KR101657840B1 - Steel having superior brittle crack arrestability and method for manufacturing the steel - Google Patents

Abstract

The present invention provides a high strength steel excellent in resistance to brittle crack propagation and a method of manufacturing the same.
According to the present invention, there is provided a steel sheet comprising 0.05 to 0.1% of C, 1.5 to 2.2% of C, 0.3 to 1.2% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Ti, , Si: 0.1 to 0.3%, P: not more than 100 ppm, S: not more than 40 ppm, and the balance of Fe and other unavoidable impurities; 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 Have; And a high strength steel having a thickness of 50 mm or more and excellent in brittle crack propagation resistance and a method of manufacturing the same.
According to the present invention, it is possible to obtain a high-strength steel having high yield strength and excellent Charpy impact transition temperature, as well as excellent brittle crack propagation resistance.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength steel material excellent in brittle crack propagation resistance and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a high strength steel excellent in brittle crack propagation 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 steel, since the central portion is not sufficiently deformed due to the decrease in the total reduction ratio in the production of extreme post material, the core structure becomes coarse, and the hardening ability is increased thereby 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 having a yield strength of 390 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.

According to one aspect of the present invention, there is provided a high strength steel excellent in brittle crack propagation resistance.

According to another aspect of the present invention, there is provided a method of manufacturing a high strength steel material excellent in brittle crack propagation resistance.

According to an aspect of the present invention, there is provided a steel sheet comprising, by weight, 0.05 to 0.1% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% To 0.5%, Si: 0.1 to 0.3%, P: not more than 100 ppm, S: not more than 40 ppm, and the balance Fe and other unavoidable impurities; 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 Have; And a high strength steel having a thickness of 50 mm or more and excellent in brittle crack propagation resistance is provided.

The steel may preferably have a grain size of less than 30 micrometers (micrometer) with a high-angle boundary of at least 15 degrees measured by the EBSD method at the center.

The steel material may have an area ratio of (100) planes of less than 40% at an angle of less than 15 degrees with respect to a plane perpendicular to the rolling direction in a thickness having a range of 20% of the steel material thickness, have.

The steel material preferably has a yield strength of 390 MPa or more and a Charpy wave fracture transition temperature at the center of the steel material thickness may be -40 占 폚 or less.

According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.05 to 0.1% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% 0.1 to 0.5% of Si, 0.1 to 0.3% of Si, less than 100 ppm of P, 40 ppm or less of S and the remaining Fe and other unavoidable impurities are reheated to 950 to 1100 ° C, ; Subjecting the roughly rolled bar to finish rolling at a temperature of 850 ° C to Ar ₃ or more to obtain a steel sheet having a thickness of 50 mm or more; And a step of cooling the steel sheet to a temperature of 700 ° C or less, wherein the temperature difference between the central portion and the surface of the slab or bar prior to rolling in the rough rolling is 70 ° C or more, and a method of manufacturing a high strength steel excellent in brittle crack propagation resistance / RTI >

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

The cooling of the steel sheet can be performed at a cooling rate of the central portion of 1.5 ° C / s or more.

The cooling of the steel sheet can be performed at an average cooling rate of 2 to 300 ° 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 having high yield strength and excellent Charpy impact transition temperature, as well as excellent brittle crack propagation resistance.

Fig. 1 shows a photograph of the center of thickness of the inventive steel 3 observed with an optical microscope.

The inventors of the present invention conducted research and experiment to improve yield strength and brittle crack propagation resistance of a thick steel having a thickness of 50 mm or more, and the present invention was proposed based on the results.

The present invention improves the yield strength and brittle crack propagation resistance of a thick steel by controlling the steel composition, structure, texture 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 controlled appropriately in order to obtain strength improvement by improving 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 ensured even at a slow cooling rate up to a central portion of a thick steel of 50 mm or more.

3) Preferably, the structure of the steel material can be made finer to improve strength and brittle crack propagation resistance. Particularly, the structure of the center portion of the steel material is made finer.

By refining the structure of the core of the steel in this manner, the strength and strength through strengthening of grains are improved, and generation and propagation of cracks are minimized, thereby improving brittle crack propagation resistance.

4) Preferably, the texture of the steel can be controlled to improve brittle crack propagation resistance.

Considering that the crack propagates in the width direction of the steel material, that is, in the direction perpendicular to the rolling direction and that the brittle fracture surface of the body-centered cubic structure (BCC) is the (100) plane, The area ratio of the (100) plane forming the angle of less than 15 degrees is minimized.

Especially, the microstructure controls the texture of the coarse central region relative to the surface.

