KR102508128B1 - Steel plate having excellent low temperature impact toughness of heat affeected zone and manufacturing mehtod for the same - Google Patents

Abstract

An object of the present invention is to provide a steel material having excellent low-temperature impact toughness in a heat-affected zone after welding with high heat input, and a method for manufacturing the same.

Description

Steel with excellent low-temperature impact toughness of the welded heat-affected zone and its manufacturing method

The present invention relates to a steel material having excellent low-temperature impact toughness of a weld heat-affected zone and a manufacturing method thereof.

Recently, due to the demand for large-scale and large-capacity storage of facilities for refining and storing crude oil, the demand for thickening of steel materials used for them is continuously increasing. In particular, due to the increased use in cold environments, The temperature is gradually decreasing.

In particular, the heat-affected zone is composed of a hard phase such as bainite or martensite, and generally has a large crystal grain size. Since the hard phase can act as an impact notch site during Charpy impact evaluation, it lowers the ductile fracture rate, and as the grain size of the hard phase increases, the DBTT increases. Therefore, it is common that the impact toughness of the heat-affected zone is inferior to that of the base material. .

On the other hand, in the case of high heat input electric gas welding (EGW, Electro Gas Welding), the welding time is about 50% compared to Flux Cored Arc Welding (FCAW) or Submerged Arc Welding (SAW) of Multi Pass. In order to improve welding productivity, the use of a large heat input method is increasing from the existing FCAW or SAW welding method.

In the case of such high heat input welding, the growth of crystal grains and the formation of brittle structures in the heat-affected zone of the weld are promoted, so that the impact toughness may be further deteriorated. Therefore, there is a need for techniques capable of preventing the deterioration of toughness in the welding heat-affected zone, particularly in the Coarse Grain Heat Affected Zone (CGHAZ) near the fusion line, which is the heat-affected zone of coarse crystal grains.

For example, in Patent Document 1, carbon (C): 0.08 to 0.20%, silicon (Si): 0.1 to 0.4%, manganese (Mn): 1.0 to 1.6 in weight% to improve the impact toughness of HAZ during high heat input welding %, available aluminum (Sol.Al): 0.005 to 0.050%, titanium (Ti): 0.005 to 0.030%, vanadium (V): 0.005 to 0.10%, boron (B): 0.0005 to 0.0050%, and nitrogen (N): Contains 0.003~0.008%, the balance is composed of Fe and other unavoidable impurities, contains 50~70% of bainite and 30~50% of acicular ferrite as a microstructure, and includes initial austenite The average grain size is 30 μm or less, and the microstructure of the heat-affected zone contains 95% or more of bainite when high-heat-input welding is performed. are starting

However, if defects such as inclusions in steel that decrease the minimum value of impact absorption energy by acting as an impact initiation point by controlling the microstructure alone increase, low-temperature impact toughness may deteriorate, and hot forming (hot forming) in the temperature range of 800 ~ 900 ℃ When TMCP materials are used for pressure vessels manufactured by -forming), the strength and impact toughness after processing may rather deteriorate. Therefore, it is not easy to secure the quality of weld heat affected zone impact toughness in the steel for pressure vessels required in the present invention.

In Patent Document 2, a large amount of N is added in steel and at the same time, the contents of Ti and B are appropriately controlled to precipitate TiN and BN, which suppresses the growth of austenite crystal grains. An improvement technique has been proposed. In addition, in Patent Document 3, Mg and Ca are added in steel to form Mg and Ca oxides, thereby acting as a nucleation site for acicular ferrite and suppressing the formation of grain boundary ferrite, thereby reducing the effect of welding heat A technique to improve the toughness of the part was proposed.

However, in the case of Ti, it exists as a crystallized substance or as a coarse precipitate in the form of TiNbC during the slab solidification process, which can deteriorate the impact toughness of the base material itself. In addition, the proposed technologies are difficult to actually mass-produce due to the high possibility of cracking the surface of the cast steel when manufacturing slabs under continuous casting due to the high N content in the steel, or the production process is complicated because the components in the steel must be precisely controlled. There is a problem of increasing cost.

Korean Patent Publication No. 10-2014-0006657 (published on January 16, 2014) Japanese Unexamined Patent Publication No. 2005-200716 (published on July 28, 2005) Japanese Unexamined Patent Publication No. 2006-241510 (published on September 14, 2006)

According to one aspect of the present invention, it is intended to provide a steel material having excellent low-temperature impact toughness of a heat-affected zone even after welding with high heat input, and a method for manufacturing the same.

The object of the present invention is not limited to the above. A person skilled in the art will have no difficulty understanding the further subject matter of the present invention from the general content of this specification.

One aspect of the present invention, in weight percent, carbon (C): 0.06 ~ 0.25%, silicon (Si): 0.05 ~ 0.50%, manganese (Mn): 1.0 ~ 2.0%, aluminum (Al): 0.005 ~ 0.40% , Phosphorus (P): 0.010% or less, Sulfur (S): 0.0015% or less, Niobium (Nb): 0.001 to 0.03%, Vanadium (V): 0.001 to 0.03%, Titanium (Ti): 0.001 to 0.03%, Chromium (Cr): 0.01~0.20%, Molybdenum (Mo): 0.05~0.15%, Copper (Cu): 0.01~0.50%, Nickel (Ni): 0.05~0.50%, Calcium (Ca): 0.0005~0.0040%, Oxygen (O): 0.0010% or less, the balance including Fe and other unavoidable impurities,

The Ceq value defined in the following relational expression 1 is less than 0.42,

The IC value defined in relational expression 2 below is greater than 100,

The prior austenite grain average size is 30 μm or less,

A steel material having an IN value of 0.1 or less defined in the following relational expression 3 may be provided.

