CN116568847A

CN116568847A - Super-thick steel plate with excellent low-temperature impact toughness and manufacturing method thereof

Info

Publication number: CN116568847A
Application number: CN202180083754.6A
Authority: CN
Inventors: 金佑谦; 金相镐; 白大雨
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2020-12-15
Filing date: 2021-11-25
Publication date: 2023-08-08
Also published as: KR102498134B1; WO2022131608A1; EP4265795A1; KR20220085575A

Abstract

The present invention relates to structural steel that can be used as a material for, for example, marine, bridge and building applications, and more particularly, to an ultra-thick steel plate having excellent low-temperature impact toughness and a method of manufacturing the same.

Description

Super-thick steel plate with excellent low-temperature impact toughness and manufacturing method thereof

Technical Field

The present disclosure relates to structural steels that can be used as materials for, for example, oceans, bridges, and construction, and more particularly, to an ultra-thick steel plate having excellent low-temperature impact toughness and a method of manufacturing the same.

Background

An ultra-thick steel plate having a certain thickness or more may be manufactured by a thick plate process, and in this case, a rolling method may be classified into general rolling, normalizing rolling, thermo-mechanical controlled rolling (thermo-mechanical controlled rolling, TMCP) and the like. Further, the heat treatment process may be performed after the rolling, and in this case, the heat treatment process includes a normalizing heat treatment process, a quenching-tempering heat treatment, and the like.

In the above rolling process, general rolling is a method of rolling without controlling the rolling temperature, which can be mainly applied to general steels that do not require impact toughness.

In contrast, TMCP performs recrystallization zone rolling and non-recrystallization zone rolling by temperature control, and strength and impact toughness can be ensured by cooling as needed. However, when an ultra-thick material is manufactured through such TMCP process, a long waiting time is required to adjust the rolling temperature, resulting in serious degradation of productivity.

Normalizing rolling is accomplished at relatively high temperatures, and therefore strength and toughness may be reduced due to grain growth during air cooling.

Therefore, when manufacturing an ultra-thick steel plate through a TMCP process, a normalizing rolling process, or a heat treatment process after rolling, it is necessary to apply a high carbon component system containing 0.12% or more of C to secure strength, but since toughness is seriously deteriorated, impact toughness can be secured at room temperature and 0 ℃, and there is a problem in that the cost caused by heat treatment increases.

Meanwhile, the ultra-thick steel plate may be applied to various structural industries such as infrastructure industries (e.g., ships); as well as various frames for offshore structures, bridges, constructions, etc.; and wind power infrastructure, etc.

Recently, in most fields such as infrastructure industry, energy industry, etc., there is a trend of a larger structure due to minimization of installation cost and deterioration of installation environment, and it is expected that in structural steel plates used in various industrial fields, demand for ultra-thick steel plates having a thickness of 100mm or more will increase with the trend of a larger structure.

However, the super-thick steel plate has a metallurgical disadvantage in that it is difficult to achieve strength and secure toughness due to a reduction in rolling amount and a limitation of a cooling process.

Due to limitations of rolling and cooling processes in manufacturing such ultra-thick steel plates, there is a tendency that alloy components are excessively added to achieve the strength of the steel plates, which may cause problems of increased costs and rapid deterioration of toughness of the steel plates.

In addition, in the case of removing alloy components adversely affecting toughness to ensure toughness of the ultra-thick steel plate, a decrease in strength is caused.

Therefore, development of a technique that can achieve both strength and toughness of an ultra-thick steel plate is required.

(patent document 1) korean patent laid-open No. 10-2014-0003010

Disclosure of Invention

Technical problem

An aspect of the present disclosure is to provide an ultra-thick steel plate having excellent strength and low temperature impact toughness by overcoming the metallurgical disadvantages of the existing ultra-thick steel plate, and a method of manufacturing the same.

The purpose of the present disclosure is not limited to the above description. The objects of the present disclosure will be understood from the entire contents of the present specification, and further objects of the present disclosure will be understood by those skilled in the art to which the present disclosure pertains without difficulty.