In this way, by controlling the texture of the steel, especially the texture of the central region of the steel, even if cracks are generated, propagation of cracks is minimized and resistance to brittle crack propagation is improved.

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

Particularly, a fine structure is secured up to the center of the steel by controlling the pressing conditions during rough rolling and ensuring a sufficient temperature difference between the center and the surface.

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

In one aspect of the present invention, there is provided a high strength steel material excellent in brittle crack propagation resistance, comprising 0.05 to 0.1% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Ti, 0.1 to 0.5% of Cu, 0.1 to 0.3% of Si, not more than 100 ppm of P, not more than 40 ppm of S, the balance of Fe and other unavoidable impurities, and the ferrite single phase structure, the bainite single phase structure, A composite structure of bainite, a complex structure of ferrite and pearlite, and a structure selected from the group consisting of a composite structure of ferrite, bainite and pearlite.

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

C (carbon): 0.05 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. 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 0.10%, the low-temperature toughness is lowered due to the generation of a large amount of on-road martensite, the high strength of the ferrite itself, and the large amount of the low-temperature transformation phase, , More preferably 0.059 to 0.081%, and still more preferably 0.065 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 to add at least 0.1%.

However, when the content of Mn is more than 2.2%, the upper bainite and martensite are promoted due to an increase in the hardenability, thereby lowering impact toughness and brittle crack propagation resistance.

Therefore, the Mn content is preferably limited to 1.5 to 2.2%, more preferably to 1.59 to 2.06%, and still more preferably to 1.7 to 2.0%.

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 0.048 to 1.11%, and still more preferably to 0.55 to 0.95%.

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

Therefore, Nb is preferably 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. Therefore, the upper limit of the Nb content is preferably limited to 0.1%.

More preferably, the content of Nb is limited to 0.014 to 0.032%, more preferably 0.017 to 0.025%.

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%.

More preferably, the content of Ti is limited to 0.017 to 0.023%, and more preferably to 0.014 to 0.018%.

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.

Si: 0.1 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 element essential for the production of clean steel. However, when added in a large amount, a coarse-phase martensite (MA) phase is formed to lower the brittle crack propagation resistance. Therefore, the upper limit of the Si content is preferably limited to 0.3%.

More preferably, the content of Si is limited to 0.16 to 0.25%, and more preferably to 0.19 to 0.23%.

Cu: 0.1 to 0.5%

Cu is a main element for improving the hardenability and enhancing the strength of the steel by enhancing the hardenability, and it is a major element for increasing the yield strength through the formation of the Epsilon Cu precipitate during the application of the tempering, so it is preferable that the Cu content is 0.1% or more. However, since a large amount of steel may cause cracking of the slab due to hot shortness in the steelmaking process, the upper limit of the Cu content is preferably limited to 0.5%.

More preferably, the content of Cu is limited to 0.25 to 0.44%, more preferably 0.25 to 0.35 %.

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 ferrite is preferably a polygonal ferrite or an acicular ferrite, and the bainite is preferably a granular bainite.

For example, as the content of Mn and Ni increases, the percentage of acicular ferrite and granular bainite increases, and accordingly the strength also increases.

When the microstructure of the steel is a composite structure containing pearlite, the percentage of pearlite is preferably limited to 20% or less.

 The steel may preferably have a grain size of 30 탆 or less with a high-angle boundary of 15 캜 or more as measured by the EBSD method at the center.

In this way, the fineness of the grain size of the central portion of the steel material to 30 탆 or less improves the strength by strengthening the crystal grains and minimizes crack generation and propagation, thereby improving brittle crack propagation resistance.

Preferably, an area ratio of a (100) plane forming an angle of less than 15 degrees with respect to a plane perpendicular to the rolling direction is 40% or more in a thickness having a range of 20% of the thickness of the steel material, ≪ / RTI >

The main reason for controlling the group organization as described above is as follows.

Cracks propagate in the width direction of the steel material, that is, the direction perpendicular to the rolling direction, and the brittle fracture surface of the body-centered cubic structure (BCC) is the (100) plane.

Accordingly, in the present invention, the area ratio of the (100) plane forming an angle of less than 15 degrees with respect to the plane perpendicular to the rolling direction is minimized.

Especially, the microstructure controls the texture of the coarse central region relative to the surface.

In this way, in the case of the texture of the steel, particularly the thickness of the (100) plane which forms an angle of less than 15 degrees with respect to the plane perpendicular to the rolling direction, in a thickness having a range of 20% To 40% or less, the propagation of cracks is minimized even if cracks are generated, and the brittle crack propagation resistance is improved.

