EP4079906A1

EP4079906A1 - Structural steel material and method for manufacturing same

Info

Publication number: EP4079906A1
Application number: EP20904167.2A
Authority: EP
Inventors: Kyung-Keun Um; Hae-Sun MUN
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2019-12-16
Filing date: 2020-12-15
Publication date: 2022-10-26
Also published as: WO2021125748A1; CN114729434A; JP2023507946A; JP7395750B2; KR102255828B1; EP4079906A4

Abstract

A structural steel material according to an aspect of the present invention contains, in weight percent, 0.12-0.18% of C, 0.02-0.5% of Si, 0.6-1.6% of Mn, 0.002-0.06% of Sol. Al, 0.001-0.05% of Nb, 0.001-0.06% of V, 0.003-0.009% of Ti, 0.0002-0.006% of Ca, 0.0002-0.0005% of B, 0.001-0.006% of N, 0.02% or less of P, and 0.003% or less of S, with the remainder comprising Fe and inevitable impurities, and satisfies relational expression 1 below. The microstructure of the structural steel material includes a composite structure in which ferrite is the main phase and pearlite is the second phase, with the remainder comprising a hard structure, wherein the average grain size of the ferrite may be 20 µm or less. [Relational expression 1] [N] - 0.3*[Ti] - 0.1*[Nb] ≤ 0.001(wt%), where [N], [Ti], and [Nb] represent the contents (wt%) of N, Ti, and Nb in the steel material, respectively.

Description

[Technical Field]

The present disclosure relates to a structural steel material and a method for manufacturing the structural steel material, and more particularly, to a normalized structural thick plate, having outstanding low-temperature impact toughness after plastic deformation, and a method for manufacturing the normalized structural thick plate.

[Background Art]

Various technical methods have been proposed to increase the economic feasibility of entire projects for large structures such as ships or offshore plants.
In the related art, welding has been mainly used to manufacture structures having a curved surface such as columns, but welding is disadvantageous in terms of production time and costs. As one way to address this problem, a technique of manufacturing a steel material having a curved surface through hot bending or cold bending has been developed. However, plastically deformed steel materials obtained by the technique have poor impact toughness and thus may fail to provide suitable physical properties for large structures such as ships or offshore plants.
In general, as a result of plastic deformation, the low-temperature impact toughness of steel materials decreases because of the following reasons. When a steel material is plastically deformed, dislocations occur in the microstructure of the steel material to absorb the deformation, and the dislocations accumulate along grain boundaries. In particular, this phenomenon mainly occurs in a ferrite structure having low strength, and the strength of the steel material increases by the occurrence and accumulation of dislocations. Thereafter, when the steel material is subjected to an impact at low temperature, the strain-absorbing capacity of the steel material is easily saturated, and premature fracture may occur. In addition, factors that further decrease low-temperature impact toughness after plastic deformation are: the formation of pearlite, which is a hard phase, or the formation of retained martensite-austenite constituents (MA); dissolved carbon (C) and nitrogen (N) that interfere with the movement of dislocations; etc.
Therefore, in the related art, there has been a technique of securing strength by reducing the amount of carbon (C) and adding expensive elements such as copper (Cu), nickel (Ni), chromium (Cr), or molybdenum (Mo), or a technique of adding a large amount of titanium (Ti) to precipitate dissolved carbon (C) and nitrogen (N) because it is difficult to control the content of nitrogen (N) below a certain limit. However, these techniques are all undesirable in terms of economic feasibility because expensive elements are added in large amounts.
In particular, compared to steel materials produced through a controlled thermomechanical process, steel materials produced through a normalizing process have a relatively coarse structure and may be disadvantageous in terms of securing low-temperature impact toughness, particularly low-temperature impact toughness after plastic deformation because a relatively large amount of carbon (C) is added to secure strength. Therefore, it is urgent to develop a steel material that is effectively prevented from decreasing in low-temperature impact toughness after plastic deformation by applying a normalizing heat treatment while guaranteeing economic feasibility by excluding the addition of large amounts of expensive alloying elements.

(Prior Art Document)

(Patent Document 1) Korean Patent Application Laid-Open Publication No. 10-2012-0087686 (laid open on August 07, 2012 )

[Disclosure]

[Technical Problem]

An aspect of the present disclosure may provide a normalized structural thick plate having outstanding low-temperature impact toughness after plastic deformation and a method for manufacturing the normalized structural thick plate.
Aspects of the present disclosure are not limited thereto. Those of ordinary skill in the art will have no difficulty in understanding other aspects of the present disclosure from the detailed description of the present specification.

