WO2011081349A2

WO2011081349A2 - High strength steel sheet having excellent brittle crack resistance and method for manufacturing same

Info

Publication number: WO2011081349A2
Application number: PCT/KR2010/009222
Authority: WO
Inventors: 김상호; 방기현; 서인식
Original assignee: 주식회사 포스코
Priority date: 2009-12-28
Filing date: 2010-12-22
Publication date: 2011-07-07
Also published as: CN102753719B; EP2520683A4; CN102753719A; KR20110075321A; EP2520683A2; EP2520683B1; KR101360737B1; WO2011081349A3

Abstract

The present invention relates to a steel sheet having excellent brittle crack resistance at a base and a welding heat affected zone, and particularly to a high strength steel sheet having excellent brittle crack resistance and a method for manufacturing the same, wherein the steel sheet comprises, by weight%, C: 0.02 to 0.06%, Si: 0.1% or lower, Mn: 1.5 to 2.0%, P: 0.012% or lower, S: 0.003% or lower, Ni: 0.5 to 1.5%, Al: 0.003 to 0.015%, Ti: 0.005 to 0.02%, Nb: 0.005 to 0.015%, N: 0.002 to 0.006%, and the balance Fe and unavoidable impurities, wherein the value of C+0.5Si-0.1Ni+6Al+3Nb is 0.1% or lower.

Description

High strength steel sheet with excellent brittle cracking resistance and manufacturing method thereof

The present invention relates to a high-strength steel sheet used in marine structures, building structures, and the like, and more particularly, to a high-strength steel sheet having excellent brittle cracking resistance in a base material and a heat affected zone (HAZ) and a method of manufacturing the same. will be.

As demand for energy increases rapidly, especially in emerging economies such as China and India, oil resource development is being promoted in extreme regions, such as Sakhalin and the Arctic Ocean, which have not been developed due to low profitability in the past.

Steel materials used in the structures to be built in the extreme cold, etc. are required to have high resistance to brittle cracking at low temperatures to ensure the safety of the structure. The crack tip opening displacement (CTOD) test based on fracture mechanics is mainly used to evaluate the resistance to brittle cracking at low temperatures.

To date, CTOD tests have been used primarily to evaluate the resistance to brittle cracking in welded heat affected zones. Alternatively, impact tests have been used in place of CTOD tests for base metal parts. However, in the case of offshore structures, such as the Sakhalin and Arctic Oceans, the high-strength thick steel plates having a thickness of 50 mm or more are used in consideration of the collision with the iceberg, and the fatigue crack generated in the weld zone is subjected to cyclic stress. According to the present invention, brittle cracks may occur from fatigue cracks under specific conditions after propagation to the base metal part. Therefore, a high level of brittle crack generation resistance is required for not only the weld heat affected part but also the base material part.

Looking at the prior art of a steel sheet excellent in resistance to brittle cracking at low temperatures, as follows.

Korean Laid-Open Patent Publication No. 2002-0028203 discloses a method of adding Mg to suppress coarsening of grains generated near the Fusion Line during welding to prevent brittle fracture from occurring in the heat affected zone. . However, the published patent guarantees the prevention of brittle fracture only at a temperature of -10 ° C. or higher, and thus cannot protect the brittle fracture at low temperatures such as -40 ° C.

In addition, Korean Laid-Open Patent Publication No. 2008-0067957 limits Al, Nb, etc. to below a certain limit to prevent sudden drop in toughness generated in the weld heat affected zone, and utilizes -40 Mn, which has little effect on the weld heat affected zone toughness. A technique for securing resistance to brittle crack generation at a welded heat affected zone even at low temperatures of ° C is disclosed. However, the disclosed patent does not describe a method of securing resistance to brittle crack generation for the base material portion unlike the welding heat affected portion.

On the other hand, Korean Laid-Open Patent Publication No. 2006-0090287 is a technology for securing the physical properties of the steel sheet by lowering the C content to suppress phase martensite and adding 0.8% or more of Cu to increase precipitation by Cu precipitates. Disclosed is a method of manufacturing a steel having excellent resistance to brittle crack generation at a low temperature at the base material and the weld heat affected zone. However, the disclosed patent requires a further aging treatment after controlled rolling and accelerated cooling in a state where a large amount of Cu is added in order to secure Cu precipitates, and thus, the manufacturing process is complicated and manufacturing costs increase.

One aspect of the present invention is to provide a high-strength steel sheet excellent in brittle cracking resistance having a yield strength of 420MPa or more and both the base material and the welding heat affected zone (HAZ) can suppress the occurrence of brittle cracks at low temperatures It is.

