KR100241306B1 - The manufacturing method for low temperature steel with hot working - Google Patents

Abstract

TECHNICAL FIELD The present invention relates to a method for manufacturing a low temperature steel material for thermal mechanical controlled rolling, and is particularly resistant to cracking and propagation due to brittleness at low temperatures. The present invention relates to a method for manufacturing a hot-rolled cold-rolled low temperature steel material capable of securing an equivalent level of mechanical properties.

In the present invention, by weight%, containing C: 0.07 to 0.09%, Si: 0.20 to 0.30%, Mn: 1.35 to 1.50%, Al: 0.030 to 0.070%, P: ≤0.020%, and S: ≤0.0050% In the range of satisfying carbon equivalent 당 Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15｝ ≤0.320%, Ti: 0.010 to 0.020%, B: 0.00050 to 0.0020% Adding to produce molten steel; Rolling down the steel obtained by continuously casting the molten steel at an intermediate control temperature of 830 to 870 ° C., a residual reduction ratio of 40 to 60%, and a rolling end temperature of 720 to 750 ° C .; Hot-rolled controlled low-temperature processing, characterized in that consisting of the step of accelerated cooling the steel obtained by the rolling at the cooling start temperature 730 ~ 700 ℃, cooling end temperature 550 ~ 520 ℃, cooling rate 7 ~ 8 ℃ / sec conditions Provides a method for manufacturing molten steel.

Description

Method for manufacturing hot-rolled low-temperature steel

1 is a view showing a manufacturing process of low-temperature steel for LPG tank.

2 is a graph showing the correlation between the carbon equivalent and the low temperature impact toughness of the welded joint.

3 is a graph showing the distribution of TiN particles according to the amount of Ti added.

4 is a graph showing the correlation between the amount of B added and the low temperature impact toughness of the thermal welding joint.

The present invention relates to a method for manufacturing a thermo mechanical controlled process type low temperature steel, and particularly excellent in resistance to cracking and propagation due to brittleness at low temperatures, and in welding joints even in high heat input welding. The present invention relates to a method for manufacturing a hot-rolled low-temperature steel for controlling heat processing that can secure mechanical properties equivalent to that of a base metal.

Steel plates for the production of liquefied gas (LPG) storage tanks related to the petrochemical industry are generally specified in the class specifications, and in particular Charpy impact toughness requirements are required under the same conditions for the base material and the weld.

In other words, the quality characteristics of steel applied to the tank of LPG carrier are required to guarantee impact toughness at -51 ℃ in consideration of LPG liquefaction temperature (-46 ℃). The requirements for characteristics depend on the experience of the ship manufacturer. Table 1 shows the quality requirements of 50kg / mm ² grade LPG carrier steels.

TABLE 1

However, in the past, ship manufacturers used their own requirements based on the experience value, and the manufacturing technology has been researched to improve the impact toughness at low temperatures, and various research results have shown that low temperature toughness such as copper (Cu) and nickel (Ni) is used. It is known that it is possible to produce steel materials with excellent low temperature impact toughness by heat treatment after general rolling in the thick plate process by adding an element that is advantageous for securing. In addition, niobium (Nb) and titanium (Ti) alloy elements are used to expand the recrystallization temperature zone to form fine pearlite due to the cumulative reduction of austenite. Formation of particles in the form of a problem of low temperature impact toughness occurs.

An object of the present invention is to provide a method for manufacturing a low temperature steel CPCP-type low temperature impact toughness and excellent fracture toughness to solve the conventional problems as described above.

In order to achieve the above object, in the present invention, in weight%, C: 0.07-0.09%, Si: 0.20-0.30%, Mn: 1.35-0.50%, Al: 0.030-0.070%, P: 0.020% or less, S: Ti: 0.010 to 0.020%, in a range of satisfying carbon equivalent 당 Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15｝ ≦ 0.320% while containing 0.0050% or less, B: adding 0.00050 to 0.0020% to prepare molten steel; Rolling down the steel obtained by continuously casting the molten steel at an intermediate control temperature of 830 to 870 ° C., a residual reduction ratio of 40 to 60%, and a rolling end temperature of 720 to 750 ° C .; Cold processing of the steel obtained by the rolling comprises a step of accelerated cooling treatment at a cooling start temperature 730 ~ 700 ℃, cooling end temperature 550 ~ 520 ℃, cooling rate 7 ~ 8 ℃ / sec conditions Provided is a method for manufacturing molten steel.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

