阅读说明：本技术 低屈强比特性优异的高强度钢材及其制造方法 (High-strength steel material having excellent low yield ratio characteristics and method for producing same ) 是由 郑欢教 于 2018-12-24 设计创作，主要内容包括：根据本发明的一方面的低屈强比特性优异的高强度钢材,其按重量％计可以包含0.06-0.12％的C、0.2-0.5％的Si、1.5-2.0％的Mn、0.003-0.05％的Al、0.01％以下的N、0.02％以下的P、0.003％以下的S、0.05-0.5％的Cr、0.05-0.5％的Mo、0.01-0.05％的Nb、0.0005-0.005％的Ca、剩余的Fe以及其他不可避免的杂质,作为微细组织包括多边形铁素体,所述多边形铁素体的面积分数为10％-30％,所述多边形铁素体的平均硬度为180Hv以下。(According to one aspect of the present invention, a high-strength steel material having excellent low yield ratio characteristics may include, in terms of wt%, 0.06 to 0.12% of C, 0.2 to 0.5% of Si, 1.5 to 2.0% of Mn, 0.003 to 0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% or less of S, 0.05 to 0.5% of Cr, 0.05 to 0.5% of Mo, 0.01 to 0.05% of Nb, 0.0005 to 0.005% of Ca, and the balance Fe and other unavoidable impurities, and may include polygonal ferrite having an area fraction of the polygonal ferrite of 10 to 30% and an average hardness of the polygonal ferrite of 180Hv or less as a fine structure.)

1. A high-strength steel material having excellent low yield ratio characteristics,

the steel comprises, in wt%, 0.06-0.12% of C, 0.2-0.5% of Si, 1.5-2.0% of Mn, 0.003-0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% or less of S, 0.05-0.5% of Cr, 0.05-0.5% of Mo, 0.01-0.05% of Nb, 0.0005-0.005% of Ca, and the balance of Fe and other unavoidable impurities,

the fine structure includes polygonal ferrite as a fine structure,

the area fraction of the polygonal ferrite is 10-30%,

the average hardness of the polygonal ferrite is 180Hv or less.

2. A high-strength steel material excellent in low yield ratio characteristics according to claim 1,

the steel material further comprises a residual structure having an average hardness of 200Hv or more,

the residual structure includes acicular ferrite, bainite, pearlite, and martensite.

3. A high-strength steel material excellent in low yield ratio characteristics according to claim 2,

the sum of the area fractions of the pearlite and the martensite is 10% or less of the entire area.

4. A high-strength steel material excellent in low yield ratio characteristics according to claim 1,

the steel further contains one or more of 0.05 to 0.3% of Ni, 0.05 to 0.3% of Cu, 0.005 to 0.02% of Ti and 0.0005 to 0.0015% of B in terms of weight%.

5. A high-strength steel material excellent in low yield ratio characteristics according to claim 1,

the Charpy impact energy of the steel at-30 ℃ is more than 200J.

6. A high-strength steel material excellent in low yield ratio characteristics according to claim 1,

the yield ratio of the steel is less than 90%.

7. A high-strength steel material excellent in low yield ratio characteristics according to claim 1,

the tensile strength of the steel is 500MPa or more.

8. A method for producing a high-strength steel material having excellent low yield ratio characteristics,

reheating a steel slab at a temperature in the range of 1100-1160 ℃, said steel slab comprising, in weight%, 0.06-0.12% C, 0.2-0.5% Si, 1.5-2.0% Mn, 0.003-0.05% Al, 0.01% or less N, 0.02% or less P, 0.003% or less S, 0.05-0.5% Cr, 0.05-0.5% Mo, 0.01-0.05% Nb, 0.0005-0.005% Ca, the remainder being Fe and other unavoidable impurities;

rough rolling the reheated steel slab at an end temperature of 1050 ℃ or higher;

starting finish rolling of the roughly rolled steel slab at a temperature of 980 ℃ or lower, and finishing finish rolling at a temperature of (Ar3+50 ℃) to 900 ℃;

first cooling the finish-rolled steel product at a first cooling rate to a temperature range of (Ar3-40 ℃ C.) - (Ar3-70 ℃ C.);

and secondly cooling the steel subjected to the primary cooling to the temperature range of 350-400 ℃ at a second cooling speed.

9. The method for producing a high-strength steel material excellent in low yield ratio characteristics according to claim 8, wherein,

the effective reduction rate of the finish rolling is more than 65%.

10. The method for producing a high-strength steel material excellent in low yield ratio characteristics according to claim 8, wherein,

the second cooling rate is 10-40 ℃/s greater than the first cooling rate.

