JP2002194490A - Steel having excellent toughness in welding heat affected zone and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel having excellent toughness in the welding heat affected zone even in the case of welding in which heat input lies in the range of 10 to 50 kJ/mm, and its production method. SOLUTION: In the steel having excellent toughness in the welding heat affected zone, Ceq expressed by Ceq=C+Mn/6+Si/24+Mo/4+Cr/5+Ni/40+V/14 (each element is the content thereof by mass%) lies in the range of 0.28 to 0.45%, and the local maximum value of the area ratio of insular martensite in the vicinity of a bond in the case of welding in which heat input lies in the range of 10 to 50 kJ/mm is <4%. Further, in the method, a slab is heated to a temperature of the Ac3 transformation point to 1,300 deg.C, is rolled at <=950 deg.C at a cumulative draft of >=40%, and is thereafter air-cooled or water-cooled to produce the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material excellent in the toughness of a heat affected zone subjected to welding prior to its use and a method for producing the same. This steel material can be used in general for welded structures such as shipbuilding, bridges, buildings, marine structures, pressure vessels, line pipes, etc., especially for structures subject to welding work with a heat input of 10 to 50 kJ / mm. Valid for use. The form of the steel material is particularly useful for steel plates, thick steel plates, steel pipes, and shaped steels used in bridges and shipbuilding.

[0002]

2. Description of the Related Art With the recent enlargement of structures, steel materials are required to have higher strength in order to reduce weight.
In order to increase the strength of the steel material, it is necessary to add a certain amount of alloying elements, but it is well known that on the other hand, it deteriorates the toughness of the heat affected zone. There are two main guidelines for improving the toughness of the heat affected zone. One is the refinement of the structure of the weld heat-affected zone, and the toughness of the weld heat-affected zone is improved by reducing the unit of the brittle fracture surface.
The other is control of an embrittlement phase that can be a starting point of brittle fracture, and the existence of island-like martensite is known as an embrittlement phase particularly problematic in a weld heat affected zone.

A method of improving HAZ toughness by controlling island martensite is disclosed in, for example, JP-A-62-214126.
JP, JP-A-62-174324, JP-A-62-174324
These are disclosed in publications such as Japanese Patent Application Laid-Open No. 151544 and Japanese Patent Application Laid-Open No. 61-270333. These are intended to improve the toughness of the heat affected zone by suppressing the amount of island martensite generated by controlling the amount thereof, and use low carbon equivalent, low Si, low Al, and the like. .

[0004]

SUMMARY OF THE INVENTION The above-mentioned JP-A-62-2
No. 14126, Japanese Patent Application Laid-Open No. 62-174324,
JP-A-62-151544, JP-A-61-270
All of the methods disclosed in publications such as 333 publication aim at reducing the amount of island martensite by reducing the amount of alloying elements. However, the reduction in the amount of island martensite due to the reduction in the amount of alloying elements simultaneously lowers the strength level, and thus impairs compatibility between high strength and high toughness of the weld heat affected zone. In addition, since the reduction in the amount of alloying elements increases the area ratio of grain boundary ferrite in the structure of the weld heat-affected zone, even if the formation of an embrittlement structure called island-like martensite is suppressed, the coarse grain boundary ferrite and its interior and its vicinity These measures alone have little effect on improving the toughness of the weld heat-affected zone, because they cause a new embrittlement factor of precipitates and carbides.

[0005] As described above, in order to improve the strength of the base material and to ensure high toughness in the heat affected zone of the weld, the formation of island martensite is suppressed as much as possible while adding appropriate alloying elements. It is necessary to select a combination of alloy components. In order to improve the strength of the base material, for example, the application of controlled rolling or controlled cooling is effective, but as an increase in the amount of alloying elements basically results in an increase in strength and a decrease in toughness of the welded heat-affected zone, It must be said that there is a limit to the method based only on the reduction of the amount of alloying elements in terms of achieving both the strength of the base metal and the toughness of the heat affected zone. An object of the present invention is to provide a steel material which solves the above-mentioned problems, secures strength, and is excellent in the toughness of a weld heat affected zone, and a method of manufacturing the same.

[0006]

Means for Solving the Problems The inventors of the present invention have examined in detail the relationship between the toughness of the welded heat-affected zone and the martensite in the form of islands. Instead of lowering the toughness, we newly discovered that the formation of locally dense regions of island-like martensite was dominant in the reduction of the toughness of the weld heat-affected zone. The present invention has been completed by limiting the upper limit value of the area ratio and the advantageous steel composition, and the gist thereof is as follows.

