JP2002060891A - Steel having excellent toughness in heat affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel having superior toughness at low temperature in a heat affected zone. SOLUTION: The steel has a composition containing, by mass, <0.03% C, 0.6-1.2% Mn and 1.0-2.3% Ni and satisfying Ni<=-2Mn+4.0 and further containing proper amounts of Si, Al, Ti and B and also has a structure in which the formation of island martensite is suppressed. Further, either or both of Nb and V and also Ca can be incorporated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material used in various fields such as construction, civil engineering, marine structures, pipes, shipbuilding, and storage tanks, and more particularly to a steel material having excellent weld heat affected zone toughness. In addition, the steel material referred to in the present invention is a steel plate, a section steel, a steel bar,
It shall include steel pipes.

[0002]

2. Description of the Related Art Generally, steel materials used in various fields such as construction, civil engineering, and marine structures are finished to have a desired shape by welding. From the viewpoint of the safety of the structure, it is required that the steel to be used be excellent not only in base metal toughness but also in weld toughness. The most problematic is that
The toughness of the weld, especially the bond. The bond is exposed to a high temperature just below the melting point, and the austenite crystal grains are most coarsened. And, by the subsequent cooling, it is easy to transform into martensite or upper bainite. The martensite structure and the upper bainite structure are fragile structures, and the formation of the martensite structure and the upper bainite structure deteriorates the bond part toughness.

[0003] As a method for preventing such toughness deterioration of the welded portion, it can be roughly divided into (1) using inclusions and precipitates to suppress the coarsening of austenite grains, and (2) improving the structure after transformation. Two methods have been considered: a toughened structure. As an example of the method of suppressing coarsening of austenite grains by using inclusions and precipitates ((1) described above), for example, Japanese Patent Application Laid-Open No.
0.1%, Mn: 0.4 to 2.0%, Si: 0.1% or less, Ti:
0.002 to 0.02%, rare earth element (REM): 0.003 to 0.05
%, Al: 0.04 ~ 0.10%, heat input 10
A low-temperature high-strength steel sheet for large heat input welding having sufficient low-temperature toughness of a weld even in large heat input welding of 0 kJ / cm or more is disclosed.

As an example of the method (2) described above, in which the transformed structure is made to have a toughened structure, for example, Japanese Patent Publication No. 59-11
No. 658 discloses that C is 0.03% or less, Si is 0.05 to 0.40%,
Mn: 0.70 to 2.50%, 2.0 to 12.0% of Ni, which is particularly effective for high heat input weld joint toughness, and 0.005 to 0.090 of sol Al
%, (P + S) 0.015% or less, (N + O) 0.009%
A high-efficiency welding low-temperature steel that can produce a steel having excellent low-temperature toughness and capable of high-efficiency welding is disclosed below. The technique described in JP-B-59-11658 strictly reduces P, S, N and O, which are harmful impurities, to a certain amount or less, and contains an effective amount of Ni for low-temperature toughness in a certain range. In order to reduce the amount of island martensite in the weld heat affected zone (HAZ) at the time of large heat input welding, C and N are reduced.

Further, Japanese Patent Publication No. 61-39392 discloses a low C
(0.005 to 0.03%), Nb added (0.005 to 0.05%) and low P (<0.005%), containing 0.5 to 4.0% Ni, 0.002 to 0.02% Ti, 0.0005 to 0.005% Ca A low temperature steel is disclosed. This low-temperature steel is said to be excellent in low-temperature toughness in a welded part, particularly in a bond part, has very little variation in COD value, and is inexpensive and has a stable and excellent low-temperature toughness.

Japanese Patent Application Laid-Open No. 61-143517 discloses a low C
(0.005 to 0.05%) and a small amount of Ti
Further, the steel to which Nb or V is added is hot-rolled so that the finish rolling end temperature is in the range of 900 to 600 ° C., quenched immediately after the finish rolling is completed, and then tempered, so-called direct quenching-tempering treatment. A method for manufacturing a high-strength steel sheet for low temperature to be applied is disclosed. The low-temperature high-strength steel sheet, the yield strength of 46kgf / mm ² or more, a tensile strength of 53kgf / mm have ² or more high strength, particularly low temperature toughness, including welds as thickness 50mm or more thick material It is said to be excellent.

