KR20150025946A

KR20150025946A - Steel sheet and method of manufacturing the same

Info

Publication number: KR20150025946A
Application number: KR20130104160A
Authority: KR
Inventors: 김규태; 강동훈; 윤동현
Original assignee: 현대제철 주식회사
Priority date: 2013-08-30
Filing date: 2013-08-30
Publication date: 2015-03-11

Abstract

Disclosed are a steel sheet, capable of reducing the contents of high-priced alloy element and improving the property of hydrogen induced cracking and low temperature toughness by controlling the alloy component and process condition, and a manufacturing method thereof. According to the present invention, the steel sheet includes 0.02-0.04 wt% of C, 0.2-0.3 wt% of Si, 1.0-1.4 wt% of Mn, 0.008 wt% or less of P, 0.001 wt% or less of S, 0.02-0.05 wt% of Al, 0.2-0.5 wt% of Ni, 0.2-0.3 wt% of Cu, 0.2-0.3 wt% of Cr, 0.1-0.2 wt% of Mo, 0.01-0.03 wt% of Nb, 0.04-0.08 wt% of V, 0.01-0.02 wt% of Ti, 0.001-0.004 wt% of Ca, and the remainder consisting of iron (Fe) and inevitable impurities. The microstructure has a composite structure including accicular ferrite, ferrite, and pearlite. The fraction of accicular ferrite is equal to or greater than 70% as a cross-section area rate.

Description

Technical Field [0001] The present invention relates to a steel sheet and a method of manufacturing the steel sheet.

The present invention relates to a steel sheet and a method of manufacturing the same, and more particularly, to a steel sheet and a steel sheet capable of improving the hydrogen-organic cracking property and the low temperature toughness while controlling the content of an expensive alloy element, And a manufacturing method thereof.

Steel sheets for line pipes used for transporting gas or crude oil containing hydrogen sulfide (H ₂ S) gas over a certain amount are vulnerable to Hydrogen Induced Cracking (HIC), which is more clean than steel plates for transporting sweet gas and oil. It must be strictly applied.

Hydrogen components contained in the H ₂ S gas penetrate into the steel, increasing the pressure with hydrogen molecules, and cracks are generated starting from the ends of inclusions such as oxides, MnS, Nb (C, N) .

The elongated inclusions such as MnS minimize the sulfur impurities and form CaS through the addition of Ca to control the shape of the inclusions. In particular, niobium carbonitrides such as Nb (C, N) Due to the segregation in the center, the sensitivity to hydrogen organic cracks is relatively increased in the formation of low-temperature transformed structure such as local bainite and martensite, and it is very easy to act as a crack origin.

Niobium is basically difficult to completely remove the formation of coarse carbonitrides in the center of the thickness, and as the reheating temperature of the prepared slab is low, the solubility in the steel decreases and the sensitivity to the occurrence of hydrogen organic crack increases. If high temperature heat is essential. Such high-temperature heating has a disadvantage in that it is difficult to ensure low-temperature toughness because it causes coarse-grained structure in the center of thickness in a relatively thick steel sheet.

Prior art related to the present invention is a high carbon hot-rolled steel sheet disclosed in Korean Patent Publication No. 10-2010-0021273 (published on Mar.

An object of the present invention is to provide a method of manufacturing a steel sheet capable of improving the hydrogen organic cracking property and the low temperature toughness while reducing the content of expensive alloying elements through the control of alloy components and the control of process conditions.

Another object of the present invention is to provide a steel sheet produced by the above method and exhibiting a tensile strength (TS) of 550 to 650 MPa, a yield strength (YS) of 450 to 550 MPa, and a ductile fracture rate of 90% will be.

