JP3091060B2 - How to strengthen steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for strengthening a steel material without causing a problem such as thermal strain. Here, the steel material includes a steel plate and other unprocessed materials and processed materials obtained by processing them by pressing or the like.

2. Description of the Related Art In recent years, demands for higher strength and lighter weight of press-formed products have been increasing more and more. However, since high-strength materials cause problems in terms of shape freezing properties, etc. There is a limit. The strength of a press-formed product can be improved by pressing a thin steel sheet into a complicated shape. However, in order to press a carbon steel sheet into a complex shape that can provide the required strength, it is necessary to use a high energy rate processing method such as an impact hydraulic forming method or an explosion forming method. There are drawbacks such as low productivity and high cost.

On the other hand, as a technique for obtaining a lightweight and high-strength press-formed product, a press-formed product such as a thin steel plate is irradiated with high-density energy such as laser or plasma to be melted linearly, and the melted portion is quenched. Technology to improve the strength of press-formed products by making the structure (quenched and hardened part)
It has been proposed as JP-A-4-72010. This technology can be applied to materials having high quench hardening ability, that is, materials having a carbon content of 0.05 wt% or more, and for press forming thin steel sheets that cannot be normally quenched due to problems such as poor shape due to thermal strain. It is possible to obtain a press-formed product with increased strength, light weight and high strength.

[0003]

The quenching and hardening ability of steel materials is generally determined by a carbon equivalent (eg, Ceq = C + Si / 24).
+ Mn / 6), but the increase in strength of the laser-treated material is also governed by the carbon equivalent, as is the quench hardening ability. According to JP-A-4-72010 described above, for example, a steel material having a carbon content of 0.05 wt% is used to form three laser hardened portions parallel to the length direction.
Regarding the tensile strength of the IS5 test piece, an increase in strength of about 20% with respect to the untreated test piece is observed. However, in the case of a steel material having a low carbon content, for example, a steel material having a carbon content of 0.03 wt%, a JIS having a laser hardened and hardened portion similar to the above is formed.
In the No. 5 test piece, only about 2% increase in strength was observed with respect to the untreated test piece. As described above,
The technique of No. 72010 has a problem that a material having a relatively low carbon content has a low rate of increase in strength and is not practically applicable. Further, even with a steel material having a high carbon content, the rate of increase in strength is at most about 20%, and further higher strength cannot be achieved.

The present invention has been made in view of such a conventional problem, and it is possible to increase the strength of a steel material in accordance with its carbon content and without causing a problem such as thermal strain.
An object of the present invention is to provide a method capable of obtaining a lighter and higher strength steel material than ever before.

In order to achieve the above object, the present invention provides a method similar to that obtained when carbon steel is quenched by externally adding nitrogen to a molten portion of a steel material by laser irradiation. It is intended to increase the strength of the steel material by forming a melt-solidified portion having a quenched structure.

[0005] The present invention provides fused portion there are formed
By irradiating the steel material with a laser at appropriate intervals in a linear manner so as to form a deep penetration fusion part with a spectacle ratio of 0.5 or more, supplying nitrogen as all or part of the shielding gas to the laser irradiation part, This is a method for strengthening a steel material, wherein nitrogen is added to a molten portion formed by irradiation, and a nitrogen-enriched bead-shaped melt-solidified portion is linearly formed at appropriate intervals . Laser irradiation in the method of the present invention uses deep penetration to effectively increase the strength and suppress the occurrence of thermal strain.
It is necessary to implement such a Mino molten form,
Therefore, as described above, the melting ratio is set so that the aspect ratio (penetration depth H / penetration width W) becomes 0.5 or more.