11. The method for producing a high-strength steel material excellent in low yield ratio characteristics according to claim 8, wherein,

the starting temperature of the first cooling is (Ar3+10 ℃ C.) - (Ar3+30 ℃ C.).

12. The method for producing a high-strength steel material excellent in low yield ratio characteristics according to claim 8, wherein,

the first cooling speed is 10-20 ℃/s.

13. The method for producing a high-strength steel material excellent in low yield ratio characteristics according to claim 8, wherein,

the second cooling is performed immediately after the first cooling.

14. The method for producing a high-strength steel material excellent in low yield ratio characteristics according to claim 8, wherein,

the second cooling speed is 30-50 ℃/s.

15. The method for producing a high-strength steel material excellent in low yield ratio characteristics according to claim 8, wherein,

and air-cooling the steel subjected to the secondary cooling to room temperature.

16. The method for producing a high-strength steel material excellent in low yield ratio characteristics according to claim 8, wherein,

the steel slab further comprises one or more of 0.05 to 0.3% of Ni, 0.05 to 0.3% of Cu, 0.005 to 0.02% of Ti and 0.0005 to 0.0015% of B in terms of weight%.

Technical Field

The present invention relates to a steel material for a pipe and a method for manufacturing the same, and more particularly, to a high-strength steel material having excellent low yield ratio characteristics and a method for manufacturing the same.

Background

The steel material for pipes is manufactured into pipes by pipe forming in various ways. When a pipe is molded, the pipe is easily plastically deformed, the plastic deformability of the entire pipe is uniform, and the pipe can be evaluated to be excellent in pipe-forming performance as long as cracking or breaking does not occur during the pipe molding process. Further, when the pipe is used, if the pipe can be deformed without breaking even if the delivery pressure is rapidly changed, the safety performance of the pipe can be evaluated to be excellent. In order to improve the pipe forming performance and the safety performance of the steel material for pipes, the physical properties required are to reduce the yield ratio. The lower the yield ratio of the steel material, the greater the difference between the yield stress and the tensile stress, because the excess stress up to fracture increases after plastic deformation occurs.

The yield ratio depends on the physical properties of the microstructure. In general, when a plurality of phases having different hardnesses are mixed, the yield ratio tends to decrease. That is, it could be confirmed that: in the microstructure consisting of ferrite and pearlite bands, which are produced by normalizing heat treatment, lower yield ratio values are obtained, on the contrary, higher yield ratio values are obtained in high-strength steels consisting of bainite only. In the case of two phases of different hardness, the yield strength will be affected by the strength and fraction of the phase with the lower hardness, while the tensile strength will be affected by the strength and fraction of the phase with the higher hardness. Therefore, when the strength and the fraction of the phase having low hardness and the phase having high hardness are optimally controlled, a low yield ratio can be secured.

However, in the case of a steel material for a pipe, particularly a steel material for a line pipe (line pipe), not only a yield ratio but also strength and toughness at a certain level or more are required, and therefore, it is not possible to control a microstructure in consideration of only the yield ratio. If the fraction of polygonal ferrite, which is a phase having a low hardness, is increased more than necessary, the yield strength is lowered; if the fraction of martensite or bainite as the high-hardness phase is increased, there is a problem that toughness deteriorates.

Therefore, in the case of a steel material for a pipe, particularly a steel material for a line pipe, it is required to ensure strength and toughness at a certain level or more, and to ensure pipe formability and safety performance by having a low yield ratio characteristic.

Disclosure of Invention

According to one aspect of the present invention, a high-strength steel material excellent in low yield ratio characteristics and a method for manufacturing the same can be provided.

The subject of the present invention is not limited to the above. Those of ordinary skill in the art will appreciate additional aspects of the invention through the entire disclosure of this specification without any difficulty.

Means for solving the problems

According to one aspect of the present invention, a high-strength steel material having excellent low yield ratio characteristics may include, in terms of wt%, 0.06 to 0.12% of C, 0.2 to 0.5% of Si, 1.5 to 2.0% of Mn, 0.003 to 0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% or less of S, 0.05 to 0.5% of Cr, 0.05 to 0.5% of Mo, 0.01 to 0.05% of Nb, 0.0005 to 0.005% of Ca, and the balance Fe and other unavoidable impurities, and may include polygonal ferrite as a fine structure, the area fraction of the polygonal ferrite being 10 to 30%, and the average hardness of the polygonal ferrite being 180Hv or less.

The steel material further includes a residual structure having an average hardness of 200Hv or more, and the residual structure may include acicular ferrite, bainite, pearlite, and martensite.

