KR100957961B1 - High strength steel plate having excellent welded zone toughness for linepipe and the method for manufacturing the same - Google Patents

Abstract

The present invention relates to a high-strength line pipe steel having excellent weld toughness and a manufacturing method thereof, in weight%, C: 0.03 to 0.1%, Si: 0.05 to 0.3%, Mn: 0.5 to 2.0%, Ti: 0.005 to 0.02% , Nb: 0.005 to 0.03%, P: 0.010% or less, S: 0.004% or less, Ca: 0.0005 to 0.005%, N: 0.008 or less, remainder Fe and other unavoidable impurities, and the TiN ratio is 2 <Ti / N < A high strength line pipe steel having excellent weld toughness satisfying the range of 4.5 and a method of manufacturing the same are provided.

According to the present invention, it is possible to produce a high-strength line pipe steel having excellent weld toughness having a tensile strength of 620-900 MPa excellent in weld properties.

Line pipe, weld toughness, cooling rate, center segregation, steel

Description

High strength steel plate having excellent welded zone toughness for linepipe and the method for manufacturing the same}

The present invention relates to a high-strength line pipe steel having a tensile strength of 620MPa or more and 900MPa or less and mainly used for the purpose of construction, pipelines and offshore structures, and a method of manufacturing the same. More specifically, the present invention relates to a steel plate for high-strength line pipe having a tensile strength of 620 MPa or more and 900 MPa or less having excellent welded properties by managing alloy elements and relations and controlling cooling rates.

In order to increase the transport capacity and efficiency of the line pipe according to the long-distance transportation of crude oil and natural gas, a high strength steel sheet is required to increase the transport pressure and transport capacity. As a result, although line pipes up to API-X80 grade have been put to practical use, demand for high-strength line pipe steel of API-X100 grade or higher is increasing.

In addition, when the steel sheet for line pipe is used at low temperatures, when the toughness of the welded part and the base material is weak, there is a risk that a large accident may occur due to rapid brittle fracture, and thus the securing of physical properties is more important.

In general, increasing the strength of a material, on the contrary, tends to reduce toughness. This is because the alloying elements added usually have a beneficial effect on strength while playing a contradictory role in inhibiting toughness. In order to solve this problem, the method of improving the strength and toughness of the steel while suppressing the adjustment of the element as much as possible, the so-called TMCP (Thermo Mechanical Controlling Process), reduces the internal grain size and improves toughness, Has been used a lot of methods to form a hard tissue to pursue high strength and high toughness.

In order to satisfy the tensile strength required in the manufacture of steel pipes, the amount of alloying elements added is increased, and the cooling conditions are severe in manufacturing. At this time, when making a welded steel pipe made of a high alloy steel sheet, the seam welding reduces the toughness of the weld heat affected zone, making it difficult to secure the stability of the steel pipe.

In Japanese Patent Laid-Open No. 2006-328523, a steel tube having excellent welded toughness was manufactured. When Mg and B were used, heat values of up to 70 KJ / cm were obtained. However, there are difficulties in adding Mg and adjusting inclusions in the steelmaking process, and difficulties in controlling boron when manufacturing steel by adding boron. In general steelmaking, deoxidation is performed in aluminum. However, in the inventive steel, Al content is difficult to be prepared to 0.01% or less in the Mg system.

Japanese Patent Laid-Open No. 2005-2476 adds boron to a thick plate when applying high heat input (200KJ / cm) to suppress the formation of ferrite side plates at the austenite grain boundary at a content of 10 ppm or less. More than 100J was secured at -40 ° C. However, the strength of the applied steel is less than 615MPa and the content of the alloy Mn added does not exceed 1.5%, so it is difficult to apply to high strength steel.

Japanese Laid-Open Patent Publication No. 2003-313628 proposes a method for manufacturing steel materials which does not harm welded toughness when applied by the electroslag method, which is subjected to super heat input 410 KJ / cm by inclusion control. However, the present invention has a problem that a large amount of expensive Ni contained.