According to the technique described in Japanese Patent Application Laid-Open No. 61-143517, the amount of Ni for making the weld toughness the best is 1.0 to 1.0.
It is 4.0%. In Japanese Patent Publication No. 6-49898, the C content is reduced (0.005 to 0.05%), the Ceq is reduced (0.36% or less), and Ti and Nb are contained (Ti: 0.005 to 0.020%).
%, Nb: more than 0.020% and 0.10% or less) is rolled by controlled rolling, and the rolling is completed at a temperature of Ar ₃ −40 ° C. to Ar ₃ + 40 ° C.
A method for producing a high yield point steel for low temperature with excellent toughness of the weld heat affected zone, which suppresses the formation of island-like martensite, by accelerated cooling to a temperature of 400 to 600 ° C. at a cooling rate of not less than 400 ° C./s. ing.

[0008]

[Problems to be solved by the invention]
In the technique described in JP-A-59-11658, it is necessary to add a large amount of expensive Ni and reduce impurity elements in order to improve the toughness of the welded portion, which increases the manufacturing cost and is economically disadvantageous. There was a problem of becoming. In addition, JP 60
According to the technique described in Japanese Patent No. -184633, it is possible to obtain a sufficient low-temperature toughness of a welded portion even in large heat input welding by effectively and finely dispersing a nitride of Ti or a sulfate of a rare earth element (REM). However, in addition to the problem that part of TiN is redissolved at high temperature and solute N increases, the toughness of the heat-affected zone decreases, and in addition, REM-based inclusions such as REM sulfates However, there is a problem that coarse inclusions are liable to cause agglomeration and coarse inclusions become a starting point of fracture and deteriorate toughness.

In the technique described in Japanese Patent Application Laid-Open No. 61-143517, B is not added, a part of TiN is redissolved at high temperature, and free N increases, so that the toughness near the weld metal is improved. There was a problem that can not be achieved. Also, Japanese Patent Publication No. 61-39
In the technique described in Japanese Patent Publication No. 392, B is not added and Ti is added.
Since part of N redissolves at high temperature and free N increases,
In addition to the problem that the toughness cannot be improved in the vicinity of the weld metal, it is necessary to perform refining by a very expensive special method to reduce P to less than 0.005%, which is disadvantageous economically. Was.

In the technique described in Japanese Patent Publication No. 6-49898, an attempt is made to suppress the formation of island-like martensite in the base material. There is a problem that new generation of martensite is unavoidable and the toughness of the heat affected zone is deteriorated. The present invention solves the above-mentioned problems of the prior art, and is inexpensive, 100 kJ /
It is an object of the present invention to propose a steel material having excellent heat affected zone toughness of large heat input welding of not less than cm.

[0011]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have proposed a method for improving the toughness of a welded portion other than a method of refining austenite grains using inclusions and precipitates. I studied diligently. As a result, they have found that by controlling the generation of hard island-like martensite in which C is enriched, the weld toughness of the ultra-low carbon steel material can be significantly improved. According to the study of the present inventors, the amount of island martensite in an ultra-low carbon steel material having a C content of less than 0.03% by mass is significantly affected by the Mn and Ni contents contained in the steel material,
It was found that excellent weld toughness was exhibited only when the Mn and Ni contents were adjusted within a certain range.

First, the experimental results on which the present invention is based will be described. Influence on maximum C solid solution in ferrite obtained using Thermo-Calc which is thermodynamic calculation software
FIG. 1 shows the effect of the amounts of Mn and Ni. The horizontal axis is Fe-C-1.5%
These are the Mn content in the Ni system and the Ni content in the Fe-C-1.4% Mn system. From FIG. 1, it can be seen that the maximum C solid solution amount in the ferrite greatly changes depending on the Mn and Ni contents, and the maximum C solid solution amount in the ferrite decreases as the Mn and Ni contents increase. From this, the present inventors have found that the amount of carbon discharged from the transformed ferrite to untransformed austenite during the cooling of the welding heat cycle in the welding heat-affected zone is Mn, Ni
It is thought that the amount of island martensite increases with the content and that the amount of island martensite increases, and it was found that the adjustment of the Mn and Ni contents can control the formation of island martensite.

