KR101505260B1 - Hot rolled steel sheet for steel pipe and method of manufacturing the steel pope - Google Patents

Abstract

A method of manufacturing a hot-rolled steel sheet and a steel pipe for a steel pipe capable of improving spherical corrosion resistance of an electric resistance welded portion by controlling an alloy component and controlling process conditions.
The method of manufacturing a steel pipe according to the present invention is characterized in that it comprises 0.05 to 0.18% of C, 0 to 0.025% of Si, 0.30 to 0.60% of Mn, 0 to 0.020% of P, (Fe) and other unavoidable impurities, in a range of from 0.001 to 0.005%, Cu: 0.08 to 0.30%, Ni: 0.08 to 0.30%, Cr: 0.08 to 0.30%, Ca: (Slab Reheating Temperature): reheating to 1220 ~ 1300 ℃; Hot-rolling the reheated plate to a finishing delivery temperature (FDT) of 860 to 900 占 폚; Cooling and winding the hot rolled plate; Uncoiling the wound sheet material and normalizing the sheet at 800 to 900 DEG C for 10 to 30 minutes; Cutting the normalized plate material and welding it by an electric resistance welding method to form a steel pipe; And post-weld heat treatment (PWHT) of the welded steel pipe at 520 to 620 캜.

Description

TECHNICAL FIELD [0001] The present invention relates to a hot rolled steel sheet for steel pipe and a method for manufacturing the steel pipe,

The present invention relates to a hot-rolled steel sheet for steel pipe and a steel pipe manufacturing method, and more particularly, to a hot-rolled steel sheet for steel pipe and a steel pipe manufacturing method capable of improving spherical corrosion resistance of an electric resistance welded portion by controlling alloy components and controlling process conditions will be.

 The inner corroded steel was developed in response to the spherical erosion that appeared as grooves along the weld in electric resistance welding (ERW) steel pipe. In order to control the spherical corrosion, it is necessary to heat-treat the ERW-welded steel pipe at about 900 ° C or more, or to reduce the content of sulfur (S) contained in the steel sheet to a very low level.

This is because the main mechanism causing the spherical corrosion is the lack of welding of the steel pipe or the welding of MnS at the time of welding and it is not completely recombined in the process of re-solidification, so that the potential difference of -60 mV is higher than that of the base metal. It is because it acts as a factor to cause.

A related prior art is Korean Patent Laid-Open Publication No. 10-2008-0057845 (Jun. 25, 2008), which discloses a hot-rolled steel sheet and a high-strength steel pipe having excellent continuous heat treatment characteristics and a manufacturing method thereof.

An object of the present invention is to provide a steel pipe manufacturing method capable of improving the spherical corrosion resistance of an electric resistance welded portion by controlling an alloy component and controlling process conditions.

It is another object of the present invention to provide a method of manufacturing a steel sheet which suppresses the change to fayalite (Fe ₂ SiO ₄ ) at a high temperature by strictly controlling the content of silicon (Si) to 0.025 wt% or less, (Cr) is added in a content ratio of 0.08 to 0.30% by weight, thereby ensuring excellent corrosion resistance.

In order to achieve the above object, a steel pipe manufacturing method according to an embodiment of the present invention comprises: 0.05 to 0.18% of C, 0 to 0.025% of Si, 0.30 to 0.60% of Mn, 0% 0.08 to 0.30%, Cr: 0.08 to 0.30%, Ca: 0.0010 to 0.0060%, and the balance of iron (Fe) and others Reheating a steel slab made of unavoidable impurities to a slab reheating temperature (SRT) of 1220 to 1300 占 폚; Hot-rolling the reheated plate to a finishing delivery temperature (FDT) of 860 to 900 占 폚; Cooling and winding the hot rolled plate; Uncoiling the wound sheet material and normalizing the sheet at 800 to 900 DEG C for 10 to 30 minutes; Cutting the normalized plate material and welding it by an electric resistance welding method to form a steel pipe; And post-weld heat treatment (PWHT) of the welded steel pipe at 520 to 620 캜.

