JP2001131679A - Joint and structure composed of hyperfine-grained steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a joint and a joined structure having excellent toughness and arresting properties as those of structures reflecting the characteristics of hyperfine-grained steel without extremely damaging excellent characteristics of the steel and excellent in safety, as for a joint of steel in which the average ferrite grain size is <=3 μm, and the characteristics of the strength, toughness, arresting properties or the like of steel are improved by the hyperfine-grained structure, a joined structure composed of the joint and moreover, a method for joining hyperfine-grained steel. SOLUTION: In hyperfine-grained steel in which the average grain size of a part or the whole of steel having a composition containing, by weight, 0.01 to 0.2% C, 0.01 to 1% Si, 0.1 to 2% Mn, 0.001 to 0.1% Al, 0.001 to 0.01% N, <=0.025% P, <=0.015% S, and the balance Fe is <=3 μm, laser welding is executed in such a manner that the width of the region having the average grain size of >=5 μm in the heat-affected zone produced in the steel sheet by welding or joining is controlled to <=0.5 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a low-temperature storage tank,
The present invention relates to a joint made of ultrafine-grained steel which is extremely excellent in low-temperature toughness as a material and is used for a structure requiring low-temperature toughness as a structure such as a structure for a cold region, and a joined structure comprising the joint.

[0002]

2. Description of the Related Art Recently, a low-temperature steel having extremely good low-temperature toughness and brittle crack propagation arrestability (arrest property) having an ultrafine grain structure with a ferrite grain size of about 2 μm or less has been put to practical use by thermomechanical treatment. Is being done. For example, JP-A-7-126
No. 798, JP-A-7-826424 and JP-A-9-1776782, the arrestability of a steel sheet is improved by providing an ultrafine grain layer of up to 33% on the front and back surfaces of the steel sheet. Various so-called surface layer ultrafine grained steels and methods for producing the same have been disclosed.

In addition, Japanese Patent Application Laid-Open Nos. 7-126797, 8-198829, 8-295982
Japanese Patent Application Laid-Open No. 9-202919 discloses a full-thickness ultrafine-grained steel based on the same technical concept as the surface ultrafine-grained steel and a method for producing the same. The full-thickness ultrafine-grained steel has an ultrafine-grained structure formed over the entire thickness of the steel sheet, thereby making it possible to further improve arrestability as compared with the surface layer ultrafine-grained steel. Since these ultrafine-grained steels have a large amount of fine-grain reinforcement, their strength can be significantly improved as compared with ferrite + pearlite steels having the same composition.

However, as typified by the above example, ultrafine-grained steel is formed by making full use of thermomechanical heat treatment or heat treatment, and in some cases, cold working such as mechanical milling. When it is joined to a structure by arc welding or latent arc welding, it is extremely thin in the heat affected zone (HAZ) exposed to high temperature by welding. Grain structure is eliminated or changed, especially melting line (fusion line)
The strength and toughness of the weld heat-affected zone in the vicinity has a disadvantage that it is reduced to the same level as steel having a normal structure.

[0005] Therefore, the ultrafine-grained steel currently in practical use guarantees the low-temperature toughness and arrestability of the base material to the last, and cannot guarantee the low-temperature toughness and arrestability of HAZ. In a structure in which toughness and HAZ arrestability (bond arrestability) determine the characteristics of the entire structure, the use of ultrafine-grained steel may not lead to an improvement in safety, and the characteristics of the steel material are extremely excellent. However, there are restrictions on their use.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a steel joint having an average ferrite grain size of 3 μm or less, which aims to improve properties such as strength, toughness and arrestability of a steel material by the ultrafine grain structure. A joint having excellent toughness and arrestability as a structure reflecting the properties of the ultrafine-grained steel without extremely impairing the excellent properties of the steel material, and having excellent safety. And a joint structure.