The sum of the area fractions of pearlite and martensite may be 10% or less of the entire area.

The steel may further contain one or more of 0.05 to 0.3% of Ni, 0.05 to 0.3% of Cu, 0.005 to 0.02% of Ti, and 0.0005 to 0.0015% of B in terms of weight%.

The Charpy impact energy of the steel at-30 ℃ may be 200J or more.

The yield ratio of the steel material may be 90% or less.

The tensile strength of the steel material may be 500MPa or more.

According to one aspect of the present invention, a high strength steel material having excellent low yield ratio characteristics, comprising, in weight%, 0.06 to 0.12% of C, 0.2 to 0.5% of Si, 1.5 to 2.0% of Mn, 0.003 to 0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% or less of S, 0.05 to 0.5% of Cr, 0.05 to 0.5% of Mo, 0.01 to 0.05% of Nb, 0.0005 to 0.005% of Ca, and the balance of Fe and other unavoidable impurities, can be reheated in a temperature range of 1100-; rough rolling the reheated steel slab at an end temperature of 1050 ℃ or higher; starting finish rolling of the roughly rolled steel slab at a temperature of 980 ℃ or less and finishing finish rolling at a temperature of (Ar3+50 ℃) to 900 ℃; first cooling the finish-rolled steel product at a first cooling rate to a temperature range of (Ar3-40 ℃ C.) - (Ar3-70 ℃ C.); the steel material after the first cooling is cooled for the second time to the temperature range of 350-400 ℃ at a second cooling speed.

The effective reduction rate of the finish rolling may be 65% or more.

The second cooling rate is 10-40 ℃/s greater than the first cooling rate.

The starting temperature of the first cooling may be (Ar3+10 ℃ C.) - (Ar3+30 ℃ C.).

The first cooling rate may be 10-20 ℃/s.

The second cooling may be performed immediately after the first cooling.

The second cooling rate may be 30-50 ℃/s.

The steel material after the second cooling may be air-cooled to room temperature.

The steel slab may further include one or more of 0.05 to 0.3% of Ni, 0.05 to 0.3% of Cu, 0.005 to 0.02% of Ti, and 0.0005 to 0.0015% of B in terms of weight%.

Effects of the invention

According to one aspect of the present invention, there is provided a steel material capable of simultaneously ensuring a tensile strength of 500MPa or more and a yield ratio of 90% or less, and a method for manufacturing the same, and therefore, capable of effectively ensuring the pipe formability and the safety performance of the steel material for pipelines.

Detailed description of the preferred embodiments

The present invention relates to a high-strength steel material having excellent low yield ratio characteristics and a method for producing the same, and preferred embodiments of the present invention will be described below. The 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. The present embodiment is provided to explain the present invention in more detail to those skilled in the art to which the present invention pertains.

The components of the steel of the present invention will be described in more detail below. Hereinafter, unless otherwise specified,% used to indicate the content of each element is based on weight.

According to one aspect of the present invention, a high-strength steel material having excellent low yield ratio characteristics may include, in terms of wt%, 0.06 to 0.12% of C, 0.2 to 0.5% of Si, 1.5 to 2.0% of Mn, 0.003 to 0, 05% of Al, 0.01% or less of N, 0, 02% or less of P, 0.003% or less of S, 0.05 to 0.5% of Cr, 0.05 to 0.5% of Mo, 0.01 to 0.05% of Nb, 0.0005 to 0.005% of Ca, and the balance Fe and other unavoidable impurities.

C：0.06-0.12％

C is an element that effectively strengthens the steel material by solid solution strengthening and precipitation strengthening. In particular, C is an element that exerts a larger influence on the tensile strength than the yield strength, and thus, effectively contributes to the reduction in the yield ratio. Therefore, the present invention can limit the lower limit of the content of C to 0.06%. However, in the case where C is excessively added, toughness may be lowered, and therefore, the present invention may limit the upper limit of the content of C to 0.12%. Therefore, the content of C in the present invention may be 0.06 to 0.12%. Further, in the case where the content of C exceeds 0.10%, cracks (cracks) frequently occur on the surface of the steel slab in the steel making continuous casting process, and thus the preferred content of C in the present invention may be 0.06-0.10%.

Si：0.2-0.5％

Si is an element that not only acts as a deoxidizer but also contributes to solid solution strengthening to improve the strength of steel. Si is an element that contributes to the generation of island Martensite (MA) during phase transformation. The island Martensite (MA) is an element contributing to the low yield ratio characteristic. Therefore, the present invention can limit the lower limit of the Si content to 0.2% in consideration of the generation of island Martensite (MA). However, in the case where Si is excessively added, there is a possibility that the surface quality after the end of rolling deteriorates as the peeling property of the scale (scale) grown in the heating furnace decreases, and the present invention can limit the upper limit of the Si content to 0.5%. Therefore, the content of Si in the present invention may be 0.2 to 0.5%.

