KR101696154B1 - Steel having excellent in resistibility of brittle crack arrestbility and manufacturing method thereof - Google Patents

Abstract

The present invention provides a structural reinforcement material excellent in brittle crack propagation resistance and a method of manufacturing the same.
According to the present invention, there is provided a steel sheet comprising, by weight%, 0.02 to 0.10% of C, 0.8 to 2.5% of Mn, 0.05 to 1.5% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Ti, balance Fe and other unavoidable impurities And a single structure selected from the group consisting of ferrite single phase structure, bainite single phase structure, complex structure of ferrite and bainite, composite structure of ferrite and paleite, and complex structure of ferrite, bainite and pearlite Which is excellent in brittle crack propagation resistance, and a method for manufacturing the same.
According to one aspect of the present invention, it is possible to obtain a steel material having a superior yield strength and an excellent brittle crack propagation resistance with an excellent core transition temperature.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent brittle crack propagation resistance and a method of manufacturing the steel material.

The present invention relates to a structural reinforcement material excellent in brittle crack propagation resistance and a method of manufacturing 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 steel, since the central portion is not sufficiently deformed due to the decrease in the total reduction rate in the production of extreme post material, the core structure becomes coarse and the hardenability thereof increases, resulting in a low temperature transformation image such as bainite.

Also, it may be difficult to obtain impact toughness at the center due to coarsened organization.

Especially, in case of brittle crack propagation resistance which shows the stability of the structure, there is an increasing number of cases in which a guarantee is required in application to main structures such as ships, but the brittle crack propagation resistance is very low when the low temperature transformation phase is formed at the center portion 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 350 MPa or more, various techniques such as application of surface cooling during finishing rolling for finer grain size of the surface layer and grain refinement by applying bending stress during rolling and surface fine- Respectively.

However, this technique is helpful for micronization of the superficial structure, but it can not be said to be a fundamental countermeasure against the brittle crack propagation resistance because the decrease in the impact toughness due to the coarsening of the core structure can not be solved.

In addition, since the technology itself is expected to be greatly reduced in productivity to be applied to a general production system, it can not be said to be a commercial application.

In addition, when a large amount of elements such as Ni is added to improve the toughness, the brittle crack propagation resistance can be improved. However, since Ni is an expensive element, it is difficult to commercialize it in terms of manufacturing cost.

Korean Patent Publication No. 2009-0069818 Korean Patent Publication No. 2002-0091844

According to an aspect of the present invention, there is provided a structural steel material having excellent brittle crack propagation resistance.

According to another aspect of the present invention, there is provided a method of manufacturing a structural steel material having excellent brittle crack propagation resistance by controlling an alloy composition and a microstructure.

According to one aspect of the present invention, there is provided a steel sheet comprising, by weight%, 0.02 to 0.10% of C, 0.8 to 2.5% of Mn, 0.05 to 1.5% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Ti, And other unavoidable impurities, and one selected from the group consisting of ferrite single phase structure, bainite single phase structure, complex structure of ferrite and bainite, composite structure of ferrite and paleite, and complex structure of ferrite, bainite and pearlite There is provided a structural reinforcement steel having excellent microstructure including brittle crack propagation resistance.

The steel may preferably have a grain size of 15 micrometers or less with a high-angle boundary of 15 degrees or greater measured by the EBSD method at the center of the plate thickness.

The steel may preferably have a yield strength of 350 MPa or more and a core impact transit temperature of -60 캜 or less.

According to another aspect of the present invention, there is provided a method for producing a steel sheet, which comprises 0.02 to 0.1% of C, 0.8 to 2.5% of Mn, 0.05 to 1.5% of Ni, 0.005 to 0.10% of Nb, 0.005 to 0.1% of Ti, And then rough rolling the slab at a temperature of 1100 to 900 占 폚; Subjecting the roughly rolled bar to finish rolling at a temperature of Ar3 or higher to obtain a steel sheet; Wherein the temperature difference between the central portion on the thickness of the slab or bar before rolling and the outer surface of the slab before rolling in the rough rolling is 100 占 폚 or more during brittle rolling; and brittle crack propagation resistance A method of manufacturing such an excellent structural steel backing material is provided.