[Technical Solution]

According to an aspect of the present disclosure, a structural steel material may include, by wt%, C: 0.12% to 0.18%, Si: 0.02% to 0.5%, Mn: 0.6% to 1.6%, sol.Al: 0.002% to 0.06%, Nb: 0.001% to 0.05%, V: 0.001% to 0.06%, Ti: 0.003% to 0.009%, Ca: 0.0002% to 0.006%, B: 0.0002% to 0.0005%, N: 0.001% to 0.006%, P: 0.02% or less, S: 0.003% or less, and a balance of Fe and inevitable impurities, and may satisfy Relational expression 1 below, wherein the structural steel material may have a multi-phase microstructure including ferrite as a main phase, pearlite as a secondary phase, and a hard structure as a remainder, and the ferrite may have an average grain size of 20 µm or less, $[N] - 0.3 * [Ti] - 0.1 * [Nb] \leq 0.001 (wt %)$
where [N], [Ti], and [Nb] respectively refer to the contents (wt%) of N, Ti, and Nb in the structural steel material.
The ferrite may be included in an area fraction of 80 area% or more.
The hard structure may include at least one selected from the group consisting of bainite, martensite-austenite constitutes, and cementite, and the hard structure may be included in an area fraction of 5 area% or less.
The ferrite may have an average grain size within a range of greater than 10 µm but less than or equal to 20 µm.
The structural steel material may further include at least one selected from the group consisting of Cu, Ni, Cr, and Mo, and the total content of Cu, Ni, Cr, and Mo may satisfy Relational expression 2 below: $0 (wt %) < [Cu] + [Ni] + [Cr] + [Mo] \leq 0.08 (wt %)$
where [Cu], [Ni], [Cr], and [Mo] respectively refer to contents (wt%) of Cu, Ni, Cr, and Mo in the structural steel material.
The structural steel material may have a yield strength of 310 MPa or more, a yield ratio of 0.75 or less, and an elongation of 25% or more.
The structural steel material may have Charpy impact absorption energy of 200 J or more at -40°C.
According to another aspect of the present disclosure, a method for manufacturing a structural steel material may include: reheating a slab to a temperature of 1080°C to 1250°C, wherein the slab may include, by wt%, C: 0.12% to 0.18%, Si: 0.02% to 0.5%, Mn: 0.6% to 1.6%, sol.Al: 0.002% to 0.06%, Nb: 0.001% to 0.05%, V: 0.001% to 0.06%, Ti: 0.003% to 0.009%, Ca: 0.0002% to 0.006%, B: 0.0002% to 0.0005%, N: 0.001% to 0.006%, P: 0.02% or less, S: 0.003% or less, and a balance of Fe and inevitable impurities, and the slab may satisfy Relational expression 1 below; providing an intermediate material by performing a controlled rolling process on the reheated slab at a finish rolling temperature of 800°C to 950°C; and providing a final product by normalizing the intermediate material within a temperature range of 850°C to 950°C for 1.3*t + (10 to 30) minutes, where t refers to a thickness of the intermediate material in millimeters (mm). $[N] - 0.3 * [Ti] - 0.1 * [Nb] \leq 0.001 (wt %)$
where [N], [Ti], and [Nb] respectively refer to contents (wt%) of N, Ti, and Nb in the slab.
The slab may further include at least one selected from the group consisting of Cu, Ni, Cr, and Mo, and the total content of Cu, Ni, Cr, and Mo may satisfy Relational expression 2 below: $0 (wt %) < [Cu] + [Ni] + [Cr] + [Mo] \leq 0.08 (wt %)$
where [Cu], [Ni], [Cr], and [Mo] respectively refer to contents (wt%) of Cu, Ni, Cr, and Mo in the slab.
When the intermediate material has a thickness greater than 25 mm, the method may further include, after the controlled rolling process, performing an accelerated cooling process on the intermediate material at a cooling rate of 5°C/s or more to a temperature of 750°C or less.
Aspects of the present disclosure are not limited to the aspects described above, and various features of the present disclosure and advantages and effects by the features may be understood in more detail with reference to specific embodiments described below.

[Advantageous Effects]

An aspect of the present disclosure may provide a normalized structural thick plate having outstanding low-temperature impact toughness after plastic deformation and economic feasibility as well, and a method for manufacturing the normalized structural thick plate.
Effects of the present disclosure are not limited thereto and may include other effects which those or ordinary skill in the art may infer from the following description.

[Best Mode]

The present disclosure relates to a structural steel material and a method for manufacturing the structural steel material, and preferred embodiments of the present disclosure will be described below. Embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. Embodiments are provided to describe details of the present disclosure to those of ordinary skill in the art to which the present disclosure pertains.
Hereinafter, reasons for limiting the contents of alloying elements in the present disclosure will be described. Herein, unless otherwise specified, "%" and "ppm" related to the contents of alloying elements are based on weight.
According to an aspect of the present disclosure, a structural steel material may include, by wt%, C: 0.12% to 0.18%, Si: 0.02% to 0.5%, Mn: 0.6% to 1.6%, sol.Al: 0.002% to 0.06%, Nb: 0.001% to 0.05%, V: 0.001% to 0.06%, Ti: 0.003% to 0.009%, Ca: 0.0002% to 0.006%, B: 0.0002% to 0.0005%, N: 0.001% to 0.006%, P: 0.02% or less, S: 0.003% or less, and a balance of Fe and inevitable impurities, and the structural steel material may satisfy Relational expression 1 below. $[N] - 0.3 * [Ti] - 0.1 * [Nb] \leq 0.001 (wt %)$
In Relational expression 1, [N], [Ti], and [Nb] respectively refer to the contents (wt%) of N, Ti, and Nb contained in the structural steel material.
In addition, the structural steel material according to the aspect of the present disclosure may further include at least one selected from the group consisting of Cu, Ni, Cr, and Mo, and the total content of Cu, Ni, Cr, and Mo may satisfy Relational expression 2 below. $0 (wt %) < [Cu] + [Ni] + [Cr] + [Mo] \leq 0.08 (wt %)$
In Relational expression 2, [Cu], [Ni], [Cr], and [Mo] respectively refer to the contents (wt%) of Cu, Ni, Cr, and Mo contained in the structural steel material, and 0% is substituted for an element which is not included.