Mn：1.5-2.0％

Mn is an element that serves to improve the strength of steel as a solid solution strengthening element and promotes the formation of a low-temperature transformation phase by improving the hardening properties of steel. In order to reduce the yield ratio, it is necessary to include a low-temperature transformation phase having an appropriate fraction in the microstructure. Therefore, in order to form a low-temperature transformation phase, the present invention may limit the lower limit of the content of Mn to 1.5%. However, in the case of excessively adding Mn, when a billet is cast, the generation of coarse bainite is promoted by center segregation, the toughness of the center portion is lowered, and the weldability of steel is also lowered, so the upper limit of the Mn content in the present invention may be limited to 2.0%. Accordingly, the Mn content of the present invention may be 1.5 to 2.0%.

Al：0.003-0.05％

Since Al is a typical deoxidizing element and a sufficient deoxidizing effect cannot be expected when the content thereof is less than 0.003%, the lower limit of the content of Al may be limited to 0.003% in the present invention. In addition, if a1 is excessively added, Al, which is a non-metal oxide, is excessively formed₂O₃Therefore, the present invention can limit the upper limit of the content of a1 to 0.05%, because the toughness of the base material and the welded portion is lowered. Therefore, the content of Al in the present invention may be 0.003 to 0.05%, and more preferably, the content of Al may be 0.005 to 0.05%.

N: less than 0.01%

N is an element that forms a nitride by bonding with Al, thereby contributing to an increase in the strength of the steel. However, in the case where N is excessively added, since N in a solid solution state hinders toughness to be a problem, the upper limit of the content of N in the present invention may be limited to 0.01%. Also, it is difficult to completely remove N from steel from industrial and economic aspects, and thus, the lower limit of the content of N may be limited to a range that can be allowed in the manufacturing process, i.e., 0.001%. Accordingly, the content of N in the present invention may be 0.01% or less, and more preferably, the content of N may be 0.001 to 0.01%.

P: less than 0.02%

P hinders weldability and toughness as an element that is inevitably contained in steel during steel making, and also hinders toughness of steel as an element that is easily segregated at the center portion and austenite grain boundary of a billet during solidification. Therefore, in order to ensure an appropriate level of toughness, the content thereof is preferably limited to a certain level or less. In particular, when the content of P exceeds 0.02%, brittle fracture in the thickness center portion of the steel material is promoted, and it is difficult to ensure low-temperature toughness, so the content of P in the present invention can be limited to 0.02% or less.

S: less than 0.003%

S is an impurity element inevitably contained in steel in the steel making process, and is an element which forms a nonmetallic inclusion such as MnS in the center of the thickness of the steel material to inhibit low-temperature toughness. When the content of S exceeds 0.003%, a large amount of non-metallic inclusions are formed in the thickness center portion of the steel material, thereby functioning as a starting point of brittle fracture and providing a path for propagating cracks. Therefore, in the present invention, the S content may be limited to 0.003% or less in order to secure toughness.

Cr：0.05-0.5％

Cr is an element that secures sufficient hardening performance upon cooling, and contributes to the effects of a second phase such as cementite and a low-temperature transformation phase, thereby effectively contributing to a decrease in yield ratio. In order to obtain such an effect, the present invention may limit the lower limit of the content of Cr to 0.05%. On the other hand, in the case where Cr is excessively added, the toughness of the welded portion may be impaired, and therefore, the upper limit of the content of Cr may be limited to 0.5% in the present invention. Therefore, the content of Cr in the present invention may be 0.05 to 0.5%, and more preferably, the content of Cr may be 0.05 to 0.45%.

Mo：0.05-0.5％

Like Cr, Mo is an element having a very high hardening property, and is an element that can promote the formation of a low-temperature transformation phase even when added in a small amount, thereby effectively contributing to the reduction of the yield ratio. By adding Mo, the fraction of bainite or martensite can be increased, and the yield ratio can be reduced. In order to obtain such an effect, the present invention may limit the lower limit of the content of Mo to 0.05%. However, Mo is an element whose alloy cost is high, and therefore, the present invention can limit the upper limit of the content of Mo to 0.5% in view of economical aspects. Accordingly, the content of Mo in the present invention may be 0.05 to 0.5%, and more preferably, the content of Mo may be 0.05 to 0.45%.