The total cumulative rolling reduction during rough rolling is preferably 40% or more

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 one aspect of the present invention, it is possible to obtain a steel material having a superior yield strength and an excellent brittle crack propagation resistance with an excellent core transition temperature.

Fig. 1 is a photograph of the center of thickness of invention steel 1 observed with an optical microscope. Fig.

The inventors of the present invention have solved the problems of the prior art and have conducted studies to secure a steel material having a very high yield strength and a high impact transition temperature as compared with the prior art. As a result, It is possible to improve the brittle crack propagation resistance of the steel material for superfine structural steel by properly controlling it, and on the basis of this, the present invention has been completed.

Hereinafter, the present invention will be described in detail with respect to a structural steel material having excellent brittle crack propagation resistance, which is one aspect of the present invention.

In one aspect of the present invention, the ultra-fine steel material having excellent brittle crack propagation resistance is characterized by containing 0.02 to 0.10% of C, 0.8 to 2.5% of Mn, 0.05 to 1.5% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1%, the balance of Fe and other unavoidable impurities, and the composite structure of ferrite single phase structure, bainite single phase structure, ferrite and bainite, complex structure of ferrite and pearlite, and ferrite, bainite and pearlite And a single tissue selected from the group consisting of complex tissues.

Such a superfine steel may have a thickness of 10 to 100 mm, more preferably 50 to 100 mm.

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

C (carbon): 0.02 to 0.10% (hereinafter, the content of each component means weight%)

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. It is preferable to add C in an amount of 0.02% or more in order to obtain such an additive effect.

However, when the content of C exceeds 0.10%, the low temperature toughness is lowered due to the generation of a large amount of on-road martensite and the high strength of the ferrite itself. Therefore, the content of C is preferably limited to 0.02 to 0.10%.

Mn (manganese): 0.8 to 2.5%

Since Mn is a useful element that improves strength by solid solution strengthening and improves hardenability so as to produce a low temperature transformation phase, it is preferable that Mn is added by 0.8% or more.

However, when the content of Mn exceeds 2.5%, the Mn content is 0.8 (%) because it promotes the formation of upper bainite and martensite due to an increase in the excess hardening ability, thereby lowering impact toughness and brittle crack propagation resistance. To 2.5%.

Ni (nickel): 0.05 to 1.5%

Ni is an important element for improving strength by improving cross-slip of dislocations at low temperature to improve impact toughness and hardenability. In order to improve impact toughness and brittle crack propagation resistance, Ni is added in an amount of 0.05% or more . However, if Ni is added in an amount exceeding 1.5%, the curing ability is excessively elevated to generate a low-temperature transformation phase, thereby lowering the toughness and raising the manufacturing cost. Therefore, the upper limit of the Ni content is preferably limited to 1.5%.

Nb (niobium): 0.005 to 0.1%

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 at the edge of the steel, so the lower limit of the Nb content is preferably limited to 0.1%.

Ti (titanium): 0.005 to 0.1%

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 in an amount exceeding 0.1%, clogging of the performance nozzle or low temperature toughness due to centering can be reduced, so that the Ti content is preferably limited to 0.005 to 0.1%.

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 has a single structure selected from the group consisting of a ferrite single phase structure, a bainite single phase structure, a complex structure of ferrite and bainite, a composite structure of ferrite and pearlite, and a complex structure of ferrite, bainite and pearlite Containing microstructure.

The ratio of pearlite in the complex structure of the ferrite, bainite and pearlite is preferably limited to 30% by volume or less.

The ferrite is preferably acicular ferrite, and the bainite is preferably granular bainite. At this time, the ferrite may be a polygonal ferrite if necessary.

For example, as the content of Mn and Ni increases, the fraction of acicular ferrite or polygonal ferrite and granular bainite increases, thereby increasing the strength.

 The steel may preferably have a grain size of 15 탆 or less having a high-angle boundary of 15 캜 or more as measured by the EBSD method at the center of the plate thickness.

The steel may preferably have a yield strength of 350 MPa or more and a core impact transit temperature of -60 캜 or less.