(1) Ceq = C + Mn / 6 + Si / 24 +
Ceq represented by Mo / 4 + Cr / 5 + Ni / 40 + V / 14 (each element contains% by mass) is 0.28 to 0.4.
The local maximum value of the area ratio of island martensite in the vicinity of the bond is less than 4% when the welding is performed in the range of 5% and the heat input is in the range of 10 to 50 kJ / mm. A steel material with excellent toughness in the heat affected zone. (2) In mass%, C: 0.005 to 0.2%, Si:
0.01 to 0.5%, Mn: 0.1 to 2%, P: 0.0
2% or less, S: 0.02% or less, Al: 0.001 to
0.1%, and V: 0.001 to 0.2
%, Nb: 0.001 to 0.1%, Cr: 0.01 to 2
%, Mo: 0.01 to 2%, W: 0.01 to 2%, B:
(1) The steel material having excellent heat-affected zone toughness as described in (1) above, wherein one or more of 0.0001 to 0.004% is contained, and the balance is Fe and inevitable impurities. (3) In mass%, Cu: 0.01 to 3%, Ni: 0.0
The steel material having excellent toughness of the weld heat-affected zone according to the above (1) or (2), comprising 1 to 5% of one or two types. (4) In mass%, Ti: 0.001 to 0.05%, Z
r: excellent in the toughness of the weld heat-affected zone according to any one of the above (1) to (3), further containing one or two kinds of 0.001 to 0.1%. Steel. (5) In mass%, Ca: 0.0005 to 0.02%, M
g: 0.0005 to 0.02%, REM: 0.001 to
The steel material having excellent toughness of the weld heat-affected zone according to any one of the above (1) to (4), further comprising 0.1% or more of one or more types. (6) Ca, Mg, one or more of REM and O,
S or both, and the particle size is 0.005 to 0.5.
(5) The steel material having excellent toughness of the weld heat-affected zone as described in (5) above, wherein 5 μm or more of 5 μm particles are dispersed per 1 mm ² . (7) In the production of the steel material according to any one of the above (1) to (6), the steel slab is made to have an Ac ₃ transformation point or more and 130 or more.
A method for producing a steel material having excellent toughness in a weld heat affected zone, characterized in that the steel is heated to a temperature of 0 ° C. or less, rolled at a cumulative draft of 40% or more at 950 ° C. or less, and then air-cooled or water-cooled. (8) The method for producing a steel material having excellent weld heat affected zone toughness according to the above (7), wherein the average value of the rolling reduction per rolling pass is 15% or more.

[0008]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail. The inventors investigated the relationship between HAZ toughness and island martensite using eight types of steel materials with different amounts of alloying elements in order to clarify the relationship between the weld heat affected zone and island martensite. The steel material used is a 590MPa-grade strength steel. After applying a heat cycle that reproduces welding with a heat input of 20kJ / mm, the fracture transition temperature (brittle fracture surface), which is one of the effective guidelines for toughness by the Charpy test, is given. Temperature at which the rate becomes 50%)
Was evaluated. FIG. 1 shows the relationship between the fracture surface transition temperature and the average island martensite area ratio, and FIG. 2 shows the relationship between the fracture surface transition temperature and the local maximum value of the island martensite area ratio.

The maximum value of the local area ratio of the island martensite described herein is measured by the following method. The structure of the weld heat affected zone of the target steel was clarified by the two-stage electrolytic corrosion method described in the 1980 Journal of the Japan Welding Society, vol. 25 tissue photographs are taken. From these, ten places where it is visually determined that the island-like martensite is dense are selected, and 50 × 50 μm is selected.
The area ratio of the island-like martensite is measured for the region (1). It is desirable that the area ratio is measured by an image analysis device or the like. However, if there is a restriction on the device, island-like martensite is formed at the intersection of the meshes by using a mesh obtained by equally dividing the region in the vertical and horizontal directions by 20. A method of calculating the area ratio based on the existence probability is also available. 10 measured in this way
Of the island martensite area ratio data, the average value of the three largest data is used as the maximum value of the local area ratio of the island martensite. The average area ratio of the island martensite is the area ratio measured for all the 25 photographs.

From the results, although there is a correlation between the HAZ toughness and the area ratio of the island-like martensite, the variation is very large, and even if the area ratio of the island-like martensite is the same, there is a large difference in the HAZ toughness. Is seen. On the other hand, regarding the maximum value of the area ratio of local island martensite,
There is a very good correlation between HAZ toughness.
It has been clarified by examination of the fracture surface and cross section after the Charpy test that the dense region of island martensite causes brittle fracture preferentially.

