KR101726082B1 - Steel having superior brittle crack arrestability and resistance brittle crack initiation of welding point and method for manufacturing the steel - Google Patents

Abstract

The purpose of the other aspect of the present invention is to provide a high-strength steel material with excellent brittle crack propagation resistance and excellent brittle crack initiation resistance; and a manufacturing method thereof. According to one aspect of the present invention, provided are a high-strength steel material with excellent brittle crack propagation resistance and excellent brittle crack initiation resistance comprising: 0.05-0.09 wt% of C, 1.5-2.2 wt% of Mn, 0.3-1.2 wt% of Ni, 0.005-0.04 wt% of Nb, 0.005-0.04 wt% of Ti, 0.1-0.8 wt% of Cu, 0.05-0.3 wt% of Si, 0.005-0.05 wt% of Al, 100 ppm or less of P, 40 ppm or less of S, and the remainder consisting of Fe and inevitable impurities. Fine structures of a central part of the high-strength steel material are formed of a mixed phase of eccentric ferrite and granular bainite, upper bainite, and the remainder consisting of at least one type selected from a group composed of ferrite, pearlite, and tempered martensite (MA). Fine structures of a surface unit in a section 2 mm or less right under a surface are formed of ferrite, and the remainder consisting of at least one type between bainite and martensite. A welding-heat influenced unit formed when welding includes 5 area% or less of tempered martensite. According to the present invention, the high-strength steel material with high yield strength, excellent brittle crack propagation resistance, and excellent brittle crack initiation resistance are obtained.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel having excellent brittle crack propagation resistance and weld brittle crack initiation resistance,

The present invention relates to a high strength steel excellent in brittle crack propagation resistance and weld brittle crack initiation resistance and a method for producing the same.

Recently, in designing structures used in domestic and overseas ships, marine, architecture, and civil engineering fields, development of ultra high strength steel having high strength characteristics is required.

When a high-strength steel is used for designing a structure, the shape of the structure can be reduced in weight and economical gain can be obtained. In addition, since the thickness of the steel sheet can be reduced, ease of machining and welding work can be secured at the same time.

Generally, in the case of high-strength steels, since the deformation rate of the extreme post-material is lowered, the deformation of the microstructure of the extreme post material does not occur due to the deformation of the material, .

Especially, in case of brittle crack propagation resistance which shows the stability of the structure, there are cases where a guarantee is required for application to major structures such as ships. However, when microstructures are coarsened, brittle crack propagation resistance is very low It is very difficult to improve the brittle crack propagation durability of super high strength steel

On the other hand, in the case of high strength steels with a yield strength of 460 MPa or more, various technologies such as surface cooling during surface finishing to reduce surface grain size and grain size control by bending stress during rolling were introduced to improve brittle crack propagation resistance.

However, this technique is helpful for micronization of the superficial structure, but it can not be said to be a fundamental countermeasure against brittle crack propagation resistance because the degradation of impact toughness due to coarsening of tissues other than the surface layer can not be solved.

In addition, with the introduction of a design concept to improve the safety of a ship by controlling brittle crack initiation itself in steel recently applied to large container ships, it is generally considered to be the most vulnerable site for brittle crack initiation There is an increasing number of cases in which the brittle crack initiation resistance of the affected part is guaranteed.

Generally, in the case of high-strength steel, the microstructure of the heat affected zone (HAZ) is composed of low-temperature transformation phases having high strength such as bainite, and thus the toughness of the heat affected zone (HAZ) Have.

Particularly, in order to evaluate the stability of the structure, in case of brittle crack initiation resistance generally evaluated by CTOD evaluation (Crack Tip Opening Displacement), it is considered that the malleous martensite generated from the untransformed austenite during the low- It is very difficult to improve the resistance to brittle cracking of high-strength steels

In the case of a conventional high-strength steel having a yield strength of 460 MPa or more, in order to improve the resistance to brittle crack initiation of the welded portion, TiN is used to refine the microstructure of the weld heat affected portion or to form ferrite in the weld heat affected portion by using oxide However, it has some effects on improving impact toughness through microstructure, but it has no great effect on reducing the fraction of graphite martensite which has a major influence on the reduction of brittle crack initiation resistance.

In addition, the brittle crack initiation resistance of the base material can be secured by transforming the martensite into another phase through tempering or the like, but in the case of a weld heat affected zone in which the effect of tempering is lost due to thermal history It is impossible to apply this.

On the other hand, in order to minimize the formation of martensite, it is necessary to reduce C and Nb elements. However, when it is reduced, it is difficult to secure the strength level. In order to secure the strength level, a large amount of high- There is a problem that economical efficiency is deteriorated.

It is an object of the present invention to provide a high strength steel excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.

Another aspect of the present invention is to provide a method of manufacturing a high strength steel excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.

According to an aspect of the present invention, there is provided a steel sheet comprising 0.05 to 0.09% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.04% of Nb, 0.005 to 0.04% 0.05 to 0.3% of Si, 0.005 to 0.05% of Al, 100 ppm or less of P, 40 ppm or less of S, and the balance of Fe and other unavoidable impurities; The microstructure of the core has an area percentage of 70% or more of a mixed phase of acicular ferrite and granular bainite, 20% or less of upper bainite, and remaining ferrite, pearlite, Martensite (MA), and the circle equivalent diameter of the effective crystal grains having a high boundary angle of 15 degrees or more measured by the EBSD method of the upper bainite is 15 mu m (micrometer) or less ; The surface microstructure of the area immediately below the surface of 2 mm or less consists of at least 20% ferrite and at least one of bainite and martensite; A high strength steel material excellent in brittle crack propagation resistance and welding brittle crack initiation resistance including on-road martensite of 5% or less is provided as the area% of the weld heat affected portion formed at the time of welding.

