JPH11293398A - Hot rolled steel plate suited to high density energy beam welding, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide superior high density energy beam weldability and excellent formability after welding by controlling the chemical composition of a steel and providing a steel-plate structure containing a base phase having a specific grain size and a secondary phase having a specific area ratio. SOLUTION: The steel has a composition consisting of, by weight, 0.001-0.20% C, 0.005-1.5% Si, 0.05-3.5% Mn, 0.005-0.15% P, <=0.02% S, 0.005-0.2% Al, <=0.02% N, further one or more kinds selected from 0.005-0.20% Nb, 0.005-0.20% Ti, and 0.005-0.20% V, and the balance Fe. The structure of steel contains a base phase of <=15 μm average crystalline grain size and a secondary phase of <=20% in an area ratio. Hot rolling is applied to the steel stock of this composition at the finishing temperature of >=800 deg.C and coiling temperature of >=500 deg.C to form a hot rolled plate having >=10% uniform elongation and >=5% local elongation. If necessary, 0.0005-0.005% B and 0.005-0.20% of one or more kinds among Cu, Ni, Cr, and Mo are incorporated into the steel composition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a hot-rolled steel sheet suitable for high-density energy beam welding, and more particularly to a hot-rolled steel sheet having excellent high-density energy beam weldability and excellent press formability after welding. The steel sheet in the present invention,
It shall include steel sheets and steel strips.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of preserving the global environment, there has been an active movement to regulate the emission of carbon dioxide gas, and attention has been focused on improving fuel efficiency by reducing the weight of automobiles. In order to reduce the weight of automobiles, it is effective to reduce the thickness of steel sheets, which account for a large proportion of the body of the automobile, and the development of high-strength automotive steel sheets has been promoted in order to reduce the thickness of steel sheets. ing. However, a thin high-strength steel sheet cannot be simply applied to a part where high rigidity of a part is required. In addition, if the thickness of another member is adapted to the thickness of a portion where high rigidity is required, the weight of the formed member becomes unnecessarily heavy.

In order to solve such a problem, conventionally, individually press-formed members are assembled by spot welding to form constituent members. However, recently, materials having different thicknesses and different materials are formed into a flat shape in a part shape. A technique called so-called tailored blank has been developed in which blanks (shear punching or shearing out) are combined, joined by high-density energy beam welding such as a laser beam, and then press-formed. The technical contents are introduced, for example, in the Preprints of the Society of Automotive Engineers of Japan, Academic Lecture Meeting 901, 1990-5, P245. As an improved technique, for example, Japanese Patent Application Laid-Open No. 63-168286 discloses a metal plate having a different thickness using laser beam butt welding without adding a filler material.
After tilting the direction of the laser beam with respect to the plate surface and welding one of the directional components of the laser beam toward the cutting edge of the thicker metal plate, press-forming the welded metal plate
A method for manufacturing a molded member to be subjected to deep drawing is disclosed.

By applying this technique called a tailored blank, it is possible to prevent an extra increase in plate thickness and to achieve a significant reduction in weight. For example, only a portion where high rigidity is required can be made thick, and other portions where high rigidity is not required can be made thin by applying a high-tensile steel plate. However, in applying this tailored blank technique, poor welding at the welded portions of members has been a problem from the viewpoint of appearance and formability. To deal with this problem of appearance, for example, Japanese Patent Application Laid-Open No. 5-84585 discloses a method in which a plurality of sheared thin steel plates are butted with the same direction of burrs, and a filler is supplied to the sagged side of the cut surface. There is disclosed a method of manufacturing an automobile panel in which a laser beam is irradiated from the sagging side and laser-welded, and a surface opposite to the laser beam-irradiating side is used as a panel outer surface.
Regarding the problem of formability, Japanese Patent Application Laid-Open No. 5-50275 discloses an abutment interval, a distance between a laser light irradiation start position and a welding start side end face of a steel sheet, a laser light irradiation end position and a steel sheet irradiation time. Laser welding of steel plate for press forming, which performs laser welding with the distance between the end face on the welding end side, the difference between the laser light irradiation start time and the filler supply start time, and the difference between the laser light irradiation end time and the filler supply end time as predetermined ranges. A method is disclosed, which alleges that the stretch flangeability is enhanced.