Technical proposal

According to one aspect of the present disclosure, there is provided an ultra-thick steel plate having excellent low temperature impact toughness, comprising, by weight: 0.06% to 0.1% of carbon (C), 0.3% to 0.5% of silicon (Si), 1.35% to 1.65% of manganese (Mn), 0.015% to 0.04% of aluminum (soluble Al), 0.015% to 0.04% of niobium (Nb), 0.005% to 0.02% of titanium (Ti), 0.15% to 0.4% of chromium (Cr), 0.3% to 0.5% of nickel (Ni), 0.002% to 0.008% of nitrogen (N), 0.01% or less (excluding 0%) of phosphorus (P), 0.003% or less (excluding 0%) of sulfur (S), and the balance of iron (Fe) and unavoidable impurities, the super-thick steel plate satisfying the following formula 1,

wherein the ultra-thick steel plate comprises in area fraction: 80% to 90% ferrite and the remainder pearlite as microstructure.

[ relation 1]

Mn+5(Ni+Ct)>3.6

Wherein each element refers to the weight content.

According to another aspect of the present disclosure, there is provided a method for manufacturing an ultra-thick steel plate having excellent low-temperature impact toughness, the method comprising the operations of: preparing a billet satisfying the alloy composition and relation 1; heating the billet at a temperature in the range of 1020 ℃ to 1150 ℃; subjecting the heated billet to rough rolling at 1000 ℃ or higher; finish hot rolling the billet after rough rolling at a temperature just above the non-recrystallization temperature (Tnr) or at a temperature in the range of Tnr to A3; and air-cooling it after finish hot rolling.

Advantageous effects

As described above, according to the present disclosure, it is possible to provide an ultra-thick steel sheet having excellent strength and low-temperature impact toughness for an ultra-thick steel sheet having a thickness of 100mm to 200mm.

As a structural material, the ultra-thick steel plate of the present disclosure may be used in various fields, such as infrastructure industry (e.g., ships); various frames for marine structures, bridges, construction, etc.; and wind power infrastructure, etc.

Drawings

Fig. 1 shows a photograph of a microstructure of an ultra-thick steel plate according to one embodiment of the present disclosure.

Detailed Description

In providing an ultra-thick steel sheet having a thickness of 100mm or more (100 mm to 200 mm) suitable for structural steel, the inventors of the present disclosure have intensively studied a method for securing excellent strength and low-temperature impact toughness.

As a result thereof, it was confirmed that it is possible to provide an ultra-thick steel plate having target physical properties by optimizing an alloy composition system and a rolling process of the ultra-thick steel plate, and thus the present disclosure is provided.

In particular, the technical meaning of the present disclosure is that the productivity problem of the existing TMCP steel, the problem of ensuring physical properties of general rolled materials and heat treatment materials, the problem of heat treatment material costs, and the like can be solved.

Hereinafter, the present disclosure will be described in detail.

According to one aspect of the present disclosure, an ultra-thick steel plate having excellent low temperature impact toughness may include, by weight: 0.06% to 0.1% carbon (C), 0.3% to 0.5% silicon (Si), 1.35% to 1.65% manganese (Mn), 0.015% to 0.04% aluminum (soluble Al), 0.015% to 0.04% niobium (Nb), 0.005% to 0.02% titanium (Ti), 0.15% to 0.4% chromium (Cr), 0.3% to 0.5% nickel (Ni), 0.002% to 0.008% nitrogen (N), 0.01% or less (excluding 0%) phosphorus (P), 0.003% or less (excluding 0%) sulfur (S).

Hereinafter, the reason why the alloy composition of the steel sheet provided in the present disclosure is limited as described above will be described in detail.

Meanwhile, in the present disclosure, unless otherwise indicated, the content of each element is based on weight, and the ratio of the tissues is based on area.

Carbon (C): 0.06 to 0.1%

Carbon (C) is an element that causes solid solution strengthening and combines with Nb or the like in steel to form carbonitrides, which is advantageous in ensuring the strength of the steel.

In order to sufficiently obtain the strength effect of C, C may be contained in an amount of 0.06% or more, but when the C content exceeds 0.1%, pearlite is excessively formed as a microstructure, and thus there is a problem that impact characteristics and fatigue characteristics deteriorate at low temperatures. Further, as the content of solid solution C increases, impact characteristics decrease.

Thus, C may be included in an amount of 0.06% to 0.1%, and more advantageously, in an amount of 0.07% or more and 0.09% or less.

Silicon (Si): 0.3 to 0.5%

Silicon (Si) is used to deoxidize molten steel together with aluminum (Al). Si has an effect of improving strength, but when the Si content is excessive, impact characteristics and fatigue characteristics at low temperature may be impaired, so that Si must be added in an appropriate amount.

When the Si content is less than 0.3%, sufficient strength may not be ensured, and on the other hand, when the Si content exceeds 0.5%, diffusion of C is hindered, so that there is a problem that formation of MA phase (martensite-austenite mixed structure) is promoted.

