JPH07305111A - High workability steel sheet and method for increasing its strength by irradiation of high density energy - Google Patents

Abstract

PURPOSE:To increase the strength of the prescribed part of a high workability steel by irradiating the prescribed part of a high workability steel sheet with high density energy to form a molten zone penetrated by the same, furthermore adding nitrogen to the molten zone and rapidly solidifying the same to regulate the concn. of nitrogen to a specified one. CONSTITUTION:Prior to the working of a high workability steel sheet or after the working, the prescribed part is irradiated with high density energy (laser beam) in an atmosphere of gaseous nitrogen to form a molten zone penetrating the prescribed part, and furthermore, nitrogen is added to the molten zone. Next, this steel sheet is rapidly solidified to regulate the nitrogen concn. in the solidified phase to 300 to 800ppm. moreover, the compsn. of the steel preferably regulated to the one contg., by weight, <=0.1% C, <=2.0% Si, <=3.0% Mn, <=100ppm N (not contg. zero in every cases), and the balance Fe with inevitable impurities. Thus, the strength of the prescribed part of the high workability steel sheet can be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for strengthening a high workability steel plate, and more particularly, a method for strengthening a desired portion without adversely affecting the high workability of the high workability steel plate, That is, if it is desired to increase the strength after processing the high workability steel sheet, select a portion to be strengthened after processing (if you want to increase the strength before processing, select a portion that is not subjected to strong processing). The present invention relates to a method of irradiating the portion with a high-density energy source to increase the strength. In the following description, the members that are one of the members for automobiles will be described as a representative example, but the steel sheet to which the present invention is applied is not limited thereto, and is a field in which both the above characteristics are required. Can be widely used against.

[0002]

2. Description of the Related Art Automotive members, particularly 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 It is necessary to modify the shape of
In addition, the number of laser irradiations required is very large, and the total processing time becomes long, which impedes practical application.

On the other hand, Japanese Patent Application Laid-Open No. 4-72010 discloses a technique of irradiating a press-formed product with laser to increase the strength. In this patent publication, it is shown that an increase in strength can be obtained when laser processing is performed using a carbon steel sheet. However, in this patent publication, only the carbon content is referred to regarding the steel plate composition, and it is completely possible to obtain the knowledge about the relationship between the alloy composition and the metal structure and the laser processing condition, and the relationship between them and the strength increase amount. Has not been done. On the other hand, 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 components and the metal structure. Therefore, it is necessary to clarify this relationship in order to obtain a large increase in strength by laser processing. As a result of various studies from this viewpoint, Japanese Patent Application Nos. 4-230569, 230570, The patent applications such as the above-mentioned 230574 to 230577 were filed.

In the prior art based on the laser hardening treatment before and after such press forming as a basic constitution, the alloying components of the target steel, the metal structure, and the irradiation conditions by the high-density energy source are the main subjects of the study. However, research has been insufficient from the viewpoint of providing a technique capable of constantly increasing the strength of the irradiated portion without specifying the alloy composition or metallographic structure of the target steel. Therefore, as mentioned above,
The alloy composition and the metal structure are remarkably limited, and as a result, the workability of the steel sheet is naturally limited. Therefore, there has been awaited the completion of a technique capable of reliably achieving the above-described high strength without being limited by the alloy composition and metal structure of the raw steel sheet.

Japanese Patent Laid-Open No. 61-232087 discloses that N _{2 is used in} laser welding using a condenser lens having a long focal length.
A technique for deepening the penetration by using a gas or a mixed gas of N ₂ and Ar is disclosed. However, this technique focuses on the state of plasma generation when N ₂ gas is irradiated with a laser, and is completely different in technical concept from the present invention described in detail below.

[0006]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention The inventors of the present invention are to achieve a high strength effect by high-density energy irradiation stably and highly without being limited by the kinds of alloying elements and metal structures. I examined possible methods. That is, even if a material that does not significantly affect the workability is selected, if the atmosphere gas in the melting region formed by high-density energy irradiation is controlled to be nitrogen-rich, that is, the steel in the melting region is It has been found that, by nitriding the steel, it is possible to achieve high strength without being greatly affected by the difference in the material steel plates, and the present invention has been completed.

