KR101467050B1 - Steel plate and method of manufacturing the same - Google Patents

Abstract

A steel material having a yield strength of 315 to 440 MPa, a tensile strength of 440 to 590 MPa and a center average absorbed energy of 340 J or more at -60 캜, and a method for producing the same.
(A) 0.05 to 0.1% of carbon (C), 0.2 to 0.4% of silicon (Si), 1.0 to 1.7% of manganese (Mn) (P): not more than 0.012%, sulfur (S): not more than 0.003%, nickel (Ni): 0.2 to 0.5%, copper (Cu): 0.05 to 0.2%, niobium (Nb): 0.01 to 0.03 The steel slab consisting of the remaining iron (Fe) and the unavoidable impurities at a temperature of 1100 to 1200 ° C is reheated at a temperature of 10 to 1200 ° C, a boron (B) content of 0.0005% or less, a titanium (Ti) content of 0.01 to 0.02%, a nitrogen (N) content of 0.005% step; Firstly rolling the reheated steel at a temperature of RST (Recrystallization Stop Temperature) to RST + 100 占 폚; Subjecting the primary rolled steel material to a secondary rolling with a compression ratio of 60% or less and a finish rolling temperature of 820 to 860 ° C; And cooling the secondary rolled steel at a cooling rate of 5 DEG C / sec or more.

Description

{STEEL PLATE AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a steel material and a manufacturing method thereof, and more particularly, to a steel material having excellent strength and impact resistance at low temperatures through alloy composition and process control, and a method of manufacturing the same.

Recently, as the drying amount of the liquefied gas carrier is increased, the amount of the low temperature steel used is increasing, and the characteristics and requirements of the steel are different from those of general steel.

In particular, the steel material used in the ammonia (NH ₃ ) acid application tank may cause stress corrosion cracking due to H ₂ S because it is in direct contact with liquefied ammonia. As a result, the low-temperature steels used for cargo tanks are limited to a minimum yield stress of 440 MPa or less at the design stage. If the yield strength of the final steel is higher than the yield strength to be designed, Heat treatment is carried out.

Further, in the case of the cargo tank steel, the thickness of the steel used for general structure is thinner, and strict thickness tolerance is required. Furthermore, the Society has to limit the yield strength to tensile strength ratio (YR ratio) to 90% or less. Accordingly, in the production of ammonia / LPG acid-applied steels, a higher control rolling technique than that of the conventional rolling technique is required.

In the production of low-temperature steels, fluctuation of the microstructure and material properties of the steel is very large in accordance with variations in the alloy composition and manufacturing method of the steel, and the required properties It is a very difficult situation to satisfy exactly. In addition, excellent steelmaking and rolling techniques for satisfying the material are required in the production of such low-temperature steels.

As a background art related to the present invention, there is a method for manufacturing a superfine steel material having excellent strength and toughness at the center of thickness disclosed in Korean Patent Registration No. 10-0782761 (published on December 12, 2007).

An object of the present invention is to provide a steel material excellent in strength and impact resistance at low temperatures.

Another object of the present invention is to provide a method of manufacturing a steel material having excellent strength and low temperature impact toughness through alloy composition and process control.

(A) 0.05 to 0.1% of carbon (C), 0.2 to 0.4% of silicon (Si), and 1.0 of manganese (Mn) as weight%, based on the total weight of the steel material. (P): more than 0% to 0.012%, sulfur (S): more than 0% to less than 0.003%, nickel (Ni): less than 0.2% (N): 0.01 to 0.03%, boron (B): more than 0% to 0.0005%, titanium (Ti): 0.01 to 0.02%, nitrogen (N) : Reheating the steel slab, which is composed of more than 0% to less than 0.005% and the remaining iron (Fe) and unavoidable impurities, at 1100 to 1200 ° C; (b) primary-rolling the reheated steel at a temperature of RST (Recrystallization Stop Temperature) to RST + 100 占 폚; (c) secondarily rolling the primary rolled steel at a compression ratio of 60% or less and a finish rolling temperature of 820 to 860 ° C; And (d) cooling the secondary rolled steel material at a cooling rate of 5 DEG C / sec or more.