Japanese Patent Laid-Open No. 2001-113374 has invented a method for manufacturing steel pipes having excellent low temperature toughness of core welds in steels exceeding API-X100. However, there is a problem that the heat input is very low at 20KJ / cm.

In Korean Laid-Open Publication No. 2004-0057240, a steel material having improved weld toughness even when high heat input is applied by suppressing austenite grain growth of HAZ part is produced. However, there is a problem that it is not suitable as a line pipe steel used in the high pressure gas transport pipe containing a lot of nitrogen more than 80ppm.

The present invention is to improve the above-mentioned problems, and to provide a high-strength line pipe steel having a high tensile strength of 620MPa or more and a method of manufacturing the same by managing alloying elements and relational expressions and controlling the cooling rate. There is this.

According to one aspect of the invention, in weight%, C: 0.03-0.1%, Si: 0.05-0.3%, Mn: 0.5-2.0%, Ti: 0.005-0.02%, Nb: 0.005-0.03%, P: 0.010 High strength with excellent weldability toughness of% or less, S: 0.004% or less, Ca: 0.0005 to 0.005%, N: 0.008 or less, balance Fe and other unavoidable impurities, and satisfying a TiN ratio of 2 <Ti / N <4.5 Linepipe steels are provided.

According to another aspect of the invention, in weight%, C: 0.03-0.1%, Si: 0.05-0.3%, Mn: 0.5-2.0%, Sol.Al: 0.005-0.05%, Ti: 0.005-0.02%, Nb : 0.005 to 0.03%, P: 0.010% or less, S: 0.004% or less, Ca: 0.0005 to 0.005%, N: 0.008 or less, balance with Fe and other unavoidable impurities, and the TiN ratio of 2 <Ti / N <4.5 Steel slab having a composition that satisfies a range and subjected to a low pressure with a reduction ratio of 2% or more and a casting process of 0.9-1.2 m / min to perform a reproducing process to produce a slab; Reheating the slab to 1150-1300 ° C .; Finishing rolling the reheated steel at 800-950 ° C .; And cooling the finished rolled steel sheet to 150-350 ° C. at a cooling rate of 3 ° C./sec or more, thereby providing a high strength line pipe steel having excellent weld toughness.

According to the present invention, it is possible to produce a high-strength line pipe steel having excellent weld toughness having a tensile strength of 620-900 MPa excellent in weld properties.

The inventors maintained the Ti / N ratio as 2 <Ti / N <4.5 and controlled Si below 0.3% in order to properly add the alloying elements and to suppress the austenite grain growth in the heat affected zone. Cr, Mo, Ni, Cu, etc. are managed below the value suggested by Ma's formula to produce slabs by controlling center segregation during steelmaking and performance, and to control the cooling rate. An excellent line pipe for steel pipes is secured and the present invention is proposed based on the results of this study.

Hereinafter, the composition range of the steel component of the present invention will be described.

The content of C is preferably 0.03 to 0.1%. The C is the most economical and effective element to strengthen the steel by causing solid solution strengthening and improving the hardenability of the steel, the weldability, formability and toughness may be reduced by adding a large amount. If the content is less than 0.03%, it is not economical because a relatively large amount of other alloy elements must be added to exert the same strength, and if it is more than 0.1%, it is not preferable because the weldability, formability and toughness are lowered.

The content of Si is preferably 0.05 to 0.3%. The Si serves to deoxidize molten steel by assisting aluminum and also has an effect as a solid solution strengthening element. If the content is less than 0.05%, it is difficult to obtain clean steel because it does not sufficiently deoxidize the molten steel.If the content exceeds 0.3%, the red scale is formed by Si during rolling, and the surface shape of the steel sheet becomes very bad and brittle fracture occurs. This is undesirable because the risk of occurrence is significantly higher. In addition, containing a large amount of Si promotes the production of M / A, which is not preferable.

The content of Mn is preferably 0.5 to 2.0%. The Mn is an element effective in strengthening the solid solution of the steel to be added at least 0.5% can exhibit high strength with the effect of increasing the hardenability. However, the addition of more than 2.0% is not preferable because the segregation at the center of thickness during the casting of the slab in the steelmaking process greatly develops and damages the weldability of the final product.