From the above findings, it is considered that even if the area ratio of martensite is the same, if the distribution state is different, the toughness of the heat affected zone is different. Specifically, it is effective to suppress local crowding of the island-like martensite, and if the area ratio is the same, the closer to a uniform distribution, the higher the weld heat affected zone toughness. The inventor found that Ceq = C + Mn / 6
+ Si / 24 + Mo / 4 + Cr / 5 + Ni / 40 + V /
For steel materials having a Ceq in the range of 0.28 to 0.45 represented by No. 14, the relationship between the amount of alloying elements, the local area ratio of island-like martensite, and the relationship between the weld heat-affected zone toughness was investigated in detail. It has been clarified that when the maximum value of the area ratio is less than 4%, high toughness of the heat affected zone is exhibited. In order to reduce the local area ratio of the island-like martensite to less than 4%, various combinations of alloy components can be considered, but the following alloy components or alloy component ratios are particularly effective.

Low C, low Si and low N. These reduce the local area ratio of the island martensite through the conventionally known effect of reducing the area ratio of the island martensite. Should be used accordingly.

Adding Ni and Cu. Cu, Ni
Is used to ensure strength, but the effect on martensite formation is different, and the amount of island-like martensite formed per unit added amount is larger in Cu than in Ni. The inventor has found that by setting the value represented by Cu / Ni to less than 0.8, it is possible to suppress the increase in the local area ratio of the island-like martensite while ensuring strength. Therefore, it is desirable that X represented by X = Cu / Ni be less than 0.8.

Adding Ti and Zr. Ti, Zr
In the heat affected zone of the weld, the nitride of the heat affected zone refines the austenite grain size in the region heated to the point ₃ or higher, and in the cooling process, the grains act as ferrite transformation nuclei from within the austenite grains, thereby reducing the weld heat affected zone. It has the effect of making the structure finer. When ferrite is formed from austenite, C is concentrated in austenite in the vicinity of ferrite, but the local concentration of the C-enriched region is suppressed by the increase of ferrite transformation nuclei, and the ferrite is formed from austenite in which C is concentrated. The maximum value of the local area ratio of island martensite is reduced. In addition, the generation of intragranular ferrite in the structure of the weld heat-affected zone, in addition to the viewpoint of island martensite, also improves the toughness of the weld heat-affected zone by reducing the unit of cleavage surface of brittle fracture. It is valid. In addition, the inventors have found that when the local area ratio of the island-like martensite is less than 4%, the combination of the following methods is also effective for improving the toughness of the heat affected zone.

[0015] To suppress the generation of grain boundary ferrite.
When a large amount of grain boundary ferrite is present and coarse grain boundary ferrite is present, the brittle crack once generated propagates through the coarse grain boundary ferrite and easily leads to brittle fracture. That is, even if the local area ratio of the island-like martensite is low, it becomes a factor of lowering the toughness. The formation of grain boundary ferrite can be suppressed by adding an appropriate amount of each of V, Nb, Cr, Mo, W and B.

Fine oxides or sulfides containing Ca, REM and Mg are dispersed in steel. These oxides, which are stable at high temperatures, make it possible to reduce the austenite grain size during heating during the production of the base material, and facilitate securing strength. That is, the addition amount of the alloy element for obtaining the required strength can be suppressed, and as a result, the toughness of the weld heat affected zone can be improved. In addition, in the heat affected zone of the weld, these oxides reduce the austenite grain size in the region heated to the point of _three or more Ac, so that the toughness of the heat affected zone is improved. Note that Ca, M here
g, fine oxides and sulfides containing REM are not only oxides and sulfides of the elements alone, but also composite oxides of the elements, composite sulfides, and oxides containing other oxide-forming elements, Includes sulfides. In order to make the heated austenite particle size of the base material fine, the number of the oxides and sulfides is important, and those having a size of 0.005 to 0.5 μm are more than 50,000 per 1 mm ^2. In some cases it has a great effect.

Production of a base material by controlled rolling and controlled cooling. By producing a steel sheet by controlled rolling and controlled cooling, it is possible to reduce the amount of alloying elements necessary for obtaining the required strength. That is, local concentration of island martensite is indirectly suppressed through reduction of the amount of alloying elements.

Increasing the average value of the rolling reduction per rolling pass. Increasing the rolling reduction per pass makes it possible to refine the final structure, and reduces the amount of alloying elements required to obtain a predetermined strength. That is, local concentration of island martensite is indirectly suppressed through reduction of the amount of alloying elements.

The reason why Ceq is limited to the range of 0.28 to 0.45 is as follows. Although the reduction of Ceq basically reduces the maximum value of the local area ratio of the island martensite, it is difficult to secure the strength. In the present invention, the lower limit is set to 0.28 because it is necessary to secure a strength level of about 500 MPa or more. On the other hand, an increase in Ceq increases the local area ratio through an increase in the average area ratio of the island-like martensite, and it is difficult to suppress the local area ratio to 4% or less at 0.45 or more. .45.

