KR101256521B1 - Method for manufacturing hot rolled steel sheet including high carbon contents with excellent surface quality using minimill process - Google Patents

Abstract

The present invention is a mini mill process that can effectively reduce the occurrence of arithmetic scales in which the scale is inserted into the base metal after pickling by optimizing the temperature and descaling conditions of rough rolling in a direct rolling process such as a mini mill process. Its main purpose is to provide a method for producing a high carbon hot rolled steel sheet having excellent surface quality.
Method for producing a high-carbon hot-rolled steel sheet of excellent surface quality using the mini-mill process of the present invention for achieving the above object, by weight% C: 0.2 ~ 1.3%, Mn: 0.2 ~ 2.0%, Si: 0.5% or less ( Continuous casting of molten steel consisting of balance Fe and other unavoidable impurities into a thin slab having a thickness of 30 to 150 mm; After descaling the continuously cast thin slab to a descaling injection pressure of 200 bar / mm ² or more and a distance of 200 cm or less (except 0 cm) between the descaler and the roughing mill entrance side, the thin slab temperature at the entrance of the roughing mill is 1050 to Rough rolling to 1100 ° C .; Finishing or rolling the rough rolled hot rolled strip after heating or heating; And winding the finish rolled hot rolled strip after cooling on a runout table.

Description

Manufacture method of high carbon hot rolled steel sheet with excellent surface quality using mini mill process {METHOD FOR MANUFACTURING HOT ROLLED STEEL SHEET INCLUDING HIGH CARBON CONTENTS WITH EXCELLENT SURFACE QUALITY USING MINIMILL PROCESS}

The present invention relates to a method for producing a high carbon hot rolled steel sheet having excellent surface quality using a mini mill process, and more particularly to a method for producing a high carbon hot rolled steel sheet having a low material deviation and excellent surface quality after pickling using a mini mill process. It is about.

According to the mini mill process consisting of continuous casting machine-> rough rolling mill-> induction heating machine-> finish rolling mill, when the scale remaining on the cast iron is inserted into the base metal during rough rolling, the scale crushed on the surface of steel sheet in rough rolling after pickling remains. The arithmetic scale that appears is generated. For this reason, the hot rolled steel sheet manufactured by the mini-mill process was not used in a product requiring high surface quality.

Korean Patent Publication No. 2007-117992 (name of the invention: a method and system for manufacturing a metal strip and sheet without a continuity solution between continuous casting and rolling) and Korean Patent Publication No. 2008-25671 ( The present invention discloses a layout in which a descaler is installed prior to rough rolling in order to prevent the appearance of surface defects due to residual scales in the rough rolling after pickling. It does not disclose specific conditions of descaling and temperature.

Therefore, in the related art, descaler operating conditions, that is, descaling and temperature conditions are not exactly known, and thus, it is difficult to effectively prevent arithmetic scales generated by indentation in rough rolling after pickling.

The present invention was developed to solve this problem, and the arithmetic scale in which the scale is inserted into the base material and remains after pickling by optimizing the temperature and descaling conditions of the rough rolling in the direct-rolling process such as the mini mill process. The main object of the present invention is to provide a method for producing a high carbon hot rolled steel sheet having excellent surface quality using a mini-mill process that can effectively reduce the occurrence of.

Method for producing a high-carbon hot-rolled steel sheet of excellent surface quality using the mini-mill process of the present invention for achieving the above object, by weight% C: 0.2 ~ 1.3%, Mn: 0.2 ~ 2.0%, Si: 0.5% or less ( However, continuous casting of molten steel consisting of balance Fe and other unavoidable impurities into a thin slab having a thickness of 30 to 150 mm; After descaling the continuously cast thin slab to a descaling injection pressure of 200 bar / mm ² or more and a distance of 200 cm or less (except 0 cm) between the descaler and the roughing mill entrance side, the thin slab temperature at the inlet of the roughing mill is 1050. Rough rolling to 1100 ° C .; Finishing or rolling the rough rolled hot rolled strip after heating or heating; And winding the finish rolled hot rolled strip after cooling on a runout table.

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Another manufacturing method according to the present invention, by weight% C: 0.2 to 1.3%, Mn: 0.2 to 2.0%, Si: 0.5% or less (except 0%) remainder molten steel consisting of Fe and other unavoidable impurities 30 Continuous casting with a thin slab of ˜150 mm; After descaling the continuously cast thin slab to a descaling injection pressure of 300 bar / mm ² or more and a distance of 400 cm or less (except 0 cm) between the descaler and the roughing mill entrance side, the thin slab temperature at the inlet of the roughing mill is 1050. Rough rolling to 1100 ° C .; Finishing or rolling the rough rolled hot rolled strip after heating or heating; And winding the finish rolled hot rolled strip after cooling on a runout table.