The steel preferably has a yield strength of at least 390 MPa.

The steel material has a thickness of 50 mm or more, preferably 50 to 100 mm, and more preferably 80 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.

Another aspect of the present invention is to provide a high strength steel material excellent in resistance to brittle crack propagation which comprises 0.05 to 0.1% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.1% of Nb, The slab containing the remaining Fe and other unavoidable impurities is reheated to 950 to 1100 ° C and then heated to 1100 ° C Rough rolling at a temperature of ~ 900 ° C; Rolling the roughly rolled bar at a temperature of 850 캜 to Ar 3 or more to obtain a steel sheet; And cooling the steel sheet to a temperature of 700 ° C or lower, wherein the temperature difference between the central portion and the surface of the slab or bar before rolling in the rough rolling is 70 ° C or higher.

Reheating slabs

Reheat the slab before rough rolling.

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, the austenite may be coarsened, so that the upper limit of the reheating temperature is preferably limited to 1100 캜.

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.

In the present invention, the temperature difference between the central portion and the surface of the slab or bar immediately before rolling during rough rolling is set to 70 ° C or more.

In this way, by providing the temperature difference between the center and the surface during rough rolling, the surface portion of the slab or bar maintains a lower temperature than the center portion. When the rolling is performed in the presence of such a temperature difference, More deformation occurs in the center portion having a higher temperature, so that the central portion grain size becomes finer. At this time, preferably, the central portion average particle size can be kept at 30 탆 or less.

This is a technique utilizing a phenomenon in which the surface portion having a relatively low temperature has a higher strength than the center portion having a relatively high temperature, and thus, a deformation is more likely to occur at the 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 higher, more preferably 100-300 ° C.

Here, the temperature difference between the center and the surface of the slab or bar means a difference in the center temperature calculated considering the surface temperature of the slab or bar measured immediately before rough rolling, the cooling condition, and the thickness of the slab or bar just before rough rolling .

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 passes or more, the temperature difference between the central portion and the surface of the slab or bar means that the temperature difference obtained by calculating the average value of the rough rolling pass passes is 100 ° C or more.

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

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.

Finish rolling

The rough-rolled bar is subjected to finish rolling at 850 ° C to Ar ₃ (ferrite transformation start temperature) or higher to obtain a steel sheet.

In order to obtain finer microstructure, it is preferable to carry out finishing rolling temperature of finishing rolling to 850 degrees or less.

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

After finishing rolling, the steel sheet has a thickness of 50 mm or more, preferably 50 to 100 mm, and more preferably 80 to 100 mm.

Cooling

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

If the cooling end temperature exceeds 700 ° C, the microstructure is not properly formed, and the yield strength may become 390 MPa or less.

The cooling of the steel sheet can be performed at a central cooling rate of 1.5 ° C / s or more. If the center cooling rate of the steel sheet is less than 1.5 ° C / s, the microstructure is not properly formed and the yield strength may become 390 MPa or less .

The cooling of the steel sheet can be performed at an average cooling rate of 2 to 300 ° 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 1060 캜, followed by rough rolling at a temperature of 1030 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 indicates the difference in the central portion temperature calculated considering the temperature of the slab or bar surface immediately before rough rolling, the quantity of water injected into the bar, and the thickness of the slab just before rough rolling, The results of calculating the average value of the whole pass by measuring the pass temperature difference of rolling.

After the rough rolling, the steel sheet was subjected to finish rolling at a finish rolling temperature of 790 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 4.5 캜 / sec.

With respect to the steel sheet produced as described above, an angle of not more than 15 degrees with respect to the plane perpendicular to the rolling direction in a thickness having a range of 20% of the plate thickness centered on 1/2 of the thickness of the microstructure, the yield strength, (Biaxial crack propagation resistance coefficient) of the (100) plane, the central portion of the impact transit temperature and the Kca value (the brittle crack propagation resistance coefficient).