[0007]

As a method of joining steel materials without impairing the characteristics of the steel materials at all, bolt joining is conceivable. However, joining all of a complicated structure by bolt joining is called welding joining. However, in view of the current situation, which is the mainstream, it is not practical in terms of work efficiency and cost. Therefore, the present inventors, at the same efficiency and cost as the current arc welding method, in the category of a practical welding method, in order to solve the above problems, the main point is that the characteristics of ultra-fine-grained steel By reducing the width of the HAZ in which is deteriorated, and by preventing the weld metal and remarkable softening from occurring as compared with steel,
The purpose of the present invention is to ensure that the characteristics of the structure as a whole are determined by the characteristics of the steel material by ensuring that the destruction of the structure always occurs from the base metal raw material portion.

(1) By weight%, C: 0.01-0.2
%, Si: 0.01 to 1%, Mn: 0.1 to 2%, A
l: 0.001 to 0.1%, N: 0.001 to 0.01
%, Further contains, as impurities, P: 0.025% or less, S: 0.015% or less, the balance being iron and unavoidable impurities, and an ultrafine-grained ferrite structure having an average grain size of 3 μm or less. A joint made of ultrafine-grained steel, wherein a steel sheet having 10 to 50% of the thickness in the thickness direction from the front surface and the back surface of the steel plate is laser-welded so that the width of the weld metal is 5 mm or less. .

(2) C: 0.01 to 0.2% by weight
%, Si: 0.01 to 1%, Mn: 0.1 to 2%, A
l: 0.001 to 0.1%, N: 0.001 to 0.01
%, Further contains, as impurities, P: 0.025% or less, S: 0.015% or less, the balance being iron and unavoidable impurities, and an ultrafine-grained ferrite structure having an average grain size of 3 μm or less. The average grain size in the heat-affected zone generated in the steel sheet by welding or joining the steel sheet having 10 to 50% of the thickness in the thickness direction from the front surface to the front and rear surfaces of the steel plate is 5 μm.
A joint made of ultrafine-grained steel, wherein the joint is welded or joined so that the width of a region of at least m is 0.5 μm or less.

(3) The steel sheet is Cr: 0.01% by weight.
-1%, Ni: 0.01-6%, Mo: 0.01-1
%, Cu: 0.01 to 1.5%, Ti: 0.003 to
0.1%, V: 0.005 to 0.5%, Nb: 0.00
3 to 0.1%, Zr: 0.003 to 0.1%, Ta:
0.005 to 0.2%, W: 0.01 to 2%, B: 0.
The joint made of ultrafine-grained steel according to any of (1) and (2), further comprising one or more of 0003 to 0.002%.

(4) The steel sheet is Mg: 0.00% by weight.
05-0.01%, Ca: 0.0005-0.01%,
REM: One or more of 0.005 to 0.1% of the above (1) to (1) are further contained.
A joint comprising the ultrafine-grained steel according to any one of (3). (5) The joint according to any of (1) to (4)
A joint structure made of ultrafine-grained steel characterized by having the above.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In a joining structure in which a molten material, a filler metal, or a steel material itself is heated or melted and solidified to join the steel materials, a heat affected zone (HAZ) in the vicinity of the joint has a slight difference. In any case, heating beyond the transformation point is inevitable. Therefore, by any means, HA of ultrafine-grained steel having thermal instability
It is impossible for Z to retain the same structure as that of the parent material, and it is inevitable that some structural change is caused by grain growth, and as a result, the properties of HAZ, especially ultrafine In addition, deterioration of strength and toughness, which are remarkably expressed, cannot be avoided.

The present invention is based on the premise that the deterioration of the material of ultra-fine-grained steel HAZ is suppressed as much as possible and on the premise of the deterioration of the material. , Which led to the invention. The basic requirement in the present invention is that a steel sheet having an ultrafine-grained ferrite structure having an average grain size of 3 μm or less from the front surface to the back surface of each of the steel plates in the thickness direction from the surface to 10% to 50% of the thickness is welded or joined. The welding or joining is performed so that the width of the region having an average particle size of 5 μm or more in the heat-affected zone is 0.5 μm or less.