Carbon (C): 0.12% to 0.18%

Carbon (C) may be the most economical element for securing the strength of steel, and thus in the present disclosure, carbon (C) may be added for this effect in an amount of 0.12% or more. The content of carbon (C) may preferably be greater than 0.12%, and more preferably 0.125% or more. In addition, carbon (C) forms pearlite, cementite, or martensite-austenite constituents (MA) in normalized steel, and is thus commonly used as an element for securing tensile strength. However, it is preferable to limit the content of carbon (C) to a certain range so as to secure low-temperature impact toughness after plastic deformation as desired in the present disclosure. The reason for this is that when the content of carbon (C) exceeds a certain range, hard phases may be formed in large amounts, and these hard phases may be present in a band shape in a rolled steel material, reducing low-temperature impact toughness. Therefore, in the present disclosure, the upper limit of the content of carbon (C) may be set to be 0.18%. The content of carbon (C) may be preferably less than 0.18%, and more preferably 0.17% or less.

Silicon (Si): 0.02% to 0.5%

Silicon (Si) is an element added for deoxidation, desulfurization, and solid solution strengthening, and in the present disclosure, silicon (Si) may be added in an amount of 0.02% or more to achieve these effects. The lower limit of the content of silicon (Si) may be preferably 0.022%, and more preferably 0.024%. In addition, when the content of silicon (Si) exceeds a certain range, weldability and low-temperature impact properties may deteriorate, and the surface of a manufactured steel sheet may easily be oxidized, excessively forming an oxide film. Thus, in the present disclosure, the upper limit of the content of silicon (Si) may be set to be 0.5%. The upper limit of the content of silicon (Si) may be preferably 0.47%, and more preferably 0.44%.

Manganese (Mn): 0.6% to 1.6%

Manganese (Mn) is an element contributing to solid solution strengthening, and thus in the present disclosure, manganese (Mn) may be added in an amount of 0.6% or more for the effect of increasing strength. The lower limit of the content of manganese (Mn) may be preferably 0.8%, and more preferably 1.0%. However, when manganese (Mn) is excessively added, low-temperature impact toughness may be greatly reduced because the formation of MnS, a non-metallic inclusion, is propelled in a thickness direction center portion of a steel sheet. Thus, in the present disclosure, the upper limit of the content of manganese (Mn) may be set to be 1.6%. The upper limit of the content of silicon (Si) may be preferably 1.57%, and more preferably 1.55%.

Soluble aluminum (sol.Al): 0.002% to 0.06%

In steelmaking processes, aluminum (Al) is used as a strong deoxidizer like silicon (Si) and manganese (Mn). In the present disclosure, aluminum (Al) may be added in an amount of 0.002% or more to achieve this effect. The lower limit of the content of aluminum (Al) may be preferably 0.005%, and more preferably 0.01%. However, when aluminum (Al) is excessively added, the deoxidation effect is saturated, and the fraction of Al₂O₃ in oxide inclusions generated as a result of deoxidation markedly increases more than necessary, thereby coarsening the oxide inclusions and excessively increasing molten steel refining process loads. Therefore, in the present disclosure, the upper limit of the content of aluminum (Al) may be set to be 0.06%. The upper limit of the content of aluminum (Al) may be preferably 0.055%, and more preferably 0.05%.

Niobium (Nb): 0.001% to 0.05%,

During slab reheating, niobium (Nb) dissolves in austenite and increases the hardenability of austenite, and during hot rolling, niobium (Nb) precipitates at high temperature as a carbonitride coherent on the matrix of steel and suppresses recrystallization, effectively contributing to the refinement of a final microstructure. In addition, even during transformation after cooling, niobium (Nb) forms fine precipitates having a size of 100 nm or less, markedly contributing to an increase in strength. Therefore, in the present disclosure, niobium (Nb) may be added in an amount of 0.001% or more to achieve these effects. However, when niobium (Nb) is excessively added, coarse precipitates are easily formed in a thickness direction center portion, and the hardenability of a weld zone is increased more than necessary, thereby decreasing low-temperature impact toughness. Thus, in the present disclosure, the upper limit of the content of niobium (Nb) may be set to be 0.05%. The content of niobium (Nb) may be preferably less than 0.05%, and more preferably 0.047% or less.

Vanadium (V): 0.001% to 0.06%

Vanadium (V) does not significantly contribute to precipitation strengthening or solid-solution strengthening during a rolling process because almost all of vanadium (V) dissolves again during a slab reheating process, but vanadium (V) precipitates as a very fine carbonitride and improves strength in a later tempering process or a post-welding heat treatment process. Therefore, in the present disclosure, vanadium (V) may be added in an amount of 0.001% or more to achieve this effect. The content of vanadium (V) may be preferably greater than 0.001%, and more preferably 0.0015% or more. However, since vanadium (V) is an expensive element, the upper limit of the content of vanadium (V) may be set to be 0.06% by considering economic feasibility. More preferably, the upper limit of the content of vanadium (V) may be set to be 0.05%.