In order to achieve the above object, steel according to an embodiment of the present invention comprises 0.05 to 0.1% of carbon (C), 0.2 to 0.4% of silicon (Si), 1.0 to 1.7% of manganese (Mn) (Al): more than 0% to 0.5%, P: more than 0% to 0.012%, S: more than 0% to less than 0.003%, nickel: 0.2 to 0.5% (B): more than 0% to 0.0005%, titanium (Ti): 0.01 to 0.02%, nitrogen (N): more than 0% 0.005% or less, and the balance of iron (Fe) and inevitable impurities, and has a yield strength of 315 to 440 MPa, a tensile strength of 440 to 590 MPa and an average center absorbed energy of 340 J or more at -60 캜.

According to the method for producing a steel material according to the present invention, it is possible to secure a material satisfying all of strength limitations such as yield strength and tensile strength in addition to low-temperature impact toughness through process control such as alloy composition, finishing rolling temperature and cooling rate .

The steel material according to the present invention satisfies the tensile strength of 440 to 590 MPa with a yield strength of 315 to 440 MPa and the center average absorbed energy of 340 J or more at -60 캜, thereby being excellent in strength and impact resistance at low temperatures.

1 is a flowchart showing a method of manufacturing a steel material according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method of manufacturing a steel product according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The steel according to the present invention may contain, by weight, 0.05 to 0.1% of carbon (C), 0.2 to 0.4% of silicon (Si), 1.0 to 1.7% of manganese (Mn) (P): more than 0% to 0.012%, sulfur (S): more than 0% to less than 0.003%, nickel (Ni): 0.2 to 0.5%, copper (Cu): 0.05 to 0.2% (Nb): 0.01 to 0.03%, boron (B): more than 0 to 0.0005%, titanium (Ti): 0.01 to 0.02%, and nitrogen (N): 0 to 0.005%.

The rest of the above components are composed of iron (Fe) and unavoidable impurities.

Hereinafter, the role and content of each component contained in the steel according to the present invention will be described.

Carbon (C)

Carbon (C) contributes to the improvement of martensite fraction and hardness in composite textured steel.

The carbon is preferably added in an amount of 0.05 to 0.1% of the total weight of the steel material. When the addition amount of carbon is less than 0.05% by weight, it is difficult to secure a strength of 440 MPa or more in tensile strength. On the other hand, when the carbon content exceeds 0.1% by weight, formation of carbide in the steel is promoted, and it is difficult to secure a desired elongation of 22% or more.

silicon( Si )

Silicon (Si) serves as a deoxidizer, and in the present invention, it is added in an amount of 0.2 to 0.4% of the total weight of the steel material, thereby improving the elongation.

When the added amount of silicon is less than 0.02% by weight of the total weight of the steel, it is difficult to secure an elongation of 22% or more. On the contrary, when the addition amount of silicon exceeds 0.4% by weight of the total weight of the steel material, the performance is deteriorated and SiMn ₂ O ₄ Or the like, and the plating performance is deteriorated.

manganese( Mn )

Manganese (Mn) contributes to the improvement of strength of steel by strengthening solid solution and increasing ingotability. It also contributes to preventing hot cracking due to sulfur (S).

The manganese is preferably added in an amount of 1.0 to 1.7% by weight based on the total weight of the steel material. When the content of manganese is less than 1.0% by weight, the effect of the addition is insufficient. On the other hand, when the content of manganese exceeds 1.7% by weight, the manganese band develops at the center in the thickness direction of the material, and the elongation rate is lowered, and the carbon equivalent is increased to deteriorate the weldability.

aluminum( Al )

Aluminum (Al) acts as a deoxidizer during steelmaking, inducing the clarification of the ferrite phase to improve elongation, and increasing the amount of carbon (C) in the austenite phase to increase the hardness of the final martensite. In addition, aluminum (Al) suppresses the formation of manganese bands in the hot-rolled steel to prevent elongation reduction.