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

Steel grade
Steel composition (% by weight) C Si Mn Ni Cu Ti Nb P (ppm) S (ppm) Inventive Steel 1 0.061 0.20 1.97 0.65 0.29 0.018 0.032 88 15 Invention river 2 0.078 0.21 2.06 0.97 0.36 0.017 0.018 78 31 Invention steel 3 0.081 0.16 1.94 1.11 0.41 0.023 0.016 67 24 Inventive Steel 4 0.074 0.21 1.67 0.48 0.35 0.015 0.014 45 19 Invention steel 5 0.069 0.23 1.78 0.76 0.27 0.019 0.022 56 21 Invention steel 6 0.059 0.18 1.59 0.84 0.44 0.021 0.023 89 49 Comparative River 1 0.066 0.17 1.84 0.81 0.38 0.011 0.023 52 8 Comparative River 2 0.13 0.25 1.79 0.45 0.29 0.013 0.012 49 11 Comparative Steel 3 0.064 0.55 1.86 0.78 0.33 0.019 0.017 87 31 Comparative Steel 4 0.070 0.22 2.56 0.89 0.47 0.018 0.024 58 23 Comparative Steel 5 0.067 0.15 1.95 1.85 0.28 0.022 0.016 57 21 Comparative Steel 6 0.059 0.16 1.85 1.11 0.31 0.012 0.017 141 74

Steel grade Center-surface temperature difference (℃) during rough rolling Product thickness (mm) * Fine
group,
Phase fraction (%) (001)
texture Yield strength (Mpa) center
Average particle size
(탆) Central impact transition temperature (캜) ^{Kca (N / mm 1 .5,} @ - 10 ℃) Inventive Steel 1 122 95 AF + GB (12%) 11 455 18.2 -65 8255 Invention river 2 101 90 AF + GB (34%) 23 506 17.1 -58 7255 Invention steel 3 187 85 PF + P (14%) 18 406 20.3 -76 8596 Inventive Steel 4 230 85 PF + P (15%) 27 399 21.1 -81 8378 Invention steel 5 189 90 AF 17 438 12.3 -86 9024 Invention steel 6 156 100 AF + GB (24%) 24 495 19.6 -71 8562 Comparative River 1 32 85 PF + P (14%) 42 404 31.1 -35 4212 Comparative River 2 178 90 UB 43 586 39.4 -25 3255 Comparative Steel 3 115 90 AF + UB (26%) 29 555 27.9 -43 4965 Comparative Steel 4 210 100 UB 45 579 36.3 -25 3356 Comparative Steel 5 166 80 GB, UB (17%) 18 532 32.1 -38 3978 Comparative Steel 6 178 90 AF + GB (11%) 22 475 24.1 -51 4879

PF: Polygonal ferrite, P: Pearlite AF: Acicular ferrite, GB: Granular bainite, UB: Upper bainite, Upper bainite, (%): volume %

As shown in Table 2, in the comparative steel 1, the center-surface average temperature difference in the rough rolling proposed in the present invention was controlled to be less than 70 캜, and since the core portion was not sufficiently deformed during rough rolling, The area ratio of the (100) plane forming the angle of less than 15 degrees with respect to the plane perpendicular to the rolling direction is 40% or less at a thickness of 20% of the plate thickness centered on 1/2 of the thickness, And the impact transition temperature is not less than -40 DEG C and the Kca value measured at -10 DEG C does not exceed 6,000 required in general steel for shipbuilding.

In the case of Comparative Steel 2, the content of C is higher than the upper limit of the C content of the present invention, and even though the grain size of the central austenite is reduced through cooling during rough rolling, the upper bainite is produced The grain size of the final microstructure is 39.4 μm and the thickness of the (100) plane forming an angle of less than 15 degrees with respect to the plane perpendicular to the rolling direction in a thickness having a range of 20% It is found that the impact transition temperature is higher than -40 ° C and the Kca value is lower than-6000 at -10 ° C due to the fact that the upper bainite having a brittleness of more than 40% .

In Comparative Steel 3, the content of Si is higher than the upper limit of the Si content of the present invention. Even though the grain size of the central austenite was reduced through cooling during rough rolling, the upper bainite And a large amount of MA is produced in a large amount as the Si is added in a large amount, and the Kca value has a value of 6000 or less at -10 ° C.

In the case of Comparative Steel 4, the Mn content is higher than the upper limit of the Mn content of the present invention, and the microstructure of the base material is the upper bainite due to the high hardenability, and the grain size of the central austenite is reduced The grain size of the final microstructure is 36.3 탆 and the thickness of the final microstructure is in the range of 20% of the plate thickness centered on 1/2 of the thickness, ) Plane is 40% or more, the impact transition temperature is -40 DEG C or more, and the Kca value is -10 DEG C to 6000 or less.

In the case of the comparative steel 5, the Ni content is higher than the upper limit of the Ni content of the present invention, and because of the high hardenability, the microstructure of the base material is granular bainite and the upper bainite, It was found that the grain size of the final microstructure was 31.2 占 퐉 in spite of the fineness of the austenite grains in the central portion and the impact transition temperature was more than -40 占 and the Kca value was less than 6000 at -10 占 폚 have.