Therefore, si may be contained in an amount of 0.3% to 0.5%.

Manganese (Mn): 1.35 to 1.65%

Manganese (Mn) is an element having a large effect of improving strength by solid solution strengthening, and may be contained in an amount of 1.35% or more. However, when the Mn content is excessive, mn may be contained in an amount of 1.65% or less in view of the problem that toughness may be deteriorated due to formation of MnS inclusions and center portion segregation.

Aluminum (soluble Al): 0.015% to 0.04%

Aluminum (soluble Al) is the primary deoxidizer of steel and contributes to the fixation of nitrogen (N) in steel. For this reason, it is advantageous to contain Al in an amount of 0.015% or more, but when the Al content exceeds 0.04%, al ₂ O ₃ The fraction and size of inclusions increase, which results in impaired low temperature toughness. Further, similar to Si, there is a problem that low temperature toughness and low temperature fatigue characteristics deteriorate due to acceleration of formation of MA phases in the base material and the weld heat affected zone.

Accordingly, al may be contained in an amount of 0.015% to 0.04%.

Niobium (Nb): 0.015% to 0.04%

Niobium (Nb) has a solid solution strengthening effect and contributes to improvement in strength by forming carbonitrides to finely form a structure to suppress recrystallization during rolling or cooling.

In order to sufficiently obtain the above effect, nb may be contained in an amount of 0.015% or more. On the other hand, when the content of Nb is excessive, C aggregation occurs due to C affinity, so that formation of MA phase is promoted, and there is a problem that toughness and fatigue characteristics at low temperature are impaired, so that the content of Nb may be limited to 0.04% or less in view of this.

Thus, nb may be included in an amount of 0.015% to 0.04%, and more advantageously, nb may be included in an amount of 0.02% or more.

Titanium (Ti): 0.005% to 0.02%

Titanium (Ti) combines with nitrogen (N), which may deteriorate impact characteristics and surface quality of steel, to form Ti-based nitride (TiN), and serves to reduce the content of dissolved N. The Ti-based precipitates contribute to refinement by suppressing coarsening of the structure, and are useful for improving toughness.

In order to sufficiently obtain the above-described effects, ti may be contained in an amount of 0.005% or more, but when the Ti content exceeds 0.02%, damage is caused due to coarsening of precipitates, and dissolved Ti remaining after bonding with N forms Ti-based carbide (TiC), so there is a problem that toughness of the base material and the weld zone is impaired.

Accordingly, ti may be contained in an amount of 0.005% to 0.02%, and more advantageously, ti may be contained in an amount of 0.01% or more.

Chromium (Cr): 0.15 to 0.4%

Chromium (Cr) is an element that contributes to improvement of strength by increasing hardenability of steel.

In order to sufficiently obtain the above-described effects, cr may be contained in an amount of 0.15% or more, but when the content of Cr exceeds 0.4%, not only weldability is deteriorated but also there is a problem that manufacturing cost is increased as an expensive element.

Thus, cr may be contained in an amount of 0.15% to 0.4%.

Nickel (Ni): 0.3 to 0.5%

Nickel (Ni) is an element that can improve both strength and toughness of steel.

In particular, in order to sufficiently obtain the effect of improving strength and toughness in the rolling process according to the present disclosure, ni may be included in an amount of 0.3% or more. However, when the Ni content exceeds 0.5%, the above effect is saturated, but there is a problem in that the manufacturing cost increases.

Thus, ni may be included in an amount of 0.3% to 0.5%.

Nitrogen (N): 0.002% to 0.008%

Nitrogen (N) combines with Ti, nb, al, etc. in steel to form precipitates, and these precipitates are effective in improving strength and toughness by forming a fine austenitic structure during reheating.

In order to sufficiently obtain the above-mentioned effects, it is advantageous to add 0.002% or more of N, but when the N content exceeds 0.008%, surface cracks are caused at high temperature, and N remaining after the formation of precipitates exists in an atomic state, resulting in impaired toughness of the steel.

Thus, N may be included in an amount of 0.002% to 0.008%.

Phosphorus (P): 0.01% or less (excluding 0%)

Phosphorus (P) is an element that causes grain boundary segregation, which may cause embrittlement of steel. Therefore, the content of P should be controlled as low as possible.

In the present disclosure, even when P is contained in a maximum amount of 0.01%, there is no problem in ensuring desired physical properties, and thus the P content may be limited to 0.01% or less. However, considering the unavoidable addition level, 0% may not be included.

