HIGH-STRENGTH STEEL HAVING EXCELLENT BRITTLE CRACK ARRESTABILITY AND WELDING PART BRITTLE CRACK INITIATION RESISTANCE, AND PRODUCTION METHOD THEREFOR

Description

[Technical Field]

[0001] The present disclosure relates to a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance, and to a method of manufacturing the same.

[Background Art]

[0002] Recently, there has been demand for the development of ultra-thick steel sheets having high strength properties in consideration of the design requirements of structures to be used in the shipping, maritime, architectural, and civil engineering fields, domestically and internationally.

[0003] In a case in which high-strength steel is included in the design of a structure, economic benefits may be obtained due to reductions in the weight of structures while processing and welding operations may be easily undertaken using a steel sheet having a relatively reduced thickness.

[0004] In general, in the case of high-strength steel, due to a reduction in a reduction ratio when thick steel plates are manufactured, sufficient deformation may not be performed, as compared with a thin steel sheet. Thus, microstructures of thick steel plates may be coarse, so that low temperature properties on which grain sizes have the most significant effect may be degraded.

[0005] In detail, in a case in which brittle crack arrestability representing stability of a structure is applied to a main structure, such as a ship's hull, the number of cases of assurances being demanded has increased. However, in a case in which microstructures become coarse, a phenomenon in which brittle crack arrestability is significantly degraded may occur. Thus, it may be difficult to improve brittle crack arrestability of an ultra-thick high-strength steel material.

[0006] In the meantime, in the case of high-strength steel having yield strength of 390 MPa or greater, various technologies, such as adjustment of grain size, by applying surface cooling during finishing milling and applying bending stress during rolling to refine the grain size of a surface portion, in order to improve brittle crack arrestability, have been introduced.

[0007] However, such technologies may contribute to refining a structure of a surface portion, but may not solve a problem in which impact toughness is degraded due to coarsening of structures other than that of the surface portion. Thus, such technologies may not be fundamental countermeasures to brittle crack arrestability.

[0008] In addition, recently, a design concept to improve safety of a ship by controlling brittle crack initiation of a steel material applied to large container ships has been introduced. Thus, in general, the number of cases of guaranteeing brittle crack initiation of a heat affected zone (HAZ), the most vulnerable portion in terms of brittle crack initiation, has increased.

[0009] In general, since, in the case of high-strength steel, the microstructure in a HAZ includes low temperature transformation ferrite having high strength, such as bainite, there is a limitation in which HAZ properties, in detail, toughness, are significantly reduced.

[0010] In detail, in the case of brittle crack initiation resistance generally evaluated through a crack tip opening displacement (CTOD) test to evaluate the stability of the structure, martensite-austenite generated from untransformed austenite, when low temperature transformation ferrite is generated, becomes an active nucleation site of brittle crack occurrence. Thus, it may be difficult to improve brittle crack initiation resistance of a high-strength steel material.

[0011] In the case of high-strength steel of the related art having a yield strength of 400 MPa or greater, in order to improve welding zone brittle crack initiation resistance, an effort to refine a microstructure in a HAZ using TiN or to form ferrite in a HAZ using oxide metallurgy has been made. However, the effort partially contributes to forming impact toughness through refining a structure, but does not have a great effect on reducing a fraction of martensite-austenite having a significant influence on reducing brittle crack initiation resistance.

[0012] In addition, in the case of brittle crack initiation resistance of a base material, martensite-austenite may be transformed to have a different phase through tempering, or the like, to secure physical properties. However, in the case of a HAZ, in which an effect of tempering disappears due to thermal history, it is impossible to apply brittle crack initiation resistance.

[0013] In the meantime, in order to minimize the formation of martensite-austenite, the amount of elements, such as carbon (C) and niobium (Nb), should be reduced. However, in this case, it may be difficult to secure a specific level of strength. To this end, a relatively large amount of high-priced elements, such as molybdenum (Mo) and nickel (Ni), should be added. Thus, there is a limitation in which economic efficiency is deteriorated.

[0014] KR2015-0112489 A discloses a steel material and a manufacturing method comprising by weight %, carbon (C): 0.05% to 0.09%, manganese (Mn): 1.5% to 2.0%, nickel (Ni): 0.3% to 0.8%, niobium (Nb): 0.005% to 0.04%, titanium (Ti): 0.005% to 0.04%, copper (Cu): 0.1% to 0.5%, silicon (Si): 0.05% to 0.3%, aluminum (Al) : 0.005% to 0.05%, phosphorus (P) : 100 ppm or less, sulfur (S): 40 ppm or less, iron (Fe) as a residual component thereof, and inevitable impurities and having a Cu/N ratio of 0.75, a yield strength of 517 MPa and a transition temperature of -65°C.

[Disclosure]

[Technical Problem]

[0015] An aspect of the present disclosure may provide a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance.

[0016] Another aspect of the present disclosure may provide a method of manufacturing a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance.

[Technical Solution]

[0017] According to an aspect of the present disclosure, a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance comprises, by wt%, carbon (C): 0.05% to 0.09%, manganese (Mn): 1.61% to 2.0%, nickel (Ni): 0.3% to 0.8%, niobium (Nb) : 0.005% to 0.04%, titanium (Ti) : 0.005% to 0.04%, copper (Cu): 0.1% to 0.5%, silicon (Si): 0.05% to 0.3%, aluminum (Al): 0.005% to 0.05%, phosphorus (P): 100 ppm or less, sulfur (S) : 40 ppm or less, iron (Fe) as a residual component thereof, and inevitable impurities, wherein a microstructure of a central portion includes, by area%, acicular ferrite in an amount of 70% or greater, pearlite in an amount of 10% or less, and one or more selected from a group consisting of ferrite, bainite, and martensite-austenite (MA), as residual components; a circle-equivalent diameter of pearlite being 15 µm or less; a surface portion microstructure in a region at a depth of 2 mm or less, directly below a surface, includes, by area%, ferrite in an amount of 30% or greater and one or more of bainite, martensite, and pearlite as residual components; and a heat affected zone (HAZ) formed during welding includes, by area%, martensite-austenite (MA) in an amount of 5% or less.

[0018] A weight ratio of Cu and Ni (a Cu/Ni weight ratio) may be set to be 0.8 or less, and in more detail, 0.6 or less.

[0019] The high-strength steel material may have yield strength of 390 MPa or greater.

[0020] The high-strength steel material may have a Charpy fracture transition temperature of -40°C or lower in a 1/2t position in a steel material thickness direction, where t is a steel sheet thickness.