According to another aspect of the present invention, there is provided a steel hot-rolled steel sheet for steel pipe comprising 0.05 to 0.18% of C, 0 to 0.025% of Si, 0.30 to 0.60% of Mn, More than 0% to 0.020%, S: more than 0% to 0.005%, Cu: 0.08 to 0.30%, Ni: 0.08 to 0.30%, Cr: 0.08 to 0.30% (TS): 380 to 480 MPa, a yield strength (YS): 215 MPa or more, and an elongation percentage (EL): 32% or more.

The hot-rolled steel sheet and the steel pipe manufacturing method for a steel pipe produced by the method according to the present invention are characterized in that the content of silicon (Si) is controlled strictly at a content ratio of 0.025% by weight or less to change into fayalite (Fe ₂ SiO ₄ ) It is possible to prevent the occurrence of abnormal precipitation during hot dip galvanizing.

Further, in the hot-rolled steel sheet for steel pipe and the method for manufacturing steel pipe according to the present invention, an oxide film composed of a compound of copper (Cu) -nickel (Ni) -crome (Cr) is formed on the surface, In addition, the local corrosion inhibition effect is excellent.

FIG. 1 is a flow chart showing a method of manufacturing a steel pipe according to an embodiment of the present invention.
Fig. 2 is a microstructure photograph showing the result of anodic dissolution acceleration test of the specimen according to Example 1. Fig.
3 is a microstructure photograph showing the result of anodic dissolution acceleration test of the specimen according to Example 2. Fig.
4 is a microstructure photograph showing the result of anodic dissolution acceleration test of a specimen 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 hot-rolled steel sheet for a steel pipe according to a preferred embodiment of the present invention and a method for manufacturing a steel pipe using the same will be described in detail with reference to the accompanying drawings.

Hot-rolled steel plate for steel pipe

The hot-rolled steel sheet for a steel pipe according to the present invention has tensile strength (TS) of 380 to 480 MPa, yield strength (YS) of 215 MPa or more and elongation (EL) of 32% or more through control of alloy components and process conditions We aim.

The hot-rolled steel sheet for steel pipe according to the present invention contains 0.05 to 0.18% of C, 0 to 0.025% of Si, 0.30 to 0.60 of Mn, 0 to 0.020% of P, S: more than 0% to 0.005%, Cu: 0.08 to 0.30%, Ni: 0.08 to 0.30%, Cr: 0.08 to 0.30%, Ca: 0.0010 to 0.0060% and balance iron (Fe) and other unavoidable impurities .

At this time, the hot-rolled steel sheet for a steel pipe according to the embodiment of the present invention having the above composition satisfies a spherical corrosion factor of 1.1 or less.

(Here, spherical corrosion factor = corrosion depth of welded part / corrosion depth of base metal.)

Further, an oxide film made of a compound of copper (Cu) -nickel (Ni) -chromium (Cr) is formed on the surface of the hot-rolled steel sheet.

Hereinafter, the role and content of each component included in the hot-rolled steel sheet for a steel pipe according to the present invention will be described as follows.

Carbon (C)

Carbon (C) is added to secure strength and is the most influential element in weldability.

The carbon (C) is preferably added in a content ratio of 0.05 to 0.18% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. If the content of carbon (C) is less than 0.05% by weight, it may be difficult to secure sufficient strength. On the contrary, when the content of carbon (C) exceeds 0.18% by weight, the toughness may be lowered and weldability may be deteriorated during electrical resistance welding (ERW).

Silicon (Si)

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

However, when a large amount of silicon (Si) is added in an amount exceeding 0.025% by weight of the total weight of the hot-rolled steel sheet in the present invention, the surface tension may be increased to reduce wettability or fayalite (Fe ₂ SiO ₄ ) Resulting in abnormal precipitation during plating, which causes a drastic decrease in the plating ability. Therefore, in the present invention, it is preferable to limit the content of silicon to a content ratio of more than 0 wt% to 0.025 wt% of the total weight of the hot-rolled steel sheet.

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 0.30 to 0.60% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of manganese (Mn) is less than 0.30% by weight, the effect of addition is insufficient. On the contrary, when the content of manganese (Mn) exceeds 0.60% 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 0.020% 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 more than 0 wt% and not more than 0.020 wt% of the total weight of the hot-rolled steel sheet.

Sulfur (S)

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

However, when the content of sulfur (S) exceeds 0.005% 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 over 0 wt% to 0.005 wt% or less of the total weight of the hot-rolled steel sheet.