[0007]

[Means for Solving the Problems] The method for achieving high strength according to the present invention means that a high workability steel sheet is densely formed on a predetermined portion of the steel sheet or the processed product before or after the working. Energy is irradiated to form a melting zone penetrating the predetermined portion, nitrogen is added to the melting zone, and then this is rapidly solidified to adjust the nitrogen concentration in the solidification phase to 300-8.
The basic constitution is to set it to 00 ppm.

The material steel sheet suitable for applying the strengthening method of the present invention is not limited by the alloy component and the metal structure, as described in the above description, but the alloy component is as follows. Those containing components such as those shown in are particularly preferred. C: 0.1% or less (0% is not included) Si: 2.0% or less (0% is not included) Mn: 2.5% or less (0% is not included) N: 100 ppm or less (0 ppm is included) ) And the balance consisting of Fe and inevitable impurities,
Furthermore, Cr: 2.5% or less (including 0%) Nb: 0.3% or less (including 0%) V: 0.3% or less (including 0%) Further, P: 0.15% or less (including 0%) Cu: 2.5% or less (including 0%) Ni: 1.5% or less (including 0%) Mo: 1.0% Below (including 0%) Ti: 0.1% or less (including 0%) Zr: 0.1% or less (including 0%) W: 0.1% or less (including 0%) B: 50 ppm or less (Including 0%) REM: 0.02% or less (including 0%) Ca: 0.02% or less (including 0%) Al: 0.1% or less (including 0%) Including the above.

[0009]

In the present invention, the main point is to add (nitriding) nitrogen into the melting region formed by irradiation with high-density energy. It is necessary to raise it. As such a nitriding method, a method of nitriding molten steel by using an inert gas containing nitrogen as an assist gas at the time of laser irradiation is exemplified as a typical means, but the present invention does not prescribe a nitriding method. Alternatively, for example, laser irradiation may be performed in the chamber as a nitrogen gas atmosphere or a nitriding atmosphere such as ammonia gas. A laser irradiation experiment was conducted using steel type 1 in Table 1 and changing the nitrogen fraction of the assist gas. The results are shown in Table 2.

[0010]

[Table 1]

[0011]

[Table 2]

Here, the laser output is 3 kw and the scanning speed is 3 m.
/ Min. As can be seen, when the amount of nitrogen in the bead portion is increased to 300 ppm or more, the amount of strength increase due to nitriding is increased, and when it is 800 ppm or more, the amount of strength increase is rather decreased.

In addition, FIG. 1 shows the results of measuring the amount of increase in strength by using steel type 1 in Table 1 and changing the laser irradiation conditions using a mixed gas of nitrogen gas 50% / argon gas 50% (test piece). The shape, size, and laser irradiation position are also shown in FIG. Here, a large amount of increase in strength is obtained by using high density energy of 100 J / mm ² or more. This is consistent with the condition that a molten layer penetrating the plate thickness is obtained, and it is understood that this condition is necessary. Table 2
Also shows the maximum hardness of the bead, but the maximum hardness is significantly increased when the nitrogen content in the bead portion is 300 ppm or more, which is considered to be the cause of the strength increase. However, the maximum hardness is not lowered even by nitriding at 800 pp or more. Therefore, the fracture surface of the tensile test piece was observed in order to investigate the cause of the decrease in the strength increase despite the hardness not decreasing. As a result, a large number of blowholes were observed on the fracture surface in the test piece having a nitrogen content of more than 800 ppm. That is, it was found that when the nitrogen content exceeds 800 ppm, a large amount of blowholes are generated during solidification, which prevents the tensile strength from increasing. Next, the reasons for selecting each of the above-listed particularly preferable alloy components in the steel sheet of the present invention and the reason for recommending the component range will be described.

The steel of the present invention must be particularly suitable for cold working such as press forming. For this purpose, the smaller the amount of C added, the more preferable. Further, the above-described effect of increasing the strength by nitriding becomes remarkable in the region where the C content is low. From such a viewpoint, the upper limit of the amount of C is set to 0.1%.

Si is added in order to improve the laser processability or according to various required characteristics of the raw steel sheet.
%, The surface becomes rough, so the upper limit is 2.0%.
And Mn is an element essential for increasing the strength by laser processing, and is added according to various required properties of the raw steel sheet. However, if too much is added, the cold formability of the steel sheet is impaired, so the upper limit of the addition is 3.0%, preferably 2.
5%. The lower limit is not particularly limited, but it is recommended to add 1.2% or more.

