WO2016105062A1

WO2016105062A1 - High-strength steel having excellent resistance to brittle crack propagation, and production method therefor

Info

Publication number: WO2016105062A1
Application number: PCT/KR2015/014054
Authority: WO
Inventors: 이학철; 장성호
Original assignee: 주식회사 포스코
Priority date: 2014-12-24
Filing date: 2015-12-21
Publication date: 2016-06-30
Also published as: EP3239331B1; KR101746999B1; WO2016105062A8; CN107109597A; CN107109597B; EP3239331A1; EP3239331A4; US20170327922A1; JP2018503744A; JP6788589B2; KR20160078926A

Abstract

The present invention provides high-strength steel having excellent resistance to brittle crack propagation and a production method therefor. Provided according to the present invention are: high-strength steel, which has excellent resistance to brittle crack propagation, comprises 0.05-0.1 wt% of C, 0.9-1.5 wt% of Mn, 0.8-1.5 wt% of Ni, 0.005-0.1 wt% of Nb, 0.005-0.1 wt% of Ti, 0.1-0.6 wt% of Cu, 0.1-0.4 wt% of Si, at most 100 ppm of P, and at most 40 ppm of S with the remainder being Fe and other inevitable impurities, and has microstructures including one structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex-phase structure of ferrite and bainite, a complex-phase structure of ferrite and pearlite, and a complex-phase structure of ferrite, bainite, and pearlite; and a production method therefor. According to the present invention, high-strength steel having high yield strength and excellent resistance to brittle crack propagation can be obtained.

Description

High strength steel with excellent brittle crack propagation resistance and manufacturing method

The present invention relates to a high strength steel having excellent brittle crack propagation resistance and a method of manufacturing the same.

In recent years, in designing structures used in domestic and overseas ships, offshore, architecture, and civil engineering, development of ultra-thick steels having high strength properties is required.

When using high strength steel when designing the structure, the structure of the structure can be reduced in weight and economical benefits can be obtained, and the thickness of the steel sheet can be reduced, thereby ensuring ease of machining and welding operations.

In general, in the case of high-strength steel, the core structure becomes coarse because sufficient deformation is not made in the center due to a decrease in the total reduction ratio during the manufacture of the ultra-thick material, and thus the hardenability is increased to generate a low temperature transformation phase such as bainite.

In addition, it may be difficult to secure the impact toughness of the center due to the coarse organization.

In particular, in the case of brittle crack propagation resistance indicating stability of the structure, there is an increasing number of cases requiring a guarantee when applied to major structures such as ships, but when the formation of low temperature transformation in the center, the brittle crack propagation resistance is very low. It is very difficult to improve the brittle crack propagation resistance phase of very thick high strength steel.

On the other hand, in the case of high strength steel with a yield strength of 390 MPa or more, various technologies such as surface cooling at the time of finishing rolling for miniaturization of the brittle crack propagation, particle size control by applying bending stress during rolling, and surface layer refinement through abnormal reverse rolling are applied. This was introduced.

However, this technique is helpful for the microstructure of the surface layer, but the impact toughness due to the coarse structure of the core cannot be solved, and thus it is not a fundamental countermeasure against brittle crack propagation resistance.

In addition, since the technology itself is expected to be significantly reduced in productivity to be applied to the general mass production system, it can be said that the technology is not suitable for commercial applications.

According to an aspect of the present invention, to provide a high strength steel excellent in brittle crack propagation resistance, an object thereof.

According to another aspect of the present invention, an object of the present invention is to provide a method for producing a high strength steel having excellent brittle crack propagation resistance.

According to an aspect of the present invention, in weight%, C: 0.05 to 0.1%, Mn: 0.9 to 1.5%, Ni: 0.8 to 1.5%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Cu: 0.1 ˜0.6%, Si: 0.1-0.4%, P: 100 ppm or less, S: 40 ppm or less and the remaining Fe and other unavoidable impurities; Microstructure including a single structure selected from the group consisting of ferrite single phase structure, bainite single phase structure, ferrite and bainite complex, ferrite and perlite complex, and ferrite, bainite and perlite complex. Have; In addition, a high strength steel having excellent brittle crack propagation resistance having a thickness of 50 mm or more is provided.

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

The steel may preferably have a particle size of 30 μm (micrometer) or less having a high-angle boundary of 15 degrees or more, as measured by the EBSD method of the center portion.