[Relationship 1]

Ceq = [C] + [Mn]/6 + ([Cr]+[Mo]+[V])/5 + ([Ni]+[Cu])/15

(Where [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the weight % of each element)

[Relationship 2]

IC = 565-3951*[C] - 82*[Mn] - 30*[Ni] - 200*[Mo]

(Where [C], [Mn], [Ni] and [Mo] mean the weight % of each element)

[Relationship 3]

IN = S1/S2

(Wherein, S1 is the sum of the areas of Ca-Al-O composite inclusions having a size of 6 μm or more measured as an equivalent circle diameter, and S2 is the sum of the areas of all Ca-Al-O composite inclusions)

The steel may further include 0.0020 to 0.0060% of nitrogen (N).

The microstructure of the steel material may consist of 30 area% or less of pearlite and the balance of ferrite.

The thickness of the steel may be 5 to 100 mm.

After welding and post-welding heat treatment (PWHT) of the steel with a high heat input of 150 to 200 kJ/cm, the ICHAZ (Inter-critical HAZ) of the heat-treated steel includes an MA phase of 15 area% or less, and the diameter of the MA phase is 10 It may be less than μm.

After welding and post-welding heat treatment (PWHT) of the steel with a large heat input of 150 to 200 kJ / cm, the ICHAZ of the heat-treated steel has a tensile strength of 450 to 620 MPa, and a low-temperature impact toughness at -40 ° C. of 80 J or more.

Another aspect of the present invention, in weight percent, carbon (C): 0.06 ~ 0.25%, silicon (Si): 0.05 ~ 0.50%, manganese (Mn): 1.0 ~ 2.0%, aluminum (Al): 0.005 ~ 0.40 %, Phosphorus (P): 0.010% or less, Sulfur (S): 0.0015% or less, Niobium (Nb): 0.001 to 0.03%, Vanadium (V): 0.001 to 0.03%, Titanium (Ti): 0.001 to 0.03%, Chromium (Cr): 0.01~0.20%, Molybdenum (Mo): 0.05~0.15%, Copper (Cu): 0.01~0.50%, Nickel (Ni): 0.05~0.50%, Calcium (Ca): 0.0005~0.0040%, Oxygen (O): 0.0010% or less, the balance including Fe and other unavoidable impurities, a Ceq value defined in the following relational expression 1 below 0.42, and an IC value defined by the following relational expression 2 above 100 Manufacturing a steel slab ;

Reheating the manufactured steel slab to a temperature range of 1150 to 1300 ° C;

Hot rolling the heated slab at a finish hot rolling temperature of 900 to 1050 ° C;

Normalizing the hot-rolled steel sheet by heating it to 850 to 950° C. and holding it for 5 to 60 minutes;

The slab manufacturing step is the step of introducing a metal Ca wire to the molten steel after the secondary refining so that the amount of Ca is 0.015 ~ 0.12 kg / ton; And after adding the Ca, bubbling cleanly for 5 to 20 min so that the amount of inert gas blown into the ladle is 10 to 50 liter/min,

The input speed of the Ca Wire is 100 to 250 meter / min, and the inert gas injection location in the ladle can provide a method for manufacturing two steel materials.

[Relationship 1]

Ceq = [C] + [Mn]/6 + ([Cr]+[Mo]+[V])/5 + ([Ni]+[Cu])/15

(Where [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the weight % of each element)

[Relationship 2]

IC = 565-3951*[C] - 82*[Mn] - 30*[Ni] - 200*[Mo]

(Where [C], [Mn], [Ni] and [Mo] mean the weight% of each element)

The slab may further include 0.0020 to 0.0060% of nitrogen (N).

The Metal Ca Wire is composed of a steel material surrounding a Ca alloy, and the thickness of the steel material may be 1.2 to 1.4 mm.

During the hot rolling, the thickness of the steel material may be 5 to 100 mm.

After the hot rolling step, a step of multi-stage cooling from 200 ° C. or higher to room temperature may be further included.

According to one aspect of the present invention, it is possible to provide a steel material excellent in low-temperature impact toughness of a welded heat-affected zone even after welding with high heat input, and a manufacturing method thereof.

According to another aspect of the present invention, it is possible to provide a steel material for a pressure vessel that can be used for petrochemical manufacturing facilities, manufacturing tanks, and the like, and a manufacturing method thereof.

Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

The present inventors have studied in depth to provide a steel material with excellent low-temperature impact toughness of the weld heat-affected zone, and improved the Ca treatment method during slab manufacturing to reduce the amount of oxidative inclusions and at the same time control the alloy composition to improve the fineness of the weld heat-affected zone. organization was limited. As a result, it was confirmed that oxidative inclusions containing Ca and Al can be effectively reduced, and characteristics excellent in low-temperature impact toughness of the weld heat-affected zone can be secured even after welding with high heat input, and the present invention has been completed.

Hereinafter, the present invention will be described in detail.

Hereinafter, the steel composition of the present invention will be described in detail.

In the present invention, unless otherwise specified, % indicating the content of each element is based on weight.

Steel material according to one aspect of the present invention, by weight%, carbon (C): 0.06 ~ 0.25%, silicon (Si): 0.05 ~ 0.50%, manganese (Mn): 1.0 ~ 2.0%, aluminum (Al): 0.005 ~ 0.40%, Phosphorus (P): 0.010% or less, Sulfur (S): 0.0015% or less, Niobium (Nb): 0.001 to 0.03%, Vanadium (V): 0.001 to 0.03%, Titanium (Ti): 0.001 to 0.03% , Chrome (Cr): 0.01~0.20%, Molybdenum (Mo): 0.05~0.15%, Copper (Cu): 0.01~0.50%, Nickel (Ni): 0.05~0.50%, Calcium (Ca): 0.0005~0.0040% , Oxygen (O): 0.0010% or less, the balance may include Fe and other unavoidable impurities.

Carbon (C): 0.06~0.25%

Since carbon (C) is the most important element for ensuring the strength of steel, it is preferably contained in steel within an appropriate range. It is preferable to add 0.06% or more of carbon (C) in order to secure a target level of strength. However, if the content exceeds 0.25%, the central segregation degree increases, and martensite or MA phase (Martensite & Austenite) is formed in the HAZ, so there is a concern that low-temperature impact toughness is lowered. In particular, when the MA phase is formed, there is a fear that the IGHAZ toughness deteriorates rapidly.