Nb：0.01-0.05％

Nb exists as precipitates in the form of carbides or nitrides in the billet, but it is dissolved in the steel in the reheating process, thereby playing a role of delaying recrystallization at the time of rolling. The delay of recrystallization promotes the formation of ferrite nuclei when ferrite phase transformation occurs after rolling, and the strength of the steel material can be improved by grain refinement. In order to achieve such an effect, the present invention may limit the lower limit of the content of Nb to 0.01%. However, the strength of the steel material increases by adding Nb, but the increase in strength due to grain refinement contributes to an increase in yield strength more than tensile strength. Therefore, in the case where Nb is excessively added, the strength is increased, but the yield ratio may be increased, and therefore, the upper limit of the Nb content may be limited to 0.05% in the present invention. Therefore, the content of Nb in the present invention may be 0.01 to 0.05%.

Ca：0.0005-0.005％

Ca plays a role of spheroidizing (spheriodizing) a nonmetallic inclusion such as MnS. Since Ca reacts with S to form CaS, the production of nonmetallic inclusions such as MnS is suppressed, and low-temperature toughness can be improved. In order to obtain the effect of spheroidizing the nonmetallic inclusions such as MnS, the present invention may limit the lower limit of the Ca content to 0.0005%. However, Ca is an element having high volatility and low yield. The present invention can limit the upper limit of the content of Ca to 0.005% in consideration of providing the load generated in the process. Therefore, the content of Ca in the present invention may be 0.0005 to 0.005%.

The high-strength steel material excellent in low yield ratio characteristics according to one aspect of the present invention may further include one or more of 0.05 to 0.3% of Ni, 0.05 to 0.3% of Cu, 0.005 to 0.02% of Ti, and 0.0005 to 0.0015% of B in terms of weight%.

Ni: 0.05-0.3% or less

Ni is an effective element that can improve both the strength and toughness of steel. Therefore, in order to obtain the effect of improving such strength and toughness, the present invention may limit the lower limit of the Ni content to 0.05%. However, since it is not economically preferable to add Ni excessively as an expensive element, the upper limit of the Ni content may be limited to 0.3% in the present invention. Therefore, the Ni content of the present invention may be 0.05 to 0.3%.

Cu：0.05-0.3％

Cu is an element that improves strength by solid solution strengthening. Therefore, in order to obtain the effect of improving the strength, the present invention may limit the lower limit of the Cu content to 0.05%. However, in the case where Cu is excessively added, surface cracks are induced during the production of the slab to reduce local corrosion resistance, and when the slab for rolling is reheated, Cu having a low melting point penetrates into the grain boundary of the steel to possibly induce cracks during hot working, so that the present invention can limit the upper limit of the Cu content to 0.3%. Therefore, the content of Cr in the present invention may be 0.05 to 0.3%.

Ti：0.005-0.02％

Precipitates of Ti in the form of TiN or TiC exist in the billet. When the billet is reheated, Nb is dissolved and re-dissolved, while Ti is not dissolved in the reheating process and exists in the austenite grain boundary in the form of TiN. The TiN precipitates present in the grain boundaries of austenite act to suppress the grain growth of austenite that occurs during reheating, and therefore ultimately contribute to the refinement of ferrite grains, thereby improving strength and toughness. In order to achieve the effect of suppressing such grain growth of austenite, the present invention may limit the lower limit of the content of Ti to 0.005%. However, when the addition amount of Ti is too large, the content of Ti with respect to N in the steel increases, and therefore, coarse precipitates may be formed. Since these coarse precipitates do not contribute to suppression of grain growth of austenite but act as a main factor for lowering toughness, the upper limit of the Ti content may be limited to 0.02% in the present invention in consideration of the N content in the steel.

B: less than 0.0005-0.0015%

B is an element having high hardenability, which can easily form a low-temperature transformation phase even when added in a small amount, and effectively contributes to an increase in strength and a decrease in yield ratio. In order to obtain the effect resulting from such hardenability, the present invention may limit the lower limit of the content of B to 0.0005%. However, since B is not only an element that is difficult to control in a steel making process, but also if an appropriate amount or more is added, brittleness of grain boundaries is induced to drastically decrease toughness, the upper limit of the content of B in the present invention can be limited to 0.0015%. Therefore, the content of B in the present invention may be 0.0005 to 0.0015%.

In the present invention, the remainder may be Fe and inevitable impurities, in addition to the components of the steel as described above. Unavoidable impurities are impurities that may be unexpectedly mixed in a general steel making process, and they cannot be completely excluded as long as they are understood by those of ordinary skill in the art of steel making. Also, the present invention does not fully exclude the addition of other components than those of the steel mentioned previously.

