel, it is possible to secure the strength to be achieved in the present invention without adding P, so the content is limited to 0.04 wt% or less of the impurity element level.

Since the amount of S in the steel is an element that forms the edge cracks of the slab by forming FeS, the content is limited to 0.015 wt% or less in order to avoid the area where the edge cracks occur during rolling.

Acid-soluble Al (Sol.Al) in steel is generally produced by managing the content of cold rolled products at about 0.02 ~ 0.07wt% in consideration of economic aspects while keeping dissolved oxygen level in steel sufficiently low. . However, in the case of the present invention, even if the carbon content is relatively high, it serves to ensure a stable deep workability. That is, the more acid-soluble Al in the steel of the present invention, the more clearly the effect of inhibiting the recrystallization inhibitory action of the solid solution C in the steel, not only promotes recrystallization but also plays a role in developing the {111} series aggregate. However, if the content is 0.40% or more, the content is limited to 0.03 to 0.40 wt% because the cost increases and the performance of performance is impaired.

In steel, N content in solid solution impairs elongation and drawability, so its content is limited to 0.007 wt% or less.

Tram elements in steel that are usually mixed from scrap are Cu, Ni, Cr, Mo, Sn, etc., and if the amount exceeds 0.12wt% or more, the elongation and drawability are large, making it difficult to secure materials for deep processing. In addition, the content is limited to less than 0.12wt% because it serves to lower the surface quality.

Carbonitride forming elements (Ti, Nb, V, etc.) are very important elements in terms of processability, and their content is 0.005-0.040 in consideration of the optimum amount and economical aspect for producing workability (especially r value) synergistic effect. It was limited to the range of wt%. As a carbonitride forming element, it is preferable to add Ti and Nb individually or in combination. The above content is much less than 0.03 to 0.07 wt% which is generally added in the conventional manufacturing method. Although the content is small as described above, in the case of the present invention, it is possible to secure good drawing property (r value). It is considered that the hot direct rolling method adopted in the present invention is much more advantageous for stabilizing invasive solid-solution elements such as carbon or nitrogen as precipitates. Turned out to be necessary.                     

Continuously cast steel composed of components in the alloy design method as described above with a slab of 50 to 100 mm thick, roughly roll at a temperature of 950 to 1100 ° C, heat and heat again, and then finish rolling. .

It is very important in the present invention to undergo a heating and heating step after rough rolling and before finishing rolling. In the case of the present invention, unlike the conventional manufacturing method (the conventional milling method) in which the slab passes through the reheating furnace after cooling, the steel solid solution C and N are adopted because the rolling process is performed immediately after the performance (hot direct rolling). The supersaturated solid solution exists in this case. In this case, in order to promote the precipitation of the solid solution elements, Ti, Nb-based high temperature precipitates (mainly nitrides, emulsions, and complex precipitates thereof) are formed at a temperature of 950-1100 ° C. where precipitation occurs actively. Rolling must be carried out. In addition, it is important to form the desired {111} series texture in order to improve the processability of the final product, the cold rolled product. To develop the texture, the precipitates of the hot rolled sheet are uniformly distributed in the size of several tens of nanometers and at the same time low temperature precipitates are formed. It is necessary to coarsen [TiC, NbC, Ti, Nb (C, N)]. As described above, in order to form the hot rolled sheet precipitate, it is necessary to undergo a heating and heat retaining step before finishing rolling, and the heating and heat retaining are preferably performed for 10 to 90 minutes in a temperature range of 1030 to 1150 ° C. If the retention time is less than 10 minutes in the precipitate is not stabilized, and if the time is too long more than 90 minutes, the risk of surface defects with the decrease in productivity is limited to 10 to 90 minutes.

