EP4640905A1

EP4640905A1 - Steel material having excellent low-temperature toughness for line pipe, and manufacturing method for same

Info

Publication number: EP4640905A1
Application number: EP23907371.1A
Authority: EP
Inventors: Jin-You KIM; Hyo-Shin Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-20
Filing date: 2023-10-26
Publication date: 2025-10-29
Also published as: CN120344704A; KR20240097648A; JP2025541920A; WO2024136085A1; EP4640905A4

Abstract

The present invention relates to a high-strength API steel material having excellent low-temperature toughness and cost-effectiveness, and a manufacturing method for same, and more specifically, relates to a high-strength API steel material which has outstanding properties and which can be used for transporting crude oil, and a manufacturing method for same.

Description

Technical Field

The present disclosure relates to a high-strength API steel material having excellent low-temperature toughness and cost-effectiveness and a high-strength API steel material having outstanding properties, which may be used for transporting crude oil, and a manufacturing method for the same.

Background Art

In the case of an API steel material for pipelines used when transporting crude oil after mining to the site of use, high strength and deformation stability of the material are required to stably protect the structure against deformation caused by external factors and external impacts such as earthquakes or the like. Therefore, thick plate steel material used in the related art as API material for transporting crude oil by adding a large amount of solid solution strengthening elements such as C, Si, Mn, Cr and the like to high purity steel with minimized impurities in the steel, or by adding hardenable elements such as Ni, Mo and the like thereto, to strengthen the strength has been mainly used.
However, as the demand for thick high strength materials has recently increased, expensive elements such as Ni, Mo and the like have been added in large quantities to secure the strength of thick plate materials. Therefore, in order to secure the aforementioned strength, the heating temperature, rolling temperature, cooling end temperature and the like have been greatly reduced, resulting in a decrease in productivity, and as the cooling end temperature has been reduced, rapid cooling structures have been created on the surface layer, resulting in problems such as hard spots or the like.

[Prior art literature]

[Patent literature]

(Patent literature 1) Korean Patent Publication No. 2014-0002273

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a thick high-strength API steel material with excellent low-temperature toughness and economic efficiency and a method for manufacturing the same.
The task of the present disclosure is not limited to the above-mentioned content. Anyone with ordinary knowledge in the technical field to which the present disclosure belongs will not have difficulty in understanding the additional task of the present disclosure from the contents throughout the specification of the present disclosure. Solution to Problem
According to an aspect of the present disclosure, a steel material for pipelines includes, by weight %, C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), and Cu: 0.010% or less (excluding 0%), and Fe and unavoidable impurities as a remainder,

wherein the steel material includes, as a microstructure, by area %, at least one selected from the group consisting of 80% or more (including 100%) of ferrite, and pearlite and bainite as remainders, and
a surface layer portion having an average grain size of 10 to 15 µm in a thickness direction.

In addition, according to another aspect of the present disclosure, a method for manufacturing a steel material for a pipeline includes,

an operation of reheating a slab at 1050 to 1250°C, the slab containing, in wt%, C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), and Cu: 0.010% or less (excluding 0%), and a remainder of Fe and unavoidable impurities;
an operation of rough rolling the reheated slab and then finish rolling at an austenite single-phase temperature, and obtaining a thick plate steel material;
a first cooling operation of water-cooling the thick plate steel material to a temperature of 700 to 800°C at an average cooling rate of 5 to 25°C/s after completion of the finish rolling;
a second cooling operation of air-cooling the first-cooled steel material; and
a third cooling operation of water-cooling the second-cooled steel material to a temperature of 300 to 500°C at an average cooling rate of more than 25°C/s and less than or equal to 60°C/s.