The content of Ti is preferably 0.005 to 0.02%. Ti is a very useful element for miniaturizing grains and is present in the steel as TiN to inhibit the growth of grains during the heating process for rolling. Also, Ti reacts with nitrogen and solidifies with carbon to form TiC. Precipitates are formed and the formation of TiC is very fine, greatly improving the strength of the steel. Therefore, it is necessary to add at least 0.005% or more in order to obtain the austenite grain growth inhibition effect by TiN precipitation and the strength increase by TiC formation. On the other hand, when the content is added in excess of 0.02%, the effect is saturated and the toughness of the weld heat affected zone is deteriorated by the re-use of TiN as the steel sheet is welded to the melting point to the melting point. It is preferable.

The content of Nb is preferably 0.01 to 0.03%. The Nb is an element that refines the austenite grain size, widens the unrecrystallized region, and contributes to the refinement and strength of the final structure. On the other hand, when it is added in excess of 0.03%, it is desirable to limit the upper limit to 0.03% because the weld heat affected zone toughness is inferior due to excessive generation of M / A.

The content of P is preferably 0.010% or less. P needs to be actively reduced because it combines with Mn to form a non-metallic inclusion to cause embrittlement of the steel. However, in order to reduce P to an extreme, the steelmaking process load is intensified and the segregation problem in the center is less than 0.010%. Since it does not occur large, it is preferable to limit the upper limit to 0.010%.

The content of S is preferably 0.004% or less. S is combined with Mn to form non-metallic inclusions to embrittle steel and cause red brittle brittleness. When S is segregated at the center, S is not good for weld toughness. It is preferable to limit the upper limit to 0.004%.

In this case, the TiN ratio is controlled to be in the range of 2 <Ti / N <4.5 in order to suppress austenite grain growth. When the ratio is less than or equal to 2, the temperature at which the weld is melted due to heat influence is low, resulting in coarse austenite grains. On the other hand, when the ratio is larger than 4.5, the precipitated TiN particles are very large and austenite grains grow and coarsen due to thermal influence.

In addition, at least one of V, Mo, Cr, Ni, and Cu may be further contained in the steel formed as described above.

The content of V is preferably 0 to 0.1%. The V is preferably added at least 0.01% to improve the strength of the base metal and the welded portion of the steel. However, when the content is added in excess of 0.1%, the problem of deterioration of toughness and weldability may occur, so the upper limit is preferably limited to 0.1%.

The content of Mo is preferably 0 to 0.5%. When adding more than 0.5% of Mo, there is an increase in steel manufacturing cost, so it is preferable to limit the upper limit to 0.5%.

The content of Cr is preferably 0.50% or less. Cr increases the hardenability of steel like Mo and is effective in corrosion resistance and hydrogen organic crack resistance, but when added in excess of 0.5%, the toughness of the base metal and the toughness of the weld heat affected zone (HAZ) may occur. have.

The content of Ni is preferably 0.5% or less. The Ni has an effect of lowering the ductile-brittle transition temperature of the base material without severe segregation, but when it is an expensive element and exceeds 0.5%, toughness of the base material and toughness of the weld heat affected zone (HAZ) may occur.

The content of Cu is preferably 0.5% or less. While Cu has an effect on corrosion resistance and hydrogen organic crack resistance, when the content is added in excess of 0.5%, the toughness of the base material and the toughness of the weld heat affected zone (HAZ) may occur.

In addition, it is preferable that the line pipe steel material of the present invention satisfies a value of 10 or less represented by Equation 1 below.

[Equation 1]

Ma = 28.7C + 25Cr + 31Mo + 0.8Mn + 2.5Cu + 157Nb-4.3

The value of Ma is microscopically related to the mixed phase of martensite and austenite. If the value of Ma exceeds 10 or more, it is undesirable because of the characteristics that promote crack initiation during impact test fracture tests.

The steel sheet of the present invention thus formed has a tensile strength of 620 MPa or more, and can ensure a high strength of API-X80 grade or more.