In the present invention, by weight%, C: 0.02 to 0.06%, Si: 0.1% or less, Mn: 1.5 to 2.0%, P: 0.012% or less, S: 0.003% or less, Ni: 0.5 to 1.5%, Al: 0.003 to 0.015%, Ti: 0.005% to 0.02%, Nb: 0.005% to 0.015%, N: 0.002% to 0.006%, the rest includes Fe and unavoidable impurities,

It provides a high strength steel sheet excellent in brittle cracking resistance having a C + 0.5 Si-0.1 Ni + 6 Al + 3 Nb value of 0.1% or less.

In addition, the present invention comprises the steps of heating a steel slab satisfying the composition range to a temperature range of 1000 ~ 1100 ℃;

Roughly rolling the heated slab with a cumulative reduction ratio of 40% or more at a temperature of 950 ° C. or higher;

Finishing rolling in a temperature range of 700 to 800 ° C. after the rough rolling; And

It provides a method of producing a high strength steel sheet excellent in brittle crack generation resistance comprising the step of cooling the rolled steel sheet.

The present invention can provide a high strength steel sheet having a yield strength of 420 MPa or more and excellent resistance to brittle cracking at low temperatures of -60 ° C. and -40 ° C. in the base material and the weld heat affected zone, respectively, and the thick steel sheet. It can be used for offshore structures, building structures, ships and tankers operating under extreme conditions.

Figure 1 is a graph showing the CTOD test results of the heat affected zone for the C + 0.5 Si-0.1 Ni + 6 Al + 3 Nb value.

2 is a graph showing the CTOD test results of the base material portion according to the effective grain size.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The inventors have recognized that the brittle cracks appearing at low temperatures in the weld heat affected zone are caused by the in-phase martensite structure generated in the weld heat affected zone. In particular, at low temperatures such as -40 ° C, very small amounts of phase martensite present in the welding heat affected zone may cause brittle fracture in the CTOD test, so it is recognized that the inhibition of phase martensite is very important. In-depth study has been made on the method of inhibiting the iconic martensite structure in the department.

In addition, the present inventors have investigated the cause of brittle cracks occurring in the base material of the thick steel plate of 50 mm or more, the brittle fracture occurs mainly in the center of the thickness of the steel sheet, microscopically, the embrittlement in relatively coarse grains among the grains of the center of the thickness As a result of discovering the fact that a crack occurs and studying in depth the method which can suppress it, it came to this invention.

Hereinafter, the composition range of the present invention will be described in detail (hereinafter,% by weight).

Carbon (C): 0.02-0.06%

Since C is an important alloying element that forms the martensite which is formed in the weld heat affected zone and causes brittle fracture, it is necessary to first limit the C content to suppress the formation of the martensite. When the C content is more than 0.06%, it is preferable to limit the upper limit to 0.06% because the island martensite is not sufficiently suppressed and the object of the present invention cannot be achieved. However, if the C content is too low, it is difficult to secure the strength of the steel sheet, so it is preferable to make 0.02% the lower limit.

Silicon (Si): 0.1% or less (excluding 0)

Si is an element necessary for increasing the tensile strength of the base material and deoxidation of the steel, but prevents the metamorphic austenite from being decomposed into ferrite and cementite when the unmodified austenite formed by the welding heat cycle is cooled to form a final structure. Since it greatly contributes to the formation of the site and greatly reduces the CTOD toughness in the weld heat affected zone, the amount of addition is preferably 0.1% or less.

Manganese (Mn): 1.5-2.0%

Since Mn is a useful element for securing strength, it should be added at least 1.5% to secure strength of the steel sheet. However, when the amount of Mn added is excessive, the center segregation is encouraged at the center of thickness, and the formation of phase martensite is locally promoted at the site where the central segregation is formed, which greatly inhibits the CTOD characteristics of the weld heat affected zone, so the upper limit is limited to 2.0%. It is desirable to.

Phosphorus (P): 0.012% or less, Sulfur (S): 0.003% or less

Since P and S are elements that cause grain embrittlement in the heat affected zone, it is necessary to reduce them as much as possible. However, P and S are limited to 0.012% or less and S to 0.003% or less because they are difficult in the steelmaking process.

