KR101412295B1 - High strength steel and method for manufacturing the same - Google Patents

Abstract

Strength steel having a tensile strength of 490 MPa or more and excellent resistance to hydrogen organic cracking, and a method for producing the same.
A method of manufacturing a high strength steel according to the present invention comprises: 0.13 to 0.19% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.0 to 1.4% of manganese (Mn) (P): more than 0% to 0.01% or less, sulfur (S): more than 0% to less than 0.001%, soluble aluminum (S-Al): 0.01 to 0.05%, copper (Cu) (Ni): 0.1 to 0.3%, Cr (Cr): 0.15 to 0.25%, Mo (Mo): more than 0% (V): more than 0% to 0.01%, calcium (Ca): 0.0015 to 0.004%, nitrogen (N): more than 0% 0.005% or less and the balance of iron (Fe) and unavoidable impurities; Reheating the steel slab at 1120 to 1220 占 폚; Hot-rolling the reheated steel at a temperature equal to or greater than Ar3; Cooling the hot rolled steel material; And heat treating the cooled steel at a temperature of 870 to 910 占 폚.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel material,

The present invention relates to a high-strength steel material and a method of manufacturing the same, and more particularly, to a high-strength steel material excellent in suppressing hydrogen-induced organic cracking due to H ₂ S contained in crude oil, .

The H ₂ S component contained in crude oil is a fatal component that can cause hydrogen organic cracking (HIC) in the pressure vessel steel for refinery plant to break the vessel.

Therefore, the pressure vessel steel material for the refinery plant should have excellent resistance to hydrogen organic cracking due to the H2S component of crude oil, and it is usually required to have a high tensile strength of 490 MPa or more in tensile strength.

However, in order to increase the strength, it is inevitable to add strength strengthening elements such as carbon (C) and niobium (Nb). However, such carbon, niobium and the like deteriorate the hydrogen organic cracking property. In particular, the MnS and Nb inclusions generated during the steelmaking process mainly serve as starting points for hydrogen organic cracking. In addition, the pearlite band produced by the manganese segregation zone serves as a site for propagating hydrogen organic cracks.

As a background art related to the present invention, there is a method for producing a steel for pipes having excellent resistance to organic cracking caused by hydrogen disclosed in Korean Patent Registration No. 10-0256347 (published on May 15, 2000).

It is an object of the present invention to provide a high strength steel having excellent strength and excellent resistance to hydrogen organic cracking through alloy components and process control, and a method for producing the same.

(A) 0.13 to 0.19% of carbon (C), 0.2 to 0.5% of silicon (Si), and 0.005 to 0.15% of carbon (C) in weight percent through steelmaking and continuous casting to achieve the above object. (S): not less than 0% to not more than 0.001%, soluble aluminum (S-Al): 0.01 to 0.05%, manganese (Mn): 1.0 to 1.4% 0.1 to 0.2% of niobium (Nb), 0.01 to 0.02% of boron (B), 0.1 to 0.3% of nickel (Ni), 0.1 to 0.3% of chromium (Cr) (Ti): not less than 0% to not more than 0.01%, vanadium (V): not less than 0% to not more than 0.01%, calcium (Ca): not more than 0.005% 0.004%, nitrogen (N): more than 0% to 0.005%, and the balance of iron (Fe) and unavoidable impurities; (b) reheating the steel slab at 1120 to 1220 占 폚; (c) hot rolling the reheated steel at a temperature equal to or greater than Ar3; (d) cooling the hot-rolled steel material; And (e) subjecting the cooled steel material to heat treatment at 870 to 910 占 폚.

At this time, the step (a) may include bubbling the calcium added for 3 minutes or more so that the [Ca] / [S] ([] represents the weight percentage of each component) have.

Further, it is preferable that the heat treatment is performed for a time determined by the following formula (1).