As the laser, any laser system that can be used for thermal processing, such as a CO ₂ laser, a CO laser, an Nd-YAG laser, a glass laser, and an excimer laser, can be applied. All or part of the shielding gas supplied to the laser irradiation part is nitrogen, and the center gas method of supplying the gas coaxially with the laser beam is relatively easy as the shielding gas supply method, but it is limited to this. It is not something to be done. When the shielding gas is a mixed gas of nitrogen and another gas, there is no particular restriction on the type of gas to be mixed.
A rare gas such as Ar or He is used.

[0007]

The operation of the present invention will be described with reference to FIG. FIG. 1 shows an example in which nitrogen is supplied to a laser irradiation unit as a shielding gas by a center gas method. Laser beam 2 (normally, energy density: 10 ⁴ to 10 ⁷ W / cm ² ) focused by a focusing lens 1 (for example, a ZnSe lens)
Irradiates steel material 3, the irradiated part instantaneously melts and evaporates,
A melt hole called a keyhole 4 is formed. The inside of the keyhole 4 is composed of iron evaporating particles, which are the main atoms constituting steel, and iron atoms in an excited / ionized state, and the temperature reaches 5000 ° C. to 10,000 ° C. The supplied nitrogen enters an excited state in the keyhole 4 and enters the molten portion 6.

[0008] As described above, nitrogen is enriched in the melted portion by laser irradiation, and the melted portion is rapidly solidified and cooled. Therefore, a melt-solidified portion having a quenched structure similar to that obtained when carbon steel is quenched. Are formed, and the hardness and strength of the melt-solidified portion are significantly increased as compared with the base material. Therefore, if at least a portion of the steel material where strength is required, the laser irradiation is performed linearly at appropriate intervals,
A linear melt-solidified portion having a quenched structure is formed at the site, and the strength of the site can be significantly increased. Further, it is needless to say that the strength of the entire steel material can be increased by linearly performing the laser irradiation on the entire steel material at appropriate intervals. Usually, the linear melt-solidified portions are formed at appropriate intervals in the form of stripes or grids.

[0009] The aspect ratio of the melt part in the present invention (penetration depth H / penetration width W) as 0.5 or more
Are therefore, the strength more effectively enhanced, moreover it is possible to suppress the generation of thermal distortion effectively. The hardness of the melt-solidified portion and the strength of the steel material can be controlled by adjusting the supply amount of nitrogen. However, when the laser output and the processing speed are kept constant, they can also be adjusted by the nozzle height and the nozzle diameter.
The strength of the steel material can also be adjusted by selecting the interval between the melt-solidified portions.

The steel materials to which the present invention is applied include general carbon steel materials and ultra-low carbon steel materials.
All kinds of steel materials are included, such as steel materials strengthened by Si and P, steel materials to which B is added for grain boundary strengthening, and ultra-low carbon steel materials in which at least a part of interstitial elements are fixed by Ti and Nb. Further, the type of steel material (processed material, unprocessed material) is not limited to a steel plate, but can be applied to all types of pipes, strips, wires, and the like. Further, the present invention can be applied to a steel material such as a steel plate having a surface plated (electroplating or hot-dip plating), and the type of plating is not limited.

It is to be noted that the addition of carbon from the outside to the molten portion of the steel material can also make the melt-solidified portion a quenched structure and increase the strength of the steel material. As a method of adding carbon to the molten portion, a method for supplying hydrocarbon gas (C _m H _n) and carbon oxide-based gas (CO _m) to the laser irradiation section, a carbon fine powder to a laser irradiation portion in a carrier gas There are a method of supplying, a method of supplying a high carbon filler wire to the laser irradiation part, and the like. Therefore, if these methods are used in combination with the method of the present invention, it is possible to more effectively increase the strength of the steel material.