[0005]

However, the present inventors have found that even in the above-mentioned welding method, when the shape accuracy of the cut surface of the steel plate to be the butt surface at the time of welding is low, it is possible to stably weld the members. It was found that there was a case where it was difficult to perform welding, and it was conceived that it was necessary to improve the shape accuracy of the butted surfaces of the members for performing high-density energy beam welding from the viewpoint of welding workability. In addition, even if they are joined without any problem in the flat plate state, when there is a problem in the soundness of the welded portion, there may be a problem such as the occurrence of press cracking in the joined portion during the subsequent press forming. Furthermore, if the heat affected zone is significantly softened, cracks and necking will occur during press forming. The problem remained.

The present invention advantageously solves the above-mentioned problems of the prior art, has excellent weldability in high-density energy beam welding, and has excellent formability after welding. The purpose is to provide steel sheets.

[0007]

Means for Solving the Problems In order to stably perform high-density energy beam welding, the present inventors first find that the shape accuracy of the cut surface of a steel plate, which is the butted surface of members, is important. I found it. In particular, in the case of a steel sheet cut to a predetermined size by shearing, the form of the fracture surface of the cut surface is important. On the other hand, a shear surface of 65% or more, preferably 75% or more, a fracture surface of 20% or less, preferably 15% or less, a "whore" of 15% or less, preferably 10% or less, and a "burr" of 10% or less It was found that it would be better to use a fractured form consisting of In general, as shown in FIG. 1, the fracture surface when a steel sheet is cut by shearing is as follows.
It consists of a shear plane followed by a fracture plane, a droop on the shear start side, and a burr on the shear end side.

Further, the inventors of the present invention performed shearing with varying shearing conditions on a hot-rolled steel sheet which had been strengthened by combining various strengthening mechanisms, observed the fracture surface morphology of the cut portion, and under normal shearing conditions. The material structure for forming a fracture surface suitable for the welding operation of the high-density energy beam welding described above was studied. As a result, it is necessary to regulate the chemical composition of steel as a material and to have a uniform and fine structure. Specifically, the structure of the steel sheet is made up of a matrix with an average crystal grain size of 15 μm or less, and an area ratio of 20% It was found that it is better to use a tissue containing the following second phase.

In order to improve the characteristics of a high energy beam welded part, the present inventors conducted high-density energy beam welding on a hot-rolled steel sheet having a high strength by combining various strengthening mechanisms. Part characteristics were studied. As a result, by adding Nb, Ti, V alone or in combination, and using precipitation strengthening by these carbonitrides, or by further increasing the strength using Cu, Ni, Cr, Mo and B, It has been found that even when high energy beam welding is performed, there is no deterioration in the strength and ductility of the welded portion and the heat affected zone, and there is no softening of the heat affected zone, and the press formability is significantly improved.

The present invention has been completed based on the above findings. That is, the present invention provides a method for preparing C:
0.001 to 0.20%, Si: 0.005 to 1.5%, Mn: 0.05 to 3.5
%, P: 0.005 to 0.15%, S: 0.02% or less, Al: 0.005%
0.20.2%, N: 0.02% or less, Nb: 0.005 00.20
%, Ti: 0.005 to 0.20%, V: 0.005 to 0.20%, one or two or more selected from the group consisting of a balance of Fe and unavoidable impurities, and a base having an average crystal grain size of 15 μm or less. High-density energy beam welding, comprising a phase and a structure containing a second phase having an area ratio of 20% or less, and having a uniform elongation of 10% or more, preferably 20% or more, and a local elongation of 5% or more. It is a suitable hot-rolled steel sheet. In addition to the above composition, B: 0.0005 to 0.005% and one or two or more kinds selected from Cu, Ni, Cr, and Mo in a total of 0.005 to 0.005%
It may contain at least one of 0.20%.