In addition, the continuous casting step is preferably such that the casting speed is 4.5 mpm or more.

In addition, the rough rolling step is preferably such that the cumulative reduction rate at the time of rough rolling is 65 ~ 90%.

In addition, the finishing rolling step is such that the rolling speed difference in one strip is 15% or less.

According to the manufacturing method of high-carbon hot rolled steel sheet having excellent surface quality using the mini mill process of the present invention configured as described above, the descaler in front of the rough mill in the mini mill process consisting of a continuous casting machine-> rough rolling mill-> induction heating-> finish rolling mill By further installing and optimizing the temperature and descaling conditions of the rough rolling, it is possible to effectively reduce the occurrence of arithmetic scales, in which the scale is inserted into the substrate after pickling. As a result, the high-carbon hot rolled steel produced in the mini mill process can be extended to products requiring excellent surface quality.

In addition, by utilizing the characteristics of the mini-mill process, which is differentiated from the existing blast furnace mill, it is possible to produce a hot rolled steel sheet which significantly reduces the material deviation in the width direction and the length direction of the strip.

In addition, the thin slab playing method can be used to melt the scrap steel, such as scrap in the electric furnace can increase the recycling of resources.

1 is a schematic diagram illustrating a minimill process of the present invention.

Hereinafter, the technical configuration of the present invention will be described in more detail.

As described above, since the present invention relates to a method for manufacturing a high carbon hot rolled steel sheet through a mini mill process, first, the mini mill process according to the present invention will be briefly described with reference to FIG.

First, in the continuous casting machine 10 to produce a thin slab (a) having a thickness of 30 ~ 150mm. This is called thin slab in comparison with slabs of 200 mm or more produced by continuous casting machines of conventional mills. Since a slab of 200 mm or more is completely cooled in a yard or the like, it has to be sufficiently reheated to a surface temperature of 1100 ° C. or more in a reheating furnace before hot rolling. On the contrary, since the thin slab is immediately transferred to the roughing mill 20 without passing through the reheating furnace, the heat slab can be used as it is, thereby reducing energy and greatly improving productivity.

According to the present invention, the high pressure injection type descaler 15 is installed within a predetermined distance in front of the roughing mill 20. The descaler 15 is configured to precisely control the descaling conditions (injection pressure and installation distance) in association with the thin slab temperature at the inlet of the subsequent rough mill 20. Specific rough rolling temperature and descaling conditions will be described later.

The descaled thin slab (a) is rolled into a hot rolled strip having a predetermined thickness or less in the rough mill (20), and in this process the temperature of the reduced strip is compensated by the heating means (30), and then the heated hot rolled strip ( b) is rolled to the desired final thickness in finishing mill 50, cooled via ROT [Run Out Table 60] (hereinafter referred to as "runout table"), and then finalized to constant temperature in winder 70 By winding, a hot rolled steel sheet of a desired material is produced. The precise temperature control can be configured to move directly from the roughing mill 20 to the finishing rolling mill 50 without the heating means 30.

At this time, in order to compensate for the difference between the playing speed and the rolling speed, the coil box 40 may be installed in front of the finish rolling mill 50 so as to be configured to firstly wind the hot rolled strip b passed through the induction heater 30. have. As a high speed playing method of 6mpm or more has been realized recently, a true continuous rolling process without using the coil box 40 has been developed.

The composition of the high-carbon hot-rolled steel sheet manufactured through the mini-mill process described above is composed of C: 0.2 to 1.3%, Mn: 0.5 to 2.0%, Si: 0.5% or less of residual Fe and other unavoidable impurities in weight%. The function and content range of each element is briefly described.

The content of carbon (C) is limited to 0.2 to 1.3%. High carbon steel is mainly used for products requiring surface hardness. According to the needs of customers, it is widely used as a product requiring high wear resistance and high strength by securing high hardness through annealing, quenching and tempering (Q & T treatment).

Since carbon has the greatest influence on the hardenability or hardness of steel, when the content is less than 0.3%, it may be difficult to secure hardenability or hardness and thus may not be used in high strength products. In addition, when the content exceeds 0.9%, the hot rolled strength is too high, so that it may be difficult to blanking and re-rolling in the customer, and defects such as thermal cracks may occur even in the hot rolling process.