Titanium (Ti): 0.003% to 0.009%

Titanium (Ti) combines with nitrogen (N) contained in steel and forms a nanosized nitride, effectively reducing the amount of nitrogen (N) dissolved in steel. The addition of titanium (Ti) reduces the amount of dissolved nitrogen (N), effectively suppressing a decrease in low-temperature impact toughness after plastic deformation. Furthermore, the addition of titanium (Ti) effectively suppresses the occurrence of cracks in the surface of the steel material. Therefore, in the present disclosure, titanium (Ti) may be added in an amount of 0.003% or more to achieve these effects. Preferably, the content of titanium (Ti) may be greater than 0.003%, and more preferably, the lower limit of the content of titanium (Ti) be 0.004%. However, when the content of titanium (Ti) is greater than a certain value, the fraction of TiN precipitates having a coarse hexagonal structure and acting as a crack initiation point may increase, thereby decreasing low-temperature impact toughness. Thus, in the present disclosure, the upper limit of the content of titanium (Ti) may be set to be 0.009%. The content of titanium (Ti) may be preferably less than 0.009%, and more preferably 0.008% or less.

Calcium (Ca): 0.0002% to 0.006%

Calcium (Ca) combines with sulfur (S) of MnS which is a non-metallic inclusion, thereby suppressing the formation of MnS and forming spherical CaS which suppresses hydrogen cracking. Therefore, in the present disclosure, calcium (Ca) may be added in an amount of 0.0002% or more to achieve these effects. The lower limit of the content of calcium (Ca) may be preferably 0.0003%, and more preferably 0.0005%. However, when calcium (Ca) is excessively added, surplus calcium (Ca) combines with oxygen (O) to form a coarse oxide inclusion, which may increase susceptibility to cracking as being stretched and fractured in a subsequent rolling process. Therefore, in the present disclosure, the upper limit of the content of calcium (Ca) may be set to be 0.006%. The upper limit of the content of calcium (Ca) may be preferably 0.005%, and more preferably 0.004%.

Boron (B): 0.0002% to 0.0005%

Boron (B) is a typical hardenability improving element, and even a small amount of boron (B) segregates along austenite grain boundaries and strongly suppresses nucleation of ferrite during cooling. That is, the addition of boron (B) markedly reduces the ferrite transformation initiation temperature, thereby lowering the growth rate of ferrite and effectively guaranteeing final ferrite grain refinement. In addition, since the present disclosure involves normalizing, the lower limit of the content of boron (B) for obtaining the effect of suppressing the nucleation of ferrite may be set to be 0.0002% by considering the grain size of austenite which regenerates at a normalizing temperature. Preferably, the content of boron (B) may be 0.0003% or more. However, when the content of boron (B) is greater than a certain value, hardenability increases greatly. In this case, bainite may be formed instead of pearlite in a base metal and also in a heat-affected zone in a later welding process, or the possibility of martensite formation increases in segregation zones, thereby deteriorating low-temperature impact toughness. Therefore, in the present disclosure, the upper limit of the content of boron (B) may be set to be 0.0005%. The content of boron (B) may be preferably 0.0004% or less.

Nitrogen (N): 0.001% to 0.006%

Nitrogen (N) forms precipitates together with added niobium (Nb) and titanium (Ti), thereby refining the grains of steel and improving the strength and toughness of a base material. Therefore, in the present disclosure, nitrogen (N) may be added in an amount of 0.001% or more to achieve these effects. The content of nitrogen (N) may be preferably 0.0015% or more, and more preferably 0.002% or more. However, when nitrogen (N) is added excessively, the amount of dissolved nitrogen increases, and thus the deformation absorption capacity of the steel material is easily saturated, thereby causing embrittlement. Therefore, in the present disclosure, the upper limit of the content of nitrogen (N) may be set to be 0.006%. The upper limit of the content of nitrogen (N) may be preferably 0.0055%, and more preferably 0.005%.
Furthermore, in the present disclosure, the relative contents of nitrogen (N), titanium (Ti), and niobium (Nb) may be limited as shown in Relational expression 1 below. $[N] - 0.3 * [Ti] - 0.1 * [Nb] \leq 0.001 (wt %)$
In Relational expression 1, [N], [Ti], and [Nb] respectively refer to the contents (wt%) of N, Ti, and Nb contained in the steel material.
Dissolved nitrogen (N) is adhered to dislocations and hinders movements of the dislocations, which may decrease the low-temperature impact toughness of steel material. Therefore, in order to reduce the amount of dissolved nitrogen (N), the content of nitrogen (N) is controlled to be as low as possible during steelmaking, and the contents of titanium (Ti) and niobium (Nb) which react with nitrogen (N) and form precipitates are considered when controlling the content of nitrogen (N). That is, in the present disclosure, the relative contents of nitrogen (N), titanium (Ti), and niobium (Nb) are limited as shown in Relational expression 1, and thus the content of dissolved nitrogen (N) may be limited to an optimal value. According to Relational expression 1, the content of dissolved nitrogen (N) may be 0.001 wt% or less, and more preferably 0 wt% or less.

