JP2688312B2 - High strength and high toughness steel plate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength and high-toughness steel plate, and more particularly, it is used for welding structures such as ships, marine structures, line pipes, low-temperature tanks or bridges which are used at low temperatures, and are welded. And toughness of weld heat affected zone (C
The present invention relates to a high strength and high toughness steel sheet having excellent TOD characteristics).

[0002]

2. Description of the Related Art Conventionally, a Cu precipitation strengthened high-strength steel sheet is well known as a steel sheet having high strength and excellent weldability. For example, US Pat. No. 3,692,514.
And the ASTM
It is specified in Standard A710.

However, with the recent increase in the size of structures and the harsh environment of use, the characteristics required for the steel materials used have become extremely severe. In particular, in order to secure the safety of the structure, it is required that the welded joint portion having good base metal toughness and the most brittle is highly tough.

As a quantitative method for evaluating fracture toughness, there is a toughness (CTOD characteristic) test of a weld heat affected zone, and in a structure in which safety is important, the fracture toughness evaluation is more realistic than the Charpy impact characteristic. That is, the toughness (CTOD characteristic) of the weld heat affected zone is required.

However, in the welded joint portion of a high strength steel sheet, a local embrittlement region is generated when multi-pass welding is performed, and this local embrittlement region becomes a starting point of fracture, resulting in a toughness (CTO) of the heat affected zone of welding.
It is difficult to stably secure good toughness (CTOD characteristics) of the heat-affected zone of the welding because it lowers the D characteristics).

It is generally said that it is important to reduce island martensite (MA) in order to improve the toughness (CTOD property) of the weld heat affected zone of the welded joint. In order to reduce the island-like martensite (MA), it is effective to reduce the carbon equivalent, C is 0.10 wt% or less, and Nb is a harmful element, usually 0.1.
It is said that it is necessary to suppress it to less than 02 wt%. However, at low carbon equivalent, low carbon, and low Nb content,
It is difficult to achieve all of high strength and high toughness of a steel sheet and high toughness (high CTOD characteristics) of a weld heat affected zone of a welded part.

[0007]

SUMMARY OF THE INVENTION As described above, the present invention has been made by the present inventor in order to solve the problem that various characteristics of conventional steel materials used for large structures cannot be achieved. As a result of intensive studies and studies, it has been found that it is effective to utilize Cu precipitation strengthening in order to increase the strength of the steel sheet at a low carbon equivalent.

However, C ≦ 0.10 wt%, Nb <0.0
Under the condition of 2 wt%, it is difficult to secure high strength, high toughness, and good weld heat affected zone toughness (CTOD characteristics) of the welded joint, even if Cu precipitation strengthening is utilized. Therefore, it is necessary to positively utilize Nb to further remove a factor harmful to the toughness (CTOD characteristic) of the heat-affected zone of welding.

The necessity of Nb <0.02 wt% is the case where the C content is relatively high such that C> 0.06 wt%, and when the C content is reduced to 0.06 wt% or less. , Even if the Nb content is increased to 0.02 wt% or more, the formation of island martensite (MA) is small in the multi-pass welding and excellent toughness (CTOD characteristics) of the heat affected zone can be obtained. There was found.

In particular, C ≦ 0.06 wt%, Nb ≧ 0.02
By setting the content to be wt%, the structure of the weld heat affected zone heated to a temperature of 1200 ° C. or higher does not become a coarse grained upper bainite structure having low toughness, and contains ferrite. Since the structure becomes a fine structure, the structure itself exhibits high toughness, and even in the portion heated to the two-phase region by the second thermal cycle, island martensite (M-
A) is small, and high toughness (high CTOD characteristics) of the weld heat affected zone can be obtained.

Further, by containing 0.02 wt% or more of Nb, the structure of the steel sheet is extremely fine ferrite (acicular ferrite, polygonal ferrite 1).
It can be a tissue mainly composed of two or more species. The structure mainly composed of fine precipitates of Nb carbonitride and TiN in the steel sheet and fine ferrite of the steel sheet prevents coarsening of the heated austenite grain size and improves the toughness (CTOD characteristic) of the weld heat affected zone. Has the effect of improving.