Copper (Cu)

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

The copper (Cu) is preferably added in a content ratio of 0.08 to 0.30% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of copper (Cu) is less than 0.08% by weight, the above effect can not be exhibited properly. On the contrary, when the content of copper (Cu) exceeds 0.30 wt%, there is a problem that the workability of the steel sheet is lowered and the susceptibility to stress relief cracking after welding is increased.

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 0.08 to 0.30% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of nickel (Ni) is less than 0.08% by weight, it may be difficult to exhibit the above-mentioned effect properly. On the contrary, when the content of nickel (Ni) exceeds 0.30 wt%, the workability of the steel sheet is lowered and the production cost is excessively increased compared to the effect of nickel addition, which is not economical.

Chromium (Cr)

Chromium (Cr) is a ferrite stabilizing element and contributes to strength improvement. Chromium (Cr) also plays a role in enlarging the delta ferrite region and shifting the hypo-peritectic region to the high carbon side to improve the slab surface quality.

The chromium (Cr) is preferably added at a content ratio of 0.08 to 0.30% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of chromium (Cr) is less than 0.08% by weight, the effect of the addition can not be exhibited properly. On the contrary, when the content of chromium (Cr) exceeds 0.30 wt%, there is a problem that the weld heat affected zone (HAZ) toughness deteriorates.

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 0.0010 to 0.0060% by weight of the total weight of the hot-rolled steel sheet according to the present invention. When the content of calcium (Ca) is less than 0.0010 wt%, the above effect can not be exhibited properly. On the contrary, when the content of calcium (Ca) exceeds 0.0060% by weight, generation of CaO inclusions becomes excessive, and electrical resistance weldability is deteriorated.

Steel pipe manufacturing method

FIG. 1 is a flow chart showing a method of manufacturing a steel pipe according to an embodiment of the present invention.

Referring to FIG. 1, the steel pipe manufacturing method includes a slab reheating step S110, a hot rolling step S120, a cooling / winding step S130, a normalizing step S140, a welding step S150, (S160). 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 pipe manufacturing method according to the present invention, the semi-finished slab plate to be subjected to the hot rolling process preferably contains 0.05 to 0.18% of C, more than 0 to 0.025% of Si, 0.30 to 0.60 of Mn, 0.30 to 0.60 of Si, More than 0% to 0.020%, S: more than 0% to 0.005%, Cu: 0.08 to 0.30%, Ni: 0.08 to 0.30%, Cr: 0.08 to 0.30%, Ca: 0.0010 to 0.0060% ) And other 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 1220 to 1300 ° C. The reason for performing the slab reheating temperature (SRT) in the high temperature region of 1220 to 1300 ° C is to sufficiently dissolve alloy components such as copper (Cu), nickel (Ni), and chromium (Cr) contained in the slab plate to be.

If the slab reheating temperature (SRT) is lower than 1220 deg. C, alloy components such as copper (Cu), nickel (Ni) and chromium (Cr) are not sufficiently dissolved in this step, 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 (SRT) is higher than 1300 ° C, the austenite grain growth can not be suppressed and the austenite grains are rapidly coarsened, making it difficult to secure the strength and low temperature toughness of the steel sheet to be produced.

Although not shown in the drawing, the steel pipe manufacturing method according to the embodiment of the present invention may further include a steel slab casting step performed before the slab reheating step.

In the steel slab casting step, steel having the above composition is steel-cast, and then steel slab is cast by playing at a casting speed of 1.3 to 2.0 m / min. In this case, when the casting speed is set to 1.3 to 2.0 m / min, the slab crack can be prevented by strictly controlling the solidification speed by lowering the performance speed during the steelmaking and playing process, 0.005 weight% or less, and calcium: 0.0060 ~ 0.0060 weight%.

In this step, when the casting speed is less than 1.3 m / min, the speed is too slow, and the productivity is drastically deteriorated. On the contrary, when the casting speed exceeds 2.0 m / min, mixing of slag may be caused and slab cracks may occur.

Hot rolling

In the hot rolling step (S120), the reheated plate is hot-rolled to FDT (Finish Delivery Temperature): 860 to 900 占 폚.

If the finish hot rolling temperature (FDT) is lower than 860 DEG C, there may occur problems such as generation of coarse gravel structure due to abnormal reverse rolling. On the other hand, when the final hot rolling temperature (FDT) is higher than 900 ° C, the austenite grains are coarsened and the ferrite grains can not be sufficiently refined after the transformation.

