JPH0673441A - High workability steel sheet excellent in high strengthening characteristic by irradiation with high density energy source - Google Patents

Abstract

PURPOSE:To obtain a steel sheet having excellent treating characteristic by specifying alloy components and constitution structure in the steel sheet under a fixed irradiating condition. CONSTITUTION:This steel sheet is the high workability steel sheet used after high strengthening by irradiating the surface of the steel sheet with a high density energy source to form the solidified zone penetrating the sheet thickness. This steel sheet contains, by weight 0.05-0.25% C, <=3.0% Si, 1.1-3.0% Mn and the balance Fe and has the structure containing ferrite and retained austenite and besides, either one or more of martensite and/or bainite. Particularly, the steel sheet having K, >=0.35, in K1 (Mn% + 0.25Si%) X C% is desirable. This steel sheet displays the excellent workability at the time of working and the excellent high by strengthening characteristic by the irradiation treatment with the high density energy source.

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: 0.05 to 0.25% Si: 3.0% or less Mn: 1.1 to 3.0%, the balance being Fe and Fe. It is characterized in that it is composed of unavoidable impurities, contains ferrite and retained austenite, and has a structure containing at least one of martensite and bainite.

The basic alloy composition in the present invention is as described above. Especially, the _one having a K ₁ value of 0.35 or more calculated by the following formula according to the contents of C, Si and Mn is laser treated. It has been confirmed that it exerts a more reliable and excellent effect in high workability before and high strength after laser treatment. K ₁ = (Mn% + 0.25 · Si%) × C%

Further, the high workability steel sheet of the present invention is the above-mentioned C, S.
In addition to i and Mn, one or more of Cr: 2.5% or less, Mo: 1.0% or less, B: 50 ppm or less may be contained 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. Further, it is desired that the K ₂ value given by the following calculation formula is also 0.35 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.

[0013]

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 (impurity element added as a deoxidizing element): 0.032%, steel material consisting of balance Fe and unavoidable impurities as a test piece (plate thickness 1.4 mm), laser irradiation conditions Although various changes were made to show the relationship with the amount of increase in strength, it can be seen that a significant increase in strength can be obtained by performing irradiation so that the energy density becomes 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. Also, by making such conditions, the strain generated in the plate thickness direction is released,
The residual strain after the treatment can be suppressed to a very small level.

Next, the relationship between the structure and the amount of increase in strength will be described. FIG. 2 shows the relationship between the material characteristics and the amount of increase in strength due to laser treatment in the case of ferrite + pearlite steel and the case of ferrite + retained austenite + (martensite and / or bainite) steel. The material property is the product of strength and elongation, and the yield stress is the index. The reason why the ferrite + pearlite steel was used as the comparative steel is that the ferrite + pearlite steel is the most general structure as a steel sheet for pressing and is widely used. As can be seen from this figure, the ferrite + retained austenite + (martensite and / or bainite) steel satisfying the structural conditions of the present invention exhibits a far superior strength increasing effect as compared with the ferrite + pearlite steel. . As a result of diligent research on this cause, it was found that it is indispensable that the alloy composition is contained in a certain condition range in addition to the steel structure in order to improve the balance between the workability and the strength increase amount. (See Examples below). Explaining this point further, in the composite structure defined in the present invention, first, carbide precipitation occurs from the martensite phase and the retained austenite phase during laser heating, but the size thereof is very small, so it is easy. In addition, since the carbides precipitated in the bainite phase are sufficiently small, the penetration is completed during the laser treatment, and a sufficient hardened area is secured by these penetrations, while the ferrite and pearlite phases are In that case, it takes a long time for the precipitated carbides to melt, and the laser does not sufficiently melt during the laser treatment, so that the hardening region is narrowed as a whole.

Further, FIG. 3 shows the relationship between the K ₁ value and the K ₂ value shown in the above formula and the increase in yield stress. K
Looking at both ₁ and K ₂ , when it is 0.35 or more, a large yield stress increase of 200 MPa is obtained. It was found that the effect of adding Cr, Mo, and B can be understood by the concept of the K ₂ value described above.