The content of Cu and Ni may be set so that the weight ratio of Cu / Ni is 0.8 or less, preferably 0.6 or less.

The steel may preferably have a yield strength of 460 MPa or more.

The steel material may preferably have a Charpy Fracture Transition Temperature of -40 DEG C or less in a steel material thickness of 1 / 2t (t: steel sheet thickness) portion in a thickness direction of the steel material.

According to another aspect of the present invention, there is provided a steel sheet comprising: 0.05 to 0.09% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.04% of Nb, 0.005 to 0.04% 0.1 to 0.8%, Si: 0.05 to 0.3%, Al: 0.005 to 0.05% P: not more than 100 ppm, S: not more than 40 ppm, and the balance Fe and other unavoidable impurities is reheated to 1000 to 1100 deg. C, followed by rough rolling at a temperature of 1100 to 900 deg. Rolling the roughly rolled bar at a temperature between Ar ₃ + 60 ° C and Ar ₃ ° C based on the center temperature to obtain a steel sheet; And cooling the steel sheet to a temperature of 500 DEG C or less, and a method of manufacturing a high strength steel having excellent brittle crack propagation resistance and weld brittle crack initiation resistance.

For the last three passes in the rough rolling, the rolling reduction per pass is preferably 5% or more and the total cumulative rolling reduction is preferably 40% or more

It is preferable to set the strain rate to 2 / sec or less for the last three passes during rough rolling

The grain size at the center of the thickness of the bar before the finish rolling after the rough rolling may be 150 탆 or less, preferably 100 탆 or less, more preferably 80 탆 or less.

The pressing force during the finish rolling may be set so that the ratio of the slab thickness (mm) / the steel sheet thickness (mm) after the finish rolling is 3.5 or more, preferably 4 or more.

Preferably, the cumulative reduction rate during the finish rolling is preferably at least 40%, and the reduction rate per pass excluding the final shape rolling is preferably maintained at 4% or more. The final shape rolling refers to the process of rolling at a low rolling reduction rate to ensure the flatness of the plate

The cooling of the steel sheet can be performed at a cooling rate of the central portion of 2 DEG C / s or more.

The steel sheet can be cooled at an average cooling rate of 3 to 300 DEG C / s.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, it is possible to obtain a high strength steel excellent in high yield strength, excellent brittle crack propagation resistance and weld brittle crack initiation resistance.

The inventors of the present invention conducted research and experiment to improve yield strength, brittle crack propagation resistance and weld brittle crack initiation resistance of a thick steel material, and the present invention was proposed based on the results.

The present invention improves the yield strength, brittle crack propagation resistance and weld brittle crack initiation resistance of a thick steel by controlling the steel composition, structure and manufacturing conditions of the steel.

The main concept of the present invention is as follows.

1) The steel composition is appropriately controlled to obtain strength improvement through strengthening employment. In particular, Mn, Ni, Cu and Si contents are optimized for solid solution strengthening.

2) The steel composition is appropriately controlled to improve the hardenability. Especially, the contents of Mn, Ni and Cu are optimized with the carbon content for the improvement of hardenability.

By improving the hardenability, a fine structure is secured up to the center portion of the thick steel even at a slow cooling rate.

And 3) the composition was appropriately controlled to control the fraction of martensite on the surface. In particular, the content of C, Si and Nb influencing on the formation of martensite is optimized.

By optimizing the steel composition in this way, excellent brittle crack initiation resistance is ensured even in the weld heat affected zone.

4) Preferably, the structure of the steel can be controlled to improve strength and brittle crack propagation resistance. Particularly, the structure of the center portion and the surface layer portion is controlled in the thickness direction of the steel material.

By controlling the microstructure in this manner, the brittle crack propagation resistance is improved by excluding the microstructure which ensures the strength required for the steel material and promotes the generation of cracks.

5) Preferably, the rough rolling conditions may be controlled to further refine the texture of the steel material.

Particularly, by controlling the pressing conditions at the time of rough rolling, a fine structure is secured in the center portion. This also promotes the formation of acicular ferrite and granular bainite.

6) Finishing rolling conditions are controlled to make the structure of steel more finer. Especially, by controlling the finishing rolling temperature and pressing conditions, a large amount of strain bands are generated in the austenite during finish rolling, and a large amount of ferrite nucleus sites are secured, whereby a fine structure is secured up to the center of the steel. This also promotes the formation of acicular ferrite and granular bainite.

Hereinafter, a high strength steel excellent in brittle crack propagation and brittle crack initiation resistance as one aspect of the present invention will be described in detail.

The high strength steel material excellent in brittle crack propagation resistance and weld brittle crack initiation resistance, which is one aspect of the present invention, contains 0.05 to 0.09% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.004 to 0.04% of Ti, 0.005 to 0.04% of Ti, 0.1 to 0.8% of Cu, 0.05 to 0.3% of Si, 0.005 to 0.05% of Al, P: not more than 100 ppm, S: not more than 40 ppm, the balance Fe and other unavoidable impurities; A mixture of 70% or more acicular ferrite and granular bainite, 20% or less of upper bainite, and a balance of ferrite, pearlite, (MA), and the circle equivalent diameter of the effective grain having a high boundary angle of 15 degrees or more measured by the EBSD method of the upper bainite is 15 mu m (micrometer) or less ; The surface microstructure of the area immediately below the surface of 2 mm or less consists of at least 20% ferrite and at least one of bainite and martensite; Incidentally, the weld heat affected zone formed in welding includes not more than 5% of on-state martensite.