The steel which has undergone the above heat treatment process is then rolled so that the finish rolling temperature is between 780 ~ 930 ℃ and wound at a temperature of 560 ~ 700 ℃. This is to ensure that the low temperature precipitate is as coarsely stabilized, the finish rolling temperature If too low, there is a problem that the rolling deformation resistance is greatly increased, and if the temperature is too high, defects are caused by sticking phenomenon, so the finish rolling temperature is limited to 780 ~ 930 ° C, and the winding temperature is 560 If the temperature is lower than or lower, the precipitate becomes fine, and if the temperature is higher than 700 or higher, there is a risk of scale defects. The winding temperature is limited to 560 to 700 캜. At this time, it is preferable that the finish rolling is performed by a tandem type rolling mill.

The hot rolled sheet manufactured in the above manner is pickled and cold rolled at a cold reduction rate of 65 to 80%, and then continuously annealed at a temperature range of 780 ° C to 860 ° C. In the case of the following, it is difficult to obtain deep workability, and in the case where the annealing temperature is higher than 860 ° C., the setting range is limited because there is a high risk of problems in the operation of strips due to high temperature annealing.

Hereinafter, embodiments of the present invention will be described.

As shown in Table 1, a slab having a composition of the present invention (made by a mini mill process) and a slab having a component system of a conventional high strength steel (35E grade) for a 35 kg / mm ² grade deep core processing. After the annealing plate was manufactured under the conditions of hot rolling, cold rolling and continuous annealing as shown in Table 2, the material properties were measured, and the results are shown in Table 3.

Fatigue test conditions for the evaluation of the fatigue properties of spot welds were set to the fatigue strength under conditions of 10 to 10 million repeated loads with a maximum load to minimum load ratio of 10. After forming the cup with drawing ratio 1.9, the minimum temperature without falling free weight and brittle fracture is obtained. Other material characteristic values are obtained by general measuring methods.

Table 1

Table 2

Table 3

As shown in Table 3, the deep-strength steel sheet manufactured by the method of the present invention is equivalent in tensile strength, elongation, and r value to the deep-strength steel sheet made of the conventional ultra low carbon steel, and is equivalent to weld fatigue. It can be seen that the strength and the secondary processing brittleness characteristics are more excellent.

As described above, the present invention makes it possible to manufacture a high-strength steel sheet for deep processing with a tensile strength of 35 kg / mm ² or more using a component system of medium and low carbon steel in a mini mill process, and the manufactured steel sheet is used for a steel sheet made using conventional ultra low carbon steel. It is a breakthrough that can prevent secondary processing brittleness, weld fatigue characteristics and plating properties deterioration, which is pointed out as a problem. In addition, by manufacturing a high-strength steel sheet for deep processing of low and medium carbon steel, it is very economical in that it can avoid the increase in equipment investment and operating costs for the production of ultra low carbon steel.

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

Claims (4)

In the method of manufacturing a steel sheet by a mini-mill process, Weight% C: 0.006 ~ 0.020%, Si: 0.01 ~ 0.02%, Mn: 0.01 ~ 0.8%, P: 0.001 ~ 0.04%, S: 0.001 ~ 0.015%, Acid Soluble Al: 0.03 ~ 0.40%, N: 0.001 ~ 0.007%, total tramp element (Cu + Ni + Cr + Mo + Sn): 0.01 to 0.12%, carbonitride forming elements (Ti and / or Nb): 0.005 to 0.040%, and the rest is made of steel Continuous casting by slab of 50 ~ 100mm to perform rough rolling at a temperature of 950 ~ 1100 ℃, Heating and heat again the rough rolled steel sheet, Immediately followed by finish rolling at a finish rolling temperature of 780-930 ° C., Winding the hot rolled sheet in the range of 560-700 ° C., Pickling the wound steel sheet, cold rolling at a reduction ratio of 65 to 80%, and A method of manufacturing a high strength steel sheet for deep processing by a mini-mill process, characterized in that it consists of a step of continuous annealing at a temperature range of 780 ℃ ~ 860 ℃. The method according to claim 1, wherein the heating and heat holding are performed for 10 to 90 minutes in a temperature range of 1030 to 1150 ° C. delete The method of manufacturing a high strength steel sheet for deep processing according to claim 1 or 2, wherein the finish rolling is performed in a tandem rolling mill.