In order to achieve the above object, a steel sheet manufacturing method according to an embodiment of the present invention comprises: (a) 0.02 to 0.04% of C, 0.2 to 0.3% of Si, 1.0 to 1.4% of Mn, 0.008% 0.2 to 0.3% of Cr, 0.2 to 0.3% of Cr, 0.1 to 0.2% of Mo, 0.01 to 0.03% of Nb, 0.01 to 0.03% of Mo, 0.001 to 0.10% of S, 0.02 to 0.05% of Al, 0.2 to 0.5% (Slab Reheating Temperature): 1050 to 1150 占 폚, wherein the slab plate comprises 0.04 to 0.08% of Ti, 0.01 to 0.02% of Ca, 0.001 to 0.004% of Ca, and the balance of Fe and unavoidable impurities. (b) subjecting the reheated plate to an FRT (Finish Rolling Temperature) at a temperature of 750 to 850 캜; And (c) cooling the hot-rolled plate to 300 to 550 ° C.

According to another aspect of the present invention, there is provided a steel sheet comprising 0.02 to 0.04% of C, 0.2 to 0.3% of Si, 1.0 to 1.4% of Mn, 0.008% or less of P, 0.001 0.2 to 0.3% of Cr, 0.2 to 0.3% of Cr, 0.1 to 0.2% of Mo, 0.01 to 0.03% of Nb, 0.01 to 0.03% of V, 0.04 to 0.08 of V, (Fe) and unavoidable impurities, and the microstructure includes accicular ferrite, ferrite, and pearlite. The steel sheet according to claim 1, wherein the steel sheet contains 0.01-0.02% Ti, 0.001-0.02% Ti, , Wherein the fraction of the acicular ferrite is 70% or more in terms of the cross-sectional area ratio.

In the present invention, the content of carbon (C) is lowered and niobium (Nb) is added in an amount of 0.01 to 0.03 wt% or less to increase the solubility of the niobium-based carbonitride to 1050 to 1150 DEG C The niobium-based carbonitride is dissolved even at the reheating temperature of the molten steel, so that the low-temperature toughness can be ensured, and the strength can be secured by reducing the content of molybdenum (Mo), which is a relatively expensive element, to 0.1 to 0.2 wt.

Accordingly, in the present invention, it is possible to manufacture a steel sheet with a satisfactory hydrogen organic cracking property and low temperature toughness through optimum adjustment of alloy components.

1 is a flowchart showing a method of manufacturing a steel sheet according to an embodiment of the present invention.
2 is a photograph showing a microstructure of a specimen according to Comparative Example 1. Fig.
3 is a photograph showing the microstructure of the specimen according to Comparative Example 1. Fig.
4 is a photograph showing the microstructure of the specimen according to Example 1. Fig.

The features of the present invention and the method for achieving the same will be apparent from the accompanying drawings and the embodiments described below. However, the present invention is not limited to the embodiments described below, but may be embodied in various forms. The present embodiments are provided so that the disclosure of the present invention is complete and that those skilled in the art will fully understand the scope of the present invention. The invention is only defined by the description of the claims.

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

Steel plate

The steel sheet according to the present invention has a tensile strength (TS) of 550 to 650 MPa, a yield strength (YS) of 450 to 550 MPa, a ductile fracture rate of 90% or more at -20 캜, and a hydrogen induced cracking index CLR Crack Length Ratio (CTR), and Crack Sensitivity Ratio (CSR) of less than 5%, less than 1%, and less than 0.5%, respectively.

The steel sheet according to the present invention preferably contains 0.02 to 0.04% of C, 0.2 to 0.3% of Si, 1.0 to 1.4% of Mn, 0.008% or less of P, 0.001% or less of S, 0.05 to 0.5% of Ni, 0.2 to 0.3% of Cu, 0.2 to 0.3% of Cr, 0.2 to 0.3% of Cr, 0.1 to 0.2% of Mo, 0.01 to 0.03% of Nb, 0.04 to 0.08% of V, 0.01 to 0.02 of Ti %, Ca: 0.001 to 0.004%, and the balance of iron (Fe) and unavoidable impurities.

At this time, the steel sheet has a composite structure in which the microstructure includes accicular ferrite, ferrite, and pearlite, and the fraction of the acicular ferrite has a cross-sectional area ratio of 70% or more.