Further, only by adjusting the composition and refining the structure of the steel sheet, the toughness of the weld heat affected zone (CTO) can be improved.
It has been found that it is difficult to stably secure the (D characteristic), and the cause is that the center segregation portion or inclusions become the starting point of fracture occurrence. Since this central segregation portion is a portion where the components are concentrated, the structure after welding is hardened and the toughness deteriorates due to an increase in island martensite (MA) and the like. Therefore, the C content in the center segregation portion is 0.06 wt%
Hereinafter, it is necessary to reduce the center segregation so that the P content is 0.015 wt% or less.

Also, the inclusions which are the starting points of the fractures are mainly Al ₂ O ₃ (alumina) -based inclusions that are continuous for a long time, and there are cases where expanded A-based inclusions such as MnS. This inclusion has a thickness of 10 μm or more and a length of 5
It has been found that when the thickness is 0 μm or more, it may be a starting point of fracture occurrence.

Further, although the inclusion of Ca is extremely effective in controlling the morphology of A-based inclusions such as MnS, Ca further reduces the amount of alumina-based inclusions that are detrimental to fracture toughness, and makes the alumina-based inclusions finer. It is clear that it has the effect of
Due to the inclusion of Ca, the toughness (C
It was found that the TOD characteristic) was improved. The present invention has developed a high strength and high toughness steel sheet based on the various findings described above.

[0015]

[Means for Solving the Problems] C 0.01 to 0.06w
t%, Si 0.01 to 0.50 wt%, Mn 0.5 to
2.0 wt%, P ≤ 0.015 wt%, S ≤ 0.0.
005wt%, Al 0.01-0.05wt%, Cu
0.6-2.0 wt%, Ni 0.3-3.5 wt%, Nb
0.02 to 0.05 wt%, Ti 0.005 to 0.02
wt%, N 0.003 to 0.008 wt%, Ca 0.0.
0005 to 0.0030 wt%, the balance Fe and unavoidable impurities, and the average chemical analysis value of the central segregated portion (sheet thickness / 50) mm thickness x 10 mm x 10 mm of the steel sheet is C ≤ 0.06 wt%, P ≤ 0.015 wt%
And a thickness of 10 μm or more and a length of 50 observed on the cross section.
High strength and high toughness characterized by containing 1 or less inclusions of 10 μm or more in 10 mm ² and acicular ferrite having a grain size of 20 μm or less or polygonal ferrite in a total content of 40% or more. A steel sheet is the first invention, C 0.01 to 0.06 wt%, Si 0.01 to 0.0.
50 wt%, Mn 0.5-2.0 wt%, P ≤ 0.0.
015 wt%, S ≤ 0.005 wt%, Al 0.0
1-0.05wt%, Cu 0.6-2.0wt%, Ni
0.3-3.5 wt%, Nb 0.02-0.05 wt
%, Ti 0.005 to 0.02 wt%, N 0.003
~ 0.008 wt%, Ca 0.0005 ~ 0.0030
wt%, Cr 0.01-0.50 wt%
%, Mo 0.01 to 0.50 wt%, V 0.005
It contains one or more selected from 0.10 wt%, and contains REM 0.005-0.050 wt%, consists of the balance Fe and inevitable impurities, and has a central segregation part ( Plate thickness / 50) mm thickness x 10 mm x 10
The average chemical analysis value of mm is C ≤ 0.06 wt%, P
≤ 0.015 wt%, thickness observed in cross section 1
One or less inclusions having a size of 0 μm or more and a length of 50 μm or more in 10 mm ² , and a steel sheet structure containing acicular ferrite having a grain size of 20 μm or less or polygonal ferrite in a total content of 40% or more. And a high strength and high toughness steel sheet according to the second invention.

Regarding the high strength and high toughness steel plate according to the present invention,
This will be described in detail below. First, the contained components and component ratios of the high strength and high toughness steel sheet according to the present invention will be described.

Since C increases the amount of island martensite (MA) in the heat affected zone of the welded joint, it is harmful to the toughness (CTOD characteristic) of the heat affected zone and also impairs the weldability. It is desirable that the content is small,
If the content is less than 0.01 wt%, the strength cannot be secured, and if the content exceeds 0.06 wt%, the weld microstructure and the steel plate base metal microstructure include a large amount of a low toughness upper bainite microstructure. As a result, the amount of island martensite (MA) in the weld becomes too large. Therefore, the C content is set to 0.01 to 0.06 wt%.

Si is a useful element for deoxidation and strengthening of steel, and if the content is less than 0.01 wt%, such an effect is small, and if it exceeds 0.50 wt%, weldability is obtained. And toughness deteriorates. Therefore, the Si content is set to 0.01 to 0.50 wt%.