Sulfur (S): 0.003% or less (excluding 0%)

Sulfur (S) mainly combines with Mn in steel to form MnS inclusions, which are factors that impair low temperature toughness.

Therefore, in order to ensure the low temperature toughness and low temperature fatigue characteristics desired in the present disclosure, the S content should be controlled to be as low as possible, and may preferably be limited to 0.003% or less. However, considering the unavoidable addition level, 0% may not be included.

The remainder of the present disclosure may be iron (Fe). However, in a general manufacturing process, unavoidable impurities may be inevitably added from raw materials or surrounding environment, and thus impurities may not be removed. Those skilled in the art of general manufacturing processes may be aware of the impurities, and thus, a description of the impurities may not be provided in the present disclosure.

Preferably, in the steel sheet of the present disclosure satisfying the above alloy composition, the relationship between Mn, ni and Cr in the steel satisfies the following relationship 1.

[ relation 1]

Mn+5(Ni+Cr)≥3.6

Wherein each element refers to the weight content.

In the present disclosure, in order to improve low temperature toughness of an ultra-thick steel plate having a thickness of 100mm to 200mm, the content of C may be limited to 0.10% or less. In the present disclosure, the relationship between Mn, ni, and Cr in steel is controlled by the relationship 1 so that the ensured strength is not adversely affected even when the C content is relatively reduced.

Specifically, when the content relationship between Mn, ni, and Cr in the alloy composition proposed in the present disclosure does not satisfy the above-described relationship 1, that is, when the value of relationship 1 is less than 3.6, the strength of the ultra-thick steel sheet having a maximum thickness of 200mm may not be obtained.

The ultra-thick steel plate of the present disclosure satisfying the above alloy composition and relation 1 may have a microstructure composed of a composite structure of ferrite and pearlite.

Specifically, it is preferable that the ultra-thick steel plate of the present disclosure contains, in area fraction: 80% to 90% ferrite and the remainder pearlite.

When the fraction of ferrite is less than 80%, it is difficult to ensure low-temperature toughness of the ultra-thick steel plate. On the other hand, when the fraction of ferrite exceeds 90%, the fraction of pearlite is insufficient, so that the strength of the target level cannot be ensured.

Further, the ultra-thick steel sheet of the present disclosure has a fine structure because the average grain size of ferrite is 50 μm or less.

Here, it should be noted that the average grain size is based on the equivalent circle diameter.

As described above, the present disclosure has an effect of being able to simultaneously secure excellent strength and low temperature toughness by finely realizing the structure of the ultra-thick steel plate.

In particular, the ultra-thick steel sheet of the present disclosure may have a yield strength of 300MPa or more and an impact toughness of 200J or more at-20 ℃, exhibiting high strength and excellent low temperature impact toughness.

Hereinafter, a method for manufacturing an ultra-thick steel plate having excellent low temperature impact toughness according to another aspect of the present disclosure will be described in detail.

In short, a steel sheet may be manufactured by preparing a steel slab satisfying the alloy composition and relation 1 set forth in the present disclosure, and then subjecting the steel slab to a [ heating-rolling-air cooling ] process. In particular, in the present disclosure, there is a technical meaning in that, as a rolling process, the rolling process is performed in a normalizing heat treatment region, without performing a separate heat treatment after the rolling process is completed.

Each process condition will be described in detail below.

[ billet heating ]

In the present disclosure, it is preferable to perform a process of heating and homogenizing the billet before performing the rolling process, in which case the heating process may be performed at a temperature ranging from 1020 to 1150 ℃.

When the heating temperature of the billet is less than 1020 ℃, ti, nb, etc. may not be sufficiently dissolved, resulting in a decrease in strength. On the other hand, when the heating temperature thereof is higher than 1150 ℃, austenite grains coarsen, so that there is a problem in that toughness of the steel may deteriorate.

The billet may have a thickness of 400mm or less to ensure a sufficient rolling amount to ensure strength and toughness while having a maximum thickness of 200mm through a subsequent rolling process.

[ Rolling Process ]

The steel slab heated as described above may be hot-rolled to produce a hot-rolled steel sheet.

In the present disclosure, hot rolling is preferably performed in the operation of [ recrystalization zone rolling (rough rolling) -non-recrystalization zone rolling (finish rolling) ].

The rough rolling may be performed at 1000 ℃ or more so that austenite may be completely recrystallized.

