JPH0673439A - 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 excellent in workability 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:The cold-rolled low carbon steel sheet containing, by weight, 0.002-0.02% C, < 2.0% Si, 0.1-2.5% Mn, either one or more of <=0.1% Ti, Nb and further either one or more of 0.06-0.2% P and <50ppm B and having >=0.01 T value expressed by (Mn% + 20P% + 250B% + 0.25Si%) X C% or, if necessary, further one or more kinds of the specific small quantities of Cu, Ni, Cr, Mo, Zr, V, W, etc., and having mainly ferrite structure and excellent workability, is irradiated a laser beam, etc., of high density energy, and by forming the melt-penetrated and solidified zone to the steel sheet, the steel sheet having extremely high strength in spite of the low carbon content is obtd.

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.

Moreover, in ultra low carbon steels (which have excellent formability due to a small amount of C) as described below, there is no known knowledge that the strength is significantly improved by laser treatment. In particular, the principle of strengthening by laser treatment is considered to be the quench hardening seen in the melted zone and the heat-affected zone around it, and even its application to ultra-low carbon steel with high formability has been considered. There wasn't. If such a strength increase is possible in a high moldability material, it is expected to greatly contribute to weight reduction of automobiles, for example, and the effect is considered to be very large.

The present technology, which is basically composed of such press molding and subsequent laser hardening treatment, is characterized in that the parts are pressed in the press line and then irradiated with a high-density energy source. 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.

[0006] Incidentally, Japanese Patent Laid-Open No. 61-261462 discloses the knowledge of a steel plate for laser processing having excellent 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 of performing rapid remelting-rapid resolidification 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 must be said that the conventionally known method is essentially different from the method described below, which is used in the present invention.

[0009]

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.

[0010]

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 the solidified region is formed, it exhibits a sufficiently high strength, so that it can 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: 0.002 to 0.02% Si: 2.0% or less Mn: 0.1 to 2.5%, the balance being Fe and Fe. It is characterized by comprising an unavoidable impurity and having a structure mainly composed of ferrite.

The basic alloy composition in the present invention is as described above, but as alloy elements other than the above-mentioned essential elements C, Si and Mn, Ti: 0.1% or less Nb: 0.1% When (A) contains at least one selected from the group consisting of:
The present invention also includes a case (B) in which any one or more of P: 0.06 to 0.2% B: 50 ppm or less is separately contained as an alloying element. However, in the case of (B), T = (Mn% + 20.P% + 250.B% + 0.25.
It is necessary that the T value given by the calculation formula of (Si%) × C% is 0.01 or more.

In the present invention, the case (C) is defined as follows when the lower limits of the C content and the Mn content are increased a little. That is, C: 0.005-0.02% Si: 2.0% or less Mn: 1.2-2.5% P: 0.06-0.2% B: 50ppm or less, and further Ti: 0.01 to 0.1% Nb: 0.005 to 0.1% or less is included, and the T value given by the above calculation formula is 0.01 or more.

Further, in the case of (D), the present invention contains C: 0.002 to 0.2% Si: 2.0% or less, Mn: 0.1 to 2.5%, and further, Cu: 2. 5% or less Ni: 1.5% or less Cr: 2.5% or less Mo: 1.0% or less P: 0.15% or less B: 50 ppm or less Nb: 0.1% or less Ti: 0.1% or less Zr : 0.1% or less V: 0.1% or less W: 0.1% or less Any one or more of the following is included, and in the case of (E), Nb and Ti in the above (D) are particularly essential elements. Including the case of including as. In this case, the Ti content is 0.1% or less and the Nb content is 0.1% or less.

[0015]

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: 51 ppm, Mn: 0.99%, T
i: 0.053%, Al (impurity element based on addition as a deoxidizer): 0.029%, 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,
Under these conditions, the strength can be greatly 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 reasons for limiting the alloy components in the steel sheet of the present invention will be explained. 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. Since the present invention aims at the extremely low carbon steel region, 0.002 to 0.0
It was set at 2%. If it is less than 0.002%, the effect of increasing the strength by laser irradiation or the like does not increase the yield stress by 20 MPa or more, so 0.002% or more must be added. On the other hand, if it exceeds 0.02%, the workability of the material itself cannot exhibit the characteristics as an ultra-low carbon steel.