Therefore, the content of carbon (C) may be 0.06 to 0.25%, more preferably 0.10 to 0.20%, more preferably 0.10 to 0.15%.

Silicon (Si): 0.05 to 0.50%

Silicon (Si) is a substitutional element that improves the strength of steel through solid solution strengthening and has a strong deoxidizing effect, so it is an essential element for manufacturing clean steel. In order to secure this effect, it is desirable to add 0.05% or more of silicon (Si), but when added in large amounts, MA phase is generated and the strength of the ferrite matrix is excessively increased, resulting in deterioration of HIC resistance and impact toughness. , the upper limit can be limited to 0.50%.

Accordingly, the content of silicon (Si) may be 0.05 to 0.50%, more preferably 0.05 to 0.40%, and more preferably 0.20 to 0.35%.

Manganese (Mn): 1.0 to 2.0%

Manganese (Mn) is a useful element for improving strength by solid solution strengthening and improving hardenability so that a low-temperature transformation phase is generated. In addition, since a low-temperature transformation phase can be generated even at a slow cooling rate due to improved hardenability, it is a major element for securing a low-temperature bainite phase during normalization. While it is preferable to add 1.0% or more of manganese (Mn) in order to obtain the above-mentioned effect, if the content exceeds 2.0%, the central segregation increases and the fraction of MnS inclusions formed with S increases, resulting in hydrogen organic Crack resistance may be reduced.

Therefore, the content of manganese (Mn) may be 1.0 to 2.0%, more preferably 1.0 to 1.7%, and more preferably 1.0 to 1.5%.

Aluminum (Al): 0.005 to 0.40%

Aluminum (Al) is a strong deoxidizer in the steelmaking process along with Si, and it is preferable to add 0.005% or more to obtain its effect. However, if the content exceeds 0.40%, the fraction of Al2O3 among the oxidative inclusions produced as a result of deoxidation increases excessively, resulting in a coarse size and difficulty in removing during refining. There is a risk of this deterioration.

Therefore, the content of aluminum (Al) may be 0.005 to 0.40%, more preferably 0.1 to 0.4%, more preferably 0.1 to 0.35%.

Phosphorus (P): 0.010% or less and Sulfur (S): 0.0015% or less

Phosphorus (P) and sulfur (S) are elements that cause brittleness at grain boundaries or form coarse inclusions to cause brittleness. The upper limit may be limited to 0.010% or 0.0015%.

Accordingly, the phosphorus (P) content may be 0.010% or less, and the sulfur (S) content may be 0.0015% or less.

Niobium (Nb): 0.001 to 0.03%

Niobium (Nb) precipitates in the form of NbC or NbCN to improve the strength of the base metal, and increases the non-recrystallization rolling reduction by increasing the recrystallization temperature, thereby miniaturizing the initial austenite grain size. In order to obtain the above-mentioned effect, it is preferable to add 0.001% or more of niobium (Nb), but if the content is excessive, undissolved niobium (Nb) is generated in the form of TiNb (C, N), resulting in UT defects and impact toughness. Along with deterioration, it becomes a factor that inhibits hydrogen induced cracking resistance, so the upper limit can be limited to 0.03%.

Accordingly, the content of niobium (Nb) may be 0.001 to 0.03%, more preferably 0.005 to 0.02%, and more preferably 0.007 to 0.015%.

Vanadium (V): 0.001 to 0.03%

Vanadium (V) is almost all re-dissolved when the slab is reheated, so the strengthening effect by precipitation or solidification in the subsequent rolling process is insignificant, while it precipitates as very fine carbonitride in the heat treatment process such as PWHT to improve strength. While it is desirable to add 0.001% or more of vanadium (V) in order to obtain the above-mentioned effects, if the content exceeds 0.03%, the strength and hardness of the welded part are excessively increased, which can act as a factor such as surface cracks during processing of pressure vessels. In addition, there is a problem in that the manufacturing cost rapidly increases and becomes economically unfavorable.

Accordingly, the content of vanadium (V) may be 0.001 to 0.03%, more preferably 0.005 to 0.02%, and more preferably 0.007 to 0.015%.

Titanium (Ti): 0.001 to 0.03%

Titanium (Ti) is an element that greatly improves low-temperature toughness by precipitating as TiN when the slab is reheated and suppressing the growth of crystal grains in the base material and heat-affected zone. In order to obtain this effect, it is preferable to add 0.001% or more of titanium (Ti), but if the content exceeds 0.03%, low-temperature toughness due to clogging of the playing nozzle or crystallization of the center may be reduced. In addition, when coarse TiN precipitates are formed in the center of the thickness by combining with N, it can act as a starting point of low-temperature impact.

Accordingly, the content of titanium (Ti) may be 0.001 to 0.03%, more preferably 0.010 to 0.025%, and more preferably 0.010 to 0.018%.

Chromium (Cr): 0.01~0.20%

Chromium (Cr) has little effect on increasing yield strength and tensile strength by solid solution, but has an effect of preventing strength decrease by delaying the decomposition rate of cementite during tempering or PWHT heat treatment. In order to secure the above effect, it is preferable to add 0.01% or more of chromium (Cr), but if the content exceeds 0.20%, the size and fraction of Cr-Rich coarse carbides such as M23C6 increase, resulting in a significant drop in impact toughness. However, there is a problem in that manufacturing cost increases and weldability deteriorates.

Accordingly, the content of chromium (Cr) may be 0.01 to 0.20%, more preferably 0.05 to 0.15%, and more preferably 0.07 to 0.13%.

Molybdenum (Mo): 0.05 to 0.15%

Molybdenum (Mo), like Cr, is an element useful for preventing strength deterioration during tempering or PWHT heat treatment, and also has an effect of preventing toughness deterioration due to grain boundary segregation of impurities such as P. In addition, as a solid-solution strengthening element in ferrite, there is an effect of increasing the strength of the matrix phase. In order to obtain this effect, it is preferable to add molybdenum (Mo) in an amount of 0.05% or more. However, when excessively added as an expensive element, the manufacturing cost may increase significantly, so the upper limit may be limited to 0.15%.