Thereafter, finish rolling may be performed in the austenite single-phase region at a temperature just above the non-recrystallization temperature (Tnr) or at a temperature in the range of Tnr to A3. In this case, in order to further promote the grain refining effect, it is advantageous to perform finish rolling at a temperature close to A3, but in order to obtain the normalizing effect, it is advantageous to perform finish rolling at a temperature just above Tnr. Temperatures just above Tnr may be expressed as a temperature range greater than Tnr to tnr+50℃.

The Tnr and A3 temperatures can be obtained by the following formula, wherein each element means a weight content.

Tnr＝887+464C+(6445Nb-644√NB)+(732V-230√V)+890Ti+363Al-357Si

A3＝910-203√C-15.2Ni+44.7Si+104V+31.5Mo-30Mn+11Cr+20Cu-700P-400Al-400Ti

When the temperature during finish rolling is lower than A3, two-phase zone rolling is performed, and the normalizing effect is insufficient, so that there may be a problem in that an additional heat treatment process is required.

More preferably, the finish rolling may be accomplished at a temperature ranging from 820 ℃ to 900 ℃.

Since the present disclosure aims to obtain an ultra-thick steel plate having a maximum thickness of 200mm by performing the above-described rolling process, it is necessary to consider the distribution of reduction during rough rolling and finish rolling in the rolling process.

In the present disclosure, it is preferable to control the remaining rolling reduction immediately after the rough rolling to 25% to 35%. When the remaining rolling reduction is less than 25%, there are problems in that the rough rolling process is prolonged and the productivity is lowered. On the other hand, when the remaining rolling reduction exceeds 35%, there is a problem in that good rolling may not be achieved due to a load generated on the rolling mill during finish rolling after rough rolling.

Here, it should be noted that the remaining rolling reduction refers to the amount of finish rolling remaining after rough rolling with respect to the target thickness.

[ air Cooling ]

The hot rolled steel sheet obtained by completing the rolling process according to the above may be cooled, in which case air cooling is preferably performed to achieve the normalizing effect.

By performing air cooling after the completion of the rolling process according to the present disclosure, not only the effect of grain refinement but also the effect of obtaining an ultra-thick steel plate having excellent strength and toughness without performing a subsequent heat treatment process can be achieved.

More specifically, when a desired microstructure is formed in the ultra-thick steel sheet of the present disclosure, for the ultra-thick steel having a thickness of 100mm to 200mm, both excellent strength and toughness characteristics can be ensured.

In order to secure strength, the carbon content of the steel sheet manufactured by the conventional normalizing heat treatment is higher than that of the TMCP steel manufactured by the controlled rolling+cooling, so that the steel manufactured by the conventional normalizing heat treatment tends to have poor impact toughness even after the heat treatment. Further, when the heat treatment temperature is too high, or the heat treatment time is too long, the strength may be lowered due to grain growth as compared with the steel sheet in a rolled state before the heat treatment.

In the case of manufacturing an ultra-thick steel plate through the TMCP process, since air cooling waiting time of several minutes is required for temperature control, productivity is lowered, and costs due to water treatment are required, which is economically disadvantageous.

The present disclosure proposes a manufacturing method capable of overcoming the drawbacks of the ultra-thick plate produced by the above-described process, and by optimizing rolling and cooling conditions of a slab having a specific alloy composition system, it is possible to provide an ultra-thick plate having excellent strength and low-temperature toughness characteristics.

Hereinafter, the present disclosure will be described in more detail by the following examples. It should be noted, however, that the following examples are only used to describe the present disclosure in detail by way of illustration and are not intended to limit the scope of the claims of the present disclosure. The reason is that the scope of the right of the present disclosure is determined by the matters described in the claims and matters reasonably inferred therefrom.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Example (example)

Billets having the alloy compositions shown in table 1 were prepared. In this case, the content of the alloy composition is weight%, and the remainder thereof contains Fe and unavoidable impurities.

The prepared billets were subjected to heating, hot rolling (rough rolling and finish rolling) and cooling (air cooling) under the conditions shown in table 2, and thus hot rolled steel sheets were produced. In this case, the rough rolling is performed at 1000 ℃ or more.

TABLE 1

TABLE 2

The microstructure and mechanical properties of each hot rolled steel sheet manufactured as above were measured, and the results thereof are shown in table 3.

In the microstructure of each hot rolled steel sheet, samples collected at 1/4t (where t means thickness (mm)) points were observed with an Optical Microscope (OM), and the same samples were subjected to a charpy impact test at-20 ℃ to evaluate impact toughness.