Hereinafter, the steel component and the component range of the present invention will be described.

C (carbon): 0.05 to 0.09% by weight (hereinafter referred to as "%"),

Since C is the most important element for securing the basic strength, it is necessary to be contained in the steel within an appropriate range. In order to obtain such an additive effect, C is preferably added in an amount of 0.05% or more.

However, when the content of C exceeds 0.09%, a large amount of on-state martensite is generated in the weld heat affected zone to lower the brittle crack initiation resistance, and the high strength of the ferrite itself of the base material and the large amount of low- The low-temperature toughness is lowered. Therefore, the content of C is preferably limited to 0.05 to 0.09%.

More preferably, the content of C is 0.055 to 0.08%, and more preferably 0.06 to 0.075%.

Mn (manganese): 1.5 to 2.2%

Mn is a useful element that improves the hardenability by enhancing strength by solid solution strengthening and producing a low temperature transformation phase. In addition, since the hardening ability is improved, the low temperature transformation phase can be formed even at a slow cooling rate, and thus it is a main element for securing the strength of the center portion of the extreme post material.

Therefore, in order to obtain such an effect, it is preferable that it is added by 1.5% or more.

However, when the content of Mn exceeds 2.2%, excessive bainite and martensite are promoted due to an increase in the hardenability, thereby lowering the impact toughness and brittle crack propagation resistance, and also the toughness of the weld heat affected zone .

Therefore, the Mn content is preferably limited to 1.5 to 2.2%.

The more preferable content of Mn is 1.6 to 2.0%, and more preferably 1.65 to 1.95%.

Ni (nickel): 0.3 to 1.2%

Ni is an important element for facilitating cross slip of dislocations at low temperature to improve impact toughness and hardenability and to improve strength. In order to obtain such an effect, it is preferable that Ni is added in an amount of 0.3% or more. However, when 1.2% or more of Ni is added, the curing ability is excessively elevated to produce a low-temperature transformation phase, thereby lowering the toughness and raising the manufacturing cost due to the high cost of Ni relative to other hardenable elements. %.

More preferably, the content of Ni is limited to 0.4 to 1.0%, and more preferably to 0.45 to 0.9%.

Nb (niobium): 0.005 to 0.04%

Nb precipitates in the form of NbC or NbCN to improve the strength of the base material.

In addition, the Nb solidified at the time of reheating at a high temperature is extremely finely precipitated in the form of NbC at the time of rolling, thereby suppressing the recrystallization of austenite, and thereby making the structure finer.

Therefore, it is preferable that Nb is added in an amount of 0.005% or more, but if it is added excessively, there is a possibility of causing a brittle crack in the edge of the steel material by lowering resistance to brittle crack initiation by promoting the generation of martensite in the welded heat affected zone , And the upper limit of the Nb content is preferably limited to 0.04%.

The content of Nb is more preferably 0.01 to 0.035%, and still more preferably 0.015 to 0.03%.

Ti (titanium): 0.005 to 0.04%

Ti is a component that precipitates as TiN upon re-heating to suppress the growth of crystal grains in the base material and the weld heat affected zone, thereby greatly improving the low-temperature toughness. In order to obtain such an effect, Ti is preferably added in an amount of 0.005% or more.

However, if Ti is added excessively, clogging of the performance nozzle and low temperature toughness due to centering can be reduced, so that the Ti content is preferably limited to 0.005 to 0.04%.

The content of Ti is more preferably 0.008 to 0.03%, and still more preferably 0.01 to 0.02%.

Si: 0.05 to 0.3%

Si is a substitutional element which improves the strength of the steel through solid solution strengthening and has a strong deoxidizing effect and is an essential element for the production of clean steel. However, when added in a large amount, a coarse-ground martensite (MA) phase is formed to reduce the brittle crack propagation and the brittle crack initiation resistance of the welded portion. Therefore, the upper limit of the Si content is preferably limited to 0.3%.

The Si content is more preferably 0.1 to 0.25%, and still more preferably 0.1 to 0.2%.

Cu: 0.1 to 0.8%

Cu is a main element for improving the hardenability and strengthening the solid solution by improving the hardenability, and it is a main element for increasing the yield strength through the formation of the epsilon Cu precipitate when the tempering is applied. . However, since a large amount of steel may cause cracks in slabs due to hot shortness during steelmaking, the upper limit of the Cu content is preferably limited to 0.8%.

More preferably, the content of Cu is limited to 0.2 to 0.6%, and more preferably to 0.25 to 0.5%.

The content of Cu and Ni may be set so that the weight ratio of Cu / Ni is 0.8 or less, preferably 0.6 or less. Still more preferably 0.5 or less.

When the Cu / Ni weight ratio is set as described above, the surface quality can be further improved.

Al: 0.005 to 0.05%

Al is a component acting as a deoxidizer, and when it is added in an excessive amount, inclusions can be formed to lower the toughness, and therefore the content thereof is preferably limited to 0.005 to 0.05%.

