KR100833066B1 - High strength steel sheet having excellent welded zone property for linepipe and the method for manufacturing the same - Google Patents

Abstract

A high strength steel sheet for linepipe is provided to obtain excellent physical properties in a welded zone and a yield strength of at least 690 MPa by especially controlling alloy elements and a relational expression and controlling the cooling rate, and a method for manufacturing the high strength steel sheet for linepipe is provided. A high strength steel sheet having excellent welded zone properties for linepipe comprises, by weight percent, 0.04 to 0.1% of C, 0.1 to 0.4% of Si, 1.0 to 2.2% of Mn, 0.01 to 0.05% of soluble Al, 0.005 to 0.02% of Ti, 0.01 to 0.06% of Nb, 0.01 to 0.1% of V, 0.1 to 0.5% of Mo, 0.015% or less of P, and 0.005% or less of S with the balance being Fe and other inevitable impurities, the Ti, N, Nb, and V satisfy 0.005<=(Ti-3.4N)+Nb+V<=0.15, and a matrix of the steel sheet has a tensile strength of 760 to 904 MPa. The steel sheet further comprises at least one of 1.0% or less of Cu, 1.0% or less of Ni, and 1.0% or less of Cr. A method for manufacturing a high strength steel sheet having excellent welded zone properties for linepipe comprises: reheating a steel slab at a temperature of 1050 to 1180 deg.C, the steel slab comprising, by weight percent, 0.04 to 0.1% of C, 0.1 to 0.4% of Si, 1.0 to 2.2% of Mn, 0.01 to 0.05% of soluble Al, 0.005 to 0.02% of Ti, 0.01 to 0.06% of Nb, 0.01 to 0.1% of V, 0.1 to 0.5% of Mo, 0.015% or less of P, and 0.005% or less of S with the balance being Fe and other inevitable impurities, the Ti, N, Nb, and V satisfying 0.005<=(Ti-3.4N)+Nb+V<=0.15; finish rolling the reheated steel slab to a reduction ratio of 70 to 82% at the austenite recrystallization temperature or lower; and cooling the rolled steel sheet at a cooling rate of 15 to 30 deg.C/sec and stopping cooling of the rolled steel sheet at a temperature of 350 deg.C or lower.

Description

High strength steel sheet having excellent welded zone property for linepipe and the method for manufacturing the same}

Japanese Patent Laid-Open No. 9-41074

The present invention relates to a steel sheet for line pipes with a yield strength of 690 MPa or more mainly used for construction, pipelines and offshore structures. More specifically, the present invention relates to a steel sheet for high strength line pipes having a yield strength of 690 MPa or more excellent in 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 metal is weak, there is a risk that a large accident may occur due to rapid brittle fracture, and the demand for toughness is also increasing.

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.

Therefore, in order to satisfy the tensile strength, the addition amount of the alloying elements is increased, and the cooling conditions during the manufacturing are severe. In addition, when welding the steel pipe (Seam), the strength of the weld heat affected zone is reduced, softening occurs and it is difficult to secure the toughness by high alloy.

As an example of the above method, in Japanese Patent Laid-Open No. 9-41074, in weight%, C: 0.05 to 0.1%, Si: 0.6% or less, Mn: 1.8 to 2.5%, P: 0.015% or less, S: 0.003 % Or less ， Mo: 0.3 ~ 0.6%, Nb: 0.01 ~ 0.1%, V: 0.03 ~ 0.1%, Al: 0.06% or less ， Ti: 0.005 ~ 0.03%, N: 0.001 ~ 0.006% It is composed of unavoidable impurities and has a martensite and bainite fraction of 20 to 90%, and has an ideal mixed structure with ferrite. A method for producing ultra high tensile steel is described.

However, the prior art is a process of performing a tempering treatment at a temperature of 400 ℃ ~ Ac ₁ or less, there is a problem that causes the occurrence of excessive cost due to the heat treatment.

The present invention is to improve the above problems, and to provide a high-strength line pipe steel sheet having a high yield strength of 690MPa or more and a method of manufacturing the same by specially managing alloy elements and relational expressions and controlling the cooling rate. There is this.