Manganese (Mn) is limited to 0.5 to 2.0%. Manganese (Mn) not only contributes to securing hardness and hardenability, but also prevents brittleness caused by sulfur (S). Therefore, when the content of Mn is less than 0.5%, it is difficult to secure the hardness and hardenability described above. However, when the content of Mn is more than 2.0%, not only the workability is lowered but also the alloying cost increases, which may cause uneconomical results. .

Silicon (Si) is limited to 0.5% or less. Si in steel contributes to securing strength as a high solid solution strengthening element, and may inevitably remain in molten steel in the deoxidation process of steelmaking. However, in the case of exceeding 0.5%, fayalite may be easily generated during the hot rolling process to generate a red scale.

According to the present invention, in order to secure other materials and hardenability, one or more of the following elements may be additionally added in weight%.

Cu: 0.2% or less

B: 0.005% or less

Ni: 2.0% or less

Cr: 1.5% or less

Mo: 1.0% or less

Al: not more than 0.5%

Ti: 0.1% or less

Nb: 0.02% or less

V: 0.5% or less

Zr: 0.5% or less

Copper (Cu) may inevitably enter the tramp element in the mini-mill process and may be added to improve strength and toughness. However, it is preferable to control it to 0.2% or less because too much may cause cracking during hot rolling.

Boron (B) is very advantageous for securing the hardenability in a small amount, it is preferable to manage below that since addition of more than 0.005% does not affect the securing of further hardenability.

Nickel (Ni) is advantageous for securing hardenability and has an effect of suppressing cracking caused by Cu during hot rolling. However, since Ni is an expensive element and a large amount is added, there is a risk of cost increase, so it is preferable to manage it to 2.0% or less.

Although chromium (Cr) is added as a hardenability securing element, it is preferable to control it to 1.5% or less because too much oxidization causes surface internal oxidation.

Molybdenum (Mo) is added in order to increase the hardenability and improve the wear resistance, but it is preferably at most 1.0% because it is an expensive element.

Aluminum (Al) is advantageous in terms of solid solution strengthening and deoxidation, and inevitably enters the surface. However, it is preferable to control it to 0.5% or less because too much addition may cause inclusions in the steel and nozzle clogging during playing.

Titanium (Ti), niobium (Nb), vanadium (V), and zirconium (Zr) contribute to the improvement of strength by inhibiting ferrite generation and miniaturization of grains, but if too much, the cost-effectiveness is reduced. Therefore, it is preferable to manage the addition amount as mentioned above, respectively.

The present invention is composed of Fe and other unavoidable impurities in addition to the above components.

Using the molten steel composed of the above components will be described in detail a method for producing a high carbon hot rolled steel sheet of excellent surface quality according to the present invention.

As described above with reference to FIG. 1, the mini mill process is a process of manufacturing a hot rolled steel sheet through continuous casting, rough rolling, heating, finishing rolling, cooling, and winding. Characteristic technical configuration of the present invention is to control the operating conditions of each step of the hot rolling process to produce a high-carbon hot rolled steel sheet having a high surface quality of the target.

The continuous casting step is to produce a thin slab of 30 ~ 150mm, the casting speed is preferably to be 4.5 mpm or more. In general, the slab has a risk of segregation as the casting speed is slower, and the segregation is limited to 4.5mpm or more because of the high risk of material deviation.

The main technical configuration of the present invention is to install the descaler 15 at the rear end of the continuous casting machine 10 as shown in FIG. 1, and optimize the descaling conditions (spray pressure and installation distance) in connection with subsequent rough rolling temperature conditions. This minimizes the generation of arithmetic scales.

First, when the rough rolling temperature, more precisely, the surface temperature of the thin slab at the entrance of the rough mill is lower than 1050 ° C., the scale growth rate on the surface of the thin slab is low, so that there is little possibility of generating an arithmetic scale. Excellent surface quality can be secured without the need for connection. However, the surface temperature of the thin slab is preferably managed at 850 ℃ or more. If the rough rolling at a temperature lower than 850 ℃ not only burden the facility by increasing the rolling load significantly, but also because there is a risk of deterioration of the surface quality due to the edge cracks.

On the other hand, when the rough rolling temperature is higher than 1050 ℃, the growth rate of the scale is very fast on the surface of the slab is very likely to be inserted into the base material. Therefore, in this case, generation of arithmetic scale should be prevented in conjunction with descaling conditions.

As the first descaling condition, the continuously cast thin slab has a descaling injection pressure of 200 bar / mm ² or more and a distance between the descaler 15 and the roughing mill 20 entrance (hereinafter referred to as “installation distance”) 200 cm or less The rough rolling can be performed after descaling. The injection pressure of the descaler and the installation distance are in inverse proportion to each other. That is, when the injection pressure is high, the scale can be effectively removed even if the installation distance is somewhat longer, and when the injection pressure range is low, it is necessary to shorten the installation distance to increase the actual injection effect.