The steel may have an area ratio of (100) planes having an angle within 15 degrees with respect to the plane perpendicular to the rolling direction at a thickness having a range of 20% of the steel thickness with respect to 1/2 part of the steel thickness. have.

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

According to another aspect of the present invention, in weight%, C: 0.05 to 0.1%, Mn: 0.9 to 1.5%, Ni: 0.8 to 1.5%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Cu: 0.1 ~ 0.6%, Si: 0.1 ~ 0.4%, P: 100ppm or less, S: 40ppm or less and reheat the slab containing the remaining Fe and other unavoidable impurities to 950 ~ 1100 ℃, and then rough-roll at a temperature of 1100 ~ 900 ℃ Doing; Finishing rolling the rough rolled bar at a temperature of at least 850 to Ar ₃ to obtain a steel sheet having a thickness of 50 mm or more; The method of manufacturing a high strength steel having excellent brittle crack propagation resistance, comprising the step of cooling the steel sheet to a temperature of 700 ° C. or lower, such that the temperature difference between the center and the surface of the slab or bar before rolling is roughly 70 ° C. or more during the rough rolling. Is provided.

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

The center grain size of the bar after the rough rolling and before the finish rolling may be 200 μm or less, preferably 150 μm or less, and more preferably 100 μm or less.

The rolling reduction ratio during the finish rolling may be set so that the ratio of slab thickness (mm) / thickness of the steel sheet after finishing rolling (mm) is 3.5 or more, preferably 3.8 or more.

The steel sheet may be cooled at a central cooling rate of 2 ° C./s or more.

Cooling of the steel sheet can be carried out at an average cooling rate of 3 ~ 300 ℃ / s.

In addition, the solution of the said subject does not enumerate all the characteristics of this invention.

Various features of the present invention and the advantages and effects thereof may be understood in more detail with 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 and excellent brittle crack propagation resistance.

1 shows a photograph obtained by observing the thickness center of the inventive steel 1 with an optical microscope.

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

The present invention is to improve the yield strength and brittle crack propagation resistance of 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) Steel composition is properly controlled to obtain strength improvement through strengthening of solid solution. In particular, Mn, Ni, Cu and Si content is optimized for solid solution strengthening.

2) Steel composition is appropriately controlled to obtain strength improvement through improvement of hardenability. In particular, Mn, Ni and Cu content is optimized with the carbon content to improve the hardenability.

By improving the hardenability, even at a slow cooling rate, the microstructure is secured to the center of the thick steel of 50 mm or more.

3) Preferably, in order to improve the strength and resistance to brittle crack propagation, it is possible to refine the structure of the steel. In particular, the core structure of the steel is refined.

Thus, by miniaturizing the central structure of the steel material, the strength of the grains is enhanced and the generation and propagation of cracks is minimized, thereby improving brittle crack propagation resistance.

4) Preferably, it is possible to control the texture of the steel in order to improve the brittle crack propagation resistance.

The crack is propagated in the width direction of the steel, that is, the direction perpendicular to the rolling direction, and the brittle wavefront of the body centered cubic structure (BCC) is the (100) plane, with respect to the surface perpendicular to the rolling direction. The area ratio of the (100) plane forming an angle within 15 degrees is to be minimized.

In particular, the microstructure controls the aggregate structure of the central region relatively coarse to the surface.

By controlling the texture of the steel, in particular, the structure of the central region of the steel, even if a crack is generated, the propagation of the crack is minimized to improve the brittle crack propagation resistance.

5) Preferably, the rough rolling conditions can be controlled in order to refine the structure of the steel.

In particular, the microstructure is secured to the center of steel by controlling the rolling reduction condition and securing a sufficient center-to-surface temperature difference during rough rolling.

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

High strength steel having excellent brittle crack propagation resistance, which is an aspect of the present invention, is weight%, C: 0.05 to 0.1%, Mn: 0.9 to 1.5%, Ni: 0.8 to 1.5%, Nb: 0.005 to 0.1%, and Ti: 0.005 ~ 0.1%, Cu: 0.1 ~ 0.6%, Si: 0.1 ~ 0.4%, P: 100ppm or less, S: 40ppm or less, containing the remaining Fe and other unavoidable impurities, and ferrite single phase structure, bainite single phase structure, ferrite and It has a microstructure comprising one tissue selected from the group consisting of a composite structure of bainite, a composite structure of ferrite and perlite, and a composite structure of ferrite, bainite and perlite.

Hereinafter, the steel component and the component range of this invention are demonstrated.