The aluminum is preferably added in an amount of more than 0 wt% to 0.5 wt% or less of the total weight of the steel material. If the content of aluminum exceeds 0.5% by weight, the weldability and the continuous casting are reduced, and aluminum nitride (AlN) in the slab is formed to cause hot cracking.

In (P)

Phosphorus (P) contributes to the improvement of the strength of steel by solid solution strengthening, and is an effective element for suppressing carbide formation, and plays a role of preventing elongation decrease due to carbide formation after cooling. In addition, manganese equivalence is improved and it is effective in securing the martensitic phase fraction. However, when phosphorus is added in excess, Fe3P stoodite structure is formed, which causes hot brittleness.

In the present invention, the content of phosphorus is limited to more than 0 wt% and not more than 0.012 wt% of the total weight of the steel material.

Sulfur (S)

Sulfur (S) inhibits toughness and weldability, and increases MnS inclusions in steel to inhibit the incombustibility effect of Mn and cause cracks in work. Also, if sulfur is excessively contained, the amount of crude inclusion is increased to deteriorate the fatigue characteristics.

Accordingly, in the present invention, the content of sulfur is limited to more than 0 wt% and 0.003 wt% or less of the total weight of the steel material, taking the above points into consideration.

nickel( Ni )

Nickel (Ni) is a very effective element for improving the low temperature toughness and increasing the strength.

The nickel (Ni) is preferably added in an amount of 0.2 to 0.5% by weight based on the total weight of the steel according to the present invention. When the addition amount of nickel is less than 0.2% by weight, it is difficult to secure a desired low temperature toughness. On the contrary, when the addition amount of nickel exceeds 0.5% by weight, the weldability of the steel material is lowered, and the production cost is greatly increased due to the high price.

Copper( Cu )

Copper (Cu) contributes to solid solution strengthening and enhances strength.

The copper is preferably added in an amount of 0.05 to 0.2% by weight based on the total weight of the steel according to the present invention. When the addition amount of copper is less than 0.05% by weight, the effect of the addition is insufficient. On the other hand, when the addition amount of copper exceeds 0.2% by weight, there is a problem that the hot workability of the steel is lowered and the susceptibility to stress relief cracking after welding is increased.

Niobium ( Nb )

Niobium (Nb) contributes to enhance the formability by forming a niobium precipitate in the steel, improving the strength of a steel material produced through solid solution strengthening in Fe, and improving grain refinement and martensite dispersibility. Further, it functions effectively as a precipitate forming element in securing high strength.

The niobium is preferably added in an amount of 0.01 to 0.03% by weight based on the total weight of the steel material. When the addition amount of niobium is less than 0.01% by weight, it is difficult to secure the bending workability of the steel material. On the contrary, when the addition amount of niobium exceeds 0.03% by weight, moldability is deteriorated.

Boron (B)

Boron (B) is a strong incombustible element and can be greatly effective in improving strength.

The boron is preferably added in an amount of more than 0 wt% to 0.0005 wt% of the total weight of the steel material. When the content of boron exceeds 0.0005% by weight of the total weight of the steel, it is segregated in the grain boundaries and acts as an element inhibiting the plating ability.

titanium( Ti )

Titanium (Ti) prevents the formation of aluminum nitride (AlN) and generates precipitates of Ti (C, N) with high stability at high temperature, thereby preventing the growth of austenite grains during welding, thereby improving the welded characteristics through refinement of the welded structure.

The titanium is preferably added in an amount of 0.01 to 0.02% by weight based on the total weight of the steel material. If the content of titanium is less than 0.01% by weight, the prevention of the formation of aluminum nitride and the improvement of the welded part characteristics are insufficient. On the other hand, when the content of titanium exceeds 0.02% by weight, coarse precipitates are formed to deteriorate the impact characteristics of the steel and to combine with carbon in steel to increase the yield ratio.