Mn is an element useful for increasing the strength, and if the content is less than 0.5 wt%, this effect is small, and if it exceeds 2.0 wt%, the weldability deteriorates. Therefore, the Mn content is set to 0.5 to 2.0 wt%.

P is an element that easily segregates and segregates at grain boundaries to deteriorate toughness. Therefore, the P content is 0.015w
t% or less.

S forms MnS and becomes a starting point of fracture in the welding heat affected zone test (CTOD test). Therefore, the S content is set to 0.005 wt% or less.

Al is a deoxidizing element and its content is 0.01
If it is less than wt%, this effect is small, and 0.05 wt.
If it is contained excessively in excess of 10%, coarse alumina-based inclusions are formed and the toughness deteriorates. Therefore, the Al content is set to 0.01 to 0.05 wt%.

Cu is an important element, and by utilizing the aging precipitation strengthening action after rolling and cooling, a low carbon equivalent,
It has the effect of making it possible to lower the carbon content, improve weldability, and improve the toughness (CTOD property) of the weld heat affected zone of the weld zone. If the content is less than 0.6 wt%, precipitation hardening occurs. The action cannot be utilized, and it is 2.0
If it is contained in excess of wt%, the toughness is lowered. Therefore, the Cu content is set to 0.6 to 2.0 wt%.

Ni is Cu in the Cu precipitation strengthened steel plate.
It is an element that has the effect of preventing cracking during hot rolling due to its content. If the content is less than 0.3 wt%, such an effect cannot be expected, and it has the effect of improving toughness. However, if the content exceeds 3.5 wt%, the effect is saturated, and the content higher than that is useless. Therefore, the Ni content is set to 0.3 to 3.5 wt%.

Nb, together with C, is an important element and generally significantly impairs the toughness of the weld heat affected zone of the weld zone, so it is said that it is necessary to limit the content to a minimum amount,
In particular, it is said that Nb <0.02 wt% is required in order to obtain good toughness (CTOD property) of the weld heat affected zone, but when C ≦ 0.06 wt%, Nb is
When ≧ 0.02 wt%, the structure of the steel sheet becomes a structure mainly composed of fine ferrite, and high strength and high toughness can be achieved, and the toughness of the weld heat affected zone of the welded joint (CTOD characteristics)
Also becomes good, and if a large amount of Nb is contained in excess of 0.05 wt%, the toughness of the welded joint deteriorates. Therefore,
The Nb content is 0.02 to 0.05 wt%.

Ti is an important element effective in improving the toughness (CTOD characteristic) of the heat-affected zone of welding, and prevents TiN precipitated in the steel sheet from coarsening austenite grains when heated by welding, and Although the transformation is promoted, if the content is less than 0.005 wt%, such an effect cannot be expected, and if it exceeds 0.020 wt%, the toughness deteriorates. Therefore, the Ti content is 0.005 to 0.
It is set to 020 wt%.

N is an element which produces Ti and TiN and has an effect of improving the toughness (CTOD characteristic) of the weld heat affected zone. If the content is less than 0.003, this effect is small,
Further, if the content exceeds 0.008 wt%, the toughness deteriorates. Therefore, the N content is 0.003 to 0.008 wt.
%.

Ca is a characteristic element that has the effect of improving the toughness (CTOD characteristics) of the weld heat affected zone of the weld by reducing, refining, and controlling the morphology of inclusions that are the starting points of fracture occurrence. Yes, in particular, it has the effect of flocculating coarse Al ₂ O ₃ (alumina) -based inclusions in the molten metal, and reduces the amount of alumina-based inclusions and finely disperses them to form MnS inclusions. If the content is controlled and the content is less than 0.0005 wt%, such effect is small,
Also, if the content exceeds 0.0030 wt%, Ca
Coarse inclusions of alumina and alumina are formed. Therefore, the Ca content is 0.0005 to 0.0030 wt%.

Cr is an element effective in increasing strength, and if the content is less than 0.01 wt%, such an effect is small,
Also, if a large amount is added in excess of 0.50 wt%, the weldability will be impaired. Therefore, the Cr content is 0.0
1 to 0.50 wt%.

Mo is an element effective for increasing strength, and if the content is less than 0.01 wt%, such an effect is small,
Also, if a large amount is added in excess of 0.50 wt%, the weldability will be impaired. Therefore, the Mo content is 0.0
1 to 0.50 wt%.