C (carbon): 0.05 to 0.10% (hereinafter, the content of each component means weight%)

Since C is the most important element for securing basic strength, it needs to be contained in steel within an appropriate range, and in order to obtain such an addition effect, it is preferable to add C 0.05% or more.

However, when the content of C exceeds 0.10%, the low temperature toughness is lowered due to the formation of a large amount of phase martensite and the high strength of ferrite itself, and the formation of a large amount of low temperature transformation phase, and thus the content of C is 0.05 to 0.10%. It is preferable to limit to 0, more preferably to 0.061 to 0.091%, even more preferably to 0.065 to 0.085%.

Mn (manganese): 0.9-1.5%

Mn is a useful element that improves the strength by solid solution strengthening and improves the hardenability so that low-temperature transformation phase is produced. In order to obtain such an effect, Mn is preferably added at least 0.9%.

However, when the Mn content exceeds 1.5%, the excessive hardening capacity increases, thereby promoting the formation of upper bainite and martensite, and causing segregation of the core to create coarse low-temperature transformations, resulting in impact toughness. And lowers brittle crack propagation resistance.

Therefore, the Mn content is preferably limited to 0.9 to 1.5%, limited to 0.97 to 1.39%, and more preferably limited to 1.15 to 1.30%.

Ni (nickel): 0.8-1.5%

Ni is an important element for facilitating cross slip of dislocations at low temperatures, improving impact toughness, improving hardenability, and improving strength, and it is preferable to add 0.8% or more to obtain such effects. However, when the Ni is added at least 1.5%, the hardenability is excessively increased to form low-temperature transformation phase, which may lower toughness and increase manufacturing cost, so the upper limit of the Ni content is preferably limited to 1.5%.

The content of Ni is more preferably limited to 0.89 to 1.42%, even more preferably 1.01 to 1.35%.

Nb (niobium): 0.005 to 0.1%

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

In addition, Nb dissolved in reheating at a high temperature precipitates very finely in the form of NbC during rolling, thereby suppressing recrystallization of austenite, thereby miniaturizing the structure.

Therefore, Nb is preferably added at least 0.005%, but if excessively added, there is a possibility of causing brittle cracks at the corners of the steel, so the upper limit of the Nb content is preferably limited to 0.1%.

The content of Nb is more preferably limited to 0.012 to 0.028%, and even more preferably 0.018 to 0.024%.

Ti (titanium): 0.005 to 0.1%

Ti is a component that precipitates with TiN upon reheating and inhibits the growth of crystal grains of the base metal and the weld heat affected zone to greatly improve low-temperature toughness. To obtain such an additive effect, Ti is preferably added at least 0.005%.

However, when Ti is added in excess of 0.1%, since the low temperature toughness due to clogging of the playing nozzle or the center portion determination may be reduced, the Ti content is preferably limited to 0.005 to 0.1%.

More preferably, the content of Ti is limited to 0.009 to 0.024%, even more preferably 0.011 to 0.018%.

P: 100 ppm or less, S: 40 ppm or less

P, S is an element that causes brittleness or forms coarse inclusions at grain boundaries, and is preferably limited to P: 100 ppm or less and S: 40 ppm or less in order to improve brittle crack propagation resistance.

Si: 0.1 ~ 0.4%

Since Si improves the strength of the steel and has a strong deoxidation effect, it is preferable to add 0.1% or more in order to obtain such an effect as an essential element for producing clean steel. However, when a large amount is added, coarse phase martensite (MA) phase may be generated to lower brittle crack propagation resistance, so the upper limit of the Si content is preferably limited to 0.4%.

The content of Si is more preferably limited to 0.22 to 0.32%, even more preferably 0.25 to 0.3%.

Cu: 0.1 ~ 0.6%

Cu is the main element to improve the hardenability and to increase the strength of the steel to increase the strength of the steel and to increase the yield strength through the generation of epsilon Cu precipitates when tempering (tempering), it is preferably added more than 0.1%. However, when a large amount is added, the slab may be cracked due to hot shortness in the steelmaking process, so the upper limit of the Cu content is preferably limited to 0.6%.

The content of Cu is more preferably limited to 0.21 to 0.51%, even more preferably 0.18 to 0.3%.

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

The remaining component of the present invention is iron (Fe).

However, in the conventional manufacturing process, impurities which are not intended from the raw materials or the surrounding environment may be inevitably mixed, and thus cannot be excluded.

Since these impurities are known to those skilled in the art, not all of them are specifically mentioned herein.