Further, the present invention relates to a method for producing a composition, in which Nb: 0.005
0.20%, Ti: 0.005 to 0.20%, V: 0.005 to 0.20%. A steel material containing at least one selected from the group consisting of a finish rolling temperature of 800 ° C or more and a winding temperature of 500%.
Hot-rolled steel sheet suitable for high-density energy beam welding, characterized in that the hot-rolled steel sheet is hot-rolled to at least 100 ° C and has a uniform elongation of 10% or more, preferably 20% or more, and a local elongation of 5% or more. It is a manufacturing method of. In the present invention, rapid cooling is preferably started within 2 seconds after the finish rolling of the hot rolling, and preferably at a cooling rate of 30 ° C./s or more.
It is preferable to quench to 750 ° C., then slowly cool for preferably 3 to 20 seconds, and then quench to the winding temperature, preferably at a cooling rate of 30 ° C./s or more. In the present invention,
The above composition is expressed in terms of% by weight: C: 0.001 to 0.20%, Si: 0.00
5 to 1.5%, Mn: 0.05 to 3.5%, P: 0.005 to 0.15%,
S: 0.02% or less, Al: 0.005 to 0.2%, N: 0.02% or less, Nb: 0.005 to 0.20%, Ti: 0.005 to 0.20%,
V: It is preferable that the composition contains one or more selected from 0.005 to 0.20% and has a balance of Fe and unavoidable impurities. In the present invention, in addition to the above composition, B : 0.0005-0.005%, Cu, Ni, Cr, Mo
0.005 in total from one or more selected from
To 0.20%.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The high-density energy beam in the present invention refers to a laser beam, an electron beam, a plasma beam and the like. When the hot-rolled steel sheet according to the present invention is used as each member and the members are formed by high-density energy beam welding between the members, the subsequent forming is facilitated.

First, the reasons for limiting the chemical composition in the present invention will be described. Nb: 0.005 to 0.20%, Ti: 0.005 to
0.20%, V: One or more selected from among 0.005 to 0.20% Nb, Ti, and V are effective elements for making the steel sheet structure uniform and fine, and each of them is added by 0.005% or more. The effect is recognized. These elements can be added alone or in combination of two or more. However, when any of the elements is added in excess of 0.20%, the effect saturates,
An effect commensurate with the amount added cannot be expected. Therefore, Nb, T
i and V are each limited to the range of 0.005 to 0.20%. In addition, when it is added in combination, if the total amount of Nb, Ti, and V exceeds 0.20%, the effect tends to be saturated, which is not preferable.

Further, Nb, Ti, and V not only act as strengthening elements, but also have the effect of setting the fracture surface shape of the cut surface and the accuracy of the cut surface when the steel sheet is sheared in a range suitable for high-density energy beam welding. have. For steel sheets precipitation-strengthened with Nb, Ti, and V, even when high-density energy beam welding is performed, the hardness distribution of the welds becomes a relatively flat distribution,
There is no extremely hardened or softened part, and there is little decrease in ductility of the welded part. Therefore, even during press forming after welding, the strain distribution becomes substantially uniform across the welding line, and the formability of the welded portion is significantly improved.

C: 0.001 to 0.20% C is an important element for securing the strength of the steel sheet.
If the C content is less than 0.001%, the crystal grains in the heat affected zone become coarse, and cracking and necking occur during press forming after welding. On the other hand, if the C content exceeds 0.20%, the weld is hardened remarkably and ductility is deteriorated, so that cracks occur during press forming after welding, and the formability is deteriorated. Therefore, C is 0.00
Limited to 1-0.20%. The content is preferably 0.002 to 0.15% from the viewpoint of further improving moldability.