From the viewpoint that the steel of the present invention must be particularly suitable for cold working applications such as press forming, the N content is preferably as small as possible. 100
The presence of N in excess of ppm significantly deteriorates the elongation of the raw steel sheet, so the upper limit was set to 100 ppm. The typical contained elements in the present invention are as described above, and the balance is Fe and unavoidable impurities. However, if desired, the following elements can be added.

Cr is effective in increasing the strength by laser treatment, especially in strength when nitriding is also used, but it is uneconomical to add more than necessary, so the upper limit was made 2.5%. The lower limit is not particularly limited, but is preferably 0.3%
The above additions are recommended.

Nb is also effective in increasing the strength by laser treatment when nitriding is also used, but it is uneconomical to add more than necessary, so the upper limit was made 0.3%. The lower limit is not particularly limited, but it is recommended to add 0.005% or more. V is also effective for increasing the strength by laser treatment when nitriding is also used, but it is uneconomical to add more than necessary, so the upper limit was made 0.3%. The lower limit is not particularly limited, but it is recommended to add 0.005% or more.

In addition to the above elements, it may be recommended to add the following elements. Although the 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. However, if a large amount is added in excess of 0.15%, the embrittlement of the steel becomes remarkable, so the addition amount is made 0.15% or less.

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 Cu and Ni
In addition to the composite addition, the addition amount is 2.5% or less for Cu and 1.5 for Ni for economic reasons.
% Or less is desirable. The lower limit is not particularly limited, but it is recommended to add Cu: 0.4% or more and Ni: 0.4% or more.

Mo is an element effective in increasing the strength by laser treatment. However, adding a large amount exceeding 1.0% is economically disadvantageous, so the upper limit is made 1.0%. Further, each element of Ti, Zr, and W is effective for increasing the strength of steel, but the upper limits are set to 0.1% due to economic constraints.

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

In the present invention, addition of other elements is allowed. As such an element, Ca may be added to control the inclusion morphology of steel. However, if added excessively, the amount of inclusions increases and the cold formability and toughness of the steel sheet deteriorate, so the upper limit is made 0.02%. It should be noted that REM may be replaced in whole or in part as an equivalent element of Ca, but the upper limit of REM is 0.02%.

The unavoidable impurity element may be Al added as a deoxidizing element. Al is an element added 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 is set to 0.1%.

As described above, the steel of the present invention exhibits excellent cold workability at the material stage, and laser irradiation is carried out by selecting a desired portion after processing once or a non-processed portion prior to processing. Since the strength is increased by the above, it is possible to significantly increase the strength under the conditions of use. The steel sheet to which the method of the present invention is applied may be either a hot-rolled steel sheet or a cold-rolled steel sheet, and may be subjected to various surface treatments such as galvanizing.

In the present invention, the hardness of that portion is increased by intensively nitriding only the portion to which high-density energy is irradiated, that is, the portion for which high strength is desired to be achieved, and a large increase in strength is obtained. In order to obtain a large strength increase by laser irradiation, it is a normal idea that it is necessary to increase the amount of carbon and alloy in the raw steel sheet,
Since strengthening by nitriding effectively acts in the present invention, it becomes unnecessary to preliminarily increase the amount of carbon and the amount of alloy in the steel material, and it becomes possible to secure the workability of the material. Therefore, for example, it is only necessary to perform laser processing in a nitrogen atmosphere only on a portion considered necessary in a member such as a member so as to exert strength, and the material steel plate itself has a deformability necessary for press forming. Can be made. On the other hand, it is very difficult to make the raw material steel material to have a high nitrogen content, and the workability of the steel material is significantly deteriorated, which makes it difficult to process the press-formed product.

Depending on the type of the molded product, only the portion that does not affect the press molding may be strengthened by the laser irradiation. In such a case, the laser irradiation is performed before the press molding. Since it is handled as a target, it is easy to perform high-density energy irradiation and the like, and therefore the irradiation processability is good, and the characteristics of the processing material can be easily secured. In this way, it is possible to increase the strength with a laser or the like before press molding.