[Formula 1]

t = A x T + 10

(t: heat treatment time (min), T: steel material thickness (mm), A = 1.4 when T = 40, A = 1.5 when 40 <T≤60, A = 1.6, 80 < T, A = 1.7)

To achieve the above object, a high strength steel according to an embodiment of the present invention includes 0.13 to 0.19% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.0 to 1.4% of manganese (Mn) (P): more than 0% to 0.01% or less, sulfur (S): more than 0% to less than 0.001%, soluble aluminum (S-Al): 0.01 to 0.05%, copper (Cu) (Ni): 0.1 to 0.3%, Cr (Cr): 0.15 to 0.25%, Mo (Mo): more than 0% (V): more than 0% to 0.01%, calcium (Ca): 0.0015 to 0.004%, nitrogen (N): more than 0% 0.005% or less, and the balance of iron (Fe) and unavoidable impurities, and has a tensile strength of 490 MPa or more and an average impact absorption energy of 290 J or more at -20 캜.

At this time, the steel material is characterized in that it contains calcium and sulfur in a range that [Ca] / [S] ([] represents the weight percentage of each component) of 2 to 3.5.

According to the method for manufacturing a high strength steel material according to the present invention, it is possible to manufacture a steel material having high strength and excellent resistance to hydrogen organic cracking through control of components such as phosphorus, sulfur, calcium and the like and process control.

1 is a flowchart schematically showing a method of manufacturing a high-strength steel material according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a high strength steel according to an embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

High strength steel

The high strength steel according to the present invention comprises 0.13 to 0.19% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.0 to 1.4% of manganese (Mn), more than 0% of phosphorus (P) 0.01% or less of sulfur (S), more than 0% to 0.001% of sulfur (S), 0.01 to 0.05% of soluble aluminum (S-Al), 0.1 to 0.2% of copper, 0.01 to 0.02% of niobium , Boron (B): more than 0 to 0.001%, nickel (Ni): 0.1 to 0.3%, chromium (Cr): 0.15 to 0.25%, molybdenum (Mo): more than 0 to 0.08% : More than 0% to 0.01%, vanadium (V): more than 0% to 0.01%, calcium (Ca): 0.0015 to 0.004%, nitrogen (N): more than 0% to less than 0.005%.

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 included in the high strength steel according to the present invention will be described.

Carbon (C)

Carbon (C) is an element contributing to strength.

The carbon is preferably added in an amount of 0.13 to 0.19 wt% of the total weight of the steel material. When the addition amount of carbon is less than 0.13% by weight, the effect of improving the strength is insufficient. On the contrary, when the addition amount of carbon exceeds 0.19% by weight, the hydrogen organic cracking resistance is lowered, and Martensite Austenite constituent (MA) is generated in the heat affected zone during welding for the production of pressure vessel, .

Silicon (Si)

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

The silicon is preferably added in an amount of 0.2 to 0.5% by weight based on the total weight of the steel material. When the addition amount of silicon is less than 0.2% by weight, the effect of addition is insufficient. On the contrary, when the amount of silicon added exceeds 0.5% by weight, the toughness and weldability of the steel are deteriorated.

Manganese (Mn)

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

The manganese is preferably added in an amount of 1.0 to 1.4% by weight based on the total weight of the steel material. If the addition amount of manganese is less than 1.0% by weight, the effect of addition thereof is insufficient. On the other hand, if the addition amount of manganese exceeds 1.4% by weight, the resistance to hydrogen organic cracking may be lowered due to the increase of MnS inclusions even by the addition of calcium.

In (P)

The phosphorus (P) deteriorates the toughness of the steel and acts as a trap site for hydrogen, which may cause hydrogen organic cracking.

Therefore, in the present invention, the content of phosphorus is limited to more than 0 wt% to 0.01 wt% or less of the total weight of the steel material. This can be obtained by setting the end point temperature to 1600 DEG C or higher and the end point oxygen to 200 to 900 ppm or so in the conversion process at the time of steelmaking.

Sulfur (S)

Sulfur (S) needs to be managed at a very low level as a substance that increases susceptibility to hydrogen organic cracking along with phosphorous (P).

Therefore, in the present invention, the sulfur content is limited to not less than 0 wt% and not more than 0.001 wt% of the total weight of the steel material. This can be achieved by lowering the content of sulfur to 0.003 wt% or less in the KR (Kanvara Reactor) process at the time of steel making, and conducting a strong bubbling in LF (Ladle Furnace) for 10 minutes or more.