P: not more than 100 ppm, S: not more than 40 ppm

P and S are elements which induce brittleness in grain boundaries or cause coarse inclusions to induce brittleness. In order to improve brittle crack propagation resistance, it is preferable to limit P to not more than 100 ppm and S to not more than 40 ppm.

The remainder of the present invention is iron (Fe).

However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded.

These impurities are not specifically mentioned in this specification, since they can be known by any ordinary person skilled in the art.

The steel material of the present invention is characterized in that the central microstructure is a mixture of 70% or more of acicular ferrite and granular bainite in the percentage of the central microstructure, 20% or less of upper bainite ), And the remainder is at least one selected from the group consisting of ferrite, pearlite and malleable martensite (MA), and the circle equivalent of the effective grain having a high boundary angle of at least 15 degrees measured by the EBSD method of the upper bainite A diameter of 15 mu m (micrometer) or less; A microstructure in an area of 2 mm or less directly below the surface is composed of at least 20% of ferrite and at least one of bainite and martensite; The weld heat affected zone formed at the time of welding includes not less than 5% of on-road martensite in terms of area%.

When the fraction of the mixture of acicular ferrite and granular bainite in the center microstructure is less than 70%, it may be difficult to secure a sufficient yield strength. For example, a yield strength of 460 MPa or more Can be difficult to secure.

More preferably, the fraction of the mixed phase of the acicular ferrite and the granular bean is limited to 75% or more, and more preferably 80% or more.

The fraction of the acicular ferrite is preferably 20 to 70%.

If the fraction of acicular ferrite is more than 70%, it may be difficult to secure a sufficient yield strength due to a decrease in strength. For example, it may be difficult to secure a yield strength of 460 MPa or more. When the fraction of acicular ferrite is less than 20% There is a possibility that impact toughness may be lowered due to high strength.

More preferably, the fraction of the acicular ferrite is limited to 30 to 50%, and more preferably to 30 to 40%.

remind The fraction of granular bainite is preferably 10 to 60%.

If the fraction of the granular bainite exceeds 60%, impact toughness may be deteriorated due to high strength. When the fraction is less than 10%, it may be difficult to obtain a sufficient yield strength due to a decrease in strength. , For example, securing a yield strength of 460 MPa or more may be difficult.

More preferably, the fraction of granularlainite is limited to 20 to 50%, and more preferably to 30 to 50%.

When the fraction of the central portion upper bainite exceeds 20%, microcracks are generated at the crack tip at the time of brittle crack propagation, thereby lowering the brittle crack propagation resistance. Therefore, the central bainite upper bainite is preferably 20% or less.

More preferably, the fraction of the upper bainite is 15% or less, and more preferably 10% or less.

When the circle equivalent diameter of the effective crystal grains having a high boundary angle of 15 degrees or more measured by the EBSD method of the central portion upper bainite exceeds 15 mu m (micrometer), cracks are easily induced despite the fraction of lower upper bainite It is preferable that the circle equivalent diameter of the effective crystal grains of the central upper bainite is 15 mu m or less.

The surface microstructure of the area immediately below the surface of 2 mm or less When ferrite is contained in an amount of 20% or more, brittle crack propagation resistance can be improved by effectively preventing crack propagation on the surface during brittle crack propagation.

The more preferable fraction of ferrite is 30% or more, and more preferably 40% or more.

The ferrite in the microstructure of the central part and the surface part means a polygonal ferrite or an elongated polygonal ferrite

If the amount of martensite in the welded heat affected zone of the steel exceeds 5%, it acts as a crack initiation start point to lower the resistance to brittle crack initiation. Therefore, it is preferable that the fraction of martensite on the welded heat affected zone is 5% or less.

The welding heat input amount during the welding may be 0.5 to 10 kJ / mm.

The welding method at the time of welding is not particularly limited, and examples thereof include FCW (Flux Cored Arc Welding) and SAW (Submerged Arc Welding).

The steel may preferably have a yield strength of 460 MPa or more.

The steel material may preferably have a Charpy Fracture Transition Temperature of -40 DEG C or less in a steel material thickness of 1 / 2t (t: steel sheet thickness) portion in a thickness direction of the steel material.

The steel may have a thickness of 50 mm or more, and preferably 50 to 100 mm.

Hereinafter, a method of manufacturing a high strength steel excellent in resistance to brittle crack propagation, which is another aspect of the present invention, will be described in detail.

A method of manufacturing a high strength steel material excellent in brittle crack propagation resistance and weld brittle fracture initiation resistance, which is another aspect of the present invention, comprises 0.05 to 0.09% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.04% of Ti, 0.001 to 0.04% of Ti, 0.1 to 0.8% of Cu, 0.05 to 0.3% of Si, 0.005 to 0.05% of Al, 100 ppm or less of P and 40 ppm or less of S and the balance of Fe and other unavoidable impurities Reheating the slab including the slab to 1000 to 1100 占 폚, followed by rough rolling at a temperature of 1100 to 900 占 폚; Rolling the roughly rolled bar at a temperature between Ar ₃ + 60 ° C and Ar ₃ ° C based on the center temperature to obtain a steel sheet; And cooling the steel sheet to a temperature of 500 DEG C or less.

Reheating slabs

Reheat the slab before rough rolling.

The slab reheating temperature is preferably 1000 ° C or higher, in order to solidify the carbonitride of Ti and / or Nb formed during the casting.

However, when reheating at an excessively high temperature, there is a possibility that the austenite is coarsened, so the upper limit of the reheating temperature is preferably 1100 ° C.