Advantageous Effects of Invention

As one of the many effects of the present disclosure, a thick high-strength API steel material with excellent low-temperature toughness and economic efficiency and a method for manufacturing the same may be provided.
The various and beneficial advantages and effects of the present disclosure are not limited to the above-described contents, and will be more easily understood in the process of explaining detailed embodiments of the present disclosure. Best Mode for Invention
Hereinafter, preferred embodiments of the present disclosure will be described. However, the embodiments of the present disclosure may be modified in various other forms, and the scope of the present disclosure is not limited to the embodiments described below. In addition, the embodiments of the present disclosure are provided to allow those of average knowledge in the relevant technical field to more completely understand the present disclosure.
In addition, the terms used in this specification are for the purpose of describing specific embodiments, and are not intended to limit the present disclosure. For example, the singular forms used in this specification also include the plural forms unless the relevant definition clearly indicates a meaning contrary thereto. In addition, the meaning of "comprising" used in the specification specifies a configuration, and does not exclude the presence or addition of other configurations.
Hereinafter, one aspect of the present disclosure will be described in detail about the steel material for pipeline (or, API steel material).
First, the reason for adding alloy components to the API thick plate steel material of the present disclosure and the content limit range will be described in detail. It should be noted in advance that the content of each component described below is based on weight% unless specifically stated.

C: 0.030 to 0.100%

Carbon (C) is the most economical and effective element for securing strength. If the carbon content is too low, it may be difficult to secure target strength even if a precipitation strengthening element such as Nb is added, so the lower limit of the carbon addition amount is set to 0.030%. On the other hand, if the carbon content is too excessive, the ductility may deteriorate due to excessive strength increase, so the upper limit of the carbon addition amount is set to 0.100%. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the carbon content may be 0.040%, or the upper limit of the carbon content may be 0.090%.

Si: 0.50% or less (excluding 0%)

Silicon (Si) is an element that contributes to the increase in strength through deoxidation and solid solution strengthening of molten steel. However, in the present disclosure, it is not intentionally added, and even if silicon is not added, there is no significant problem in terms of securing physical properties, and considering the case in which this element is inevitably included during the manufacturing process, 0% is excluded as the lower limit of the silicon content. On the other hand, if the silicon content is excessively excessive, red scale due to Si is formed on the surface of the thick plate steel material, which may deteriorate the surface quality and weldability, so the content is set to 0.50%. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the silicon content may be 0.01%, or the upper limit of the silicon content may be 0.40%.

Mn: 0.50 to 2.50%

Manganese (Mn) is an effective element for strengthening steel by solid solution strengthening, and 0.50% or more needs to be added to secure appropriate strength. However, if the manganese content is excessively high, there is a risk of central segregation occurring during the continuous casting process, so the upper limit of the manganese content is set to be 2.5%. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the manganese content may be 0.60%, or the upper limit of the manganese content may be 2.40%.

Nb: 0.070% or less (excluding 0%)

Niobium (Nb) is a precipitation strengthening element that is effective in securing strength by forming NbC series precipitates and refining grains. On the other hand, if the niobium content is excessively excessive, there is a disadvantage in that it is difficult to implement a low yield ratio by increasing the yield strength compared to the tensile strength due to the effect of grain refinement, so in the present disclosure, the niobium content is controlled to 0.070% or less. However, considering the case in which this element is unavoidably included, 0% is excluded from the lower limit of the niobium content. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the niobium content may be 0.010%, or the upper limit of the niobium content may be 0.060%.

V: 0.030% or less (excluding 0%)

Vanadium (V) is also a precipitation-strengthening element, and is an effective element for securing the strength of steel. However, if the vanadium content is excessively excessive, the low-temperature toughness and weldability are reduced due to a large amount of precipitates, which increases the cost of the alloy. Therefore, in the present disclosure, the vanadium content is managed to 0.030% or less. However, considering the case in which this element is unavoidably included, 0% is excluded from the lower limit of the vanadium content. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the vanadium content may be 0.010%, or the upper limit of the vanadium content may be 0.020%.

