KR100356706B1 - Hot rolled steel sheet with excellent workability and weather resistance and manufacturing method - Google Patents

Abstract

In the present invention, C: 0.03% to 0.06%, Mn: 0.25% to 0.70%, Si: 0.4% or less, P: 0.05% to 0.15%, S: 0.006% or less, Cr: 0.6% or less, Ni: 0.4% or less, Cu: 0.5% or less, Sn: 0.001% to 0.04%, acid processing Al: 0.04% to 0.10%, N: 150ppm or less, Ca: 0.0002% to 0.008%, the rest is composed of Fe and inevitable impurities , Providing a hot rolled steel sheet with excellent workability and weather resistance satisfying the conditions of Mn / S ratio ≥30 (1) and Cu / Ni≥0.5 (2), and continuously dissolving steel satisfying the above-mentioned composition components and conditions Firstly rolling the slabs after casting them into slabs, heating the hot-rolled steel strip at a temperature of 1080 ° C. or lower in an induction furnace, and maintaining a coil box atmosphere temperature of 1080 ° C. or lower and a temperature of 870 ° C. or higher. Hot rolled steel excellent in workability and weather resistance by performing a finish rolling under the conditions and winding at a temperature of 550 ° C. or higher. The method of manufacturing the plate is presented.

Description

Hot rolled steel sheet with excellent workability and weather resistance and manufacturing method

The present invention relates to a hot rolled steel sheet having excellent workability and weather resistance, and a method for manufacturing the same, wherein the hot rolled steel sheet has excellent surface properties and weather resistance using a tramp element as a useful element for suppressing weather resistance and surface cracks and a high content of nitrogen as a strength reinforcement source. And to a method for producing the same.

Unlike the blast furnace method of steelmaking, the mini mill process melts scrap metal in an electric furnace and casts it into a thin slab of 100 mm or less using a continuous casting machine, and secures a rolling temperature through an intermediate facility to perform direct hot rolling. It is a friendly process. In recent years, the generation of scrap metal after automobile scrapping has been on the rise, and such scrap metal contains hard-to-refining tramp elements such as Cu, Sn, Pb, and Ni. When the scrap metal of automobiles is used as an iron source, the tramp elements remain in the steel, which increases the strength due to the tramp elements and increases the possibility of surface cracks due to the low melting point elements Cu, Sn, and Pb. Since segregation occurs in the grain boundary and the workability is lowered, proper control of these tramp elements is necessary.

However, the problem is that these tramp elements are poorly refined elements that cannot be refined during the refining process because they are thermodynamically stable compared to Fe. Therefore, it is necessary to classify scrap iron as an iron source and use scrap iron containing an appropriate tramp element as an iron source. In addition, in the electric furnace refining, unlike the refining using the converter method, it is difficult to control nitrogen introduced into the atmosphere by a facility that removes gas. In addition, since the nitrogen concentration in steel is higher than that of blast furnace, there is a problem in that yield strength and tensile strength increase and elongation increases.

Therefore, manufacturing hot rolled steel sheets for general processing in a mini mill process is at a disadvantage compared to blast furnace mills. As described above, efforts to change the weakness of the mini mill process to strengths of the mini mill process in comparison with the blast furnace mill are necessary, and as described above, it is necessary to use tram elements such as Cu, Ni, and Mo Cr contained in the scrap metal used as the iron source of the mini mill process. Steel grade development is needed.

The present invention solves the problems of the conventional mini-mill process, hot-rolled steel sheet using the elements such as Cu, Cr, etc. contained in the scrap metal as a corrosion resistance element, Ni as a surface crack suppression element, high concentration nitrogen in the steel as a strength increase source And its manufacturing method.

The present invention provides a hot rolled steel sheet excellent in workability and weather resistance by appropriately controlling the alloy component of steel and its content, heating temperature after primary rolling and coil box holding conditions, finishing rolling and winding temperature.