The soluble aluminum (S- Al )

Soluble aluminum (S-Al) forms AlN nitride and contributes to grain refinement.

The soluble aluminum is preferably added in an amount of 0.01 to 0.05% by weight based on the total weight of the steel material. If the amount of the soluble aluminum is less than 0.01% by weight, the effect of the addition is insufficient. On the other hand, when the content of soluble aluminum exceeds 0.05% by weight, inclusions are increased to deteriorate the ductility and toughness of the steel and deteriorate the cleanliness.

Copper (Cu)

Copper precipitates on the surface of the steel to form a protective coating on the surface, which can prevent absorption of hydrogen by the protective coating. Therefore, copper plays a role in improving hydrogen organic cracking resistance.

The copper is preferably added in an amount of 0.1 to 0.2% by weight based on the total weight of the steel material. When the addition amount of copper is less than 0.1% by weight, the effect of the addition is insufficient. On the contrary, when the addition amount of copper exceeds 0.2% by weight, surface defects of the steel are caused.

Niobium (Nb)

Niobium (Nb) contributes to enhancement of strength and improvement of impact toughness at low temperatures through formation of precipitates and grain refinement.

The niobium is preferably added in an amount of 0.01 to 0.02% by weight based on the total weight of the steel material. When the addition amount of niobium is less than 0.01% by weight, the effect of adding niobium can not be sufficiently exhibited. On the contrary, when the addition amount of niobium exceeds 0.02 wt%, the hydrogen organic cracking resistance of the steel is lowered.

Boron (B)

Boron (B) is an element which increases solubility when solidified, and also reduces solute N when precipitated as BN, thereby improving the toughness of the weld heat affected zone (HAZ).

The boron is preferably added in an amount of more than 0 wt% to 0.001 wt% of the total weight of the steel material. If the addition amount of boron exceeds 0.001% by weight, the strength is good but the impact resistance at low temperature is lowered.

Nickel (Ni)

Nickel (Ni) is effective for improvement in toughness while improving incineration.

The nickel is preferably added in an amount of 0.1 to 0.3% by weight based on the total weight of the steel material. When the addition amount of nickel is less than 0.1% by weight, the addition effect is insignificant. On the other hand, when the addition amount of nickel exceeds 0.3% by weight, the workability of the steel material can be lowered.

Chromium (Cr)

Chromium (Cr) lowers the yield strength and also contributes to the improvement of hydrogen organic cracking resistance.

The chromium is preferably added in an amount of 0.15 to 0.25% by weight based on the total weight of the steel material. When the addition amount of chromium is less than 0.15% by weight, the effect of addition thereof is insufficient. On the contrary, when the addition amount of chromium exceeds 0.25% by weight, the hydrogen organic cracking resistance may be deteriorated.

Molybdenum (Mo)

Molybdenum (Mo) inhibits propagation of hydrogen-induced cracks by forming bainite in the pearlite band. In addition, molybdenum contributes to strength improvement through bainite formed in the pearlite band.

The molybdenum is preferably added in an amount of more than 0 wt% to 0.08 wt% of the total weight of the steel material. If the amount of the molybdenum added exceeds 0.08% by weight, the low-temperature impact toughness is deteriorated.

Titanium (Ti)

Titanium (Ti) contributes to the refinement of the crystal grains of the steel material and the improvement of impact toughness at low temperatures.

The titanium is preferably added in an amount of more than 0 wt% to 0.01 wt% or less of the total weight of the steel material. If the addition amount of titanium exceeds 0.01% by weight, the solid solution titanium reacts with the carbon (C) to form a carbide, which lowers the impact toughness at low temperature.

Vanadium (V)

Vanadium (V) is a carbide-generating element and is effective in increasing the strength.

The vanadium (V) is preferably added in an amount of more than 0% by weight to 0.01% by weight or less of the total weight of the steel material. If the added amount of vanadium exceeds 0.01% by weight, there is a problem that the susceptibility to hydrogen organic cracking increases.

Calcium (Ca)

Calcium (Ca) is superior to manganese (Mn) in affinity with sulfur (S), and forms CaS inclusions in spheres, thereby reducing MnS inclusions that adversely affect hydrogen organic cracking.

