JPH0673440A - High workability steel sheet excellent in high strengthening characteristic by irradiation with high density energy source - Google Patents

Abstract

PURPOSE:To develop a steel sheet having workability and excellent strength by irradiating a low carbon steel sheet having excellent workability and the specific composition with a high density energy source and forming the solidified zone after its melt-penetration of the steel sheet. CONSTITUTION:By applying a high energy laser beam irradiation, etc., to the cold rolled steel sheet containing, by weight, 0.02-0.3% C, <1.5% Si, <2.5% Mn and >=0.01 K1 value expressed by (Mn% + 0.25Si%) X C% and having mainly ferrite structure coexisted with pearite and cementite structures or containing <2.5% Cr, <1.0% Mo, <50 ppm B and >= 0.05 K2 value expressed by (Mn% + Cr% + Mo% + 250B% + 0.25Si%) X C%, and further, containing one or more kinds of the specific quantities of Cu, Ni, P, Nb, Ti, Zr, V and W as the alloy metal elements in both of them, the melt-penetrated and solidified zone is formed to the steel sheet to produce the steel sheet having excellent workability and strength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet which has excellent working characteristics during working and which can be used after being strengthened by irradiation from a high-density energy source. In the following description, the members, which are one of the members for automobiles, will be described as a representative example, but the application target of the steel sheet of the present invention is not limited by this, and is a field in which both the above characteristics are required. Can be widely used for.

[0002]

2. Description of the Related Art Automotive members, especially members and the like, are required to have two contradictory characteristics, that is, workability and strength. In other words, it is necessary to have excellent workability in order to arrange the members so that they follow the smooth curve of the car body. From the standpoint of exerting an excellent protective action, it is necessary that the prescribed part be strengthened to the desired strength. Therefore, a technique has been proposed in which a mild steel sheet having high workability is press-formed and then irradiated with a high-density energy source to increase the strength of a predetermined portion of the press-formed part (Japanese Patent Laid-Open No. 61-99629). . However, according to the irradiation conditions described in the above-mentioned patent publication, for example, when laser irradiation is performed from a high intensity energy source, the degree of thermal influence in the plate thickness direction becomes uneven and distortion occurs in the shape, and after laser processing Practical use has been hindered in that the shape correction of No. 1 is required and the number of irradiations required for laser processing is very large, and the total processing time becomes long.

The present technology, which is basically composed of such press molding and subsequent laser hardening treatment, is characterized in that after the parts are pressed in the press line, irradiation with a high-density energy source is performed, but they have been studied so far. In the range that has been set, how to devise the combination of the irradiation condition by the high-density energy source and the target steel structure can reduce the strain and obtain a sufficient increase in strength. No knowledge has been obtained.
Therefore, it has been earnestly desired to establish a knowledge about a preferable combination of a treatment condition with a high-density energy source and a steel structure. In other words, it has been desired to develop a material steel sheet that has sufficient workability during press forming, and whose strength can be significantly increased by processing with a high-density energy source after working.

Japanese Unexamined Patent Publication (Kokai) No. 4-72010 also discloses a technique for increasing the strength by irradiating a press-formed product with a laser. In this patent publication, it is shown that the increase in strength can be obtained by performing the laser treatment using a carbon steel sheet. However, in this patent publication,
Regarding the steel sheet composition, it only refers to the amount of carbon, and does not refer to the alloy components other than carbon or the microstructure of the steel sheet at all, and therefore the relationship between the alloy components and microstructure and the laser processing conditions, as well as their strength. No knowledge has been obtained regarding the relationship between the amount of increase. The research conducted by the present inventors has revealed that the increase in strength during laser processing largely depends not only on the laser processing conditions but also on the alloy composition and structure. Therefore, it was necessary to clarify this relationship in order to obtain a large increase in strength by laser processing.

Incidentally, Japanese Patent Laid-Open No. 61-261462 discloses the knowledge of a steel plate for laser processing which is excellent in workability. Here, the workability in the case of performing processing such as press forming after performing laser cutting. Is a problem. On the other hand, the present invention is intended for the curing treatment by laser irradiation, and the same laser irradiation is not intended for the cutting process as in the above-mentioned publication, and the technical field is also technical content. Is also completely different.