Hot rolled steel sheet according to the present invention by weight% C: 0.03% to 0.06%, Mn: 0.25% to 0.70%, Si: 0.4% or less, P: 0.05% to 0.15%, S: 0.006% or less, Cr: 0.6% Ni: 0.4% or less, Cu: 0.5% or less, Sn: 0.001% to 0.04%, acid processing Al: 0.04% to 0.10%, N: 150 ppm or less, Ca: 0.0002% to 0.008%, the rest is Fe and inevitable It is composed of impurities and satisfies the conditions of the ratio of Mn / S ≧ 30 (1) and Cu / Ni ≧ 0.5 (2).

Hot rolled steel sheet according to the present invention by weight% C: 0.03% to 0.06%, Mn: 0.25% to 0.70%, Si: 0.4% or less, P: 0.05% to 0.15%, S: 0.006% or less, Cr: 0.6% Ni: 0.4% or less, Cu: 0.5% or less, Sn: 0.001% to 0.04%, acid processing Al: 0.04% to 0.10%, N: 150 ppm or less, Ca: 0.0002% to 0.008%, the rest is Fe and inevitable It is composed of impurity, melting the steel that satisfies the condition of the ratio of Mn / S ≥ 30 (1), Cu / Ni ≥ 0.5 (2) and then cast continuously to make a slab and to firstly roll the slab and Heating the hot-rolled steel strip at a temperature of 1080 ° C. or lower in an induction furnace, maintaining a coil box atmosphere temperature of 1080 ° C. or lower, and performing finish rolling at a temperature condition of 870 ° C. or higher, and winding at a temperature of 550 ° C. or higher. It is prepared by taking the step of taking.

1 is a graph depicting temperature trends in a minimill process.

Hot rolled steel sheet according to the present invention by weight% C: 0.03% to 0.06%, Mn: 0.25% to 0.70%, Si: 0.4% or less, P: 0.05% to 0.15%, S: 0.006% or less, Cr: 0.6% Ni: 0.4% or less, Cu: 0.5% or less, Sn: 0.001% to 0.04%, acid processing Al: 0.04% to 0.10%, N: 150 ppm or less, Ca: 0.0002% to 0.008%, the rest is Fe and inevitable It is composed of impurities and satisfies the conditions of the ratio of Mn / S ≧ 30 (1) and Cu / Ni ≧ 0.5 (2).

The reason which limited each component which comprises a hot rolled sheet steel as above is demonstrated.

1. C is an invasive element contained in steel and is an element for improving strength. However, continuous casting for producing thin slabs of 100 mm or less is difficult to manufacture flawless casts because the slabs are thinner and the casting speed is faster than conventional casting methods. In particular, when C is more than 0.06%, slab shrinkage occurs during the solidification reaction during solidification, and part is released from the copper casting mold to suppress heat transfer. At this time, the solidification layer is re-dissolved and the reaction solid layer is destroyed, which causes the cast iron to rupture. Therefore, the content of C in the production of low carbon steel by the thin slab casting method was limited to 0.06% or less.

2. Mn is an element which is contained in steel and improves strength. It fixes S, which is an impurity in steel, with MnS to suppress cracks caused by S compound which is a low melting point compound during hot rolling. In the thin slab casting method, since the intermediate rolling is performed at a high temperature immediately after continuous casting, edge cracking due to the S compound is more serious than the conventional casting method. Therefore, the Mn / S ratio is set to 30 or more.

3. Si is an element contained in steel to improve strength and weather resistance. Since Si is associated with hot rolled scale formation, it is limited to 0.4% or less. In steels containing a large amount of Si, fayalite is formed in a continuous casting process and a high-temperature furnace atmosphere, and melts at a melting point of 1,180 ° C. or higher of the Si compound to exist on the scale and the ferrous interface. Therefore, it is difficult to remove the scale when the hot rolled scale is removed by water sand, and the element remains in the form of primary scale or red scale in the final hot rolled steel sheet, thereby deteriorating the surface shape. However, it is limited to 0.4% or less because it is an efficient element for improving weather resistance and strength.

