KR20120097162A - Thick steel plate and method of manufacturing the thick steel plate - Google Patents

Abstract

PURPOSE: A thick plate and a manufacturing method thereof are provided to obtain an average impact toughness of 250J or greater at yield strength of 420MPa or greater and the temperature of -40 °C. CONSTITUTION: A method for manufacturing a thick plate comprises the steps of: reheating a slab panel, which comprises C of 0.065-0.95wt.%, Si of 0.25-0.35wt.%, Mn of 1.50-1.60wt.%, Al of 0.02-0.06wt.%, Ti of 0.01-0.02wt.%, Nb of 0.01-0.03wt.%, Ni of 0.2-0.4wt.%, and Fe and inevitable impurities of the remaining amount(S110), rolling the reheated panel in an austenite recrystallization range first(S120), rolling the first rolled panel in a non-recrystallization range below the austenite recrystallization stop temperature secondly(S130), and cooling the second rolled panel to a banite temperature range(S140). [Reference numerals] (AA) Start; (BB) Air cooling; (CC) End; (S110) Reheating slab; (S120) First rolling; (S130) Second rolling; (S140) Cooling

Description

Thick sheet and its manufacturing method {THICK STEEL PLATE AND METHOD OF MANUFACTURING THE THICK STEEL PLATE}

The present invention relates to a thick steel sheet manufacturing technology, and more particularly, to a yield strength 420MPa grade steel sheet used in marine structures and the like and a manufacturing method thereof.

Thick steel plate is mainly applied to the fields requiring high strength and high toughness such as offshore structures.

In general, the thick steel sheet is manufactured through a slab reheating process, a rolling process and a cooling process.

In the slab reheating process, the steel slab, which is semifinished, is reheated.

In the rolling process, the reheated slabs are rolled using a rolling roll.

In the cooling process, the plate finished rolling is cooled.

An object of the present invention is to provide a method for producing a yield strength 420MPa grade steel sheet excellent in low temperature impact characteristics, through the control of the alloy components and rolling process conditions.

Another object of the present invention is to provide a thick steel sheet which can have a yield strength of 420 MPa grade and excellent low temperature impact toughness without containing copper (Cu).

Method for manufacturing a thick steel sheet according to an embodiment of the present invention for achieving the above object is carbon (C): 0.065 ~ 0.95% by weight, silicon (Si): 0.25 ~ 0.35% by weight, manganese (Mn): 1.50 ~ 1.60 Weight%, aluminum (Al): 0.02 to 0.06 weight%, titanium (Ti): 0.01 to 0.02 weight%, niobium (Nb): 0.01 to 0.03 weight%, nickel (Ni): 0.2 to 0.4 weight% and remaining Fe A slab reheating step of reheating the slab plate made of inevitable impurities; A primary rolling step of rolling the reheated plate in an austenite recrystallization zone; A secondary rolling step of rolling the first rolled sheet in an unrecrystallized region below an austenite recrystallization stop temperature; And a cooling step of cooling the second rolled plate to a bainite temperature range.

In this case, the slab plate may include phosphorus (P): 0.012% by weight or less, sulfur (S): 0.003% by weight or less, and nitrogen (N): 50 ppm or less.

In addition, the slab reheating step is preferably carried out at a slab reheating temperature: 1100 ~ 1200 ℃.

In addition, the primary rolling step is carried out at a temperature of 930 ℃ or more, preferably carried out so that the residual pressure reduction rate of the secondary rolling step is 50 ~ 70%.

In addition, the secondary rolling step is preferably the finish rolling temperature is carried out at 770 ~ 860 ℃.

In addition, the cooling step is preferably carried out at a cooling rate of 5 ~ 12 ℃ / sec to the secondary rolled plate to 470 ~ 600 ℃.

The thick steel plate according to an embodiment of the present invention for achieving the above another object is carbon (C): 0.065 ~ 0.95% by weight, silicon (Si): 0.25 ~ 0.35% by weight, manganese (Mn): 1.50 ~ 1.60% by weight, Aluminum (Al): 0.02 to 0.06% by weight, titanium (Ti): 0.01 to 0.02% by weight, niobium (Nb): 0.01 to 0.03% by weight, nickel (Ni): 0.2 to 0.4% by weight and the rest of Fe and unavoidable impurities It has a composite structure containing ferrite and bainite, and has a yield strength of 420 MPa or more and an average impact toughness of 250 J or more at -40 ° C.