[0021] According to another aspect of the present disclosure, a method of manufacturing a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance comprises rough rolling a slab at a temperature of 900°C to 1100°C after reheating the slab at 1000°C to 1100°C, including, by wt%, C: 0.05% to 0.09%, Mn: 1.61% to 2.0%, Ni: 0.3% to 0.8%, Nb: 0.005% to 0.04%, titanium (Ti): 0.005% to 0.04%, copper (Cu): 0.1% to 0.5%, silicon (Si): 0.1% to 0.3%, aluminum (Al): 0.005% to 0.05%, phosphorus (P): 100 ppm or less, sulfur (S): 40 ppm or less, iron (Fe) as a residual component thereof, and inevitable impurities; obtaining a steel sheet by finish rolling a bar obtained from the rough rolling a slab, at a temperature in a range of Ar₃ + 60°C to Ar₃°C, based on a temperature of a central portion; and cooling the steel sheet to 700°C or lower, and welding the steel sheet in such a manner that a heat affected zone (HAZ) formed therein includes 5 area% or less of martensite-austenite (MA), and wherein a reduction ratio per pass of three final passes during the rough rolling a slab is 5% or greater, and a total cumulative reduction ratio of 40% or greater, and wherein a strain rate of three final passes during the rough rolling a slab is 2/sec or lower, and wherein a reduction ratio during the finish rolling is set such that a ratio of a slab thickness (mm) to a steel sheet thickness (mm) after the finish rolling is 3.5 or greater, and wherein a cumulative reduction ratio during the finish rolling is maintained to be 40% or greater, and a reduction ratio per pass, not including skin pass rolling, is maintained to be 4% or greater, and wherein the cooling the steel is performed at a cooling rate of the central portion of 1.5°C/s or higher.

[0022] A grain size of a central portion in a bar thickness direction before finish rolling after the rough rolling a slab may be 100 µm or less, and more specifically, 80 µm or less.

[0023] A reduction ratio during the finish rolling may be set such that a ratio of a slab thickness (mm) to a steel sheet thickness (mm) after the finish rolling may be 4 or greater.

[0024] The skin pass rolling is performed to secure a shape of a sheet (to secure a flat sheet) at a relatively low reduction rate, less than 5% in 1 to 2 passes of finish rolling.

[Advantageous Effects]

[0025] According to an aspect of the present disclosure, a high-strength steel material having a relatively high level of yield strength, as well as excellent brittle crack arrestability and welding zone brittle crack initiation resistance may be provided.

[Description of Drawings]

[0026] FIG. 1 is an image captured using an optical microscope, illustrating a central portion of Inventive Steel 3 in a thickness direction.

[Best Mode for Invention]

[0027] The inventors of the present disclosure conducted research and experiments to improve yield strength, brittle crack arrestability, and welding zone brittle crack initiation resistance of a thick steel material and proposed the present disclosure based on results thereof.

[0028] In an exemplary embodiment, a steel composition, a structure, and manufacturing conditions of a steel material may be controlled, thereby improving yield strength, brittle crack arrestability, and welding zone brittle crack initiation resistance of the thick steel material.

[0029] A main concept of an exemplary embodiment is as follows .

1) The steel composition is appropriately controlled to improve strength through solid solution strengthening. In detail, contents of manganese (Mn), nickel (Ni), copper (Cu), and silicon (Si) are optimized for solid solution strengthening.

2) The steel composition is appropriately controlled to improve strength by increasing hardenability. In detail, the contents of Mn, Ni, and Cu, as well as a carbon (C) content are optimized to increase hardenability.
A fine structure is secured in a central portion of the thick steel material even at a relatively slow cooling rate.

3) In detail, a weight ratio of Cu to Ni may be controlled.
In a case in which the weight ratio of Cu to Ni is controlled as described above, surface quality may be improved.

4) A composition is appropriately controlled to control a fraction of martensite-austenite in a heat affected zone (HAZ) formed during welding. In detail, contents of C, Si, and niobium (Nb), affecting generation of martensite-austenite, are optimized.
As such, the steel composition may be optimized, thereby securing excellent brittle crack initiation resistance even in the HAZ.

5) More specifically, a structure of the steel material may be controlled to improve strength and brittle crack arrestability. In detail, a structure of the central portion and a surface layer region is controlled in a direction of a steel material thickness.
As such, a microstructure may be controlled, thereby securing strength required in the steel material, while the microstructure facilitating generation of a crack may be excluded, thereby improving brittle crack arrestability.

6) In detail, rough rolling conditions may be controlled to refine the structure of the steel material. In detail, the fine structure is secured in the central portion by controlling rolling conditions during rough rolling. Using a process described above, the generation of acicular ferrite is also facilitated.

7) Finish rolling conditions may be controlled to further refine the structure of the steel material. In detail, the fine structure is secured in the central portion by controlling rolling conditions during rough rolling. As such, the generation of acicular ferrite is also facilitated.

[0030] Hereinafter, the high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance according to an aspect of the present disclosure will be described in detail.

[0031] According to an aspect of the present disclosure, a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance comprises, by wt%, carbon (C): 0.05% to 0.09%, manganese (Mn): 1.61% to 2.0%, nickel (Ni): 0.3% to 0.8%, niobium (Nb) : 0.005% to 0.04%, titanium (Ti) : 0.005% to 0.04%, copper (Cu): 0.1% to 0.5%, silicon (Si): 0.05% to 0.3%, aluminum (Al): 0.005% to 0.05%, phosphorus (P): 100 ppm or less, sulfur (S) : 40 ppm or less, iron (Fe) as a residual component thereof, and inevitable impurities, wherein a microstructure of a central portion includes, by area%, acicular ferrite in an amount of 70% or greater, pearlite in an amount of 10% or less, and one or more selected from a group consisting of ferrite, bainite, and martensite-austenite (MA), as residual components; a circle-equivalent diameter of pearlite being 15 µm or less; a surface portion microstructure in a region at a depth of 2 mm or less, directly below a surface, includes, by area%, ferrite in an amount of 30% or greater and one or more of bainite, martensite, and pearlite as residual components; and a heat affected zone (HAZ) formed during welding includes, by area%, martensite-austenite (MA) in an amount of 5% or less.

[0032] Hereinafter, a steel component and a component range of an exemplary embodiment will be described.

Carbon (C): 0.05 wt% to 0.09 wt% (hereinafter, referred to as "%")

[0033] Since C is the most significant element used in securing basic strength, C is required to be contained in steel within an appropriate range. In order to obtain an effect of addition, C may be added in an amount of 0.05% or greater.

[0034] However, in a case in which a C content exceeds 0.09%, a large amount of martensite-austenite is generated in the HAZ to degrade brittle crack initiation resistance. Low temperature toughness is degraded due to a relatively high level of strength of ferrite of a base material and the generation of a relatively large amount of low temperature transformation ferrite. Thus, the C content is limited to 0.05% to 0.09%. In detail, the C content may be limited to 0.061% to 0.085%, and more specifically, to 0.065% to 0.075%.

Manganese (Mn): 1.61% to 2.0%

[0035] Mn is a useful element improving strength through solid solution strengthening and increasing hardenability to generate low temperature transformation ferrite. In addition, since Mn may generate low temperature transformation ferrite even at a relatively low cooling rate due to improved hardenability, Mn is a main element used to secure strength of a central portion of a thick steel plate.

[0036] Therefore, in order to obtain an effect described above, Mn may be added in an amount of 1,61% or greater.

[0037] However, in a case in which a Mn content exceeds 2.0%, generation of upper bainite and martensite may be facilitated due to an increase in excessive hardenability, thereby degrading impact toughness and brittle crack arrestability and toughness of the HAZ.

[0038] Therefore, the Mn content is limited to 1.61% to 2.0%. In detail, the Mn content may be limited to 1.7% to 1.9%.

