KR20140072246A

KR20140072246A - Steel sheet and method of manufacturing the same

Info

Publication number: KR20140072246A
Application number: KR1020120136624A
Authority: KR
Inventors: 김규태; 박기정; 윤동현
Original assignee: 현대제철 주식회사
Priority date: 2012-11-29
Filing date: 2012-11-29
Publication date: 2014-06-13

Abstract

A steel sheet having excellent hydrogen-organic cracking properties by minimizing the formation of pearlite bands, which is a main cause of hydrogen induced cracking, through control of alloy components and process conditions, and a method for producing the same.
A steel sheet manufacturing method according to the present invention comprises the steps of: (a) mixing 0.18 to 0.22% of C, 0.2 to 0.4% of Si, 0.8 to 1.2% of Mn, 0.008% or less of P, (Slab reheating temperature) of 1150 to 1250 占 폚 is used as a slab plate composed of 0.1 to 0.5% of Nb, 0.005 to 0.020% of Ca, 0.001 to 0.004% of Ca, 0.1 to 0.3% of Mo and the balance of Fe and unavoidable impurities. &Lt; / RTI > (b) subjecting the reheated plate to an FRT (Finish Rolling Temperature) at a temperature of 840 to 900 캜; And (c) cooling the hot-rolled plate material.

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 manufacturing method thereof, and more particularly, to a steel sheet having excellent hydrogen-organic cracking characteristics through control of alloy components and process conditions and a method of manufacturing the same.

The steel plate for pressure vessels used for gas or crude oil refining containing more than a certain amount of hydrogen sulfide (H ₂ S) gas is more vulnerable to hydrogen induced cracking (HIC) The organizational control that becomes the basis must be strictly applied.

Hydrogen components contained in the hydrogen sulfide (H ₂ S) gas permeate into the steel to increase the pressure of the hydrogen molecules. The cracks develop along the nonmetallic inclusions, such as MnS, and the relatively mild pearlite band, Lt; / RTI >

Although elongated inclusions such as MnS act as major defects that cause cracking even at relatively low hydrogen pressures, they minimize impurities such as phosphorus (P) and sulfur (S) that generate segregation and inclusions during steelmaking, By adding Ca to form CaS, the shape of the inclusions can be controlled to reduce the cause.

Steel plates for use in pressure vessels are subjected to a relatively poor working environment such as high temperature and cryogenic temperature, so that normalizing or post-heat treatment for stress relief is essential for the removal of internal stress and formation of uniform structure. However, in the heat treatment process, formation of a pearlite band through carbon diffusion in the matrix of the steel sheet can not be avoided, and the pearlite band acts as a pathway for propagation of the hydrogen organic crack propagation.

In addition, the carbon content is necessarily increased to increase the strength in the material after 60 t or more, and the increase of the carbon content increases the segregation of the center of the thickness, so that the formation of the pearlite band is inevitably further intensified.

A related art is Korean Patent Laid-Open Publication No. 10-2011-0060449 (published on June 6, 2011), which discloses a steel sheet for pressure vessels excellent in low temperature toughness and hydrogen organic cracking resistance and a method for manufacturing the same .

It is an object of the present invention to provide a method for manufacturing a steel sheet having excellent hydrogen-organic cracking characteristics by minimizing the formation of pearlite bands, which is a main cause of hydrogen induced cracking, through control of alloy components and control of process conditions.

Another object of the present invention is to provide a method for producing a pearlite structure having a tensile strength (TS) of 500 MPa or more and a yield strength (YS) of 300 MPa or more, Is defined as a pearlite band, an average distance between the pearlite bands is 200 m or more.

(A) 0.18 to 0.22% of C, 0.2 to 0.4% of Si, 0.8 to 1.2% of Mn, 0.008% or less of P of the steel sheet, , S: 0.0008% or less, Ni: 0.1 to 0.5%, Nb: 0.005 to 0.020%, Ca: 0.001 to 0.004%, Mo: 0.1 to 0.3%, and the balance iron (Fe) and unavoidable impurities. Slab Reheating Temperature: Reheating to 1150 ~ 1250 ℃; (b) subjecting the reheated plate to an FRT (Finish Rolling Temperature) at a temperature of 840 to 900 캜; And (c) cooling the hot-rolled plate material.