Si is added according to the strength required for the steel plate. If too much is added, the surface will be roughened significantly, so the upper limit was made 2.0%. Mn is also added according to the strength required for the steel plate, but if it is added in a too large amount, the cold formability of the steel sheet is impaired, so the upper limit of the addition amount is 2.5%. On the other hand, regarding the lower limit amount, the strength improvement effect by laser irradiation is meaningfully exerted (the yield stress is 20 MPa.
The content was set to 0.1% for the purpose of (the above increase), but it is desired to add 1.2% or more in order to more reliably exert the strength increasing effect. 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.

First, P is added according to the required strength of the steel sheet. However, if too much is added, secondary work embrittlement appears due to a decrease in grain boundary strength, so the upper limit is made 0.2%. The lower limit is preferably 0.06% from the viewpoint of the effect of increasing the strength by laser irradiation.

Next, B is recommended to be added as an element which is particularly useful for exerting the strength increasing effect by laser irradiation. However, 50
If it exceeds ppm, the ductility of the base material will be significantly deteriorated, so 5
It is set to 0 ppm. Although there is no particular need to set the lower limit value, addition of 5 ppm or more can be recommended.

The next most important elements are Ti and Nb. In the present invention, the formability as a base steel sheet is excellent, while increasing the strength after laser treatment is an important issue. From this point of view, a very low carbon steel to which a carbonitride forming element is added is a useful means. The carbonitride forming element is added from the viewpoint of precipitating and fixing C and N in steel and imparting excellent formability. Ti and Nb are the most effective for such purposes, and the suitable addition amount thereof is 0.01 to 0.1% for Ti and 0.005 to 0.
1%. The upper limits of these were set from the viewpoint of economic efficiency.

Next, other important elements will be described.
Cr is effective in increasing the strength by laser treatment, but excessive addition of Cr beyond necessity is economically disadvantageous, so the upper limit was made 2.5%. Mo is effective in increasing strength by laser treatment, but addition of a large amount impairs economic efficiency, so the upper limit is made 1.0%.

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 them in an amount of 2.5% or less with respect to Cu, and Ni in the amount of 1.5% or less in consideration of the cost.

Although each element of Zr, V and W is effective in increasing the strength of steel, the upper limit is set to 0.1% due to economic constraints. Further, REM and Ca may be added in order to control the inclusion morphology of the steel, but if added in excess, the amount of inclusions increases and the cold workability and toughness of the steel sheet deteriorate, so the upper limits are each 0.02 %.

Mg has an effect of preventing hydrogen embrittlement, and may be added for the purpose of preventing hydrogen embrittlement in the laser-treated portion. However, for economic reasons, the upper limit is made 0.01%. Examples of the unavoidable impurity element include N, O and the like, and Al that may be added as a deoxidizing element. Especially in the case of aluminum killed steel, it is inevitably mixed, but
If it exceeds 1%, a large amount of c-based inclusions are generated and cause surface scratches, so the upper limit is set to 0.1%.

Next, the relationship between the components and the amount of increase in strength will be described. 2A and 2B show the following calculation formula: T = (Mn% + 20.P% + 250.B% + 0.25.
It shows the relationship between the T value given by (Si%) × C% and the yield stress increase amount. Note that (b) is an enlarged view of a main part of (a).

First, according to (b), the C content is 0.002%.
In the region of less than 1% and in the region of less than 0.1% of Mn (comparative steel), the amount of increase in yield stress remains at about 8 to 10 PMa. It can be seen from FIG. 2 that the T value is preferably 0.01 or more. According to (a), T
There is a portion that shows a significant increase in strength around the value of 0.06, but in this portion, r value (index of deep drawability)
Is decreased to 1.1. It is explained that this is because the C content is as high as 0.03%. FIG. 3 shows the relationship between the amount of C and the r value, but when C exceeds 0.02%, the r value remarkably decreases.

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, etc. It is possible to increase the strength.

Since the laser irradiation or the like in the present invention enhances the strength of the steel sheet, it is necessary to appropriately select a necessary portion and select an irradiation portion. 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. 4, 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), respectively, (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.