In this regard, as the second descaling condition applicable when the rough rolling temperature is higher than 1050 ° C, the continuous cast thin slab is descaled to 300 bar / mm ² or more, and the installation distance of the descaler 15 is 400 cm or less. The rough rolling can be performed after descaling.

The above two descaling conditions are optimum conditions that can prevent the surface scale from regrowing even after descaling and getting into the base material under a relatively high rough rolling temperature. At this time, the injection pressure of the descaler is supercooled thin slab so that the rolling load in the rough rolling process can be increased or decreased within the limit without affecting the material of the steel sheet. In addition, the installation distance of the descaler is determined by the layout of the equipment, the distance that can be as close as possible to the roughing mill is determined as the lower limit is not particularly specified.

Even when the rough rolling temperature is higher than 1050 ° C, it is desirable to manage not to exceed 1100 ° C. This is because not only the energy cost for the temperature rises but also the tendency for defects such as surface cracks to increase.

On the other hand, the rough rolling step is preferably such that the cumulative reduction rate at the time of rough rolling is 65 ~ 90%. The cumulative reduction rate at the time of rough rolling plays an important role in obtaining a product having a uniform material targeted in the present invention. In other words, the higher the reduction ratio during rough rolling, the more uniform the microscopic distribution of elements such as Mn, Si, etc. in the steel, the smaller the temperature gradient in the width direction and the thickness direction of the strip is very effective in obtaining a uniform material. However, if the cumulative reduction ratio is less than 65%, the above effects are not sufficiently exhibited. If the cumulative reduction ratio is greater than 90%, the resistance of the rolling deformation is greatly increased, thereby increasing the manufacturing cost, so that the cumulative reduction ratio is 65 to 90%. It is preferable to roll.

The finish rolling step heats and heats the roughly rolled hot rolled strip to a constant temperature and then finish rolls to the desired final thickness. According to the mini-mill process, the heated and heated hot rolled strip may be wound or finish rolled immediately.

It is preferable to make the rolling speed difference in one strip be 15% or less at the time of finish rolling. If the rolling speed difference exceeds 15% in the finishing mill consisting of multiple stands, it is difficult to obtain a uniform cooling rate and target winding temperature in the subsequent runout table, resulting in a large material deviation in the width or length direction of the strip. It causes the occurrence.

In the conventional blast furnace process, when reheating steels with carbon content of 0.2% or more, temperature variation in the width direction and length direction of the slab occurs in the heating furnace, which makes it difficult to control cooling in the runout table. It has been a major cause of directional and longitudinal material deviations. However, in the mini mill process of the present invention, the continuous cast thin slab is directly rolled without heating and cooling to room temperature, and is heated and maintained before finishing rolling, thereby entering the runout table at a uniform temperature of the strip. It is easy to control cooling and has the advantage of reducing material deviation in the width direction and the length direction.

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

Steel number C Si Mn Al Cr Mo P S N One 0.451 0.20 0.69 0.01 - - 0.003 0.003 0.008 2 0.849 0.21 0.41 0.01 - - 0.004 0.003 0.008 3 0.350 0.20 0.68 0.03 1.03 0.21 0.003 0.003 0.008 4 0.521 0.22 0.89 0.03 1.01 - 0.003 0.003 0.009