Phosphorus (P): 0.02% or less

Phosphorus (P) somewhat contributes to increasing the strength of steel, but segregates along grain boundaries and thus greatly reduces low-temperature toughness. Thus, it is preferable to adjust the content of phosphorus (P) to be as low as possible. However, phosphorus (P) is an inevitable impurity element, and it costs a lot to completely remove phosphorus (P) in steelmaking processes. Thus, in the present disclosure, the upper limit of the content of phosphorus (P) may be set to be 0.02%.

Sulfur (S): 0.003% or less

Sulfur (S) combines with manganese (Mn) and forms MnS inclusion in a thickness direction center portion of a steel sheet, thereby reducing low-temperature impact toughness and being considered a typical factor promoting the occurrence and propagation of hydrogen-induced cracks. Therefore, in order to secure the low-temperature impact toughness and hydrogen-induced cracking resistance of the steel material, it is preferable to adjust the content of sulfur (S) to be as low as possible. However, sulfur (S) is also an inevitable impurity element, and it costs a lot to completely remove phosphorus (P) in steelmaking processes. Thus, in the present disclosure, the upper limit of the content of sulfur (S) may be set to be 0.003%. Preferably, the upper limit of the content of sulfur (S) may be 0.002%.
Copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo): 0.08% or less in total.
Copper (Cu) may greatly improve the strength of the steel material by solid solution strengthening and precipitation strengthening, and has an effect of suppressing corrosion of the steel material in a wet hydrogen sulfide atmosphere. However, copper (Cu) is expensive, and the addition of copper (Cu) may cause surface cracks. Thus, in the present disclosure, copper (Cu) is not intentionally added.
Nickel (Ni) does not significantly contribute to increasing the strength of the steel material, but is effective in improving low-temperature impact toughness. However, since nickel (Ni) is an expensive element, nickel (Ni) is not intentionally added in the present disclosure.
Chromium (Cr) has a small effect of increasing strength by solid solution strengthening, but has an effect of preventing a decrease in strength by lowering the rate of cementite decomposition during a tempering process or a post-welding heat treatment process. However, since chromium (Cr) is an expensive element, chromium (Cr) is not intentionally added in the present disclosure.
Like chromium (Cr), molybdenum (Mo) is an effective alloying element for preventing a decrease in strength during a tempering process or a post-welding heat treatment process. However, since molybdenum (Mo) is also an expensive element, chromium (Cr) is not intentionally added in the present disclosure.
That is, in the present disclosure, the total content of copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo) is limited to 0.08% or less as shown in Relational expression 2 below to secure economic feasibility, and strength and low-temperature impact toughness are guaranteed to be equal to or greater than certain levels by controlling the contents of other elements other than aforementioned elements and process conditions. Preferably, the total content of copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo) may be 0.06% or less, and more preferably 0.04% or less. In addition, according to Relational expression 2, the total content of copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo) is greater than 0%. However, this condition does not mean intentional addition of these elements, but only indicates inevitable inclusion of these elements during steelmaking processes. $0 (wt %) < [Cu] + [Ni] + [Cr] + [Mo] \leq 0.08 (wt %)$
In Condition 2, [Cu], [Ni], [Cr], and [Mo] respectively refer to the contents (wt%) of Cu, Ni, Cr, and Mo in the steel material.
According to the aspect of the present disclosure, the structural steel material may include a balance of Fe and other inevitable impurities in addition to the above-described elements. However, impurities contained in raw materials or surroundings may be unintendedly introduced into the structural steel material during normal manufacturing processes, and such impurities may not be entirely excluded. Such impurities are known to those of ordinary skill in the art, and thus may not be particularly specified in the present disclosure. In addition to the elements described above, other effective elements may also be added.
According to the aspect of the present disclosure, the structural steel material may have a multi-phase microstructure, which includes ferrite as a primary phase, pearlite as a secondary phase, and a hard structure as a remainder.
The fraction of ferrite, which is a main phase, is 80 area% or more, and the upper limit thereof is not specifically limited. In order to secure the strength and low-temperature impact toughness of the steel material, the average grain size of ferrite may be preferably 20 µm or less, and more preferably 18 µm or less. The lower limit of the average grain size of ferrite is not particularly limited. However, since the steel material of the present disclosure is manufactured through normalizing, the average grain size of ferrite inevitably has a certain value or more. Therefore, in the present disclosure, the average grain size of ferrite may be greater than 10 µm, and more preferably, the average grain size of ferrite may be greater than 12 µm.
The hard structure, which is a remainder, may include at least one selected from the group consisting of bainite, martensite-austenite constitutes, and cementite. Although the hard structure effectively contributes to improving the strength of the steel material, the hard structure is a major cause of lowering the low-temperature impact toughness of the steel material. Thus, in the present disclosure, the fraction of the hard structure may be limited to 5 area% or less. More preferably, the upper limit of the fraction of hard tissue may be 4 area%. Furthermore, in the present disclosure, the lower limit of the fraction of the hard structure is not particularly specified. However, in a non-limiting example, the fraction of the hard structure may be 3 area% or more.
According to the aspect of the present disclosure, the structural steel material may have a yield strength of 310 MPa or more, a yield ratio of 0.75 or more, an elongation of 25% or more, and Charpy impact absorption energy of 200 J or more at -40°C.
Hereinafter, the manufacturing method of the present disclosure will be described in more detail.
According to an aspect of the present disclosure, a method for manufacturing a structural steel material may include: reheating a slab to a temperature of 1080°C to 1250°C, wherein the slab includes, by wt%, C: 0.12% to 0.18%, Si: 0.02% to 0.5%, Mn: 0.6% to 1.6%, sol.Al: 0.002% to 0.06%, Nb: 0.001% to 0.05%, V: 0.001% to 0.06%, Ti: 0.003% to 0.009%, Ca: 0.0002% to 0.006%, B: 0.0002% to 0.0005%, N: 0.001% to 0.006%, P: 0.02% or less, S: 0.003% or less, and a balance of Fe and inevitable impurities, wherein the slab satisfies Relational expression 1 below and further includes at least one selected from the group consisting of Cu, Ni, Cr, and Mo, and the total content of Cu, Ni, Cr, and Mo satisfies Relational expression 2 below: providing an intermediate material by performing a controlled rolling process on the reheated slab at a finish rolling temperature of 800°C to 950°C; and providing a final product by normalizing the intermediate material within a temperature range of 850°C to 950°C for 1.3*t + (10 to 30) minutes, where t refers to a thickness of the intermediate material in millimeters (mm). $[N] - 0.3 * [Ti] - 0.1 * [Nb] \leq 0.001 (wt %)$
In Relational expression 1, [N], [Ti], and [Nb] respectively refer to the contents (wt%) of N, Ti, and Nb in the slab. $0 (wt %) < [Cu] + [Ni] + [Cr] + [Mo] \leq 0.08 (wt %)$
In Relational expression 2, [Cu], [Ni], [Cr], and [Mo] respectively refer to the contents (wt%) of Cu, Ni, Cr, and Mo in the slab.