The reasons for limiting the chemical components in the present invention will be described. C is an element indispensable for securing the strength, so its addition amount is set to 0.005% or more. However, on the other hand, the increase in the amount of C increases the maximum value of the local area ratio of the island-like martensite due to the promotion of micro-segregation of the alloying element. Therefore, the upper limit is set to 0.2%, preferably 0.15%. However, it is desirable to reduce the required strength level as much as possible. Since Si is an element necessary for securing strength and deoxidizing, the amount of Si added is set to 0.01% or more.
However, on the other hand, an increase in the Si content increases the maximum value of the local area ratio through an increase in the area ratio of the island-like martensite, so the upper limit is set to 0.5%. If possible in consideration of the strength level, it is desirable to set it to 0.1% or less. Mn is useful as a strengthening element, but excessive addition lowers the weldability, so the range was made 0.1% or more and 2.0% or less. P is an impurity element, and it is desirable that P is low.
2% or less. In particular, since P dissolved in ferrite lowers the ductility of the base material, it is desirably 0.015% or less. S is an impurity element and is preferably low.
0.02% or less. S reduces the base material toughness due to generation of MnS, so is desirably 0.01% or less. Al is useful for fixing the deoxidizer and N, but when added in a large amount, forms an oxide and lowers the base material toughness. Therefore, the amount of Al is 0.001 to 0.1%
Defined in

V, Nb, and B suppress the formation of grain boundary ferrite and improve the toughness of the weld heat affected zone, and further improve the base metal toughness and the weld heat affected zone toughness by fixing free N. Further, since it has the effect of improving the strength, it is added as necessary, but the amount of each element and N is limited for the following reasons. V is an element useful for improving the toughness of the weld heat affected zone by suppressing the formation of grain boundary ferrite, and further improving the base metal toughness and the weld heat affected zone toughness by fixing N. It is also effective for increasing the strength. In order to exhibit the effect, 0.001% or more must be added. On the other hand, addition exceeding 0.2% lowers the base material toughness through formation of coarse precipitates. For this reason, the amount of V is 0.1.
001 to 0.2%. Nb is an element that suppresses the generation of grain boundary ferrite, improves the toughness of the weld heat affected zone, and is useful for improving the base metal toughness and the weld heat affected zone toughness by fixing N. It is also effective for increasing the strength. In order to exhibit the effect, 0.001% or more must be added. On the other hand, the addition exceeding 0.1% lowers the base material toughness through formation of coarse precipitates. Therefore, the Nb content is in the range of 0.001 to 0.1%. In addition, from the viewpoint of ensuring the low-temperature toughness of the heat affected zone, the Nb content is 0.05%.
% Is preferable. B is an element that suppresses the generation of grain boundary ferrite, improves the toughness of the weld heat affected zone, and is useful for improving the base metal toughness and the weld heat affected zone toughness by fixing N. It is also effective for increasing the strength. In order to exhibit the effect, it is necessary to add 0.0001% or more. On the other hand, the addition exceeding 0.004% lowers the base material toughness through formation of coarse precipitates. For this reason, the B content is in the range of 0.0001 to 0.004%.

Further, in order to suppress the generation of grain boundary ferrite and to improve the toughness of the heat affected zone by welding and to secure the strength,
If necessary, one, two or more of Cr, Mo and W can be contained. The amount of each element is limited for the following reasons. Cr is an element that suppresses generation of grain boundary ferrite, improves the toughness of the weld heat affected zone, and is useful for improving the base metal toughness and the weld heat affected zone toughness by fixing N. It is also an element useful for increasing the strength of the base material. In order to exhibit the effect, it is necessary to add 0.01% or more. On the other hand, the addition exceeding 2% lowers the toughness and weldability of the base material. For this reason, the amount of Cr is 0.01 to
The range is 2%. From the viewpoint of ensuring the low-temperature toughness of the heat affected zone, the Cr content is preferably 1% or less. Mo is an element that suppresses the generation of grain boundary ferrite, improves the toughness of the weld heat affected zone, and is useful for improving the base metal toughness and the weld heat affected zone toughness by fixing N. It is also an element useful for increasing the strength of the base material. In order to exhibit the effect, it is necessary to add 0.01% or more. 2%
Addition of more than 10% reduces the toughness and weldability of the base material. For this reason, the Mo amount is in the range of 0.01 to 2%. In addition, from the viewpoint of ensuring the low-temperature toughness of the weld heat affected zone, the Mo content is 1
% Is preferable. W is an element that suppresses the generation of grain boundary ferrite, improves the toughness of the weld heat affected zone, and is useful for improving the base metal toughness and the weld heat affected zone toughness by fixing N. It is also an element useful for increasing the strength of the base material. In order to exhibit the effect, it is necessary to add 0.01% or more. On the other hand, addition exceeding 2% lowers the toughness of the base material. For this reason, the W amount is in the range of 0.01 to 2%. In addition, from the viewpoint of ensuring the low-temperature toughness of the weld heat affected zone, the W content is preferably 1% or less.