Nickel (Ni): 0.3% to 0.8%

[0039] Ni is a significant element used in improving impact toughness by facilitating a dislocation cross slip at a relatively low temperature and increasing strength by improving hardenability. In order to obtain an effect described above, Ni may be added in an amount of 0.8% or greater. However, in a case in which Ni is added in an amount of 1.2% or greater, hardenability may be excessively increased to generate low temperature transformation ferrite, thereby degrading toughness, and manufacturing costs may be increased due to a relatively high cost of Ni, as compared with other hardenability elements. Thus, an upper limit value of the Ni content is limited to 0.8%.

[0040] In detail, the Ni content may be limited to 0.37% to 0.71%, and more specifically, to 0.4% to 0.6%.

Niobium (Nb): 0.005% to 0.04%

[0041] Nb is educed to have a form of NbC or NbCN to improve strength of a base material.

[0042] In addition, Nb solidified when being reheated at a relatively high temperature is significantly finely educed to have the form of NbC during rolling to suppress recrystallization of austenite, thereby having an effect of refining a structure.

[0043] Therefore, Nb: is added in an amount of 0.005% or greater. However, in a case in which Nb is added excessively, generation of martensite-austenite in the HAZ may be facilitated to degrade brittle crack initiation resistance and cause a brittle crack in an edge of the steel material. Thus, an upper limit value of an Nb content: is limited to 0.04%.

[0044] In detail, the Nb content may be limited to 0.012% to 0.031%, and more specifically, to 0.017% to 0.03%.

Titanium (Ti): 0.005% to 0.04%

[0045] Ti is a component educed to be TiN when being reheated and inhibiting growth of the base material and a grain in the HAZ to greatly improve low temperature toughness. In order to obtain an effect of addition, Ti is added in an amount of 0.005% or greater.

[0046] However, in a case in which Ti is added excessively, low temperature toughness may be degraded due to clogging of a continuous casting nozzle or crystallization of the central portion. Thus, a Ti content is limited to 0.005% to 0.04%.

[0047] In detail, the Ti content may be limited to 0.012% to 0.023%, and more specifically, to 0.014% to 0.018%.

Silicon (Si): 0.05% to 0.3%

[0048] Si is a substitutional element improving strength of the steel material through solid solution strengthening and having a strong deoxidation effect, so that Si may be an element essential in manufacturing clean steel. Thus, Si is added in an amount of 0.05% or greater. However, when a relatively large amount of Si is added, a coarse martensite-austenite phase may be formed to degrade brittle crack arrestability and welding zone brittle crack initiation resistance. Thus, an upper limit value of a Si content is limited to 0.3%.

[0049] In detail, the Si content may be limited to 0.1% to 0.27%, and more specifically, to 0.19% to 0.25%.

Copper (Cu): 0.1% to 0.5%

[0050] Cu is a main element used in improving hardenability and causing solid solution strengthening to enhance strength of the steel material. In addition, Cu is a main element used in increasing yield strength through the generation of an epsilon Cu precipitate when tempering is applied. Thus, Cu: is added in an amount of 0.1% or greater. However, when a relatively large amount of Cu is added, a slab crack may be generated by hot shortness in a steelmaking process. Thus, an upper limit value of a Cu content is limited to 0.5%.

[0051] In detail, the Cu content may be limited to 0.15% to 0.31%, and more specifically, to 0.2% to 0.3%.

[0052] Contents of Cu and Ni may be set such that the weight ratio of Cu to Ni may be 0.8 or less, and in more detail, 0.6 or less. More specifically, the weight ratio of Cu to Ni may be limited to 0.5 or less.

[0053] In a case in which the weight ratio of Cu to Ni is set as described above, surface quality may be improved.

Aluminum (Al): 0.005% to 0.05%

[0054] Al is a component functioning as a deoxidizer. In a case in which an excessive amount of Al is contained, an inclusion may be formed to degrade toughness. Thus, an Al content is limited to 0.005% to 0.05%.

Phosphorus (P): 100 ppm or less, Sulfur (S): 40 ppm or less

[0055] P and S are elements causing brittleness in a grain boundary or forming a coarse inclusion to cause brittleness. In order to improve brittle crack arrestability, a P content is limited to 100 ppm or less, while an S content is limited to 40 ppm or less.

[0056] A residual component of an exemplary embodiment is Fe.

[0057] However, since, in a manufacturing process of the related art, unintended impurities may be inevitably mixed from a raw material or an external source, which may not be excluded.

[0058] Since the impurities are apparent to those skilled in the art, all the contents thereof are not specifically described in the present disclosure.

[0059] In the case of a steel material of an exemplary embodiment, a microstructure of a central portion includes, by area%, acicular ferrite in an amount of 70% or greater, pearlite in an amount of 10% or less, and one or more selected from a group consisting of ferrite, bainite, and martensite-austenite (MA), as residual components; a circle-equivalent diameter of pearlite being 15 µm or less; a surface portion microstructure in a region at a depth of 2 mm or less, directly below a surface, includes, by area%, ferrite in an amount of 30% or greater and one or more of bainite, martensite, and pearlite as residual components; and a heat affected zone (HAZ) formed during welding includes, by area%, martensite-austenite (MA) in an amount of 5% or less.

[0060] Ferrite refers to polygonal ferrite, while bainite refers to granular bainite and upper bainite.

[0061] In a case in which a fraction of acicular ferrite of the microstructure of the central portion is less than 70%, generation of coarse bainite may cause degradation of toughness.

[0062] In detail, the fraction of acicular ferrite may be 75% or greater, and more specifically, may be limited to 80% or greater.

[0063] In a case in which a fraction of pearlite in the central portion exceeds 10%, a microcrack may be generated in a front end of a crack during brittle crack propagation, thereby degrading brittle crack arrestability. Thus, the fraction of pearlite in the central portion may be 10% or less.

[0064] In detail, the fraction of pearlite may be limited to 8% or less, and more specifically, to 5% or less.

[0065] In a case in which the circle-equivalent diameter of pearlite in the central portion exceeds 15 µm, there is a problem in which a crack may be easily generated despite a relatively low fraction of pearlite being present in the central portion. Thus, the circle-equivalent diameter of pearlite in the central portion is 15 µm or less.

[0066] In a case in which the surface portion microstructure in the region at a depth of 2 mm or less, directly below the surface, includes ferrite in an amount of 30% or greater, crack propagation may be effectively prevented on the surface at a time of brittle crack propagation, thereby improving brittle crack arrestability.

[0067] In detail, the fraction of ferrite may be limited to 40% or greater, and more specifically, to 50% or greater.

[0068] In a case in which a fraction of martensite-austenite in the HAZ formed when the steel material is welded exceeds 5%, martensite-austenite functions as a starting point of a crack, thereby degrading brittle crack initiation resistance. Thus, the fraction of martensite-austenite in the HAZ may be 5% or less.

[0069] Welding heat input during welding may be 0.5 kJ/mm to 10 kJ/mm.

[0070] A welding method during welding is not specifically limited and may include, for example, flux cored arc welding (FCAW), submerged arc welding (SAW), and the like.

[0071] The steel material may have yield strength of 390 MPa or greater.

[0072] The steel material may have a Charpy fracture transition temperature of -40°C or lower in a 1/2t position in a steel material thickness direction, where t is a steel sheet thickness.

[0073] The steel material have a thickness of 50 mm or greater, in detail, a thickness of 60 mm to 100 mm, and more specifically, 80 mm to 100 mm.