Mo: 0.05% or less (excluding 0%)

Molybdenum (Mo) is a representative element that improves the hardenability of steel, and greatly improves the ability to create low-temperature structures even at low cooling rates. This is an effective element for securing the strength of steel by forming low-temperature structures such as bainite or the like. Molybdenum is not only a relatively expensive element, but also, if the content thereof is excessively high, toughness may deteriorate. Therefore, in the present disclosure, it is not added artificially for economic reasons, and the upper limit of the molybdenum content is set to be 0.05%. However, considering the case in which this element is unavoidably included, 0% is excluded from the lower limit of the molybdenum content. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the molybdenum content may be 0.005%, or the upper limit of the molybdenum content may be 0.04%.

Ni: 0.05% or less (excluding 0%)

Nickel (Ni) plays a role in simultaneously improving the strength and toughness of steel, but in the present disclosure, even if nickel is not added, there is no significant problem in terms of securing physical properties, so it does not need to be added artificially. On the other hand, if the nickel content is excessive, the economic efficiency is reduced, so in the present disclosure, the content is 0.05% or less. However, considering the case in which this element is unavoidably included, 0% is excluded from the lower limit of the nickel content. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the nickel content may be 0.005%, or the upper limit of the nickel content may be 0.04%.

Cr: 0.10 to 0.30%

Chromium (Cr) strengthens the steel by solid solubilization and delays the bainite phase transformation during cooling, helping to form equiaxed ferrite, and especially when added together with Mo, it effectively increases the hardenability. In the present disclosure, it is preferable to add 0.10% or more to obtain this effect. On the other hand, if the chromium content is excessive, the weldability and brittleness deteriorate, so the content is set to 0.30% or less in the present disclosure. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the chromium content may be 0.12%, or the upper limit of the chromium content may be 0.20%.

P: 0.03% or less (excluding 0%)

Phosphorus (P) is an impurity that is inevitably included in steel, and it is desirable to manage the content thereof as low as possible. In particular, if the phosphorus content is excessive, the risk of weldability deterioration and steel brittleness increases, so in the present disclosure, the phosphorus content is managed to 0.03% or less. However, considering the case in which this element is inevitably included, 0% is excluded from the lower limit of the phosphorus content. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the phosphorus content may be 0.001%, or the upper limit of the phosphorus content may be 0.025%.

S: 0.050% or less (excluding 0%)

Sulfur (S) is an impurity that is inevitably included in steel, and it is desirable to manage the content thereof as to be as low as possible. In particular, if the content is excessive, it may combine with Mn or the like to form non-metallic inclusions, and the risk of steel brittleness increases, so the sulfur content is managed to 0.050% or less in the present disclosure. However, considering the case in which this element is unavoidably included, 0% is excluded from the lower limit of the sulfur content. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the sulfur content may be 0.001%, or the upper limit of the sulfur content may be 0.045%.

Al: 0.050% or less (0% is excluded)

Aluminum (Al) contributes to the deoxidation of molten steel, but it is not intentionally added in the present disclosure, and even if aluminum is not added, there is no significant problem in terms of securing physical properties. Meanwhile, if the aluminum content is excessive, nozzle clogging or the like may occur during continuous casting, so the aluminum content is managed to be 0.050% or less in the present disclosure. However, considering the case in which this element is unavoidably included, 0% is excluded from the lower limit of the aluminum content. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the aluminum content may be 0.010%, or the upper limit of the aluminum content may be 0.040%.

N: 0.010% or less (excluding 0%)

Nitrogen (N) contributes to improving the strength of steel, but in the present disclosure, it is not intentionally added, and even if nitrogen is not added, there is no significant problem in terms of securing the physical properties. Meanwhile, if the nitrogen content is excessive, the risk of steel brittleness increases, so in the present disclosure, the content is managed to be 0.010% or less. However, considering the case in which this element is unavoidably included, 0% is excluded from the lower limit of the nitrogen content. Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the nitrogen content may be 0.001%, or the upper limit of the nitrogen content may be 0.009%.

