KR101344640B1 - High strength steel plate and method for manufacturing the same - Google Patents

Abstract

Disclosed are a high strength steel sheet having excellent strength and low temperature impact toughness, and having a resistance ratio, and a method of manufacturing the same.
High-strength steel sheet manufacturing method according to the present invention by weight, carbon (C): 0.04 ~ 0.15%, silicon (Si): 0.15 ~ 0.35%, manganese (Mn): 1.2 ~ 1.6%, phosphorus (P): 0.02% Sulfur (S): 0.005% or less, Niobium (Nb): 0.04 to 0.06%, Molybdenum (Mo): 0.05 to 0.1%, Titanium (Ti): 0.005 to 0.02%, Vanadium (V): 0.02 to 0.06% And a hot rolling step of firstly rolling the slab plate composed of the remaining iron (Fe) and unavoidable impurities in the austenite recrystallization region, and then secondly rolling in the austenite microcrystallization region. And a cooling step of cooling the hot rolled sheet material. The secondary rolling is characterized in that the cumulative reduction ratio is 50% or less and the end temperature is 860 ° C or more.

Description

High strength steel sheet and its manufacturing method {HIGH STRENGTH STEEL PLATE AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high-strength steel sheet that can be used as a material such as line pipes and a method of manufacturing the same, and more specifically, to the alloy component such as molybdenum and the like, and having a tensile strength of 600 MPa or more and a resistance of 0.8 or less. It relates to a high strength steel sheet having a composite ratio and a manufacturing method thereof.

The line pipe means a steel pipe buried in the ground mainly for transportation of crude oil or natural gas, and high pressure is usually applied because such high pressure crude oil or gas flows in the line pipe.

Such line pipes are being expanded to polar regions and cold regions due to resource depletion. In order to use the line pipe in such polar regions and cold regions, the material must have high strength and low temperature toughness, and high deformation ability due to earthquakes and other tectonic fluctuations is required.

In order for the material to have high deformation capacity, the yield ratio, which means the ratio of yield strength (YS) to tensile strength (TS), must be low.

Background art related to the present invention is a method of manufacturing a resistance-ratio-type high toughness hot rolled steel disclosed in Republic of Korea Patent Publication No. 10-2002-0051254 (2002.06.28. Publication).

One object of the present invention is to provide a method of manufacturing a high strength steel sheet having a yield ratio of 0.8 or less while having a high strength of 600 MPa or more through an alloy component control and process control such as molybdenum (Mo).

Another object of the present invention is to provide a high strength steel sheet that can be utilized as a material such as line pipe by having a high strength of 600MPa or more and a resistance ratio of 0.8 or less.

High-strength steel sheet manufacturing method according to an embodiment of the present invention for achieving the above one object by weight, carbon (C): 0.04 ~ 0.15%, silicon (Si): 0.15 ~ 0.35%, manganese (Mn): 1.2 ~ 1.6%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Niobium (Nb): 0.04 ~ 0.06%, Molybdenum (Mo): 0.05 ~ 0.1%, Titanium (Ti): 0.005 ~ 0.02 %, Vanadium (V): hot rolling step of firstly rolling the slab plate composed of 0.02 to 0.06% and the remaining iron (Fe) and inevitable impurities in the austenite recrystallization zone, and then second rolling in the austenite microcrystallization zone; And a cooling step of cooling the hot rolled sheet; wherein the secondary rolling includes a cumulative reduction ratio ((AB) / AX 100), where A is a sheet thickness at the start of secondary rolling, and B is a secondary. The thickness of the sheet at the end of rolling) is 50% or less and the end temperature is 860 ° C or more.

 At this time, the hot rolling step is preferably carried out so that the end temperature of the secondary rolling is 860 ~ 890 ℃.

In addition, the hot rolling step is preferably carried out so that the cumulative reduction of the secondary rolling is 20 to 50%.