Si: 0.005 to 1.5% Si is an element effective for strengthening the steel sheet while minimizing the decrease in ductility of the steel sheet. This effect is observed at 0.005% or more. However, if the addition exceeds 1.5%, the strength of the steel sheet is significantly increased, so that the load in a steel sheet manufacturing process such as hot rolling is large, which is an obstacle to the manufacture. For this reason, Si is limited to the range of 0.005 to 1.5%.
In addition, from the viewpoint of weldability, the content is preferably limited to 0.05 to 1.0%.

Mn: 0.05-3.5% Mn is an element effective for refining the structure of a steel sheet and forming a low-temperature transformation structure. Such an effect is observed when the addition is 0.05% or more, but when the addition exceeds 3.5%, the effect is saturated, and the load of a steel sheet manufacturing process such as hot rolling increases. For this reason, Mn was limited to 0.05 to 3.5%.

P: 0.005 to 0.15% P is an element effective for solid solution strengthening of a steel sheet.
To achieve this effect, 0.005% or more must be added. On the other hand, if added in excess of 0.15%, the weldability of the steel sheet decreases. Therefore, P is limited to the range of 0.005 to 0.15%. In addition, it is preferably 0.01 to 0.10%.

S: 0.02% or less S is preferably reduced as much as possible to reduce the ductility of the steel sheet. From the viewpoint of ensuring ductility, it is allowable up to 0.02%. If high ductility is required, 0.01
It is preferably set to 0% or less. Al: 0.005 to 0.2% Al acts as a deoxidizing element, and the addition of 0.005% or more can sufficiently reduce the amount of oxides in steel. When added in excess of 0.2%, alumina clusters are formed, surface defects are frequently generated, and hot ductility is reduced. Therefore, Al is 0.005 to
Limited to the range of 0.2%. In addition, from the viewpoint of surface properties
It is preferably in the range of 0.005 to 0.15%.

N: 0.02% or less N is an element that forms a solid solution in steel and increases the strength of the steel sheet. However, since N deteriorates aging resistance, it is added within a range that does not deteriorate aging resistance to increase the strength. Can be achieved. In addition, excessive addition causes blowholes on the steel sheet surface, so N is limited to 0.02% or less. For applications requiring ductility, N is preferably set to 0.007% or less.

B: 0.0005% to 0.005% B is an element that increases the hardenability and is a useful element for forming a bainite structure as the second phase, and can be added as necessary. As the second phase, a structure having a bainite structure is effective for improving high-density energy beam weldability and formability after high-density energy beam welding. B
If it is less than 0.0005%, the above effects cannot be expected. On the other hand, if it exceeds 0.005%, the surface properties of the steel sheet deteriorate. For this reason, B is preferably set in the range of 0.0005 to 0.005%. The content is more preferably 0.0010 to 0.0020% from the viewpoint of stabilizing the material.

One or more selected from Cu, Ni, Cr and Mo in a total amount of 0.005 to 0.20% Cu, Ni, Cr and Mo are elements that increase the hardenability,
One or more of these elements can be added as necessary to increase the strength of the steel sheet. Cu, Ni, Cr, and Mo are effective when added at 0.005% or more, respectively.
%, Hardening of the steel sheet becomes remarkable, and high-density energy beam weldability and formability after high-density energy beam welding deteriorate. Therefore, Cu, Ni, Cr, and Mo are each preferably in the range of 0.005 to 0.20%. In addition, when adding in a combined manner, if the total amount exceeds 0.20%, the high-density energy beam weldability and the formability after high-density energy beam welding deteriorate. Therefore, the total amount of each element is preferably limited to 0.20% or less.