Steel of the present invention is a single structure selected from the group consisting of ferrite single phase structure, bainite single phase structure, ferrite and bainite complex structure, ferrite and perlite complex structure, and ferrite, bainite and perlite complex structure. It has a microstructure that contains.

The ferrite is preferably polygonal ferrite or acicular ferrite, and bainite is preferably granular bainite.

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

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

The steel may preferably have a particle size of 30 μm or less having a high-angle boundary of 15 degrees or more, as measured by the EBSD method of the center portion.

Thus, by miniaturizing the grain size of the central structure of the steel material to 30㎛ or less, the strength of the grain through the strengthening of the grain is enhanced, and the generation and propagation of cracks is minimized to improve the brittle crack propagation resistance.

Preferably, the area ratio of the (100) plane which forms an angle within 15 degrees with respect to the plane perpendicular to the rolling direction at a thickness having a range of 20% of the steel thickness with respect to 1/2 part of the steel thickness is 40%. It may be:

The main reasons for controlling the collective structure as described above are as follows.

The crack propagates in the width direction of the steel, that is, the direction perpendicular to the rolling direction, and the brittle wavefront of the body centered cubic structure BCC is the (100) plane.

Thus, in the present invention, the area ratio of the (100) plane which forms an angle within 15 degrees with respect to the plane perpendicular to the rolling direction is minimized.

The area ratio of the (100) plane which forms an angle within 15 degrees with respect to the plane perpendicular to the rolling direction at a thickness having a range of 20% of the steel thickness, particularly about 1/2 of the thickness of the steel, in particular. By controlling to less than 40%, even if a crack is generated, the propagation of the crack is minimized to improve the brittle crack propagation resistance.

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

The steel may have 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 having excellent brittle crack propagation resistance, which is another aspect of the present invention, will be described in detail.

Another aspect of the present invention is a method of manufacturing a high strength steel having excellent brittle crack propagation resistance by weight% C: 0.05 ~ 0.1%, Mn: 0.9 ~ 1.5%, Ni: 0.8 ~ 1.5%, Nb: 0.005 ~ 0.1%, Ti : 0.005 ~ 0.1%, Cu: 0.1 ~ 0.6%, Si: 0.1 ~ 0.4%, P: 100ppm or less, S: 40ppm or less, and reheat the slab containing the remaining Fe and other unavoidable impurities to 950 ~ 1100 ℃ to 1100 Rough rolling at a temperature of ˜900 ° C .; Obtaining a steel sheet by finishing rolling the roughly rolled bar at a temperature of 850 ° C. to Ar ₃ or higher; And cooling the steel sheet to a temperature of 700 ° C. or less, so that the temperature difference between the center and the surface of the slab or bar before rolling during the rough rolling is 70 ° C. or more.

Reheat slab

Reheat the slab prior to rough rolling.

The slab reheating temperature is preferably set at 950 ° C. or higher, in order to solidify the carbonitrides of Ti and / or Nb formed during casting. Moreover, in order to fully solidify the carbonitride of Ti and / or Nb, it is more preferable to heat to 1000 degreeC or more. However, when the slab is reheated at an excessively high temperature, austenite may coarsen, so the upper limit of the reheating temperature is preferably limited to 1100 ° C.

Rough rolling

Crimp the reheated slab.

It is preferable to make rough rolling temperature more than the temperature (Tnr) at which recrystallization of austenite stops. By casting, the casting structure such as the dendrite formed during casting is destroyed, and the effect of reducing the size of austenite can also be obtained. In order to obtain such an effect, the rough rolling temperature is preferably limited to 1100 ~ 900 ℃.

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

As such, by providing the temperature difference between the center and the surface during the rough rolling, the surface of the slab or the bar is kept at a lower temperature than the central part. More strain occurs in the high temperature center portion, whereby the center grain size becomes finer. At this time, preferably, the central mean particle size may be maintained at 30 μm or less.

This is a technology that utilizes the phenomenon that more deformation occurs in the center of relatively low strength because the surface of the relatively low temperature has higher strength than the center of relatively high temperature. It is preferable that the temperature difference between surfaces is 100 degreeC or more, and a more preferable temperature difference is 100-300 degreeC.

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

The measurement of the surface temperature and the thickness of the slab is carried out before the first rough rolling, and the measurement of the surface temperature and the thickness of the bar is carried out from the two rough rolling before the rough rolling.

When the rough rolling is performed for two or more passes, the temperature difference between the center and the surface of the slab or the bar means that the temperature difference obtained by measuring the average temperature of each pass of the rough rolling is calculated to be 70 ° C. or more.