Further, depending on the desired strength and toughness level, one or two of Ni and Cu can be contained as necessary. The amount of each element is limited for the following reasons. Ni is added as necessary to ensure strength and toughness. If the addition is less than 0.01%, the effect is small. On the other hand, if the addition exceeds 5%, the weldability is reduced, so the range is set to 0.01 to 5%.
Incidentally, from the viewpoint of ensuring the low-temperature toughness of the weld heat affected zone, N
The i amount is preferably 2% or less. Cu is an element useful for improving the strength and toughness of the base material. In order to exhibit the effect, it is necessary to add 0.01% or more. On the other hand, addition exceeding 3% causes a problem in hot workability. For this reason, the Cu content is in the range of 0.01 to 3%. In addition, from the viewpoint of ensuring the low-temperature toughness of the heat affected zone, the Cu content is 1%.
The following is preferred. With respect to the addition amounts of Ni and Cu, X represented by X = Cu / Ni is set to 0.8 in order to reduce the local area ratio of the island martensite.
It is desirable to be less than.

Ti and Zr are added as necessary because they have the effect of improving the base material toughness and further improving the strength through fixing, but the amount of each element is limited for the following reasons. . Ti forms nitrides,
The local area ratio of island martensite through reduction of the amount of C enriched in austenite near grain boundary ferrite and reduction of the C enriched region by refining the grain size of reheated austenite in the heat affected zone Reduce the maximum value of. In addition, the base material toughness is also improved by fixing N. 0.001%
If the addition is less than 0.05%, the effect is small. On the other hand, if the addition exceeds 0.05%, the base material toughness is lowered through formation of coarse precipitates. Z
r is an element useful for fixing N and improves the base material toughness. It is also effective for increasing the strength. In order to exhibit the effect, 0.001% or more must be added. On the other hand, the addition exceeding 0.1% lowers the base material toughness through formation of coarse precipitates. Therefore, the amount of Zr is 0.00
The range is 1 to 0.1%.

Further, if necessary, Ca, Mg, REM
Or one or more of these. The amount of each element is limited for the following reasons.
By adding one or more of Ca, Mg, and REM, the heated austenite in the heat affected zone can be refined. In order to exhibit this effect, it is necessary to add 0.0005% or more of Ca and Mg and 0.001% or more of REM. On the other hand, if added excessively, the sulfides and oxides become coarse and reduce the base material toughness and ductility, so the upper limit is 0.02% for Ca and Mg and 0.1% for REM.
And

In the present invention, any method can be selected for the production conditions, but from the viewpoint of securing the strength of the base material, if necessary, rolling at 950.degree. It is effective to carry out water cooling.
Further, if necessary, by setting the average value of the rolling reduction per rolling pass to 15% or more, it is possible to secure higher strength with the same component, and to improve the toughness of the weld heat affected zone by reducing the amount of the alloy component. Becomes possible.

[0027]

EXAMPLES Yield stress, tensile strength, local area ratio of island-like martensite of a base material for steel plates having a thickness of 25 mm to 100 mm manufactured under various manufacturing conditions using test steel materials of various chemical components. 0.005 to 0.5 of oxides and sulfides
The number per 1 mm ² of the sample having a size of μm and the toughness of the heat affected zone were evaluated. Steel sheet chemical composition and Ce
q. Table 1 shows the production conditions and the yield stress, tensile strength,
Table 2 shows the local area ratio of the island martensite, the number of oxides and sulfides having a size of 0.005 to 0.5 μm per 1 mm ² , and the toughness of the heat affected zone by welding.