In the present invention, the addition of special elements such as Ti and B in addition to the five major elements (C, Si, Mn, P, S), and low carbon equivalent (Ceq) was managed to ensure low-temperature toughness and excellent weldability In the chemical targets, C: 0.08%, Mn: 1.45%, Si: 0.25%, Ti: 0.015%, B: 0.001%, and the carbon equivalent is about 0.312%, similar to that of a typical 40kg / mm grade ² steel. Set it to the level.

2 shows low-temperature impact toughness of the welded joint according to the carbon equivalent (Ceq).

The reason for setting the target for each chemical component described above will be described below.

In the case of carbon (C), the strength is the largest contributing factor, and the strength is increased in proportion to the increase in the amount added. However, since the impact toughness is reduced, the carbon (C) is set at about 0.07% to 0.09%, similar to the conventional level. .

Silicon (Si) improves yield strength and deoxidizes simultaneously, but when excessively added, it forms SiO ₂ -based inclusions (Silicate), which can reduce brittle cracking and toughness at low temperatures, and scales on the product surface after hot rolling. Since the formation of (Scale) is limited to about 0.2% to 0.3% level.

Manganese (Mn) is an element that can improve strength and toughness at the same time, forms carbide (Mn ₃ C), and unrecrystallized crystals in which austenite does not grow due to a decrease in transformation temperature (Ar ₃ ) during hot rolling. It is possible to refine the structure by increasing the area of the unrecrystallized cumulative reduction by increasing the area, and by reinforcing fine carbide in the ferrite mouth, the ferrite can be remarkably strengthened to increase the strength and toughness simultaneously. (Lath Martensite) is formed to reduce the ductility and low-temperature toughness, and when a small amount of addition is unstable to secure the strength, the manganese addition amount of 5kg / mm grade ² steel sheet for low temperature toughness guarantee is set to about 1.35% to 1.5%.

In addition, phosphorus (P) is the biggest impurity element that impairs the low temperature impact toughness of the steel sheet is segregated in the center part during casting in the casting process, so defects such as lamination during steel processing degrade the internal quality. 0.02% or less, sulfur (S) is an element that inhibits the quality of the steel sheet, such as phosphorus, causing cracks and segregation in the casting process casting process, combined with manganese (Mn) when excessively added The inclusion (MnS) shape inhibits the quality of the steel in the thickness direction, so about 0.005% or less is an optimal condition.

On the other hand, aluminum (Al) is a deoxidation element, which is very effective for refining grains, and when the content is high, it is evenly distributed throughout the steel, and does not require diffusion energy to bond with nitrogen when cooling from high temperature to low temperature. It is easier to bond with nitrogen than B), so that a sufficient amount of boron (B) remains in the steel material to finally help the formation of the BC compound. About 0.030% to 0.070%, which is higher than usual, is added.

Titanium (Ti) contributes to the refinement of ferrite particles by preventing the coarsening of austenite by forming a compound (TiN) that is stable at high temperatures during casting, and TiN is usually complete at about Ti: N = 3.2: 3.4. By forming a precipitate at the austenite grain boundary at high temperatures during slab reheating fixing and high heat input welding, it inhibits the growth of the initial austenite crystal blotter, while acting as a nucleation site of ferrite during transformation. A partial increase in and a significant improvement in toughness occur, where titanium is set at about 0.01% to 0.02%, taking into account the management range of nitrogen. In particular, FIG. 3 is a distribution diagram of Ti particles depending on the amount of titanium (Ti) added.

Here, titanium is added to form BC (Boron Carbide), which is the main component of the material design of the present invention. In the steel casting process and the welded joint, the nitrogen atom is primarily bonded to titanium having the largest bonding force and not bonded. The excess nitrogen of the second bond with boron (B) in the order of binding force, so that sufficient titanium is added to prevent the formation of BN compounds that impair the mechanical properties.