[0074] Hereinafter, a method of manufacturing a high-strength steel material having excellent brittle crack arrestability according to another aspect of the present disclosure will be described in detail.

[0075] According to another aspect of the present disclosure, the method of manufacturing a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance comprises rough rolling a slab at a temperature of 900°C to 1100°C after reheating the slab at 1000°C to 1100°C, including, by wt%, C: 0.05% to 0.09%, Mn: 1.61% to 2.0%, Ni: 0.3% to 0.8%, Nb: 0.005% to 0.04%, Ti: 0.005% to 0.04%, Cu: 0.1% to 0.5%, Si: 0.1% to 0.3%, Al: 0.005% to 0.05%, P: 100 ppm or less, S: 40 ppm or less, Fe as a residual component thereof, and inevitable impurities; obtaining a steel sheet by finish rolling a bar obtained from the rough rolling a slab, at a temperature in a range of Ar₃ + 60°C to Ar₃°C, based on a temperature of a central portion; and cooling the steel sheet to 700°C or lower.

Reheating a slab

[0076] A slab is reheated before rough rolling.

[0077] A reheating temperature of the slab may be 1000°C or higher so that a carbonitride of Ti and/or Nb, formed during casting, may be solidified.

[0078] However, in a case in which the slab is reheated at a significantly high temperature, austenite may become coarse. Thus, an upper limit value of the reheating temperature may be 1100°C.

Rough rolling

[0079] A reheated slab is rough rolled.

[0080] A rough rolling temperature may be a temperature Tnr at which recrystallization of austenite is halted, or higher. Due to rolling, a cast structure, such as a dendrite formed during casting, may be destroyed, and an effect of reducing a size of austenite may also be obtained. In order to obtain the effect, the rough rolling temperature is limited to 900°C to 1100°C.

[0081] In more detail, the rough rolling temperature may be 950°C to 1050°C.

[0082] In an exemplary embodiment, in order to refine a structure of the central portion during rough rolling, a reduction ratio per pass of three final passes during rough rolling is 5% or greater, and a total cumulative reduction ratio is 40% or greater.

[0083] In the case of a structure recrystallized by initial rolling during rough rolling, grain growth occurs due to a relatively high temperature. However, when three final passes are performed, a bar is air cooled while waiting for a rolling process, so that grain growth speed may be decreased. Thus, during rough rolling, a reduction ratio of the three final passes has the greatest impact on a grain size of a final microstructure.

[0084] In addition, in a case in which the reduction ratio is reduced per pass of rough rolling, sufficient deformation may not be transmitted to the central portion, so that toughness may be degraded due to coarsening of the central portion. Therefore, the reduction ratio per pass of the three final passes : is limited to 5% or greater.

[0085] In detail, the reduction ratio per pass may be 7% to 20%.

[0086] In the meantime, in order to refine a structure of the central portion, the total cumulative reduction ratio during rough rolling is set to be 40% or greater.

[0087] In detail, the total cumulative reduction ratio may be 45% or greater.

[0088] A strain rate of the three final passes during rough rolling is 2/sec or lower.

[0089] In general, rolling is difficult at a relatively high reduction ratio due to a relatively great thickness of the bar during rough rolling. Thus, there is a limitation in which it is difficult to transmit a rolling reduction to the central portion of a thick steel plate, thereby allowing an austenite grain size in the central portion to be coarsened. However, as the strain rate is reduced, deformation is transmitted to the central portion even at a relatively low rolling reduction ratio. Thus, the grain size may be refined.

[0090] Therefore, in terms of the three final passes having the greatest impact on the final grain size during rough rolling, the strain rate is limited to 2/sec or lower, thereby refining the grain size of the central portion. Thus, generation of acicular ferrite may be facilitated.

Finish rolling

[0091] A rough rolled bar may be finish rolled at a temperature of Ar₃ (a ferrite transformation initiation temperature) + 60°C to Ar₃°C to obtain a steel sheet so that a further refined microstructure may be obtained.

[0092] In a case in which rolling is performed at a temperature higher than Ar₃, a relatively large amount of strain bands may be generated in austenite to secure a relatively large number of ferrite nucleation sites, thereby obtaining an effect of securing a fine structure in the central portion of a steel material.

[0093] In addition, in order to effectively generate a relatively large amount of strain bands in austenite, a cumulative reduction ratio during finish rolling is maintained to be 40% or greater. The reduction ratio per pass, not including skin pass rolling, is maintained to be 4% or greater.

[0094] In detail, the cumulative reduction ratio may be 40% to 80%.

[0095] In detail, the reduction ratio per pass may be 4.5% or greater.

[0096] In a case in which a finish rolling temperature is reduced to Ar₃ or lower, coarse ferrite is generated before rolling and is elongated during rolling, thereby reducing impact toughness. In a case in which finish rolling is performed at a temperature of Ar₃ + 60°C or higher, the grain size is not effectively refined, so that the finish rolling temperature during finish rolling is set to be a temperature of Ar₃ + 60 °C to Ar₃°C.

[0097] In an exemplary embodiment, a reduction ratio in an unrecrystallized region may be limited to 40% to 80% during finish rolling.

[0098] As described above, since the reduction ratio in the unrecrystallized region is controlled, thereby increasing a number of nucleation sites of acicular ferrite, generation of structures described above may be facilitated.

[0099] In a case in which the reduction ratio in the unrecrystallized region is significantly low, acicular ferrite may not be sufficiently secured. In a case in which the reduction ratio in the unrecrystallized region is significantly high, strength may be reduced due to generation of pro-eutectoid ferrite caused by a relatively high reduction ratio.

[0100] The grain size of the central portion of the bar in a thickness direction after rough rolling before finish rolling may be 150 µm or less, in detail, 100 µm or less, and more specifically, 80 µm or less.

[0101] The grain size of the central portion of the bar in a thickness direction after rough rolling before finish rolling may be controlled depending on a rough rolling condition, or the like.

[0102] As described above, in a case in which the grain size of the bar after rough rolling before finish rolling may be controlled, a final microstructure is refined due to refinement of an austenite grain. Thus, an advantage of improving low temperature impact toughness may be added.

[0103] The reduction ratio during finish rolling is set such that a ratio of a slab thickness (mm) to a steel sheet thickness (mm) after finish rolling may be 3.5 or greater, and in detail, 4 or greater.

[0104] As described above, in a case in which the reduction ratio is controlled, as the rolling reduction is increased during rough rolling and finish rolling, an advantage of improving toughness of the central portion may be added by increasing yield strength/tensile strength, improving low temperature toughness, and decreasing the grain size of the central portion in the thickness direction through refinement of the final microstructure.

[0105] After finish rolling, the steel sheet may have a thickness of 50 mm or greater, in detail, 60 mm to 100 mm, and more specifically, 80 mm to 100 mm.

Cooling

[0106] The steel sheet is cooled to a temperature of 700°C, or lower, after finish rolling.

[0107] In a case in which a cooling end temperature exceeds 700°C, a microstructure may not be properly formed, so that sufficient yield strength may be difficult to secure. For example, yield strength of 390 MPa or greater may be difficult to secure.

[0108] The cooling end temperature may be 300°C to 600°C.

[0109] In a case in which the cooling end temperature is lower than 300 °C, an increase in a generation amount of bainite may degrade toughness.