In the present invention, in order to refine the structure of the center part at the time of rough rolling, it is preferable that the reduction rate per pass is 5% or more and the total cumulative reduction rate is 40% or more for the last three passes during rough rolling.

In the present invention, in order to refine the structure of the center part during the rough rolling, the rolling reduction rate per pass is preferably 5% or more and the total cumulative reduction rate is 40% or more for the last three passes during rough rolling.

In the early rolling during the rough rolling, the recrystallized structure causes grain growth due to the high temperature, but during the last three passes, the grain growth rate is slowed down as the bar is air-cooled in the rolling atmosphere. The rate of reduction of the pass is greatest for the particle size of the final microstructure.

In addition, when the rolling reduction per pass of the rough rolling is lowered, sufficient deformation is not transmitted to the center, and thus toughness may be reduced due to the coarsening of the center. Therefore, it is desirable to limit the rolling reduction per pass of the last three passes to 5% or more.

On the other hand, the total cumulative reduction rate during rough rolling is preferably set to 40% or more in order to refine the structure of the central portion.

Finish rolling

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

In order to obtain a finer microstructure, it is preferable to perform finishing rolling temperature of finishing rolling at 850 degreeC or less.

In finish rolling, the austenite structure becomes a deformed austenite structure.

The grain size of the center portion of the bar after the rough rolling and before the finish rolling may be 200 μm or less, preferably 150 μm or less, and more preferably 100 μm or less.

The grain size of the center portion of the bar after rough rolling and before finishing rolling may be controlled according to rough rolling conditions.

As described above, in the case of controlling the size of the central grain of the bar before the rough rolling after the rough rolling as described above, the final microstructure according to the miniaturization of the austenite grain may be refined, thereby increasing yield / tensile strength and improving low temperature toughness.

As described above, in the case of controlling the rolling reduction ratio during finishing rolling, as the rolling reduction during rough rolling and finishing rolling increases, yield / tensile strength can be increased and low temperature toughness can be improved through the miniaturization of final microstructure, and also a decrease in the thickness of the center of thickness Can improve the toughness of the core through.

After finishing rolling, the steel sheet may have 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 lower.

If the cooling end temperature exceeds 700 ℃, the microstructure is not formed properly, there is a possibility that the yield strength is 390Mpa or less.

The cooling of the steel sheet can be performed at a central cooling rate of 2 ° C / s or more. If the central cooling rate of the steel sheet is less than 2 ° C / s, the microstructure is not formed properly, and the yield strength may be 390 Mpa or less. .

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

Hereinafter, the present invention will be described in more detail with reference to the following examples.

However, it is necessary to note that the following embodiments are only intended to describe the present invention by way of example, not to limit the scope of the present invention.

This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example 1)

The steel slab having a thickness of 400 mm having the composition shown in Table 1 was reheated to a temperature of 1050 ° C, and then rough-rolled at a temperature of 1020 ° C to prepare a bar. Surface roughness average temperature difference of the slab during rough rolling was as shown in Table 2, and the cumulative rolling rate was equally applied to 50%.

The center-surface average temperature difference in rough rolling in Table 2 represents the difference between the temperature of the slab or bar surface measured immediately before rough rolling, the center temperature calculated in consideration of the quantity sprayed on the bar and the slab thickness just before rough rolling. It is the result of calculating the average value of the whole by measuring the pass temperature difference of each rolling.

The rough rolled bar had a thickness of 180 mm, and the rough grain size before rough rolling was 80 μm.

After the rough rolling, a finish rolling was performed at a finish rolling temperature of 770 ° C. to obtain a steel sheet having the thickness shown in Table 2, and then cooled to a temperature of 700 ° C. or less at a cooling rate of 5 ° C./sec.

For the steel sheet manufactured as described above, the microstructure, the yield strength, the average particle size of the core measured by EBSD, and the thickness perpendicular to the rolling direction at a thickness having a range of 20% of the plate thickness with respect to 1/2 part of the thickness were measured. The area ratio of the (100) plane and the Kca value (brittle crack propagation resistance coefficient) which form angles within degrees are investigated, and the results are shown in Table 2 below.

Kca value of Table 2 is the value evaluated by performing ESSO test on the steel sheet.