Slab Reheat

A slab having a given composition is prepared and reheated to the temperature range of 1080% to 1250°C. The composition of the slab corresponds to the composition of the steel material described above, and thus the above description of the composition of the steel sheet may be referred to for understanding the composition of the slab.
The lower limit of the slab reheating temperature may be set to be a certain value or more in order to re-dissolve carbides or the like formed in the slab during continuous casting. In particular, since titanium (Ti), niobium (Nb), and the like are added in the present disclosure, the lower limit of the slab reheating temperature may be set to be 1080°C so as to sufficiently re-dissolve these elements. However, when the slab reheating temperature exceeds a certain range, austenite grains may be coarsely formed, deteriorating mechanical properties of a final steel material such as tensile strength and low-temperature impact toughness. Thus, in the present disclosure, the upper limit of the slab reheating temperature may be set to be 1250°C.

Controlled Rolling

The reheated slab may be controlled rolled at a finish rolling temperature of 800°C to 950°C to provide an intermediate material. If normal rolling is applied to the reheated slab, rolling finishes at an excessively high temperature, and thus sufficient grain refinement may not be achieved. In addition, if the controlled rolling is performed within an excessively low temperature range, redissolved niobium (Nb) or the like precipitates as a carbonitride which markedly decreases the effect of suppressing the growth of austenite grains in a subsequent normalizing heat treatment process, and moreover, coarse composite inclusions generated in a refining process are segmented into small inclusions or are elongated by the controlled rolling to result in a decrease in low-temperature impact toughness. Therefore, in the present disclosure, controlled rolling may be performed on the slab while limiting the finish rolling temperature of the controlled rolling to the range of 800°C to 950°C by considering the above-mentioned matters.
In addition, when the thickness of the intermediate material exceeds 25 mm, the method may further include, after the controlled rolling, an accelerated cooling process to cool the intermediate material to a temperature of 750°C or less at a cooling rate of 5°C/s or more. Since the accelerated cooling process is performed at a cooling rate greater than that of air cooling, the growth of austenite in which strain energy is accumulated may be suppressed after ferrite nucleation, thereby obtaining smaller grains. Therefore, the effect of grain refinement may remain even after a final normalizing heat treatment, and as a result, strength and toughness may both be improved.

Normalizing

The rolled intermediate material is subjected to a normalizing heat treatment within the temperature range of 850°C to 950°C for 1.3*t + (10 to 30) minutes (here, t refers to the thickness (mm) of the intermediate material) to provide a final product. The lower limit of the normalizing heat treatment temperature may be set to be 850°C in order to secure the strength of the steel material by re-dissolving solute elements. In addition, the upper limit of the normalizing heat treatment temperature may be set to be 950°C in order to prevent a decrease in low-temperature impact toughness which is caused by grain growth. In addition, if the normalizing heat treatment time is excessively short, it is difficult to obtain a uniform microstructure, and if the normalizing heat treatment time is excessively long, it is undesirable in terms of productivity. Thus, in the present disclosure, the normalizing heat treatment time may be limited to 1.3*t + (10 to 30) minutes (here, t refers to the thickness (mm) of the intermediate material).
The steel material manufactured by the manufacturing method described above may have a composite microstructure including ferrite as a main phase, pearlite as a secondary phase, and a hard structure as a remainder, wherein the fraction of the ferrite may be 80 area% or more, and the fraction of the hard structure may be 5% or less. Here, the hard structure may include at least one selected from the group consisting of bainite, martensite-austenite constituents, and cementite.
In addition, the steel material manufactured by the manufacturing method described above may have a yield strength of 310 MPa or more, a yield ratio of 0.75 or more, an elongation of 25% or more, and Charpy impact energy of 200 J or more at -40°C.