First, the microstructure requirements of the steel to be joined were determined as follows: an ultrafine grain structure having an average ferrite grain size of 3 μm or less was applied to each of the front and back surfaces of the steel plate in the thickness direction from the front surface to 10 to 50% of the plate thickness.
The reason for having the above is as follows. The structure having an average ferrite grain size of more than 3 μm can be achieved by normalizing or ordinary controlled rolling, and is thermally stable, and it is not necessary to apply the joint of the present invention. If it exceeds 3 μm, the strength and toughness obtained are limited,
This is because it is relatively easy to make the strength and toughness of the HAZ equal to those of the base metal raw material portion, even if the invention is not used.

Next, the ultrafine-grained structure is made
Each has 10 to 50% of the thickness in the thickness direction from the surface
Limited to less than 10% of each surface as above
The strength and toughness that can be obtained with steel that only has an ultrafine grain structure
There is a limit to the strength, and even if it is not according to the present invention, the strength of HAZ
It is relatively easy to make the degree and toughness equal to those of the base metal
That's why. Ultrafine grains with an average ferrite grain size of 3 μm or less
The thickness of the structure in the thickness direction from the front surface to the back surface
The steel sheet having up to 10 to 50% of the
If the conditions are satisfied at the same time, the Charpy impact of the surface layer
Characteristics are -120 ° C or less at fracture surface transition temperature (vTrs), steel
The arrest characteristic of the whole board is K_CaIs 400kgf / mm
^1.5Temperature (T_KCa400) And below -100 ° C
Good characteristics can be sufficiently achieved.

Although the present invention is directed to an ultrafine-grained ferritic steel having an average grain size of 3 μm or less, it may inevitably contain a structure other than ferrite, such as pearlite, bainite, martensite. Even after the very good toughness and arrestability of the base material have become a structure, the properties of the entire structure are required to maintain the properties of the base material, and the necessary requirements arise in the steel sheet by welding or joining. Welding or joining such that the width of the region having an average particle size of 5 μm or more in the heat-affected zone is 0.5 μm or less.

In a joint structure in which a steel material is joined by heating, melting, or solidifying a molten material, a filler metal, or a steel material itself, a heat-affected zone (HAZ) in the vicinity of the joint portion.
It is inevitable that the steel material will be heated above the transformation point, though to some extent. Therefore, by any means, it is impossible for HAZ of ultrafine-grained steel having thermal instability to maintain the same structure as that of the base metal raw material part, and some kind of grain growth is representative. It is unavoidable that a structural change is caused, and as a result, the properties of the HAZ, particularly the strength and toughness, which are remarkably developed by ultra-fine graining, are unavoidable.

However, as a result of detailed experiments and analysis, the present inventors can secure the safety of the joint or the entire structure by controlling the size and characteristics of the portion where the structure of the HAZ has changed. Was found. That is, after defining the chemical composition of the steel sheet and setting the width of the coarse structure whose material has deteriorated to 0.5 mm or less in each HAZ, the characteristics of the entire joint substantially reflect the characteristics of the base metal. Note that the coarse structure refers to a structure having an average particle size of 5 μm or more in which the material difference from the base material having an average particle size of 3 μm or less is significant. Here, the grain size refers to the ferrite grain size in the case of a ferrite-based structure, and the former austenite grain size in the case of a bainite or martensite-based structure.

