KR100406413B1 - Method of continuous casting weathering steel for cold rolling - Google Patents

Abstract

The present invention relates to a manufacturing method of weathering steel used as a material for industrial boilers and heat exchangers, the object of the present invention is a casting method that can solve the occurrence of surface defects by thinning (cold rolled) the existing thick material (hot rolled material) It is about.

The present invention for achieving the above object, in the weight%, C: 0.06 to 0.08%, Mn: 0.90 to 1.0%, Si: 0.17 to 0.24%, P: 0.02% or less, S: 0.01% or less, Cu: 0.2 Managing the superheat degree in the tundish at 17 to 23 ° C. for molten steel composed of ˜0.3%, Ni: 0.2 to 0.3% and the remaining Fe and other inevitable impurities;

Injecting the molten steel into a mold having a taper set at 1.00 to 1.05% and casting at a speed of 1.12 to 1.17 m / min;

The present invention relates to a method for casting cold-rolled weathering steel, comprising: cooling the cast steel passed through the mold to a non-aqueous quantity of 0.29 to 0.35 l / kg in a secondary cooling zone.

Description

Method of continuous casting weathering steel for cold rolling

The present invention relates to a manufacturing method of weathering steel used as a material for industrial boilers and heat exchangers, and more particularly, casting which can solve the occurrence of surface defects due to thinning of a conventional thick material (hot rolled material) (cold rolled material). It is about a method.

In the early period when weathering steel is exposed to the atmosphere, rust occurs similarly to ordinary steel, but with the passage of time, part of the rust gradually adheres to the base metal to form dense rust, and the rust layer acts as a protective film to the environment, thereby inhibiting corrosion progress. Compared to ordinary steel, the resistance to atmospheric corrosion is much better. Atmospheric corrosion resistance is almost dependent on chemical composition rather than microstructure or manufacturing process, and corrosion resistance (C.I) according to chemical composition is defined in Equation 1 below in ASTM G101.

[Relationship]

CI = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) -7.29 (% Cu) (% Ni) -9.10 (% Ni) (% P ) -33.39 (% Cu) ²

Weather-resistant steel is also used for industrial boilers and heat exchangers, and thick materials (hot rolled materials) are all used. The component type is typical of steel grades containing C: 0.09%, Mn: 0.3-0.5%, Si: 0.4-0.5%, and weathering enhancing elements P, Cu, and Ni. Cu is required to be added in terms of weather resistance, but it is infiltrated at the grain boundaries during high temperature heating and thus acts as a cause of cracking during high temperature processing. Therefore, an appropriate amount of Ni is added to produce a high melting point copper compound to prevent high temperature cracking.

Recently, a thin material (cold rolled material) is also required for an industrial boiler or a heat exchanger, but when a cold rolled thick material (hot rolled material) is manufactured as a thin material, cracks and surface defects occur, and thus, actual production is not possible. . This is pointed to because of the surface defects in the continuous casting cast. There are surface defects in the continuously cast slab, and the surface defects are scarfed in order to prevent the surface defects from occurring due to defects such as a cap during hot rolling. Scarping is not a problem in hot rolled materials, but cold rolling is different. At present, the solution is to fundamentally lower the surface defect index of cast steel (about 0.1-0.2).

In the continuous casting method of weather resistant steel containing C: 0.09%, Mn: 0.3-0.5%, Si: 0.4-0.5%, and P, Cu, Ni, the casting speed is set to 1.2 (m / min) in consideration of productivity. In addition, the specific quantity in the secondary cooling stand is also adjusted to 0.47-2.49 (ℓ / kg). The secondary cooling zone is a section for cooling the slab that has passed through the mold (primary cooler), and is a zone for cooling the slab through direct spraying on the slab.

The present invention aims to reduce the surface defects in the cast steel of weathering steel used as a material for heat exchangers or industrial boilers, and to make the steel thinner. It was completed as a derivation.

1 is a Fe-C system equilibrium diagram

2 is a graph showing the relationship between carbon concentration and austenite particle size.

3 is a schematic diagram of a continuous casting mold;

Casting method of the present invention for achieving the above object, in weight%, C: 0.06 to 0.08%, Mn: 0.90 to 1.0%, Si: 0.17 to 0.24%, P: 0.02% or less, S: 0.01% or less, Managing the superheat degree in the tundish at 17 to 23 ° C. for molten steel comprising Cu: 0.2 to 0.3%, Ni: 0.2 to 0.3%, and remaining Fe and other inevitable impurities;

Injecting the molten steel into a mold having a taper set at 1.00 to 1.05% and casting at a speed of 1.12 to 1.17 m / min;

And cooling the slab conveyed through the mold to a non-aqueous quantity of 0.29 to 0.35 l / kg in a secondary cooling zone (secondary cooling zone).

Hereinafter, the present invention will be described in detail.

The present invention is a method for thinning weathering steel, and has the greatest feature in deriving casting conditions for minimizing surface defects of cast steel. In parallel with this, the components of C, Mn, and Si are adjusted in the steel component system so as to suppress the occurrence of surface cracks of the cast steel, which will be described first.

In the present invention, the steel is basically made of weather resistance elements Cu: 0.20 to 0.30%, Ni: 0.20 to 0.30%, and P: 0.020% or less to ensure an atmospheric corrosion resistance index of 5.5 or more. In addition, while adjusting the components of C and Si affecting crack induction, the components of Mn were upwardly adjusted to secure a tensile strength of 45 kg / mm 2.