Accordingly, the content of molybdenum (Mo) may be 0.05 to 0.15%, more preferably 0.07 to 0.13%, and more preferably 0.10 to 0.12%.

Copper (Cu): 0.01 to 0.50%

Copper (Cu) is an element advantageous for greatly improving the strength of the matrix phase by solid solution strengthening in ferrite, and is preferably added in an amount of 0.01% or more to secure this effect. However, if the addition amount is excessive, the possibility of hot-shortness surface defects due to copper (Cu) increases, and if the strength is excessively increased, there is a risk of impact toughness deterioration, so the upper limit can be limited to 0.50%. .

Therefore, the content of copper (Cu) may be 0.01 to 0.50%, more preferably 0.05 to 0.35, more preferably 0.10 to 0.25%.

Nickel (Ni): 0.05 to 0.50%

Nickel (Ni) is an important element for increasing strength by improving impact toughness and hardenability by easily forming cross slip of dislocations by increasing stacking faults at low temperatures. It is preferable to add more. However, if the content exceeds 0.50%, there is a risk of increasing manufacturing cost due to high cost compared to other hardenability improving elements, and since Ceq increases, the HAZ hard phase fraction can be increased to reduce low-temperature impact toughness.

Accordingly, the content of nickel (Ni) may be 0.05 to 0.50%, more preferably 0.10 to 0.40%, and more preferably 0.10 to 0.30%.

Calcium (Ca): 0.0005 to 0.0040%

When calcium (Ca) is added after deoxidation by Al, it combines with S forming MnS inclusions to suppress the production of MnS, and at the same time, it forms spherical CaS to suppress the occurrence of cracks due to hydrogen-induced cracking. there is. In the present invention, in order to sufficiently form S contained as an impurity into CaS, it is preferable to add 0.0005% or more of calcium (Ca), but if the added amount is excessive, calcium (Ca) remaining after forming CaS is combined with O to form a coarse Oxidative inclusions are generated, which may be elongated or destroyed during rolling to promote hydrogen induced cracking, so the upper limit may be limited to 0.0040%.

Accordingly, the content of calcium (Ca) may be 0.0005 to 0.0040%, more preferably 0.001 to 0.0025%.

Oxygen (O): 0.0010% or less

In the steel of the present invention, extreme desulfurization must be applied to suppress MnS production, and for desulfurization, the concentration of oxygen (O) in molten steel must be extremely low to ensure excellent efficiency. Therefore, the content of oxygen (O) dissolved in molten steel is very low, and the total amount of oxygen (O) contained in inclusions is equal to the total amount of oxygen (O) in the product. Although the size of inclusions is limited in the present invention, it is preferable to limit the total amount of fine inclusions, so the content of oxygen (O) is limited to 0.0010% or less. In the present invention, the lower the content of oxygen (O) is controlled, the more preferable it is.

Accordingly, the content of oxygen (O) may be 0.0010% or less, more preferably 0.0008%.

The steel of the present invention may include the remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing field, not all of them are specifically mentioned in this specification.

Steel according to an aspect of the present invention may further include 0.0020 to 0.0060% of nitrogen (N).

Nitrogen (N) has the effect of improving the toughness of CGHAZ by combining with Ti to form precipitates during one-pass high heat input welding such as EGW (Electro Gas Welding) of steel.

The steel material of the present invention may have a Ceq value of less than 0.42 defined in relational expression 1 below.

If the Ceq value is more than 0.42, there is a concern that the fraction of hard tissue such as MA in the weld heat affected zone may increase, and accordingly, there is a problem that the impact toughness of ICHAZ may be lowered.

[Relationship 1]

Ceq = [C] + [Mn]/6 + ([Cr]+[Mo]+[V])/5 + ([Ni]+[Cu])/15

(Where [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the weight % of each element)

The steel material of the present invention may have an IC value greater than 100 defined in the following relational expression 2.

Relational Equation 2 shows the low-temperature impact toughness absorbed energy value according to the alloy components, and as the fraction of the MA phase increases, the impact absorbed energy value may decrease.

If the IC value of relational expression 2 is less than 100, there may be a problem in that the low-temperature impact toughness and absorbed energy value in ICHAZ required in the present invention cannot be properly secured. In the present invention, in order to effectively secure the impact toughness of ICHAZ, it may be more preferably 130 or more.

In the present invention, when the relational expression 1 and the relational expression 2 are simultaneously satisfied, it may be advantageous to secure the impact toughness of the heat-affected zone targeted in the present invention after welding and post-weld heat treatment.

[Relationship 2]

IC = 565-3951*[C] - 82*[Mn] - 30*[Ni] - 200*[Mo]

(Where [C], [Mn], [Ni] and [Mo] mean the weight % of each element)

Hereinafter, the steel microstructure of the present invention will be described in detail.

In the present invention, % representing the fraction of the microstructure is based on the area unless otherwise specified.

The microstructure of the steel according to one aspect of the present invention may consist of 30 area% or less of pearlite and the balance of ferrite.

If pearlite exceeds 30% as a microstructure, it may exceed the appropriate tensile strength range required by the present invention, and accordingly, it is impossible to secure low-temperature impact toughness of 80J or more at -40 ° C. Therefore, it is preferable that the fraction of pearlite is 30% or less.

In the steel material according to one aspect of the present invention, the prior austenite average grain size may be 30 μm or less.

In the present invention, impact toughness can be secured by controlling the average size of prior austenite grains. When the prior austenite average grain size exceeds 30 μm, the brittle-ductile transition temperature becomes high, which may cause a decrease in impact toughness. Therefore, in the present invention, the prior austenite grain average size may be 30 μm or less, more preferably 25 μm or less.

The steel material of the present invention may have an IN value of 0.1 or less defined in the following relational expression 3.

In the present invention, the size distribution of composite oxides containing Ca and Al is limited as a means for securing the low-temperature impact toughness of the weld heat-affected zone. The composition of the inclusion is a composite oxide containing both Ca and Al, and when the diameter of the inclusion is turned into a circle, the ratio of the inclusion having a particle diameter of 6 μm or more is controlled. As the coarse complex inclusions are significantly reduced, the occurrence of defects can be reduced as the hydrogen adsorption source in the steel material, which is crushed during rolling and used as a hydrogen adsorption source to act as a defect of hydrogen induced cracking, is reduced.