Further, Japanese Patent Application Laid-Open No. 1-259118 discloses a technique for performing a rapid remelting-rapid resolidifying treatment on a portion of a press material to be strengthened so as to make crystal grains finer and to have higher strength. . However, the invention disclosed in this publication only melts a portion which becomes the back surface at the time of use, and unlike the through-melting method of the present invention which will be described in detail later, a large residual strain occurs, and a sufficient strength increasing effect is obtained. I can't get it. Further, in the above-mentioned Japanese Laid-Open Patent Publication, the strengthening mechanism lies in the refinement of crystal grains, and a quenched structure is not obtained. This point is also distinguished from the present invention in which the formation of a quenched structure is a mechanism. As described above, it should be said that the conventionally known method is essentially different from the method described below as employed in the present invention.

[0007]

DISCLOSURE OF THE INVENTION The inventors of the present invention have earnestly studied the influence of the type and structure of alloying elements on the processability by a high-density energy source, and as a result, under certain irradiation conditions, steel sheets were produced. The present invention has been completed by finding out that by setting the alloy component of (1) to a specific range and defining the constitutional structure, it is possible to obtain excellent processing characteristics that cannot be obtained in the conventional steel sheet.

[0008]

The steel sheet provided by the present invention has excellent workability during processing and is capable of irradiating from a high-density energy source such as laser irradiation to penetrate the plate thickness. When it forms a large solidification region, it exhibits a sufficiently high strength, which allows it to be used in a wide range of applications, and is a high workability steel sheet excellent in high strength characteristics.

The alloy composition of the high workability steel sheet according to the present invention contains C: 0.02 to 0.3% Si: 1.5% or less Mn: 2.5% or less, with the balance being Fe and unavoidable impurities. And under the condition that the K ₁ value calculated by the following formula according to the contents of C, Si and Mn satisfies 0.01 or more (more preferably 0.05 or more), K ₁ = (Mn% + 0.25 · Si%) × C% Furthermore, by having a structure in which pearlite and / or cementite coexist in addition to ferrite, high workability before laser processing and high strength after laser processing are achieved. It has been confirmed that it exhibits more reliable and excellent effects.

The basic alloy composition in the present invention is as described above, and the high workability steel sheet of the present invention is
In addition to Si and Mn, Cr: 2.5% or less, Mo: 1.0% or less, B: 50 ppm or less may be included as an essential component. However, the calculation formula for obtaining the K ₁ value in the case of containing such an additional component is changed as follows. The K ₂ value given by the following formula is also desired to be 0.05 or more. K ₂ = (Mn% + Cr% + Mo% + 250 · B% + 0.
25 ・ Si%) × C%

Further, the high workability steel sheet of the present invention is the above-mentioned C, S.
Other than i and Mn, Cr: 2.5% or less Mo: 1.0% or less B: 50 ppm or less and / or Cu: 2.5% or less Ni: 1.5% or less P: 0 .15% or less Nb: 0.2% or less Ti: 0.2% or less Zr: 0.1% or less V: 0.1% or less W: 0.1% or less Any one or more of these are included. May be.

[0012]

First, the irradiation conditions by the high intensity energy source will be described. Although an example in which a laser is used as the high-density energy source is shown here, plasma or the like can also be used. In FIG. 1, C: 0.10%, Si: 0.01%, M
n: 0.90%, Al (unavoidable impurities due to addition as deoxidizing element): 0.032%, steel material consisting of balance Fe and unavoidable impurities as a test piece (plate thickness 1.4 mm)
Although the laser irradiation conditions were variously changed and the relationship with the intensity increase amount was shown, it can be seen that a significant intensity increase can be obtained by performing the irradiation such that the energy density is 100 J / mm ² or more. This range is a condition for forming a molten phase that penetrates the plate thickness, and under such a condition, the strength can be significantly increased. In addition, since the strain generated in the plate thickness direction is released under such a condition, the residual strain after the treatment can be suppressed to be extremely small.