Cu: 0.010% or less (excluding 0%)

Copper (Cu) forms fine precipitates to increase strength, but in the present disclosure, there is no significant problem in terms of securing physical properties even if copper is not added. On the other hand, if the content is excessive, cracks occur in the surface portion and room temperature workability deteriorates, so the content is controlled to 0.010% or less in the present disclosure. However, considering the case in which this element is unavoidably included, 0% is excluded from the lower limit of the copper content. On the other hand, in terms of further improving the aforementioned effect, the lower limit of the copper content may be 0.0001%, or the upper limit of the copper content may be 0.009%.
In addition to the above composition, the remainder is Fe. However, since unavoidable impurities that are not intended from raw materials or the surrounding environment may inevitably be mixed in during a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone with ordinary knowledge in this technical field, not all thereof are specifically mentioned in this specification, but representative impurities are as follows.
Meanwhile, in the steel material, the carbon equivalent (Ceq) defined by the following Relationship 1 may be controlled to 0.45 or less (excluding 0), which is for securing an appropriate level of weldability. Ceq = [C] + [Mn]/6 + ([Cu]+[Ni])/15 + ([Cr]+[Mo]+[V])/5
(In the above Relationship 1, each of the [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] represents the content (weight %) of the element in the corresponding parentheses.)
In the present disclosure, the microstructure of the steel material is not particularly limited, but, for example, the steel material may include ferrite, pearlite, and bainite as the microstructure. In particular, according to an embodiment of the present disclosure, the steel material may include, as the microstructure, at least one selected from the group consisting of 80% or more (including 100%) of ferrite, and the remainder pearlite and bainite, in terms of area %. If the area ratio of ferrite is less than 80%, the possibility of crack occurrence during expansion after pipe forming may increase due to deterioration of workability.
According to an embodiment of the present disclosure, the steel material may have an average grain size of 10 to 15 µm in the thickness direction of the surface layer portion. The thick plate steel material has one feature that the grain sizes of the surface layer portion and the central portion are different due to the multi-stage cooling described below. When the grain size distribution of the steel material is different in the thickness direction, the nucleation and crack propagation speeds are slowed down between fine grains and normal-sized grains, thereby improving toughness. At this time, it is more advantageous to secure toughness when the grains are distributed differently in the thickness direction than when the grains are fine overall. Accordingly, the present disclosure aims to secure strength and toughness at the same time by refining the grain size of the surface layer portion in the first cooling and securing appropriate grains and microstructure in the second cooling.
Meanwhile, in terms of further improving the aforementioned effect, the lower limit of the average grain size of the surface layer portion may be 11 µm, or the upper limit of the average grain size of the surface layer portion may be 14.5 µm.
Meanwhile, in the present specification, the grain size represents the average size for a phase including ferrite, pearlite, and bainite, and the size represents the equivalent diameter of a circle.
In addition, according to an embodiment of the present disclosure, the definition of the surface layer portion is not particularly limited, and may be specified through a method conventional in the relevant technical field. As an embodiment, the surface layer portion may represent a region corresponding to 15% of the total thickness in the thickness direction from at least one surface of the steel material. Therefore, it may mean a region corresponding to 30% in total, including 15% of the total thickness in the thickness direction based on both surfaces of the steel material. In addition, the average grain size may be measured by respectively measuring the grain sizes for several points (for example, n points; n is a positive constant) at equal intervals in the thickness direction for the surface layer portion described above, and calculating the average value thereof.