High strength steel sheet according to an embodiment of the present invention for achieving the other object by weight, carbon (C): 0.04 ~ 0.15%, silicon (Si): 0.15 ~ 0.35%, manganese (Mn): 1.2 ~ 1.6% , Phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, niobium (Nb): 0.04-0.06%, molybdenum (Mo): 0.05-0.1%, titanium (Ti): 0.005-0.02%, vanadium (V): 0.02 to 0.06% and the remaining iron (Fe) and inevitable impurities, more than 70vol% of acicular ferrite (acicular ferrite), less than 10vol% bainite and polygonal ferrite, martensitic austenite It is characterized by having a microstructure comprising less than 10 vol% of one or more secondary phases of knight (Martensitic Austenite).

In this case, the steel sheet may have a tensile strength of 600 MPa or more, Charpy average impact absorption energy of 290 J or more, and a yield ratio of 0.8 or less at -40 ° C.

High-strength steel sheet manufacturing method according to the present invention by controlling the components such as molybdenum (Mo), and controlling the process conditions such as hot rolling, cooling, etc., a high-strength steel sheet having a high strength of at least 600MPa tensile strength, low-temperature impact toughness, and a resistance ratio Can be prepared.

Therefore, the manufactured steel sheet can be utilized as a material such as line pipes of polar regions and cold regions through such characteristics as strength, low temperature impact toughness, and resistance ratio.

1 is a flow chart schematically showing a method of manufacturing a high strength steel sheet according to an embodiment of the present invention.
Figure 2 shows the optical structure picture of the steel sheet prepared according to Example 1.
Figure 3 shows an optical structure picture of a steel sheet prepared according to Comparative Example 2.

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. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey 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 high strength steel sheet according to an embodiment of the present invention and a manufacturing method thereof will be described in detail with reference to the accompanying drawings.

High strength steel plate

High-strength steel sheet according to the present invention, in weight%, carbon (C): 0.04 ~ 0.15%, silicon (Si): 0.15 ~ 0.35%, manganese (Mn): 1.2 ~ 1.6%, phosphorus (P): 0.02% or less , Sulfur (S): 0.005% or less, niobium (Nb): 0.04 to 0.06%, molybdenum (Mo): 0.05 to 0.1%, titanium (Ti): 0.005 to 0.02%, vanadium (V): 0.02 to 0.06% Include.

The rest of the above components are composed of iron (Fe) and impurities inevitably included in the steelmaking process and the like.

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

Carbon (C)

In the present invention, carbon (C) is added to secure the strength of the steel sheet.

The carbon is preferably added in 0.04 ~ 0.15% by weight of the total weight of the steel sheet. If the amount of carbon added is less than 0.04% by weight, the strength of the steel sheet is insufficient. On the other hand, if the addition amount of carbon exceeds 0.15% by weight, the low temperature impact toughness and weldability of the steel sheet are deteriorated.

Silicon (Si)

Silicon (Si) is added as a deoxidizer to remove oxygen in steel during the steelmaking process. Silicon also contributes to the strength improvement of the steel sheet through solid solution strengthening.

The silicon is preferably added in 0.15 ~ 0.35% by weight of the total weight of the steel sheet. When the amount of silicon added is less than 0.15% by weight, the deoxidation effect due to the addition of silicon is insufficient. On the contrary, when the amount of added silicon exceeds 0.35% by weight, there is a problem in that a large amount of oxide is formed on the surface of the steel sheet to hinder the plating property of the steel sheet and lower the weldability.

Manganese (Mn)

Manganese (Mn) is an austenite stabilizing element and serves to improve the strength and impact resistance at low temperatures by making the grain finer.

The manganese is preferably added in 1.2 ~ 1.6% by weight of the total weight of the steel sheet. When the addition amount of manganese is less than 1.2% by weight, the effect of addition thereof is insufficient. On the contrary, when the amount of manganese exceeds 1.6% by weight, there is a problem of lowering the low temperature impact toughness.