Next, the relationship between the structure and the amount of increase in strength will be described. Generally, for cold working, a soft material, for example, one having a structure composed of pearlite and ferrite is used, and when a softer material is desired, one having a coarse spheroidized cementite structure is selected.

In FIG. 2, ferrite + perlite (and / or
Or cementite: Perlite is also represented below)
The relationship between the strength (tensile stress) and the amount of increase in strength by laser treatment is shown for steel and for ferrite + coarse spheroidized cementite steel. As can be seen from this figure, in the case of ferrite + pearlite steel, the amount of increase in strength at the same strength is superior to that in the case of ferrite + coarse spheroidized cementite steel. That is, from the viewpoint of the balance between the workability and the subsequent increase in strength, it can be seen that the ferrite + pearlite steel is superior. As a result of earnest research on this cause, in addition to the carbide size,
It has been found that inclusion of alloy components within a certain condition range is indispensable for improving the balance between workability and increase in strength (see Examples below).

FIG. 3 shows the relationship between the short side length of the carbide size and the strength increase amount. Regarding the size of the carbide, the cross-sectional structure was observed by SEM, and the short side (diameter in the case of a circular cross section) of the carbide was measured on the photograph. As is clear from FIG. 3, if the size of the short side of the carbide exceeds 1 μm, the strength increase amount decreases. In other words, it was found that by forming the pearlite structure, the carbide size was made finer, and the alloy composition was specified, and then an excellent balance between the workability retention and the significant strength increase could be obtained.

The cause can be considered as follows. One is that the area of the cured phase formed by laser irradiation becomes large when the above conditions are satisfied. That is, when a cross-sectional structure after laser processing was observed for a solidified phase that penetrated the plate thickness in the laser processing experiment, it was a ferrite + pearlite structure and satisfied the above conditions (FIG. 10).
(See (a)), the area of the hardened phase was large, but even if the ferrite + pearlite structure was coarsely spheroidized (see FIG. 10 (b)), the above conditions were not satisfied. Therefore, the area of the curing phase was small. That is, it is considered that when the short side size of the carbide is 1 μm or less and the alloy component is included in a certain range, difference in the penetration of the carbide or the like is caused, and as a result, the hardened area is increased.

Further, regarding the data of the steel sheet of ferrite + pearlite structure, the relation between the value obtained by the calculation formula of K ₁ = (Mn% + 0.25 · Si%) × C% and the strengthening amount is summarized, and the result is shown in FIG. It was shown to. In FIG. 4, (b) is an enlarged view of a main part of (a). As is clear from FIG. 4, when the K ₁ value is 0.01 or more, a large increase in strength is obtained. As long as the amount of strengthening by laser irradiation does not increase by 50 MPa or more, it cannot be evaluated as practically effective. However, the only increase in the amount of strengthening by less than 50 MPa is that the amount of carbon is 0.02. Less than%, and M
The amount of n is less than 0.3%. After all, as can be seen from these figures, the K ₁ value is 0.01 or more, preferably 0.0
It can be seen that setting the number to 5 or more is effective.

The essential additional elements of the present invention are C and S described above.
Although i and Mn, as will be described later, in addition to these essential elements, three elements of Cr, Mo, and B can be added as the same-effect elements. The action and effect can be shown as in FIG. That is, FIG. 5 shows the effect of addition of each element as follows: K ₂ = (Mn% + Cr% + Mo% + 250 · B% + 0.
25 · Si%) × C% and the relationship between the amount of strengthening and Cr, M
The effect of adding o and B can be summarized by the value of K ₂ .
It can be seen that when the value of K ₂ is 0.05 or more, a large increase in strength is possible.