Although not particularly limited, according to an embodiment of the present disclosure, the steel material for pipeline having the aforementioned alloy component has the advantage of excellent economic efficiency by reducing the content of expensive alloying elements such as nickel (Ni), molybdenum (Mo) and the like, and according to a non-limiting example, the steel material of the present disclosure exhibits physical properties such as a yield strength of 550 to 700 MPa, a tensile strength of 600 to 800 MPa, and an elongation of 30 to 60%, and may have a product thickness of 30 to 50 mmt.
Although not particularly limited, according to an embodiment of the present disclosure, the average grain size of the central portion in the thickness direction may be larger than the average grain size of the surface layer portion. As described above, in the present disclosure, by managing the grain sizes of the central portion and the surface layer portion differently, the nucleation and crack propagation speed between the fine grains and the normal-sized grains is slowed down, thereby exerting an effect of improving toughness.
At this time, the definition of the central portion is not particularly limited, and may be specified through a method conventional in the relevant technical field. As an example of implementation, the central portion may represent a central region excluding the surface layer portion with respect to the entire thickness in the thickness direction of the steel material (for example, a region corresponding to 70% of the entire thickness in the thickness direction, excluding the surface layer portions, which are regions corresponding to 15% from respective both surfaces described above (corresponding to a total of 30% based on the two surfaces)). Meanwhile, when measuring the average grain size of the central portion described above, the measurement may be made in the same way as the measurement of the average grain size of the surface layer portion described above, and for example, the average grain size described above may be measured by respectively measuring the grain sizes of several points (for example, n points; n is a positive constant) at equal intervals in the thickness direction with respect to the central portion described above, and calculating the average value thereof.
Meanwhile, according to an embodiment of the present disclosure, in terms of further improving the aforementioned effect, the average grain size of the central portion may be greater than 15 µm and 30 µm or less, or the lower limit of the average grain size of the central portion may be 16 µm, or the upper limit of the average grain size of the central portion may be 25 µm.
Although not particularly limited, according to an embodiment of the present disclosure, the difference between the average grain size of the central portion and the average grain size of the surface layer portion may be managed to exceed 5 µm. By managing the grain sizes of the central portion and the surface layer portion differently, the nucleation and crack propagation speeds between the fine grains and the normal-sized grains are slowed down, so that toughness may be improved. However, if the difference in size is too excessive, it may rather harm the physical properties, so it may be managed within an appropriate range. In terms of further improving the aforementioned effect, the difference between the average grain size of the central portion and the average grain size of the surface layer portion may be 6 µm or more, or may be 10 µm or less.
The steel material of the present disclosure described above may be manufactured in various ways, and the manufacturing method is not particularly limited. However, as a preferred example, it may be manufactured by the following method.
First, the slab having the aforementioned composition is reheated to a temperature of 1050 to 1250°C. If the reheating temperature is less than 1050°C, the rolling load may become excessively large in the subsequent rolling process, and the precipitate-generating element, Nb or the like, may not be sufficiently dissolved and remain, so that it does not contribute to the formation of precipitates in the subsequent process, which may result in a decrease in strength. On the other hand, if the reheating temperature exceeds 1250°C, there is a concern that the grain size of the final microstructure may not be uniform due to partial coarsening caused by abnormal growth of some austenite grains. Meanwhile, in the present disclosure, the slab reheating time is not particularly limited, and any normal conditions may be sufficient. As a non-limiting example, the slab reheating time may be 100 to 400 minutes.
Next, the reheated slab is subjected to rough rolling and then finish rolling at an austenite single-phase temperature to obtain a thick plate steel material.
Here, rough rolling refers to a series of intermediate rolling processes performed before finish rolling. In the present disclosure, there is no particular limitation on the detailed conditions of rough rolling, and any normal conditions may be sufficient. The rough rolling temperature may be set to a sufficiently high temperature at which the finish rolling temperature may be secured.