Cooling / Winding

In the cooling / winding step (S130), the hot-rolled plate is cooled to a CT (Coiling Temperature): 550 to 650 ° C and wound.

In this step, when the coiling temperature is lower than 550 캜, there is a problem that the surface quality of the steel sheet is deteriorated due to a rapid difference between the finish rolling temperature and the coiling temperature. On the other hand, when the coiling temperature exceeds 650 ° C, the grain refinement effect deteriorates, so that it may be difficult to ensure sufficient strength.

On the other hand, the cooling rate in the cooling / winding step (S130) is preferably 10 to 40 占 폚 / min. In this step, if the cooling rate is less than 10 캜 / min, it may be difficult to secure strength. On the other hand, when the cooling rate exceeds 40 占 폚 / min, there is a problem that the structure is weakened and impact toughness is lowered.

Normalizing

In the normalizing step (S140), the rolled sheet is uncoiled and normalized at 800 to 900 DEG C for 10 to 30 minutes.

In this step, when the normalizing heat treatment temperature is less than 800 ° C, it is difficult to reuse the solid solute elements, so that it may be difficult to secure sufficient strength. On the other hand, when the normalizing heat treatment temperature exceeds 900 캜, crystal grains are grown to deteriorate low-temperature toughness.

In addition, 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 plate material is cut and then welded by an electric resistance welding method to form a steel pipe. By performing this welding step (S150), the steel pipe can be manufactured for an API oil used for transportation and storage of crude oil resources in a poor 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 steel pipe is subjected to post-weld heat treatment (PWHT) at 520 to 620 ° C.

At this time, when the post-weld heat treatment temperature is less than 520 ° C, it is not easy to remove the residual stress at the welded part or the like. On the other hand, if the post-weld heat treatment temperature exceeds 620 占 폚, it may be difficult to secure sufficient strength.

It is preferable that the heat treatment time after welding is 1 to 15 hours per 20 to 25 mm thickness since it is not easy to remove the residual stress in the weld part when the heat treatment time after welding is out of the above range to be.

The steel pipe manufacturing method according to the embodiment of the present invention can be ended through the above-described processes (S110 to S160).

At this time, the hot-rolled steel sheet for steel pipe, which is a plate manufactured after the cooling / winding step (S130) and before the normalizing step (S140), has tensile strength TS: 380 to 480 MPa, yield strength YS: (EL): 32% or more.

Particularly, the hot-rolled steel sheet for a steel pipe satisfies the spherical corrosion factor of 1.1 or less through the control of alloy components and process conditions, thereby improving the spherical corrosion resistance of the electric resistance welded portion.

(Here, spherical corrosion factor = corrosion depth of welded part / corrosion depth of base metal.)

That is, the hot-rolled steel sheet for a steel pipe according to the present invention can suppress the change to fayalite (Fe ₂ SiO ₄ ) at a high temperature by strictly controlling the content of silicon (Si) to 0.025 wt% The occurrence of abnormal precipitation can be prevented in advance. In addition, the hot-rolled steel sheet for steel pipe according to the present invention was added with chromium (Cr) exhibiting excellent effects in seawater-resistant steel at a content ratio of 0.08 to 0.30% by weight to secure the overall corrosion resistance of the hot-rolled steel sheet.

In addition, the hot-rolled steel sheet for a steel pipe manufactured by the above process can form an oxide film composed of a compound of copper (Cu) -nickel (Ni) -chromium (Cr) on its surface, There is an excellent property of inhibiting phosphorus corrosion.

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 4 and Comparative Examples 1 to 3 were prepared with the compositions shown in Table 1 and the process conditions shown in Table 2. At this time, in the case of the hot-rolled samples according to Examples 1 to 4 and Comparative Examples 1 to 3, the ingots having the respective compositions were prepared and heated, hot-rolled and cooled using a rolling simulation tester, After that, a tensile test was performed on each specimen.