Further, FIGS. 6 and 7 show hardness distributions of the cross section in the laser irradiation portion when the alloy composition conditions of the present invention are satisfied (invention steel) and when they are not satisfied (comparative steel). Indicated. The comparative steel (No. 4) in FIG. 6 has a K ₁ value of 0.01 or less, and although the steel structure is finished with ferrite + pearlite, it has insufficient hardenability and therefore has a sufficient hardness. Has not been obtained. Further, in the case of the comparative steel (No. 22) in FIG. 7, the maximum hardness is sufficiently obtained because the present invention is satisfied in terms of alloy composition conditions, but ferrite + coarse spheroidized cementite structure Therefore, the width of the cured area is small. This is not a phenomenon that can be understood simply from the hardenability, but it is considered that the width of the hardened zone has become smaller due to the difference in the transformation point due to the alloy components and the difference in the penetration method due to the difference in the carbide size.

FIG. 8 shows the relationship between the carbon content and the amount of increase in strength by laser treatment. It can be seen that there is variation in the amount of increase in strength even with the same amount of carbon. The reason for this is explained by the fact that not only the difference in carbon content but also the influence of other elements must be comprehensively judged.

Next, the reasons for limiting the alloy components in the steel sheet of the present invention will be described. The steel of the present invention must be particularly suitable for cold working applications such as press forming. For this purpose, the smaller the amount of C added, the more preferable. However, on the other hand, the increase in strength due to laser irradiation or the like is an important issue.
% Addition is required. For example, when the amount of C added is about 0.01%, the effect of improving the strength by laser irradiation cannot be obtained so much as described later. On the other hand, if too much C is added, the workability of the steel sheet and further the weldability are significantly deteriorated, so the upper limit of C is made 0.30%.

Si is added to improve the laser processability, but if it exceeds 1.5%, the surface will be roughened, so the upper limit was made 1.5%. Mn is an element essential for increasing the strength by laser processing, and contributes to the strength increase depending on the addition amount thereof, but if added in too much amount, the cold formability of the steel sheet is impaired, so the upper limit of the addition amount is 2.5. Defined as%. In order to effectively bring out the effect of adding Mn, it is recommended to add 0.3% or more.

The essential contained elements in the steel of the present invention are as described above, and the balance is Fe and inevitable impurities, but if desired, the following elements can be added. Cr is effective for increasing the strength by laser treatment, but it is uneconomical to add it in a larger amount than necessary, so the upper limit was made 2.5%.

Mo is effective in increasing the strength by laser treatment, but addition of a large amount impairs economic efficiency, so the upper limit is 1.0.
%. B is an element effective in increasing the strength by laser processing, but if 50 ppm or more is added, the ductility of the base material is significantly deteriorated, so the upper limit was made 50 ppm.

The above-mentioned three elements are particularly significant as those having a large addition effect and have an important influence on the above-mentioned K ₂ value. In addition to these, the following elements are further added. You can also go.

Cu has the function of ensuring the strength of the material by aging precipitation, and can improve the corrosion resistance of the base material, and is therefore an effective element for improving the characteristics of the material. However, when added in a large amount, surface defects are generated in the steel sheet, and therefore it is necessary to improve the effect by adding Ni together. Therefore, in the steel of the present invention, it is desirable to add Cu and Ni in combination, and to add Cu in an amount of 2.5% or less and Ni in an amount of 1.5% or less in consideration of the cost.

Although cold workability can be improved by reducing the content of P, it is expected as a strengthening element for steel, so P may be added if necessary. But 0.
If a large amount is added in excess of 15%, the embrittlement of the steel becomes remarkable, so the addition amount is made 0.15% or less.

Each element of Nb and Ti is effective for increasing the strength of steel, but the upper limit is 0.2% due to economic constraints. Z
Each element of r, V, and W is similarly effective in increasing the strength of steel, but the upper limit is 0.1% due to economic constraints.

REM and Ca may be added in order to control the morphology of inclusions in the steel, but if added in excess, the amount of inclusions increases and the cold workability and toughness of the steel sheet deteriorate. It is set to 0.02%. Mg has an effect of preventing hydrogen embrittlement, and may be added for the purpose of preventing hydrogen embrittlement in the laser-processed portion. However, for economic reasons, the upper limit is made 0.01%.