The finish rolling is performed at an austenite single-phase temperature, which is intended to increase the uniformity of the structure. As an example, during rolling, the finish rolling temperature may be 800 to 950°C. If the finishing rolling temperature is less than 800°C, the rolling load may increase, which may lower productivity, and there is a concern that the grains may become excessively fine or coarse unrecrystallized austenite may be generated. On the other hand, if it exceeds 950°C, the austenite grains of the slab may become excessively coarse, making it difficult to secure the target strength.
Next, the thick plate steel material is cooled at an accelerated rate. The cooling operation is performed in three stages to generate fine ferrite in the surface layer and ferrite of an appropriate size in the central portion. At this time, when the grain size ratio of the surface layer portion and the central portion is appropriately adjusted, toughness may be improved.
First, after the finishing rolling is completed, the thick plate steel material is first cooled by water cooling the same to a temperature of 700 to 800°C at an average cooling rate of 5 to 25°C/s. At this time, although not particularly limited, according to an embodiment of the present disclosure, the cooling start temperature of the first cooling that initiates cooling may be managed in the range of 750 to 850°C. If the cooling start temperature is initiated at a temperature exceeding 850°C, the possibility of forming coarse bainite increases as the bainite area ratio increases, and thus there is a high possibility that toughness characteristics will deteriorate. On the other hand, if the cooling start temperature is initiated at less than 750°C, air-cooled ferrite structures may be formed, causing a decrease in tensile strength.
It is preferable that the cooling end temperature during the first cooling be limited to the range of 700 to 800°C. At the corresponding temperature, most of the austenite is transformed into ferrite in the surface layer portion, and untransformed austenite still exists in the central portion. At this time, if the cooling end temperature during the first cooling exceeds 800°C, sufficient microferrite is not formed in the surface layer portion, resulting in low-temperature toughness deterioration. In addition, if the cooling end temperature of the first cooling is less than 700°C, excessive ferrite is formed in the surface layer portion, making it difficult to secure a sufficient fraction of bainite in the central portion and thus resulting in a decrease in strength.
Next, second cooling is performed to cool the first-cooled steel material in air. Then, third cooling may be performed to cool the second-cooled steel material in water to a temperature of 300 to 500°C at an average cooling rate of more than 25°C/s and 60°C/s or less. At this time, although there is no particular limitation, the steel material may be maintained for 3 seconds or more under the air cooling conditions during the second cooling.
The cooling end temperature during the third cooling is managed to be in the range of 300 to 500°C. At the corresponding temperature, the austenite remaining in the central portion after the first cooling transforms into ferrite and bainite, and at this time, grain growth occurs after the first cooling, resulting in the generation of ferrite of an appropriate size. At this time, if the cooling end temperature during the third cooling exceeds 500°C, excessive ferrite growth occurs in the central portion and bainite formation is insufficient, resulting in a decrease in strength. In addition, if the cooling end temperature is less than 300°C, sufficient ferrite is not secured, resulting in a decrease in low-temperature toughness. Meanwhile, according to an embodiment of the present disclosure, in terms of further improving the aforementioned effect, the average cooling speed in the third cooling may be 30 to 60°C/s.
Although not particularly limited, according to the aforementioned manufacturing method, the content of expensive alloying elements such as nickel (Ni) and molybdenum (Mo) may be reduced, thereby improving profitability, while reducing the load of the process by applying general management standards to the rolling and cooling processes. In addition, the steel material for pipeline of the present disclosure manufactured according to the manufacturing method described above exhibits physical properties such as a yield strength of 550 to 700 MPa, a tensile strength of 620 to 820 MPa, an elongation of 30 to 60%, and a DWTT ductile fracture ratio of 95% or more at - 30°C, and thus may be suitably used as a material for pipeline.

