KR101175413B1 - Continuous casting method for rolled steel products - Google Patents

Abstract

The present invention relates to a continuous casting method of the slab for hot-rolled steel sheet for crack reduction to control the composition of the molten metal produced in the blast furnace in the steelmaking process to suppress the occurrence of horizontal and corner cracks during continuous casting.

The steel material of the present invention is a weight%, C: 0.15 to 0.20%, Mn: 0.5 to 2.0%, Si: 0.01 to 0.5%, Nb: 0.005 to 0.1%, Ti: 0.02 to 0.05%, P: 0.02% or less , S: 0.01% or less, Al: 0.02 ~ 0.04%, B: 0.0010 ~ 0.0040%, Cu: 0.02% or less, Ni: 0.2% or less, including at least one of the remaining residues, Fe and other unavoidable impurities It is created.

Accordingly, the present invention has a useful effect of suppressing the occurrence of cracks in the slab in the straightening temperature range during continuous casting to improve the quality of the product.

Description

Continuous casting method of slab for hot rolled steel plate for crack reduction {CONTINUOUS CASTING METHOD FOR ROLLED STEEL PRODUCTS}

The present invention relates to a continuous casting method of slabs for hot-rolled steel sheets for crack reduction, in particular, by controlling the composition of titanium and boron to reduce the crack sensitivity of the austenite grain boundaries during grain boundary ferrite transformation due to continuous casting operation and increase the rate of cross-sectional reduction It is to be able to suppress.

Generally, liquid molten steel is cooled while passing through a continuous casting facility and solidified into a solid slab.

In the continuous casting facility, the molten steel in the liquid is solidified by 20% while passing through the mold, and then the remaining 80% is completely cooled as it passes through the strand to become a solid slab as described above.

Therefore, in the continuous casting method, the thermal stress due to the temperature gradient occurring on the inside and the surface of the slab due to the rapid cooling and the mechanical stress due to the periodic contact between the slab and the roll are combined to form various forms on the inside and the surface of the slab. There is a problem that cracks occur.

Particularly, in the case of the vertical curved player, the slab that was bent in the temperature range (700 to 900 ° C.) to be straightened is stretched, and the tensile stress acts on the upper surface and the compressive stress acts on the lower surface.

Therefore, in the case of low-temperature ductility steel, surface cracks tend to be formed in the transverse direction of the cross section at the upper and corners of the slab by the tensile stress of the upper surface, and the cracks generated at this time are called horizontal cracks and corner cracks.

Therefore, these cracks are generated by combining the characteristics of the facility and the high temperature properties of the steel material, and tight control is required in terms of manufacturing conditions and alloy design to suppress cracking.

In addition, these cracks generated in the straightening section cause plate breakage during the hot rolling process, and thus, a process of removing the surface cracks by using a torch or the like after cooling the slab to room temperature is required to prevent this. And there is a problem that the cost rise and production efficiency is lowered by the energy loss.

Lateral and corner cracks are mostly progressed along grain boundaries in steel grades with weak grain boundaries, and occur frequently in steels with precipitation-forming elements such as niobium (Nb), vanadium (V), and aluminum (Al).

In particular, the occurrence frequency of 50kg or more high strength steel containing niobium 0.02 ~ 0.05wt% is very high.

In the continuous casting process, cracks generated in the straightening region where the slab is spread correspond to grain boundary fractures that progress along the grain boundaries, which is caused by the decrease in the ductility of the hot ductile embrittlement section of the steel.

Therefore, in order to suppress the occurrence of cracks, it is necessary to improve the ductility in the embrittlement section, and the decrease in the ductility in the embrittlement section is caused by the concentration of strain in the grain boundary ferrite formed at the grain boundary, thus improving the characteristics of the grain boundary ferrite. It is important.

Among them, in the case of niobium alone-added steel, niobium carbonitride which is first precipitated at the grain boundary tends to form voids, and since niobium carbonitride precipitated at the base strengthens the matrix, the concentration of grain boundary strain is increased to increase the crack generation rate.

The present invention has been proposed to solve the above problems, the purpose of which is to reduce the cracks to reduce the horizontal and corner cracks of the slab by improving the high temperature cross-sectional reduction rate during continuous casting of 60kg / mm2 high strength steel sheet It is to provide a continuous casting method of slabs for hot rolled steel sheet for.

The present invention for achieving the above object by weight%, C: 0.15 ~ 0.20%, Mn: 0.5 ~ 2.0%, Si: 0.01 ~ 0.5% or less, Nb: 0.005 ~ 0.1%, Ti: 0.02 ~ 0.05%, P: 0.02% or less, S: 0.01% or less, Al: 0.02 ~ 0.04%, B: 0.0010 ~ 0.0040%,

Cu: 0.02% or less, Ni: 0.2% or less of one or two,

The remaining balance is characterized in that steel grades composed of Fe and other unavoidable impurities are manufactured into slabs through a continuous casting process.