[Mode for Invention]

Hereinafter, the present disclosure will be described in more detail through an example. However, it should be noted that the example described below is for illustrative purposes only and is not intended to limit the scope of the present disclosure.

(Example)

Slabs having the compositions as shown in Tables 1 and 2 below were prepared and subjected to slab reheating, controlled rolling, and normalizing under the conditions shown in Table 3 below to prepare samples.

[Table 1]

Steels	Composition (wt%)
Steels	C	Si	Mn	P	S	sol.Al	Ti	Nb	V	B	N	Ca
A	0.13	0.36	1. 54	0.008	0.0012	0.028	0.007	0.027	0.002	0.0003	0.0029	0.0014
B	0.15	0.27	1.24	0.011	0.0018	0.034	0.005	0.045	0.013	0.0004	0.0035	0.0015
C	0.19	0.35	1. 45	0.005	0.0008	0.036	0.004	0.021	0.005	0.0003	0.0031	0.0011
D	0.12	0.17	1.55	0.021	0.0021	0.022	0.001	0.032	0.031	0.0004	0.0041	0.0015
E	0.14	0.45	1. 45	0.015	0.0007	0.032	0.014	0.022	0.021	0.0005	0.0044	0.0021
F	0.15	0.35	1.22	0.005	0.0013	0.021	0.001	0.033	0.004	0	0.0032	0.0013
G	0.11	0.42	1. 34	0.005	0.0003	0.016	0.001	0.027	0.028	0.0012	0.0028	0.0011
H	0.12	0.22	1. 54	0.008	0.0022	0.025	0.004	0.018	0.001	0.0004	0.0052	0.0012

[Table 2]

Steels	Composition (wt%)
Steels	Cu	Ni	Cr	Mo	[Relational expression 1]	[Relational expression 2]
A	0.01	0.01	0.02	0.004	-0.0019	0.044
B	0.02	0.01	0.01	0.006	-0.0025	0.046
C	0.01	0.02	0.03	0.009	-0.0002	0.069
D	0.01	0.01	0.01	0.003	0.0006	0.033
E	0.02	0.03	0.01	0.005	-0.002	0.065
F	0.01	0.02	0.03	0.009	-0.0004	0.069
G	0.03	0.01	0.01	0.01	-0.0002	0.06
H	0.02	0.02	0.01	0.01	0.0022	0.06

[Table 3]

Sample No.	Steels	Product thickness (mm)	Slab reheatin g temp. (°C)	Finish rolling temp. (°C)	Cooling rate (°C/s) after rolling	Cooling stop temp. (°C)	Normalizing temp. (°C)	Normalizing time (minute)
1	A	16	1134	888	Air cooling	-	896	44
2	B	48	1123	855	Air cooling	-	893	90
3	B	65	1158	823	8°C/s	510	896	111
4	A	73	1172	1010	Air cooling	-	905	116
5	B	48	1138	882	Air cooling	-	830	88
6	C	18	1160	851	Air cooling	-	896	46
7	D	22	1151	825	Air cooling	-	894	56
8	E	35	1138	935	Air cooling	-	892	69
9	F	48	1169	920	Air cooling	-	908	89
10	G	15	1154	826	Air cooling	-	903	47
11	H	16	1138	855	Air cooling	-	901	40

The microstructure of each sample was analyzed, and mechanical properties of each sample was measured. Results thereof are showing in Table 4 below. Each sample was prepared by ASTM E3 and etched by ASTM E407, and the types and fractions of microstructures of the samples were analyzed by ASTM E1245. A tensile test was performed at room temperature by ASTM E8, and Charpy impact absorption energy was measured at -40°C under the conditions specified in ASTM E23. In addition, the occurrence of cracks in each sample was observed, and the occurrence of surface cracks having a depth of 0.1 mm or more was marked with O.

[Table 4]

Sample No.	Microstructure				YS (MPa)	TS (MPa)	YR	E (%)	IAE (J) (@-40°C)	Surface cracking
Sample No.	Ferrite fraction (area%)	Pearlite fraction (area%)	Hard structure faction (area%)	Average Ferrite grain size (µm)	YS (MPa)	TS (MPa)	YR	E (%)	IAE (J) (@-40°C)	Surface cracking
1	85.3	11.1	3.4	16.9	331	474	0.70	27.7	232	X
2	82.7	13.2	3.9	16.6	366	531	0.69	26.7	205	X
3	82.7	13.4	3.9	15.5	367	532	0.69	26.7	201	X
4	85.3	11.0	3.4	27.7	305	448	0.68	27.7	232	X
5	82.7	12.6	3.9	18.3	365	530	0.69	26.7	20	X
6	74.7	14.8	5.6	14.7	320	518	0.62	25.6	17	X
7	85.3	10.3	3.1	13.3	344	476	0.72	28.3	253	O
8	84.0	11.4	3.6	18.9	319	473	0.67	27.1	22	×
9	82.7	13.1	3.9	26.3	303	468	0.65	26.7	301	∘
10	88.0	4.5	7.2	18.7	285	531	0.54	29.1	13	∘
11	86.7	9.5	3.1	24.8	305	437	0.70	28.3	25	×