The above HAZ microstructure requirements do not depend on the means for achieving the microstructure, but more specifically, an ultrafine ferrite microstructure having an average grain size of 3 μm or less is applied to each of the front and rear surfaces of the steel sheet in the thickness direction from the front surface to the sheet thickness direction. A steel plate having up to 10 to 50% of the plate thickness,
It is preferable to join by laser welding so that the width of the weld metal is 5 mm or less. That is, in order to minimize the coarsening of the ultrafine grain structure as much as possible, a method in which the input energy is as small as possible and the cooling rate after the joining is low is preferable. When the amount of heat input is reduced by general arc welding, the width of the heat effect in one welding pass is small and the cooling rate is large, but thick materials are inevitably multi-layered, so they are repeatedly heated to high temperatures. Therefore, it is difficult to suppress grain growth. In addition, in order to suppress the grain growth of the ultrafine grain structure to such an extent that the characteristics of the joined body do not deteriorate, it is necessary to minimize the heat input, but in such a case, the number of layers in the welding pass becomes extremely large. In addition, the welding efficiency is extremely deteriorated and is not practical.

The present inventors have not shown the above drawbacks,
It has been found that laser welding is preferable as a joining method capable of suppressing a structural change. In other words, laser welding can effectively perform small heat input welding and can be joined in one pass up to thick materials. Therefore, among the joining methods for melting steel, the change in ultrafine grain structure is the most effective. Can be smaller. Even with laser welding, the structure of ultrafine-grained steel cannot be preserved exactly as it is in the original base metal part, but in structures, the occurrence and propagation of fractures is such that the ultrafine-grained structure is damaged. If it is generated in the HAZ to the base material raw portion near the base material where the heating temperature is low and the superfine grain structure is preserved without being generated by the metal to HAZ, the characteristics of the entire structure can be changed to the base material. It becomes possible to obtain the characteristics of fine-grained steel.

For this purpose, it is necessary to reduce the width of the weld metal and the HAZ to a certain value or less, and to make the hardness of the region where the ultrafine grain structure is damaged higher than that of the base metal material. A joint structure that satisfies these requirements prevents extreme deterioration of the material in the weld metal to HAZ region in which the ultrafine grain structure has been impaired, and suppresses deformation in the region to prevent fracture in any mode. It can be generated from the raw material part.

Specifically, the width of the weld metal formed by laser welding must be 5 mm or less. By setting the width of the weld metal to 5 mm or less, the width of the HAZ also becomes correspondingly narrow, and the width of the region having an average particle size of 5 μm or more in the HAZ becomes 0.5 μm or less and is sufficiently quenched. The hardness of the metal to HAZ is higher than that of the base metal raw portion. Although the above description of the present invention is based on the assumption of laser welding without using a filler metal, a filler metal within a limited range of steel material components described later, or a steel material constituting a structure of the present invention as a weld metal. Any filler metal that can exhibit equivalent characteristics may be used.

As described above, the present invention is limited to laser welding capable of joining in the air, assuming that the present invention can be applied to all joining structures including large structures. It is also possible in principle to produce a joint structure made of the ultrafine-grained steel of the present invention by beam welding. The above are the basic requirements for the joint made of ultrafine-grained steel and the structure made of the same according to the present invention. The weld metal formed by laser welding when no filler metal is used, and the extreme deterioration of properties of the HAZ In order to prevent this and to secure the properties of the base metal raw material portion and to exhibit more effects, it is necessary to limit each chemical component of the steel material for the following reasons.

That is, C is contained as an effective component for improving the strength of steel. If it is less than 0.01%, it is difficult to secure the strength required for structural steel, but it exceeds 0.2%. Excessive content lowers the toughness and weld crack resistance of the base metal and the welded portion, so the content was made 0.01 to 0.2%. Next, Si is an element effective as a deoxidizing element and for securing the strength of the base material. However, if the content is less than 0.01%, deoxidation becomes insufficient and disadvantageous for securing the strength. Conversely, an excessive content exceeding 1% forms a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is 0.01 to
1%.

Further, Mn is an element necessary for securing the strength and toughness of the base material, and it is necessary to contain at least 0.1% or more. For example, the base material toughness, the toughness of the welded portion, the weld cracking property, and the like are deteriorated due to the above factors. Al is an element effective for deoxidation, austenite grain size reduction, etc., but 0.001%
It is necessary to contain the above. On the other hand, if it is contained in excess of 0.1%, a coarse oxide is formed and the ductility is extremely deteriorated. Therefore, it is necessary to limit the content to the range of 0.001 to 0.1%.