Non-metallic inclusions with an equivalent circle diameter of 0.5 μm or more were analyzed by EDS, the sum of the areas of inclusions with a diameter of 6 μm or more was represented by S1, and the sum of the total areas of Ca-Al-O composite inclusions was represented by S2. was When the ratio of the sum of the areas of inclusions having a diameter of 6 μm or more to the total area of the complex inclusions exceeds 0.1, the number of inclusions that are crushed increases during rolling, and hydrogen-induced crack resistance cannot be secured.

[Relationship 3]

IN = S1/S2

(Wherein, S1 is the sum of the areas of Ca-Al-O composite inclusions having a size of 6 μm or more measured as an equivalent circle diameter, and S2 is the sum of the areas of all Ca-Al-O composite inclusions)

ICHAZ (Inter-critical HAZ) after welding and post-welding heat treatment (PWHT) of 150 ~ 200 kJ / cm high heat input to the steel according to one aspect of the present invention may contain less than 15 area% of the MA phase as a microstructure, The MA phase may have a diameter of 10 μm or less.

When the MA phase is 15 area% or less and the diameter is 10 μm or less as the microstructure of ICHAZ after welding and post-weld heat treatment, it may be advantageous to secure the impact toughness of the heat-affected zone aimed at in the present invention. This may be more advantageous when the above relational expressions 1 and 2 are simultaneously satisfied.

Hereinafter, the steel manufacturing method of the present invention will be described in detail.

The steel material according to one aspect of the present invention may be manufactured by manufacturing, reheating, hot rolling, cooling, and normalizing a steel slab satisfying the above-described alloy composition, and the slab preparation step may further include Ca input and clean bubbling steps. there is.

slab manufacturing

A steel slab satisfying the above alloy composition can be manufactured.

In the present invention, when manufacturing a slab, after secondary refining, Ca input and clean bubbling steps may be included. Specifically, when preparing the slab, after the secondary refining, Metal Ca Wire may be injected into the molten steel so that the amount of Ca added is 0.015 to 0.12 kg/ton. The Metal Ca Wire is composed of a steel material surrounding a Ca alloy, the thickness of the steel material may be 1.2 ~ 1.4mm, the input speed of the Ca Wire may be 100 ~ 250meter / min.

After adding the Ca, clean bubbling may be performed so that the amount of inert gas blown into the ladle is 10 to 50 liter/min. The number of inert gas injection points in the ladle may be two, and the clean bubbling time may be 5 to 20 min.

In the present invention, the process before the secondary refining is not particularly limited, and any conventional method can be applied. In the present invention, the process after the Ca injection and clean bubbling process is not particularly limited, and the slab can be manufactured by cooling the molten steel under normal conditions. In addition, when the amount of Al2O3 in molten steel increases, inclusions containing both Ca and Al are generated and coarsened, resulting in increased crushing inclusions during rolling, making it impossible to secure hydrogen-induced cracking resistance. The total amount of inclusions in molten steel may be limited to 2 to 5 ppm.

Ca input

When manufacturing steel slabs, after secondary refining, Metal Ca Wire can be injected into the molten steel so that the amount of Ca added can be 0.015 to 0.12 kg/ton. The Metal Ca Wire is composed of a steel material surrounding a Ca alloy, the thickness of the steel material may be 1.2 ~ 1.4mm, the input speed of the Ca Wire may be 100 ~ 250meter / min.

When adding Ca, if the thickness of the steel material surrounding the Ca alloy of the Metal Ca Wire is less than 1.2mm, Ca melts at the top of the ladle, reducing the effect of iron pressure, resulting in inferior Ca real rate and increasing the amount of input. On the other hand, if the thickness exceeds 1.4mm, the Ca wire is brought into contact with the base of the ladle, causing dissolution of the ladle's refractories, making it impossible to secure operational stability.

When adding Ca into the molten steel, the feeding speed of the wire into the molten steel should be controlled to ensure the real rate of Ca together with the thickness of the metal Ca wire. If the input speed of the wire is less than 100 m/min, the Ca melt is melted at the top of the ladle, and the effect of the iron pressure is reduced, resulting in a poor Ca real rate and an increase in the input amount. On the other hand, if the input speed of the wire exceeds 250 meters / min, there is a problem that the safety of the operation cannot be secured because the Ca wire is contacted to the base of the ladle and the refractory material of the ladle is lost. In addition, a more preferable input rate may be 120 to 200 meters/min, more preferably 140 to 180 meters/min.

If the amount of Ca input is too small, MnS is generated in the center during solidification, making it impossible to secure hydrogen induced cracking resistance. There is a problem that stability cannot be secured. Therefore, considering the above problems, the amount of Ca input may be limited to 0.015 to 0.12 kg/ton, more preferably 0.015 to 0.10 kg/ton, and more preferably 0.050 to 0.10 kg/ton.

clean bubbling

After adding the Ca, clean bubbling may be performed for 5 to 20 min so that the amount of inert gas blown into the ladle is 10 to 50 liter/min. The number of inert gas injection points in the ladle may be two.

If the amount of inert gas blown into the ladle is too small, the amount of Al2O3 Clusters attached to and removed by the inert gas and the complex inclusions containing Ca and Al are reduced, resulting in poor cleanliness and hydrogen-induced cracking resistance. there is. On the other hand, when the amount is excessive, the agitating force becomes strong, and the surface of the molten steel becomes dirty and slag is mixed at the same time, resulting in poor cleanliness. Therefore, in the present invention, the gas injection amount may be 10 to 50 liter/min, more preferably 15 to 40 liter/min, and more preferably 20 to 30 liter/min.

If there is only one inert gas injection point in the ladle, there is an uneven area in the molten steel, and the removal ability of the Al2O3 Cluster and complex inclusions containing Ca and Al are inferior. As a result, the agitation power becomes stronger, and the surface of the molten steel becomes dirty and slag is mixed at the same time, resulting in poor cleanliness.