* In Table 4, YS refers to yield strength, TS refers to tensile strength, YR refers to yield ratio, E refers to elongation, and IAE refers to impact absorption energy

As shown in Tables 1 to 4, Samples 1 to 3 satisfying the compositions and process conditions specified in the present disclosure have microstructures and mechanical properties as proposed in the present disclosure, whereas Samples 4 to 11 not satisfying one or more of the composition and process conditions specified in the present disclosure do not have the microstructure or mechanical properties proposed in the present disclosure.
Although embodiments of the present disclosure have been described in detail, other embodiments may also be possible. Therefore, the spirit and scope of the present disclosure defined by the appended claims are not limited to the embodiments.

Claims

A structural steel material comprising, by wt%, C: 0.12% to 0.18%, Si: 0.02% to 0.5%, Mn: 0.6% to 1.6%, sol.Al: 0.002% to 0.06%, Nb: 0.001% to 0.05%, V: 0.001% to 0.06%, Ti: 0.003% to 0.009%, Ca: 0.0002% to 0.006%, B: 0.0002% to 0.0005%, N: 0.001% to 0.006%, P: 0.02% or less, S: 0.003% or less, and a balance of Fe and inevitable impurities, and satisfying Relational expression 1 below,
wherein the structural steel material has a multi-phase microstructure comprising ferrite as a main phase, pearlite as a secondary phase, and a hard structure as a remainder, and

the ferrite has an average grain size of 20 µm or less, $[N] - 0.3 * [Ti] - 0.1 * [Nb] \leq 0.001 (wt %)$

where [N], [Ti], and [Nb] respectively refer to contents (wt%) of N, Ti, and Nb in the structural steel material.
The structural steel material of claim 1, wherein the ferrite is included in an area fraction of 80 area% or more.
The structural steel material of claim 1, wherein the hard structure comprises at least one selected from the group consisting of bainite, martensite-austenite constitutes, and cementite, and
the hard structure is included in an area fraction of 5 area% or less.
The structural steel material of claim 1, wherein the ferrite has an average grain size within a range of greater than 10 µm but less than or equal to 20 µm.
The structural steel material of claim 1, further comprising at least one selected from the group consisting of Cu, Ni, Cr, and Mo, and a total content of Cu, Ni, Cr, and Mo satisfies Relational expression 2 below: $0 (wt %) < [Cu] + [Ni] + [Cr] + [Mo] \leq 0.08 (wt %)$
where [Cu], [Ni], [Cr], and [Mo] respectively refer to contents (wt%) of Cu, Ni, Cr, and Mo in the structural steel material.
The structural steel material of claim 1, wherein the structural steel material has a yield strength of 310 MPa or more, a yield ratio of 0.75 or less, and an elongation of 25% or more.
The structural steel material of claim 1, wherein the structural steel material has Charpy impact absorption energy of 200 J or more at -40°C.
A method for manufacturing a structural steel material, the method comprising:
reheating a slab to a temperature of 1080°C to 1250°C, wherein the slab comprises, by wt%, C: 0.12% to 0.18%, Si: 0.02% to 0.5%, Mn: 0.6% to 1.6%, sol.Al: 0.002% to 0.06%, Nb: 0.001% to 0.05%, V: 0.001% to 0.06%, Ti: 0.003% to 0.009%, Ca: 0.0002% to 0.006%, B: 0.0002% to 0.0005%, N: 0.001% to 0.006%, P: 0.02% or less, S: 0.003% or less, and a balance of Fe and inevitable impurities, and the slab satisfies Relational expression 1 below;

providing an intermediate material by performing a controlled rolling process on the reheated slab at a finish rolling temperature of 800°C to 950°C; and

providing a final product by normalizing the intermediate material within a temperature range of 850°C to 950°C for 1.3*t + (10 to 30) minutes, where t refers to a thickness of the intermediate material in millimeters, $[N] - 0.3 * [Ti] - 0.1 * [Nb] \leq 0.001 (wt %)$

where [N], [Ti], and [Nb] respectively refer to contents (wt%) of N, Ti, and Nb in the slab.
The method of claim 8, wherein the slab further comprises at least one selected from the group consisting of Cu, Ni, Cr, and Mo, and a total content of Cu, Ni, Cr, and Mo satisfies Relational expression 2 below: $0 (wt %) < [Cu] + [Ni] + [Cr] + [Mo] \leq 0.08 (wt %)$
where [Cu], [Ni], [Cr], and [Mo] respectively refer to contents (wt%) of Cu, Ni, Cr, and Mo in the slab.
The method of claim 8, wherein when the intermediate material has a thickness greater than 25 mm, the method further comprises, after the controlled rolling process, performing an accelerated cooling process on the intermediate material at a cooling rate of 5°C/s or more to a temperature of 750°C or less.