Nickel (Ni): 0.5 to 1.5%

Ni promotes the formation of martensite by increasing the hardenability, but since the toughness strengthening effect of the matrix structure is greater than that of the alloy, Ni has an effect of improving the toughness of the weld heat affected zone when added. In addition, since the effect of improving the matrix structure toughness by Ni is exerted on the base material part, it is also effective in strengthening the toughness of the base material part. In addition, it is necessary to add 0.5% or more in order to secure the strength of the steel sheet required by the present invention in a state where C and Si are extremely limited. However, if an excessively large amount is added, the upper limit of toughness of the matrix structure is saturated, so the upper limit is preferably limited to 1.5%.

Aluminum (Al): 0.003-0.015%

Al, similar to Si, prevents the formation of ferrite and cementite from unmodified austenite during the welding heat cycle, and contributes to the formation of phase martensite. When Al is added in excess of 0.015%, the toughness of the weld heat affected zone is greatly reduced. Therefore, it is preferable to limit the upper limit to 0.015%. However, Al is a very effective element for deoxidation of steel, and in the present invention, when the Si content is limited to 0.1% or less, if the Al content is too low, the deoxidation of the steel may not be sufficiently performed, and thus the cleanliness of the steel may be greatly deteriorated. It is preferable to add more.

Titanium (Ti): 0.005 ~ 0.02%

Ti combines with N to form fine nitride and prevents grain coarsening near the weld melting line, thereby improving the toughness of the weld heat affected zone. If the Ti content is too small, Ti nitride is not sufficiently formed, and thus it is not possible to prevent grain coarsening near the weld melting line, and therefore, 0.005% or more is preferably added. However, when Ti is added in excess of 0.02%, Ti carbide may be formed together with Ti nitride, and due to the precipitation hardening effect of Ti carbide, the hardness of the base material portion and the weld heat affected zone increases, thereby increasing the possibility of brittle cracking. It is preferable to make an upper limit into 0.02%.

Niobium (Nb): 0.005 to 0.015%

Nb is an alloying element that lowers the brittle fracture resistance of the welded heat affected zone during addition, but it is an important element to increase the brittle fracture resistance of the base metal part because it greatly contributes to making the structure fine in the controlled rolling-accelerated cooling process. In particular, in a thick steel sheet having a thickness of 50 mm or more, it is difficult to secure an effective grain size of 30 μm or less, which is required in the present invention, even when controlled rolling-acceleration cooling is not accompanied by micronization of the structure by Nb. Therefore, in order to ensure brittle fracture resistance of the base material portion required by the present invention, it is preferable to add 0.005% or more. However, since too much addition may promote formation of island martensite and damage the toughness of the weld heat affected zone, the upper limit thereof is preferably limited to 0.015%.

Nitrogen (N): 0.002 ~ 0.006%

N combines with Ti to form TiN particles to prevent grain coarsening near the weld melting line. Therefore, it is necessary to include 0.002% or more in order to achieve such an effect, but if it is added too much, the toughness of the base material portion and the welding influence part may be impaired by free N atoms not bonded with Ti, so the upper limit is preferably set to 0.006%. Do.

In the present invention, the physical properties can be sufficiently secured even with the basic composition described above, but Cu may be added to further improve the properties of the steel sheet. The content of Cu is preferably 0.35% or less. The Cu is an alloying element which can secure the strength of the steel sheet while relatively harming the toughness of the weld heat affected zone, but when added excessively, the strength of the steel sheet is too high due to the precipitation of Cu, so that the CTOD toughness cannot be secured stably in the base material portion. Since Cu cracks may be generated on the surface of the slab and the steel sheet, the upper limit thereof is preferably 0.35%.

The rest consists of Fe and unavoidable impurities.

In the present invention, the C + 0.5Si-0.1Ni + 6Al + 3Nb value in the composition is preferably 0.1% or less.

The present inventors have conducted in-depth studies on the alloying elements that affect the formation of martensite in the weld heat affected zone. As a result, the weld heat affected under low to medium heat input welding conditions where the welding heat input was 0.8 to 4.5 kJ / mm. We have come up with ways to minimize the generation of in-phase martensite in wealth.

In order to derive the correlation between the alloy element and the weld heat affected zone based on the following findings, the present inventors have identified an intercritically reheated coarse known as the region where the most martensite is generated in the weld heat affected zone. To simulate the heat affected zone, the weld heat affected zone simulation experiment was performed as follows.

First, a small specimen having a thickness of 10 mm, a width of 10 mm, and a length of 60 mm is heated to 1400 ° C., and then the temperature section between 800 ° C. and 500 ° C. is cooled at a cooling rate of 20 ° C./s, and then reheated to an ideal region. After cooling the temperature range between the maximum heating temperature and 500 ℃ at a cooling rate of 20 ℃ / s to simulate the ideal zone reheating grain coarsening heat affected zone. The CTOD test was performed at -40 ° C after the fatigue cracks were introduced to the thermally affected zone simulated specimens up to 50% of the specimen width. The correlation between the alloying element and the CTOD toughness of the weld heat affected zone is derived from this experiment and the results are shown in FIG. 1.