C is indispensable in terms of strength compensation of the material because high strength compensation effect can be obtained even by adding a small amount. However, it is important to add an appropriate amount because excessive addition is a direct cause of casting cracks during continuous casting. As shown in the Fe-C-based equilibrium diagram of FIG. 1, as the molten steel solidifies, the phase transformation is performed from the ferrite (δ) at high temperature (1450 ° C.) to the austenite (γ) phase while the particle size is reduced. Coarsening will cause cracks in the cast. 2 shows the relationship between the carbon concentration and the particle size of austenite. Therefore, in the present invention, in consideration of the effect of C on the strength and surface cracks, it is strictly managed at 0.06 to 0.08%.

In the present invention, the strength is low because C is lowered for the same reason as described above. This is corrected by the upward adjustment of the Mn component, and Mn also plays an effective role in improving mechanical properties such as hardenability and weldability. In this respect, Mn is 0.90 to 1.00%.

Next, Si dissolves in ferrite phase to increase elastic modulus, toughness, and remove oxygen in molten steel in molten steel, but when it contains excessively, it increases the size of particles and decreases weldability. Considering this, the ratio is 0.17 to 0.24%.

The steel formed as described above is subjected to a normal converter, secondary refining (degassing), and received into a tundish and continuously cast. In the present invention, the molten steel superheat of the tundish is managed at 17 to 23 ° C. Molten steel superheat is very important for securing the internal quality and castability of cast steel. If the molten steel superheat is too high, the central segregation is likely to be inferior due to the development of columnar structure and the decrease of equiaxed crystal structure. The cleanliness of steel may be deteriorated due to heavy nozzle clogging and inclusion separation. In particular, since the present invention is close to the component system in which a positive reaction occurs during solidification to the γ phase, generation of cracks due to coarsening of particles during solidification becomes easy. Therefore, in consideration of this aspect, it is preferable to manage at 17 to 23 ° C., which is somewhat lower than the superheat degree of low carbon steel.

The taper (χ) of the mold during continuous casting is determined by the mold top end width A and the mold bottom end width A1 as shown in FIG. The taper of a mold is usually determined by the following equation.

Where χ is taper, A: mold top end width, A1: mold bottom end width, B: taper amount

In the present invention, the taper (χ) is set to 1.00 to 1.05% in consideration of the degree of solidification expansion. In the case of general medium carbon steel, it is about 1.0%. In addition, the lower end width of the mold is set to +8 to +10 mm than the width of the slab as in the usual case.

And, the casting speed is lowered to 1.12 ~ 1.17m / min to ensure good internal quality. When the low speed casting is performed, the internal quality of the cast can be improved because the expansion of the molten steel in the mold and the cooling of the cast are made uniform, thereby reducing the expansion at the bottom of the mold by securing sufficient solidification shell.

The castings continuously cast under the conditions described above and passed through the molds are cooled to a specific quantity of 0.29 to 0.35 l / kg when they reach the secondary cooling zone (secondary cooling zone).

Even if the cast is cast in accordance with the present invention, the surface of the cast may have partial surface defects and is covered with an oscillation mark and scale. Therefore, in order to prevent flaws such as a cap due to surface defects during hot rolling, it is preferable to perform front scarfing to inspect and remove defects in a cast state.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The weatherproof steel of the composition as shown in Table 1 below was subjected to a converter blow, degassing for 20 minutes or more in a low vacuum of less than 2torr and continuously cast under the conditions of Table 2 to prepare a cast steel of 1300mm width. At this time, the bottom width of the mold was set to +8 ~ + 10mm than the width of the slab. The surface test of this cast was carried out and the results are shown in Table 2.

Chemical composition (% by weight) C Si Mn P S Al Cu Ni Cr 0.08 0.21 1.05 0.016 0.005 0.019 0.27 0.27 -

division Casting speed (m / min) Specific quantity (ℓ / kg) Superheat degree (℃) Scarping Surface Defect Index Hot rolled caps (pieces / m) Inventive Example 1 1.12 0.29 18 Muscarping 0.01 0.5 to 1 Inventive Example 2 1.17 0.35 19 Front 0.02 0 Inventive Example 3 1.15 0.32 20 Front 0.02 0 Comparative Example 1 1.20 0.37 20 Front 0.1 to 0.2 2 to 5 Comparative Example 2 1.18 0.35 22 Muscaping 0.1 to 0.12 6-9 Comparative Example 3 1.10 0.32 19 Front 0.12 to 0.15 4 to 8

As shown in Table 2, in the case of the invention example (1), the surface defect index is good, and in the case of no scarfing, defects remained on the surface after hot rolling. In the case of the invention (2, 3), the entire surface was scarfed, there was no cap after hot rolling, which was cold-rolled to be a material but there was no surface defect problem.

division Tensile Strength (kg / ㎡) Yield strength (kg / ㎡) Elongation (%) Atmospheric Corrosion Resistance Index Product surface goal 45 or more 32 or more 22 or more 5.5 and above Satisfaction in Quality Inspection Standard Psalm 1 50.4 34.0 31.0 5.7 Psalm 2 49.5 36.0 31.5 5.7 Psalm 3 50.1 34.5 31.8 5.6

As described above, the present invention has a useful effect of providing a cast steel that can be manufactured from a material (cold rolled material) because there is no surface defect in a hot rolled product state.

Claims (1)

By weight%, C: 0.06 to 0.08%, Mn: 0.90 to 1.0%, Si: 0.17 to 0.24%, P: 0.02% or less, S: 0.01% or less, Cu: 0.2 to 0.3%, Ni: 0.2 to 0.3% And managing superheat at a tundish of 17 to 23 ° C. for molten steel composed of the remaining Fe and other inevitable impurities. Injecting the molten steel into a mold having a taper set at 1.00 to 1.05% and casting at a speed of 1.12 to 1.17 m / min; A method of casting cold-rolled weathering steel, comprising: cooling a cast steel that is passed through a mold to a non-aqueous quantity of 0.29 to 0.35 l / kg in a secondary cooling zone (secondary cooling zone).