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

Carbon (C)

Carbon (C) is added to ensure strength.

The carbon (C) is preferably added in an amount of 0.02 to 0.04% by weight based on the total weight of the steel sheet according to the present invention. When the content of carbon (C) is less than 0.02% by weight, it may be difficult to secure sufficient strength. On the contrary, when the content of carbon (C) exceeds 0.04% by weight, toughness may be lowered.

Silicon (Si)

Silicon (Si) acts as a deoxidizer in the steel and contributes to securing strength.

The silicon (Si) is preferably added in an amount of 0.2 to 0.3% by weight based on the total weight of the steel sheet according to the present invention. When the content of silicon (Si) is less than 0.2% by weight, the effect of addition is insufficient. On the contrary, when the content of silicon (Si) exceeds 0.3% by weight, the toughness and weldability of the steel sheet deteriorate.

Manganese (Mn)

Manganese (Mn) is an element useful for improving strength without deteriorating toughness.

The manganese (Mn) is preferably added in a content ratio of 1.0 to 1.4% by weight based on the total weight of the steel sheet according to the present invention. When the content of manganese (Mn) is less than 1.0% by weight, the effect of addition is insufficient. On the contrary, when the content of manganese (Mn) exceeds 1.4% by weight, the sensitivity to temper embrittlement increases.

In (P)

Phosphorus (P) is an element contributing to strength improvement.

However, when the content of phosphorus (P) exceeds 0.008% by weight in the present invention, fine segregation is formed as well as center segregation, which adversely affects the material and may deteriorate the weldability. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.008% by weight or less based on the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) is an element contributing to improvement of processability.

However, when the content of sulfur (S) exceeds 0.001% by weight in the present invention, there is a problem that the weldability is greatly deteriorated. Therefore, in the present invention, the content of sulfur (S) is limited to 0.001% by weight or less based on the total weight of the steel sheet.

Aluminum (Al)

Aluminum (Al) acts as a deoxidizer to remove oxygen in the steel.

The aluminum (Al) is preferably added in a content ratio of 0.02 to 0.05% by weight based on the total weight of the steel sheet according to the present invention. When the content of aluminum (Al) is less than 0.02% by weight, the effect of adding silicon can not be exhibited properly. On the contrary, when the content of aluminum (Al) exceeds 0.05% by weight, Al ₂ O ₃ , which is a non-metallic inclusion, is formed to lower the impact resistance at low temperatures.

Nickel (Ni)

Nickel (Ni) is an element effective for improving toughness while improving toughness.

The nickel (Ni) is preferably added at a content ratio of 0.2 to 0.5% by weight based on the total weight of the steel sheet according to the present invention. When the content of nickel (Ni) is less than 0.2% by weight, the addition effect is insignificant. On the contrary, when the content of nickel (Ni) exceeds 0.5% by weight, the workability of the steel sheet is lowered and the manufacturing cost is increased.

Copper (Cu)

Copper (Cu) together with nickel (Ni) serves to improve the hardenability of the steel and the impact resistance at low temperatures.

The copper (Cu) is preferably added in an amount of 0.2 to 0.3% by weight based on the total weight of the steel sheet according to the present invention. When the content of copper (Cu) is less than 0.2% by weight, the effect of adding copper can not be exhibited properly. On the contrary, when the content of copper (Cu) exceeds 0.3% by weight, it exceeds the solubility limit, it does not contribute to the increase in the strength, and there is a problem of causing the redispersible brittleness.

Molybdenum (Mo)

Molybdenum (Mo) is a substitutional element and improves the strength of steel by solid solution strengthening effect. In addition, molybdenum (Mo) serves to improve the hardenability of the steel.

The molybdenum (Mo) is preferably added in an amount of 0.1 to 0.2% by weight based on the total weight of the steel sheet according to the present invention. If the content of molybdenum (Mo) is less than 0.1% by weight, the above effect can not be exhibited properly. On the contrary, when the content of molybdenum (Mo) exceeds 0.2% by weight, there is a problem of raising the manufacturing cost without any further effect.