The yield stress and tensile strength of the base metal are 1 /
It was measured from a tensile test using a JIS No. 4 subsize round bar tensile test piece taken from the 4t portion in parallel with the rolling direction. The maximum value of the local area ratio of the island-like martensite was measured according to the method described above. 0.005 to 0.005 of oxides and sulfides
Regarding the number of pieces per 1 mm ² having a size of 0.5 μm, bright field images of 10 visual fields were observed by transmission electron microscope observation, and the average number density was measured. The toughness of the heat affected zone was evaluated by sampling a Charpy test specimen from a joint that had been subjected to electrogas welding at a heat input of 35 kJ / mm in parallel with the rolling direction. In the Charpy test piece, the notch is oriented in the thickness direction from the weld heat affected zone corresponding to 1 / 4t of the base material so that the longitudinal direction of the test piece is perpendicular to the welding direction, and the notch bottom is bonded to the bond. Collect as equivalent, -4
Absorbed energy at 0 ° C. was measured. Three tests were performed under the same conditions, and the average value was adopted.

Steel numbers A1 to A3, A5, A6, A8, A
9, A16 is a steel sheet for which alloy components and manufacturing methods have been determined so that the yield stress is about 350 MPa. Each component has a low Si content, an increase in the local area ratio of the island-like martensite is suppressed, and the toughness of the weld heat-affected zone is improved. In addition, elements such as Ti and Zr are added as necessary, and the effect of improving the toughness of the weld heat-affected zone directly by the formation of intragranular ferrite and the local area ratio of island martensite by reducing the local C-enriched region. The effect of improving the toughness of the weld heat affected zone through reduction is obtained. Furthermore, Ca, Mg, REM
And other elements are added as necessary, and the toughness of the weld heat-affected zone is improved by reducing the local C-enriched region by reducing the alloying elements by refining the structure of the base metal and by refining the structure of the weld heat-affected zone. are doing. Also, V, Nb, Cr,
The addition of elements such as Mo, W, and B ensures the toughness of the weld heat-affected zone through the suppression of grain boundary ferrite generation, while ensuring the strength while suppressing the increase in the local area ratio of island martensite due to the addition of elements such as Ni and Cu. It is planned. In the manufacturing process, controlled rolling and controlled cooling are performed to secure a tensile strength of about 500 MPa at a low carbon equivalent.

On the other hand, in steel number B1, the local area ratio of the island-like martensite due to the high Si content is increased, and the weld heat affected zone toughness is reduced.

The steel numbers A10 to A13 correspond to the above steel numbers A
9. In addition to the method used in 9
This is a steel sheet to which the average reduction per pass is applied. The weld heat-affected zone toughness is improved by reducing the amount of alloying elements added through the structure refinement effect of the base metal and by the structure refinement effect of the welded portion.

Steel numbers A4, A7, A14, A15, A1
7 to A20, A24, A25, A27, A28, A30
-A32 and A35 are steel sheets for which alloy components and manufacturing methods have been determined so that the yield stress is about 420 MPa or more.
Each component has a low Si content, an increase in the local area ratio of the island-like martensite is suppressed, and the toughness of the weld heat-affected zone is improved. In addition, elements such as Ti and Zr are added as necessary, and the effect of improving the toughness of the weld heat-affected zone directly by the formation of intragranular ferrite and the local area ratio of island martensite by reducing the local C-enriched region. The effect of improving the toughness of the weld heat affected zone through reduction is obtained. Further, Ca, M
g, REM, etc. are added as needed, and the local heat-affected zone is reduced by reducing the local C-enriched region by reducing the alloying element by refining the microstructure of the base metal and by refining the microstructure of the weld heat-affected zone. Improved toughness. In addition, if necessary, V, N
b, Cr, Mo, W, B, etc. The toughness of the weld heat-affected zone through the suppression of grain boundary ferrite formation by adding elements such as Ni, Cu
The strength is secured while suppressing an increase in the local area ratio of the island-like martensite due to the addition of elements such as. In the manufacturing process, controlled rolling and controlled cooling are carried out in order to secure a tensile strength of about 550 MPa at a low carbon equivalent.

On the other hand, in steel No. B2, Ni is added to suppress the increase of the local area ratio of the island martensite and to secure the strength, but since the Si content is high, the local area ratio of the island martensite increases. As a result, the toughness of the weld heat affected zone is reduced. Furthermore, despite the addition of Cu and Ni and the addition of Ca and Mg, steel No. B3 has a high Si content, so that the local area ratio of island martensite increases, and the weld heat affected zone toughness increases. Is declining. Similarly, steel number B
4 also contains Nb, Cu, Ni and Ti,
Since the amount of Si is high, the local area ratio of the island-like martensite increases, and the toughness of the weld heat-affected zone decreases.

The steel numbers A21 to A23 correspond to the above steel numbers A
20 is a steel sheet to which, in addition to the technique used in No. 20, a large amount dispersion of a trace amount of oxide and an increase in average draft per pass of rolling are applied.
The weld heat-affected zone toughness is improved by reducing the amount of alloying elements added through the structure refinement effect of the base metal and by the structure refinement effect of the welded portion.