4. P is an effective element contained in steel to increase weather resistance. P contained in the steel acts as an accelerating element to amorphize rust generated when the hot rolled steel sheet is rusted by atmospheric exposure, and inhibits corrosion by inhibiting supply of moisture and oxygen to the rust layer. However, P contained in steel forms a low melting process compound such as Fe ₃ P during continuous casting, which acts as a factor in deepening the polarization rate during continuous casting, thereby decreasing weldability. Therefore, it was limited to 0.05% to 0.15% to secure the weldability while increasing the weather resistance.

5. Cu, together with P, is an effective element that increases the weather resistance of steel. Cu contained in steel suppresses rust generation by thermodynamic stability during atmospheric exposure, or acts as an element to amorphize rust generated, thereby further suppressing water and oxygen supply to the rust layer generated and inhibiting corrosion. However, because Cu is thermodynamically stable compared to Fe when heated, oxidation does not proceed even when the Fe base layer is oxidized to form a concentrated Cu layer between the hot rolled scale layer and the branch iron. When the thickened Cu layer has a low melting point and hot deformation is performed above the melting point, the liquid Cu layer invades the grain boundaries first, and the infiltrated grain boundaries in which the liquid phase is infiltrated are accompanied by hot working cracks as cracks are generated by stress.

Therefore, the surface cracking by the selective oxidation and low melting point Cu thickening layer is proposed to adjust the Cu content, but since Cu is an excellent element for improving weather resistance, it is hot due to the increase in the melting temperature of the thickening layer due to the addition of Ni. There is a process crack suppression method. Therefore, in the present invention, the content of Cu is limited to 0.5% or less so as to have the best weather resistance.

6. Ni is an element that suppresses hot surface cracks due to the formation of a low melting concentration layer by selective oxidation of Cu and the like. Cu, Sn, Pb, etc. remaining in the scrap iron is thermodynamically stable compared to Fe, iron is selectively oxidized during the oxidation of the iron, these elements are concentrated and concentrated on the scale and the surface of the iron. The concentrated layer of these elements is a low melting point material, which exists in a molten state at a hot rolling temperature, and induces hot cracks by eroding austenite grain boundaries when the deformation is accompanied. The low melting point thickening layer is electrothermally dissolved when Ni is added, thereby increasing the melting point of the thickening layer and thus acting as a solid at hot deformation temperature, thereby suppressing hot cracking. However, when Ni exceeds 0.04%, the action reaches a saturation point, thereby losing economical efficiency. Therefore, Ni content was limited to 0.4% or less. When the Ni content increases, the shape between the scale and the iron is rough, which hinders descaling.

7. Al acts as a deoxidizer in the steel refining process and lowers the oxygen content in the steel. The extra Al in the nitrogen is fixed to AlN to suppress the aging phenomenon, lowering the yield strength, and to suppress the occurrence of coarse grains due to abnormal grain growth during winding. Therefore, the acid value Al was limited to 0.03% to 0.10%.

8. Ca acts to suppress the clogging of the playing nozzle by adjusting the shape of the inclusions. In the case of thin slab continuous casting, nozzle clogging is serious because the diameter is smaller than that of the conventional blast furnace mill. Therefore, the playability is improved by adding Ca to spherical inclusions, and the content thereof is preferably 0.002% to 0.08%.

9. S is an impurity which forms an emulsion-based inclusion and degrades workability. In addition, in the thin slab casting method, an element causing edge cracks is formed by forming an emulsion-based low melting point compound during hot working. In addition, in the mini mill process, Cu is precipitated as a compound of the Cu ₂ S-based steel to suppress the effect of improving the weather resistance of Cu. Therefore, it is good to fix MnS by Mn or to manage S below 80ppm.

By adjusting the component range as described above it is possible to manufacture a hot rolled steel sheet excellent in workability and weather resistance in the mini-mill process. In addition, it is possible to produce hot rolled steel sheet having good surface quality by suppressing occurrence of hot surface cracks due to Cu during hot rolling.

The hot rolled steel sheet having the above characteristics is manufactured through the following process.