[0012]

【Example】

[Example 1] The composition of the material steel sheet was C: 0.01 to 0.1.
20 wt%, Si: 0.02 wt%, Mn: 0.69 w
At t%, a linear laser irradiation was performed on a galvannealed steel sheet having a sheet thickness of 1.4 mm using a CO ₂ laser. In the example of the present invention, nitrogen was used as the shielding gas, while in the comparative example, Ar was used as the shielding gas. The laser irradiation conditions are as follows. Laser output: 3.0 kW Processing speed: 3 m / min Focal length of condenser lens: 254 mm Focus position: -0.5 mm Aspect ratio: 1.4 Nozzle diameter: 3 mm Nozzle height: 5 mm Type of center gas: Ar, N ₂ Center gas flow rate: 20 l / min

The micro Vickers hardness Hv (measuring load 50) of a bead-shaped melt-solidified portion obtained by laser irradiation
gf) was measured. Further, three bead-shaped melt-solidified portions were formed in parallel with the tensile direction on a JIS No. 5 test piece as shown in FIG. 2 according to each test condition, and the tensile strength of each test piece was measured. . FIG. 3 shows the relationship between the carbon content of the steel sheet (base material) and the micro-Vickers hardness Hv of the melt-solidified portion. According to this, the micro Vickers hardness of the melt-solidified portion increases as the carbon content of the base material increases regardless of the type of the shielding gas, but when nitrogen is used as the shielding gas under the conditions of the method of the present invention. Is markedly increased in micro Vickers hardness as compared with the case where Ar is used. In particular, when the carbon content of the base material is 0.01% by weight or more, the hardness is significantly increased.

FIG. 4 shows the carbon content and JI of the steel sheet (base material).
The relationship between the tensile strength of the S5 test piece and the rate of increase in strength with respect to the untreated material is shown. According to this, similarly to the result shown in FIG. 3, the tensile strength increases as the carbon content of the base material increases, regardless of the type of the shielding gas .
When nitrogen is used as shielding gas under the conditions, A
The rate of increase in strength is higher than when r was used. In a steel sheet having a carbon content of 0.03 wt%, the strength increase rate is only about 2% when Ar is used.
When nitrogen is used, the strength increase is about 15%.
When nitrogen is used for a steel sheet having a carbon content of 0.05 wt%, the strength increase rate reaches as much as about 25%. Thus, it can be seen that by supplying nitrogen as a shielding gas under the conditions of the method of the present invention , nitrogen penetrates into the molten and solidified portion and the solidified structure becomes a martensite structure, thereby increasing the hardness and tensile strength.

Example 2 Component composition: C: 0.07 wt.
%, Si: 0.11 wt%, Mn: 1.65 wt%, and a 1.6 mm-thick cold-rolled steel sheet was subjected to linear laser irradiation with an Nd-YAG laser. In the example of the present invention, nitrogen was used as a shielding gas, while in the comparative example, Ar was used as a shielding gas.

In this embodiment, the laser beam diameter is set to 0.4 to 8
mm, the test pieces having different aspect ratios (penetration depth H / penetration width W) of the melt-solidified portion were prepared, and their tensile strength, thermal strain, and micro Vickers hardness H
v (measuring load 50 gf) was measured. Tensile test is based on JI
The test was performed by forming three linear melt-solidified portions as shown in FIG. 2 on the S5 test piece. The measurement of thermal strain was 300 (l).
Three melt-solidified portions were formed on a × 25 (w) mm test piece, and the amount of warpage (h) in the longitudinal direction of the test piece was measured.
× 100 (%) was evaluated. The laser irradiation conditions are as follows. Laser power: 2.5 kW Processing speed: 3 m / min Focal length of condenser lens: 127 mm Focus position: 0 to 20 mm Laser beam diameter on steel plate: 0.4 to 8 mm Nozzle diameter: 5 mm Nozzle height: 7 to 27 mm Shield gas type: Ar, N ₂ Shield gas flow rate: 20 l / min

FIG. 5 shows the relationship between the rate of increase in strength of the test material supplied with nitrogen as a shielding gas with respect to the untreated material, the thermal strain, and the aspect ratio. According to this, the strength increase rate is relatively small in a steel sheet having a surface melting type melt-solidified portion having an aspect ratio of less than 0.5, whereas the strength increase rate increases when the aspect ratio is 0.5 or more. ing. In the case of a surface-melting type having an aspect ratio of less than 0.5, deformation due to thermal strain is large, while an aspect ratio of 0.
In the deep penetration type of 5 or more, thermal deformation is appropriately suppressed.