According to another aspect of the present invention, there is provided a steel sheet comprising 0.18 to 0.22% of C, 0.2 to 0.4% of Si, 0.8 to 1.2% of Mn, 0.008% or less of P, (Fe) and unavoidable impurities, and the microstructure is composed of ferrite and pearlite. The amount of the ferrite and the pearlite is in the range of 0.1 to 5% by mass, the content of Ni is 0.1 to 0.5%, the content of Nb is 0.005 to 0.020%, the content of Ca is 0.001 to 0.004% And has a tensile strength (TS) of 500 MPa or more and a yield strength (YS) of 300 MPa or more.

The present invention minimizes the formation of pearlite bands, which are the main cause of hydrogen-induced cracking, through the addition of molybdenum (Mo), thereby securing excellent hydrogen-organic cracking characteristics and minimizing the carbon equivalent (Ceq) The welding characteristics can be improved.

Therefore, the steel sheet produced by the method according to the present invention has an average distance between pearlite bands: 200 m or more, so that it can have excellent welding characteristics while having excellent hydrogen-organic cracking characteristics.

1 is a flowchart schematically showing a method of manufacturing a steel sheet according to an embodiment of the present invention.
FIG. 2 is a flowchart specifically illustrating a steel sheet manufacturing method according to an embodiment of the present invention.
3 is a photograph showing the final microstructure of the specimen prepared according to Example 1. Fig.
Fig. 4 is a photograph showing the final microstructure of the specimen prepared according to Comparative 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 500 MPa or more and a yield strength (YS) of 300 MPa or more through control of alloy components and process conditions and has an average distance between perlite structures of 5 μm or less and an average length Of the pearlite band is defined as a pearlite band, the average distance between the pearlite bands is set to be 200 mu m or more.

The steel sheet according to the present invention preferably contains 0.18 to 0.22% of C, 0.2 to 0.4% of Si, 0.8 to 1.2% of Mn, 0.008% or less of P, 0.0008% or less of S, (Fe) and unavoidable impurities, wherein the microstructure has a composite structure containing ferrite and pearlite, and the ferrite and pearlite are mixed with each other at a ratio of 0.5 to 0.5%, Nb: 0.005 to 0.020%, Ca: 0.001 to 0.004% A tensile strength (TS) of 500 MPa or more and a yield strength (YS) of 300 MPa or more.

It is more preferable that the steel sheet contains carbon (C), manganese (Mn), nickel (Ni) and molybdenum (Mo) within a range satisfying the following formula (1).

[C] + [Mn] / 6 + [Ni] / 15 + [Mo] / 5? 0.430

(Where [] is the weight percentage of each element)

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 secure strength and is the most influential element in weldability. At this time, the influence of alloying elements other than carbon (C) may be expressed as carbon equivalent (Ceq) equivalent to carbon (C).

The carbon (C) is preferably added in an amount of 0.18 to 0.22% 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 0.18% by weight, it may be difficult to secure sufficient strength. On the other hand, if the content of carbon (C) exceeds 0.22% by weight, the toughness may be lowered and the weldability may be deteriorated during the SAW welding.

The steel sheet according to the present invention preferably further contains carbon (C), manganese (Mn), nickel (Ni) and molybdenum (Mo) within a range satisfying the following formula (1).

This is because, at the time of SAW welding for steel pipe manufacturing,

[C] + [Mn] / 6 + [Ni] / 15 + [Mo] / 5? 0.430

(Where [] is the weight percentage of each element), the occurrence of cracks in welds is significantly reduced if the carbon content is within a certain range.

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 0.4% 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 0.4% 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 an amount of 0.8 to 1.2% 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 0.8% by weight, the effect of the addition is insufficient. On the contrary, when the content of manganese (Mn) exceeds 1.2% by weight, there is a problem that the sensitivity to temper embrittlement is increased.

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, in the present invention, when the content of sulfur (S) exceeds 0.0008 wt%, there is a problem that the weldability is greatly deteriorated. Therefore, in the present invention, the content of sulfur (S) is limited to 0.0008 wt% or less of the total weight of the steel sheet.

Nickel (Ni)

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

The nickel (Ni) is preferably added in an amount of 0.1 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 0.1% 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.

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 0.005-0.020 wt% of the total weight of the steel sheet according to the present invention. When the content of niobium is less than 0.005% by weight, it may be difficult to exhibit the above effects properly. On the other hand, when a large amount of niobium (Nb) is added in an amount exceeding 0.020 wt%, the strength and low temperature toughness due to an increase in the amount of niobium (Nb) are not improved any more and are present in a solid state in ferrite, There is a risk of degradation.

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. When the content of Ca is less than 0.001% by weight, it is necessary to control the content of sulfur to be as small as possible in order to satisfy Equation 2, that is, 2.0? [Ca] / [S]? 4.0. 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.