The present invention for achieving the above object, in weight%, C: 0.04 ~ 0.1%, Si: 0.1 ~ 0.4%, Mn: 1.0 ~ 2.2%, Sol.Al: 0.01 ~ 0.05%, Ti: 0.005 ~ 0.02% , Nb: 0.01% to 0.06%, V: 0.01% to 0.1%, Mo: 0.1% to 0.5%, P: 0.015% or less, and S: 0.005% or less, and are composed of the remaining Fe and other unavoidable impurities, and the Ti, N , Nb and V relates to a steel sheet for high-strength line pipe having excellent welded properties, satisfying 0.005 ≦ (Ti−3.4N) + Nb + V ≦ 0.15.

In addition, the present invention is a weight%, C: 0.04 ~ 0.1%, Si: 0.1 ~ 0.4%, Mn: 1.0 ~ 2.2%, Sol.Al: 0.01 ~ 0.05%, Ti: 0.005 ~ 0.02%, Nb: 0.01 ~ 0.06%, V: 0.01% to 0.1%, Mo: 0.1% to 0.5%, P: 0.015% or less, S: 0.005% or less, and are composed of the remaining Fe and other unavoidable impurities, and the Ti, N, Nb and V are Reheat the steel slab satisfying 0.005≤ (Ti-3.4N) + Nb + V≤0.15 at 1050 ~ 1180 ℃, finish rolling at a reduction rate of 70% or more below the austenite recrystallization temperature, and then 15 ~ 30 ℃ / sec The present invention relates to a method for producing a high strength line pipe steel sheet having excellent welded part physical properties including cooling at a rate of 350 ° C. and stopping cooling at 350 ° C. or lower.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present inventors secured line pipes with high strength of API-X100 grade and excellent weld properties by properly adding alloying elements, managing Ti, N, Nb and V relations and controlling the cooling rate. Based on the results, the present invention is proposed.

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

The content of C is preferably 0.04 to 0.1%.

The C is the most economical and effective element to strengthen the steel by hardening and improving the hardenability of the steel, the weldability, formability and toughness may be reduced by the addition of a large amount. If the content is less than 0.04%, 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 0.1 to 0.4%.

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 0.1%, it is difficult to obtain clean steel because it does not sufficiently deoxidize the molten steel. If the content exceeds 0.4%, the red scale formed by Si is formed during rolling, and the surface of the steel sheet becomes very bad and brittle fracture occurs. This is undesirable because the risk of occurrence is significantly higher.

The content of Mn is preferably 1.0 to 2.2%.

The Mn should be added 1.0% or more as an effective element to solidify the steel to exhibit high strength with an increase in the hardenability. However, the addition of more than 2.2% is not preferable because the segregation at the center of the thickness during the casting of the slab in the steelmaking process greatly develops and damages the weldability of the final product.

The content of Sol.Al is preferably 0.01 to 0.05%.

Since Sol.Al is a major deoxidizer of steel, it needs to be added 0.01% or more. However, when it is added in excess of 0.05%, the deoxidation effect is saturated, so it is preferable to limit the upper limit to 0.05%.

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 0.06%.

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 added in excess of 0.06%, the effect is not expected to be increased any longer, and due to excessive precipitation of Nb carbonitrides, the austenite microcrystallization temperature is too high. Considering the cost, it is preferable to limit the upper limit to 0.06%.

The content of V is preferably 0.01 to 0.1%.

The V needs to be 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 0.1 to 0.5%.

The Mo needs to be added 0.1% or more in order to improve the strength of the base metal and the welded portion of the steel. However, when the content is added in excess of 0.5%, it is preferable to limit the upper limit to 0.5% because there is an increase in steel manufacturing cost.

The content of P is preferably 0.015% or less.

The P needs to be actively reduced in combination 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 problem is greatly caused at 0.015% or less. Since it is not, it is preferable to limit the upper limit to 0.015%.

The content of S is preferably 0.005% or less.