Conventionally, surface modification techniques using laser irradiation for the purpose of improving the mechanical properties, heat resistance, and the like of metal materials are known. In these techniques, a metal material is heated to a temperature just above the Ac ₃ point, or Has been melted. However, all of these conventional techniques only thinly treat the vicinity of the surface of the metal material, and even when melting, it is a surface melting type laser processing, and the aspect ratio of the laser processing layer is less than 0.5. . According to the test results, although such an aspect ratio of surface melting type laser treatment but one <br/> response of less than 0.5 effect can be obtained, high strength and thermal distortion of the suppression of the steel is sufficiently not, in order to effectively achieve the inhibition of high strength and thermal distortion of the aspect ratio it can be seen that it is necessary to be 0.5 or more.

Table 1 shows the results of thermal strain, micro-Vickers hardness Hv, and rate of increase in strength for a typical example of processing in this embodiment and an example in which transformation quenching was performed. According to the same table, when nitrogen is supplied as a shielding gas, a significant increase in hardness is recognized in both the deep penetration melting and the surface melting type, but the molten volume is not sufficient in the surface melting type, The strength increase rate is smaller than that of the deep penetration melting type, and the thermal strain is also large.

[0020]

[Table 1]

[0021]

According to the present invention described above, a carbon steel material,
Regardless of the type of the ultra-low carbon steel material, the strength of the steel material can be effectively increased, and a lightweight and high-strength steel material can be obtained more than before. In particular, an aspect ratio of 0.5
By the laser irradiation conditions such as more deep penetration weld portion is formed, it is possible to increase the strength more effectively, efficiently suppressing that the generation of thermal distortion which cause such defective shape Thus, a steel material having higher strength and higher dimensional accuracy can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of an implementation state of the present invention.

FIG. 2 is a plan view showing a test piece used in a tensile test according to an example of the present invention.

FIG. 3 shows a case where nitrogen is used as a shielding gas and Ar
Is a graph showing the relationship between the carbon content of the base material and the micro-Vickers hardness Hv of the melt-solidified portion in the case of using

FIG. 4 shows a case where nitrogen is used as a shielding gas and Ar
Is a graph showing the relationship between the carbon content of the base material, the tensile strength of the test piece, and the rate of increase in strength with respect to the untreated material when using

FIG. 5 is a graph showing a relationship between an aspect ratio of a molten portion by laser irradiation, a rate of increase in strength of a test piece with respect to an untreated material, and thermal strain.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Condensing lens, 2 ... Laser beam, 3 ... Steel material, 4 ... Keyhole, 5 ... Shielding gas, 6 ... Fused part

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Makoto Kabazawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yukio Maho 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Inside Steel Tube Co., Ltd. (72) Inventor Aohi Tsuyama 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Inside Tube Co., Ltd. (72) Inventor Hiroyuki Tsunoda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Tube Co., Ltd. (56) References JP-A-56-116820 (JP, A) JP-A-59-179776 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 1 / 09,1 / 34 C23C 8 / 24,8 / 26

Claims (1)

(57) [Claims] 1. A method according to claim 1, wherein the melting portion is formed with an aspect ratio of 0.5.
Appropriate laser is applied to the steel material to form the above deep penetration and fusion zone.
By supplying nitrogen as all or a part of the shielding gas to the laser irradiation part while irradiating linearly at an appropriate interval, nitrogen was added to a molten part formed by laser irradiation, and nitrogen was enriched. during the bead-shaped molten-solidified portion suitable
A method for strengthening a steel material, wherein the steel material is formed linearly at intervals .