Figure 1 shows the relationship between the C + 0.5 Si-0.1 Ni + 6 Al + 3 Nb value and -40 ℃ limit CTOD test value obtained from the heat affected zone simulation specimens. It can be seen that the lower the C + 0.5Si-0.1Ni + 6Al + 3Nb value, the -40 ° C limit CTOD value of the weld heat affected zone increases. When the C + 0.5Si-0.1Ni + 6Al + 3Nb value exceeded 0.2%, brittle fracture occurred in all specimens. It can be seen from FIG. 1 that C + 0.5Si-0.1Ni + 6Al + 3Nb should be 0.1% or less in order for the limit CTOD value measured at −40 ° C. to be 0.25 mm or more.

The alloying elements of C, Si, Al, and Nb in the formula C + 0.5Si-0.1Ni + 6Al + 3Nb promote brittle cracks in the weld heat affected zone when the alloy is added, but only Ni has the opposite effect. This is because the toughening effect of the matrix structure is increased more than the effect of decreasing the toughness by increasing the phase martensite in the weld heat affected zone as the hardenable element.

In the steel sheet of the present invention, at least 5 or more crystal grains defined as boundaries having a crystal orientation difference of 15 degrees or more measured by the Electro Back-Scattered Pattern (EBSP) method at the center of the thickness of the steel sheet are in the top 5%. It is preferable that the average circle equivalent diameter of belonging crystal grains is 30 micrometers or less. In the present invention, the thickness center is defined within ± 1 mm in the thickness direction from the 1/2 point of the steel sheet thickness.

In general, to measure the particle size, an image analysis method is used based on an optical microscope image, but the image analysis method is relatively accurate only when the microstructure is composed of polygonal ferrite and pearlite. In the microstructure mixed with knight, the grain boundary is unclear, which makes it difficult to accurately measure the particle size.

Accordingly, the present inventors used the Electro Back-Scattered Pattern (EBSP) method based on the Kikuchi pattern to more accurately measure the grain size of the center of thickness. Using the electron backscattering pattern method has the advantage of quantitatively analyzing the orientation difference between grains regardless of the microstructure. When defining grains in this way, the boundary where the orientation difference between measured crystals is 15 degrees or more is defined as diagonal boundary.

As a result of comparing the particle size distribution of the thickness center and the CTOD characteristics obtained using the electron backscattering pattern method, it is brittle by the grains belonging to the top 5% of the total grain size distribution rather than the grain size of the whole grains defined by the diagonal grain boundary. It was found that the crack initiation resistance was determined. That is, in order to increase the brittle crack generation resistance of the base material portion, it is very important to suppress a few coarse grains in the structure of the thickness center portion.

In the present invention, among the at least 5000 crystal grains defined as the boundary (diagonal grain boundary) whose crystal orientation difference measured by the electron rear scattering pattern method in the thickness center of the steel sheet is 15 degrees or more, the size of the crystal grains (effective grains) belonging to the top 5% The average equivalent circle diameter was defined as the effective grain size.

In order to derive a correlation between the effective grain size defined in the present invention and the brittle fracture resistance of the base metal part, specimens having various particle sizes by changing heating and rolling conditions of slabs having a composition of 0.05C-0.04Si-1.62Mn-0.95Ni The specimens were subjected to CTOD test at various temperatures and 0.25mm limit CTOD transition temperature was obtained. Here, the 0.25 mm limit CTOD transition temperature represents the transition temperature when the measured limit CTOD value is 0.25 mm. The relationship between the effective grain size measured from each specimen and the 0.25 mm limit CTOD transition temperature is shown in FIG. 2.

It can be seen from FIG. 2 that a steel sheet having a limit CTOD value of at least 0.25 mm at -60 ° C. can be obtained only when the effective grain size defined in the present invention is 30 μm or less. If the effective grain size exceeds 30 μm, the -60 ° C. limit CTOD value of the steel plate base material portion is 0.25 mm or less, and thus the object of the present invention cannot be satisfied.

In this case, the basic microstructure of the thickness center portion is preferably ferrite, bainite, or a complex structure thereof except for martensite. The reason is that even though the martensitic structure has a fine particle size, the hardness is too high to easily pop-in at cryogenic temperatures such as -60 ° C, making it difficult to obtain a target limit CTOD value.