S is an element that combines with Mn to form a non-metallic inclusion to embrittle steel, and causes red brittleness. Like S, it is preferable to limit the upper limit to 0.005% in consideration of steelmaking process load.

The present invention is composed of Fe and other unavoidable impurities in addition to the above components.

In the present invention, the component ratios of Ti, N, Nb, and V may be controlled to prevent softening of the weld.

Preference is given to 0.005 ≦ (Ti−3.4N) + Nb + V ≦ 0.15.

When the relational expressions of Ti, N, Nb, and V satisfy 0.005 to 0.15, softening may be prevented due to the strengthening effect of precipitation during cooling. When the value of the relational expression in which excessive alloying elements are added is 0.15 or more, it may cause the deterioration of toughness due to the central segregation with the increase of the forced manufacturing cost.

In the present invention, at least one of Cu, Ni, and Cr may be further added to the steel formed as described above.

The content of Cu is preferably 1.0% or less.

While Cu has an effect on corrosion resistance and hydrogen organic crack resistance, when the content is added in excess of 1.0%, the toughness of the base metal and the toughness of the weld heat affected zone (HAZ) may occur.

The content of Ni is preferably 1.0% or less.

The Ni has an effect of lowering the soft-brittle transition temperature of the base material without severe segregation of the base material. However, when Ni is more than 1.0%, the toughness of the base material and the toughness of the weld heat affected zone (HAZ) may occur.

The content of Cr is preferably 1.0% or less.

The Cr increases the hardenability of the steel, such as Mo, and is effective in corrosion resistance and hydrogen organic crack resistance, but when added in excess of 1.0%, the toughness of the base metal and the toughness of the weld heat affected zone (HAZ) may occur. have.

In addition, the steel sheet of the present invention has a tensile strength of 760 MPa or more, and can ensure a high strength of API-X100 grade or more.

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

First, the steel slab formed as described above is reheated at 1050 ~ 1180 ℃.

The heating process of the steel is a process of heating the steel so as to smoothly perform the subsequent rolling process and obtain the properties of the target steel sheet, so that the heating process should be performed within an appropriate temperature range according to the purpose. If the reheating temperature of the steel is less than 1050 ° C., Nb or V may not be re-used in the steel, making it difficult to achieve high strength of the steel sheet, and partial recrystallization may occur, resulting in uneven austenite grains. On the other hand, when the reheating temperature exceeds 1180 ° C., the austenite grains are excessively coarse, thereby providing a cause of an increase in grain size of the steel sheet, and as a result, the toughness of the steel sheet may be extremely deteriorated. Therefore, the reheating temperature is preferably limited to 1050 ~ 1180 ℃.

The reheated slabs are finish rolled to a reduction ratio of at least 70% below the austenite recrystallization temperature.

In order for the steel sheet to have low temperature toughness, internal grains must be present in a fine size, which may be possible by controlling the rolling temperature. The grain size reduction effect by the rolling temperature control is slightly different in two temperature ranges. First, when rolling is performed at more than Ar ₃ below the recrystallization temperature, a dislocation due to deformation develops in the austenite, which is followed by Ar. _In the rolling or cooling process below ₃ , austenite becomes a nucleation site that transforms into bainite.

This is possible only by securing a reduction ratio of 70% or more below the austenite recrystallization temperature. The reason is that the recrystallization continues even if rolling is carried out in the recrystallization temperature range above the austenite unrecrystallization temperature, so that the grain refining effect cannot be obtained. There is.

After finishing the said rolling, it cools at the speed of 15-30 degree-C / sec, and stops cooling at 350 degrees C or less.

Cooling rate is an important factor to improve the toughness and strength of the steel sheet. This is because the faster the cooling rate, the finer the grains of the internal structure of the steel sheet can be to improve the toughness, and the hard structure can be developed therein to improve the strength. If the cooling rate is less than 15 ° C./sec, coarse ferrites formed during cooling are mixed, which is disadvantageous in strength and toughness. Therefore, the cooling rate of the steel sheet after rolling should be at least 15 ° C / sec to produce a steel sheet with improved toughness and strength target in the present invention. However, in the case of exceeding 30 ° C./sec, due to the characteristics of the steel sheet targeted by the present invention, the cooling water amount control limit is confronted with water cooling equipment, and the warpage of the steel sheet occurs due to the excessive amount of cooling water. It becomes bad.