Rough rolling

The reheated slab is rough-rolled.

The rough rolling temperature is preferably set to be not lower than the temperature (Tnr) at which recrystallization of the austenite is stopped. It is possible to obtain an effect of reducing the size of austenite and breaking the cast structure such as dendrites formed during casting by rolling. In order to obtain such an effect, the rough rolling temperature is preferably limited to 1100 to 900 ° C.

The more preferable rough rolling temperature is 1050 to 950 占 폚.

In the present invention, in order to miniaturize the structure of the center portion in rough rolling, it is preferable that the rolling reduction per pass is not less than 5% and the total cumulative rolling reduction is not less than 40% for the last three passes during rough rolling.

A more preferable reduction ratio per pass is 7 to 20%

A more preferable total cumulative reduction ratio is 45% or more.

During the rough rolling, the grain recrystallized due to the initial rolling causes the grain growth due to the high temperature. However, when the last 3 passes are performed, the grain growth rate is slowed as the bars are air-cooled in the rolling atmosphere, The reduction rate of the pass is the largest in the grain size of the final microstructure.

Also, when the reduction rate per pass of the rough rolling becomes low, sufficient deformation is not transmitted to the center portion, which may cause toughness degradation due to center coarsening. Therefore, it is preferable to limit the reduction rate per pass of the last three passes to 5% or more.

On the other hand, in order to miniaturize the structure of the center portion, it is preferable to set the cumulative rolling reduction ratio at the time of rough rolling to 40% or more.

It is preferable that the strain rate is not more than 2 / sec for the last three passes during rough rolling.

In general, since it is difficult to roll at a high rolling reduction due to a thick bar thickness during rough rolling, it is difficult to transfer a reduction amount to the center of the extreme post material, thereby causing a problem that the grain size of the center austenite is coarsened. However, as the strain rate is lowered, deformation is transmitted to the center part even at a small rolling reduction, and the grain size can be miniaturized.

Therefore, by limiting the deformation rate to less than 2 / sec for the last 3 pass which has the greatest effect on the final grain size in the rough rolling, the grain size of the core is made finer and the generation of the acicular ferrite and granular bainite . ≪ / RTI >

Finish rolling

The roughly rolled bars are finish-rolled at Ar ₃ (ferrite transformation start temperature) + 60 ° C to Ar ₃ ° C to obtain a steel sheet.

This is to obtain finer microstructures. When the steel is rolled at a temperature of Ar ₃ , a large amount of strain bands are formed in the austenite, thereby securing a large number of ferrite nucleus sites, thereby securing a fine structure to the center of the steel Can be obtained.

Further, in order to effectively generate a large amount of strain bands in the austenite, it is preferable to maintain the cumulative rolling reduction rate at the finish rolling at 40% or more and the rolling reduction per pass excluding the final shape rolling at 4% or more.

A more preferable cumulative reduction rate is 40 to 80%

A more preferable reduction ratio per pass is 4.5% or more.

The finish rolling temperature was set to Ar ₃ , The coarse ferrite is formed before rolling, and the impact toughness is lowered as the steel is elongated during rolling. If the steel is finely rolled at a temperature higher than Ar ₃ + 60 ° C, it is not effective in fineness of the grain size. Therefore, Is preferably set to Ar ₃ + 60 ° C to Ar ₃ .

In the present invention, the reduction ratio of the non-recrystallized region But is preferably limited to 40 to 80%.

As described above, since the nucleation sites of acicular ferrite and granular bainite are increased by controlling the reduction ratio of the non-recrystallized region, the formation of these tissues can be further promoted.

If the non-recrystallized zone reduction ratio is too low, acicular ferrite and granular bainite can not be sufficiently secured. If the non-recrystallized zone reduction ratio is too high, the strength is lowered due to the generation of cornerstone ferrite due to a high reduction ratio There is a concern.

The grain size at the center of the thickness of the bar before the finish rolling after the rough rolling may be 150 탆 or less, preferably 100 탆 or less, more preferably 80 탆 or less.

The grain size at the center of the thickness of the bar before the finish rolling after the rough rolling can be controlled according to the rough rolling conditions and the like.

As described above, when the grain size of the bar before the finish rolling is controlled after the rough rolling, the final microstructure is miniaturized by the fineness of the austenite grain size, so that an advantage of improving the low temperature impact toughness can be added.

The pressing force during the finish rolling may be set so that the ratio of the slab thickness (mm) / the steel sheet thickness (mm) after the finish rolling is 3.5 or more, preferably 4 or more.

As described above, in the case of controlling the compression ratio, an increase in yield and tensile strength due to finer microstructure and an improvement in low-temperature toughness and a reduction in the center portion toughness by increasing the reduction amount during rough rolling and finishing rolling, Can be added.

After finish rolling, the steel sheet may have a thickness of 50 mm or more, and preferably 50 to 100 mm.

Cooling

After finishing rolling, the steel sheet is cooled to 500 ° C or less.

If the cooling termination temperature exceeds 500 ° C, the microstructure is not appropriately formed, and it may be difficult to secure a sufficient yield strength. For example, it may be difficult to secure a yield strength of 460 MPa or more.

If the cooling end temperature exceeds 400 ° C, the amount of the acicular ferrite (AF) and granulobenite (GB) may be reduced and the strength may be lowered due to the auto tempering effect.

The preferred cooling end temperature is 400 占 폚 or less.