Next, based on these findings, the present inventors confirmed the effect of the Mn and Ni contents on the formation of island martensite. In mass%, C: 0.02%, Si: 0.
20%, Mn: 1.4%, Al: 0.03%, Ti: 0.01%, B: 0.00
A steel material containing 1% as a basic component and having a Ni content varied in the range of 0 to 3%, C: 0.02%, Si: 0.20%, Mn: 0.
9%, Al: 0.03%, Ti: 0.01%, B: 0.001%, and using Ni: 1.5% as a base material. After heating these steel materials to 1150 ° C, unrecrystallized area Then, hot rolling was performed at a cumulative rolling reduction of 50% or more and a rolling end temperature of 850 ° C. to obtain a 16 mm thick steel plate.

From these thick steel plates, reproducible heat cycle test pieces were sampled, and subjected to a reproducible heat cycle of welding at a maximum heating temperature of 1400 ° C. and a cooling time of 800 to 500 ° C .: 110 s. The area ratio of the generated island-like martensite was determined for the test piece after the welding reproduction heat cycle was applied. These results are shown in FIG. 2 as the relationship between the area ratio of island martensite and the Ni content.

From FIG. 2, the area ratio of the island martensite is
It can be inferred that it increases with the Ni content and corresponds to the change in the amount of C solid solution in ferrite. Then, from the test specimen to which the above-mentioned welding reproduction thermal cycle was given, JIS
No. 4 impact test specimen was collected and subjected to ductile brittle fracture transition temperature (vT
rs). The result is shown in FIG.

From FIG. 3, it can be inferred that it is possible to improve the weld toughness by setting the Ni content and further the Mn content to a predetermined value or less. From these facts, the present inventors restricted the total amount of Mn + Ni content because excessive addition of Mn and Ni both reduced the solid solubility limit of C in ferrite and increased island martensite. I learned that it is necessary to do it.

The present invention has been completed based on the above-mentioned findings and further studies. That is, according to the present invention, in mass%, C: less than 0.03%, Si: 0.50% or less, M
n: 0.6 to 1.2%, Ni: 1.0 to 2.3%, Al: 0.005 to 0.10
%, Ti: 0.005 to 0.02%, B: 0.0005 to 0.0030%, and Mn and Ni are represented by the following formula (1): Ni ≦ −2Mn + 4.0 (1) (where Ni, Mn: each element) Content (mass%)), and has excellent toughness in the weld heat-affected zone, characterized by having a composition comprising the balance of Fe and unavoidable impurities. In addition to the composition, by mass%, Nb: 0.005 to 0.04%, V: 0.005 to 0.04%
%, And preferably contains one or two kinds selected from the above-mentioned compositions. In the present invention, it is preferable that the composition further contains 0.0005 to 0.005% of Ca in addition to the above-mentioned components.

[0018]

First, the reasons for limiting the composition of the steel material of the present invention will be described. In addition, mass% is simply described as%. C: less than 0.03% C is an important element that controls the structure of the base metal and the welded portion of the steel material. In the present invention, the formation of the pearlite phase in the equilibrium state is eliminated, and the toughness is deteriorated even in the weld heat affected zone. In order to suppress the formation of island martensite,
It was less than 0.03%. From the viewpoint of toughness, the content is preferably 0.01% or more.

Si: 0.50% or less Si is required as a deoxidizing agent at the time of refining, but if it exceeds 0.50%, the base material toughness is significantly deteriorated. For this reason,
Si was limited to 0.50% or less. Mn: 0.6 to 1.2% Mn is an element that increases the strength of a steel material, and at least 0.6% must be contained in order to secure the desired strength of the steel material. Mn is an element having the effect of decreasing the amount of solute C in ferrite and increasing the amount of island martensite. Excess of more than 1.2% promotes the formation of island martensite, and Deteriorates toughness. Therefore, the lower limit of Mn is set to 0.6% and the upper limit is set to 1.2%.

