JP3503148B2 - Steel with excellent toughness in the heat affected zone - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is required to have particularly excellent weld heat-affected zone toughness, such as welding steel used in bridges, shipbuilding, and construction, and steel for line pipes that require welding during pipe manufacturing. Related to welded steel.

[0002]

2. Description of the Related Art Generally, it is known that when a steel material is welded, the grain size of the weld heat affected zone (hereinafter referred to as HAZ) becomes coarse and the toughness deteriorates, and the performance of the welded structure deteriorates. Many attempts have been made to improve the HAZ toughness.

Among them, in the steel disclosed in Japanese Examined Patent Publication No. 55-47100, the effect of promoting the γ-α transformation by lowering the Al content and T
Combined with the combined addition of i and B and the control of N amount, HAZ
It proposes to improve toughness. In this case, B precipitates in the γ grains in the form of BN during cooling in the HAZ part, and γ-α
Since it functions as a nucleation site in the γ grains of the transformation, even when the super-high heat input welding in which the γ grains become significantly coarse, its HA
The Z part toughness can be maintained.

However, in order to secure the toughness, in addition to TiN,
Since BN is used, unless the added amounts of elements such as B, N, Ti, and Al are accurately controlled during manufacturing, the amount of B that is not fixed as a precipitate increases, and the HAZ toughness is rather deteriorated. Also, a relatively large number of N
Since it contains, the high temperature ductility of the steel is deteriorated and continuous casting becomes difficult, which causes difficulties in actual production.

Further, a series of patents including JP-A-57-51243, JP-A-59-185760 and the like show that, in addition to selecting a low Al-based component and utilizing TiN, BN
In place of the above, Ti oxide or a composite of Ti oxide and MnS is dispersed, and these dispersoids function as precipitation nuclei of ferrite, thereby improving HAZ toughness, which can be called low Al-Ti oxide dispersed steel. Proposing means.

This means is not necessarily limited to Japanese Patent Publication No. 55-471.
Although it does not give HAZ performance superior to that of the steel of JP-A-00-00, since B is not essential, it can be used as a material for line pipes, and this point is more preferable.

However, JP-A-60-245768, JP-A-61-79745 and JP-A-62-18.
As disclosed in Japanese Patent Publication No. 42 etc., in the production of such a steel material, Ti, which has a weaker deoxidizing power than Al, is used to perform final deoxidation and to disperse an oxide necessary for improving the HAZ toughness. I have to let you. In such a case, it is not easy to sufficiently reduce the oxygen activity in the product steel, and deterioration of steel quality is likely to occur.

That is, in order to improve the HAZ toughness, Ti oxide must be contained in the steel,
Since Ti oxide can serve as an oxygen source in steel, it can also cause deterioration of steel quality. In order to prevent this, Ti must be added excessively to increase the amount of Ti not forming an oxide. On the other hand, since such excess Ti deteriorates both the toughness of the base metal and the HAZ, it should not be present in a large amount especially in the steel material for welding. Thus, in order to obtain the desired performance, the Ti content must be controlled to a value satisfying these mutually contradictory requirements.

In consideration of this point, Japanese Patent Application Laid-Open No. 2-175815 gives an appropriate relational expression between the contents of O, N, etc. forming Ti and a compound, and the optimum range. Is prescribed.

However, this relational expression does not take into consideration the influence of aluminum oxide that fixes an oxygen amount that cannot be ignored in practice. In addition, the range of Ti as a whole is specified to be small, and even if the Ti in the steel is all Ti oxide, it is highly likely that deterioration of steel quality due to insufficient deoxidation is caused, which is extremely dangerous.

Under these circumstances, it is difficult to stably exhibit the desired performance in the low Al-Ti oxide dispersion steel, and it has been necessary to improve such a situation.

[0012]

In view of the above situation, the object of the present invention is to control the composition of dispersoid particles and to determine the optimum production conditions and component ranges.
It is to obtain a means for stably producing a steel showing excellent base material toughness and HAZ toughness in a low Al-oxide dispersed steel.

[0013]

Means for Solving the Problems The inventors of the present invention have made extensive studies in order to solve the problems and have found the following facts.

Conventionally, with respect to Ti in which Ti oxide is not formed, in low Al--Ti oxide dispersed steel,
Recognition as a cause of deterioration of HAZ toughness is not so strong, and T
There is even a view that the HAZ toughness is improved by forming iN.