If the time to apply clean bubbling is too short even though the amount of inert gas injected into the ladle is limited, the amount of Al2O3 Clusters and composite inclusions containing Ca and Al are reduced, which are removed by being attached to the inert gas, resulting in inferior cleanliness. It becomes impossible to ensure hydrogen induced cracking property. On the other hand, if the time is excessive, the temperature drop in the molten steel increases and a temperature gradient in the ladle is generated, resulting in poor cleanliness. Therefore, in the present invention, the clean bubbling time may be limited to 5 to 20 min, more preferably 7 to 17 min, and more preferably 10 to 15 min.

reheat

The slab may be reheated to a temperature range of 1150 to 1300 ° C.

In order to maximize the grain size of austenite, by re-dissolving carbonitrides of Ti or Nb or coarse crystallized TiNb(C,N) formed during casting, and heating and maintaining austenite to a temperature higher than the recrystallization temperature after sizing rolling, the slab It is preferable to limit the heating temperature to 1150°C or higher. On the other hand, if the temperature is excessively high, problems may occur due to oxide scale at high temperatures, and manufacturing costs may be excessively increased due to cost increases due to heating and maintenance, so the upper limit may be limited to 1300 ° C.

hot rolled

The heated slab may be hot rolled at a finish hot rolling temperature of 900 to 1050 ° C.

In the present invention, 900 ° C. corresponds to a region equal to or higher than the recrystallization temperature. When the finish hot rolling temperature is less than 900 ° C, the complex inclusions produced in the refining process must accept the deformation caused by rolling as the strength of the steel sheet increases as the rolling temperature decreases, and as a result, it is crushed or segmented into small-sized inclusions or Inclusions such as MnS can be elongated. Since the inclusions crushed, segmented, or stretched with such small-sized inclusions act as a direct cause of the initiation and propagation of low-temperature impact toughness, finish hot rolling can be performed at 900 ° C. or higher, which is a recrystallization temperature or higher at which work hardening does not occur. However, if the temperature exceeds 1050 ℃, since austenite grain growth continues to occur even after the rolling is finished, the impact toughness may decrease due to the increase in DBTT. In addition, the hot-rolled steel sheet may be air-cooled to room temperature.

The thickness of the steel sheet after finish hot rolling may be 5 to 100 mm, more preferably 5 to 70 mm, and more preferably 5 to 40 mm.

normalizing

The hot-rolled steel sheet may be normalized by heating to 850 to 950° C. and holding for 5 to 60 minutes.

During normalizing, if the heating temperature is less than 850℃ or the holding time is shorter than 5 minutes, the carbide generated during cooling after rolling or impurity elements segregated at the grain boundary do not re-dissolve smoothly, and the low-temperature toughness of the steel after normalizing may be greatly reduced. can On the other hand, if the temperature exceeds 950 ° C or the holding time exceeds 60 minutes, toughness may decrease due to coarsening of austenite and coarsening of precipitated phases such as Nb(C,N) and V(C,N). can

In the present invention, the holding time is preferably carried out for 5 to 60 minutes from the time the steel plate center temperature reaches the target temperature.

In the present invention, when the amount of dissolved hydrogen in the molten steel is 1.3 ppm or more when manufacturing the slab, after hot rolling before normalizing, multi-stage dropwise cooling from a temperature of 200 ° C. or more to room temperature may be further included.

In the case of multi-stage cooling, by releasing dissolved hydrogen in the steel, internal microcracks caused by hydrogen can be more effectively suppressed, and finally, the low-temperature impact toughness of the base material and the heat-affected zone of the weld can be improved.

The steel of the present invention manufactured as described above may have a thickness of 5 to 100 mm, and after 150 to 200 kJ / cm large heat input welding and post-weld heat treatment (PWHT), ICHAZ (Inter-critical HAZ) is a microstructure of MA phase 15 area%, the diameter of the MA phase is 10 μm or less, the tensile strength of ICHAZ after post-weld heat treatment is 450-620 MPa, the low-temperature impact toughness of the base material and HAZ at -40 ° C is 80 J or more, and the welding heat effect It can have excellent characteristics of negative impact toughness.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the present invention.

(Example)

In Table 1 below, alloy compositions according to steel types are disclosed, and relational expressions 1 and 2 are calculated and shown. The content of oxygen is related to the process and inclusions, and is not separately disclosed in the alloy composition of Table 1. Steel plates having a thickness of 5 to 100 mm were manufactured using the steel of Table 1 below under the process conditions shown in Table 2 below. Conditions other than the process conditions described in Table 2 were equally applied as conditions satisfying the scope of the present invention.

steel grade Alloy component (% by weight) Relation 1
(Ceq) Relation 2
(IC) C Si Mn Al P* S* Nb V Ti Cr Mo Cu Ni Ca* A 0.07 0.15 1.1 0.03 80 10 0.013 0.015 0.011 0.02 0.10 0.20 0.25 25 0.31 170.73 B 0.08 0.2 1.2 0.03 80 10 0.015 0.015 0.013 0.05 0.10 0.08 0.20 25 0.33 124.52 C 0.07 0.17 1.07 0.03 85 12 0.013 0.017 0.012 0.014 0.08 0.02 0.23 22 0.29 177.79 D 0.09 0.23 1.05 0.02 81 10 0.015 0.02 0.012 0.019 0.06 0.04 0.18 21 0.30 105.91 E 0.09 0.35 1.03 0.03 83 11 0.012 0.013 0.015 0.05 0.07 0.15 0.21 20 0.31 104.65 F 0.18 0.32 1.17 0.02 82 13 0.018 0.015 0.001 0.13 0.08 0.12 0.28 22 0.45 -266.52 G 0.09 0.29 1.1 0.02 85 12 0.018 0.013 0.010 0.11 0.11 0.20 0.19 23 0.35 91.51 H 0.11 0.22 1.09 0.03 84 10 0.019 0.047 0.015 0.08 0.09 0.18 0.27 22 0.37 14.91