The thick steel sheet manufacturing method according to the present invention can reduce the expensive nickel (Ni) content and also reduce the expensive nickel (Ni) content to reduce the thick steel sheet manufacturing cost.

In addition, the method for manufacturing a thick steel sheet according to the present invention provides an effect capable of providing a thick steel sheet having a yield strength of 420 MPa or more and an average impact toughness of 250 J or more at −40 ° C. through control of another alloy component and a rolling process.

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

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail 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. Like reference numerals refer to like elements throughout the specification.

Hereinafter, a thick steel plate according to a preferred embodiment of the present invention and a manufacturing method thereof will be described in detail with reference to the accompanying drawings.

Thick steel plate

The thick steel sheet according to the present invention is carbon (C): 0.065 to 0.95% by weight, silicon (Si): 0.25 to 0.35% by weight, manganese (Mn): 1.50 to 1.60% by weight, aluminum (Al): 0.02 to 0.06% by weight , Titanium (Ti): 0.01 to 0.02 wt%, niobium (Nb): 0.01 to 0.03 wt%, nickel (Ni): 0.2 to 0.4 wt%.

In addition, the thick steel plate according to the present invention may include phosphorus (P): 0.012% by weight or less, sulfur (S): 0.003% by weight or less, and nitrogen (N): 50ppm or less.

The rest is composed of iron (Fe) and inevitable impurities generated during steelmaking.

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

Carbon (C)

Carbon (C) is added to secure the strength of the thick steel sheet according to the invention.

The carbon is preferably added in a content ratio of 0.065 to 0.95% by weight of the total weight of the thick steel sheet according to the present invention. If the carbon is added less than 0.065% by weight, the fraction of the second phase structure is lowered, thereby lowering the strength of the thick steel sheet. On the contrary, when the content of carbon exceeds 0.95% by weight, the strength of the thick steel sheet increases, but there is a problem in that low-temperature impact toughness and weldability decrease.

Silicon (Si)

Silicon (Si) is added as a deoxidizer to remove oxygen in the steel during the steelmaking process. Silicon is also an effective element for the effect of solid solution strengthening.

The silicon (Si) is preferably added in a content ratio of 0.25 to 0.35% by weight of the total weight of the thick steel sheet according to the present invention. If the content of silicon is less than 0.25% by weight, the above effect of adding silicon is insufficient. On the contrary, when the content of silicon exceeds 0.35% by weight, the toughness of the thick steel sheet manufactured due to excessive formation of nonmetallic inclusions is lowered.

Manganese (Mn)

Manganese (Mn) is an austenite stabilizing element that lowers the Ar ₃ temperature to enlarge the controlled rolling region. Through this, manganese serves to improve the strength and toughness by miniaturizing the grains by rolling.

The manganese is preferably added in a content ratio of 1.5 to 1.6% by weight of the total weight of the thick steel sheet according to the present invention. If manganese is added at less than 1.5% by weight, the formation of phase 2 tissue is insufficient, which does not contribute to the strength improvement. On the contrary, when the content of manganese exceeds 1.6% by weight, there is a problem in that sulfur solid solution dissolved in steel is precipitated with MnS to lower impact toughness.

Aluminum (Al)

Aluminum (Al), together with silicon (Si), serves as a deoxidizer to remove oxygen in the steel.

The aluminum is preferably added in a content ratio of 0.02 to 0.06% by weight of the total weight of the thick steel sheet according to the present invention. When the amount of aluminum added is less than 0.02% by weight, the deoxidation effect due to aluminum addition is insufficient. On the contrary, when the addition amount of aluminum exceeds 0.06% by weight, Al ₂ O ₃ , which is a non-metallic inclusion, is formed to deteriorate impact toughness.

Titanium (Ti)

Titanium (Ti) forms TiN upon slab reheating and serves to inhibit austenite grain growth.

The titanium is preferably added in an amount of 0.01 to 0.02% by weight of the total weight of the thick steel sheet according to the present invention. If the addition amount of titanium is less than 0.01% by weight, the above titanium addition effect is insufficient. On the contrary, when the content of titanium exceeds 0.02% by weight, TiN precipitates are coarsened, so that the effect of inhibiting austenite grain growth is rather reduced.

Niobium (Nb)

Niobium (Nb) combines with carbon (C) and nitrogen (N) to form carbonitrides. The formed niobium-based carbonitride suppresses grain growth during rolling to refine the grains, thereby contributing to the improvement of strength and low temperature toughness of the thick steel sheet produced.