However, the present inventors have found that this is a mistake, and that excessive Ti, even if it forms TiN, is γ-α in the low Al--Ti oxide dispersed steel.
It has been found that it inhibits transformation and inhibits the refinement of the HAZ structure. That is, in the low Al—Ti oxide dispersed steel, the HAZ toughness is further improved by suppressing the Ti amount that does not become Ti oxide to such an extent that deoxidation deficiency does not occur.

Aluminum oxide, which has hitherto been considered only to be reduced by the recognition that it is an ineffective inclusion, is indispensable for achieving both the base material toughness and the HAZ toughness, and there is an optimum amount. That is, if the total amount of oxygen in the steel is the same, it is possible to obtain excellent base material toughness by fixing a part of it in the form of alumina, rather than fixing it in the form of Ti oxide.
Moreover, the effect of Ti oxide dispersion is not lost unless the amount of alumina, that is, the amount of insol.Al exceeds the limit.

The present invention has been made on the basis of these facts, and has the following problems (1).
% By weight, C: 0.01 to 0.25%, Si: 0.6%
Hereinafter, Mn: 0.3 to 2.2%, P: 0.02% or less,
S: 0.001 to 0.010%, N: 0.0010 to 0.0
0075%, O: 0.0010 to 0.011%, sol.
Al: 0.005% or less, insol. Al: 0.001 to 0.00
007%, sol.Ti: 0.003 to 0.012%, ins
ol.Ti: 0.001 to 0.017%, with a composition of balance Fe and inevitable impurities, and N, sol.T.
i is a restricted expression,

[Formula 1] −0.0025 ≦ N− (sol.Ti / 3.4) ≦ 0.0030 (A) Within the range satisfying the condition (A), the inclusions having a diameter exceeding 30 μm are A steel material having excellent toughness in the weld heat-affected zone, characterized in that the average number of polished steel pieces per mm ² is less than 0.3. Is one of the solutions,

(2) The steel according to claim 1 further comprises Cr:
1.0%, Mo: 0.7% or less, Nb: 0.08%, C
u: 2.0% or less, Ni: 2.0% or less, V: 0.10
% Or less, B: 0.0020% or less, and at least one kind is contained, and the steel material excellent in weld heat affected zone toughness according to the above (1). Is the second solution.

[0019]

[Operation] As the above-mentioned value of excess Ti, it is considered that oxygen is fixed in the form of Ti ₂ O ₃ , and it is possible to estimate it by subtracting the oxygen content from the total Ti content. In addition, Ti does not always form an oxide stoichiometrically, a correct calculation cannot be performed, and since the presence of Al oxide is essential in the present invention, it is necessary to estimate excess Ti. Will cause a large deviation.

Therefore, in the present invention, a portion (hereinafter, sol.Ti: corresponding to excess Ti) that is soluble and a portion (hereinafter, insol.Ti:
(Corresponding to the oxide portion) and these are considered to be the portion contained in the steel in the form other than oxide and the portion contained in the steel in the form of oxide.

Regarding the conventionally known balance of the Ti-N amount, it is necessary to manage the balance of sol.Ti and the N amount as shown in the formula (A), not just the balance of Ti-N.

This [N- (sol.Ti / 3.4)] is-
It should be in the range 0.0025 to 0.0030. If it is less than -0.0025, the miniaturization of the HAZ structure is inhibited, or even if the HAZ structure is miniaturized, T
HAZ toughness is not good because iC is formed.
If it exceeds 0.0030, the hot ductility of the steel deteriorates due to excessive N, which causes manufacturing difficulties.

As a method of adding insol.Ti and insol.Al, it is preferable to use Ti oxide and Al oxide produced by deoxidizing steel with Ti and Al. This is because even if titania and alumina are added from the outside, even if they are well dispersed in the steel, H
This is because it does not always contribute effectively to the improvement of AZ toughness.

Deoxidation by adding Ti and Al
When producing steel containing insol.Ti and insol.Al, if the cooling rate during solidification is about 0.3 ° C / min or more,
The HAZ toughness of the product is secured. However, this cooling rate is a value that is easily achieved as long as casting is performed under normal operating conditions using a continuous casting apparatus that is currently common in the production of steel products. Do not add.

Even when a relatively coarse inclusion having a diameter of about 5 to 10 μm is included, the large inclusion tends to precipitate a larger amount of ferrite and contribute to the improvement of the HAZ toughness. If it is spherical, it is not necessarily HA
It does not directly lead to deterioration of Z toughness. In some cases, it is better to disperse such medium size inclusions.
It may exhibit Z toughness.