Mode for Invention

Hereinafter, the present disclosure will be described more specifically through examples. However, it should be noted that the following examples are only intended to explain the present disclosure through examples and are not intended to limit the scope of the rights of the present disclosure. This is because the scope of the rights of the present disclosure is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

(Example)

A slab having the compositions of Tables 1 and 2 below was reheated at 1100°C for 200 minutes, and then rough-rolled and finish-rolled to satisfy the conditions described above in the specification to obtain a hot-rolled steel material. At this time, in all examples, the thickness of the rough-rolled slab was kept constant at 20% compared to the thickness of the reheated slab. Thereafter, the steel material was subjected to first (water cooling), second (air cooling, for 3 seconds), and third cooling (water cooling) under the conditions described in Table 3 below.

Afterwards, the microstructure of the manufactured steel material was analyzed by SEM to measure the area ratio, and as mechanical properties, the tensile strength, yield strength, and elongation were measured using a room temperature tensile tester, and the DWTT ductile fracture ratio was measured by measuring the fracture surface of the specimen at -30°C using a drop tester with a capacity of 100,000J. The results are illustrated in Table 4 below. For reference, in all examples, the remaining structures other than ferrite were pearlite and/or bainite. At this time, the average grain size in the surface layer portion and the central portion was measured in the same manner as described above in the specification.

[Table 1]

Steel Grade	Alloy Composition (wt%)
Steel Grade	C	Si	Mn	Nb	V	P	S
Invent ion Steel 1	0.062	0.25	1.60	0.040	0.001	0.01	0.004
Invent ion Steel 2	0.064	0.24	1.59	0.041	0.002	0.01	0.004
Compar ative Steel 1	0.160	0.25	1.50	0.030	0.001	0.01	0.004
Compar ative Steel 2	0.064	0.24	1.45	0.041	0.002	0.01	0.004

[Table 2]

Steel Grade	Alloy Composition (wt%)						Ceq
Steel Grade	Al	N	Cr	Ni	Mo	Cu	Ceq
Invent ion Steel 1	0.025	0.005	0.15	0.01	0.01	0.001	0.38
Invent ion Steel 2	0.025	0.005	0.14	0.01	0.01	0.001	0.36
Compar ative Steel 1	0.025	0.005	0.15	0.01	0.01	0.001	0.44
Compar ative Steel 2	0.025	0.005	0.14	0.10	0.08	0.001	0.36

[Table 3]

Steel Grade	Coolin g start temperature (°C) during first coolin g	Cooling end temperature (°C )during first cooling	Average cooling rate (°C/s) during first cooling	Cooling end temperat ure (°C) during third cooling	Average cooling rate (°C/s) during third cooling	Remark
Invent ion Steel 1	780	730	14	430	42	Inventiv e Example 1
	780	650	17	450	40	Comparat ive Example 1
	780	430	43	-	-	Comparat ive Example 2
Invent ion Steel 2	780	730	15	430	41	Inventiv e Example 2
	780	650	16	450	41	Comparat ive Example 3
	780	430	44	-	-	Comparat ive Example 4
Compar ative Steel 1	780	730	14	430	43	Comparat ive Example 5
Compar ative Steel 2	780	730	15	430	42	Comparat ive Example 6

[Table 4]

Steel Grade	Microstructu re Area Ratio (%)			Average Grain Size (µm)		Mechanical Properties				Remark
	F	B	P	Surfa ce Layer Porti on	Centr al Porti on	YS (MPa)	TS (MPa)	El (%)	DWTT ductil e fractu re ratio (%) at -30°C
Invention Steel 1	88	12	-	12	18	568	655	46	100	Inventive Example 1
	84	9	7	9	12	543	615	51	70	Compara tive Example 1
	72	28	-	8	10	577	659	41	40	Compara tive Example 2
Inven tion Steel 2	87	13	-	11	19	561	653	45	100	Inventi ve Example 2
	82	9	9	9	12	533	605	49	73	Compara tive Example 3
	71	29	-	8	10	573	655	40	40	Compara tive Example 4
Compa rativ e Steel 1	60	40		7	9	598	680	30	23	Compara tiv Example 5e
Compa rativ e Steel 2	71	29	-	8	11	573	654	40	95	Compara tive Example 6