N works effectively with the refining of austenite grains in combination with Al and Ti, so that a small amount of N works effectively on mechanical properties. Further, it is impossible to industrially completely remove N in steel, and it is not preferable to reduce N more than necessary because an excessive load is applied to a manufacturing process. Therefore, the lower limit is set to 0.001% as a range in which industrial control is possible and load on the manufacturing process can be tolerated. If it is contained excessively, the solute N increases, which may adversely affect ductility and toughness.
%.

P and S are impurity elements which deteriorate ductility and toughness, and are preferably reduced as much as possible.
The upper limit of P is limited to 0.025%, and the upper limit of S is limited to 0.015%, as the amounts of material deterioration that are not large and are allowable.
The above is the reason for limiting the basic components of the steel material of the present invention. In the present invention, Cr, Ni, Mo, Cu, Ti, V, Nb, Z
One, two or more of r, Ta, W, and B can be contained. Both Cr and Mo are effective elements for improving the strength of the base material, but are required to be 0.1% for producing a clear effect.
The content is required to be not less than 01%. On the other hand, if added in excess of 1.0%, the toughness and weldability tend to be deteriorated.

Ni is a very effective element that can simultaneously improve the strength and toughness of the base material, but must be contained at 0.01% or more in order to exert its effect. When the content is increased, the strength and toughness are improved, but the effect is saturated even if added over 6%, but the weld metal of the laser welded portion and the hardness of the weld heat affected zone become excessively high, and the weldability is increased. Because of deterioration, the upper limit is set to 6%. Further, if the addition exceeds 6%, the hardenability becomes excessive, and the formation of ferrite is suppressed, so that an undesired effect on ultrafine graining of ferrite also becomes apparent.

Next, Cu also has substantially the same effect as Ni, but if it exceeds 1.5%, there is a problem in hot workability. Therefore, it is limited to the range of 0.01 to 1.5%. Ti contributes to the improvement of the base metal strength by precipitation strengthening, and is an element effective in reducing the austenite grain size by formation of TiN, and is also effective in improving the toughness. A content of 0.003% or more is required.
On the other hand, if it exceeds 0.1%, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so the upper limit is made 0.1%.

V is an element effective for improving the strength by forming VN. However, if it is contained excessively, the toughness is deteriorated due to precipitation embrittlement. Therefore, the range in which the effect can be exhibited without causing significant deterioration in toughness is limited to the range of 0.005 to 0.5%. Nb is an element effective for improving strength and toughness by forming Nb (C, N). However, if Nb is excessively contained, toughness deteriorates due to precipitation embrittlement. Therefore, the range in which the effect can be exhibited without inducing the deterioration of toughness is set to 0.003 to 0.
Limit to 1% range.

Zr is also an element forming nitride, and Tr
It has the same effect as i, but it requires 0.003% or more to exhibit the effect. On the other hand, 0.1
%, Coarse precipitates and inclusions are formed to deteriorate toughness and ductility, similarly to Ti.
Limit to 1% range. Ta is also an element effective in improving the strength and toughness, but is required to have an effect of 0.005 to exhibit the effect.
% Or more is required. On the other hand, if it exceeds 0.2%, precipitation embrittlement and toughness degradation due to coarse precipitates and inclusions occur, so the upper limit is made 0.2%.

W is effective for increasing the strength of the base material by solid solution strengthening and precipitation strengthening.
01% or more is required. On the other hand, if the content exceeds 2%, the toughness deteriorates significantly, so the upper limit is set to 2%. B is an element effective for improving toughness by fixing solid solution N and improving strength and toughness by improving hardenability, because it is surely linked to N in a trace amount.
03% or more is required. On the other hand, if it is contained in excess of 0.002%, BN becomes coarse, which adversely affects ductility and toughness. In addition, the upper limit is set to 0.0
02%.