Further, the tensile strength, yield strength and elongation of the test pieces collected according to JIS No. 5 standard were measured using a universal tensile tester.

TABLE 3

As shown in tables 1 to 3, in invention examples 1 to 3 satisfying all of the alloy compositions, relation 1 and manufacturing conditions set forth in the present disclosure, it was confirmed that steel sheets have a yield strength of 300MPa or more and an impact toughness of 200J or more at-20 ℃ which has high strength and excellent low temperature impact toughness.

On the other hand, in the case of comparative example 1 satisfying the alloy composition system proposed in the present disclosure, but having an excessively high termination temperature during finish rolling, coarse ferrite is formed, resulting in poor strength and toughness.

In comparative example 2, in which the C content in the steel was too large, pearlite was excessively formed to secure strength, but toughness was greatly deteriorated.

In comparative example 3, which deviates from relation 1 proposed in the present disclosure, it was confirmed that the strength was lowered even if the microstructure as desired in the present disclosure was formed. This demonstrates that it is difficult to ensure the target strength without optimizing the content of hardenable elements in the steel according to relation 1 of the present disclosure.

Fig. 1 is a photograph of the microstructure of invention example 3, and it was confirmed that a composite structure having pearlite and fine ferrite phase as main phases was formed.

Claims

1. An ultra-thick steel plate having excellent low-temperature impact toughness, comprising by weight: 0.06% to 0.1% of carbon (C), 0.3% to 0.5% of silicon (Si), 1.35% to 1.65% of manganese (Mn), 0.015% to 0.04% of aluminum (soluble Al), 0.015% to 0.04% of niobium (Nb), 0.005% to 0.02% of titanium (Ti), 0.15% to 0.4% of chromium (Cr), 0.3% to 0.5% of nickel (Ni), 0.002% to 0.008% of nitrogen (N), 0.01% or less (excluding 0%) of phosphorus (P), 0.003% or less (excluding 0%) of sulfur (S), and the balance of iron (Fe) and unavoidable impurities, the super-thick steel plate satisfying the following formula 1,

wherein the ultra-thick steel plate comprises in area fraction: 80 to 90% of ferrite and the remainder of pearlite as a microstructure,

[ relation 1]

Mn+5(Ni+Cr)≥3.6

Wherein each element refers to the weight content.

2. The ultra-thick steel plate with excellent low temperature impact toughness according to claim 1, wherein the average grain size of the ferrite is 50 μm or less.

3. The ultra-thick steel plate with excellent low-temperature impact toughness according to claim 1, wherein the steel plate has a yield strength of 300MPa or more and an impact toughness of 200J or more at-20 ℃.

4. The ultra-thick steel plate with excellent low temperature impact toughness according to claim 1, wherein the thickness of the steel plate is 100mm to 200mm.

5. A method for manufacturing an ultra-thick steel plate having excellent low-temperature impact toughness, comprising the operations of:

preparing a billet comprising by weight: 0.06% to 0.1% of carbon (C), 0.3% to 0.5% of silicon (Si), 1.35% to 1.65% of manganese (Mn), 0.015% to 0.04% of aluminum (soluble Al), 0.015% to 0.04% of niobium (Nb), 0.005% to 0.02% of titanium (Ti), 0.15% to 0.4% of chromium (Cr), 0.3% to 0.5% of nickel (Ni), 0.002% to 0.008% of nitrogen (N), 0.01% or less (excluding 0%) of phosphorus (P), 0.003% or less (excluding 0%) of sulfur (S), and the balance of iron (Fe) and unavoidable impurities, the steel slab satisfying the following relational expression 1,

heating the steel billet at a temperature in the range of 1020 ℃ to 1150 ℃;

subjecting the heated billet to rough rolling at 1000 ℃ or higher;

after the rough rolling, finish-hot rolling the steel slab at a temperature just above a non-recrystallization temperature (Tnr) or at a temperature in the range of Tnr to A3; and

air-cooling the finish-hot rolled product after it has been subjected to the finish-hot rolling,

[ relation 1]

Mn+5(Ni+Cr)≥3.6

Wherein each element refers to the weight content.

6. The method of claim 5, wherein the finish hot rolling ends at a temperature ranging from 820 ℃ to 900 ℃.

7. The method of claim 5, wherein the remaining rolling reduction after the rough rolling is 25% to 35%.

8. The method of claim 5, wherein the thickness of the steel sheet is 100mm to 200mm.