V is an element effective for increasing strength, and if the content is less than 0.005 wt%, such an effect is small,
Further, if it is contained in a large amount exceeding 0.10 wt%, the weldability and toughness will be impaired. Therefore, the V content is set to 0.005 to 0.10 wt%.

REM has an effect of controlling the morphology of inclusions of MnS and has a small effect of reducing the amount of alumina-based inclusions such as Ca, so that it can be added additionally, and the content is 0.005 wt%. If it is less than the above, this effect is small, and if it is contained in a large amount exceeding 0.050 wt%, harmful coarse inclusions are formed. Therefore, the REM content is 0.005 to 0.050 wt%.

The central segregated portion becomes a starting point of fracture occurrence due to hardening of the constituent element due to concentration of constituent elements and embrittlement. Particularly, C and P are easily segregated, and when C exceeds 0.06 wt%. Island martensite (MA) increases. Therefore, the C content in the center segregation portion is 0.06 wt%
It is necessary to: Further, since P deteriorates the toughness due to the segregation of grain boundaries, the P content in the center segregated portion is 0.
It is necessary to make it 015 wt% or less.

The components of the central segregation portion greatly differ depending on the area to be analyzed, and the analysis in a minute area by EPMA and CMA gives a high-concentration analysis result. Therefore,
In the center segregation part of this steel plate, first, the most segregated part after macro etching, (plate thickness / 50) mm thickness × 10 m
The average chemical analysis values in m × 10 mm are C content ≦ 0.06 wt% and P content ≦ 0.0, regardless of the components of the steel sheet.
The steel plate has a center segregation level of 015 wt%.

The inclusions are the starting points of the fracture occurrence, and the inclusions of the fracture initiation point are B type inclusions in which massive inclusions containing Al ₂ O ₃ (alumina) as a main component are connected. There are many things, and fracture may occur from the A type inclusions in which MnS is elongated.

In particular, alumina-based inclusions become the starting point of fracture due to the peeling of the interface due to heat and stress caused by welding. Breakage often occurs from inclusions having a thickness of 100 μm or more and a length of 50 μm or more observed in a cross section. Therefore, the number of inclusions having a thickness of 10 μm or more and a length of 50 μm or more observed in the cross section is set to be 1 or less within 10 mm ² .

The structure of the steel sheet also affects the toughness after welding. That is, the part where the toughness of the weld heat affected zone is most deteriorated is
A portion of the austenite coarse grain region (C) heated to a temperature higher than 1200 ° C. in the vicinity of the welding line (Fusion Line)
It is said that this is the eastern part of the city.

However, the fracture in the welding heat-affected zone test (CTOD test) of the actual weld does not always occur from the coarse grain region as a starting point, and eventually 120
It is necessary to maintain good toughness even in the region heated to a temperature lower than 0 ° C. This is affected by the structure of the steel sheet and the dispersed state of Nb carbonitride.

In order to finely disperse the Nb carbonitride, accelerated cooling after rolling or DQ is performed at the time of manufacturing the steel sheet.
It is necessary to perform relatively fast cooling as described above, and the structure at that time is a structure containing one or two of fine acicular ferrite and polygonal ferrite.

Further, in order to improve the toughness of the portion heated to the two-phase region, the structure of the steel sheet is not a bainite structure or a martensite structure but a structure mainly composed of ferrite and island martensite (MA) is used. It is better to disperse finely. For that purpose, it is necessary to have a structure containing one or two kinds of acicular ferrite or polygonal ferrite having a particle diameter of 20 μm or less in total of 40% or more. And this structure is also a structure which makes a steel plate have high strength and high toughness.

[0041]

[Examples] Examples of high strength and high toughness steel sheets according to the present invention will be described together with comparative examples.

[0042]

[Examples] Steels having the components and component ratios shown in Table 1 were melted by a conventional method, continuous casting was performed, and the produced slabs were heated, rolled and cooled, and then subjected to Cu precipitation strengthening treatment, A test steel plate was produced.

From the produced steel sheet, the structural fraction, the component analysis of the center segregation portion, the observation of inclusions in the steel sheet cross section and the tensile test and the Charpy impact test were conducted, and further, after welding in the K groove, the welding line on the vertical bead side. A notch was formed in the HAZ portion in the vicinity, and a Charpy impact test and a test of the weld heat affected zone at a thickness center portion of 30 mm were performed.