The cooling of the steel sheet can be performed at a central cooling rate of 2 DEG C / s or more. If the center cooling rate of the steel sheet is less than 2 DEG C / s, microstructure is not properly formed, For example, it may be difficult to secure a yield strength of 460 MPa or more.

The cooling of the steel sheet can be performed at an average cooling rate of 3 to 300 DEG C / s.

Hereinafter, the present invention will be described more specifically by way of examples.

It should be noted, however, that the following examples are intended to illustrate the present invention by way of illustration and not to limit the scope of the present invention.

The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

A 400 mm thick steel slab having the composition shown in the following Table 1 was reheated to a temperature of 1045 DEG C and then rough rolling was started at a temperature of 1020 DEG C to prepare a bar. The cumulative rolling reduction rate during rough rolling was 52%.

The thickness of the rough-rolled bar was 192 mm. As shown in Table 2, the grain size before roughing after rough rolling was 66 to 82 탆. During the rough rolling, the reduction rate of the last 3 passes was within 7.9 ~ 14.1%, and the strain rate during rolling was in the range of 1.22 ~ 1.68 / s.

After the rough rolling, finish rolling was performed at a cumulative reduction ratio of 50% at a reduction rate of 4.2 to 5.6% per pass at a temperature of the difference between the finish rolling temperature and the Ar ₃ temperature shown in Table 2 below, The steel sheet was then cooled to a temperature of 241 to 378 DEG C at a cooling rate of 3.8 to 5.0 DEG C / sec in the central portion.

The microstructure, yield strength, Kca value (brittle crack propagation resistance coefficient) and CTOD value (brittle crack initiation resistance coefficient) of the steel sheet thus prepared were examined, and the results are shown in Tables 3 and 4 below.

The surface properties shown in Table 3 below were measured to determine whether surface cracking occurred due to hot shortness caused by the Cu / Ni addition ratio.

In addition, the Kca value in the following Table 4 is evaluated by ESSO test for the steel sheet, and CTOD value was subjected to FCAW (1.0 kJ / mm) welding, and tissue analysis and CTOD evaluation were performed on the weld heat affected portion Results.

Steel grade Steel composition (% by weight) C Si Mn Ni Cu Ti Nb Al P (ppm) S (ppm) Cu / Ni
Weight ratio Inventive Steel 1 0.078 0.21 1.82 0.73 0.32 0.023 0.032 0.030 63 18 0.44 Invention river 2 0.069 0.19 1.72 0.66 0.39 0.012 0.022 0.031 72 15 0.59 Invention steel 3 0.057 0.22 2.05 0.57 0.26 0.017 0.027 0.025 56 16 0.46 Inventive Steel 4 0.072 0.21 1.83 0.62 0.33 0.022 0.019 0.035 49 13 0.53 Invention steel 5 0.084 0.17 1.58 1.06 0.49 0.016 0.033 0.040 66 12 0.46 Invention steel 6 0.071 0.23 1.93 0.36 0.19 0.018 0.028 0.025 43 23 0.53 Invention steel 7 0.066 0.18 1.82 0.79 0.36 0.019 0.012 0.020 39 31 0.46 Comparative River 1 0.13 0.19 1.88 0.63 0.29 0.021 0.029 0.021 79 13 0.46 Comparative River 2 0.067 0.45 1.96 0.59 0.22 0.011 0.038 0.032 88 9 0.37 Comparative Steel 3 0.071 0.19 2.44 0.88 0.32 0.013 0.021 0.029 65 23 0.36 Comparative Steel 4 0.082 0.18 2.02 1.62 0.52 0.022 0.026 0.034 55 19 0.32 Comparative Steel 5 0.072 0.27 1.89 0.62 0.44 0.043 0.049 0.030 48 22 0.71 Inventive Steel 8 0.062 0.18 1.93 0.59 0.63 0.015 0.029 0.031 65 16 1.07 Comparative Steel 6 0.042 0.22 1.36 0.55 0.21 0.019 0.033 0.027 43 18 0.38

Example No. 2.
Steel grade
(≪ RTI ID = 0.0 > m) < / RTI & The average rolling reduction (%) of the last 3 passes during rough rolling The average strain rate (/ s) of the last 3 passes during rough rolling Average rolling reduction per pass during finish rolling (%) Finishing rolling temperature
-Ar ₃ Temperature (캜) Cooling rate in the center (℃ / sec) Cooling end temperature (캜) Inventory 1 Inventive Steel 1 78 11.3 1.61 4.2 35 4.1 324 Inventory 2 Invention river 2 66 9.6 1.35 4.5 43 4.4 285 Inventory 3 Invention steel 3 79 10.3 1.44 5.1 29 4.3 296 Honorable 4 Inventive Steel 4 75 8.9 1.46 5.3 41 3.8 335 Inventory 5 Invention steel 5 69 13.2 1.67 4.8 23 3.9 342 Inventory 6 Invention steel 6 73 14.1 1.32 4.2 15 4.3 312 Comparative Example 1 Invention steel 7 79 12.2 1.22 4.7 93 4.8 256 Comparative Example 2 Comparative River 1 76 10.3 1.68 5.6 28 4.4 330 Comparative Example 3 Comparative River 2 68 13.1 1.55 5.2 26 4.2 351 Comparative Example 4 Comparative Steel 3 77 7.9 1.54 5.3 39 4.1 241 Comparative Example 5 Comparative Steel 4 81 10.1 1.39 4.5 18 4.6 378 Comparative Example 6 Comparative Steel 5 82 10.9 1.41 4.9 13 4.7 312 Honorable 7 Inventive Steel 8 73 11.3 1.43 5.1 46 4.1 333 Comparative Example 7 Comparative Steel 6 72 9.8 1.52 5.3 53 5.0 316