However, when excessively coarse inclusions having a diameter of more than 30 μm are dispersed in the steel in an amount of 0.3 / mm ² or more, the inclusions serve as a starting point of fracture and significantly deteriorate the toughness. Therefore, in the present invention, it is necessary to limit the upper limit of the amount of excessively large inclusions to less than 0.3 / mm ² .

The reasons for limiting each component are as follows. inso
l.Al and insol.Ti form a complex of inclusions and are dispersed in the steel to form γ-α transformation nuclei and contribute to the improvement of HAZ toughness. HAZ when only insol.Al is present in the steel
No toughness improving effect is observed, and insol.Ti is the main component, and insol.
When Al hardly exists, it becomes difficult to secure the base material toughness for the reason described above.

Therefore, the lower limit of the content is 0.001%. If either one of them falls below this lower limit, it becomes difficult to stably produce a steel material having both excellent base material toughness and HAZ toughness.

These two substances improve the HAZ structure as the amount of dispersion increases, but insol.Al> 0.0
When 07% and insol.Ti> 0.017%, even if the manufacturing conditions are adjusted, it is not possible to sufficiently suppress the formation of coarse inclusions having a diameter of more than 30 μm, and the HAZ toughness and base material toughness are significantly deteriorated. To do. Therefore, the upper limit amount shown here is set.

Unlike insol.Al, sol.Al is not an essential component for the present invention, but when it is present in a large amount, it reduces insol.Ti to 0.001% or less, and sol.Al. Since Al itself deteriorates the HAZ toughness,
Its content should be carefully controlled and should not be present in the steel in excess of 0.005%.

Sol.Ti is T if it does not form TiN.
It becomes iC and deteriorates the HAZ toughness. Therefore, the excessive content of N must be limited, and the present invention defines the balance between N-sol.Ti (formula A). However, even if the formula (A) is satisfied, if the content of sol.Ti exceeds 0.012%, the fineness of the HAZ structure is hindered and the HAZ toughness is not improved. Therefore, this value was set as the upper limit.

On the other hand, when sol.Ti is less than 0.003%, lower oxides tend to remain in the steel, resulting in deterioration of both the base metal toughness and HAZ toughness. Therefore, sol.Ti ≧
It must be 0.003%.

C is an element necessary for securing strength, and is 0.0
If 1% is not contained, it is not possible to produce steel having practical strength. However, since C is an element that causes deterioration of the HAZ toughness, its upper limit is 0.2.
Set 5%.

Si is an element effective for preliminary deoxidation of molten steel, but excessive addition causes the formation of island-like martensite (high carbon martensite-austenite mixed phase) in the HAZ part, so the upper limit is 0. .6%

Mn is an element necessary for ensuring strength, and
3% or more must be added. However, excessive addition causes a significant deterioration in HAZ toughness, so 2.2%
Should not be added over.

Although P is an unavoidable impurity, it is an element that causes intergranular cracking in the HAZ part, so in the present invention, the upper limit was 0.02%.

Since S forms inclusions such as MnS which can be a starting point of cracking, S should not be contained in an amount exceeding 0.010%. However, when the amount is reduced to less than 0.001%, the production cost increases and the HAZ toughness tends to deteriorate, so this amount is made the lower limit.

When a large amount of N that does not form TiN is present, N causes deterioration of the toughness of the steel material, but if the excess / deficiency is within the range of the formula (A), the HAZ toughness does not deteriorate.
However, N is less than 0.0010%, or 0.007
If it exceeds 5%, the HAZ toughness deteriorates, so the N content must be within the range of 0.0010 to 0.0075%.

Most of the O in the steel exists in the form of Ti oxide or Al oxide, and if the total oxygen content in the steel is not more than 0.0010%, the oxide necessary for ensuring the HAZ toughness is obtained. We cannot secure the quantity. Even if the restrictions of sol.Ti, insol.Al, etc. are satisfied, it is difficult to suppress the formation of coarse inclusions when the total oxygen content exceeds 0.011%, and the toughness of both the base metal and HAZ is high. Deteriorates. Therefore, the total oxygen content in the steel is 0.0010-0.
Within the range of 011%.

Cr and Mo both increase the hardenability of the steel material and are effective for securing the strength. However, when Cr is contained in an amount of more than 1.0% and Mo is added in an amount of more than 0.7%, welding cold cracking and HA
This value is set as the upper limit because it causes deterioration of Z toughness.