Next, the reasons for limiting the organization will be described. Mother phase: average crystal grain size of 15 μm or less The hot-rolled steel sheet of the present invention has a structure in which ferrite is a parent phase, and the area ratio of the parent phase is preferably at least 80% or more in order to ensure formability. The average crystal grain size of the parent phase is 15
μm or less. If the average crystal grain size of the parent phase exceeds 15 μm, when the steel sheet is cut by shearing, dripping on the fracture surface and formation of burrs become remarkable, and the area ratio of the fracture surface increases, and the accuracy of the cut surface increases. As a result, the high-density energy beam welding cannot be performed stably. The average crystal grain size is
Average of the total thickness in the thickness section parallel to the rolling direction (however,
Excluding from the outermost surface up to 100 μm).

Second phase: not more than 20% in area ratio It is preferable that the second phase be substantially bainite, martensite, or a mixture thereof from the viewpoint of achieving both strength and ductility. In addition, if any of the above is included, the mixture of retained austenite or pearlite is also allowed. When the area ratio of the second phase is more than 20%, ductility is reduced, and the state of the processed surface is significantly deteriorated particularly when shearing is performed with a small clearance in order to obtain a high-precision shearing surface shape. Become like The second phase is preferably formed at an area ratio of 5% or more from the viewpoint of improving strength.

With the above composition and structure, the hot-rolled steel sheet has a uniform elongation of 10% or more and a local elongation of 5% or more, and is excellent in ductility and formability, and is suitable for high-density energy beam welding. It shows excellent shearing fracture surface morphology, excellent high density energy beam welding properties and formability after high density energy beam welding. The ratio of each phase is the thickness
S of thickness section parallel to rolling direction at 1/4 to 1/2 depth
It was measured by an EM image.

Next, the conditions for producing the hot-rolled steel sheet of the present invention will be described. The molten steel having the above-described composition is smelted by a commonly known smelting method such as a converter or an electric furnace, and is made into a steel material using a solidifying means such as a continuous casting method. The steel material is then subjected to hot rolling without or with heating. The heating temperature for hot rolling is not particularly limited, but may be 1000 to 1300
C. is preferred. If it is less than 1000 ° C., it is difficult to obtain a predetermined finish rolling temperature, and if it exceeds 1300 ° C.,
The crystal grains are remarkably coarsened.

In the present invention, in order to make the average crystal grain size of the parent phase 15 μm or less, in addition to the steel composition, the finish rolling temperature of the hot rolling conditions is set to 800 ° C. or more. C or higher. Finish rolling temperature of hot rolling is 800 ℃
If it is less than 1, it becomes difficult to obtain a uniform and fine structure, and there is a problem that the fracture surface after shearing becomes non-uniform. Further, the amount of the second phase and the amount of carbide cannot be stably controlled. In addition, it is preferable that the rolling reduction of the hot rolling is 80% or more in total in the temperature range of 1000 ° C. to 850 ° C. from the viewpoint of uniformity and fineness of the final product structure.

Winding temperature: 500 ° C. or more After hot rolling to give a hot rolled sheet, winding is performed at 500 ° C. or more, preferably 700 ° C. or less. When the winding temperature exceeds 700 ℃,
The generation of scale is remarkable, the pickling efficiency is deteriorated, and the material varies in the longitudinal and width directions of the coil. On the other hand, if the winding temperature is lower than 500 ° C., the shape of the steel sheet is deteriorated.

The hot-rolled sheet which has been hot-rolled and wound up as described above may be subjected to temper rolling or the like. The temper rolling is preferably performed at a rolling reduction of 5% or less from the viewpoint of material stability. When the rolling reduction exceeds 5%, the yield point rapidly increases, and the problem that the ductility rapidly decreases occurs. In order to precipitate carbides of Ti, Nb, and V and obtain a second phase (substantially martensite or bainite or a mixture thereof) of 20% or less, the cooling pattern after hot rolling is controlled. Is preferred. Specific conditions vary depending on the steel composition and the target material (strength, ductility, etc.), but after rolling, rapidly cool to 600 to 750 ° C (pre-cooling), and then slowly cool for preferably 3 to 20 seconds (for example, air cooling). ), And then rapidly cooled to the winding temperature (post-stage cooling). A cooling rate of 30 ° C./s or more is preferable for both the first stage cooling and the second stage cooling, and the first stage cooling is preferably started within 2 seconds after the hot rolling.