Steel numbers A26, A29, A33, A34,
A36 to A39 are steel sheets for which alloy components and manufacturing methods have been determined so that the yield stress is about 460 MPa or more. Each component has a low Si content, an increase in the local area ratio of the island-like martensite is suppressed, and the toughness of the weld heat-affected zone is improved. In addition, elements such as Ti and Zr are added as necessary, and the effect of improving the toughness of the weld heat-affected zone directly by the formation of intragranular ferrite and the local area ratio of island martensite by reducing the local C-enriched region. The effect of improving the toughness of the weld heat affected zone through reduction is obtained. Further, Ca, Mg,
Elements such as REM are added as necessary, and the toughness of the weld heat affected zone is reduced by reducing the local C enrichment region by reducing the alloy elements by refining the structure of the base metal and by reducing the structure of the weld heat affected zone. Has improved. Also, if necessary, V, Nb,
Welding heat affected zone toughness through suppression of grain boundary ferrite generation by addition of elements such as Cr, Mo, W, B, etc., and strength after suppressing increase in local area ratio of island martensite by addition of elements such as Ni, Cu, etc. It is being secured. In the manufacturing process, controlled rolling and controlled cooling are carried out in order to secure a tensile strength of about 550 MPa at a low carbon equivalent.

On the other hand, in steel No. B5, Cu and Ni are added to suppress the increase of the local area ratio of the island-like martensite and secure the strength. However, since the Si content is high, the local area ratio of the island-like martensite is increased. And the toughness of the weld heat-affected zone is reduced. Further, although the steel No. B6 added Cu, Ni and Ti, the local area ratio of the island martensite was increased due to the high Si content, and the weld heat affected zone toughness was reduced.

The steel numbers A40 to A42 correspond to the above steel numbers A
39 is a steel sheet to which the technique used in No. 39, a large amount dispersion of a trace amount of oxide and an increase in average draft per rolling pass are applied.
The weld heat-affected zone toughness is improved by reducing the amount of alloying elements added through the structure refinement effect of the base metal and by the structure refinement effect of the welded portion.

Steel numbers A43 to A46 have a yield stress of 48
This is a steel sheet whose alloy components and manufacturing method are determined so as to be about 0 MPa or more. Each component has a low Si content, an increase in the local area ratio of the island-like martensite is suppressed, and the toughness of the weld heat-affected zone is improved. Also, if necessary,
Elements such as i, Zr, etc. are added, and the effect of welding heat-affected zone is directly improved by the formation of intragranular ferrite and the effect of welding heat is reduced by reducing the local area ratio of island martensite by reducing the local C-enriched region. The effect of improving the toughness is obtained.
In addition, elements such as Ca, Mg, and REM are added as necessary, and by reducing the alloying element by reducing the structure of the base material, the local C-enriched region is reduced, and the structure of the weld heat affected zone is reduced. The toughness of the heat affected zone is improved. In addition, if necessary, the addition of elements such as V, Nb, Cr, Mo, W, and B causes the toughness of the weld heat-affected zone through the suppression of grain boundary ferrite formation to increase the locality of island martensite by the addition of elements such as Ni and Cu. Strength is ensured while suppressing an increase in the area ratio. In the manufacturing process, controlled rolling and controlled cooling are carried out in order to secure a tensile strength of about 550 MPa at a low carbon equivalent.

On the other hand, in steel No. B7, the amount of Nb was increased to suppress the formation of grain boundary ferrite, and further, Cu, Ni, Zr
However, since the amount of Si is high, the local area ratio of the island-like martensite increases, and the toughness of the weld heat-affected zone decreases.

The steel numbers A47 to A49 correspond to the above steel numbers A
39 is a steel sheet to which the technique used in No. 39, a large amount dispersion of a trace amount of oxide and an increase in average draft per rolling pass are applied.
The weld heat-affected zone toughness is improved by reducing the amount of alloying elements added through the structure refinement effect of the base metal and by the structure refinement effect of the welded portion.

Steel numbers A50 to A52 have a yield stress of 53
This is a steel sheet whose alloy components and manufacturing method are determined so as to be about 0 MPa or more. Each component has a low Si content, an increase in the local area ratio of the island-like martensite is suppressed, and the toughness of the weld heat-affected zone is improved. Also, if necessary,
Elements such as i, Zr, etc. are added, and the effect of welding heat-affected zone is directly improved by the formation of intragranular ferrite and the effect of welding heat is reduced by reducing the local area ratio of island martensite by reducing the local C-enriched region. The effect of improving the toughness is obtained.
In addition, elements such as Ca, Mg, and REM are added as necessary, and by reducing the alloying element by reducing the structure of the base material, the local C-enriched region is reduced, and the structure of the weld heat affected zone is reduced. The toughness of the heat affected zone is improved. In addition, if necessary, the addition of elements such as V, Nb, Cr, Mo, W, and B causes the toughness of the weld heat-affected zone through the suppression of grain boundary ferrite formation to increase the locality of island martensite by the addition of elements such as Ni and Cu. Strength is ensured while suppressing an increase in the area ratio. In the manufacturing process, controlled rolling and controlled cooling are carried out in order to secure a tensile strength of about 550 MPa at a low carbon equivalent.