Boron (B) is an element that is combined with iron (Fe) and nitrogen to form boron carbide (BC) during slab casting and high heat input welding heat cycle in the regeneration process, and the BC compound is a grain boundary of austenite and Precipitates in the mouth and acts as nucleation site (Site) of the ferrite during transformation, thereby forming a fine ferrite.

However, if the nitrogen content is high, free nitrogen remaining after bonding with titanium combines with boron (B) to form a BN compound, which adversely affects mechanical properties such as intergranular embrittlement, thus maintaining titanium at a sufficiently high content.

Therefore, as shown in Figure 4, the boron (B) content of the material according to the present invention is maintained at about 0.0005% to 0.002% in consideration of the mechanical properties of the high heat input welded joint.

Looking at the manufacturing process according to the present invention, in order to reduce the low-temperature toughness and mechanical properties adverse elements in the molten iron pre-treatment step by performing the molten iron stirring process by the mill scale (Mill scale), the dephosphorization (P) operation, carbide ( Carry out desulfurization (S) work by adding carbide.

After the converter operation, vacuum degassing (RH-OB) treatment is performed to reduce inclusions and impurities in the molten steel, and calcium (C) is used to reduce sulfur content and shape control, which is easy to generate internal cracks and central segregation. Powder injection is performed by adding Ca).

In addition, during the casting process of the casting process, electro magnetic stirring is performed to suppress the occurrence of central segregation.

On the other hand, the heavy plate manufacturing process has various problems such as delay of production air, increase of manufacturing cost (Cost), and deterioration of weldability due to the addition of high alloy after conventional rolling and quenching and tempering processes. The material according to the invention overcomes the above-mentioned disadvantages by TMCP rolling, and in particular, in order to improve low-temperature fracture toughness, by applying rolling under low temperature two-phase (Austenite & Ferrite Region) during TMCP rolling to form a connective structure, By reducing the size of the effective ferrite grains, the resistance to generation and propagation stop characteristics against brittle fracture cracking are improved.

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

EXAMPLE

Table 2 shows the results of comparing the compounding between the material according to the present invention and the conventional material, phosphorus (P), sulfur (S) among the chemical components of the crack (Site) generation site (Site) inside the steel sheet Carefully manages and conducts electronic stirring to minimize central segregation.

And Figure 1 shows the manufacturing process of the low-temperature molten steel according to the present invention and the main control items for each process.

TABLE 2

In addition, the control rolling conditions of the thick plate process is as shown in Table 3, when the thickness of the low-temperature molten steel according to the present invention is 16mmt, the intermediate control temperature 830 ℃, residual pressure reduction rate 50% and rolling finish temperature 730 ℃, 730 ℃ in accelerated cooling process To 520 ° C., and a cooling rate of 7 ° C./sec to 8 ° C./sec similar to that of a conventional TMCP rolled material is applied.

TABLE 3

On the other hand, Table 4 shows the changes in the quality characteristics of the heat input welding part (0.07C-1.40Mn-0.0050N) according to the content of titanium (Ti) and boron (B).

TABLE 4

As described above, the present invention imparts excellent resistance to brittle fracture cracking and propagation at low temperature to the steel for manufacturing the storage tank of the LPG carrier, and provides the same level of mechanical properties as the base material in the high heat input welded joint.

Claims (1)

By weight%, C: 0.07 to 0.09%, Si: 0.20 to 0.30%, Mn: 1.35 to 1.50%, Al: 0.030 to 0.070%, P: ≤0.020%, S: ≤0.0050% = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15｝ ≤0.320% in the range of satisfying Ti: 0.010 ~ 0.020%, B: 0.00050 ~ 0.0020% Manufacturing step; Rolling down the steel obtained by continuously casting the molten steel at an intermediate control temperature of 830 to 870 ° C., a residual reduction ratio of 40 to 60%, and a rolling end temperature of 720 to 750 ° C .; Hot-rolled controlled low-temperature processing, characterized in that consisting of the step of accelerated cooling the steel obtained by the rolling at the cooling start temperature 730 ~ 700 ℃, cooling end temperature 550 ~ 520 ℃, cooling rate 7 ~ 8 ℃ / sec conditions Method of manufacturing molten steel.