F: Ferrite, B: Bainite, P: Pearlite

As can be seen from Table 4 above, in the case of Inventive Examples 1 and 2, which satisfy both the alloy composition and manufacturing conditions proposed by the present disclosure, excellent strength, elongation, and low-temperature ductility fracture rate were exhibited even without adding expensive elements such as Ni or the like.
In contrast, in the case of Comparative Examples 1 to 5, at least one of the alloy composition and manufacturing conditions deviates from the conditions proposed by the present disclosure, and thus the strength was inferior to the specifications.
In particular, in the case of Comparative Example 6, the material was satisfactory, but Ni and Mo were added in excessive amounts, resulting in poor economic efficiency.
As described above, in the detailed description of the present disclosure, preferred embodiments of the present disclosure have been described, but it is obvious that various modifications are possible within the scope of the present disclosure by those skilled in the art to which the present disclosure belongs. Therefore, the scope of the rights of the present disclosure should not be limited to the described embodiments, but should be defined by not only the claims described below but also equivalents thereof.

Claims

A steel material for pipelines, comprising:
by weight %, C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), and Cu: 0.010% or less (excluding 0%), and Fe and unavoidable impurities as a remainder,

wherein the steel material comprises,

as a microstructure, by area %, at least one selected from the group consisting of 80% or more (including 100%) of ferrite, and pearlite and bainite as remainders, and

a surface layer portion having an average grain size of 10 to 15 µm in a thickness direction.
The steel material of claim 1, wherein an average grain size of a central portion of the steel in the thickness direction is larger than the average grain size of the surface layer portion.
The steel material of claim 1, wherein an average grain size of a central portion of the steel in the thickness direction is more than 15µm and less than or equal to 30µm.
The steel material of claim 1, wherein the average grain size of the surface layer portion is 11 to 14.5µm.
The steel material of claim 1, wherein the average grain size of a central portion of the steel in the thickness direction is 16 to 25µm.
The steel material of claim 2, wherein a difference between the average grain size of the central portion and the average grain size of the surface layer portion exceeds 5µm.
The steel material of claim 2, wherein a difference between the average grain size of the central portion and the average grain size of the surface layer portion is 6µm or more.
The steel material of claim 1, wherein the surface layer portion represents a region corresponding to 15% of a total thickness of the steel material in the thickness direction from at least one surface of the steel material.
The steel material of claim 2, wherein the surface layer portion represents a region corresponding to 15% of a total thickness of the steel material in the thickness direction from at least one surface of the steel material, and
the central portion represents a remaining center region excluding the surface layer portion for the total thickness in the thickness direction of the steel material.
The steel material of claim 1, wherein a carbon equivalent (Ceq) defined by the following relationship 1 satisfies 0.45 or less (excluding 0), $Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5,$ where each of the [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] represents a content (weight %) of an element in a corresponding parentheses.
A method for manufacturing a steel material for a pipeline, comprising:
an operation of reheating a slab at 1050 to 1250°C, the slab comprising, in wt%, C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), and Cu: 0.010% or less (excluding 0%), and a remainder of Fe and unavoidable impurities;

an operation of rough rolling the reheated slab and then finish rolling at an austenite single-phase temperature, and obtaining a thick plate steel material;

a first cooling operation of water-cooling the thick plate steel material to a temperature of 700 to 800°C at an average cooling rate of 5 to 25°C/s after completion of the finish rolling;

a second cooling operation of air-cooling the first-cooled steel material; and

a third cooling operation of water-cooling the second-cooled steel material to a temperature of 300 to 500°C at an average cooling rate of more than 25°C/s and less than or equal to 60°C/s.
The method for manufacturing a steel material for a pipeline of claim 11, wherein an average cooling rate in the third cooling is 30 to 60°C/s.
The method for manufacturing a steel material for a pipeline of claim 11, wherein in the second cooling operation, the steel material is maintained for 3 seconds or more under conditions of the air cooling.
The method for manufacturing a steel material for a pipeline of claim 11, wherein a cooling start temperature in the first cooling operation is 750 to 850°C.