The calcium is preferably added in an amount of 0.0015 to 0.004% by weight based on the total weight of the steel material. When the addition amount of calcium is less than 0.0015% by weight, the effect of addition is insufficient. On the contrary, when the addition amount of calcium exceeds 0.004% by weight, a large amount of CaO is formed and the weldability of the steel is deteriorated.

At this time, it is preferable that calcium and sulfur are contained in a range satisfying 2.0? [Ca] / [S]? 3.5 where [Ca] and [S] are weight percentages of Ca and S, respectively. When [Ca] / [S] < 2.0, the amount of CaS generated is insufficient and the effect of improving the hydrogen organic cracking resistance by CaS formation is insufficient. Conversely, if [Ca] / [S]> 3.5 is exceeded, excessive CaO may be produced.

Nitrogen (N)

Nitrogen (N), if included too much, deteriorates the toughness of the steel significantly.

Therefore, in the present invention, the content of nitrogen is limited to not less than 0 wt% and not more than 0.005 wt% (70 ppm) of the total weight of the steel material.

The steel having the above composition has a reduced tensile strength of 490 MPa or more while decreasing the sulfur and phosphorus content to a minimum and adding a certain amount of calcium to the sulfur content to form spherical CaS And can also have excellent low temperature impact toughness.

How to make high strength steels

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

1, the illustrated method of manufacturing a steel material for a pressure vessel includes a steel slab manufacturing step S110, a slab reheating step S120, a hot rolling step S130, a cooling step S140 and a heat treatment step S150 do.

Manufacture of steel slabs

In the steel slab manufacturing step (S110), a steel slab having the above composition is manufactured through steelmaking and continuous casting.

In the case of steelmaking, as is well known, it may include a KR (Kanvara Reactor) process, a converter process, a Ladle Furnace process, a vacuum degassing process, and the like.

More specifically, calcium is added so that the [Ca] / [S] ([] is the weight percentage of each component) in the vacuum degassing process at the time of steelmaking, 2.0 to 3.5, and then bubbled for 3 minutes or more . When the bubbling time was more than 3 minutes, the spherical CaS formation efficiency was the best.

Reheating slabs

In the slab reheating step (S120), the produced steel slab is reheated to improve the reuse and homogenization of the precipitate.

At this time, it is preferable that the slab reheating is performed at 1120 to 1220 ° C. When the reheating temperature of the slab is less than 1120 DEG C, the temperature of the steel slab is lowered after reheating, which causes a problem that the rolling load becomes large. On the other hand, when the heating temperature is higher than 1220 DEG C, grain coarsening and economical efficiency may become a problem.

Also, it is preferable that the slab reheating is performed for 1 to 3 hours in the above temperature range. If the slab reheating time is less than 1 hour, the re-use effect of the precipitate may be insufficient. Conversely, if the slab reheating time exceeds 3 hours, economical efficiency may be a problem due to excessive heating.

Hot rolling

Next, in the hot rolling step (S130), the reheated steel is hot-rolled at a temperature equal to or higher than Ar3.

The hot rolling is preferably carried out at a finishing rolling temperature of 890 to 950 캜. When the rolling finish temperature exceeds 950 DEG C, it is difficult to obtain strength and toughness due to recrystallization and crystal grain coarsening. On the other hand, when the rolling finish temperature is less than 890 DEG C, the toughness deterioration and yield ratio due to abnormal reverse rolling can be increased.

Cooling

Next, in the cooling step (S140), the hot-rolled steel material is cooled.

The cooling is preferably carried out at an average cooling rate of 5 DEG C / sec or less, and more preferably, it can be carried out by air cooling. If the average cooling rate during cooling exceeds 5 DEG C / sec, the strength of the produced steel can be increased, but the components such as hydrogen present in the steel during cooling can not be sufficiently diffused, have.

Further, the cooling can be performed up to room temperature.

Heat treatment

Next, in the heat treatment step (S150), the cooled steel material is heat-treated at 870 to 910 ° C to refine and homogenize the structure of the steel material and improve the low-temperature impact toughness.