Table 1

Steel grade	Steel composition (% by weight)
Steel grade	C	Si	Mn	Ni	Cu	Ti	Nb	P (ppm)	S (ppm)	Cu / Ni weight ratio
Inventive Steel 1	0.061	0.23	1.25	0.89	0.35	0.015	0.019	75	16	0.39
Inventive Steel 2	0.082	0.31	1.36	0.95	0.44	0.016	0.017	77	25	0.46
Invention Steel 3	0.054	0.32	1.09	1.26	0.36	0.009	0.023	82	34	0.29
Inventive Steel 4	0.072	0.22	1.39	1.13	0.21	0.024	0.012	65	19	0.19
Inventive Steel 5	0.069	0.29	1.17	1.21	0.45	0.02	0.019	68	22	0.36
Inventive Steel 6	0.091	0.31	0.97	1.42	0.51	0.019	0.028	71	31	0.36
Comparative Steel 1	0.072	0.25	1.21	0.97	0.36	0.017	0.026	69	16	0.37
Comparative Steel 2	0.12	0.29	1.32	1.12	0.39	0.017	0.023	59	13	0.35
Comparative Steel 3	0.068	0.61	1.39	1.08	0.45	0.019	0.027	55	25	0.42
Comparative Steel 4	0.077	0.32	1.95	1.32	0.21	0.026	0.019	67	26	0.16
Comparative Steel 5	0.062	0.19	1.21	2.2	0.35	0.021	0.031	49	30	0.16
Comparative Steel 6	0.072	0.22	1.06	1.11	0.48	0.016	0.022	130	65	0.43

TABLE 2

Steel grade	Center-surface temperature difference at rough rolling (℃)	Product thickness (mm)	* Microstructure, Percentage (%)	(001) texture	Yield strength (Mpa)	Center Average Particle Size (㎛)	Kca (N / mm ^1.5 , @-10 ℃)
Inventive Steel 1	165	85	PF + P (16%)	23	396	21.2	9012
Inventive Steel 2	203	90	AF	18	442	12.7	8554
Invention Steel 3	112	85	AF + GB (24%)	26	509	15.6	7356
Inventive Steel 4	215	85	AF + GB (20%)	19	492	13.9	7855
Inventive Steel 5	188	90	AF + GB (38%)	21	521	17.7	6918
Inventive Steel 6	196	100	PF + P (17%)	16	401	20.9	6522
Comparative Steel 1	21	85	PF + P (18%)	43	398	35.4	4564
Comparative Steel 2	116	90	UB	42	579	38.3	3866
Comparative Steel 3	154	85	AF + UB (21%)	32	534	25.6	4211
Comparative Steel 4	201	90	UB	42	607	34.2	3901
Comparative Steel 5	165	90	GB, UB (22%)	31	551	31.2	3244
Comparative Steel 6	123	95	AF + GB (17%)	29	498	23.1	4855

* PF: Polygonal ferrite, P: Pearlite, AF: Acicular ferrite, GB: Granular bainite, UB: Upper bainite, upper Fraction (%): Volume%

As shown in Table 2, in the case of Comparative Steel 1, the difference between the center-surface average temperature during rough rolling of the present invention is controlled to be less than 70 ° C., and thus, the center portion is not provided with sufficient deformation in the center during rough rolling. The area ratio of the (100) plane which forms an angle within 15 degrees with respect to the plane perpendicular to the rolling direction at a thickness having a particle size of 35.4 μm and having a range of 20% of the plate thickness centering on one half of the thickness is 40%. Above, and also, it can be seen that the Kca value measured at −10 ° C. does not exceed 6000 required for general shipbuilding steels.

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, although the upper bainite is produced even though the grain size of the central austenite is refined through cooling during rough rolling. The area of the (100) plane with a particle size of 38.3 占 퐉, which forms an angle within 15 degrees with respect to the plane perpendicular to the rolling direction at a thickness ranging from 20% of the plate thickness to a half of the thickness. It can be seen that the Kca value has a value of 6000 or less at −10 ° C. due to the upper bainite having a rate of 40% or more and brittleness easily occurring as a matrix.

In the case of Comparative Steel 3, the content of Si is higher than the upper limit of the content of Si of the present invention, and although the upper bainite is partially formed in the center even though the grain size of the core austenite is refined through cooling during rough rolling, It can be seen that as a large amount of Si is added and a large amount of the MA structure is generated, the Kca value also has a value of 6000 or less at -10 ° C.