Another aspect of the present invention is to provide a method for producing a superfine steel material excellent in brittle crack propagation resistance, which comprises the steps of: C: 0.02 to 0.1%; Mn: 0.8 to 2.5%; Ni: 0.05 to 1.5%; Nb: 0.005 to 0.10% 0.1%, the remaining Fe and other unavoidable impurities is reheated to 950 to 1100 캜 and then rough-rolled at a temperature of 1100 to 900 캜; Subjecting the roughly rolled bar to finish rolling at a temperature of Ar3 or higher to obtain a steel sheet; The temperature difference between the central portion on the thickness of the slab or bar before rolling and the outer surface of the slab or bar is at least 100 캜 before the rolling in the rough rolling.

Slab reheating temperature: 950 ~ 1100 ℃

The slab reheating temperature is preferably 950 DEG C or higher, in order to solidify the carbonitride of Ti and / or Nb formed in the casting. Further, in order to sufficiently solidify the carbonitride of Ti and / or Nb, it is more preferable to heat it to 1000 DEG C or more. 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 temperature: 1100 to 900 占 폚 and a temperature difference between the central portion on the thickness of the slab or bar and the outer surface of the slab or bar before roughing: 100 占 폚 or more

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.

In the present invention, the temperature difference between the central portion on the thickness of the slab or bar immediately before rolling and the outer surface of the slab or bar immediately before rolling during rough rolling is 100 ° C or more.

Such a center-surface temperature difference can be obtained, for example, by cooling a heated slab or bar using a cooling device. The cooling device is not particularly limited, and for example, at least one of water, air, liquid coolant, and gas phase coolant may be used as the cooling medium.

By providing the temperature difference between the central portion on the thickness of the slab or the bar and the outer surface of the slab or bar during the rough rolling, the surface portion of the slab or bar is maintained at a lower temperature than the central portion. The more deformation occurs in the center portion having a relatively higher temperature than the surface portion having a relatively lower temperature, so that the center particle size becomes finer. Preferably, the center average particle size can be maintained at 15 탆 or less.

This is a technique that utilizes a phenomenon that a surface portion having a relatively low temperature has a higher strength than a center portion having a relatively high temperature and thus more deformation occurs at a center portion of a relatively low strength. In order to effectively impart more deformation to the center portion, - The temperature difference between the surfaces is preferably 100 ° C or more, more preferably 100 to 300 ° C.

Here, the temperature difference between the central portion on the thickness of the slab or the bar and the outer surface of the slab or bar is determined by considering the surface temperature of the slab or bar measured immediately before rough rolling and the thickness of the slab or bar immediately before the rough rolling Means a difference in the calculated central temperature.

The surface temperature and thickness of the slab are measured before the initial rough rolling, and the surface temperature and thickness of the bar are measured twice before rough rolling.

When the rough rolling is performed in two or more passes, the temperature difference between the central portion on the thickness of the slab or the bar and the outer surface of the slab or bar is measured by pass temperature difference of the rough rolling passages, Or more.

In the present invention, in order to miniaturize the structure of the center portion during rough rolling, the total cumulative rolling reduction during rough rolling is preferably 40% or more

Finishing rolling temperature: Ar3 (ferrite transformation start temperature) or more

The rough-rolled bars are finely rolled at a temperature higher than Ar3 to obtain a steel sheet.

The austenite structure is transformed into austenite structure during finish rolling.

Cooling after rolling: cooling to 700 ° C or less

After finish rolling, the steel sheet is cooled to 700 ° C or less.

If the cooling termination temperature exceeds 700 캜, the microstructure is not appropriately formed, and the yield strength may become 350 MPa or less.

The cooling of the steel sheet can be performed at a central cooling rate of 2 DEG C / s or more, and if the cooling rate of the central portion of the steel sheet is less than 2 DEG C / s, the microstructure is not properly formed and the yield strength may become 350 MPa or less .

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)

The steel slab having the composition shown in the following Table 1 was reheated at a temperature of 1070 캜, followed by rough rolling at a temperature of 1050 캜. The surface-to-center temperature difference during the rough rolling of the slab at the time of pressure regulation is as shown in Table 2, and the cumulative rolling reduction rate is 50%.