Niobium (Nb)

Niobium (Nb) combines with carbon (C) at high temperature to form carbide. Niobium carbide improves the strength and low-temperature toughness of a steel sheet by suppressing grain growth during hot-rolling and making crystal grains finer.

The niobium (Nb) is preferably added in an amount of 0.01 to 0.03% by weight based on the total weight of the steel sheet according to the present invention. If the content of niobium is less than 0.01% by weight, it may be difficult to exhibit the above effects. On the other hand, when a large amount of niobium (Nb) is added in excess of 0.03 wt%, the strength and low temperature toughness due to the increase in niobium content are not improved any more, but are present in a solid state in ferrite, There is a risk of degradation.

Vanadium (V)

Vanadium (V) plays a role in improving the strength of steel through precipitation strengthening effect by precipitate formation.

The vanadium (V) is preferably added in an amount of 0.04 to 0.08% by weight based on the total weight of the steel sheet according to the present invention. When the content of vanadium (V) is less than 0.04% by weight, the precipitation strengthening effect due to vanadium addition is insufficient. On the other hand, when the content of vanadium (V) exceeds 0.08% by weight, the low-temperature impact toughness is deteriorated.

Titanium (Ti)

Titanium (Ti) has the effect of improving the toughness and strength of hot-rolled steel sheet by making Ti (C, N) precipitates having high stability at high temperatures, thereby finishing the austenite grain growth and refining the texture of the welded portion.

The titanium (Ti) is preferably added in an amount of 0.01 to 0.02% by weight based on the total weight of the steel sheet according to the present invention. When the content of titanium (Ti) is less than 0.01% by weight, there arises a problem that aging hardening occurs because of the remaining solid carbon and nitrogen employed without precipitation. On the contrary, when the content of titanium (Ti) exceeds 0.02% by weight, coarse precipitates are produced, which lowers the low-temperature impact properties of the steel and raises manufacturing costs without further effect of addition.

Calcium (Ca)

Calcium (Ca) is added for the purpose of improving electrical resistance weldability by inhibiting the formation of MnS inclusions by forming CaS inclusions. That is, calcium (Ca) has a higher affinity with sulfur than manganese (Mn), so CaS inclusions are formed and CaS inclusions are reduced when calcium is added. Such MnS is stretched during hot rolling to cause hook defects and the like in electrical resistance welding (ERW), so that electrical resistance weldability can be improved.

The calcium (Ca) is preferably added in an amount of 0.001 to 0.004% by weight based on the total weight of the steel sheet according to the present invention. If the content of calcium (Ca) is less than 0.001% by weight, it may be difficult to exhibit the above effects. On the contrary, when the content of calcium exceeds 0.004% by weight, the generation of CaO inclusions is excessively generated, which deteriorates performance and electrical resistance weldability.

Steel plate manufacturing method

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

Referring to FIG. 1, a steel sheet manufacturing method according to an embodiment of the present invention includes a slab reheating step (S110), a hot rolling step (S120), and a cooling step (S130). At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to perform the slab reheating step (S110) in order to obtain effects such as the reuse of the precipitate.

In the steel sheet manufacturing method according to the present invention, the semi-finished slab plate to be subjected to the hot rolling process is composed of 0.02 to 0.04% of C, 0.2 to 0.3% of Si, 1.0 to 1.4% of Mn, 0.008% or less of P 0.2 to 0.3% of Cr, 0.2 to 0.3% of Cr, 0.1 to 0.2% of Mo, 0.01 to 0.03% of Nb, 0.01 to 0.03% of Mo, 0.001 to 0.10% of S, 0.02 to 0.05% of Al, 0.2 to 0.5% : 0.04 to 0.08%, Ti: 0.01 to 0.02%, Ca: 0.001 to 0.004%, and the balance of iron (Fe) and unavoidable impurities.

Reheating slabs

In the slab reheating step S110, the slab plate having the above composition is reheated to a slab reheating temperature (SRT) of 1050 to 1150 占 폚. Here, the slab plate can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. At this time, in the slab reheating step (S110), the slab plate obtained through the continuous casting process is reheated to reuse the segregated components during casting.