[0110] The steel sheet is cooled at a cooling rate of the central portion of 1.5°C/s or higher. In a case in which the cooling rate of the central portion of the steel sheet is lower than 1.5°C/s, the microstructure may not be properly formed, so that it may be difficult to secure sufficient yield strength. For example, yield strength of 390 MPa or greater may be difficult.

[0111] In addition, the steel sheet may be cooled at an average cooling rate of 2°C/s to 300°C/s.

[Industrial Applicability]

[0112] Hereinafter, the present disclosure will be described in more detail through exemplary embodiments . However, an exemplary embodiment below is intended to describe the present disclosure in more detail through illustration thereof, but not to limit right scope of the present disclosure, as the scope of rights thereof is determined by the contents of the appended claims and those able to be reasonably inferred therefrom.

(Exemplary Embodiment)

[0113] A steel slab having a composition illustrated in Table 1 below, which is 400 mm in thickness, was reheated to a temperature of 1060°C, and then rough rolling was performed at a temperature of 1025°C, thereby manufacturing a bar. A cumulative reduction ratio of 50% during rough rolling was equally applied to an entirety of steel grades.

[0114] A thickness of a bar having been rough rolled was 200 mm, while a grain size of a central portion after rough rolling before finish rolling, as illustrated in Table 2, was 75 µm to 89 µm. A reduction ratio of three final passes during rough rolling was within a range of 7.2% to 14.3%. A strain rate during rolling was within a range of 1.29/s to 1.66/s.

[0115] After rough rolling, finish rolling was performed at a temperature equal to a difference between a finish rolling temperature and an Ar3 temperature, illustrated in Table 2 below to obtain a steel sheet having a thickness illustrated in Table 3 below, and then the steel sheet was cooled to a temperature of 412°C to 496°C at a cooling rate of 4.5°C/sec.

[0116] In terms of the steel sheet manufactured as illustrated above, a microstructure, yield strength, a Kca value (a brittle crack arrestability coefficient), and a crack tip opening displacement (CTOD) value (a brittle crack initiation resistance) were examined, and results thereof are illustrated in Tables 3 and 4 below.

[0117] The Kca value in Table 4 is a value derived by performing an ESSO test on the steel sheet.

[0118] A FCAW (0.7 kJ/mm) welding process was performed to carry out structure analysis and a CTOD test on the HAZ, and results thereof were illustrated in Tables 3 and 4 below.

[0119] Surface properties illustrated in Table 3 below were measured to determine whether a star crack in a surface portion was generated by hot shortness occurring depending on a Cu to N addition ratio.

[Table 1]

Steel Grade	Steel Composition (wt%)
	C	Si	Mn	Ni	Cu	Ti	Nb	Al	P(pp m)	S(ppm)	Cu/Ni Weight Ratio
Inventive Steel 1	0.06 1	0.22	1.88	0.63	0.21	0.023	0.01 8	0.029	55	17	0.33
Inventive Steel 2	0.07	0.18	1.65	0.52	0.3	0.012	0.012	0.035	65	11	0.58
Inventive Steel 3	0.059	0.22	1.92	0.45	0.26	0.017	0.025	0.030	79	23	0.58
Inventive Steel 4	0.077	0.25	1.78	0.62	0.29	0.022	0.023	0.032	81	22	0.47
Inventive Steel 5	0.085	0.16	1.61	0.55	0.31	0.016	0.031	0.025	59	25	0.56
Inventive Steel 6	0.066	0.21	1.82	0.37	0.15	0.018	0.028	0.020	46	24	0.41
Inventive Steel 7	0.068	0.25	1.78	0.71	0.29	0.019	0.026	0.040	57	22	0.41
Comparative Steel 1	0.12	0.16	1.88	0.52	0.21	0.021	0.019	0.021	49	9	0.40
Comparative Steel 2	0.067	0.58	1.79	0.67	0.31	0.011	0.016	0.034	69	19	0.46
Comparative Steel 3	0.071	0.22	2.35	0.71	0.29	0.013	0.021	0.023	78	28	0.41
Comparative Steel 4	0.055	0.25	1.89	1.59	0.41	0.021	0.015	0.037	65	16	0.26
Comparative Steel 5	0.062	0.19	1.69	0.44	0.24	0.042	0.051	0.030	57	12	0.55
Inventive Steel 8	0.062	0.19	1.72	0.42	0.43	0.015	0.014	0.027	72	18	1.02
Comparative Steel 6	0.048	0.22	1.47	0.39	0.18	0.019	0.018	0.025	59	12	0.46

[Table 2]

Exemplary Embodiment No.	Steel Grade	Grain Size of Central Portion after Rough Rolling before Finish Rolling (µm)	Average Reduction Ratio of Three final passes during Rough Rolling (%)	Average Strain Rate of Three final passes during Rough Rolling (/s)	Finish Rolling Temperature -Ar3 Temperature (°C)	Cooling End Temperature (°C)
Inventive Example 1	Inventive Steel 1	78	8.8	1.55	15	453
Inventive Example 2	Inventive Steel 2	85	9.6	1.35	23	432
Inventive Example 3	Inventive Steel 3	83	12.3	1.56	2	488
Inventive Example 4	Inventive Steel 4	82	7.2	1.43	36	496
Inventive Example 5	Inventive Steel 5	88	13.3	1.29	13	412
Inventive Example 6	Inventive Steel 6	77	12.8	1.32	8	423
Comparative Example 1	Inventive Steel 7	75	10.1	1.66	89	456
Comparative Example 2	Comparative Steel 1	89	9.6	1.32	28	439
Comparative Example 3	Comparative Steel 2	82	14.3	1.59	8	440
Comparative Example 4	Comparative Steel 3	77	12.9	1.46	16	472
Comparative Example 5	Comparative Steel 4	86	9.3	1.43	4	465
Comparative Example 6	Comparative Steel 5	82	8.9	1.35	12	452
Inventive Example 7	Inventive Steel 8	83	10.3	1.43	28	444
Comparative Example 7	Comparative Steel 6	81	11.2	1.39	44	477

[Table 3]

Exemplary Embodiment No.	Steel Grade	Surface Properties	Steel Sheet Thickness (mm)	Microstructure Phase Fraction of Central Portion (area%)			Ferrite Phase Fraction in Surface Portion(area%)	Martensite-Austenite Fraction in HAZ(area%)
				Acicular Ferrite	Pearlite(Average Grain Size: µm)	Remainder(One or more of pearlite/bainite/MA)
Inventive Example 1	Inventive Steel 1	None	95	73	5.2(3.6)	21.8	45	2.3
Inventive Example 2	Inventive Steel 2	None	95	78	4.8(5.1)	17.2	51	1.6
Inventive Example 3	Inventive Steel 3	None	90	86	6.2(4.5)	7.8	68	1.9
Inventive Example 4	Inventive Steel 4	None	90	79	3.1(3.2)	17.9	49	2.8
Inventive Example 5	Inventive Steel 5	None	85	82	5.6(2.9)	12.4	59	3.1
Inventive Example 6	Inventive Steel 6	None	100	73	2.8(4.6)	24.2	72	2.2
Comparative Example 1	Inventive Steel 7	None	95	49	7.9(12.2)	43.1	17	3.5
Comparative Example 2	Comparative Steel 1	None	95	36	3.8(6.5)	60.2	39	6.9
Comparative Example 3	Comparative Steel 2	None	100	71	6.6(7.6)	22.4	68	6.8
Comparative Example 4	Comparative Steel 3	None	80	32	2.7(2.8)	65.3	59	4.7
Comparative Example 5	Comparative Steel 4	None	85	42	2.4(5.3)	55.6	76	2.9
Comparative Examp	Comparative Steel 5	None	100	72	4.9(4.2)	23.1	51	6.3
Inventive Example 7	Inventive Steel 8	Occurrence	95	73	5.6(5.8)	21.4	50	2.1
Comparative Example 7	Comparative Steel 6	None	100	15	11.8(16.2)	73.2	39	3.6