FIG. 4 shows hardness distributions of cross sections of the laser-processed portions both when the conditions of the present invention are satisfied and when the conditions are not satisfied. Since the comparative steel has a ferrite + pearlite structure, the width of the hardened area is narrow. This is not something that can be understood simply by the hardenability, but it is thought that the width of the hardening zone was narrowed due to the overall influence of the difference in the transformation point due to the alloy composition and the difference in the penetration method due to the difference in the carbide size. Be done.

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. On the other hand, however, the increase in strength due to laser irradiation or the like is an important issue, and particularly from the viewpoint of obtaining a composite structure of ferrite + retained austenite + (martensite and / or bainite), addition of 0.05% or more is necessary. On the other hand, if C is added excessively, the workability of the steel sheet and further the weldability are significantly deteriorated, so the upper limit of C is 0.25%.

Si is also ferrite + retained austenite +
Although it is an element useful in obtaining a composite structure of (martensite and / or bainite), if it exceeds 3.0%, remarkable surface roughening occurs, so the upper limit was made 3.0%.

Mn is an element essential for increasing the strength by laser processing, and is also an element useful for obtaining the composite structure, and at least 1.1% must be added. However, if added in a too large amount, the cold formability of the steel sheet is impaired, so the upper limit of the added amount is 3.0%.

The essential contained elements in the steel of the present invention are as described above, and the balance is Fe and unavoidable 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 together, and to add Cu in an amount of 2.5% or less and in Ni, 1.5% or less due to economic constraints.

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 effective for increasing the strength of steel, but the upper limit is 0.1% due to economic constraints.

Further, REM and Ca may be added to control the morphology of inclusions in steel, but if added in excess, the amount of inclusions increases and the cold workability and toughness of the steel sheet deteriorate, so the respective upper limits are set. 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%.

Examples of the unavoidable impurities that may be contained in the steel of the present invention include N, O and the like, as well as Al which can be added as a deoxidizing element. Al is an additive element particularly in the case of aluminum-killed steel, but if it exceeds 0.1%, a large amount of c-based inclusions are generated and cause surface scratches, so the upper limit was set to 0.1%.

As described above, the steel of the present invention exhibits excellent cold workability at the material stage, and once processed, the desired portion is strengthened by laser irradiation or the like, so under the conditions of use it is significantly 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 a necessary part and select an 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. 5, 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, (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.

[0032]

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 which 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 is 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.

[0034]

[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 test 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. .

[0036]

[Table 1]

[0037]

[Table 2]

Since Comparative Steels 1 to 5 have a structure of ferrite + pearlite, a sufficient improvement in strength cannot be obtained.
The other steels satisfied the conditions of the present invention, and excellent strength increase rates were obtained.

[Brief description of drawings]

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

[Fig. 2] In the case of ferrite + pearlite steel, as well as ferrite + retained austenite + (martensite and /
(Or bainite), the relationship between the material characteristics and the yield stress increase due to laser treatment.

FIG. 3 is a diagram showing a relationship between a K ₁ value or a K ₂ value and an increase amount of yield stress.

FIG. 4 is a diagram showing a hardness (Hv) distribution of a laser irradiation part.

FIG. 5 is a diagram showing processing and laser processing in an example.

 ─────────────────────────────────────────────────── ─── 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. : 0.05 to 0.25% (meaning% by weight; the same applies hereinafter) Si: 3.0% or less Mn: 1.1 to 3.0% with the balance Fe and unavoidable impurities and ferrite In addition to containing retained austenite and having a structure containing one or more of martensite and bainite, high processing characterized by exhibiting excellent strength-enhancing properties by high-density energy source irradiation treatment Steel sheet. 2. K ₁ = (Mn% + 0.25 · Si%) ×
The high workability steel sheet according to claim 1, wherein the K ₁ value given by the calculation formula of C% is 0.35 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 0.35 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.