Then, in order to examine the welding characteristics of the specimens according to Examples 1 to 4 and Comparative Examples 1 to 3, they were cut into pieces each having a size of 25 mm (width) * 60 mm (length) * 5 mm (thickness) by plasma cutting Two specimens according to Examples 1 to 4 and Comparative Examples 1 to 3, respectively, were obtained. Next, each of the two specimens prepared according to Examples 1 to 4 and Comparative Examples 1 to 3 was immersed in an immersion solution (10 wt% NaCl + HCl, pH 0.85) maintained at 30 DEG C for 168 hours, , And the corrosion depth between the weld and the base material was measured by electrostatic polarization at -550 mV for 144 hours in a 3% NaCl aqueous solution maintained at 30 ° C.

At this time, the welded portion used for the butt welding is composed of 0.06% of C, 0.40% of Si, 1.00% of Mn, 0.015% of P, 0.011% of S, 0.30% of Ni, 0.03% 0.025% of Ti, 0.025% of Al, 0.015% of Al, 0.02% of V, 0.0003% of B, 0.2% of Cu, 0.012% of Nb and the balance of Fe and other impurities.

[Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

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

 [Table 3]

With reference to Tables 1 to 3, the tensile strength (TS) of 380 to 480 MPa, the yield strength (YS) of 215 MPa or more and the elongation (EL): 32% or more.

On the other hand, in the case of the specimens according to Comparative Examples 1 to 3, the yield strength (YS) and the elongation (EL) satisfied the target value, but the tensile strength TS fell below the target value.

Particularly, in the case of the specimens produced according to Examples 1 to 4, it can be confirmed that the spherical corrosion coefficient corresponding to the target value satisfies all of 1.1 or less. On the other hand, it was confirmed that the specimens prepared according to Comparative Examples 1 to 3 had a spherical corrosion coefficient of 1.71 to 1.80.

FIG. 3 is a microstructure photograph showing the result of anodic dissolution acceleration test of the specimen according to Example 2, and FIG. 4 is a microstructure photograph showing the result of comparison 1 is a microstructure photograph showing the result of anodic dissolution acceleration test of the sample according to Example 1. Fig.

As shown in FIGS. 2 and 3, in the case of the specimens manufactured according to Examples 1 and 2, it can be seen that there is almost no corrosion between the weld portion and the base material portion. On the other hand, as shown in FIG. 4, it can be seen that corrosion of the specimen prepared according to Comparative Example 1 was significantly improved.

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

Claims (6)

P: not less than 0% to not more than 0.020%, S: not less than 0% to not more than 0.005%, Cu: not more than 0.05% Steel slabs composed of 0.08 to 0.30% of Ni, 0.08 to 0.30% of Cr, 0.08 to 0.30% of Cr, 0.0010 to 0.0060% of Ca and balance of iron (Fe) and other unavoidable impurities are mixed in a slab reheating temperature (SRT) Reheating to 1300 占 폚;
Hot-rolling the reheated plate to a finishing delivery temperature (FDT) of 860 to 900 占 폚;
Cooling and winding the hot rolled plate;
Uncoiling the wound sheet material and normalizing the sheet at 800 to 900 DEG C for 10 to 30 minutes;
Cutting the normalized plate material and welding it by an electric resistance welding method to form a steel pipe; And
And a post-weld heat treatment (PWHT) step of welding the welded steel pipe at 520 to 620 ° C.
The method according to claim 1,
Before the slab reheating step,
Further comprising the step of casting steel slabs by performing steel at a casting speed of 1.3 to 2.0 m / min after steel having the above composition is steel-casted.
The method according to claim 1,
In the cooling step,
The cooling
CT (Coiling Temperature): 550 to 650 占 폚.
P: not less than 0% to not more than 0.020%, S: not less than 0% to not more than 0.005%, Cu: not more than 0.05% 0.08 to 0.30% of Ni, 0.08 to 0.30% of Ni, 0.08 to 0.30% of Cr, 0.0010 to 0.0060% of Ca, and the balance of Fe and other unavoidable impurities,
, A tensile strength (TS) of 380 to 480 MPa, a yield strength (YS) of 215 MPa or more and an elongation (EL) of 32% or more.
5. The method of claim 4,
The hot-
And a spherical corrosion factor: 1.1 or less.
(Here, spherical corrosion factor = corrosion depth of welded part / corrosion depth of base metal.)
5. The method of claim 4,
The hot-
Wherein an oxide film composed of a compound of copper (Cu) -nickel (Ni) -chromium (Cr) is formed on the surface of the steel sheet.

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