Impurity elements that can be contained in the steel of the present invention include N, O, etc., and A which may be added as a deoxidizing element.
One can also mention l. Particularly in the case of aluminum killed steel, it is inevitably mixed, but if it exceeds 0.1%, a large amount of c-type inclusions are generated and cause surface scratches, so the upper limit is set to 0.1%.

As described above, the steel of the present invention exhibits excellent cold workability in the material stage, and once processed, the desired portion is strengthened by laser irradiation and the like, so under the conditions of use, it is significantly increased. It is possible to increase the strength.

Since the laser irradiation and the like in the present invention enhances the strength of the steel sheet, it is necessary to appropriately select the necessary part and select the irradiation part. Therefore, (1) When the processing required portion and the strength increasing required portion partially overlap with each other, the material steel plate is processed into a predetermined shape in advance, and then laser irradiation is performed aiming at the portion requiring strength increasing. It is recommended that (2) if the processing required part and the strength increase required part can be sufficiently distinguished in parts, laser irradiation is performed on the material steel sheet aiming at the part requiring strength increase, and after that You may make it process.

An example of the latter case is shown in FIG. In FIG. 9, 1 is a material steel plate, 2 is a mountain crease line, 3 is a valley crease line, 4 is a laser irradiation part, 5 is a molded product (the member), (a) is a plan view of the material steel plate, and (b) is Plane explanatory view showing the positioning of the processing unit and the laser irradiation unit,
(C) is a perspective explanatory view showing the external appearance of the molded product. First, as shown in (b), laser irradiation is performed while avoiding the mountain fold line 2 and the valley fold line 3 which are the processed parts, and then as shown in (c). It is processed into a predetermined shape. Even in the case of the illustrated shape, the material steel plate is first processed into a predetermined shape, and thereafter,
It goes without saying that the laser irradiation may be performed on a necessary portion.

The steel sheet of the present invention can be manufactured by either a hot rolling mill or a cold rolling mill, and the steel sheet of the present invention is subjected to various surface treatments such as galvanizing. Can also be provided as.

[0035]

EFFECTS OF THE INVENTION When the steel of the present invention is irradiated with a laser to form a solidified region penetrating the plate thickness, a quench-hardened portion is formed not only in the bead portion but also in the adjacent region of the bead. On the other hand, in the case where rapid heating and high temperature holding are not performed as in laser irradiation, the time for achieving the penetration of carbides and the homogenization of alloy components is usually insufficient. Therefore, in the present invention, the structure and alloy composition of steel, which is a raw material, are made to be a composition and structure effective for penetration and homogenization. In particular, it is very important to have a composition and structure corresponding to the above laser processing conditions. By doing so, it is not necessary to unnecessarily increase the amount of carbon or the amount of alloy, and it becomes possible to secure the workability of the material as well. In the case of the steel of the present invention, since the above-mentioned effect is exhibited, the area to be hardened can be widened, and therefore the strength is significantly increased. Therefore, for example, when a member such as a press-molded member is laser-processed only in a necessary portion, the deformability required for maintaining the workability can be provided at the time of processing the member while maintaining the strength. .

Depending on the type of the molded product, only the portion that does not affect the press molding may be strengthened by laser irradiation or the like. In such a case, irradiation of the laser or the like may be performed before press molding. Since it is possible to perform the treatment in the flat plate state, it is possible to perform the irradiation treatment, and it is easy to ensure the reliability of the characteristics of the treated material. It is also possible to combine the strength of the product with the workability during press molding.

[0037]

[Examples] The materials having the components shown in Table 1 were melted and rolled into a plate having a thickness of 1.4 mm, and the structure was adjusted. The evaluation of the characteristics was performed on two types of samples, a sample not irradiated with laser and a sample irradiated with laser. In particular, since the evaluation of the formability is concerned with the formability of the material, the sample before laser irradiation was performed. Laser irradiation was performed linearly and three irradiations were performed at 5 mm intervals. The laser output at that time was 3 kW, the scanning speed was 3 m / min, the focal position of the laser was within the plate, and scanning was performed with the molten phase penetrating the plate thickness. A JIS No. 5 tensile test piece was processed so that the laser irradiation line was located at the center of the test piece, and a tensile test was performed.