Ni: 1.0 to 2.3% Ni is an element having an effect of improving the strength and toughness of the steel material, reducing the amount of solute C in ferrite, and increasing the amount of island martensite. In order to ensure the strength and toughness of the steel, the content of at least 1.0% is required, but an excessive content exceeding 2.3% promotes the formation of island martensite and deteriorates the weld toughness.
Therefore, the lower limit of Ni is 1.0% and the upper limit is 2.3%.

Ni ≦ −2Mn + 4.0 (1) where, Ni and Mn: content (%) of each element In the present invention, Ni and Mn are in the above-mentioned range and in the form of island martensite. Is adjusted to satisfy the expression (1) in order to suppress the generation of. If the contents of Ni and Mn do not satisfy the expression (1), the toughness of the welded portion is significantly deteriorated. In the extremely low carbon region, Mn has a greater effect on the formation of island martensite. For this reason, in order to suppress the formation of island-like martensite, it is preferable to use low Mn and high Ni.

FIG. 5 shows the relationship between the contents of Ni and Mn and the toughness of the weld bond. Fig. 5 shows the use of a test piece of ultra-low carbon steel with varied contents of Ni and Mn to form a bevel with the shape shown in Fig. 4 and create a welded joint by submerged arc welding with a heat input of 100 kJ / cm. Then, a Charpy impact test specimen (JIS No. 4 test specimen) was taken from the welded joint bond part, and the test temperature-
The Charpy absorbed energy vE- ₅₀ at 50 ° C. was determined, and the toughness of the weld bond was evaluated based on the relationship between the Ni content and the Mn content. The numbers in the figure are the Charpy absorbed energy vE- ₅₀ (J) of the weld bond.

Ni and Mn are as described above: Mn: 0.6 to 1.2%, N
i: A high vE- ₅₀ value is exhibited only within the range of 1.0 to 2.3% and within the range satisfying the expression (1). Outside this range, a low vE _-50 value is exhibited and the weld toughness deteriorates. For these reasons, the Mn and Ni contents satisfying the expression (1) are limited. Al: 0.005 to 0.10% Al acts as a deoxidizing agent, and the present invention requires a content of 0.005% or more. On the other hand, if the content exceeds 0.10%,
Al oxide-based inclusions increase in steel and reduce the base metal toughness. For this reason, Al was limited to the range of 0.005 to 0.10%.

Ti: 0.005 to 0.02% Ti combines with N to reduce the amount of solute N in the steel, contributes to an increase in the amount of solute B, and has the effect of ensuring the effect of increasing the strength of B. Such an effect is recognized at a content of 0.005% or more. On the other hand, when the content exceeds 0.02%, coarse precipitates are formed and the toughness of the base material is impaired. Therefore, Ti is 0.005 to 0.02
%.

B: 0.0005% to 0.0030% B is an important element for increasing the strength by forming a bainitic ferrite structure in a very low carbon steel.
In the present invention, 0.0005% or more is required. on the other hand,
Even if the content exceeds 0.0030%, no remarkable effect is obtained. For this reason, B was limited to the range of 0.0005 to 0.0030%.

Nb: 0.005 to 0.04%, V: 0.005 to 0.04%
One or two Nb and V selected from
It is an effective element as a strengthening element for strengthening steel, and can be selected and contained as necessary. In order to exert such an effect, it is necessary that both Nb and V are contained at 0.005% or more. On the other hand, if both Nb and V contain more than 0.04%, the toughness of the weld heat affected zone deteriorates. Therefore, Nb: 0.005 to 0.04
%, V: preferably limited to 0.005 to 0.04%.

Ca: 0.0005-0.005% Ca is an element having an effect of improving toughness by fixing S,
It can be contained as needed. In order to exhibit such an effect, it is preferable to contain at least 0.0005%,
Even if the content exceeds 0.005%, the effect is saturated. For this reason, in the present invention, Ca is preferably limited to the range of 0.0005 to 0.005%.

The molten steel having the above-described composition is melted by a usual method such as a converter, an electric furnace, a vacuum melting furnace and the like, and is rolled with a rolling material such as a slab by a generally known casting method such as a continuous casting method or an ingot casting method. Is preferred. Then, the rolling material is re-heated to a temperature of 1000 to 1300 ° C., or hot-rolled to end the rolling at a temperature of 700 ° C. or more without being re-heated. After accelerated cooling, it is made into a product (steel material). The hot rolling conditions are not particularly limited, but it is preferable to perform controlled rolling in which rolling is performed at a cumulative draft of 50% or more in the unrecrystallized region.