In addition, in order to control the internal structure of the steel sheet, it is necessary to cool it to a temperature at which the effect of the cooling rate is sufficiently expressed. If the cooling stop temperature, which is the temperature at which the cooling stops, exceeds 350 ° C, it is difficult to form a hard phase composed of fine grains and bainite or martensite sufficiently inside the steel sheet, so the upper limit of the cooling stop temperature is limited to 350 ° C. It is preferable.

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

EXAMPLE

Inventive steels (A to F) and comparative steels (G to K), which are formed as shown in Table 1, were manufactured by vacuum dissolution (50 Kg) or slab (240 mmt), and then rolling and cooling were performed under the conditions shown in Table 2 below. Was carried out. In addition, the mechanical properties of the base metal and the welded heat affected zone (HAZ) were evaluated after simulation, assuming that the heat affected by seam welding was 35 KJ / cm.

Steel grade C Si Mn Mo Ti Nb V N (Ti-3.4N) + Nb + V Etc Inventive Steel A 0.07 0.10 1.9 0.2 0.01 0.03 0.06 0.004 0.0864 Ni: 0.2 Inventive Steel B 0.06 0.30 1.8 0.2 0.009 0.06 0.01 0.005 0.062 Cu: 0.3 Invention Steel C 0.05 0.40 1.9 0.3 0.017 0.04 0.03 0.003 0.077 Ni: 0.3 Inventive Steel D 0.08 0.2 1.7 0.1 0.013 0.03 0.03 0.004 0.08 Cr: 0.5 Inventive Steel E 0.06 0.25 2.1 0.4 0.01 0.04 0.05 0.005 0.077 - Inventive Steel F 0.06 0.30 1.8 0.15 0.014 0.06 0.04 0.005 0.097 - Comparative Steel G 0.12 0.25 1.5 0.1 0.01 0.03 0.03 0.003 0.059 - Comparative Steel H 0.07 0.30 2.5 0.1 0.007 0.05 0.04 0.005 0.060 - Comparative Steel I 0.04 0.25 2.0 - 0.01 0.03 0.02 0.004 0.066 Cr: 0.4 Comparative Steel J 0.07 0.25 1.8 0.2 0.02 0.01 0.06 0.005 0.033 - Comparative Steel K 0.06 0.27 1.9 0.2 0.025 0.08 0.06 0.003 0.155 -

Steel grade division Slab reheating temperature (℃) Undetermined rolling reduction rate (%) Cooling rate (℃ / sec) Cooling stop temperature (℃) Inventive Steel A Invention 1 1136 76 22 315 Inventive Steel B Invention 2 1124 74 25 265 Inventive Steel B Invention 3 1112 74 21 284 Invention Steel C Invention 4 1150 75 17 270 Invention Steel C Invention 5 1160 82 28 290 Inventive Steel D Invention 6 1102 74 26 235 Inventive Steel E Invention Material7 1096 74 20 278 Inventive Steel F Invention Material 8 1139 75 19 302 Inventive Steel B Comparative Material 1 1200 71 16 294 Inventive Steel B Comparative Material 2 1145 60 23 246 Inventive Steel D Comparative Material 3 1167 79 10 251 Inventive Steel D Comparative Material 4 1108 70 40 150 Inventive Steel D Comparative Material 5 1117 72 27 453 Comparative Steel G Comparative Material 6 1122 75 20 305 Comparative Steel H Comparative Material7 1140 74 24 289 Comparative Steel I Comparative Material 8 1156 78 26 330 Comparative Steel J Comparative Material 9 1165 75 22 310 Comparative Steel K Comparative Material 10 1134 75 19 288