The high-strength steel material excellent in the low yield ratio characteristic according to an embodiment of the present invention may include polygonal ferrite and a residual structure as a fine structure. The average grain size of the polygonal ferrite may be 10 μm or less, and the preferable average grain size of the polygonal ferrite may be 6 μm or less. The area fraction of polygonal ferrite with respect to the cross section of the steel material may be 10 to 30%, and the average hardness of the polygonal ferrite may be 180Hv or less. The residual structure may include acicular ferrite, bainite, pearlite, and martensite, and the average hardness of the residual structure may be 200Hv or more. The total fraction of pearlite and martensite in the residual structure may be 10% or less of the entire cross-sectional area of the steel material.

According to an embodiment of the present invention, the high-strength steel material having excellent low yield ratio characteristics may have a charpy impact energy at-30 ℃ of 200J or more and a tensile strength of 500MPa or more. In addition, according to the high-strength steel material excellent in low yield ratio characteristics of the embodiment of the present invention, the yield ratio of the steel material may be 90% or less, and more preferably 88% or less.

The production method of the present invention will be described in more detail below.

According to the method for producing a high-strength steel material having excellent low yield ratio characteristics of one aspect of the present invention, a billet containing, in terms of wt%, 0.06 to 0.12% of C, 0.2 to 0.5% of Si, 1.5 to 2.0% of Mn, 0.003 to 0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% or less of S, 0.05 to 0.5% of Cr, 0.05 to 0.4% of Mo, 0.01 to 0.05% of Nb, 0.0005 to 0.005% of Ca, and the balance Fe and other unavoidable impurities is reheated, and rough rolled and finish rolled; can be cooled for the first time at a first cooling speed to a temperature range of (Ar3-40 ℃) to (Ar3-70 ℃; the second cooling may be performed at a second cooling rate to a temperature range of 350-400 deg.C. The steel slab of the present invention may further contain one or more of 0.05 to 0.3% of Ni, 0.05 to 0.3% of Cu, 0.005 to 0.02% of Ti, and 0.0005 to 0.0015% of B in terms of weight%.

The composition of the steel slab of the present invention corresponds to the composition of the steel material described above, and the above description of the reason for limiting the composition of the steel slab is used instead of the description of the reason for limiting the composition of the steel slab of the present invention.

Reheating of billets

The steel slab having the above composition is reheated within a temperature range of 1100-. In order to form an atmosphere in which the crystal grains of the final microstructure are refined by the solid-solution Nb, it is necessary to sufficiently decompose the NbC precipitates in the billet by heating the billet to a certain temperature or higher. Therefore, in order to achieve the effect of refining the crystal grains by the solid-solution Nb, the present invention may limit the lower limit of the reheating temperature of the billet to 1100 ℃. When the reheating temperature of the billet is increased, although it is easy to ensure solid-solution Nb by decomposition of NbC precipitates, grain growth of austenite rapidly occurs, and finally, the grain size of ferrite may increase. When the amount of Nb dissolved in the steel is too large, the temperature of the unrecrystallized region increases, and thus, even if the rolling is performed at a high temperature, the grain size cannot be sufficiently reduced, and the low yield ratio property is deteriorated. Therefore, the upper limit of the reheating temperature of the slab of the present invention may be limited to 1160 ℃.

Rough rolling

The rough rolling may be performed after reheating of the billet. When rough rolling is performed, grain refinement of austenite by recrystallization phenomenon is achieved. In the case where the rough rolling temperature is less than 1050 ℃, partial recrystallization will occur, whereby the grain size of austenite inside the steel material may become non-uniform. In particular, the uneven shape of the crystal grain size of austenite in the central portion of the thickness is increased, and thus the fine structure in the central portion eventually forms a coarse bainite fine structure, and the low-temperature toughness deteriorates. Therefore, the present invention can limit the finish temperature of rough rolling to 1050 ℃ or higher.

Finish rolling

The finish rolling may be performed after the rough rolling. In order to effectively refine the grain size of austenite in the finish rolling process, it is necessary to accumulate rolling energy applied in the finish rolling process by formation of a deformation band (deformation band) of austenite grains or formation of displacement, and for this purpose, it is necessary to limit the start temperature of finish rolling. When the start temperature of finish rolling exceeds 980 ℃, most of the effect of accumulation of rolling energy in the finish rolling process is lost by annealing, and thus effective austenite crystal grains cannot be sufficiently refined, and thus the effect of refining ferrite crystal grains cannot be sufficiently achieved. Therefore, in the present invention, the start temperature of finish rolling may be limited to 980 ℃ or more in order to secure a desired microstructure.