If the pre-stage cooling or the post-stage cooling is insufficient, the possibility that the area ratio of the second phase becomes excessive increases. On the other hand, if the cooling temperature range is out of the range of 600 to 750 ° C., it may not be possible to obtain carbide precipitation sufficient to secure weldability. When the scale on the hot rolled sheet surface is peeled off, pickling may be performed. The pickling performed for the purpose of peeling the scale of the hot-rolled sheet may be performed under generally known conditions. For example,
It is preferred to use 60 DEG C.-3-15% hydrochloric acid.

These hot rolled sheets may be used as they are, or may be further subjected to hot rolled sheet annealing if necessary, and used as a base sheet for cold rolled sheets.

[0032]

EXAMPLES (Example 1) Molten steel having the chemical composition shown in Table 1 was used.
It was melted in a converter and cast into a slab by a continuous casting method. After heating these slabs to the slab heating temperature shown in Table 2, hot rolled steel sheets having a thickness of 1.8 mm and 1.4 mm were obtained under the hot rolling conditions shown in Table 2. Almost the same structure and mechanical properties were confirmed for steel plates of any thickness. In addition, uniform elongation and local elongation were measured from a stress-strain curve when a tensile test was performed on these steel sheets as original sheets. After pickling, the hot-rolled steel sheets were sheared to have a knife clearance of 5% of the sheet thickness to form steel sheets of a predetermined size, and the cut fracture surfaces were observed. The evaluation of the fracture surface morphology was as follows: ○: good, Δ: slightly poor, ×: poor,
I went in. ○ (Good) indicates that the shear surface ratio is 75% of the total thickness
As mentioned above, the fracture surface ratio is 15% or less, and who and the burrs are 10% or less. %, X (poor) indicates less than 65% shear surface ratio, fracture surface ratio
It is considered to be one of the cases exceeding 20%, anyone exceeding 15%, and returning more than 10%.

The sheared surfaces of these hot-rolled steel sheets were butted and formed into a forming member by butt welding using a carbon dioxide gas laser. The laser welding conditions are as follows. Laser output: 4.0kW Welding speed: 5m / min Focal length of condenser lens: 250mm Focus position: -0.5mm (Positive inside the plate from the plate surface) Shield gas: Ar Shield gas flow: 20 l / min Butt welding was performed under the conditions described above, and the stability of the construction was investigated. The stability of the welding operation was evaluated by observing the external appearance of the entire welding length of 500 mm, and those with undercuts and holes were rated as x (defect), and those without were rated as good (good).

Further, a hardness test piece was sampled from the welded portion of the forming member, and a hardness test of the welded portion was conducted to investigate a hardness distribution of the welded portion. In addition, a hydraulic bulge test was performed to investigate the formability of the weld. The hydraulic bulge test was performed under the following conditions. Overhang diameter: 150mm constant Welded part: Place in the center of overhang circle This method can evaluate deformation characteristics of welded joint correctly because there is no uneven deformation due to friction. The formability of the weld was evaluated by plastic strain just before breaking, that is, deformation limit strain. The bulge processing was performed on each of 10 samples, and the critical strain was compared with the comparative value (a plate without welding (1.4 mm
The case where even one sheet was significantly lower (less than 1/2) than the deformation limit strain) in the thickness) was evaluated as x. Table 2 shows the results.