The reduction rate of the cross section of the slab is 50% or more capable of continuous casting.

The present invention is to control the composition of the molten metal produced in the blast furnace in the steelmaking process to suppress the occurrence of horizontal and corner cracks during continuous casting, according to the present invention is crack generation of the slab in the straightening temperature range during continuous casting It has a useful effect to improve the quality of the product by suppressing.

Hereinafter, preferred embodiments of the present invention will be described in detail.

First, the steel material of the present invention in weight%, C: 0.15 to 0.20%, Mn: 0.5 to 2.0%, Si: 0.01 to 0.5%, Nb: 0.005 to 0.1%, Ti: 0.02 to 0.05%, P: 0.02 % Or less, S: 0.01% or less, Al: 0.02 to 0.04%, B: 0.0010 to 0.0040%,

Cu: 0.02% or less, Ni: at least one of 0.2% or less,

The remainder is composed of Fe and other unavoidable impurities.

The continuous casting process of the steel grade material having such a composition is performed by refining the water produced through the blast furnace process in ladle through the steelmaking process to adjust to have the alloying element as described above, and removing impurities, followed by a tundish and mold as a continuous casting facility. Pass through and cooled to produce a semi-finished slab slab.

At this time, the process of cooling into a slab in the continuous casting process is cooled while some solidified molten steel passing through the mold is transferred through a plurality of rolls.

Hereinafter, the components of the alloying elements of the steel material of the present invention and the critical significance thereof will be described.

Carbon [C]: 0.15 ~ 0.20wt%

Carbon is an indispensable element to give high strength to the steel sheet, and depending on its content and manufacturing method, carbon may be a solid solution carbon in the material structure, and carbon may be combined with elements having a very high bond property to carbon to form carbide.

Carbon is used in an amount of 0.15 to 0.20% by weight, and in this case, when the amount of carbon used is less than the above range, there is a problem that the strength is lowered.

Manganese [Mn]: 0.5-2.0 wt%

Manganese is an element that improves the hardenability and increases strength. It suppresses the formation of pearlite phase and promotes austenite formation and carbon enrichment inside, contributing to the formation of residual austenite.

If the manganese content is less than 0.5%, a very fast cooling rate is required, and industrially it is impossible to prevent the production of pearlite.If the manganese content exceeds 2.0%, manganese band structure is formed and segregation increases rapidly, which impairs the workability and weldability of the steel. Regulate in the range of 0.5 ~ 2.0%.

Silicon [Si]: 0.01 to 0.5wt%

Silicon is a ferrite stabilizing element employed in ferrite, which contributes to strength, and the most important role is deoxidation. When the addition amount is less than 0.01wt%, the effect of improving strength is insignificant, and when it is added more than 0.5%, the weldability is lowered.

Niobium [Nb]: 0.005 to 0.1 wt%

Niobium is a strong carbonitride-forming element in steel and is the most important element added to control microstructure with carbon in high-strength low-alloy steel. Formation of precipitated phase at high temperature suppresses recrystallization of austenite and makes grains fine. Form a phase to increase strength.

The precipitation temperature depends on the manufacturing conditions and other additives, and the precipitation behavior is mainly explained by the solid solution of these components as the re-precipitated elements are dissolved at the stocking temperature for hot rolling.

Niobium cannot achieve the target strength when added below 0.005%, and lowers the low temperature toughness when added above 0.1%, and therefore, the range of 0.005 to 0.1 wt% is preferable.

Titanium [Ti]: 0.02 ~ 0.05wt%

Titanium is an element which prevents AlN precipitation which forms TiN in steel and reduces hardenability. It is preferable to contain Ti: 0.03 to 0.05% for the composition having N: 0.008 to 0.012%, and to contain Ti: 0.02 to 0.03% for the composition having N: 0.004 to 0.008% through the VD-OB process. Do.

In the present invention, titanium is added to niobium-added steel for the purpose of suppressing the occurrence of slab cracks, thereby improving the high temperature ductility. This is because the niobium precipitated phase is coarsened by the addition of titanium, and the gap between the precipitated phases increases, thereby reducing the formation of voids.

Boron [B]: 0.0010-0.0040 wt%

Boron is an element that retards austenite-ferrite transformation in steel and has a great influence on the properties of ferrite. Due to this property, even when only a small amount is added to the steel, coarse boron carbide (Fe ₂₃ (B, C) ₆ ) precipitates are formed in the austenite grain boundary or in the mouth, which becomes a nucleation site during the austenite-ferrite transformation. As the elongation increases, the slag cracks are reduced during continuous casting.