Nitrogen (N)

Nitrogen (N) can contribute to grain refinement.

However, when the addition amount of nitrogen exceeds 0.005% by weight of the total weight of the steel material, the solid nitrogen is increased to deteriorate the impact property and elongation of the steel material, deteriorate the formability of the steel material, and can also significantly deteriorate the toughness of the welded part.

Therefore, it is preferable that the content of nitrogen is limited to not less than 0 wt% and not more than 0.005 wt% of the total weight of the steel which is insignificant to the mechanical properties.

The steel material according to the present invention can be a composite structure including acicular ferrite, bainite and pearlite, by the above composition and the rolling process control described below. At this time, the acicular ferrite and pearlite have a grain size of 10 mu m or less.

The steel according to the present invention is characterized in that it has a yield strength of 315 to 440 MPa, a tensile strength of 440 to 590 MPa and an average center absorbed energy of 340 J or more at -60 캜, in terms of mechanical properties. This is because the slab is heated at less than 1200 ° C and then controlled and rolled to maximally suppress the growth of the austenite initially generated to maximize the deformation of the center portion under the pressure during rolling and to accelerate the cooling rate at the center portion.

Further, the steel material according to the present invention has an elongation of 22% or more in terms of mechanical properties.

The steel material satisfying the yield strength, tensile strength and low-temperature impact toughness in the above-mentioned range can be used for outer sheathing for cargo tanks of liquefied gas bulk carriers and the like.

Hereinafter, a method of manufacturing a steel material according to the present invention having the above characteristics will be described.

1 is a flowchart showing a method of manufacturing a steel material according to an embodiment of the present invention.

Referring to FIG. 1, the illustrated method of manufacturing a steel product includes a slab reheating step S110, a primary rolling step S120, a secondary rolling step S130, and a cooling step S140.

Reheating slabs

In the slab reheating step (S110), the steel slab having the above composition is reheated to improve the reuse and homogenization of the precipitate while suppressing the growth of the austenite initially produced.

At this time, it is preferable that the slab reheating is performed at 1100 to 1200 ° C. When the reheating temperature of the slab is less than 1100 ° C, the temperature of the steel slab is lowered after reheating, which causes a problem that the rolling load becomes large. On the other hand, when the heating temperature exceeds 1200 ° C, crystal grain growth and economical efficiency may be a problem.

Also, it is preferable that the slab reheating is performed for 1 to 3 hours in the above temperature range. If the slab reheating time is less than 1 hour, the re-use effect of the precipitate may be insufficient. Conversely, if the slab reheating time exceeds 3 hours, economical efficiency may be a problem due to excessive heating.

Primary rolling

Next, in the primary rolling step (S120), the reheated steel is primarily rolled at RST (Recrystallization Stop Temperature) to RST + 100 ° C to form a fine austenitic structure.

That is, it is preferable that the rolling finish temperature at the time of primary rolling is a temperature immediately above the RST, that is, a temperature immediately above the recrystallization stop temperature. If the primary rolling finish temperature exceeds RST + 100 ° C, it is difficult to obtain strength and toughness due to the coarsening of the austenite grains. On the other hand, when the primary rolling finish temperature is less than RST, it takes a long time to lower the temperature of the steel, which leads to a decrease in productivity, and also to a deterioration in toughness and a yield ratio during the cooling time.

Secondary rolling

Next, in the secondary rolling step (S130), the primary rolled steel is secondary rolled at a finish rolling temperature of 820 to 860 캜 at a pressing ratio of 60% or less according to the following formula (1). This is because control of the compression ratio and finishing rolling temperature is important in order to satisfy the yield strength and tensile strength requirements of the final steel.

[Formula 1]

(T ₁ -T _f ) × 100 / T _f

Where T ₁ is the primary rolling finish thickness and T _f is the final rolling finish thickness.