The niobium is preferably added in an amount of 0.01 to 0.03% by weight of the total weight of the thick steel sheet according to the present invention. When the addition amount of niobium is less than 0.01% by weight, the above niobium addition effect cannot be properly exhibited. On the contrary, when the added amount of niobium exceeds 0.03% by weight, there is a risk that the impact toughness of the thick steel sheet produced rather exists in a solid solution state in the ferrite. In addition, when the addition amount of niobium exceeds 0.03% by weight, weldability of the thick steel sheet may be impaired.

Nickel (Ni)

Nickel (Ni) refines grains and is also dissolved in austenite and ferrite to serve to strengthen the matrix. In particular, nickel is an effective element for improving low temperature impact properties.

The nickel is preferably added in an amount of 0.2 to 0.4% by weight of the total weight of the thick steel sheet according to the present invention. If the amount of nickel added is less than 0.2% by weight, the nickel addition effect cannot be properly exhibited. On the contrary, when the amount of nickel exceeds 0.4% by weight, there is a problem of causing red brittleness.

Phosphorus (P), sulfur (S), nitrogen (N)

Phosphorus (P) is a representative element for lowering the impact toughness. The lower the content, the better. However, phosphorus is an element that is inevitably contained in the steelmaking process. Therefore, in the present invention, the content of phosphorus is limited to 0.012% by weight or less of the total weight of the thick steel sheet according to the present invention.

Sulfur (S) is an element inevitably contained in the production of steel. Sulfur forms MnS and degrades impact toughness. Therefore, in the present invention, in order to secure excellent low-temperature impact properties, the content of sulfur was limited to 0.003% by weight or less of the total weight of the thick steel sheet according to the present invention.

Nitrogen (N) reduces the internal quality of the thick steel sheet produced by generating inclusions in the steel. Therefore, it is preferable to manage nitrogen at extremely low content ratio. However, there are many difficulties in managing the nitrogen content at an extremely low level, and as a result, the manufacturing cost of the thick steel sheet may increase significantly. Therefore, in the present invention, the content of nitrogen (N) was limited to 50 ppm or less.

The thick steel sheet according to the present invention may have a composite structure including ferrite and bainite through the above-described component system and rolling condition control described later. In addition, the thick steel sheet according to the present invention showed a yield strength of 420 MPa or more and a high average impact toughness of 250 J or more at -40 ° C.

Hereinafter, a method for manufacturing a thick steel sheet according to the present invention will be described.

Thick steel plate manufacturing method

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

Referring to FIG. 1, the illustrated thick steel sheet manufacturing method includes a slab reheating step S110, a first rolling step S120, a second rolling step S130, and a cooling step S140.

Slab reheating stage

In the slab reheating step (S110), carbon (C): 0.065 to 0.95 wt%, silicon (Si): 0.25 to 0.35 wt%, manganese (Mn): 1.50 to 1.60 wt%, aluminum (Al): 0.02 to 0.06 wt% Reheating the slab plate made of titanium (Ti): 0.01 to 0.02% by weight, niobium (Nb): 0.01 to 0.03% by weight, nickel (Ni): 0.2 to 0.4% by weight and the remaining iron (Fe) and unavoidable impurities.

The slab sheet may be obtained through a continuous casting process after obtaining molten steel of a desired composition through a steelmaking process. Through reheating of the slab sheet material, re-stocking of segregated components and castings may occur.

The slab plate may include phosphorus (P): 0.012 wt% or less, sulfur (S): 0.003 wt% or less, and nitrogen (N): 50 ppm or less.

It is preferable that slab reheating temperature is 1100-1200 degreeC. When the slab reheating temperature is less than 1100 ° C., the slab plate material may have a low temperature, thereby increasing the rolling load.

Conversely, when the slab reheating temperature exceeds 1200 ° C., titanium (Ti) precipitates are dissolved to make it difficult to suppress austenite grain growth. Therefore, in this case, the austenite grains are coarsened, so that the strength and low temperature impact characteristics of the thick steel sheet to be manufactured may be difficult.

1st rolling stage

In the primary rolling step (S120), in order to adjust the reduction ratio of the secondary rolling in the austenite uncrystallized region described later, the reheated slab plate is rolled at a temperature of 930 ° C or more, which is a temperature equal to or higher than the austenite recrystallization stop temperature. More specifically, the primary rolling may be carried out at a temperature of 930 ~ 1050 ℃ corresponding to the austenite recrystallization region.