The center-surface average temperature difference in the rough rolling in Table 2 represents the difference in the center temperature calculated considering the temperature of the slab or bar surface just before the rough rolling, the quantity injected to the bar, and the thickness of the slab just before rough rolling, The roughness of each pass is measured and the mean value of the whole is calculated.

After the rough rolling, the steel sheet was subjected to finish rolling at a finish rolling temperature of 780 캜 to obtain a steel sheet having the thickness shown in Table 2, and then cooled to a temperature of 700 캜 or less at a cooling rate of 5 캜 / sec.

The microstructure, yield strength, center average particle size, core impact transition temperature, and Kca value (brittle crack propagation resistance coefficient) of the steel sheet thus prepared were examined, and the results are shown in Table 2 below.

The Kca value in Table 2 is the value obtained by performing the ESSO test on the steel sheet.

* PF: Polygonal ferrite, P: Pearlite AF: Acicular ferrite, Granular bainite (GB), Upper bainite (UB) The thickness indicates that the material was evaluated as a post-war material.

As shown in Table 2, in the comparative steels 1 and 2, the average temperature difference between the central portion and the outer surface of the steel sheet during rough rolling was controlled to be less than 100 ° C, and sufficient deformation was imparted to the center portion during rough rolling It can be seen that the core particle size is 25.3 탆 and 29.6 탆, respectively, and thus the impact transition temperature of the center portion is less than -60 캜. Also, it can be seen that the value of Kca measured at -10 ° C does not exceed the required 6000 in general steel for shipbuilding.

The comparative steels 3 and 5 have a higher value than the upper limits of C and Mn proposed in the present invention, and the upper bainite is produced even though the grain size of the central austenite is reduced through cooling during rough rolling The microstructure of the final microstructure is 32 and 38 탆 or more, and the bending moment of the upper bainite is in the range of -60 캜 or more.

Therefore, it can be seen that the value of Kca has a value of less than 6000 at -10 ° C.

The comparative steel 4 has a higher value than the upper limit of the Ni content shown in the present invention, and it can be seen that the microstructure of the base material is the granular bainite and the upper bainite due to the high hardenability.

As a result, even though the grain size of the center austenite was miniaturized through cooling during the rough rolling, the final microstructure had a grain size of 26 μm and the upper bainite which is easily brittle was the base structure, -60 ° C or higher.

Also, it can be seen that the value of Kca has a value of less than 6000 at -10 ° C.

On the other hand, in the case of inventive steels 1 to 6 in which the particle size of the central area austenite is reduced through cooling during the rough rolling while satisfying the composition range of the present invention, the yield strength is not less than 350 MPa and the center part size is not more than 15 μm. Light structure or acicular ferrite single phase structure, or composite structure of acicular ferrite or polygonal ferrite and gabbianite, acicular ferrite, pearlite and gabbianite as a microstructure.

As a result, it can be seen that the core impact transition temperature is less than -60 ° C and the Kca value is more than 6000 at -10 ° C.

As can be seen from FIG. 1, which shows a photograph of the center of thickness of invention steel 1 observed with an optical microscope, it can be seen that the core structure of Invention steel 1 is finer.

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 (5)

And the balance Fe and other unavoidable impurities, and further contains, by weight, 0.02 to 0.10% of C, 0.8 to 2.5% of Mn, 0.05 to 1.5% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Ti, A microstructure including a ferrite single phase structure, a bainite single phase structure, a complex structure of ferrite and bainite, a complex structure of ferrite and pearlite, and a complex structure of ferrite, bainite and pearlite And having a grain size of 15 탆 or more and a grain size of 15 탆 or less as measured by an ESBD method at the central portion of the plate thickness, is excellent in brittle crack propagation resistance.
The method according to claim 1,
Wherein the ferrite is an acicular ferrite or a polygonal ferrite and the bainite is a granular bainite. 2. The structural steel according to claim 1, wherein the bainite is granular bainite.
delete The method according to claim 1,
Wherein the steel has a yield strength of 350 MPa or more and a core impact transition temperature of -60 占 폚 or less.
The method according to claim 1,
And having a thickness of 10 to 100 mm, and having excellent brittle crack propagation resistance.

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