At this time, when the finish rolling temperature in the secondary rolling step (S130) exceeds 860 DEG C, it is difficult to secure strength and toughness due to crystal grain coarsening. On the other hand, when the finishing rolling temperature in the secondary rolling step (S130) is less than 820 占 폚, the rolling load may increase due to an excessively low temperature, and toughness deterioration and yield ratio may be increased.

In addition, when the reduction ratio exceeds 60%, the requirements of the yield strength and tensile strength of the final steel can not be satisfied.

Cooling

Next, in the cooling step (S140), the secondary rolled steel is cooled.

The cooling is preferably performed by accelerated cooling using a cooling rate of 5 DEG C / sec or more so as to obtain a fine needle-like ferrite phase to the center portion. When the average cooling rate exceeds 5 [deg.] C / sec during cooling, the strength of the produced steel can be increased. However, since components such as hydrogen present in the steel during cooling are difficult to sufficiently diffuse, .

In addition, the cooling start average temperature may be approximately 780 to 860 ° C.

On the other hand, the cooling end temperature is preferably 400 to 550 ° C. If the cooling end temperature is less than 400 ° C, the flatness and tensile strength of the steel produced due to excessive cooling may exceed specifications. On the other hand, when the cooling termination temperature exceeds 550 캜, it may become difficult to secure sufficient strength.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

A steel specimen of the final 20 mm thickness according to Examples 1 to 3 and Comparative Example 1 was prepared with the composition shown in Table 1 and the process conditions shown in Table 2.

[Table 1]

[Table 2]

2. Property evaluation

Table 3 shows the tensile test results of the steels produced in accordance with Examples 1 and 2 and Comparative Example 1, measured at 1 / 2T and 1 / 4T of the steel, , That is, the center portion in the thickness direction of the steel, and 1 / 4T means the 1/4 portion in the thickness direction of the steel.

The impact absorption energy was expressed by the average impact absorption energy obtained by performing the Charpy impact test based on KS B 0810 three times at -60 ° C against the center portion in the thickness direction of the steel material produced according to Examples 1 to 3 and Comparative Example 1.

[Table 3]

Referring to Table 3, all the specimens according to Examples 1 to 3 satisfied the desired physical properties with a yield strength of 351 MPa or more, a tensile strength of 479 MPa or more, and an elongation of 25% or more and the specimen according to Comparative Example 1 also had a yield strength of 481 MPa Or higher, a tensile strength of 518 MPa or more, and an elongation of 28% or more.

In all of the specimens according to Examples 1 to 3, the core average absorbed energy satisfies the target value of 340 J or more at -60 ° C. On the other hand, the specimen according to Comparative Example 1 did not satisfy the desired physical properties at an average center absorption energy of 286J at -60 ° C.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Slab reheating step
S120: Primary rolling step
S130: Secondary rolling step
S140: cooling step

Claims (6)

delete delete (Al): not less than 0% to not more than 0.5%, and phosphorus (P) as a main component, in terms of% by weight, carbon (C): 0.05 to 0.1%, silicon (Si): 0.2 to 0.4%, manganese : More than 0% to 0.012%, sulfur (S): more than 0% to less than 0.003%, nickel (Ni): 0.2 to 0.5%, copper (Cu): 0.05 to 0.2%, niobium (Nb) (Fe) and inevitable impurities, and more preferably not more than 0.03%, boron (B): not less than 0% to not more than 0.0005%, titanium (Ti): 0.01 to 0.02% ,
A yield strength of 315 to 440 MPa, a tensile strength of 440 to 590 MPa and an average center absorbed energy of 340 J or more at -60 캜.
The method of claim 3,
The steel
Characterized in that the microstructure comprises acicular ferrite, bainite and pearlite.
5. The method of claim 4,
The needle-like ferrite and the pearlite
And a grain size of 10 mu m or less.
The method of claim 3,
The steel
And an elongation of 22% or more.

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