In the case of Comparative Steel 4, the Mn content has a higher value than the upper limit of the Mn content of the present invention. Due to the high hardenability, the microstructure of the base material is upper bainite, and the grain size of the central austenite is refined through cooling during rough rolling. Despite this, the final microstructure has a particle size of 34.2㎛ and forms an angle within 15 degrees with respect to the plane perpendicular to the rolling direction at a thickness ranging from 20% of the plate thickness to a half of the thickness (100). The area ratio of the plane) is 40% or more, and the Kca value also has a value of 6000 or less at -10 ° C.

In the case of Comparative Steel 5, the Ni content is higher than the upper limit of the Ni content of the present invention. Due to the high hardenability, the microstructure of the base material is granular bainite and upper bainite, and is cooled during rough rolling. Although the particle size of the central austenite was refined, the final microstructure had a particle size of 31.2 μm, and the Kca value also had a value of 6000 or less at -10 ° C.

In the case of Comparative Steel 6, the content of P and S has a higher value than the upper limit of the P and S content of the present invention. Although all other conditions satisfy the conditions of the present invention, brittleness occurs due to high P and S. Thus, it can be seen that the Kca value has a value of 6000 or less at -10 ° C.

On the contrary, in the case of the invention steels 1 to 6 in which the component range of the present invention is satisfied and the grain size of the central austenite is refined through cooling during rough rolling, the yield strength of 390 MPa or more and the central particle size of 30 μm or less are satisfied and ferrite and pearls are satisfied. It can be seen that the microstructure has a light structure or acicular ferrite single phase tissue, or a complex structure of acicular ferrite and granular bainite, and a complex structure of acicular ferrite, pearlite and granular bainite.

Further, the area ratio of the (100) plane which forms an angle within 15 degrees with respect to the plane perpendicular to the rolling direction at a thickness having a range of 20% of the plate thickness centered on 1/2 part of the thickness is 40% or less, Kca It can be seen that the value also satisfies a value of 6000 or more at -10 ° C.

1 shows a photograph of the thickness center of the inventive steel 1 observed with an optical microscope. As can be seen from FIG. 1, the thickness center structure is minute.

(Example 2)

Except that the Cu / Ni weight ratio of the steel slab was changed as shown in Table 3 below, the steel sheet was manufactured under the same composition and manufacturing conditions as the inventive steel 2 of Example 1, and the surface characteristics of the manufactured steel sheet were investigated and the results were obtained. It is shown in Table 3 below.

In Table 3, the surface characteristics of the steel sheet are measured by the occurrence of surface cracks by hot shortness.

TABLE 3

Steel grade	Steel composition (% by weight)										Surface characteristics
Steel grade	C	Si	Mn	Ni	Cu	Ti	Nb	P (ppm)	S (ppm)	Cu / Ni weight ratio	Surface characteristics
Inventive Steel 7	0.082	0.31	1.36	0.84	0.41	0.016	0.017	77	25	0.48	Not Occurred
Inventive Steel 2				0.95	0.44					0.46	Not Occurred
Inventive Steel 8				0.37	0.12					0.32	Not Occurred
Inventive Steel 9				0.28	0.10					0.35	Not Occurred
Comparative Steel 7				0.23	0.18					0.78	Occur
Comparative Steel 8				0.48	0.33					0.71	Occur

As shown in Table 3 below, it can be seen that the surface characteristics of the steel sheet are improved when the Cu / Ni weight ratio is properly controlled.

(Example 3)

After rough rolling, the grain size (μm) before finishing rolling was changed to the same composition and manufacturing conditions as those of Inventive Steel 1 of Example 1, except that the grain size (μm) was changed as shown in Table 4 below. Was investigated and the results are shown in Table 4 below.

Table 4

Steel grade	Grain size after rough rolling and before finish rolling (㎛)	Center Average Particle Size (㎛)
Inventive Steel 1	80	21.2
Inventive Steel 10	125	29.7
Inventive Steel 11	107	25.6
Inventive Steel 12	75	19.8
Inventive Steel 13	155	21.5
Inventive Steel 14	110	24.5

As shown in Table 4, it can be seen that as the core grain size decreases in the bar state after rough rolling, the core mean particle size becomes finer, thereby improving the brittle crack propagation resistance.

Although described with reference to the embodiments, it will be understood by those skilled in the art that the present invention may be variously modified and changed without departing from the spirit and scope of the invention as set forth in the claims below. Could be.