[0031]

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 member can have the deformability necessary for maintaining the workability while being processed into 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 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.

[0033]

[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. .

[0035]

[Table 1]

[0036]

[Table 2]

Since the steels 28 and 29 have a small amount of C and Mn, the strength upper layer effect by the laser treatment is small. Steel 27 has a large amount of C and thus has a low r value of 1.1. Steels 1 and 2 are aluminum-killed steels without addition of Ti, but since the amounts of C and Mn are also small, the strength upper layer effect by laser treatment is small. FIG. 5 shows an electron micrograph (15,000 times) of the laser irradiation portion of Invention Steel 11.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a relationship between a T value and an amount of increase in yield stress.

FIG. 3 is a diagram showing the relationship between the C concentration and the r value of Nb and Ti-added steel.

FIG. 4 is a diagram showing processing and laser processing in an example.

FIG. 5 is a drawing-substituting photograph showing a metallographic structure of a laser-irradiated portion of Inventive Steel 11.

Front Page Continuation (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 Co., Ltd. (72) Invention Koichi Makii 1 Kanazawa-machi, Kakogawa-shi, Hyogo Prefecture Kamido Steel Works, Ltd. Kakogawa Steel Works (72) Inventor Tetsuo Tatsuda Kanazawa-machi, Kakogawa City, Hyogo Prefecture Kakogawa Works (72) Invention Tomohiro Kase 1 Kanazawa-cho, Kakogawa-shi, Hyogo Prefecture Kadogawa Steel Works Kakogawa Works

Claims (6)

[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. : 0.002 to 0.02% (meaning% by weight, the same applies hereinafter) Si: 2.0% or less Mn: 0.1 to 2.5%, with the balance being Fe and inevitable impurities, and ferrite A high workability steel plate characterized by exhibiting excellent strength-enhancing properties by a high-density energy source irradiation treatment, which is characterized by having a structure mainly composed of. 2. The high workability steel sheet according to claim 1, further comprising, as an alloying element, any one or more of Ti: 0.1% or less and Nb: 0.1% or less. 3. An alloying element further containing any one or more of P: 0.06 to 0.2% B: 50 ppm or less, and T = (Mn% + 20.P% + 250.B% + 0.25.・
The high workability steel sheet according to claim 1, wherein a T value given by a calculation formula of (Si%) × C% is 0.01 or more. 4. 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. : 0.005-0.02% Si: 2.0% or less Mn: 1.2-2.5% P: 0.06-0.2% B: 50ppm or less, and further Ti: 0.01 -0.1% Nb: 0.005-0.1% or less Any one or more types are included, and T = (Mn% + 20 * P% + 250 * B% + 0.25 *).
The high workability steel sheet according to claim 1, wherein a T value given by a calculation formula of (Si%) × C% is 0.01 or more. 5. 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. : 0.002-0.02% Si: 2.0% or less Mn: 0.1-2.5% is included, Furthermore, Cu: 2.5% or less Ni: 1.5% or less Cr: 2.5 % Or less Mo: 1.0% or less P: 0.15% or less B: 50 ppm or less Nb: 0.1% or less Ti: 0.1% or less Zr: 0.1% or less V: 0.1% or less W: Excellent in high-density energy source irradiation treatment, characterized by containing at least one of 0.1% or less, the balance being Fe and unavoidable impurities, and having a structure mainly composed of ferrite. Highly workable steel sheet characterized by exhibiting high strength characteristics. 6. 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. : 0.002 to 0.02% Si: 2.0% or less Mn: 0.1 to 2.5% inclusive, and further Ti: 0.1% or less Nb: 0.1% or less , Cu: 2.5% or less Ni: 1.5% or less Cr: 2.5% or less Mo: 1.0% or less P: 0.15% or less B: 50 ppm or less Zr: 0.1% or less V: 0 1% or less W: 0.1% or less Any one kind or more, the balance is Fe and inevitable impurities, and is characterized by having a structure mainly composed of ferrite, high density High strength characterized by exerting excellent strength-enhancing properties by energy source irradiation treatment Sex steel plate.