That is, the steel sheet of the present invention satisfies the -60 ℃ limit CTOD value of 0.25 mm or more in the base material portion, and the -40 ℃ limit CTOD value of 0.25 mm or more in the welding heat affected zone (HAZ) during welding, the weld heat Not only the affected part but also the base material part has excellent low temperature brittle crack resistance.

Hereinafter, the manufacturing method of the present invention will be described in detail.

Steel slab that satisfies the composition is heated to a temperature range of 1000 ~ 1100 ℃.

It is preferable that the slab uses a continuously cast slab. Since the continuous casting process has a faster solidification rate and cooling rate after solidification than the ingot process, finer TiN particles can be secured in the material, which is advantageous in increasing resistance to brittle cracks in the base material and the weld heat affected zone.

The slab heating temperature is a major factor affecting the particle size of the final structure. If the slab heating temperature exceeds 1100 ° C., the final structure cannot be sufficiently refined, and the TiN particles in the structure are coarsened to reduce the toughness of the weld heat affected zone. Therefore, it is preferable to make the upper limit into 1100 degreeC. On the contrary, when the slab heating temperature is less than 1000 ° C, the alloying elements are not sufficiently dissolved and sufficient rolling becomes difficult at or above the recrystallization temperature.

After heating the slab, rough rolling is performed with a cumulative reduction ratio of 40% or more at a temperature of 950 ° C or higher. At temperatures above 950 ° C, recrystallization of the austenite grains occurs actively, which is advantageous for reducing the particle size. In addition, the cumulative reduction ratio of 40% or more is because when the cumulative reduction ratio is less than 40%, recrystallization does not sufficiently occur, resulting in mixed grains of the final structure.

It is preferable that finish rolling is made in the temperature range of 700-800 degreeC. If the finish rolling temperature is higher than 800 ℃, it is difficult to secure the brittle crack generation resistance due to insufficient microstructure of the thickness center portion, and the lower the finish rolling temperature, the better the microstructure of the thickness center portion structure, but if it is too low, the rolling productivity decreases excessively. Since it is difficult to apply industrially, it is preferable to set the minimum to 700 degreeC.

In addition, the finish rolling cumulative reduction ratio is preferably performed at least 40% or more in order to further refine the final structure.

After the control rolling is cooled, wherein the cooling rate and the cooling stop temperature is preferably 3 ~ 20 ℃ / s and 350 ~ 550 ℃. If the strength is too high compared to the target, brittle cracking is easy to occur, so it is important not to have too high strength. From this point of view, the cooling rate and the cooling stop temperature should be 20 ° C / s or less and 350 ° C or more, respectively. However, if the cooling is not sufficient, the target strength of the present invention cannot be obtained. For this purpose, the cooling rate is 3 ° C / s. The cooling end temperature is preferably 550 ° C. or lower.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following examples.

(Example)

According to the composition shown in Table 1, a molten steel was made in a 300 ton converter and a 300 mm slab was made by continuous casting. As shown in Table 2, the slabs were heated, and the steel sheets were finally accelerated and cooled by rough rolling and finish rolling to prepare steel sheets.

In order to measure the effective grain size in the manufactured steel sheet, an electronic backscattering pattern (EBSP) device mounted on a scanning electron microscope (SEM) was used. The magnification used was about 300 to 500 times, the step size was 0.75 μm, and the thickness center of the cross section consisting of the rolling direction and the thickness direction of the steel sheet was observed. In order to obtain a statistically significant value, at least 5000 grains defined as boundaries having a crystal orientation difference of 15 degrees or more are included. The effective grain size defined in the present invention was calculated using software capable of analyzing the orientation difference measured by the electronic backscattering pattern method.

Specimens were taken from steel sheets prepared by the methods shown in Tables 1 and 2, and tensile tests were performed. CTOD tests were performed to evaluate the brittle fracture resistance of the base material. Tensile specimens were processed into rod-shaped specimens by taking the specimens from the surface at a quarter of the thickness of the steel sheet so that the direction perpendicular to the rolling direction was the length of the specimens. CTOD specimens were machined to full thickness specimens in accordance with BS7448 specifications and the lengths of the specimens were perpendicular to the rolling direction. After the notch was given to the CTOD specimen by the discharge machining, the fatigue crack was generated to 50% of the specimen width, and the CTOD test was performed three times at a temperature of -60 ° C, and the minimum value was evaluated.