Hereinafter, the manufacturing method of the steel plate which has the steel comprised as mentioned above is demonstrated in detail.

Steel slab formed as described above is subjected to low pressure at a rate of less than 2%, and a slab is produced by applying a casting speed of 0.9-1.2 m / min. When performing at low pressure drop ratio and casting speed within the range as described above during the playing process it can be suppressed that the center segregation is formed.

The slab is then reheated to 1100-1300 ° C. At the reheating temperature of 1100 ° C. or less, the breakage of the solidified structure formed during casting is insufficient, and the center segregation is well developed. Thus, mixing of the finally formed crystal grains occurs and workability and impact toughness are significantly reduced. In addition, if the reheating temperature exceeds 1300 ℃, the formation of scale by oxidization is promoted, and the thickness reduction of slab is large and grain coarsening occurs when reheating.The economical loss due to the increase of heating unit is large, so the management range is 1100 ~. It was limited to 1300 degreeC.

The reheated steel is then finish rolled at 800-950 ° C. If the finish hot rolling temperature is higher than 950 ° C, uniform hot rolling is not performed throughout the thickness, resulting in insufficient grain refinement, resulting in a drop in impact toughness due to grain coarsening. On the contrary, when the hot rolling is finished at a temperature of less than 800 ° C. as the hot rolling is finished in the low temperature region, it is preferable to limit the finish hot rolling temperature to 800 ° C. to 950 ° C. as the hybridization of crystal grains proceeds rapidly, resulting in deterioration of corrosion resistance and workability.

Then, the finished rolled steel sheet is cooled to 150-350 ° C. at a cooling rate of 15 to 30 ° C./second to produce a line pipe steel of the present invention. When cooling, the cooling rate is preferably limited to 15 ° C./sec or more, because when the cooling rate is less than 15 ° C./sec, ferrite of equiaxed crystals may be formed at the center of the thickness, thereby making it difficult to secure strength. Above 30 ° C./sec, impact toughness deteriorates due to the generation of a large amount of hard phase.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The slab was prepared by melting the steel ingot so as to satisfy the composition shown in Table 1 by applying the reduced pressure ratio and the casting speed under the conditions shown in Table 2 to prepare a slab, and reheated the slab to 1150 ° C. Next, the reheated steel was finish rolled at 900 ° C., and the finish rolled steel sheet was cooled to 300 ° C. at a cooling rate of 25 ° C./sec to prepare a linepipe steel. The yield strength, tensile strength, impact toughness and HAZ part toughness were evaluated for each manufactured steel and the results are shown in Table 2. In the toughness evaluation test of the HAZ part, the heat input amount was set to 30 kJ / cm.

[Table 1] Comparison of Chemical Compositions between Invented and Comparative Steels

Steel grade C Si Mn Mo Ti Nb V N Ni Cu Cr A 0.07 0.2 1.5 0 0.01 0.02 0 0.004 0.2 0.2 0.2 Invention steel B 0.05 0.1 1.8 0.1 0.015 0.015 0.01 0.004 0.2 0.2 0.1 Invention steel C 0.06 0.1 1.9 0.2 0.012 0.03 0.03 0.005 0.2 0.3 0 Invention steel D 0.05 0.15 2 0.25 0.013 0.02 0 0.005 0.3 0.2 0 Invention steel E 0.05 0.1 1.9 0.2 0.013 0.02 0.04 0.005 0.1 0.1 0.1 Invention steel F 0.06 0.25 1.5 0.2 0.01 0.03 0.03 0.002 0.2 0 0 Comparative steel G 0.07 0.3 2.5 0.1 0.015 0.025 0.02 0.004 0 0 0 Comparative steel H 0.05 0.5 2 0.3 0.01 0.03 0.04 0.004 0.2 0.1 0 Comparative steel I 0.07 0.25 1.8 0.2 0.015 0.05 0.02 0.005 0 0 0 Comparative steel J 0.06 0.3 1.8 0.3 0.01 0.04 0.03 0.005 0.2 0.3 0.3 Comparative steel