* Unit is ppm

[Relationship 1]

Ceq = [C] + [Mn]/6 + ([Cr]+[Mo]+[V])/5 + ([Ni]+[Cu])/15

(Where [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the weight % of each element)

[Relationship 2]

IC = 565-3951*[C] - 82*[Mn] - 30*[Ni] - 200*[Mo]

(Where [C], [Mn], [Ni] and [Mo] mean the weight% of each element)

Psalter
number steel grade slab manufacturing reheat hot rolled normalizing Ca input clean bubbling temperature
(℃) finish
hot rolled
temperature
(℃) heating temperature
(℃) holding time
(minute) Ca input
(kg/ton) speed
(m/min) gas intake
(liter/min) hour
(minute) intake point
(dog) One A 0.018 153 13 17 2 1159 908 867 7 2 B 0.054 201 12 18 2 1165 1012 910 12 3 C 0.081 153 23 15 2 1196 954 913 13 4 D 0.103 176 25 7 2 1174 932 911 15 5 E 0.085 209 36 16 2 1184 998 884 20 6 A 0.194 223 34 15 2 1207 956 860 19 7 A 0.053 65 33 17 2 1186 908 859 33 8 B 0.031 165 6 19 2 1158 966 884 34 9 B 0.024 170 19 39 2 1164 918 932 41 10 C 0.028 181 24 11 One 1180 975 915 52 11 C 0.035 209 28 13 2 1023 963 933 9 12 D 0.019 212 31 10 2 1238 1115 928 18 13 D 0.042 235 30 8 2 1188 982 981 33 14 E 0.051 241 29 7 2 1153 967 899 83 15 F 0.029 118 20 15 2 1159 965 854 53 16 G 0.053 243 28 8 2 1154 966 884 41 17 H 0.031 164 40 18 2 1157 965 883 37

The microstructure and mechanical properties of the prepared steel sheet were measured and are shown in Table 3. Microstructures and complex inclusions were observed for microstructures of each specimen using a scanning electron microscope. Composite inclusions were analyzed by EDS for non-metallic inclusions with a particle diameter of 0.5 μm or more, which were detected when observing an area of 72 mm ² with a scanning electron microscope in the base material and HAZ. When the composition of the inclusion is a composite inclusion containing both Ca and Al, and the diameter of the inclusion is converted into a circle, the sum of the areas of inclusions with a particle diameter of 6㎛ or more is represented by S1, and the sum of the total area of complex inclusions is represented by S2, which is the value of S1/S2 showed In addition, the manufactured steel was welded with a high heat input of 150 to 200 kJ/cm and subjected to post-weld heat treatment (PWHT), and then the microstructure and mechanical properties of ICHAZ were measured. The microstructure was measured in the same way as the previous steel sheet measurement method, and the mechanical properties were measured for tensile strength of each specimen through a tensile test, and impact toughness at -40 ° C was measured in a conventional manner.

Psalter
number steel grade microstructure
(area%) Ca-Al-O
complex inclusion ICHAZ microstructure Physical properties after PWHT division F P Prior austenite grain average size (μm) S1 S2 S1/S2 MA fraction
(area%) MA diameter
(μm) tensile strength
(MPa) impact toughness
(-40℃, J) One A 77 23 19.8 624 7126 0.09 3.5 8.1 512 213 Invention example 1 2 B 78 22 23.7 485 6105 0.08 4.5 2.3 532 222 Invention Example 2 3 C 74 26 22.6 214 5324 0.04 6.2 2.5 498 251 Invention example 3 4 D 75 25 22.4 183 5625 0.03 3.9 3.6 514 234 Invention Example 4 5 E 79 21 23.1 115 4563 0.03 2.5 4.3 532 209 Invention example 5 6 A 80 20 22.8 480 2315 0.21 1.8 6.5 506 15 Comparative Example 1 7 A 72 28 24.2 352 1354 0.26 2.7 3.7 514 16 Comparative Example 2 8 B 74 26 24.0 231 1685 0.14 5.5 4.1 522 23 Comparative Example 3 9 B 74 26 23.9 334 1565 0.21 6.3 5.5 508 28 Comparative Example 4 10 C 75 25 20.5 493 2557 0.19 7.3 6.1 532 27 Comparative Example 5 11 C 76 24 45.2 289 3458 0.08 8.1 1.8 509 51 Comparative Example 6 12 D 77 23 39.8 212 4862 0.04 6.9 2.8 511 38 Comparative Example 7 13 D 75 25 41.7 699 9823 0.07 2.9 6.5 524 50 Comparative Example 8 14 E 71 29 39.5 357 5662 0.06 3.3 4.6 529 18 Comparative Example 9 15 F 73 27 22.7 765 8882 0.09 20.9 15.7 583 14 Comparative Example 10 16 G 78 22 20.9 804 9985 0.08 16.5 8.3 509 23 Comparative Example 11 17 H 74 26 20.4 934 9715 0.10 15.7 9.4 531 26 Comparative Example 12

P: pearlite, F: ferrite

As shown in Table 3, inventive examples satisfying the alloy composition and manufacturing method proposed in the present invention secure all the mechanical properties targeted in the present invention.

On the other hand, Comparative Examples 1 and 2 do not satisfy the scope of the present invention in the Ca input amount and input rate when manufacturing slabs, and in Comparative Examples 1 and 2, composite inclusions containing Ca and Al having a particle size of 6 μm or more are excessive. Therefore, the impact toughness after heat treatment after welding did not achieve the desired level in the present invention.

In Comparative Examples 3 to 5, when manufacturing slabs, the gas injection amount, time, and injection location, which are clean bubbling conditions, do not satisfy the present invention, and inclusions having a size of 6 μm or more among Ca-Al-O composite inclusions are excessively generated. After welding heat treatment, the impact toughness was inferior.

In Comparative Example 6, the reheating temperature was less than the range of the present invention, the average grain size of prior austenite was coarse, and the impact toughness desired in the present invention was not secured.

In Comparative Example 7, the finish hot rolling temperature exceeded the range of the present invention, and the prior austenite crystal grains continued to grow, so the impact toughness did not satisfy the range of the present invention.