In order to evaluate the resistance to brittle crack generation in the weld heat affected zone of the manufactured thick steel sheet, evaluation was performed according to the API RP 2Z rule. A single improvement was made according to the API RP 2Z rule and then welded by Flux Cored Arc Welding and Submerged Arc Welding with weld heat inputs of 0.8 and 4.5 kJ / mm, respectively. Welded specimens are processed into full thickness specimens in accordance with BS7448 standards, and the fatigue cracks are inserted in the grain coarsening area near the weld melting line, and CTOD tests are performed three times at -40 ℃. Evaluated.

The yield strength and tensile strength of the steel sheet obtained through the tensile test, and the limit CTOD values evaluated at -60 ° C and -40 ° C for the base metal part and the welded part, respectively, are shown in Table 3. Here, the limit CTOD value shown in Table 3 represents the lowest value among the three test values, CTOD-60 is the -60 ℃ CTOD test value evaluated for the base material portion, CTOD-40 is evaluated for the weld heat affected zone Mean -40 ° C CTOD test value.

Table 1

division	C	Si	Mn	P	S	Ni	Al	Ti	Nb	N	Cu	C + 0.5Si-0.1Ni + 6Al + 3Nb
Inventive Example 1	0.039	0.05	1.57	0.003	0.003	1.30	0.007	0.012	0.013	0.0058	-	0.014
Inventive Example 2	0.045	0.04	1.80	0.004	0.002	0.68	0.012	0.008	0.006	0.0036	-	0.086
Inventive Example 3	0.023	0.07	1.75	0.005	0.003	1.14	0.010	0.006	0.013	0.0051	-	0.045
Inventive Example 4	0.046	0.04	1.73	0.003	0.003	1.41	0.012	0.007	0.006	0.0055	-	0.015
Inventive Example 5	0.045	0.10	1.59	0.006	0.002	1.14	0.003	0.005	0.014	0.0022	0.15	0.039
Inventive Example 6	0.024	0.03	1.82	0.003	0.001	1.12	0.013	0.010	0.014	0.0056	-	0.047
Inventive Example 7	0.046	0.08	1.77	0.006	0.002	0.82	0.009	0.009	0.010	0.0043	-	0.087
Inventive Example 8	0.025	0.04	1.73	0.006	0.001	1.02	0.006	0.007	0.011	0.0029	0.29	0.014
Inventive Example 9	0.044	0.09	1.76	0.004	0.001	1.45	0.014	0.011	0.005	0.0050	-	0.044
Inventive Example 10	0.037	0.05	1.77	0.003	0.002	0.84	0.009	0.009	0.009	0.0052	-	0.058
Inventive Example 11	0.044	0.07	1.78	0.007	0.002	1.09	0.014	0.007	0.007	0.0031	-	0.077
Inventive Example 12	0.042	0.03	1.55	0.004	0.002	1.31	0.009	0.012	0.012	0.0059	-	0.014
Inventive Example 13	0.050	0.03	1.65	0.003	0.002	1.05	0.006	0.016	0.015	0.0049	-	0.038
Inventive Example 14	0.056	0.07	1.52	0.003	0.002	1.44	0.013	0.010	0.013	0.0029	-	0.064
Inventive Example 15	0.040	0.06	1.71	0.005	0.001	0.83	0.013	0.007	0.009	0.0047	0.23	0.092
Inventive Example 16	0.020	0.07	1.85	0.005	0.002	1.30	0.011	0.014	0.014	0.0034	-	0.034
Comparative Example 1	0.047	0.07	1.77	0.006	0.001	0.78	0.013	0.009	0.015	0.0041	-	0.127
Comparative Example 2	0.056	0.15	1.88	0.003	0.002	0.85	0.021	0.009	0.009	0.0047	-	0.199
Comparative Example 3	0.047	0.08	1.71	0.003	0.003	0.71	0.013	0.013	0.021	0.0050	-	0.160
Comparative Example 4	0.069	0.05	1.67	0.004	0.001	0.69	0.005	0.008	0.012	0.0051	-	0.093
Comparative Example 5	0.038	0.09	1.72	0.005	0.001	0.38	0.013	0.008	0.012	0.0033	-	0.158
Comparative Example 6	0.037	0.09	1.77	0.007	0.002	1.47	0.007	0.009	0.013	0.0037	-	0.014
Comparative Example 7	0.045	0.03	1.69	0.003	0.003	1.33	0.008	0.009	0.012	0.0059	-	0.012
Comparative Example 8	0.043	0.09	1.70	0.003	0.003	1.08	0.004	0.011	0.014	0.0049	-	0.048
Comparative Example 9	0.055	0.08	1.82	0.006	0.002	0.92	0.015	0.005	0.014	0.0031	-	0.135