At this time, the primary rolling is preferably carried out so that the residual pressure reduction rate of the secondary rolling is 50 to 70%. Here, the thickness or rolling reduction of the sheet after primary rolling can be obtained from the thickness of the sheet before rolling, the thickness of the sheet after secondary rolling, and the residual reduction rate of secondary rolling. For example, if the thickness of the plate before rolling is 100mm, the thickness after the end of the secondary rolling is 20mm, and the cumulative reduction ratio of the secondary rolling is 50%, the plate thickness after the primary rolling should be 40mm (100mm → 40mm). . Therefore, the cumulative reduction ratio of the primary rolling is 60% (100 mm → 40 mm).

When the primary rolling is performed such that the residual pressure reduction rate of the secondary rolling is less than 50%, it is difficult to ensure uniform and fine structure, and the structure of the central part is coarsened, thereby deteriorating the low temperature impact characteristics. On the contrary, when the primary rolling is performed such that the residual reduction ratio of the secondary rolling exceeds 70%, there is a problem in that the productivity of the secondary rolling process is long due to a long time.

Secondary rolling stage

In the secondary rolling step (S130), the first rolled sheet is secondary rolled in the unrecrystallized region below the austenite recrystallization stop temperature.

Since the end temperature of the secondary rolling is closely related to the strength and low temperature toughness of the thick steel sheet to be manufactured, it is very important to control it properly.

In the present invention, the end temperature of the secondary rolling is preferably 770 ~ 860 ℃. If the end temperature of the secondary rolling is less than 770 ℃ abnormal reverse rolling occurs to form a non-uniform structure there is a problem that greatly lowers the low temperature impact characteristics. On the contrary, when the end temperature of the secondary rolling exceeds 860 ° C., the ductility and toughness are excellent, but there is a problem in that the strength of the thick steel sheet produced cannot be sufficiently secured.

When the first and second rolling conditions of the present invention are applied, strain bands are formed in the austenite grains, thereby forming a large amount of ferrite nucleation sites in the austenite grains to form fine grains after the end of rolling.

Cooling stage

In the cooling step (S140), the secondary rolled sheet is cooled to the bainite temperature region by a water cooling method, thereby suppressing grain growth to form a matrix having fine ferrite grains, and forming a fine bainite structure as a secondary phase. High strength and low temperature impact properties are carried out to ensure excellent material properties.

In this invention, it is preferable that cooling end temperature is 470-600 degreeC. If the cooling end temperature is less than 470 ℃ there is a problem that the low-temperature transformation structure is formed a large amount of low temperature impact properties. On the contrary, when the cooling end temperature exceeds 600 ° C, there is a problem that the strength is lowered due to the formation of coarse microstructure.

Moreover, it is preferable to cool at the cooling rate of 5-12 degreeC / sec to said cooling end temperature. In addition, when the cooling rate is less than 5 ℃ / sec is promoted grain growth is difficult to secure the strength. On the contrary, when the cooling rate exceeds 12 ° C / sec, the bainite fraction is increased and effective for increasing the strength, but there is a problem in that the low temperature impact characteristics are lowered.

After the cooling step (S140) is finished, the cooled plate can be air cooled to about 15 ~ 35 ℃ room temperature.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. 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.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Manufacturing of thick steel sheet

A thick steel sheet according to Examples 1 to 3 and Comparative Examples 1 to 3 was prepared under the composition shown in Table 1 and the process conditions described in Table 2.

In the case of Examples 1 to 3 and Comparative Examples 1 to 3 each slab was reheated at 1150 ° C. for 2 hours.

 [Table 1] (unit:% by weight)

 [Table 2]

2. Characterization

Table 3 shows the mechanical properties and impact toughness of the thick steel sheet prepared according to Examples 1-3 and Comparative Examples 1-3. In Table 3, the impact toughness was measured three times at -40 ° C (charpy) impact value was expressed as the average value.

In Table 3, 1 / 2t means a center point in the thickness T direction of the manufactured thick steel plate, and 1 / 4t means 1/4 point in the thickness T direction of the manufactured thick steel plate.

[Table 3]

Referring to Table 3, in Examples 1 to 3 in which copper (Cu) was not added to the alloy components, the yield strength was about 450 to 480 MPa, which satisfies the yield strength of 420 MPa. In addition, in Examples 1-3, the average impact toughness at -40 degreeC showed 270J or more, and it was possible to ensure the target impact toughness of 250J or more.