Comparative Examples 8 and 9 did not satisfy the normalizing temperature and time conditions of the present invention, and the prior austenite average grain size was coarse, so the desired impact toughness value in the present invention could not be secured.

In Comparative Examples 10 to 12, the relational expression 1 or the relational expression 2 of the present invention was not satisfied, and the MA fraction of ICHAZ after welding and post-welding heat treatment exceeded the target in the present invention, and as a result, the impact toughness value could not be secured. did

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (11)

By weight %, carbon (C): 0.06 to 0.25%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.0 to 2.0%, aluminum (Al): 0.005 to 0.40%, phosphorus (P): 0.010 % or less, Sulfur (S): 0.0015% or less, Niobium (Nb): 0.001 to 0.03%, Vanadium (V): 0.001 to 0.03%, Titanium (Ti): 0.001 to 0.03%, Chromium (Cr): 0.01 to 0.20 %, Molybdenum (Mo): 0.05 to 0.15%, Copper (Cu): 0.01 to 0.50%, Nickel (Ni): 0.05 to 0.50%, Calcium (Ca): 0.0005 to 0.0040%, Oxygen (O): 0.0010% or less , balance Fe and other unavoidable impurities,
The Ceq value defined in the following relational expression 1 is less than 0.42,
The IC value defined in relational expression 2 below is greater than 100,
The prior austenite grain average size is 30 μm or less,
The IN value defined in the following relational expression 3 is 0.1 or less,
ICHAZ (Inter-critical HAZ) of the heat-treated steel after welding and post-weld heat treatment (PWHT) with a large heat input of 150 to 200 kJ/cm is a steel containing less than 15 area% of the MA phase.
[Relationship 1]
Ceq = [C] + [Mn]/6 + ([Cr]+[Mo]+[V])/5 + ([Ni]+[Cu])/15
(Where [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the weight% of each element)
[Relationship 2]
IC = 565-3951*[C] - 82*[Mn] - 30*[Ni] - 200*[Mo]
(Where [C], [Mn], [Ni] and [Mo] mean the weight % of each element)
[Relationship 3]
IN = S1/S2
(Wherein, S1 is the sum of the areas of Ca-Al-O composite inclusions having a size of 6 μm or more measured as an equivalent circle diameter, and S2 is the sum of the areas of all Ca-Al-O composite inclusions)
According to claim 1,
The steel material further contains 0.0020 to 0.0060% of nitrogen (N).
According to claim 1,
The microstructure of the steel material is a steel material consisting of pearlite and the balance ferrite of 30 area% or less.
According to claim 1,
The steel material has a thickness of 5 to 100 mm.
According to claim 1,
After welding and post-welding heat treatment (PWHT) of the steel with a large heat input of 150 to 200 kJ / cm, ICHAZ (Inter-critical HAZ) of the heat-treated steel is a steel with a diameter of 10 μm or less in the MA phase.
According to claim 1,
After welding and post-welding heat treatment (PWHT) of the steel with a high heat input of 150 to 200 kJ / cm, the ICHAZ of the heat-treated steel has a tensile strength of 450 to 620 MPa and a low-temperature impact toughness of 80 J or more at -40 ° C. Steel.
By weight %, carbon (C): 0.06 to 0.25%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.0 to 2.0%, aluminum (Al): 0.005 to 0.40%, phosphorus (P): 0.010 % or less, Sulfur (S): 0.0015% or less, Niobium (Nb): 0.001 to 0.03%, Vanadium (V): 0.001 to 0.03%, Titanium (Ti): 0.001 to 0.03%, Chromium (Cr): 0.01 to 0.20 %, Molybdenum (Mo): 0.05 to 0.15%, Copper (Cu): 0.01 to 0.50%, Nickel (Ni): 0.05 to 0.50%, Calcium (Ca): 0.0005 to 0.0040%, Oxygen (O): 0.0010% or less , Fe and other unavoidable impurities, the Ceq value defined in relational expression 1 below is less than 0.42, and the IC value defined in relational expression 2 below is greater than 100;
Reheating the manufactured steel slab to a temperature range of 1150 to 1300 ° C;
Hot rolling the heated slab at a finish hot rolling temperature of 900 to 1050 ° C;
Normalizing the hot-rolled steel sheet by heating it to 850 to 950° C. and holding it for 5 to 60 minutes;
The slab manufacturing step is the step of introducing a metal Ca wire to the molten steel after the secondary refining so that the amount of Ca is 0.015 ~ 0.12 kg / ton; And after adding the Ca, bubbling cleanly for 5 to 20 min so that the amount of inert gas blown into the ladle is 10 to 50 liter/min,
The input speed of the Ca Wire is 100 to 250 meter / min, and the inert gas injection points in the ladle are two,
The prior austenite grain average size is 30 μm or less,
After welding and post-weld heat treatment (PWHT) with a high heat input of 150 to 200 kJ/cm, ICHAZ (Inter-critical HAZ) of the heat-treated steel is a method for manufacturing steel containing less than 15 area% of the MA phase.
[Relationship 1]
Ceq = [C] + [Mn]/6 + ([Cr]+[Mo]+[V])/5 + ([Ni]+[Cu])/15
(Where [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the weight % of each element)
[Relationship 2]
IC = 565-3951*[C] - 82*[Mn] - 30*[Ni] - 200*[Mo]
(Where [C], [Mn], [Ni] and [Mo] mean the weight% of each element)
According to claim 7,
The slab is a method of manufacturing a steel material further containing 0.0020 to 0.0060% of nitrogen (N).
According to claim 7,
The Metal Ca Wire is composed of a steel material surrounding a Ca alloy, and the thickness of the steel material is a method of manufacturing a steel material having a thickness of 1.2 to 1.4 mm.
According to claim 7,
A method of manufacturing a steel material such that the thickness of the steel material is 5 to 100 mm during the hot rolling.
According to claim 7,
Method for producing a steel material further comprising the step of multi-stage cooling from 200 ° C. or more to room temperature after the hot rolling step.