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 0.3% 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 0.3% by weight, there is a problem that the production cost is increased without any further effect.

Steel plate manufacturing method

1 is a flowchart schematically 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 0.18 to 0.22% of C, 0.2 to 0.4% of Si, 0.8 to 1.2% of Mn, 0.008% or less of P 0.001 to 0.004% of Ca, 0.1 to 0.3% of Mo, and the balance of iron (Fe) and unavoidable impurities.

The slab plate preferably includes carbon (C), manganese (Mn), nickel (Ni), and molybdenum (Mo) within a range satisfying the following formula (1).

[C] + [Mn] / 6 + [Ni] / 15 + [Mo] / 5? 0.430

(Where [] is the weight percentage of each element)

Reheating slabs

In the slab reheating step S110, the slab plate having the above composition is reheated to a slab reheating temperature (SRT) of 1150 to 1250 ° C. 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.

At this stage, when the slab reheating temperature (SRT) is less than 1150 DEG C, there is a problem that the reheating temperature is low and the rolling load becomes large. In addition, since the Nb-based precipitates NbC and NbN can not reach the solid solution temperature, they can not be precipitated as fine precipitates upon hot rolling, and the austenite grain growth can not be suppressed, resulting in a rapid coarsening of the austenite grains. On the other hand, when the slab reheating temperature exceeds 1250 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): 840 to 900 ° C.

In this step, when the rolling finish temperature (FRT) is lower than 840 占 폚, there may occur problems such as the occurrence of blistering due to abnormal reverse rolling. On the other hand, if the rolling finish temperature (FRT) is higher than 900 캜, the austenite grains are coarsened and the ferrite grains are not sufficiently refined after the transformation, which may make it difficult to secure strength.

At this time, in the present invention, the average rolling reduction per pass is preferably 5 to 15% so that sufficient rolling can be performed for each pass. If the average rolling reduction per pass is less than 5%, strain can not be sufficiently applied to the center of the thickness, so that it may be difficult to secure fine crystal grains after cooling. On the other hand, when the average reduction rate per pass is more than 15%, there is a problem that the production becomes impossible due to the load of the rolling mill.

Cooling

In the cooling step (S130), the hot rolled plate is cooled. Here, the cooling may be performed by air cooling which is performed in a natural cooling manner up to room temperature.

The cooling rate may be 1 to 100 ° C / sec, but is not limited thereto. When the cooling rate is less than 1 DEG C / sec, it is difficult to secure sufficient strength and toughness. On the other hand, when the cooling rate exceeds 100 DEG C / sec, cooling control is difficult, and the economical efficiency may be lowered due to excessive cooling.

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

Referring to FIG. 2, a steel sheet manufacturing method according to an embodiment of the present invention may further include a normalizing step S140, a welding step S150, and a post-welding heat treatment step S160. At this time, the normalizing step is performed after the cooling step.

Normalizing

In the normalizing step 140, the cooled plate is normalized at 890 to 910 DEG C for 10 to 30 minutes.

At this time, when the normalizing heat treatment temperature is less than 890 DEG C, it is difficult to reuse the solute elements, so that it may be difficult to secure sufficient strength. On the other hand, when the normalizing heat treatment temperature exceeds 910 ° C, crystal grains are grown to deteriorate low-temperature toughness.

Further, when the normalizing heat treatment time is less than 10 minutes, it may be difficult to obtain a uniform structure. On the other hand, when the normalizing heat treatment time exceeds 30 minutes, there is a problem of raising the production cost without further synergistic effect.

welding

In the welding step (S150), the normalized steel sheet is welded. At this time, laser welding, mesh seam welding or the like may be used for welding. By performing this welding step (S150), the plate can be manufactured for API oil used for transporting and storing crude oil resources in a harsh environment such as sand oil, or can be manufactured as a pressure vessel for a refinery plant or the like.

Heat treatment after welding

In the post-weld heat treatment step (S160), the welded plate is subjected to post-weld heat treatment (PWHT) at 580 to 620 ° C.

At this time, when the heat treatment temperature after welding is less than 580 DEG C, it is not easy to remove the residual stress at the welded portion. On the other hand, if the post-weld heat treatment temperature exceeds 620 占 폚, it may be difficult to secure sufficient strength. On the other hand, the heat treatment time after welding is preferably 1 to 10 hours per 20 to 27 mm of thickness, because if the heat treatment time after welding is out of the above range, it is not easy to remove the residual stress in the weld to be.