TABLE 2

division	Slab heating temperature (℃)	Rough rolling cumulative reduction rate (%)	Finish rolling temperature (℃)	Finish rolling cumulative reduction rate (%)	Accelerated cooling stop temperature (℃)	Cooling rate (℃ / s)	Steel plate thickness (mm)
Inventive Example 1	1075	55	751	52	493	4.4	92
Inventive Example 2	1074	41	713	53	394	6.0	76
Inventive Example 3	1057	50	715	54	501	4.0	76
Inventive Example 4	1049	48	782	42	522	7.9	70
Inventive Example 5	1050	50	765	42	443	7.3	72
Inventive Example 6	1095	45	759	48	522	6.6	79
Inventive Example 7	1043	49	734	48	411	5.7	83
Inventive Example 8	1048	51	703	42	500	3.7	94
Inventive Example 9	1075	45	736	47	504	6.9	71
Inventive Example 10	1094	47	770	48	411	5.1	72
Inventive Example 11	1090	59	749	44	456	4.0	87
Inventive Example 12	1079	57	755	55	533	4.2	85
Inventive Example 13	1088	42	789	43	474	3.1	83
Inventive Example 14	1078	52	793	50	397	4.6	71
Inventive Example 15	1063	46	760	43	408	7.1	82
Inventive Example 16	1071	41	768	42	391	4.5	95
Comparative Example 1	1057	41	744	54	396	5.0	81
Comparative Example 2	1096	43	775	47	519	4.8	87
Comparative Example 3	1096	59	775	42	527	5.1	82
Comparative Example 4	1048	48	727	50	380	4.9	92
Comparative Example 5	1057	50	744	48	440	7.3	87
Comparative Example 6	1087	30	734	50	422	3.6	83
Comparative Example 7	1071	42	790	31	642	7.5	85
Comparative Example 8	1156	42	756	45	435	4.1	91
Comparative Example 9	1091	41	709	49	537	2.1	76

TABLE 3

division	Mother and child department				Welding heat affected zone
division	Effective grain size (㎛)	Yield strength (MPa)	Tensile Strength (MPa)	CTOD-60 (mm)	0.8 kJ / mm CTOD-40 (mm)	4.5kJ / mm CTOD-40 (mm)
Inventive Example 1	17	435	533	0.89	0.88	0.64
Inventive Example 2	26	453	557	0.45	0.43	0.28
Inventive Example 3	21	441	558	0.57	0.60	0.41
Inventive Example 4	12	432	547	1.02	0.61	0.57
Inventive Example 5	26	458	551	0.49	0.67	0.55
Inventive Example 6	14	432	539	0.90	0.48	0.50
Inventive Example 7	27	456	553	0.35	0.55	0.26
Inventive Example 8	11	439	547	0.99	0.70	0.56
Inventive Example 9	15	449	548	0.86	0.49	0.34
Inventive Example 10	29	458	566	0.28	0.68	0.42
Inventive Example 11	11	447	543	1.08	0.44	0.41
Inventive Example 12	29	429	531	0.31	0.72	0.62
Inventive Example 13	25	441	547	0.45	0.68	0.46
Inventive Example 14	13	458	566	0.89	0.58	0.41
Inventive Example 15	15	444	553	0.81	0.31	0.30
Inventive Example 16	13	437	536	0.92	0.44	0.47
Comparative Example 1	20	442	549	0.42	0.29	0.14
Comparative Example 2	21	431	528	0.43	0.09	0.05
Comparative Example 3	19	427	532	0.75	0.12	0.07
Comparative Example 4	22	439	541	0.54	0.17	0.10
Comparative Example 5	35	401	487	0.23	0.19	0.04
Comparative Example 6	42	448	548	0.14	0.40	0.60
Comparative Example 7	39	395	509	0.18	0.37	0.43
Comparative Example 8	45	437	535	0.11	0.42	0.29
Comparative Example 9	16	388	491	0.71	0.31	0.14

Inventive Examples 1 to 16, which correspond to the composition and preparation method of the present invention, have an effective grain size defined in the present invention of 30 µm or less, and a limit CTOD value of 0.25 mm or more of the base metal part evaluated at -60 ° C. Under the conditions, the minimum value of -40 ℃ CTOD of the welded heat affected zone was 0.25mm or more, indicating very good brittle cracking resistance.