The steel whose composition is adjusted within the above composition range is dissolved in an electric furnace, continuously cast into slabs in a thin slab continuous casting machine, and the slabs are intermediately rolled above the Ar3 temperature. Then, in order to ensure a finishing temperature of 950 ℃ or less in the induction heating furnace heated to 1,100 ℃ or less, coiled in a bar coil state in a coil box of 1,080 ℃ or less temperature, and finish rolling at an Ar3 temperature or more. Thereafter, the hot rolled coil is wound at a temperature in the range of 550 ° C to 650 ° C. The temperature in each process as mentioned above was set for the following reason.

1. Medium rolling temperature

The intermediate rolling process is a process of firstly rolling a continuously cast slab to a bar state, and affects the final quality of the hot rolled steel sheet according to the hot working state of the bar. Hot work cracks, which tend to appear in the mid-zinc temperature range, cause surface cracks in the final hot rolled steel sheet, or edge cracks induce saw saws and the like. If the intermediate rolling process is carried out below the Ar3 temperature, the stress is concentrated in the transformed ferrite first, causing cracks at the ferrite and austenite interface, which may cause edge cracks or surface cracks. Therefore, intermediate rolling is carried out above the Ar3 temperature.

2. Heating temperature of induction furnace

In the mini mill process, the heating process through an induction furnace is an important process because it has a great influence on the material and the surface as a process for securing an appropriate finishing rolling temperature. After the intermediate rolling, it is necessary to heat the cooled bar to 1,100 ° C. or less in order to secure the finish rolling temperature and to suppress the surface cracks caused by the low melting concentrated layer produced by selective oxidation. Therefore, in the present invention, the heating temperature of the induction furnace is set to 1,100 ℃ or less.

3. Coil box heating temperature

In the mini mill process, a coil box facility is used to wind up the intermediate rolled bar in the bar state to drastically reduce the space between the intermediate rolling and the finish rolling process and to suppress the bar coil temperature drop. The coil box temperature should be properly controlled because it determines the bar temperature and affects the dissolution of the low melting thickening layer that occurs after surface selective oxidation. It is necessary to heat the coil box to 1,080 ° C. or lower in order to secure a proper finishing temperature and to suppress dissolution of the low melting concentration layer.

4. Finish rolling temperature

In the present invention, finish rolling is performed at an Ar3 temperature or more. If the finish rolling temperature is less than the Ar3 temperature, it is a strained structure, since coarse grains and mixed grains remain, which impairs workability. Therefore, the lower limit of finish rolling temperature was set to Ar3 temperature.

5. Winding temperature

In order to ensure the favorable weather resistance and workability of the hot rolled steel sheet which finish-rolled, it winds up at the temperature of 550 degreeC or more and 650 degrees C or less.

1 is a graph showing the temperature in each of the above-described processes, wherein the change in temperature shows the change in temperature.

Example

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

Table 1 shows the components of the inventive steel and the comparative steel.

division number Chemical content (% by weight) Remarks C Si Mn P S S.Al N Cu Ni Cr Mo CA Invention One 0.04 0.3 0.4 0.1 0.005 0.04 0.01 0.3 0.3 0.4 0.01 0.003 standard 2 0.04 0.4 0.3 0.08 0.005 0.04 0.01 0.4 0.3 0.4 0.01 0.003 Low P 3 0.05 0.3 0.4 0.1 0.005 0.04 0.01 0.3 0.2 0.6 0.01 0.003 High Cr 4 0.04 0.3 0.3 0.1 0.005 0.04 0.008 0.3 0.3 0.4 0.01 0.003 Cold winding Comparative example One 0.05 0.03 0.3 0.1 0.005 0.04 0.01 0.3 - 0.6 0.01 0.003 Surface crack 2 0.12 0.3 0.3 0.08 0.005 0.04 0.012 0.3 0.3 0.4 0.01 0.003 Not casting 3 0.06 0.01 0.3 0.01 0.008 0.04 0.01 0.1 0.05 0.05 0.01 0.003 SPHD

division number Medium rolling temperature Induction heating temperature Coil box temperature Finish rolling temperature Coiling temperature TS (kgf / mm ² ) YP (kgf / mm ² ) EI (%) Weather resistance Surface quality Inventive Example One 920 1070 1050 870 600 54.5 31 ○ ○ 2 910 1070 1050 870 620 53.5 32 ○ ○ 3 920 1060 1050 870 600 54.0 31 ○ ○ 4 910 1070 1050 870 550 55.0 30 ○ ○ Comparative example One 920 1070 1050 870 600 54.0 31 ○ × 2 920 1070 1050 870 600 58.0 26 ○ × 3 920 1080 1060 880 640 × ○