Nb and V are components that precipitate as carbonitrides during hot rolling, enhance the effect of controlled rolling, and greatly contribute to the improvement of base material strength and toughness.
Since the tendency to impair the toughness of the HAZ part is strong, it is 0.0 in Nb.
The upper limit is 8% and V is 0.10%.

Cu and Ni are components which both enhance the strength and toughness of the steel material and have little adverse effect on the toughness of the HAZ part. However, if the addition amount of Cu exceeds 2.0%, it is in continuous casting and rolling. Many slab surface cracks occur, making stable and efficient production difficult. Therefore, the upper limit is 2.0%.

When the Ni content exceeds 2.0%, the manufacturing cost is significantly increased.
The upper limit is 2.0%.

B is a component effective in increasing the hardenability of the base material and ensuring the strength. It precipitates as BN in the γ grains and also functions as a nucleation site for the γ-α transformation to improve the HAZ toughness. Although it also has a function, when it is contained in excess, it causes deterioration of HAZ toughness due to an increase in hardenability, so the upper limit is made 0.0020%.

[0045]

EXAMPLES Table 1 shows the basic alloy components of the steels of the present invention and comparative examples. In Table 2, O for each steel type listed in Table 1,
N, Ti, Al content, amount of coarse inclusions dispersed,
The element of the final deoxidation is shown. Table 3 shows rolling conditions, base material performance, and reproduced HAZ toughness test results of these steel types, respectively.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

The reproduced HAZ test was carried out according to the conditions shown in Table 3, W11 mm cut from a 40 mm-thick plate rolled.
* B11mm x L60mm test piece, maximum heating temperature 140
After being kept in the temperature range of 1400 to 800 ° C. at 0 ° C. for 40 seconds, (800 to 500) ° C./60 seconds and (800 to 5)
After applying a heat cycle of cooling to room temperature at a cooling rate of 00) ° C / 120 seconds, a full-size Charpy test piece was processed and used for the test.

As shown in Tables 1 to 3, the steels of the present invention (1 to 1)
In 4), even in steel types having relatively high alloy component contents, the contents of O, N, Ti, and Al and the formula (A) = f
It is clear that it is possible to secure high HAZ toughness by adjusting N. On the other hand, Comparative Example (15
~ 23) steel, sol.Ti, insol.Ti, sol.Al,
Since any of the value of insol.Al, the value of fN and the number of coarse inclusions is outside the range of the present invention, the steel of the present invention (1
It is clear that the HAZ toughness of the high heat input welded portion is significantly deteriorated as compared with 14), leaving anxiety in actual use. Especially when the amount of sol.Ti is too small (steel types 19, 2
3), when sol.Ti is too much (steel grade 18, 22), in
When the amount of sol.Al is too small (steel types 20 and 21), the tendency of deterioration of toughness is remarkable.

[0051]

According to the present invention, it is possible to secure high base metal toughness and HAZ toughness in a welding steel material, and as a result, the welding workability of the welding steel material and the safety of the welded structure are greatly improved. It became possible to do.

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Claims (5)

(57) [Claims] 1. C: 0.01 to 0.25% by weight,
Si: 0.6% or less, Mn: 0.3 to 2.2%, P:
0.02% or less, S: 0.001 to 0.010%, N:
0.0010 to 0.0075%, O: 0.0010 to
0.011%, sol.Al: 0.005% or less, insol.A
l: 0.001 to 0.007%, sol.Ti: 0.003
~ 0.012%, insol.Ti: 0.001 to 0.017
%, The balance Fe and unavoidable impurities, and
N and sol.Ti are within a range satisfying the restriction formula [Formula 1] −0.0025 ≦ N− (sol.Ti / 3.4) ≦ 0.0030 (A), and further, a diameter of 30 μm Inclusions exceeding the average size of 0.1 per 1 mm ^{2 of} polished steel surface.
A steel material having excellent weld heat affected zone toughness, characterized in that it is less than 3. 2. Further, Cr: 1.0% or less and Mo:
The steel material excellent in weld heat affected toughness according to claim 1, wherein the steel material contains at least one of 0.7% or less. 3. Further, Nb: 0.08% or less and V:
The steel material excellent in welding heat affected toughness according to claim 1 or 2, characterized in that it contains one or more of 0.1% or less. 4. Further, Cu: 2.0% or less and Ni:
The steel material excellent in welding heat affected toughness according to any one of claims 1 to 3, wherein the steel material contains at least one of 2.0% or less. 5. The steel material excellent in weld heat affected toughness according to claim 1, further comprising B: 0.0020% or less.