[0029]

Next, the effects of the present invention will be described based on embodiments. After heating the steel ingot having the composition shown in Table 1 to 1150 ° C,
Apply a rolling reduction of 50% or more in the unrecrystallized area, 800 ℃
The rolling was completed as described above to obtain a 20 mm steel sheet. The obtained steel plate was subjected to a tensile test and a Charpy impact test.

The tensile test was carried out from the center of the thickness of each steel sheet using JI
S No. 4 tensile test specimen was collected and yield strength YS, tensile strength TS,
The elongation El was determined. In the Charpy impact test, a JIS No. 4 impact test piece was sampled from the center of the thickness of each steel sheet, and the energy transition temperature (vTr _E ) was determined. In addition, a joint test piece having the shape shown in FIG.
Welded joints were made by submerged arc welding with a heat input of 100 kJ / cm. A JIS No. 4 impact test piece with the notch position as the bond part was sampled from these welded joints, and a Charpy impact test was conducted at a test temperature of -50 ° C, and the absorbed energy vE- ₅₀
I asked. In the vicinity of the bond portion of the weld joint, 2
The structure was observed by a scanning electron microscope using a step etching method, and the amount of island martensite was measured.

The results are shown in Table 2.

[0032]

[Table 1]

[0033]

[Table 2]

The example of the present invention has a tensile strength of TS 490 MPa class, an energy transition temperature vTr _E of −78 ° C. or less, is excellent in low-temperature toughness of the base material, and has an island-like martensite in the heat affected zone of welding. With a small amount of sites and a vE- _{50 of the} bond portion of the submerged arc welded joint with a heat input of 100 kJ / cm of 100 J or more, it is a steel material having excellent weld heat affected zone toughness even when large heat input welding is performed. On the other hand, the comparative examples out of the scope of the present invention show that the base metal toughness is degraded when vTr _E is −56 ° C. or more, or the weld heat affected zone toughness is vE _{-50 of the} weld bond portion being 27 J or less. Has deteriorated.

[0035]

As described above, according to the present invention,
The formation of island-like martensite in the weld heat affected zone is suppressed, steel materials having excellent weld heat affected zone toughness can be manufactured at low cost even when subjected to large heat input welding, and the welding efficiency can be significantly improved. It has a remarkable industrial effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the maximum C solid solution amount in ferrite and the amounts of Ni and Mn.

FIG. 2 is a graph showing the relationship between the area ratio of island martensite in a reproduced weld bond portion and the amounts of Ni and Mn.

Fig. 3 Ductile brittle fracture transition temperature (v
4 is a graph showing the relationship between (Trs) and the amounts of Ni and Mn.

FIG. 4 is a sectional view showing a groove shape used for manufacturing a welded joint.

FIG. 5 is a view showing the relationship between the amounts of Ni and Mn exerted on vE- ₅₀ of a weld bond portion of a submerged arc welded joint having a heat input of 100 kJ / cm 2.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Bunmaru Kawabata 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Pref. 1-chome (without address) Inside Kawasaki Steel Corporation Mizushima Works

Claims (3)

[Claims] 1. mass%, C: less than 0.03%, Si: 0.50% or less, Mn: 0.6 to 1.2%, Ni: 1.0 to 2.3%, Al: 0.005 to 0.10%, Ti: 0.005 to 0.02%, B : A steel material containing 0.0005 to 0.0030%, containing Mn and Ni so as to satisfy the following formula (1), and having a composition consisting of a balance of Fe and unavoidable impurities; . Note Ni ≦ −2Mn + 4.0 (1) where, Ni, Mn: content of each element (mass%) 2. In addition to the above composition, Nb:
2. The composition according to claim 1, wherein the composition contains one or two selected from 0.005 to 0.04% and V: 0.005 to 0.04%.
A steel material having excellent toughness in the heat-affected zone of the welding described in 1. 3. The composition according to claim 1, further comprising:
2. The composition according to claim 1, which contains 0.0005 to 0.005%.
Or a steel material excellent in the toughness of the weld heat-affected zone according to 2.