Particularly, in the present invention, since the solubility of NbC and NbN is increased even if reheating is performed at a relatively low temperature of 1050 to 1150 ° C by lowering the carbon content, a relatively expensive alloy element It is possible to improve both the hydrogen organic cracking property and the low temperature toughness while reducing the content.

At this time, when the slab reheating temperature (SRT) is lower than 1050 DEG C, there is a problem that the reheating temperature is low and the rolling load becomes large. On the other hand, when the slab reheating temperature exceeds 1150 DEG C, the austenite grains are rapidly coarsened and it is difficult to secure the strength and low temperature toughness of the steel sheet to be produced.

Hot rolling

In the hot rolling step (S120), the reheated plate is subjected to finishing hot rolling under the conditions of FRT (Finishing Rolling Temperature): 750 to 850 占 폚.

In this step, when the rolling finish temperature (FRT) is lower than 750 캜, a problem such as occurrence of blistering due to abnormal reverse rolling may occur. On the other hand, when the rolling finish temperature (FRT) exceeds 850 DEG C, the austenite grains are coarsened and the ferrite grains can not be sufficiently refined after the transformation, which may make it difficult to ensure strength.

At this time, the hot rolling is preferably carried out such that the cumulative rolling reduction is 30 to 70%. If the cumulative rolling reduction is less than 30%, it is difficult to obtain a uniform but fine structure, which may cause a significant variation in strength and impact toughness. On the other hand, when the cumulative rolling reduction exceeds 70%, there is a problem that the rolling process time is prolonged and the fishy property is deteriorated.

Cooling

In the cooling step (S130), the hot rolled plate is cooled to 300 to 550 deg.

At this time, if the cooling end temperature (FCT) is less than 300 ° C, the cost of producing the steel increases, low-temperature structure is generated, which is advantageous in securing strength but is vulnerable to low-temperature toughness. On the other hand, when the cooling end temperature (FCT) exceeds 550 캜, it may be difficult to secure sufficient strength.

In this step, the cooling rate is preferably 15 to 25 DEG C / sec. When the cooling rate is less than 15 DEG C / sec, it is difficult to secure sufficient strength and toughness. On the other hand, when the cooling rate exceeds 25 DEG C / sec, cooling control is difficult, and excessive cooling may adversely affect the shape of the steel sheet.

The steel sheet produced in the above steps S110 to S130 has a low content of carbon (C) and addition of niobium (Nb) in an amount of 0.01 to 0.03% by weight or less so that the solubility of the niobium- The niobium-based carbonitride is dissolved even at the reheating temperature of 1100 to 1150 ° C, which corresponds to a low temperature, so that low-temperature toughness can be ensured and the content of molybdenum (Mo), which is a relatively expensive element, is reduced to 0.1 to 0.2 weight It is possible to secure strength.

Therefore, the steel sheet produced by the method according to the present invention can satisfy both the hydrogen organic cracking property and the low temperature toughness through the optimum alloy component control.

Further, the steel sheet produced by the method according to the present invention has a composite structure in which the microstructure includes accicular ferrite, ferrite and pearlite, wherein the fraction of the acicular ferrite is 70 , A tensile strength (TS) of 550 to 650 MPa, a yield strength (YS) of 450 to 550 MPa, a ductile wavefront ratio at -20 캜 of 90% or more and a crack length ratio (CLR) of 5% .

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

Specimens according to Examples 1 to 3 and Comparative Examples 1 to 3 were prepared with the compositions of Tables 1 and 2 and the process conditions of Table 3.

[Table 1] (unit:% by weight)

[Table 2] (unit:% by weight)

[Table 3]

2. Evaluation of mechanical properties

Table 4 shows the results of evaluation of mechanical properties of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 to 3.

[Table 4]

Tensile Strength (TS): 550 to 650 MPa, Yielding Strength (YS): 450 to 550 MPa, and -20 DEG C, which correspond to the target values, are shown in Tables 1 to 4, Of 90% or more and a crack length ratio (CLR) of 5% or less.