On the other hand, steel number B8 is Cr, Mo, V, C
Although u, Ni and Ti are used, the local area ratio of the island-like martensite increases due to the high Si content, and the toughness of the weld heat affected zone decreases.

The steel numbers A53 to A55 correspond to the above steel numbers A
39 is a steel sheet to which the technique used in No. 39, a large amount dispersion of a trace amount of oxide and an increase in average draft per rolling pass are applied.
The weld heat-affected zone toughness is improved by reducing the amount of alloying elements added through the structure refinement effect of the base metal and by the structure refinement effect of the welded portion.

From the above examples, it is clear that the steel sheets of steel numbers A1 to A55, which are the steel materials manufactured according to the present invention, have high toughness in the heat affected zone in a wide range of strength. According to the present invention, it is possible to obtain a steel material having high weld heat affected zone toughness in a wide range of strength.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

[Table 6]

[0051]

Industrial Applicability The present invention relates to a steel material used for a structure to be welded prior to its use, and particularly to the toughness of a weld heat affected zone when a weld heat input of 10 to 50 kJ / mm is received. Excellent steel materials can be manufactured without using special alloying elements or manufacturing methods, and it can be said that industrial value is extremely high.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the average island martensite area ratio and toughness (vTrs).

FIG. 2 is a diagram showing a relationship between a local maximum value of an areal martensite area ratio and toughness (vTrs).

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shuji Awaihara 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Minoru Ito 20-1 Shintomi, Futtsu-shi, Chiba Japan-made 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA12 AA14 AA15 AA16 AA19 AA20 AA22 AA23 AA24 AA27 AA31 AA35 AA36 AA38 AA40 BA01 CA02 CA03 CB01 CB02 CD05

Claims (8)

[Claims] 1. Ceq = C + Mn / 6 + Si / 24 + Mo
Ceq expressed as / 4 + Cr / 5 + Ni / 40 + V / 14 (the content of each element is% by mass) is 0.28 to 0.45%
And when the heat input is in the range of 10 to 50 kJ / mm, the local maximum value of the area ratio of the island martensite in the vicinity of the bond is less than 4%. Steel material with excellent weld heat affected zone toughness. 2. In mass%, C: 0.005 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.1 to 2%, P: 0.02% or less, S: 0.02% or less, Al: 0.001 to 0.1%, V: 0.001 to 0.2%, Nb: 0.001 to 0.1%, Cr: 0.01 to 2%, Mo: 0.01 to 2%, W: 0.01 to 2%, B: 0.0001 to 0.004%, and the balance is Fe and unavoidable impurities. The steel material having excellent heat-affected zone toughness according to claim 1, wherein: 3. The welding heat according to claim 1, wherein one or two of Cu: 0.01 to 3% and Ni: 0.01 to 5% are contained by mass%. Steel with excellent toughness in the affected zone. 4. The composition according to claim 1, further comprising one or two kinds of Ti: 0.001 to 0.05% and Zr: 0.001 to 0.1% by mass%. 4. The steel material according to any one of the items 3 which is excellent in toughness of a heat affected zone. 5. One or more of Ca: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, REM: 0.001 to 0.1% by mass%, The steel material according to any one of claims 1 to 4, further comprising: 6. A composition containing one or more of Ca, Mg, and REM and one or both of O and S, and having a particle size of 0.00
5~0.5μm particles, excellent HAZ toughness according to claim 5, characterized in that it dispersed 50,000 or more per 1 mm ² steel. 7. The method of manufacturing a steel material according to claim 1, wherein the steel slab is made to have an Ac ₃ transformation point or more and 13 or more.
A method for producing a steel material having excellent toughness in a heat-affected zone of a weld, characterized in that the steel is heated to a temperature of not more than 00 ° C., rolled at a cumulative reduction rate of not less than 40% at a temperature of not more than 950 ° C., and then air-cooled or water-cooled. 8. An average value of the rolling reduction per rolling pass is 1
The method for producing a steel material having excellent toughness in a weld heat-affected zone according to claim 7, wherein the content is 5% or more.