Example No. 2.
Steel grade
Surface property
Steel plate thickness
(mm)
Percentage of microstructure in the center (area%) Surface portion ferrite phase fraction (%) The acicular ferrite (AF) Granular Bainite (GB) Upper bainite (average particle size, μm) The remaining phase fraction (at least one of ferrite / perlite / MA Inventory 1 Inventive Steel 1  Not occurring 85 37.9 45.4 13.9 (12.2) 2.8 45 Inventory 2 Invention river 2  Not occurring 80 49.3 41.9 7.2 (9.5) 1.6 39 Inventory 3 Invention steel 3  Not occurring 75 36.5 45.1 16.8 (9.6) 1.6 53 Honorable 4 Inventive Steel 4  Not occurring 95 39.8 53.2 5.1 (8.6) 1.9 26 Inventory 5 Invention steel 5  Not occurring 90 42.6 45.6 9.2 (11.3) 2.6 51 Inventory 6 Invention steel 6  Not occurring 100 29.7 48.8 15.6 (11.6) 5.9 66 Comparative Example 1 Invention steel 7  Not occurring 90 32.6 46.6 12.4 (12.2) 8.4 0 Comparative Example 2 Comparative River 1  Not occurring 90 20.5 39.3 32 (18.3) 8.2 59 Comparative Example 3 Comparative River 2  Not occurring 85 51.2 31 11.3 (9.7) 6.5 52 Comparative Example 4 Comparative Steel 3  Not occurring 80 16.8 32.1 43.8 (17.1) 7.3 36 Comparative Example 5 Comparative Steel 4  Not occurring 90 13.2 30.6 49.8 (19.0) 6.4 61 Comparative Example 6 Comparative Steel 5  Not occurring 75 37.5 41.6 13.7 (13.3) 7.2 59 Honorable 7 Inventive Steel 8  Occur 90 53.9 38.7 6.1 (9.6) 1.3 31 Comparative Example 7 Comparative Steel 6  Not occurring 90 13.2 20.3 6.2 (13.2) 60.3 33

Example No. 2. Steel grade Yield strength
(Mpa) ^{Kca (N / mm 1 .5,} @ - 10 ℃) Weld Heat Affected Duct Martensite Fraction (%) Weld heat affected zone
CTOD value (mm) Inventory 1 Inventive Steel 1 512 7120 1.8 0.56 Inventory 2 Invention river 2 489 6988 1.9 0.49 Inventory 3 Invention steel 3 504 7286 2.6 0.55 Honorable 4 Inventive Steel 4 518 7057 2.3 0.43 Inventory 5 Invention steel 5 485 7516 2.1 0.68 Inventory 6 Invention steel 6 523 7030 3.2 0.56 Comparative Example 1 Invention steel 7 501 5549 3.9 0.47 Comparative Example 2 Comparative River 1 579 4256 6.8 0.12 Comparative Example 3 Comparative River 2 496 6775 7.8 0.16 Comparative Example 4 Comparative Steel 3 577 4356 3.1 0.22 Comparative Example 5 Comparative Steel 4 562 4150 2.1 0.59 Comparative Example 6 Comparative Steel 5 532 6554 7.2 0.12 Honorable 7 Inventive Steel 8 516 7211 1.3 0.54 Comparative Example 7 Comparative Steel 6 435 5026 2.1 0.62

As shown in Tables 1 to 4, in the case of Comparative Example 1, the finishing rolling temperature-Ar3 temperature difference in the finish rolling proposed in the present invention was controlled to 60 占 폚 or higher, and rolling at a high temperature, As the cooling starts at a high temperature, no more than 20% of ferrite is produced on the surface portion, so that the value of Kca measured at -10 ° C does not exceed 6,000 required in general steel for shipbuilding.

In the case of Comparative Example 2, the content of C was higher than the upper limit of the C content of the present invention. As a result, a large amount of coarse upper bainite was formed at the center portion during rough rolling, Value of 6000 or less. Also, a large amount of on-state martensite (MA) structure is generated in the weld heat affected zone, and the CTOD value is 0.25 mm or less.

In the case of Comparative Example 3, the content of Si is higher than the upper limit of the Si content of the present invention. As a large amount of Si is added, a large amount of MA structure is generated in the weld heat affected portion and the CTOD value is 0.25 mm or less .

In the case of Comparative Example 4, since the Mn content is higher than the upper limit of the Mn content of the present invention, and because of the high hardening ability, the Kca value is in the range of -10 ° C to 6,000 or less Able to know. In addition, the high Ceq value indicates that the CTOD value is less than 0.25 even though the MA phase is small in the weld heat affected zone.

In the case of Comparative Example 5, the Ni content was higher than the upper limit of the Ni content of the present invention, and a large amount of the upper bainite was generated at the center portion with a high hardenability. As a result, the Kca value was decreased from -10 DEG C to less than 6000 . ≪ / RTI > However, the CTOD value is excellent due to the high Ni content.

Comparative Example 6 In the case where the content of Nb and Ti is higher than the upper limit of the content of Nb and Ti of the present invention and all other conditions satisfy the conditions of the present invention, A large amount of MA structure is generated in the part, and the value of CTOD is 0.25 mm or less

In the case of Inventive Example 7, it was confirmed that there was a star crack on the surface and the surface quality was abnormal due to the fact that it had a component exceeding the Cu / Ni ratio as suggested in a preferred aspect of the present invention, .