Further, in order to improve ductility and joint toughness, one or more of Mg, Ca and REM can be contained as necessary. Mg, Ca, REM
Are effective for improving ductility by suppressing the elongation of sulfide during hot rolling. It works effectively to improve the joint toughness by making the oxide finer. The lower limit contents for exhibiting the effect are 0.0005% for Mg and Ca, REM
Is 0.005%. On the other hand, if it is contained excessively, sulfides and oxides are coarsened and ductility and toughness are deteriorated. Therefore, the upper limits are respectively 0.01% for Mg and Ca and 0.1% for REM.

[0034]

The above has been a description of the requirements of the present invention. The effects of the present invention will be further shown based on examples. Using steel pieces having the chemical composition shown in Table 1, ultrafine-grained steel was manufactured by the methods shown in Tables 2 and 3. In Table 1, billet numbers 13 to 15 are those whose chemical compositions do not satisfy the present invention. The methods in Tables 2 and 3 are described in JP-A-7-17-1.
No. 26798 and Japanese Patent Application Laid-Open No. 7-126797, which are based on the method for producing a superficial layer or a super-thin ultra-fine grain steel.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

Table 4 shows the proportion of the ultrafine grain layer of the manufactured steel sheet,
It shows the ferrite grain size of the ultrafine grain layer, the material of the steel sheet, and the like. The ratio of the ultrafine-grain layer is an average value obtained by observing the optical microstructure in the cross-sectional direction at any 10 places of the steel sheet. In this case, the value is obtained by observing 5 to 10 visual fields with a scanning electron microscope with a magnification of 5,000 times. Tensile properties, 2 mm V notch Charpy impact properties were obtained at the center of the sheet thickness in the direction perpendicular to the rolling direction (C direction), and temperature gradient type ESSO characteristics (K _Ca was 600 kgf / mm ^1.5
Is the value measured in the C direction.

[0039]

[Table 4]

From Table 4, it can be seen that the ultrafine grain structure having the chemical composition of the present invention and having an average ferrite grain size of 3 μm or less is present in each of the front and rear surfaces of the steel sheet in the thickness direction from the surface to 10 to 50% of the sheet thickness. The steel plate has a very good material. In particular, it is excellent in toughness and crack propagation stopping characteristics (arrest characteristics) of the base material. On the other hand, it is clear that the steel sheets of steel numbers B1 to B3 whose chemical compositions do not satisfy the present invention are significantly inferior in toughness and arrest characteristics even to the ultrafine grain structure as compared with the steel materials A1 to A12 of the present invention.

Table 5 shows the results of examining the joint characteristics together with the laser welding conditions, in order to evaluate the material as the joined body, to produce a butt joint by laser welding the steel plates of Table 4, and to show the joint characteristics. In laser welding, laser-cut end faces were butt-welded. Welding machine is 20kW CO
_Two laser welding machines were used. In order to maintain the excellent properties of ultrafine-grained steel even at joints, the weld metal width of laser welding needs to be 5 mm or less. However, in the examples, the root gap and the laser welding conditions are set so that no defects occur. Was appropriately changed to adjust the width of the weld metal. When a filler metal is used, C: 0.06%, Si: 0.1%, M
n: 1.4%, P: 0.01%, S: 0.003%, N
i: A filler wire drawn from a high Ni-based steel having a composition of 3.5% was used.

The characteristics of the joined body were evaluated by a joint tensile test, a joint 2 mm V notch Charpy impact test, and a hybrid ESSO test. In the joint Charpy impact test, the notch position is from the center of the plate thickness to the weld metal center and Fusion Lin.
e (FL). Hybrid E
In the SOO test, a test piece 1 shown in FIG. 1 was prepared, and a brittle crack generated from the approaching plate (S45C) 3 at a test temperature of −120 ° C. was caused to penetrate into the laser welded portion 2, and this brittle crack was The arrest characteristics of the zygote were evaluated based on whether it propagated or not. S45C was used for the bridging plate 3 so that brittle cracks were easily generated and propagated, and welding between the bridging plate and the test joint was performed by GMAW welding 4 having a heat input of about 2 kJ / mm. Reference numeral 5 denotes a notch, and reference numeral 6 denotes a side groove.