division Mechanical Properties of the Base Material Mechanical Properties of HAZ Yield strength (MPa) Tensile Strength (MPa) Impact Toughness (-20 ℃, Joule) DWTT Ductility Rate (-20 ℃,%) Tensile Strength (MPa) Impact Toughness (-30 ℃) Invention 1 750 882 266 99 816 121 Invention 2 734 863 267 99 805 107 Invention 3 732 861 303 99 788 121 Invention 4 711 836 325 99 761 130 Invention 5 757 891 251 99 777 100 Invention 6 745 877 266 98 812 106 Invention Material7 768 904 238 99 847 112 Invention Material 8 712 838 328 97 799 131 Comparative Material 1 722 850 256 54 765 90 Comparative Material 2 690 812 226 68 731 131 Comparative Material 3 570 738 328 99 701 131 Comparative Material 4 808 950 187 65 808 75 Comparative Material 5 636 748 352 99 711 141 Comparative Material 6 609 716 328 75 680 46 Comparative Material7 820 965 150 43 820 32 Comparative Material 8 769 905 220 78 723 53 Comparative Material 9 746 878 221 99 691 105 Comparative Material 10 766 902 220 87 812 27

As shown in Table 3, in the case of the invention material (1-8) manufactured according to the production method of the present invention using the invention steel (A ~ F) satisfying the component range of the present invention, the yield strength of the base material 710MPa With above 835MPa of tensile strength, 238 ~ 328J impact toughness, 97% of DWTT ductile fracture rate, over 97% of tensile strength of HAZ part, and over 100J impact toughness of HAZ part, it has excellent mechanical properties in the base material and welding heat affected zone. It was.

However, in the case of the comparative materials (6 to 10) manufactured by using the comparative steel (G ~ K) that does not satisfy the component range of the present invention, the strength of the base material and the welding heat affected zone to which the present invention is aimed Failed to secure toughness.

In addition, even in the case of the comparative materials (1 to 5) that satisfies the component range of the present invention but does not satisfy the manufacturing conditions of the present invention, the poor characteristics were shown.

As described above, according to the present invention, a steel sheet for high strength line pipe having a yield strength of 690 MPa or more and excellent in the strength and toughness of the welded portion can be provided.

Claims (5)

By weight%, C: 0.04 to 0.1%, Si: 0.1 to 0.4%, Mn: 1.0 to 2.2%, Sol.Al: 0.01 to 0.05%, Ti: 0.005 to 0.02%, Nb: 0.01 to 0.06%, V: 0.01 to 0.1%, Mo: 0.1 to 0.5%, P: 0.015% or less, S: 0.005% or less, and is composed of the remaining Fe and other unavoidable impurities, and the Ti, N, Nb, and V are 0.005≤ (Ti- 3.4N) + Nb + V≤0.15 High strength line pipe steel sheet excellent in welded properties, characterized in that the tensile strength of the base material is 760 ~ 904MPa. The steel sheet for high-strength line pipe of claim 1, wherein the steel sheet further includes at least one of Cu: 1.0% or less, Ni: 1.0% or less, and Cr: 1.0% or less. delete By weight%, C: 0.04 to 0.1%, Si: 0.1 to 0.4%, Mn: 1.0 to 2.2%, Sol.Al: 0.01 to 0.05%, Ti: 0.005 to 0.02%, Nb: 0.01 to 0.06%, V: 0.01 to 0.1%, Mo: 0.1 to 0.5%, P: 0.015% or less, S: 0.005% or less, and is composed of the remaining Fe and other unavoidable impurities, and the Ti, N, Nb, and V are 0.005≤ (Ti- Steel slabs satisfying 3.4N) + Nb + V≤0.15 are reheated at 1050-1180 ° C, finish rolled to 70-82% reduction below the austenite recrystallization temperature, and then at a rate of 15-30 ° C / sec. The manufacturing method of the high strength line pipe steel plate excellent in the weld part physical property which consists of cooling and stopping cooling at 350 degrees C or less. The method according to claim 4, wherein the steel sheet further comprises at least one of Cu: 1.0% or less, Ni: 1.0% or less, and Cr: 1.0% or less. .