[Table 4]

Exemplary Embodiment No.	Steel Grade	Yield Strength (Mpa)	Kca (N/mm1-5, @-1 0°C)	CTOD Value in HAZ (mm)
Inventive Example 1	Inventive Steel 1	506	7943	0.65
Inventive Example 2	Inventive Steel 2	513	7962	0.45
Inventive Example 3	Inventive Steel 3	468	7588	0.78
Inventive Example 4	Inventive Steel 4	459	7951	0.46
Inventive Example 5	Inventive Steel 5	512	9633	0.52
Inventive Example 6	Inventive Steel 6	467	8051	0.67
Comparative Example 1	Inventive Steel 7	467	5761	0.64
Comparative Example 2	Comparative Steel 1	583	5123	0.13
Comparative Example 3	Comparative Steel 2	532	6013	0.21
Comparative Example 4	Comparative Steel 3	568	4687	0.29
Comparative Example 5	Comparative Steel 4	548	5631	0.56
Comparative Example 6	Comparative Steel 5	512	6891	0.11
Inventive Example 7	Inventive Steel 8	499	7012	0.79
Comparative Example 7	Comparative Steel 6	395	4123	0.64

[0120] As illustrated in Tables 1 to 4, in the case of Comparative Example 1, a steel composition satisfies an exemplary embodiment, but the difference between the finish rolling temperature during finish rolling and the Ar₃ temperature, proposed in an exemplary embodiment, was controlled to be 60°C or higher. Since sufficient reduction was not applied to the central portion, a fraction of acicular ferrite (AF) in the central portion is less than 50%. In addition, cooling was started in an initial stage, so that ferrite of 30% or greater was not generated in a surface portion. Thus, it can be confirmed that the Kca value measured at a temperature of -10°C may not exceed 6000 required in a steel material for shipbuilding of the related art.

[0121] In the case of Comparative Example 2, a C content had a value higher than an upper limit value of a C content of an exemplary embodiment. It can be confirmed that a relatively large amount of bainite was generated in the central portion during rough rolling, so an AF fraction of a final microstructure is less than 50%. Therefore, the Kca value measured at a temperature of -10° C was 6000 or less. It can be confirmed that a relatively large amount of a martensite-austenite (MA) structure was also generated in the HAZ, so the CTOD value was 0.25 mm or less.

[0122] In the case of Comparative Example 3, a Si content had a value higher than an upper limit value of a Si content of an exemplary embodiment. It can be confirmed that a relatively large amount of Si was added to generate a relatively large amount of a coarse MA structure, so the microstructure in the central portion contains a relatively large amount of AF. However, the Kca value has a relatively low value similar to 6000 at a temperature of -10°C. It can be confirmed that a relatively large amount of MA is also generated in the HAZ, so the CTOD value is 0.25 mm or less.

[0123] In the case of Comparative Example 4, a Mn content has a value higher than an upper limit value of a Mn content of an exemplary embodiment. It can be confirmed that due to having a relatively high level of hardenability, a microstructure in a base material is provided as upper bainite, thereby allowing the fraction of AF to be less than 50%. Thus, the Kca value is 6000 or less at a temperature of -10°C.

[0124] In the case of Comparative Example 5, an Ni content had a value higher than an upper limit value of an Ni content of an exemplary embodiment. It can be confirmed that due to a relatively high level of hardenability, the microstructure of the base material is provided as granular bainite and upper bainite, and the fraction of acicular ferrite is less than 50%. Thus, the Kca value is 6000 or less at a temperature of -10°C.

[0125] In the case of Comparative Example 6, an Nb and Ti content has a value higher than an upper limit value of an Nb and Ti content of an exemplary embodiment. It can be confirmed that an entirety of other conditions satisfies a condition suggested in an exemplary embodiment, but due to a relatively high Nb and Ti content, a relatively large amount of the MA structure is generated in the HAZ, thereby allowing the CTOD value to be 0.25 mm or less.

[0126] Inventive Example 7 includes a component exceeding a ratio of Cu to Ni suggested in an aspect of the present disclosure. It can be confirmed that despite having other, significantly excellent physical properties, a star crack was generated, thereby causing a defect in surface quality.

[0127] In the case of Comparative Example 7, a C and Mn content has a value lower than a lower limit value of a C and Mn content of an exemplary embodiment. It can be confirmed that due to a relatively low level of hardenability, AF in the central portion is formed in an amount of less than 50%, and most structures have ferrite and a pearlite structure in an amount of 10% or greater. As pearlite has an average grain size of 15 µm or greater, the Kca value is 6000 or less at a temperature of -10°C.

[0128] On the other hand, it can be confirmed that, in the case of Inventive Examples 1 to 6, satisfying a composition range, a manufacturing range, and the Cu to Ni ratio of an exemplary embodiment, the fraction of AF of the microstructure in the central portion is 70% or greater, the fraction of pearlite in the central portion is 10% or less, a circle-equivalent diameter of pearlite in the central portion is 15 µm or less, and a fraction of MA phase in the HAZ is less than 5%.

[0129] It can be confirmed that, in Inventive Examples 1 to 6, yield strength satisfies 390 MPa or greater, the Kca value satisfies a value of 6000 or greater at a temperature of -10°C, and the CTOD value also represents a relatively high value of 0.25 mm or greater.

[0130] FIG. 1 illustrates an image of a central portion of Inventive Steel 2 in a thickness direction, captured using an optical microscope. As illustrated in FIG. 1, it can be confirmed that a microstructure in the central portion includes a relatively large amount of acicular ferrite (AF) structures, and pearlite is finely distributed.

Claims

1. A high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance, comprising:

by wt%, carbon (C): 0.05% to 0.09%, manganese (Mn): 1.61% to 2.0%, nickel (Ni) : 0.3% to 0.8%, niobium (Nb) : 0.005% to 0.04%, titanium (Ti): 0.005% to 0.04%, copper (Cu): 0.1% to 0.5%, silicon (Si): 0.05% to 0.3%, aluminum (Al): 0.005% to 0.05%, phosphorus (P): 100 ppm or less, sulfur (S): 40 ppm or less, iron (Fe) as a residual component thereof, and inevitable impurities,

wherein a microstructure of a central portion includes, by area%, acicular ferrite in an amount of 70% or greater, pearlite in an amount of 10% or less, and one or more selected from a group consisting of ferrite, bainite, and martensite-austenite (MA), as residual components; a circle-equivalent diameter of pearlite being 15 µm or less; a surface portion microstructure in a region at a depth of 2 mm or less, directly below a surface, includes, by area%, ferrite in an amount of 30% or greater and one or more of bainite, martensite, and pearlite as residual components; and a heat affected zone (HAZ) formed during welding includes, by area%, martensite-austenite (MA) in an amount of 5% or less.