Phosphorus (P)

Phosphorus (P) contributes partly to the strength improvement, but it is a representative element that lowers impact toughness at low temperatures. The lower the content is, the better.

Therefore, in the present invention, the phosphorus content is limited to 0.02% by weight or less of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) is an element that is inevitably contained in the production of steel together with phosphorus (P), and forms an emulsion-based inclusion (MnS) to lower the low-temperature impact toughness.

Therefore, in the present invention, the content of sulfur was limited to 0.005% by weight or less of the total weight of the steel sheet.

Niobium (Nb)

Niobium (Nb) combines with carbon (C) and nitrogen (N) to form carbides or nitrides. This improves the strength and low temperature toughness by inhibiting grain growth during rolling to refine grains.

The niobium is preferably added in 0.04 to 0.06% by weight of the total weight of the steel sheet. When the amount of niobium added is less than 0.04% by weight, the above niobium addition effect cannot be sufficiently exhibited. On the contrary, when the addition amount of niobium exceeds 0.06% by weight, the weldability of the steel sheet is lowered, and there is a risk of lowering the impact toughness.

Molybdenum (Mo)

Molybdenum (Mo) improves steel strength and leads to continuous yielding behavior.

The molybdenum is preferably added at 0.05 to 0.1% by weight of the total weight of the steel sheet. When the addition amount of molybdenum is less than 0.05% by weight, it is difficult to sufficiently exhibit the above-described molybdenum addition effect. On the contrary, when the addition amount of molybdenum exceeds 0.1% by weight, it is advantageous to achieve strength and resistance ratio, but there is a problem in that low-temperature impact toughness of the steel sheet is lowered due to an increase in secondary phase such as M / A (Martensitic Austenite).

Titanium (Ti)

Titanium (Ti) contributes to finer crystal grains in the steel sheet and to improvement in impact resistance at low temperatures.

The titanium is preferably added in 0.005 to 0.02% by weight of the total weight of the steel sheet. When the addition amount of titanium is less than 0.005% by weight, the effects such as improvement in impact toughness at low temperatures and the like are insufficient. On the other hand, if the added amount of titanium is more than 0.02 wt%, the solid solution titanium reacts with the carbon (C) to form a carbide, which may result in deterioration of toughness.

Vanadium (V)

Vanadium (V) is an element that forms precipitation and contributes to the improvement of strength and improves the hardenability.

The vanadium is preferably added at 0.02 to 0.06% by weight of the total weight of the steel sheet. When the amount of vanadium added is less than 0.02% by weight, the effect of addition is insufficient. On the contrary, when the added amount of vanadium exceeds 0.06% by weight, the brittleness of the steel sheet increases.

The high strength steel sheet according to the present invention is martens, also referred to as acicular ferrite 70vol% or more, bainite less than 10vol% and polygonal ferrite, phase martensite by controlling the above components and process conditions described below. It may have a microstructure comprising less than 10 vol% of one or more secondary phases of Martensitic Austenite. The bainite and the secondary phase is increased in strength as the fraction increases, but when bainite or the secondary phase is included in 10 vol% or more, there is a problem in that low-temperature impact toughness is greatly reduced.

In addition, the high strength steel sheet according to the present invention may have a tensile strength of 600 MPa or more, a Charpy average impact absorption energy of 290 J or more, and a yield ratio of 0.8 or less in terms of mechanical properties.

High strength steel plate manufacturing method

1 schematically shows a method of manufacturing a high strength steel sheet according to an embodiment of the present invention.

Referring to Figure 1, the high strength steel sheet manufacturing method according to the present invention includes a hot rolling step (S110) and a cooling step (S120). The hot rolling step further includes a first rolling step S111 and a second rolling step S112.