Comparative Example 7 In the case where the content of C and Mn is lower than the lower limit of the contents of C and Mn of the present invention, the fraction of AF + GB is very low in the center portion due to the low hardenability and a large amount of polygonal ferrite and 10 % Or more pearlite structure, and thus the value of Kca has a value of less than 6000 at -10 ° C

On the other hand, in Inventive Examples 1 to 6 satisfying the composition range and the manufacturing range of the present invention, the AF + GB of the central microstructure was 70% or more, the fraction of the central upper bainite was 20% The equivalent circle diameter of effective grains having a high boundary angle of 15 degrees or more of bainite is 15 占 퐉 or less and the MA phase fraction of the weld heat affected zone is less than 5%.

Examples 1 to 6 show a yield strength of 460 MPa or more, a Kca value of 6,000 or more at -10 ° C, and a CTOD value of 0.25 mm or more.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (17)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.09% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.04% of Nb, 0.005 to 0.04% of Ti, 0.1 to 0.8% 0.3%, Al: 0.005 to 0.05%, P: not more than 100 ppm, S: not more than 40 ppm, and the balance Fe and other unavoidable impurities; The microstructure of the core has an area percentage of 70% or more of a mixed phase of acicular ferrite and granular bainite, 20% or less of upper bainite, and remaining ferrite, pearlite, Martensite (MA), and the circle equivalent diameter of the effective crystal grains having a high boundary angle of 15 degrees or more measured by the EBSD method of the upper bainite is 15 mu m (micrometer) or less ; The surface microstructure of the area immediately below the surface of 2 mm or less consists of at least 20% ferrite and at least one of bainite and martensite; And a high heat resistant steel material excellent in brittle crack propagation resistance and welding brittle crack initiation resistance including not more than 5%
The high-strength steel material according to claim 1, wherein the steel has a thickness of 50 mm or more, and is excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.
The high strength steel material according to claim 1, wherein the content of Cu and Ni is such that the weight ratio of Cu / Ni is 0.8 or less, and is excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.
The high strength steel material according to claim 1, wherein the weld heat input amount in welding is 0.5 to 10 kJ / mm, and is excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.
The high strength steel material according to claim 4, wherein the welding method during welding is Flux Cored Arc Welding (FCAW) or Submerged Arc Welding (SAW), which is excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.
The high strength steel material according to claim 1, wherein the steel has a yield strength of 460 MPa or more and is excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.
The steel material according to any one of claims 1 to 6, wherein the steel material has a Kca value (brittle crack propagation resistance coefficient) measured at -10 캜 of 6000 or more, and has excellent brittle crack propagation resistance and weld brittle crack initiation resistance High strength steel.
The brittle crack propagation resistance and weld brittle crack initiation characteristics according to claim 1, wherein the steel material has a Charpy fracture transition temperature in a steel material thickness of 1 / 2t (t: steel sheet thickness) High strength steel with excellent resistance.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.09% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.04% of Nb, 0.005 to 0.04% of Ti, 0.1 to 0.8% 0.3%, Al: 0.005 to 0.05% P: not more than 100 ppm, S: not more than 40 ppm, and the balance Fe and other unavoidable impurities is reheated to 1000 to 1100 deg. C, followed by rough rolling at a temperature of 1100 to 900 deg. Rolling the roughly rolled bar at a temperature between Ar ₃ + 60 ° C and Ar ₃ ° C based on the center temperature to obtain a steel sheet; And cooling the steel sheet to a temperature of 500 ° C or lower. The brittle crack propagation resistance and the welded brittle crack in which the strain rate is less than 2 / sec for the last three passes during rough rolling A method of manufacturing a high strength steel excellent in initiation resistance.
The method of manufacturing a high strength steel material according to claim 9, wherein the thickness of the finish-rolled steel sheet is 50 mm or more, and is excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.
The brittle crack propagation resistance and weld brittle cracks according to claim 9, characterized in that the reduction rate per pass is at least 5% and the total cumulative rolling reduction is at least 40% for the last three passes during rough rolling. A method of manufacturing a high strength steel excellent in initiation resistance.
delete The method of manufacturing a high strength steel material according to claim 9, wherein the grain size of the center of gravity of the bar before the finish rolling after the rough rolling is 150 탆 or less is excellent in the brittle crack propagation resistance and the weld brittle crack initiation resistance.
The high strength steel material according to claim 9, wherein the ratio of the slab thickness (mm) to the steel sheet thickness (mm) after finish rolling is 3.5 or more. The brittle crack propagation resistance and the weld brittle crack initiation resistance Gt;
The brittle crack propagation resistance and weld brittle cracks according to claim 9, characterized in that the cumulative reduction ratio during the final rolling is maintained at 40% or more and the reduction ratio per pass excluding the final shape rolling is maintained at 4% A method of manufacturing a high strength steel excellent in initiation resistance.
The method of manufacturing a high strength steel material according to claim 9, wherein the cooling of the steel plate is performed at a cooling rate of the central portion of 2 占 폚 / s or more and excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.
10. The method of manufacturing a high strength steel material according to claim 9, wherein the cooling of the steel sheet is performed at an average cooling rate of 3 to 300 DEG C / s and is excellent in brittle crack propagation resistance and weld brittle crack initiation resistance.