The results are shown in Tables 2 and 3. It can be seen that the high-strength, high-toughness steel sheet according to the present invention has high strength, high toughness, and excellent Charpy impact characteristics and weld heat-affected zone toughness (CTOD characteristics) in the weld joint.

That is, N of the high strength and high toughness steel sheet according to the present invention
No. 1, No. 5, No. 6, No. 8, and No. 10 all have a yield strength of 460 MPa or more, an absorbed energy (vE) at −80 ° C. of 100 J or more, and a transition temperature (vTr).
s) is -80 ° C or lower, and the toughness of the welded joint is -60.
It shows excellent characteristics with absorbed energy of 100 J or more at ℃ and CTOD value of 0.25 mm or more at -30 ℃.

On the other hand, Comparative Example No. 11 has a high C content and a low Nb content, so that the toughness of the steel plate and the toughness of the welded joint are low. In Comparative Examples No. 12 and No. 14, Ca is Since it is not contained, the number of inclusions is large, and the toughness and CTOD value of the welded joint are low.

In Comparative Example No. 13, since the Cu content is low, the strength of the steel sheet is low. Since Comparative Example No. 15 does not contain Ti, the toughness of the steel sheet and the toughness of the welded joint are low.
Comparative Example No. 16 has a large P content and is segregated at the center, so the CTOD value of the welded joint is low.

In Comparative Examples No. 2, No. 3 and No. 7, the steel sheet does not have a fine ferrite-based structure, so that the steel sheet has low toughness. In Comparative Examples No. 4 and No. 9, C and P are concentrated in the central segregation portion, so that the CTOD value of the welded joint portion is low.

[0049]

[Table 1]

[Table 2]

[0051]

[Table 3]

[0052]

As described above, since the high-strength and high-toughness steel sheet according to the present invention has the above constitution, the yield strength 460 is
It has high strength of MPa or more and high toughness of transition temperature (vTrs) -80 ° C. or less, and also has excellent Charpy impact characteristics in the welded joint and excellent toughness of the weld heat affected zone (C
It becomes possible to obtain TOD characteristics), and the industrial effect is extremely large.

Claims (2)

(57) [Claims] 1. C 0.01 to 0.06 wt%, Si 0.0.
01 to 0.50 wt%, Mn 0.5 to 2.0 wt%, P ≤ 0.015 wt%
%, S ≤ 0.005 wt%, Al 0.01 to 0.05 w
t%, Cu 0.6 to 2.0 wt%, Ni 0.3 to 3.5 wt
%, Nb 0.02 to 0.05 wt%, Ti 0.005 to 0.
02 wt%, N 0.003 to 0.008 wt%, Ca 0.0005
To 0.0003 wt%, the balance Fe and unavoidable impurities, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness x 10 mm
The average chemical analysis values of × 10 mm are C ≦ 0.06 wt% and P ≦ 0.015 wt%, and the thickness observed on the cross section is 10 μm or more and the length is 50 μm.
High strength and high toughness characterized by containing 1 or less inclusions of 10 μm or more in 10 mm ² and acicular ferrite having a grain size of 20 μm or less or polygonal ferrite in a total content of 40% or more. steel sheet. 2. C 0.01 to 0.06 wt%, Si 0.04.
01 to 0.50 wt%, Mn 0.5 to 2.0 wt%, P ≤ 0.015 wt%
%, S ≤ 0.005 wt%, Al 0.01 to 0.05 w
t%, Cu 0.6 to 2.0 wt%, Ni 0.3 to 3.5 wt
%, Nb 0.02 to 0.05 wt%, Ti 0.005 to 0.
02 wt%, N 0.003 to 0.008 wt%, Ca 0.0005
.About.0.0030 wt%, Cr 0.01-0.50 wt%, Mo 0.01-0.5
0 wt%, V 0.005 to 0.10 wt% selected from 1 or 2 or more, and REM 0.005 to 0.050 wt%, the balance Fe and inevitable impurities, And the center segregated part of the steel plate (plate thickness / 50) mm thickness x 10 mm
The average chemical analysis values of × 10 mm are C ≦ 0.06 wt% and P ≦ 0.015 wt%, and the thickness observed on the cross section is 10 μm or more and the length is 50 μm.
High strength and high toughness characterized by containing 1 or less inclusions of 10 μm or more in 10 mm ² and acicular ferrite having a grain size of 20 μm or less or polygonal ferrite in a total content of 40% or more. steel sheet.