Among other elements,

P: 0.01 wt% or less, S: 0.01 wt% or less

In the steel grade material of the present invention, phosphorus (P) is preferably managed at 0.020% by weight or less as an impurity. Sulfur (S) exhibits brittleness in the structure of the material, causing defects in the material, so it is desirable to strictly limit to 0.01% by weight or less. In addition to the above components, the other components together with other unavoidable impurities are made of iron (Fe).

Al: 0.02 ~ 0.04wt%

Aluminum (Al) is an element that functions as a deoxidizer, and when it is added below 0.02 wt%, it is difficult to expect a deoxidation effect, and when it is added above 0.04 wt%, deterioration of workability decreases productivity and cracks due to AlN precipitates. In order to generate | occur | produce, it is preferable to regulate in 0.02-0.04 wt%.

Moreover, 1 type or 2 types of Cu: 0.2 wt% or less and Ni: 0.2 wt% or less are included.

In the steel material of the present invention, Cu and Ni are preferably managed at 0.02% or less as impurities.

The slab, which is a semi-finished product manufactured by continuously casting a steel material having the above composition, can be evaluated for crack sensitivity as a cross-sectional reduction rate through the following test.

In order to accurately simulate the thermal-stress history of the continuous casting process in which solidification and solidification deformation simultaneously occur for high temperature tensile test specimens, a gleeble tester that can simultaneously control temperature and deformation is used.

The test conditions were the heating / maintenance up to the temperature near the liquid line, and the tensile reduction was evaluated at the same time as cooling, and in detail, the tensile test was held at 1350 ° C. for 5 minutes, and then maintained at the test temperature for 5 minutes and tensioned. After tensioning, quench with compressed air. The section reduction rate measured the diameter of the fracture surface end before and after the test.

Table 1 below shows the main composition of the alloying elements contained in Examples and Comparative Examples of the present invention.

(Unit is wt%) division C Mn Nb Ti B N (ppm) Remarks existing 0.18 1.5 0.5 - - 60 Comparative Example 1 0.18 1.5 0.5 - 0.002 60 Comparative Example 2 0.18 1.5 0.5 0.03 - 60 Example 0.18 1.5 0.5 0.03 0.002 60

As a result of testing the prior art, the comparative example, and the embodiments of the present invention having such a composition, as shown in FIG. 1, in the prior art, as described above, the rate of cross-sectional reduction decreases in a straightened temperature range, resulting in cracking. In addition, boron is added alone, so the section reduction rate is good in sections other than the linearization temperature section, but the section reduction rate is lowered in the straightening temperature section.

On the other hand, in Comparative Example 2 in which titanium was added, the reduction rate of the cross section was improved compared to Comparative Example 1 described above, but crack sensitivity was not satisfactory in the straightening temperature range.

On the contrary, the embodiment of the present invention in which titanium and boron are added has a cross-sectional reduction rate of more than 70% by increasing the cross-sectional reduction rate of the straightening temperature range and decreasing the crack sensitivity, exceeding 50% of the cross-section reduction rate that can be continuously cast.

2 to 5 are photographs showing the test results of the respective test pieces for the conventional technology described above and Comparative Examples 1, 2 and Examples of the present invention.

As described above, the embodiment of the present invention can obtain good results in which no crack is generated regardless of the test temperature.

1 is a graph showing the cross-sectional reduction rate for each temperature of the Examples and Comparative Examples of the present invention.

Figure 2 is a photograph showing the test results of the test piece prepared by the existing technology for each temperature,

3 and 4 are photographs showing the test results for each temperature of the test piece corresponding to Comparative Examples 1 and 2,

Figure 5 is a photograph showing the test results for each temperature of the test piece corresponding to the embodiment of the present invention.

Claims (2)

By weight%, C: 0.15 ~ 0.20%, Mn: 0.5 ~ 2.0%, Si: 0.01 ~ 0.5%, Nb: 0.005 ~ 0.1%, Ti: 0.02 ~ 0.05%, P: 0.02% or less, S: 0.01% or less , Al: 0.02 ~ 0.04%, B: 0.0010 ~ 0.0040%, Cu: 0.02% or less, Ni: 0.2% or less of one or two, A continuous casting method of slabs for hot rolled steel sheets for crack reduction, which includes manufacturing steel grades having a balance of Fe and other unavoidable impurities into slabs through a continuous casting process. The method according to claim 1, Continuous casting method of the slab for hot-rolled steel sheet for crack reduction, characterized in that the cross-sectional reduction rate of the slab is 50% or more capable of continuous casting.

Images