In the comparative steel 6, the contents of P and S are higher than the upper limits of the P and S contents of the present invention. Even though all other conditions satisfy the conditions of the present invention, , It can be seen that the value of Kca has a value of 6000 or less at -10 ° C.

On the other hand, in the case of inventive steels 1 to 6 in which the particle size of the central portion of theustenite is reduced through cooling during the rough rolling while satisfying the composition range of the present invention, the steel sheet satisfies a yield strength of not less than 390 MPa and a center portion size of not more than 30 μm, Light structure or acicular ferrite single phase structure, or composite structure of acicular ferrite and gabanibenite, needle-like ferrite, pearlite, and gabenite.

In addition, in the case of the thickness having a range of 20% of the plate thickness centered on 1/2 of the thickness, the area ratio of the (100) plane forming the angle of less than 15 degrees with respect to the plane perpendicular to the rolling direction is 40% It can be seen that the transition temperature is more than -40 ° C and the Kca value is more than 6000 at -10 ° C.

FIG. 1 shows a photograph of the center of the thickness of the inventive steel 3 observed by an optical microscope. As can be seen from FIG. 1, the thickness center texture 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 (14)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.1% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Ti, 0.1 to 0.5% 0.3%, P: not more than 100 ppm, S: not more than 40 ppm, the balance Fe and other unavoidable impurities; 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 Have;
A thickness of at least 50 mm;
The grain size having a high-angle boundary of not less than 15 degrees measured by the ESBD method at the center of the steel material thickness is 30 占 퐉 or less; And
A brittle crack propagation resistance of not more than 40% in the area ratio of the (100) plane forming an angle of not more than 15 degrees with respect to the plane perpendicular to the rolling direction in a thickness having a range of 20% This excellent high strength steel.
The method according to claim 1,
Wherein the ferrite is an acicular ferrite or a polygonal ferrite, and the bainite is a granular bainite. The bainite crack propagation resistance of the high strength steel is excellent.
The method according to claim 1,
And the percentage of pearlite is 20% or less when the microstructure of the steel is a composite structure containing pearlite. The high strength steel excellent in brittle crack propagation resistance.
delete The method according to claim 1,
Wherein the steel has a yield strength of at least 390 MPa and a Charpy fracture transition temperature at the center of the plate thickness is -40 캜 or less.
delete The method according to claim 1,
The steel material has a thickness of 80 to 100 mm. The steel material has excellent brittle crack propagation resistance.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.1% of C, 1.5 to 2.2% of Mn, 0.3 to 1.2% of Ni, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Ti, 0.1 to 0.5% 0.3%, P: not more than 100 ppm, S: not more than 40 ppm, and the balance Fe and other unavoidable impurities is reheated to 950 to 1100 占 폚 and then rough-rolled at a temperature of 1100 to 900 占 폚; Subjecting the roughly rolled bar to finish rolling at a temperature of 850 ° C to Ar3 or higher to obtain a steel sheet having a thickness of 50 mm or more; And cooling the steel sheet to a temperature of 700 DEG C or less, wherein the temperature difference between the central portion and the surface of the slab or bar before rolling during the rough rolling is 70 DEG C or more, and the ESBD method A grain size of not more than 30 占 퐉 with a grain boundary of not less than 15 degrees and an angle of not more than 15 degrees with respect to a plane perpendicular to the rolling direction in a thickness having a range of 20% (100) surface area of not more than 40% is produced by a method of producing a high strength steel material excellent in brittle crack propagation resistance.
The method of claim 8,
Wherein a difference in temperature between a center portion on the thickness of the slab or the bar and an outer surface of the slab or bar is 100 to 300 占 폚.
The method of claim 8,
The temperature difference between the central portion on the thickness of the slab or bar and the outer surface of the slab or bar is calculated in consideration of the surface temperature of the slab or bar measured immediately before rough rolling and the thickness of the slab or bar just before the rough rolling Wherein the difference in the temperature of the center portion is the difference in the brittle crack propagation resistance.
The method of claim 8,
And 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 a temperature difference obtained by measuring the pass temperature difference of the rough rolling passages and calculating the average value of the entire pass. Which is excellent in brittle crack propagation resistance.
The method of claim 8,
Wherein the rolling reduction per pass is at least 5% and the total cumulative rolling reduction is at least 40% for the last three passes during rough rolling.
The method of claim 8,
And cooling the steel sheet at a center cooling rate of 1.5 DEG C / s or more.
The method of claim 8,
Wherein the steel sheet is cooled at an average cooling rate of 2 to 300 DEG C / s.

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