On the other hand, Comparative Example 1, in which copper (Cu) is added to the alloy components, has excellent low-temperature impact properties and yield strengths of 420 MPa, but yield strength and impact toughness are somewhat higher than those of Examples 1 to 3. Low. In addition, in the case of Comparative Example 2 and Comparative Example 3, the impact toughness was relatively low compared to Examples 1 to 3.

In addition, referring to Example 3 and Comparative Example 3, in Example 3, the low temperature shock characteristics at the 1 / 2t and 1 / 4t points in the thickness direction were similar. However, in the case of Comparative Example 3 it can be seen that the difference in low temperature shock characteristics at the 1 / 2t point and 1 / 4t point in the thickness direction. This means that in the case of the present invention, it has a homogeneous structure in the thickness direction of the thick steel plate.

As described above, the thick steel sheet manufactured according to Comparative Examples 1 to 3 satisfies the yield strength of 420 MPa class, but contains a large amount of expensive copper (Cu) and nickel (Ni), and thus the manufacturing cost is relatively high. In addition, in the case of Comparative Examples 1 to 3, the rolling end temperature is relatively low, the productivity is low.

On the other hand, in the case of the thick steel sheet manufactured according to Examples 1 to 3 corresponding to the present invention, copper was not added, and the amount of nickel added was relatively low. Nevertheless, through the control of other alloy components and the rolling process, yield strength and impact toughness equal to or higher than those of the thick steel sheets manufactured according to Comparative Examples 1 to 3 were shown, and a homogeneous material was secured in the thickness direction.

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 (11)

Carbon (C): 0.065 to 0.95 wt%, Silicon (Si): 0.25 to 0.35 wt%, Manganese (Mn): 1.50 to 1.60 wt%, Aluminum (Al): 0.02 to 0.06 wt%, Titanium (Ti): 0.01 A slab reheating step of reheating the slab plate composed of 0.02% by weight, niobium (Nb): 0.01% to 0.03% by weight, nickel (Ni): 0.2% to 0.4% by weight and the remaining Fe and unavoidable impurities;
A primary rolling step of rolling the reheated plate in an austenite recrystallization zone;
A secondary rolling step of rolling the first rolled sheet in an unrecrystallized region below an austenite recrystallization stop temperature; And
And cooling the secondary rolled plate to a bainite temperature range.
The method of claim 1,
The slab plate
Phosphorus (P): 0.012% by weight or less, sulfur (S): 0.003% by weight or less and nitrogen (N): 50 ppm or less.
The method of claim 1,
The slab reheating step
Re-heating the slab plate at 1100 ~ 1200 ℃ thick steel plate manufacturing method characterized in that.
The method of claim 1,
The first rolling step is
A thick steel sheet manufacturing method, characterized in that carried out at a temperature of 930 ℃ or more.
The method of claim 1,
The first rolling step is
The thick steel sheet manufacturing method characterized in that it is carried out so that the residual pressure reduction rate of the secondary rolling step is 50 to 70%.
The method of claim 1,
The finish rolling temperature of the secondary rolling step is
The thick steel sheet manufacturing method characterized in that the 770 ~ 860 ℃.
The method of claim 1,
The cooling step
The second steel sheet is a thick steel sheet manufacturing method characterized in that for cooling to 470 ~ 600 ℃.
The method of claim 7, wherein
The cooling step
The thick steel sheet manufacturing method characterized in that carried out at a cooling rate of 5 ~ 12 ℃ / sec.
The method of claim 1,
After the cooling step,
The thick steel plate manufacturing method characterized in that the cooled plate to air cooled to 15 ~ 35 ℃.
Carbon (C): 0.065 to 0.95 wt%, Silicon (Si): 0.25 to 0.35 wt%, Manganese (Mn): 1.50 to 1.60 wt%, Aluminum (Al): 0.02 to 0.06 wt%, Titanium (Ti): 0.01 ~ 0.02% by weight, niobium (Nb): 0.01 to 0.03% by weight, nickel (Ni): 0.2 to 0.4% by weight and the remaining Fe and inevitable impurities,
Has a complex structure containing ferrite and bainite,
A thick steel sheet having a yield strength of at least 420 MPa and an average impact toughness of at least 250 J at -40 ° C.
The method of claim 10,
The thick steel plate
Phosphorus (P): 0.012% by weight or less, sulfur (S): 0.003% by weight or less and nitrogen (N): 50 ppm or less thick steel sheet, characterized in that it is included.

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