Hot rolling

In the hot rolling step (S110), the composition, that is by weight%, carbon (C): 0.04 ~ 0.15%, silicon (Si): 0.15 ~ 0.35%, manganese (Mn): 1.2 ~ 1.6%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, niobium (Nb): 0.04 to 0.06%, molybdenum (Mo): 0.05 to 0.1%, titanium (Ti): 0.005 to 0.02%, vanadium (V): 0.02 to A slab sheet made of 0.06% and the remaining iron (Fe) and unavoidable impurities is hot rolled. In the case of slab plate, it may be reheated for about 1 to 3 hours at a temperature of about 1150 ~ 1250 ℃ for the purpose of reusing and homogenizing the precipitate.

Hot rolling step (S110) in the present invention includes a primary rolling step (S111) carried out in the austenite recrystallization region and a secondary rolling step (S112) carried out in the austenite uncrystallized region.

At this time, the secondary rolling step (S112), the cumulative reduction ratio ((AB) / AX 100) in the secondary rolling step, where A is the plate thickness at the start of the secondary rolling, B is the plate at the end of the secondary rolling Thickness) is 50% or less, and finish temperature is 860 degreeC or more. This is because when the cumulative reduction ratio of the secondary rolling exceeds 50% or when the end temperature of the secondary rolling is less than 860 ° C, it is difficult to secure a target resistance yield ratio of 0.8 or less due to the increase in yield strength.

On the other hand, it is more preferable that the finishing temperature of secondary rolling is 860-890 degreeC. If the end temperature of the secondary rolling exceeds 890 ° C., it may be difficult to secure sufficient strength, and excessive cooling may be required to improve this.

Moreover, it is more preferable that the cumulative reduction ratio of secondary rolling is 20 to 50%. When the cumulative reduction ratio of the secondary rolling is less than 20%, it is difficult to secure a uniform and fine structure, and coarse structure of the central portion in the thickness direction of the steel sheet may lower the low-temperature impact toughness.

Cooling

Next, in the cooling step (S120) to cool the hot rolled plate, to form a final microstructure.

In addition, the cooling is preferably carried out at a cooling rate of 5 ~ 20 ℃ / sec. If the cooling rate is less than 5 ° C / sec is promoted grain growth is difficult to secure strength. On the contrary, when the cooling rate exceeds 20 ° C / sec, the bainite fraction increases to increase the strength, but there is a problem in that low-temperature impact toughness is lowered.

At this time, the cooling is preferably carried out so that the cooling end temperature is 450 ~ 650 ℃ based on the cooling rate. If the cooling end temperature is less than 450 ℃ there is a problem that the low temperature transformation toughness is formed a large amount low temperature impact toughness. On the contrary, when the cooling end temperature exceeds 650 ° C, there is a problem that the strength is insufficient due to the formation of coarse microstructure.

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. Details that are not described herein will be omitted since the description can be inferred by those skilled in the art.

1. Manufacture of steel sheet

The steel plate was manufactured by the composition of Table 1, and the process conditions of Table 2.

[Table 1]

[Table 2]

ARR: Cumulative reduction rate of secondary rolling FRT: Secondary rolling end temperature

FCT: Cooling end temperature C / R: Cooling rate

YS: Yield Strength TS: Tensile Strength

El: Elongation CVN: Charpy impact absorption energy (3 times average)

YR: Yield Ratio

2. Property evaluation result

Referring to Table 2, the steel sheets prepared according to Examples 1 and 2 satisfying the composition range and process conditions presented in the present invention yield a tensile strength of 600 MPa or more, a -40 ° C Charpy average impact absorption energy of 290 J or more, and a yield of 0.8 or less. All of the rain was satisfied.

However, in the case of the steel sheet according to Comparative Example 1 in which molybdenum was not added and the cumulative reduction ratio of the secondary rolling and the cooling end temperature of the secondary rolling were outside the ranges proposed in the present invention, the yield ratio showed very high value of 0.91. It was.