[0043]

[Table 5]

As is clear from Table 5, the joint structure (test numbers C1 to C5) produced according to the present invention has good joint strength and joint Charpy impact characteristics, and can be used even at a very low temperature of -120 ° C. It is evident that the alloy has sufficient arrest characteristics and retains the good characteristics of ultrafine-grained steel as a bonded structure. On the other hand, in the case of the test numbers D1 to D5 of the comparative examples, since the joints do not satisfy the present invention, the characteristics are inferior to those of the present invention.
In particular, even in Test Nos. D4 and D5 in which joints were manufactured using extremely good steel materials, the characteristics after joining were significantly deteriorated from the characteristics of the steel materials. It is clear that the extremely excellent properties of the grain steel can be effectively used.

[0045]

As described above, according to the present invention,
It is possible to produce a bonded structure without impairing the extremely good toughness and crack propagation arrest characteristics of ultrafine-grained steel, and as a result, it is possible to dramatically improve the safety of the structure The industrial effect is extremely large.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a view showing a shape and a preparation procedure of a hybrid ESSO test for evaluating a crack propagation stopping characteristic of a laser welded joint.

[Explanation of Signs] 1 Specimen 2 Laser welded part 3 Approaching plate 4 GMAW welding 5 Notch 6 Side groove

Claims (5)

[Claims] C .: 0.01 to 0.2%, Si: 0.01 to 1%, Mn: 0.1 to 2%, Al: 0.001 to 0.1%, N : 0.001 to 0.01%, and further, as impurities, P: 0.025% or less, S: 0.015% or less, the balance being iron and unavoidable impurities, and the average particle size is 3 μm. A steel sheet having the following ultrafine grained ferrite structure in each of the front and rear surfaces of the steel sheet in the thickness direction from the surface to 10 to 50% of the sheet thickness is laser-welded so that the width of the weld metal is 5 mm or less. Made of ultrafine-grained steel. 2. In% by weight, C: 0.01 to 0.2%, Si: 0.01 to 1%, Mn: 0.1 to 2%, Al: 0.001 to 0.1%, N : 0.001 to 0.01%, and further, as impurities, P: 0.025% or less, S: 0.015% or less, the balance being iron and unavoidable impurities, and the average particle size is 3 μm. A steel sheet having the following ultrafine grained ferrite structure in each of the front and rear surfaces of the steel sheet in the thickness direction from the surface to 10 to 50% of the sheet thickness has an average grain size of 5 μm in a heat-affected zone generated in the steel sheet by welding or joining.
A joint made of ultrafine-grained steel, which is welded or joined so that the width of the above region is 0.5 μm or less. 3. The steel sheet according to claim 1, further comprising Cr: 0.01 to 1%, Ni: 0.01 to 6%, Mo: 0.01 to 1%, Cu: 0.01 to 1% by weight. 0.5%, Ti: 0.003-0.1%, V: 0.005-0.5%, Nb: 0.003-0.1%, Zr: 0.003-0.1%, Ta: 3. The composition according to claim 1, further comprising one or more of 0.005 to 0.2%, W: 0.01 to 2%, and B: 0.0003 to 0.002%. A joint made of the ultrafine-grained steel according to any one of the above. 4. The steel sheet may further comprise one or more of: 0.0005-0.01% Mg: 0.0005-0.01%, REM: 0.005-0.10% by weight. The joint made of ultrafine-grained steel according to any one of claims 1 to 3, wherein the joint comprises two or more types. 5. A joint structure made of ultrafine-grained steel, comprising one or more joints according to claim 1.