Table 2 shows the results. The values shown as “before irradiation” in Table 2 are the results of the tensile test on the test piece not subjected to the laser irradiation, and the workability index (r value) shows the test result on the test piece not subjected to the laser irradiation. .

[0039]

[Table 1]

[0040]

[Table 2]

Steels 1 to 7 have a K ₁ value lower than 0.01, so that a sufficient increase in strength is not obtained. Steel 22 has a large K _1, but the structure has a spheroidized structure, so that sufficient strength increase is not obtained. Although the structure of Steel 26 is ferrite + pearlite, the K ₁ value is as small as 0.002, so that the strength increase is not sufficiently obtained. Steel 8, 9
The material strength is improved by adding Nb or P, Cu, Ni, and the K ₁ value is 0.01 or more, so that the amount of strength increase is sufficient.

[Brief description of drawings]

FIG. 1 shows the relationship between laser irradiation conditions and intensity increase rate.

FIG. 2 is a diagram showing a comparison of laser processing characteristics of ferrite + pearlite steel (including spheroidized steel).

FIG. 3 is a diagram showing a relationship between a carbide grain size (short side size) and a strength increase rate during laser irradiation.

FIG. 4 is a diagram showing the relationship between the K ₁ value and the amount of increase in tensile stress.

FIG. 5 is a diagram showing the relationship between the K ₂ value and the amount of increase in tensile stress.

FIG. 6 is a diagram showing a hardness (Hv) distribution of a laser irradiation part.

FIG. 7 is a diagram showing a hardness (Hv) distribution of a laser irradiation part.

FIG. 8 is a diagram showing the relationship between the carbon content and the amount of increase in strength due to laser processing.

FIG. 9 is a diagram showing processing and laser processing in an example.

FIG. 10 is a drawing-substitute photograph showing a metallographic structure of a laser irradiation portion.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Suzuki, 1 Toyota Town, Toyota City, Aichi Prefecture, Toyota Motor Co., Ltd. (72) Inventor: Shinichiro Nakamura, 1 Toyota Town, Toyota City, Aichi Prefecture, Toyota Motor Corporation ( 72) Inventor Koichi Makii 1 Kanazawa-machi, Kakogawa-shi, Hyogo Prefecture Kadoto Steel Works Kakogawa Steel Works (72) Inventor Tetsuo Tatsuda 1-Kanazawa-machi, Kakogawa City Hyogo Prefecture Kakogawa Steel Works Co., Ltd.

Claims (4)

[Claims] 1. A high workability steel sheet having excellent strength-enhancing properties, which is used by increasing the strength by irradiating the surface of the steel sheet with a high-density energy source to form a solidified region penetrating the thickness of the steel sheet. : 0.02 to 0.3% (meaning% by weight, the same applies hereinafter) Si: 1.5% or less Mn: 2.5% or less, the balance consisting of Fe and inevitable impurities, and K ₁ = ( Mn% + 0.25 · Si%) × C% given a K ₁ value of 0.01 or more and having a structure in which pearlite and / or cementite coexist in addition to ferrite. A high workability steel sheet characterized by exhibiting excellent strength-enhancing properties by irradiation treatment with a high-density energy source. 2. K ₁ = (Mn% + 0.25 · Si%) ×
The high workability steel sheet according to claim 1, wherein the K ₁ value given by the C% calculation formula is 0.05 or more. 3. The alloying element further contains any one or more of Cr: 2.5% or less, Mo: 1.0% or less, B: 50 ppm or less, and K ₂ = (Mn% + Cr% + Mo% + 250. B% + 0.
The high workability steel sheet according to claim 1 or 2, wherein a K ₂ value given by a calculation formula of 25 · Si%) × C% is 0.05 or more. 4. As an alloying element, further Cu: 2.5% or less Ni: 1.5% or less P: 0.15% or less Nb: 0.2% or less Ti: 0.2% or less Zr: 0. 1% or less V: 0.1% or less W: 0.1% or less Any one or more types of high workability steel plate in any one of Claims 1-3.