On the other hand, the tensile strength (TS), the yield strength (YS) and the crack length ratio (CLR) of the specimen according to Comparative Example 1 satisfied the target value, but the ductile fracture ratio at -20 ° C was only 15% . In particular, in the case of Comparative Example 1, since the slab reheating temperature is 1200 ° C, which is outside the range suggested by the present invention, there is a problem that productivity is deteriorated.

In the case of the specimen according to the comparative example 2, the ductile waveguide ratio at -20 ° C satisfied the target value, but the tensile strength TS and the yield strength YS were below the target value, and the crack length ratio (CLR) Was measured at 75%. Therefore, it can be seen that the specimen according to Comparative Example 2 has poor hydrogen organic cracking properties.

In the case of the specimen according to Comparative Example 3 in which niobium (Nb) was not added, the physical properties satisfied all of the target values. However, in the case of Comparative Example 3, there is a problem that productivity is deteriorated due to an increase in alloy design cost due to excessive addition of carbon (C) content and molybdenum (Mo) addition amount beyond the content range disclosed in the present invention.

Fig. 2 is a photograph showing the microstructure of the specimen according to Comparative Example 1, Fig. 3 is a photograph showing the microstructure of the specimen according to Comparative Example 1, Fig. 4 is a photograph showing the microstructure of the specimen according to Example 1 This is the picture shown.

As shown in FIG. 2 to FIG. 4, in the case of the specimen according to Example 1 and Comparative Example 1, it can be seen that cracks did not occur as a result of the HIC test.

On the other hand, in the case of the specimen according to Comparative Example 2, it was confirmed that a crack occurred as a result of the HIC test. At this time, in the case of the test piece according to Comparative Example 2, the Crack Length Ratio (Crack Ratio), the Crack Thickness Ratio (CTR) and the Crack Sensitivity Ratio (CSR) were measured to be 75%, 1.1% and 0.1%, respectively.

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: Hot rolling step
S130: cooling step

Claims

(a) 0.001% or less, S: 0.001% or less, Al: 0.02-0.05%, Ni: 0.2-0.0% The steel sheet is made up of at least one member selected from the group consisting of Cu, Cr, and Cr in an amount of 0.2 to 0.3%, 0.2 to 0.3 wt%, 0.1 to 0.2 wt%, 0.1 to 0.2 wt%, Nb to 0.01 to 0.03 wt%, V to 0.04 to 0.08 wt%, Ti to 0.01 to 0.02 wt% 0.004% and the remaining iron (Fe) and unavoidable impurities to a slab reheating temperature (SRT) of 1050 to 1150 占 폚;
(b) subjecting the reheated plate to an FRT (Finish Rolling Temperature) at a temperature of 750 to 850 캜; And
(c) cooling the hot-rolled plate to 300 to 550 占 폚.

The method according to claim 1,
In the step (c)
The cooling
At a rate of 15 to 25 占 폚 / sec.

0.001% or less of S, 0.001% or less of S, 0.02-0.05% of Al, 0.2-0.5% or less of Ni, 0.2 to 0.3% of Si, 0.2 to 0.3% of Cu, 0.2 to 0.3% of Cr, 0.1 to 0.2% of Mo, 0.01 to 0.03% of Nb, 0.04 to 0.08% of V, 0.01 to 0.02% of Ti, 0.001 to 0.004% of Ca and (Fe) and inevitable impurities,
Wherein the microstructure has a composite structure including accicular ferrite, ferrite and pearlite, wherein the fraction of the acicular ferrite has a cross-sectional area ratio of not less than 70%.

The method of claim 3,
The steel sheet
A tensile strength (TS) of 550 to 650 MPa, a yield strength (YS) of 450 to 550 MPa, and a ductile waveguide ratio at -20 캜 of 90% or more.

The method of claim 3,
The steel sheet
CLR (Crack Length Ratio): 5% or less.