The heat treatment temperature is preferably 870 to 910 占 폚. When the heat treatment temperature is lower than 870 占 폚, the effect of the heat treatment is insufficient. Conversely, when the heat treatment temperature exceeds 910 占 폚, the strength of the steel material may be lowered.

Meanwhile, the inventors of the present invention have found that when the heat treatment time according to the thickness of the steel material satisfies Equation (1), the impact resistance at low temperatures is further improved.

[Formula 1]

t = A x T + 10

(t: heat treatment time (min), T: steel material thickness (mm), A = 1.4 when T = 40, A = 1.5 when 40 <T≤60, A = 1.6, 80 < T, A = 1.7)

After the heat treatment step (S150), the steel material can be cooled to room temperature by air cooling or the like.

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. Manufacture of steels

The steel slabs according to Examples 1 to 2 and Comparative Example 1 having the compositions shown in Table 1 were reheated at 1170 占 폚 for 2 hours and then hot rolled at a finish rolling temperature of 920 占 폚 and then cooled to 25 占 폚 Respectively. Thereafter, the steel material was heated to 890 占 폚 and subjected to heat treatment and then cooled to 25 占 폚 in an air-cooling manner to prepare a steel material having a thickness of 70 mm.

The steel material thickness (T) = 70 mm and A = 1.6 were applied at the equation 1, that is, t = A x T +10, so that the heat treatment time (t) was 122 minutes.

[Table 1] (unit:% by weight)

2. Property evaluation

Table 2 shows the evaluation results of the physical properties of the steels produced according to Examples 1 and 2 and Comparative Example 1.

In Table 2, the absorbed energy means the average impact absorption energy of Charpy three times, and the hydrogen organic cracking property is evaluated as to whether the steel material cracks after standing the steel material for 4 days at H ₂ S saturation condition and pH 5.0.

[Table 2]

Referring to Table 2, the steel material produced according to Examples 1 and 2 exhibited a tensile strength of 490 MPa or more, excellent impact at low temperature, and in particular, no hydrogen-induced cracking occurred.

On the other hand, in the case of the steel according to Comparative Example 1 in which chromium was added and the content of calcium was less than 2 times the content of sulfur, the strength was good, but some cracks occurred in the evaluation of hydrogen organic cracking and the cold shock toughness was relatively poor 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.

Claims (8)

(a) 0.13 to 0.19% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.0 to 1.4% of manganese (Mn), 1.0 to 1.4% of phosphorus (P) (S): 0.01 to 0.05%, copper (Cu): 0.1 to 0.2%, niobium (Nb): 0.01 to 5.0% (Ni): 0.1 to 0.3%, Cr: 0.15 to 0.25%, molybdenum (Mo): 0 to less than 0.08%, boron (B): more than 0 to 0.001% (Ti) of more than 0% to 0.01% or less, vanadium (V) of more than 0% to 0.01%, calcium (Ca) of 0.0015 to 0.004%, nitrogen (N) of more than 0% to less than 0.005% (Fe) and inevitable impurities;
(b) reheating the steel slab at 1120 to 1220 占 폚;
(c) hot rolling the reheated steel at a temperature equal to or greater than Ar3;
(d) cooling the hot-rolled steel material; And
(e) heat treating the cooled steel at 870 to 910 &lt; 0 &gt; C.
The method according to claim 1,
The steel making in the step (a)
Wherein the calcium is added so that [Ca] / [S] ([] represents the weight percentage of each component) of 2.0 to 3.5, followed by bubbling for 3 minutes or more.
The method according to claim 1,
The slab reheating
Wherein said step (c) is carried out for 1 to 3 hours.
The method according to claim 1,
The hot rolling
And a finishing rolling temperature of 890 to 950 占 폚.
The method according to claim 1,
The cooling
Wherein the cooling water is cooled and air-cooled.
The method according to claim 1,
The heat treatment
Is performed for a time determined by the following formula (1).
[Formula 1]
t = A x T + 10
(t: heat treatment time (min), T: steel material thickness (mm), A = 1.4 when T = 40, A = 1.5 when 40 <T≤60, A = 1.6, 80 < T, A = 1.7) delete delete

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