In addition, in the case of the steel sheets according to Comparative Examples 2 to 3 in which molybdenum was added in excess of 0.1 wt%, and the cumulative reduction ratio of secondary rolling and the cooling end temperature of secondary rolling were outside the ranges proposed in the present invention, the yield ratio was Although lower than the steel sheet according to Comparative Example 1, it still exceeded the target of 0.8. In particular, in the case of the steel sheet according to Comparative Examples 2 to 3, the Charpy average impact absorption energy was very low.

In addition, the composition range proposed in the present invention is satisfactory, but in the case of the steel sheet according to Comparative Example 4 in which the cumulative reduction ratio of the secondary rolling and the cooling end temperature of the secondary rolling were outside the ranges proposed in the present invention, the yield ratio was 0.86. The target exceeded 0.8.

3. Microstructure Evaluation Results

Figure 2 shows an optical tissue picture of the steel sheet prepared according to Example 1, Figure 3 shows an optical tissue picture of the steel sheet prepared according to Comparative Example 2.

Referring to Figure 2, in the case of the steel sheet according to Example 1, it can be seen that the acicular ferrite of the needle-shaped well formed. On the other hand, in the case of the steel sheet according to Comparative Example 2 in which molybdenum is added relatively much, it can be seen that a large amount of secondary phases such as martensitic austenite (M / A), also referred to as island martensite, are formed. .

Due to the difference in microstructure, the steel sheet according to Example 1 may be seen to exhibit excellent low-temperature impact toughness with a resistance ratio compared to the steel sheet according to Comparative Example 2.

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. Therefore, the scope of the present invention will be determined by the claims described below.

Claims (7)

By weight%, carbon (C): 0.04 to 0.15%, silicon (Si): 0.15 to 0.35%, manganese (Mn): 1.2 to 1.6%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% Niobium (Nb): 0.04 to 0.06%, Molybdenum (Mo): 0.05 to 0.1%, Titanium (Ti): 0.005 to 0.02%, Vanadium (V): 0.02 to 0.06% and the remaining iron (Fe) and inevitable impurities A hot rolling step of slab plate material consisting of a first roll in the austenite recrystallized region, and then a second roll in the austenite uncrystallized region; And
A cooling step of cooling the hot rolled sheet;
The secondary rolling has a cumulative reduction ratio ((AB) / AX 100), where A is the sheet thickness at the start of the secondary rolling, B is the sheet thickness at the end of the secondary rolling, and the end temperature is 860. A high strength steel sheet manufacturing method, characterized in that it is carried out to be ℃ or more.
The method of claim 1,
The hot rolling step
The method of producing a high strength steel sheet, characterized in that the end temperature of the secondary rolling is 860 ~ 890 ℃.
The method of claim 1,
The hot rolling step
The method for producing a high strength steel sheet, characterized in that the cumulative reduction ratio of the secondary rolling is 20 to 50%.
The method of claim 1,
The cooling step
High strength steel sheet manufacturing method characterized in that carried out at a cooling rate of 5 ~ 20 ℃ / sec.
5. The method of claim 4,
The cooling step
High strength steel sheet manufacturing method characterized in that the cooling end temperature is carried out to be 450 ~ 650 ℃.
By weight%, carbon (C): 0.04 to 0.15%, silicon (Si): 0.15 to 0.35%, manganese (Mn): 1.2 to 1.6%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% Niobium (Nb): 0.04 to 0.06%, Molybdenum (Mo): 0.05 to 0.1%, Titanium (Ti): 0.005 to 0.02%, Vanadium (V): 0.02 to 0.06% and the remaining iron (Fe) and inevitable impurities Made up of
Having a microstructure comprising at least 70 vol% of acicular ferrite, less than 10 vol% of bainite and less than 10 vol% of at least one secondary phase of polygonal ferrite, Martensitic Austenite High strength steel sheet, characterized in that.
The method according to claim 6,
The steel sheet
A high strength steel sheet having a tensile strength of 600 MPa or more, a Charpy average impact absorption energy of 290 J or more and a yield ratio of 0.8 or less at -40 ° C.

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