2. The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of claim 1, wherein, a weight ratio of Cu to Ni (a Cu/Ni weight ratio) is 0.8 or less.

3. The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of claim 1, comprising yield strength of 390 MPa or greater.

4. The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of claim 1, comprising a Kca value measured at a temperature of -10°C of 6000 N/mm^1.5 or greater.

5. The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of claim 1, comprising a Charpy fracture transition temperature of -40°C or lower in a 1/2t position in a steel material thickness direction, where t is a steel sheet thickness.

6. A method of manufacturing a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance, comprising:

rough rolling a slab at a temperature of 900°C to 1100°C after reheating the slab at 1000°C to 1100°C, including, by wt%, C: 0.05% to 0.09%, Mn: 1.61% to 2.0%, Ni: 0.3% to 0.8%, Nb: 0.005% to 0.04%, Ti: 0.005% to 0.04%, Cu: 0.1% to 0.5%, Si: 0.1% to 0.3%, Al: 0.005% to 0.05%, P: 100 ppm or less, S: 40 ppm or less, Fe as a residual component, and inevitable impurities;

obtaining a steel sheet by finish rolling a bar obtained from the rough rolling a slab, at a temperature in a range of Ar₃ + 60°C to Ar₃°C, based on a temperature of a central portion; and

cooling the steel sheet to 700°C or lower; and

welding the steel sheet in such a manner that a heat affected zone (HAZ) formed therein includes 5 area% or less of martensite-austenite (MA), and

wherein a reduction ratio per pass of three final passes during the rough rolling a slab is 5% or greater, and a total cumulative reduction ratio of 40% or greater, and

wherein a strain rate of three final passes during the rough rolling a slab is 2/sec or lower, and

wherein a reduction ratio during the finish rolling is set such that a ratio of a slab thickness (mm) to a steel sheet thickness (mm) after the finish rolling is 3.5 or greater, and

wherein a cumulative reduction ratio during the finish rolling is maintained to be 40% or greater, and a reduction ratio per pass, not including skin pass rolling, is maintained to be 4% or greater, and

wherein the cooling the steel is performed at a cooling rate of the central portion of 1.5°C/s or higher.

7. The method of claim 8, wherein welding heat input during welding is 0.5 kJ/mm to 10 kJ/mm.

8. The method of claim 7, wherein in the welding, a welding method includes flux cored arc welding (FCAW) or submerged arc welding (SAW).

Ansprüche

1. Hochfestes Stahlmaterial mit ausgezeichneter Sprödrissauffangfähigkeit und Sprödrissentstehungsbeständigkeit der Schweißzone, das Folgendes umfasst:

nach Gew.-%: Kohlenstoff (C): 0,05 % bis 0,09 %, Mangan (Mn): 1,61 % bis 2,0 %, Nickel (Ni): 0,3 % bis 0,8 %, Niob (Nb): 0,005 % bis 0,04 %, Titan (Ti): 0,005 % bis 0,04 %, Kupfer (Cu): 0,1 % bis 0,5 %, Silicium (Si): 0,05 % bis 0,3 %, Aluminium (AI): 0,005 % bis 0,05 %, Phosphor (P): 100 ppm oder weniger, Schwefel (S): 40 ppm oder weniger, Eisen (Fe) als Restbestandteil davon; und unvermeidbare Verunreinigungen,

wobei eine Mikrostruktur eines mittleren Abschnitts in Flächen-% nadelförmiges Ferrit in einer Menge von 70 % oder mehr, Perlit in einer Menge von 10 % oder weniger und eines oder mehrere, ausgewählt aus einer Gruppe, bestehend aus Ferrit, Bainit und Martensitaustenit (MA) als Restbestandteil beinhaltet; wobei ein kreisäquivalenter Perlitdurchmesser von 15 µm oder weniger beträgt; eine Oberflächenabschnittmikrostruktur in einem Bereich in einer Tiefe von 2 mm oder weniger direkt unter einer Oberfläche in Flächen-% Ferrit in einer Menge von 30 % oder mehr und eines oder mehrere von Bainit, Martensit und Perlit als Restbestandteile beinhaltet; und eine Wärmeeinflusszone (HAZ), die während eines Schweißens gebildet wird, Martensitaustenit (MA) in Flächen-% in einer Menge von 5 % oder weniger beinhaltet.

2. Hochfestes Stahlmaterial mit ausgezeichneter Sprödrissauffangfähigkeit und Sprödrissentstehungsbeständigkeit der Schweißzone nach Anspruch 1, wobei ein Gewichtsverhältnis von Cu zu Ni (ein Cu/Ni-Gewichtsverhältnis) 0,8 oder weniger beträgt.

3. Hochfestes Stahlmaterial mit ausgezeichneter Sprödrissauffangfähigkeit und Sprödrissentstehungsbeständigkeit der Schweißzone nach Anspruch 1, das eine Streckgrenze von 390 MPa oder höher umfasst.

4. Hochfestes Stahlmaterial mit ausgezeichneter Sprödrissauffangfähigkeit und Sprödrissentstehungsbeständigkeit der Schweißzone nach Anspruch 1, das einen bei einer Temperatur von -10 °C gemessenen Kca-Wert von 6000 N/mm^1,5 oder höher umfasst.

5. Hochfestes Stahlmaterial mit ausgezeichneter Sprödrissauffangfähigkeit und Sprödrissentstehungsbeständigkeit der Schweißzone nach Anspruch 1, das eine Charpy-Bruchübergangstemperatur von -40 °C oder weniger bei einer Position von 1/2 t in Richtung der Stahlmaterialdicke umfasst, wobei t eine Stahlblechdicke ist.

6. Verfahren zum Herstellen eines hochfesten Stahlmaterials mit ausgezeichneter Sprödrissauffangfähigkeit und Sprödrissentstehungsbeständigkeit der Schweißzone, das Folgendes umfasst:

Vorwalzen einer Bramme bei einer Temperatur von 900 °C bis 1100 °C nach Wiedererhitzen der Bramme bei 1000 °C bis 1100 °C, die nach Gew.-% C: 0,05 % bis 0,09 %, Mn: 1,61 % bis 2,0 %, Ni: 0,3 % bis 0,8 %, Nb: 0,005 % bis 0,04 %, Ti: 0,005 % bis 0,04 %, Cu: 0,1 % bis 0,5 %, Si: 0,1 % bis 0,3 %, Al: 0,005 % bis 0,05 %, P: 100 ppm oder weniger, S: 40 ppm oder weniger, Fe als Restbestandteil und unvermeidbare Verunreinigungen beinhaltet;

Erhalten eines Stahlblechs durch Endwalzen einer aus dem Vorwalzen erhaltenen Bramme bei einer Temperatur in einem Bereich von Ar₃ + 60 °C bis Ar₃ °C, basierend auf einer Temperatur eines mittleren Abschnitts; und

Abkühlen des Stahlblechs auf 700 °C oder weniger; und

Schweißen des Stahlblechs derart, dass eine darin gebildete Wärmeeinflusszone (WEZ) 5 Flächen-% oder weniger Martensitaustenit (MA) beinhaltet, und

wobei ein Reduktionsverhältnis pro Durchgang von drei Enddurchgängen während des Vorwalzens einer Bramme 5 % oder mehr beträgt und ein kumulatives Gesamtverringerungsverhältnis 40 % oder mehr beträgt, und

wobei eine Dehnungsrate von drei Enddurchgängen während des Vorwalzens einer Bramme 2/s oder weniger beträgt, und

wobei ein Verringerungsverhältnis während des Endwalzens so eingestellt wird, dass ein Verhältnis einer Brammendicke (mm) zu einer Stahlblechdicke (mm) nach dem Endwalzen 3,5 oder mehr beträgt, und

wobei ein kumulatives Verringerungsverhältnis während des Endwalzens auf 40 % oder mehr gehalten wird und ein Verringerungsverhältnis pro Durchgang ohne Berücksichtigung des Hautdurchgangswalzens auf 4 % oder mehr gehalten wird, und

wobei das Abkühlen des Stahls mit einer Abkühlrate des mittleren Abschnitts von 1,5 °C/s oder höher durchgeführt wird.