[Table 2] Characterization of Invented and Comparative Steels

division Steel grade Ceq Ti / N Ma Casting speed Pressure drop rate Yield strength The tensile strength Impact toughness HAZ Toughness m / min Rolling reduction (%) (MPa) (MPa) (-30 ℃, Joule) -30 ℃ Inventive Steel 1 A 0.39 2.5 6.9 0.9 2.1 593 697 339 166 Inventive Steel 2 B 0.42 3.8 6.2 1.1 3 614 722 340 186 Inventive Steel 3 C 0.46 2.4 9.7 1.2 2.5 606 808 285 159 Inventive Steel 4 D 0.47 2.6 9.4 One 2.1 618 824 285 163 Inventive Steel 5 E 0.45 2.6 9.5 One 2.3 626 835 251 151 Comparative Steel 1 B2 0.42 3.8 6.2 0.9 One 611 719 340 97 Comparative Steel 2 D2 0.47 2.6 9.4 1.5 2.5 620 826 238 65 Comparative Steel 3 F 0.37 5.0 8.9 1.2 2.1 574 676 336 81 Comparative Steel 4 G 0.51 3.8 4.7 1.1 2 645 861 275 46 Comparative Steel 5 H 0.47 2.5 12.0 1.2 2.5 609 811 226 53 Comparative Steel 6 I 0.41 3.0 11.8 1.2 2 594 699 332 74 Comparative Steel 7 J 0.52 2.0 21.9 1.1 2.7 650 866 262 21

As shown in Table 2, for the inventive steels 1-5 where the chemical composition and the manufacturing conditions satisfy the scope of the present invention, yield strength of 590 MPa or more, tensile strength of 690 MPa or more, impact toughness of 250 Joule or more, and HAZ toughness of 150 or more are secured. Could. Therefore, according to the present invention, it was possible to produce a steel sheet excellent in the toughness of the weld.

Claims (4)

By weight%, C: 0.03-0.1%, Si: 0.05-0.3%, Mn: 0.5-2.0%, Ti: 0.005-0.02%, Nb: 0.005-0.03%, P: 0.010% or less, S: 0.004% or less , Ca: 0.0005 to 0.005%, N: 0.008 or less, Mo: 0.5% or less, Cr: 0.5% or less, Ni: 0.5% or less, Cu: 0.5%, balance Fe and other unavoidable impurities, and the TiN ratio is 2 < A high-strength line pipe steel having a good welded toughness that satisfies the range of Ti / N <4 and satisfies the value of Ma represented by Equation 1 below 10. [Equation 1] Ma = 28.7C + 25Cr + 31Mo + 0.8Mn + 2.5Cu + 157Nb-4.3 The high strength linepipe steel of claim 1, wherein the weld line toughness further comprises 0.1% or less of V. By weight%, C: 0.03-0.1%, Si: 0.05-0.3%, Mn: 0.5-2.0%, Ti: 0.005-0.02%, Nb: 0.005-0.03%, P: 0.010% or less, S: 0.004% or less , Ca: 0.0005 to 0.005%, N: 0.008 or less, remainder Fe and other unavoidable impurities, steelmaking steel having a composition with a TiN ratio satisfying the range of 2 <Ti / N <4.5. Performing slab under low pressure and applying a casting speed of 0.9-1.2 m / min to produce a slab; Reheating the slab to 1100-1300 ° C .; Finishing rolling the reheated steel at 800-950 ° C .; And Cooling the finish-rolled steel sheet to 150-350 ° C. at a cooling rate of 15-30 ° C./sec or more; Method for producing a high-strength line pipe steel with excellent weld toughness comprising a. 4. The steel according to claim 3, wherein the steel is further selected from the group consisting of V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Ni: 0.5% or less and Cu: 0.5% or less as further other elements. A method for producing a high strength line pipe steel having excellent weld toughness, comprising at least one species and having a value of Ma represented by Equation 1 below 10 or less. [Equation 1] Ma = 28.7C + 25Cr + 31Mo + 0.8Mn + 2.5Cu + 157Nb-4.3