Steel number Slab
thickness
(mm) Week
(mpm) Roughening Insertion Side
Temperature
(℃) Injection pressure
(bar / mm ² ) Installation distance
(Cm) Arithmetic scale generation Inventive Steel 1 One 80 6.0 950 Unapplied Unapplied Not occurring Invention river 2 One 80 6.0 1000 Unapplied Unapplied Not occurring Invention steel 3 One 80 6.0 1030 Unapplied Unapplied Not occurring Inventive Steel 4 One 80 6.0 1100 200 150 Not occurring Invention steel 5 One 80 6.0 1100 300 250 Not occurring Inventive Steel 6 2 80 5.0 950 Unapplied Unapplied Not occurring Invention steel 7 2 80 5.0 1000 Unapplied Unapplied Not occurring Inventive Steel 8 2 80 5.0 1030 Unapplied Unapplied Not occurring Invention river 9 2 80 5.0 1100 200 150 Not occurring Invented Steel 10 2 80 5.0 1100 300 250 Not occurring Invention steel 11 3 80 5.8 950 Unapplied Unapplied Not occurring Invention steel 12 3 80 5.8 1000 Unapplied Unapplied Not occurring Invention steel 13 3 80 5.8 1030 Unapplied Unapplied Not occurring Invented Steel 14 3 80 5.8 1100 200 150 Not occurring Invented Steel 15 3 80 5.8 1100 300 250 Not occurring Invented Steel 16 4 80 5.5 950 Unapplied Unapplied Not occurring Inventive Steel 17 4 80 5.5 1000 Unapplied Unapplied Not occurring Inventive Steel 18 4 80 5.5 1030 Unapplied Unapplied Not occurring Inventive Steel 19 4 80 5.5 1100 200 150 Not occurring Inventive Steel 20 4 80 5.5 1100 300 250 Not occurring Comparative River 1 One 80 6.0 1080 Unapplied Unapplied Occur Comparative River 2 2 80 5.0 1080 Unapplied Unapplied Occur Comparative Steel 3 3 80 5.8 1080 Unapplied Unapplied Occur Comparative Steel 4 4 80 5.5 1080 Unapplied Unapplied Occur Comparative Steel 5 One 80 6.0 1100 200 400 Occur Comparative Steel 6 2 80 5.0 1100 200 400 Occur Comparative Steel 7 3 80 5.8 1100 200 400 Occur Comparative Steel 8 4 80 5.5 1100 200 400 Occur Comparative Steel 9 One 80 6.0 1100 300 450 Occur Comparative Steel 10 2 80 5.0 1100 300 450 Occur Comparative Steel 11 3 80 5.8 1100 300 450 Occur Comparative Steel 12 4 80 5.5 1100 300 450 Occur

As shown in Table 1, four steel grades belonging to the component system according to the present invention were prepared, and hot rolled steel sheets were prepared according to the operating conditions disclosed in Table 2, respectively. That is, the inventive steels 1 to 20 correspond to the rough rolling temperature and descaling conditions (injection pressure and installation distance) of the present invention, and the comparative steels 1 to 4 are 1050 where the rough rolling temperature is 1080 ° C. Although descaling was not carried out despite exceeding ℃, Comparative steel 5 to 12 is a case of descaling conditions out of the range of the installation distance, among them, the descaling conditions corresponding to the high rough rolling temperature.

As a result of the experiment, as shown in Table 2, the invention steels 1 to 20 did not generate an arithmetic scale, but Comparative Examples 1 to 12 all caused an arithmetic scale to cause a problem in surface quality.

10: continuous casting machine 15: descaler
20: roughing mill 30: heating means
40: coil box 50: finishing rolling mill
60: runout table 70: winder

Claims (6)

delete Continuous casting of molten steel consisting of balance Fe and other unavoidable impurities by weight slab of C: 0.2 to 1.3%, Mn: 0.2 to 2.0%, Si: 0.5% or less (excluding 0%). step;
After descaling the continuously cast thin slab to a descaling injection pressure of 200 bar / mm ² or more and a distance of 200 cm or less (except 0 cm) between the descaler and the roughing mill entrance side, the thin slab temperature at the entrance of the roughing mill is 1050 to Rough rolling to 1100 ° C .;
Finishing or rolling the rough rolled hot rolled strip after heating or heating; And
The method of manufacturing a high carbon hot rolled steel sheet using a mini-mill process, comprising: winding the finished rolled hot rolled strip on a runout table and then winding it up. Continuous casting of molten steel consisting of balance Fe and other unavoidable impurities by weight slab of C: 0.2 to 1.3%, Mn: 0.2 to 2.0%, Si: 0.5% or less (excluding 0%). step;
After descaling the continuously cast thin slab to a descaling injection pressure of 300 bar / mm ² or more and a distance of 400 cm or less (except 0 cm) between the descaler and the roughing mill entrance side, the thin slab temperature at the entrance of the roughing mill is 1050 to Rough rolling to 1100 ° C .;
Finishing or rolling the rough rolled hot rolled strip after heating or heating; And
The method of manufacturing a high carbon hot rolled steel sheet using a mini-mill process, comprising: winding the finished rolled hot rolled strip on a runout table and then winding it up. The method according to any one of claims 2 to 3,
The continuous casting step is a method of manufacturing a high carbon hot rolled steel sheet using a mini-mill process, characterized in that the casting speed is 4.5 mpm or more. The method according to any one of claims 2 to 3,
The rough rolling step is a method of manufacturing a high carbon hot rolled steel sheet using a mini-mill process, characterized in that the cumulative reduction rate during rough rolling is 65 ~ 90%. The method according to any one of claims 2 to 3,
The finishing rolling step is a method for producing a high carbon hot rolled steel sheet using a mini-mill process, characterized in that the rolling speed difference in one strip to 15% or less.

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