7. Verfahren nach Anspruch 8, wobei der Schweißwärmeeintrag während eines Schweißens 0,5 kJ/mm bis 10 kJ/mm beträgt.

8. Verfahren nach Anspruch 7, wobei es sich bei dem Schweißverfahren um Lichtbogenschweißen mit Fülldraht (FCAW) oder Unterpulverschweißen (UP-Schweißen) handelt.

Revendications

1. Matériau en acier à haute résistance doté d'une excellente capacité d'arrêt de propagation de fissures cassantes et d'une résistance à l'amorçage de fissures cassantes de la zone de soudage, comprenant :

en pourcentage en poids, du carbone (C) entre 0,05 et 0,09 %, du manganèse (Mn) entre 1,61 et 2,0 %, du nickel (Ni) entre 0,3 et 0,8 %, du niobum (Nb) entre 0,005 et 0,04 %, du titane (Ti) entre 0,005 et 0,04 %, du cuivre (Cu) entre 0,1 et 0,5 %, du silicium (Si) entre 0,05 et 0,3 %, de l'aluminium (Al) entre 0,005 et 0,05 %, du phosphore (P) à 100 ppm ou moins, du soufre (S) à 40 ppm ou moins, du fer (Fe) comme composant résiduel de celui-ci, et des impuretés inévitables,

dans lequel une microstructure d'une partie centrale comprend, en pourcentage en surface, de la ferrite aciculaire en une quantité de 70 % ou plus, de la perlite en une quantité de 10 % ou moins, et un ou plusieurs éléments sélectionnés dans un groupe constitué de ferrite, de bainite et de martensite-austénite (MA), comme composants résiduels ; un diamètre équivalent à un cercle de perlite étant de 15 µm ou moins ; dans lequel une microstructure de partie de surface dans une région à une profondeur de 2 mm ou moins, directement en dessous d'une surface, comprend, en pourcentage en surface, de la ferrite en une quantité de 30 % ou plus et un ou plusieurs des éléments que sont la bainite, la martensite et la perlite comme composants résiduels ; et dans lequel une zone affectée thermiquement (ZAT) formée pendant le soudage comprend, en pourcentage en surface, de la martensite-austénite (MA) en une quantité de 5 % ou moins.

2. Matériau en acier à haute résistance doté d'une excellente capacité d'arrêt de propagation de fissures cassantes et d'une résistance à l'amorçage des fissures cassantes de la zone de soudage selon la revendication 1, dans lequel, un rapport pondéral de Cu au Ni (un rapport pondéral Cu/Ni) est inférieur ou égal à 0,8.

3. Matériau en acier à haute résistance doté d'une excellente capacité d'arrêt de propagation de fissures cassantes et d'une résistance à l'amorçage des fissures cassantes de la zone de soudage selon la revendication 1, présentant une limite d'élasticité supérieure ou égale à 390 MPa.

4. Matériau en acier à haute résistance doté d'une excellente capacité d'arrêt de propagation de fissures cassantes et d'une résistance à l'amorçage des fissures cassantes de la zone de soudage selon la revendication 1, ayant une valeur Kca supérieure ou égale à 6 000 N/mm^1,5, mesurée à une température de -10 °C.

5. Matériau en acier à haute résistance doté d'une excellente capacité d'arrêt de propagation de fissures cassantes et d'une résistance à l'amorçage des fissures cassantes de la zone de soudage selon la revendication 1, comprenant une température de transition de rupture Charpy inférieure ou égale à -40 °C en position 1/2t (demi-épaisseur) dans une direction d'épaisseur de matériau en acier, où t représente une épaisseur de tôle d'acier.

6. Procédé de fabrication d'un matériau en acier à haute résistance doté d'une excellente capacité d'arrêt de propagation de fissures cassantes et d'une résistance à l'amorçage des fissures cassantes de la zone de soudage, consistant à :

laminer de manière brute une brame à une température comprise entre 900 et 1 100 °C après réchauffage de la brame entre 1 000 et 1 100 °C, comprenant, en pourcentage en poids, C entre 0,05 et 0,09 %, Mn entre 1,61 et 2,0 %, Ni entre 0,3 et 0,8 %, Nb entre 0,005 et 0,04 %, Ti entre 0,005 et 0,04 %, Cu entre 0,1 et 0,5 %, Si entre 0,1 et 0,3 %, Al entre 0,005 et 0,05 %, P à 100 ppm ou moins, S à 40 ppm ou moins, Fe comme composant résiduel, et des impuretés inévitables ;

obtenir une tôle d'acier par laminage de finition d'une barre obtenue à partir du laminage brut d'une brame, à une température comprise entre Ar₃ +60 °C et Ar₃ °C, sur la base d'une température d'une partie centrale ;

laisser refroidir la tôle d'acier à 700 °C ou moins ; et à

souder la tôle d'acier de sorte qu'une zone affectée thermiquement (ZAT) formée dans celle-ci comporte de la martensite-austénite (MA) à 5 % en surface ou moins,

dans lequel un rapport de réduction par passage de trois derniers passages pendant le laminage brut d'une brame est de 5 % ou plus, et un rapport total cumulatif de réduction est de 40 % ou plus,

dans lequel un taux de déformation de trois derniers passages pendant le laminage brut d'une brame est de 2/sec ou moins,

dans lequel un rapport de réduction pendant le laminage de finition est déterminé de sorte qu'un rapport d'une épaisseur de brame (mm) à une épaisseur de tôle d'acier (mm) après le laminage de finition soit de 3,5 ou plus,

dans lequel un rapport cumulatif de réduction pendant le laminage de finition est maintenu à 40 % ou plus, et un rapport de réduction par passage, sans compter le laminage à froid, est maintenu à 4 % ou plus, et

dans lequel le refroidissement de l'acier est effectué à une vitesse de refroidissement de la partie centrale de 1,5 °C/s ou plus.

7. Procédé selon la revendication 8, dans lequel l'apport de chaleur de soudage pendant le soudage est compris entre 0,5 et 10 kJ/mm.

8. Procédé selon la revendication 7, dans lequel lors du soudage, un procédé de soudage comprend le soudage à l'arc avec fil fourré (FCAW) ou le soudage à l'arc submergé (SAW).

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