In addition, when the finish rolling temperature is lower than the temperature (Tnr) of the recrystallization zone, a deformed band of austenite grains can be formed. That is, the reduction ratio applied at a temperature (Tnr) lower than the recrystallization zone, that is, the effective reduction ratio, is an important factor that affects the improvement of the strength and toughness more than the reduction ratio applied in the entire finish rolling. When the effective reduction ratio in the finish rolling process is insufficient, not only fine crystal grains cannot be generated but also effective austenite crystal grains become coarse when ferrite is transformed, and thus the fraction of bainite due to hardenability is excessively increased, and toughness and yield ratio may be deteriorated. Therefore, the effective reduction rate of the finish rolling of the present invention may be 65% or more. For reference, the effective rolling reduction and the temperature of the recrystallization zone (Tnr) can be theoretically derived from the following formulas 1 and 2, and the temperature of the recrystallization zone (Tnr) of the present invention may refer to a temperature at which recrystallization of austenite stops.

[ formula 1]

Effective reduction (%) - (thickness of steel at Tnr temperature-final thickness (mm) of steel/thickness (mm) of steel at Tnr temperature) × 100

[ formula 2]

Tnr(℃)＝887+464*C+890*Ti+363*Al-357*Si+(6445*Nb-644*Nb^1/2)+(732*V-230*V^1/2)

The rolling energy applied in the finish rolling is accumulated in the austenite grains by the formation of a deformed band or displacement. As the finish rolling temperature becomes lower, the generation of the deformed zone is promoted, and the generation position of ferrite nuclei is increased, whereby the crystal grains can be finally refined. Further, since the higher the finish rolling temperature becomes, the more easily the displacement is eliminated, the rolling energy is not accumulated and can be easily lost. Therefore, in the present invention, the finish rolling should be finished at least at 900 ℃ or less in order to secure low temperature toughness in consideration of the limited components, the effective reduction ratio of the finish rolling, and the like. When the steel material is moved for cooling after finish rolling, the temperature of the steel material is lowered by air cooling. In order to prevent the cooling start temperature from decreasing to Ar3+10 ℃ or lower by air cooling before the start of cooling, the finish temperature of finish rolling may be limited to (Ar3+50 ℃) or higher. Therefore, the finish rolling temperature of the finish rolling of the present invention may be (Ar3+50 ℃) to 900 ℃. For reference, the temperature of Ar3 can be derived theoretically from equation 3 below.

[ formula 3]

Ar3 (. degree.C.). 910. C-80. Mn-20. Cu-15. Cr-55. Ni-80. Mo + 0.35. steel thickness-8 (mm)

First cooling

The final microstructure of the steel material can be determined by controlling the transformation from austenite to ferrite by cooling after finish rolling. In order to secure a desired yield ratio, a plurality of phases having different hardness should be compositely present at an appropriate ratio, and particularly, the fraction of ferrite and the hardness of the high hardness phase should satisfy an appropriate range. In order to introduce fine polygonal ferrite generated by the first cooling after the finish rolling, the first cooling should be started at a temperature slightly higher than Ar 3. When the first cooling is started at a temperature higher than Ar3 by about 10 to 30 ℃, fine polygonal ferrite having an optimum ratio can be introduced. Therefore, the starting temperature of the first cooling of the present invention may be (Ar3+10 ℃ C.) - (Ar3+30 ℃ C.).

In order to ensure the low yield ratio characteristics of the steel material, a microstructure in which fine ferrite and a low-temperature transformation phase are mixed should be obtained. However, if the fraction of fine ferrite is too large, low yield ratio characteristics can be ensured, but high strength characteristics cannot be ensured due to a decrease in yield strength. In the case where the finish temperature of the first cooling is less than Ar3-70 ℃, the fraction of ferrite will excessively increase, whereby the securing of the high strength characteristics becomes difficult; on the other hand, when the end temperature of the first cooling exceeds Ar3-40 ℃, the fraction of ferrite decreases, and it becomes difficult to secure low yield ratio characteristics. Therefore, the present invention can limit the cooling end temperature of the first cooling to (Ar3-70 ℃ C.) - (Ar-40 ℃ C.).

During the first cooling, it is necessary to control the first cooling rate not to be excessively fast so that bainite does not start phase transformation and polygonal ferrite can be generated. Therefore, the first cooling rate can be limited to 10-20 ℃/s. When the first cooling rate is less than 10 ℃/s, the yield strength is reduced because the water-cooled ferrite is relatively coarse, and when the first cooling rate exceeds 20 ℃/s, the amount of polygonal ferrite produced is not reached, and the fraction of the plurality of low-temperature transformation phases increases, so that the low yield ratio property cannot be secured. Therefore, the first cooling rate of the present invention can be limited to 10-20 deg.C/s.