[0035]

[Table 1]

[0036]

[Table 2]

From Table 2, it can be seen that the example of the present invention has a desired form of the fracture surface in the shearing process, and thus has high stability in welding. Also, the softening of the weld heat affected zone is not excluded except steel plate No.15, and the hardness of the welded portion is appropriate without excessive hardness.It has excellent high-density energy beam weldability. No occurrence was observed, and the moldability was good. Note that the steel sheet No. 15 of the present invention had a so-called pre-quenching type cooling pattern in which the cooling rate after cooling was reduced by eliminating the slow cooling part in cooling after hot rolling. Although the former and latter quenching types are more likely to obtain better characteristics, high-energy beam weldability can be improved if the cooling conditions of the former quenching type are optimized according to the steel composition.

Example 2 From the steel sheets shown in Table 2, Table 3
The original sheet 1 and the original sheet 2 were selected from the combination of the steel sheets shown in (1) and (2) were butt-welded using a carbon dioxide laser under the same conditions as in Example 1 to produce a forming member. In the same manner as in Example 1, welding workability, weld hardness, and press formability by a hydraulic bulge test were investigated for these members, and high-density energy beam weldability was evaluated. Table 3 shows the results. In addition, as a comparative value of the critical strain in bulging after welding,
The lower value of the two types of welded original plates was adopted. The presence or absence of a softened portion was also evaluated based on the lower base material hardness. All combinations of the steel sheets of the present invention showed good results. In addition, the combination of the steel sheet of the present invention and the comparative steel was better than the welding of the comparative steels. That is, there was no problem in press molding with the number of samples tested (10). However, since the softened part was slightly observed, the overall evaluation was evaluated as “△”.

[0039]

[Table 3]

[0040]

According to the present invention, the weight of an automobile body can be reduced and the yield of members can be improved without impairing the formability, and the industrial effects are remarkably improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a fracture surface of a steel sheet cut surface formed by shearing.

Claims (5)

[Claims] C. 0.001 to 0.20%, Si: 0.005 to 1.5%, Mn: 0.05 to 3.5%, P: 0.005 to 0.15%, S: 0.02% or less, Al: 0.005 to 0.2%, by weight%. N: 0.02% or less, Nb: 0.005 to 0.20%, Ti: 0.005 to 0.20%, V: 0.005
Of at least one selected from the group consisting of Fe and unavoidable impurities, a mother phase having an average crystal grain size of 15 μm or less, and a second phase having an area ratio of 20% or less.
A hot-rolled steel sheet suitable for high-density energy beam welding, comprising a structure containing a phase and having a uniform elongation of 10% or more and a local elongation of 5% or more. 2. In addition to the above composition, B: 0.0005 to
2. The heat according to claim 1, wherein the heat content is at least one of 0.005% and 0.005 to 0.20% in total of one or more selected from Cu, Ni, Cr and Mo. Rolled steel sheet. 3. Nb: 0.005 to 0.20% by weight, Ti: 0.
005 to 0.20%, V: 0.005 to 0.20% steel material with a composition containing one or more selected from the group consisting of heat treatment with a finish rolling temperature of 800 ° C or more and a winding temperature of 500 ° C or more. A method for producing a hot-rolled steel sheet suitable for high-density energy beam welding, wherein hot-rolled steel sheet is subjected to cold rolling to have a uniform elongation of 10% or more and a local elongation of 5% or more. 4. The composition has the following composition by weight: C: 0.001 to 0.20%, Si: 0.005 to 1.5%, Mn: 0.05 to 3.5%, P: 0.005 to 0.15%, S: 0.02% or less, Al: 0.005% 0.2%, N: 0.02% or less, Nb: 0.005 to 0.20%, Ti: 0.005 to 0.20%, V: 0.005
The method for producing a hot-rolled steel sheet according to claim 3, wherein the composition contains one or two or more kinds selected from the group consisting of -0.20% and a balance of Fe and unavoidable impurities. 5. In addition to the above composition, B: 0.0005 to
5. The method according to claim 3, wherein the composition contains at least one of 0.005% and at least one of at least one selected from Cu, Ni, Cr and Mo in an amount of 0.005 to 0.20%. Production method of hot rolled steel sheet.