[0028]

[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. Laser irradiation was performed linearly, and three irradiations were performed at 5 mm intervals. The laser output at that time was 3 kW and the scanning speed was 3 m / min. 5 focal length for focusing laser
An inch lens was used, the focal position was within the plate, and scanning was performed with the molten phase penetrating the plate thickness. The assist gas was flown as a center gas, and the flow rate was 20 liter / min. JIS so that the laser scanning line is located in the center of the test piece.
A No. 5 tensile test piece was processed and a tensile test was performed.

First, the assist gas is a mixed gas of argon and nitrogen, the mixing ratio is changed to change the nitriding degree of the bead portion, and the strength increase amount due to the nitriding is compared and shown in Table 2 (described above). . Here, an experiment was conducted using steel type 1 in Table 1. From this table, the bead portion nitriding amount is 300 to 80.
It can be seen that a large amount of nitriding enhancement is obtained at 0 ppm.

Next, the influence of the composition was investigated by using all the steel types shown in Table 1. The nitriding strengthening amount was obtained from the difference between the case where the assist gas was 100% argon gas and the case where the assist gas was 50% argon / 50% nitrogen. The results are shown in Table 3, and the difference in the strengthening based on nitriding was observed depending on the carbon amount, and the experiment No. in which the carbon amount exceeded 0.1%. In Experiment No. 10, Experiment No. Although the amount of nitriding is not different from that of No. 9, a very large amount of nitriding strengthening is not obtained. Therefore, it is recommended that the carbon content is preferably 0.1% or less.

[0031]

[Table 3]

Table 4 shows the results of investigating the effect of plating by using the steel type 1 of Table 1, but no effect of plating was observed, and the strength-increasing steel sheets obtained by the present invention were It turns out that there are no major restrictions on the application.

[0033]

[Table 4]

[0034]

EFFECTS OF THE INVENTION The present invention is configured as described above, and when forming a melt-solidification region that penetrates the plate thickness by irradiation from a high-density energy source, only the melted portion is nitrided By increasing the amount of nitrogen, the strength of the irradiated part can be remarkably enhanced. Further, according to the present invention, it is possible to select and use a material steel having good workability as the material steel itself without largely limiting the component composition and structure of the material steel sheet, and to perform laser irradiation nitriding only on a portion requiring strengthening. It is possible to easily obtain a molded product in which any position is reinforced.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the energy density of laser irradiation and the amount of increase in intensity.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication C22C 38/58 C23C 8/26 (72) Inventor Koichi Makii Ikeda, Ikeda, Kamikawa, Kakogawa, Hyogo Pioneering 2222 Address 1 Kobe Steel Works, Kakogawa Research Area (72) Inventor Akito Sato 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Shinichiro Nakamura 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation

Claims (4)

[Claims] 1. A high workability steel plate, prior to processing,
Alternatively, after processing, a predetermined area of the steel plate or the processed product is irradiated with high-density energy to form a molten area penetrating the predetermined area, nitrogen is added to the molten area, and then this is rapidly solidified. And the nitrogen concentration in the solidification phase is adjusted to 30
A method of increasing the strength of a high workability steel sheet by irradiating high-density energy, which is characterized in that the content is 0 to 800 ppm. 2. As an alloying element, C: 0.1% or less (meaning% by weight; the same applies hereinafter) (0%
Si: 2.0% or less (0% is not included) Mn: 3.0% or less (0% is not included) N: 100 ppm or less (0 ppm is included) and the balance is Fe and unavoidable A highly workable steel sheet which is made of impurities and which can be strengthened by the method according to claim 1. 3. As an alloying element, Cr: 2.5% or less (including 0%) Nb: 0.3% or less (including 0%) V: 0.3% or less (including 0%) The highly workable steel sheet according to claim 2, which comprises any one or more of them. 4. As an alloying element, P: 0.15% or less (including 0%) Cu: 2.5% or less (including 0%) Ni: 1.5% or less (including 0%) Mo : 1.0% or less (including 0%) Ti: 0.1% or less (including 0%) Zr: 0.1% or less (including 0%) W: 0.1% or less (including 0%) ) B: 50 ppm or less (including 0%) REM: 0.02% or less (including 0%) Ca: 0.02% or less (including 0%) Al: 0.1% or less (including 0%) The method according to claim 2 or 3, which comprises any one or more of
Highly workable steel sheet described in.