Second cooling

The end temperature of the second cooling should be sufficiently lower than the transformation end temperature of bainite so that austenite that has failed to be transformed during the first cooling can be transformed into a low-temperature transformation phase such as bainite. Therefore, the present invention can limit the finish temperature of the second cooling to 400 ℃ or lower. On the other hand, if the finish temperature of the second cooling is too low, the amount of martensite having high brittleness increases, and thus the yield ratio decreases, and toughness may decrease. Therefore, in order to suppress the generation of martensite, the finish temperature of the second cooling of the present invention may be limited to 350 ℃ or more. Therefore, the end temperature of the second cooling of the present invention may be 350-. After the second cooling is finished, cooling to room temperature may be performed by air cooling.

The second cooling rate may be controlled to a rate greater than the first cooling rate such that all of the plurality of austenite that fails to be phase-transformed into ferrite in the first cooling is transformed into bainite. The second cooling rate may be 10-40 deg.c/s greater than the first cooling rate, and a preferred second cooling rate of the present invention may be 30-50 deg.c/s.

In addition, from the viewpoint of fine structure, the second cooling is preferably performed immediately after the first cooling.

Detailed Description

The present invention will be described in more detail below with reference to specific examples.

(examples)

Steel slabs satisfying the compositions as described in table 1 below were produced, and steel materials were produced from the respective slabs according to the process conditions of table 2.

[ TABLE 1]

[ TABLE 2]

Tensile test pieces were prepared by cutting the steel materials manufactured according to the process conditions of table 2 in the longitudinal direction, and the tensile test pieces were subjected to a tensile test, whereby yield strength, tensile strength, and charpy impact energy at-30 ℃ were evaluated, and the results thereof are shown in table 3 below. Further, each of the steel materials manufactured according to the process conditions of table 2 was etched, and thereafter, the fine structure was observed and the hardness of each structure was measured, and the fraction and hardness of polygonal ferrite and the fraction and hardness of the remaining multiple phases (pearlite + martensite) other than the polygonal ferrite of each steel material are shown in table 4 below.

[ TABLE 3]

[ TABLE 4 ]

In the case of invention examples 1 to 16, it was confirmed that: all satisfy the composition, microstructure and process conditions of the steel of the present invention, and all satisfy Charpy impact energy at-30 ℃ of 200J or more, tensile strength of 500MPa or more, and yield ratio of 90% or less. In particular, in the cases of invention examples 1 to 16, it was confirmed that the yield ratio was less than 90%.

In the case of comparative example 1, although the component contents satisfied the component contents of the present invention, the first cooling and the second cooling were not divided, and only the first cooling was performed, and the starting temperature of the first cooling was out of the range of the present invention, and thus, it was confirmed that the conditions of the microstructure of the present invention were not satisfied. Therefore, it was confirmed that comparative example 1 could not ensure the low temperature toughness desired in the present invention.

In the case of comparative example 2, although the component contents satisfied the component contents of the present invention, the effective reduction ratio and the first cooling rate of the finish rolling did not satisfy the ranges of the present invention, and thus, it was confirmed that the conditions of the fine structure of the present invention were not satisfied. Therefore, it was confirmed that comparative example 2 could not ensure the tensile strength and low-temperature toughness desired in the present invention.

In the case of comparative example 3, although the component contents satisfied the component contents of the present invention, the reheating temperature, the finish temperature of the rough rolling, the finish temperature of the finish rolling, and the start temperature and the finish temperature of the first cooling did not satisfy the ranges of the present invention, and thus, it was confirmed that the conditions of the fine structure of the present invention were not satisfied. Therefore, it was confirmed that comparative example 3 could not ensure the low temperature toughness desired in the present invention.

In addition, in the cases of comparative examples 4 to 13, the components, the microstructure, and the process conditions of the steel of the present invention were not satisfied, and thus it was confirmed that the physical properties achieved by the present invention could not be secured.

Therefore, according to the high-strength steel material excellent in low yield ratio characteristics and the manufacturing method thereof of the embodiment of the invention, it is possible to provide a steel material for a pipe and a manufacturing method thereof capable of ensuring safety performance and pipe manufacturing performance by satisfying both low yield ratio characteristics and high strength performance.

The present invention has been described in detail with reference to the embodiments, but embodiments having different forms from the embodiments may be implemented. Therefore, the technical spirit and scope of the claims to be described are not limited to these examples.