The addition of molybdenum (Mo) minimizes the formation of pearlite bands, which are the main cause of hydrogen-induced cracking, and ensures excellent hydrogen-organic cracking properties, and weldability It is possible to improve the welding characteristic by minimizing the carbon equivalent (Ceq) which affects the welding process.

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. Thereafter, tensile tests were conducted on the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 to 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) of 500 MPa or more, yield strength (YS) of 300 MPa or more, HIC length of 3% or more, which corresponds to the target value in the case of the samples prepared according to Examples 1 to 3, And the average distance between pearlite bands: 200 탆 or more.

On the other hand, compared with Example 1, most of the alloy components were added in a similar amount, but the molybdenum (Mo) was not added and instead, chromium (Cr) and copper (Cu) The tensile strength (TS) and the yield strength (YS) of the specimens satisfied the target value, but the average distance between the HIC length and the pearlite band did not satisfy the target value.

Compared with Example 1, most of the alloying elements were added in similar amounts, but in Comparative Example where molybdenum (Mo) was below the content range of the present invention and chromium (Cr) and copper (Cu) 2, the tensile strength (TS) and the yield strength (YS) satisfied the target values, but the average distance between the HIC length and the pearlite band did not satisfy the target value.

FIG. 3 is a photograph showing the final microstructure of the specimen produced according to Example 1, and FIG. 4 is a photograph showing the final microstructure of the specimen prepared according to Comparative Example 1. FIG.

As shown in Figs. 3 and 4, in the case of the specimens produced according to Example 1 and Comparative Example 1, it was found that the final microstructure had a complex structure including both ferrite and pearlite have. At this time, it was confirmed that the pearlite band continuously formed in the specimen prepared according to Comparative Example 1 to form a long band, whereas the specimen prepared according to Example 1 had a pearlite band distance And the pearlite bands are separated from each other.

As can be seen from the above experimental data, in the case of the samples prepared according to Examples 1 to 3, the addition of molybdenum (Mo) minimized the formation of pearlite band, which is a main cause of hydrogen organic cracking, And has a cracking property. In addition, in the case of the specimens produced according to Examples 1 to 4, the welding property can be improved by minimizing the carbon equivalent (Ceq) affecting the weldability.

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
S140: normalizing step
S150: welding step
S160: Post-welding heat treatment step

Claims

(a) 0.18 to 0.22% of C, 0.2 to 0.4% of Si, 0.8 to 1.2% of Mn, 0.008% or less of P, 0.0008% or less of S, 0.1 to 0.5% of Ni, 0.005 (Slab reheating temperature) of 1150 to 1250 占 폚, the slab plate comprising 0.001 to 0.020% of Ca, 0.001 to 0.004% of Ca, 0.1 to 0.3% of Mo, and the balance of Fe and unavoidable impurities;
(b) subjecting the reheated plate to an FRT (Finish Rolling Temperature) at a temperature of 840 to 900 캜; And
(c) cooling the hot-rolled plate material.

The method according to claim 1,
The slab plate
(C), manganese (Mn), nickel (Ni) and molybdenum (Mo) in the range satisfying the following formula (1).
[C] + [Mn] / 6 + [Ni] / 15 + [Mo] / 5? 0.430
(Where [] is the weight percentage of each element)

The method according to claim 1,
After the step (c)
(d) normalizing the cooled plate at 890 to 910 캜 for 10 to 30 minutes;
(e) welding the normalized steel sheet; And
(f) a step of post-weld heat treatment (PWHT) the welded plate at 580 to 620 캜.

0.008% or less of S, 0.0008% or less of S, 0.1 to 0.5% of Ni, 0.005 to 0.020% of Nb, 0.10 to 0.25% of Sn, 0.2 to 0.4% of Si, , Ca: 0.001 to 0.004%, Mo: 0.1 to 0.3%, and the balance of iron (Fe) and unavoidable impurities,
Wherein the microstructure has a composite structure including ferrite and pearlite, and has a tensile strength (TS) of 500 MPa or more and a yield strength (YS) of 300 MPa or more.

5. The method of claim 4,
The steel sheet
(C), manganese (Mn), nickel (Ni) and molybdenum (Mo) in a range satisfying the following formula (1).
[C] + [Mn] / 6 + [Ni] / 15 + [Mo] / 5? 0.430
(Where [] is the weight percentage of each element)

5. The method of claim 4,
The steel sheet
Wherein an average distance between the ferrite bands is not less than 200 mu m when an average distance between pearlite structures is 5 mu m or less and an average length of connected pores is 500 mu m or more is defined as a pearlite band.