On the contrary, in Comparative Example 1, since the C + 0.5Si-0.1Ni + 6Al + 3Nb value exceeded 0.1%, the CTOD value of the weld heat affected zone did not exceed 0.25 mm. In Comparative Example 2, Si and Al did not satisfy the scope of the present invention, and the C + 0.5Si-0.1Ni + 6Al + 3Nb value was also high as 0.199%, and the CTOD characteristics of the weld heat affected zone at -40 ° C were not very good.

In Comparative Example 3, Nb is out of the scope of the present invention, and the C + 0.5Si-0.1Ni + 6Al + 3Nb value is 0.1% or more, and in Comparative Example 4, the C + 0.5Si-0.1Ni + 6Al + 3Nb value is 0.1% or less. As satisfies the object of the present invention, but the C content is higher than the range specified in the present invention, the toughness of the weld heat affected zone was insufficient. In Comparative Example 5, the strength of the steel sheet was insufficient due to the lack of Ni content, and both the base material portion and the weld heat affected zone toughness were not sufficient.

In Comparative Examples 6 to 8, the alloy component is in the scope of the present invention, the C + 0.5 Si-0.1 Ni + 6 Al + 3 Nb value is also 0.1% or less, the toughness of the weld heat affected zone was not bad, but the manufacturing conditions required by the present invention It was not satisfied that the effective grain size was 30㎛ or more, and Comparative Example 7 also did not reach the strength of the present invention. In Comparative Example 9, the C + 0.5Si-0.1Ni + 6Al + 3Nb value exceeded 0.1%, so that the toughness of the welded heat affected zone fell, and the cooling rate was insufficient in the manufacturing conditions, so that the yield strength of the steel sheet did not reach 420 MPa.

Claims

By weight%, C: 0.02-0.06%, Si: 0.1% or less, Mn: 1.5-2.0%, P: 0.012% or less, S: 0.003% or less, Ni: 0.5-1.5%, Al: 0.003-0.015%, Ti: 0.005% to 0.02%, Nb: 0.005% to 0.015%, N: 0.002% to 0.006%, the rest includes Fe and unavoidable impurities,

High strength steel sheet with excellent brittle cracking resistance with C + 0.5Si-0.1Ni + 6Al + 3Nb value of 0.1% or less.
The method according to claim 1,

The steel sheet is a high strength steel sheet excellent in brittle cracking resistance further comprising Cu: 0.35% or less.
The method according to claim 1,

The average circle equivalent diameter of the crystal grains belonging to the upper 5% among the at least 5000 crystal grains defined by the boundary of the crystal orientation difference measured by the electron rear scattering pattern method at the center of the thickness of the steel sheet is 15 degrees or more has a diameter of 30 μm or less High strength steel sheet with excellent brittle cracking resistance.
The method according to claim 3,

The thick central structure of the steel sheet is a high strength steel sheet having excellent resistance to brittle cracking, which is any one of ferrite, bainite, and a composite structure thereof.
The method according to claim 1,

A high strength steel sheet having excellent brittle crack generation resistance in which a base material portion of the steel sheet satisfies a -60 ° C limit CTOD value of 0.25 mm or more, and a welded heat affected zone (HAZ) satisfies a -40 ° C limit CTOD value of 0.25 mm or more.
By weight%, C: 0.02-0.06%, Si: 0.1% or less, Mn: 1.5-2.0%, P: 0.012% or less, S: 0.003% or less, Ni: 0.5-1.5%, Al: 0.003-0.015%, Ti: 0.005% to 0.02%, Nb: 0.005% to 0.015%, N: 0.002% to 0.006%, the rest includes Fe and unavoidable impurities,

Heating a steel slab having a C + 0.5Si-0.1Ni + 6Al + 3Nb value satisfying 0.1% or less in a temperature range of 1000 to 1100 ° C;

Roughly rolling the heated slab with a cumulative reduction ratio of 40% or more at a temperature of 950 ° C. or higher;

Finishing rolling in a temperature range of 700 to 800 ° C. after the rough rolling; And

Cooling the rolled steel sheet

Method for producing a high strength steel sheet having excellent brittle crack generation resistance comprising a.
The method according to claim 6,

The steel slab is a method of producing a high strength steel sheet excellent in brittle cracking resistance further comprises Cu: 0.35% or less.
The method according to claim 6,

The finish rolling is a method of producing a high strength steel sheet excellent in brittle cracking resistance is performed at a cumulative reduction of 40% or more.
The method according to claim 6,

In the cooling step, the cooling rate and the cooling stop temperature is 3 ~ 20 ℃ / s and 350 ~ 550 ℃ the method of producing a high strength steel sheet excellent resistance to brittle cracking.