Claims

By weight%, C: 0.05-0.1%, Mn: 0.9-1.5%, Ni: 0.8-1.5%, Nb: 0.005-0.1%, Ti: 0.005-0.1%, Cu: 0.1-0.6%, Si: 0.1- 0.4%, P: 100 ppm or less, S: 40 ppm or less, remaining Fe and other unavoidable impurities; Microstructure including a single structure selected from the group consisting of ferrite single phase structure, bainite single phase structure, ferrite and bainite complex, ferrite and perlite complex, and ferrite, bainite and perlite complex. Have; And high strength steel with excellent brittle crack propagation resistance more than 50mm thick.
The method according to claim 1,

The Cu and Ni content is a high strength steel with excellent brittle crack propagation resistance, characterized in that the Cu / Ni weight ratio is set to 0.6 or less.
The method according to claim 1,

The ferrite is acicular ferrite or polygonal ferrite, and bainite is brittle crack propagation resistance, characterized in that it is granular bainite.
The method according to claim 1,

When the microstructure of the steel is a composite structure containing pearlite, the fraction of pearlite is high strength steel having excellent brittle crack propagation resistance, characterized in that 20% or less.
The method according to claim 1,

The steel is a high-strength steel with excellent brittle crack propagation resistance, characterized in that the particle size having a high-angle boundary of more than 15 degrees measured by the ESBD method of the center of the steel thickness is 30㎛ or less.
The method according to claim 1,

The steel is a high strength steel with excellent brittle crack propagation resistance, characterized in that the yield strength of 390MPa or more.
The method according to claim 1,

An area ratio of the (100) plane which forms an angle within 15 degrees with respect to the plane perpendicular to the rolling direction at a thickness having a range of 20% of the steel thickness with respect to 1/2 part of the steel thickness is characterized by 40% or less. High strength steel with excellent brittle crack propagation resistance.
The method according to claim 1,

High strength steel with excellent brittle crack propagation resistance, characterized in that the steel thickness is 80 ~ 100mm.
By weight%, C: 0.05-0.1%, Mn: 0.9-1.5%, Ni: 0.8-1.5%, Nb: 0.005-0.1%, Ti: 0.005-0.1%, Cu: 0.1-0.6%, Si: 0.1- Reheating the slab containing 0.4%, P: 100 ppm or less, S: 40 ppm or less, remaining Fe, and other unavoidable impurities to 950-1100 ° C., followed by rough rolling at a temperature of 1100-900 ° C .; Finishing roughly rolling the bar 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 cooling the steel sheet to a temperature of 700 ° C. or less, wherein the temperature difference between the center and the surface of the slab or bar before rolling during the rough rolling is 70 ° C. or more.
The method according to claim 9,

The content of Cu and Ni is a method of producing a high strength steel with excellent brittle crack propagation resistance, characterized in that the Cu / Ni weight ratio is set to 0.6 or less.
The method according to claim 9,

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 100 ~ 300 ℃ characterized in that the brittle crack propagation resistance is excellent manufacturing method of high strength steel.
The method according to claim 9,

The temperature difference between the center on the thickness of the slab or bar and the outer surface of the slab or bar is calculated by considering the surface temperature of the slab or bar measured immediately before rough rolling, the cooling conditions and the thickness of the slab or bar immediately before rough rolling. A method for producing a high strength steel having excellent brittle crack propagation resistance, characterized by a difference in core temperature.
The method according to claim 9,

The rough rolling is performed for two or more passes, and the temperature difference between the center of the thickness of the slab or the bar and the outer surface of the slab or the bar is a temperature difference obtained by measuring the average temperature of each pass of the rough rolling. A method for producing high strength steels with excellent brittle crack propagation resistance.
The method according to claim 9,

The method of manufacturing high strength steel having excellent brittle crack propagation resistance, characterized in that the rolling reduction per pass is 5% or more and the total cumulative rolling reduction is 40% or more for the last three passes during rough rolling.
The method according to claim 9,

The grain size of the central portion of the bar after the rough rolling before the finish rolling is less than 200㎛ characterized in that the brittle crack propagation resistance is excellent manufacturing method of high strength steel.
The method according to claim 9,

The rolling reduction ratio during the finish rolling is set to a ratio of slab thickness (mm) / thickness of the steel sheet after finishing rolling (mm) of 3.5 or more, characterized in that the brittle crack propagation resistance is excellent manufacturing method of high strength steel.
The method according to claim 9,

Cooling of the steel sheet is a method of producing a high strength steel with excellent brittle crack propagation resistance, characterized in that performed at a central cooling rate of 2 ℃ / s or more.
The method according to claim 9,

Cooling of the steel sheet is a method of producing a high strength steel with excellent brittle crack propagation resistance, characterized in that performed at an average cooling rate of 3 ~ 300 ℃ / s.