(Temperature unit: ℃)

Table 2 is a table showing the temperature conditions in the process of manufacturing the inventive steel and comparative steel each having the components of Figure 1, the tensile-stretching characteristics of the inventive steel and comparative steel (hot-rolled steel sheet) prepared under the above conditions, Weather resistance and surface quality are shown, respectively.

At this time, the tensile test was carried out by collecting the JIS No. 5 tensile test specimen in the C direction, and the weather resistance characteristics were measured by the corrosion promoting test method and compared with the general steel. In addition, the surface quality was determined by the presence of surface cracks occurring on the hot-rolled surface.

As can be seen from Table 2, the steel of the present invention basically secured tensile strength ≥ 53kgf / mm ² , elongation ≥ 30% in the state of no surface cracks, weather resistance is more than twice as good as the comparative steel .

Invented steel 1 manufactured by adding Cu and P, which are weather resistance enhancing elements based on low carbon component, and Ni, as a method of suppressing surface cracks, is a hot-rolled steel sheet having good weatherability and workability. Good characteristics were also exhibited in the invention steel 2, the invention steel 3 with the Cr component increased, and the invention steel 4 with low temperature winding.

On the other hand, Comparative Steel 1, which has a large number of surface cracks, and Comparative Steel 2, in which casting defects are caused by a shot reaction, cannot be manufactured in a mini mill process. Comparative steel 3 is a general steel and shows poor weather resistance.

The present invention as described above can produce a hot rolled steel sheet excellent in weatherability and workability by appropriate combination of steel components and appropriate control of continuous rolling process conditions. Compared to the existing blast furnace mills, by actively utilizing the characteristics of mini mills using scrap steel, it is possible to obtain a competitive effect by manufacturing a competitive hot rolled steel sheet for blast mills.

Claims (2)

By weight% C: 0.03% to 0.06%, Mn: 0.25% to 0.70%, Si: 0.4% or less, P: 0.05% to 0.15%, S: 0.006% or less, Cr: 0.6% or less, Ni: 0.4% or less , Cu: 0.5% or less, Sn: 0.001% to 0.04%, acid processing Al: 0.04% to 0.10%, N: 150 ppm or less, Ca: 0.0002% to 0.008%, the remainder is composed of Fe and inevitable impurities, Mn / A hot rolled steel sheet excellent in workability and weather resistance satisfying the conditions of the ratio S of 30 (1) and Cu / Ni ≥ 0.5 (2). By weight% C: 0.03% to 0.06%, Mn: 0.25% to 0.70%, Si: 0.4% or less, P: 0.05% to 0.15%, S: 0.006% or less, Cr: 0.6% or less, Ni: 0.4% or less , Cu: 0.5% or less, Sn: 0.001% to 0.04%, acid processing Al: 0.04% to 0.10%, N: 150 ppm or less, Ca: 0.0002% to 0.008%, the remainder is composed of Fe and inevitable impurities, Mn / After melting the steel that satisfies the conditions of the ratio ≥ 30 (1), Cu / Ni ≥ 0.5 (2) and casting continuously to produce a slab, the slab is first rolled, 1080 ℃ in an induction furnace Workability and weather resistance consisting of heating the hot rolled steel strip at the following temperature, maintaining the coil box atmosphere temperature at 1080 ° C or less and performing finish rolling at a temperature condition of 870 ° C or higher, and winding at a temperature of 550 ° C or higher. This excellent hot rolled steel sheet manufacturing method.