JP3073630B2 - How to strengthen ultra low carbon 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 increasing the strength of ultra-low carbon steel without causing problems such as thermal strain. Here, the ultra-low carbon steel material includes a steel plate and other unprocessed materials and processed materials obtained by processing them by pressing or the like.

[0002]

2. Description of the Related Art As a technique for obtaining a light-weight 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 into a linear shape.
Japanese Patent Application Laid-Open No. 4-72010 proposes a technique for improving the strength of a press-formed product by making the molten portion a quenched structure (hardened portion). 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]

However, in recent years, demands for higher strength and lighter weight of press-formed products have been increasing more and more. In order to meet such demands, thin steel sheets have been pressed into more complicated shapes. It is necessary to process and improve the strength of the material. In order to deal with such a complicated press shape, it is conceivable to use a high energy rate processing method such as an impact hydraulic molding method or an explosion molding method from the processing technology side.
These processing methods have drawbacks of low productivity and high cost. Therefore, in order to cope with the complexity of the press shape, it is necessary to use a high r-value ultra-low carbon steel sheet excellent in press workability. However, since this type of steel sheet has a low quench hardening ability, there is a problem that even if the above-described laser quenching is performed, the strength of the press-formed product cannot be increased. That is, the hardenability of a steel material is generally defined by a carbon equivalent (for example, Ceq = C + Si / 24 + Mn / 6), but ultra-low carbon steel (generally, a carbon content: less than 0.005 wt%) is determined by Mn, Si, and the like. Ce
Even if q is increased, quench hardening does not occur, and the quenchability is essentially low. For this reason, even if the above-mentioned laser quenching technique is used, an improvement in strength can hardly be expected.

The present invention has been made in view of such a conventional problem, and it is possible to increase the strength of an ultra-low carbon steel excellent in press formability without causing a problem such as thermal strain. It is an object of the present invention to provide a method capable of obtaining a lightweight and high-strength steel material.

[0005]

In order to achieve the above object, the present invention provides a method similar to that obtained when carbon steel is quenched by adding carbon from the outside to a molten portion of a steel material by laser irradiation. It is intended to increase the strength of the ultra-low carbon steel by forming a melt-solidified portion having a quenched structure. That is, in the present invention, C: 0.005 wt%
Laser at appropriate intervals on ultra-low carbon steel containing
While illuminating the Jo, in the laser irradiation unit, by supplying one or gases containing two or more selected from the group consisting of hydrocarbon gas and carbon oxide-based gas, it is formed by laser irradiation The carbon in the gas component is added to the molten portion, and a carbon-enriched bead-shaped melt-solidified portion is appropriately formed.
This is a method for strengthening an ultra-low carbon steel material characterized by being formed linearly at an appropriate interval .

[0006] The laser irradiation in the method of the present invention is carried out so as to have a deep penetration melting shape in order to effectively increase the intensity and suppress the occurrence of thermal strain . It is preferable that the aspect ratio (penetration depth H / penetration width W) of the fusion part be set to 0.5 or more. As the laser can be applied CO ₂ lasers, CO lasers, Nd-YAG laser, a glass laser, excimer laser, any laser system that can be used for thermal processing.

[0007] Gas supplied to the laser irradiation part, there hydrocarbon (C _m H _n) based gas and one or gases containing two or more selected from the group consisting of carbon oxides (CO _m) based gas Therefore, the supplied gas may be a gas composed of only a hydrocarbon (C _m H _n ) -based gas or / and a carbon oxide (CO _m ) -based gas, For example, a mixed gas with Ar, He or the like may be supplied. As the hydrocarbon (C _m H _n) based gas methane (CH _4), propane (C ₃ H _8), ethane (C ₂ H
₆ ), butane (C ₄ H ₁₀ ), ethylene (CH ₂ ＣＨCH ₂ ) and the like, and carbon oxide (CO _m ) based gases such as CO, C
O ₂ or the like can be used, and these gases can be used alone or in combination of two or more. When two or more kinds are mixed, two kinds of hydrocarbon-based gas or carbon oxide-based gas are used.
A mixture of two or more species may be used, or a mixture of one or more hydrocarbon gases and one or more carbon oxide gases may be used.

As a method for supplying these gases, it is relatively easy to supply the gas as a center gas (or a part thereof) of a center gas system for supplying a gas coaxially with a laser beam. Any method can be used as long as a gas can be supplied into the keyhole, and for example, it can be supplied as an after gas.

[0009]

The operation of the present invention will be described with reference to FIG. FIG. 1 shows an example in which gas is supplied to a laser irradiation unit by a center gas method. When a laser beam 2 (normally, energy density: 10 ⁴ to 10 ⁷ W / cm ² ) condensed by a condensing lens 1 (for example, a ZnSe lens) is applied to the ultra-low carbon steel material 3, the irradiated portion is instantaneously melted. It evaporates to form a molten hole called a keyhole 4. The inside of the keyhole 4 is composed of evaporated particles of iron, which is a main atom constituting steel, and iron atoms in an excited / ionized state, and the temperature is 5000.
℃ to 10,000 ℃. In ordinary laser welding, a rare gas such as Ar or He is used as the center gas 5, but in the present invention, a gas containing a hydrocarbon-based gas and / or a carbon oxide-based gas is supplied as the center gas 5. These supplied gases are thermally decomposed in a keyhole to become mainly carbon in an excited state as shown in the following formula, and enter the molten portion 6. _{CH 4 → C * + 4H 2} /2 CO 2 → C * + 2O 2/2 where, C * is a carbon atom in an excited state

[0010] As described above, since the molten portion by laser irradiation is enriched with carbon and the solidified portion is rapidly solidified and cooled, the molten solidified portion has 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 the portion of the ultra-low carbon steel material required for strength is linearly subjected to the laser irradiation at appropriate intervals, a linear melt-solidified portion having a quenched structure is formed at the portion. , The strength of the portion 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.

Further, by setting the aspect ratio (penetration depth H / penetration width W) of the fusion zone to 0.5 or more,
The strength can be more effectively increased, and the occurrence of thermal strain can be effectively suppressed. The hardness of the melt-solidified part and the strength of the steel material can be adjusted by changing the mixing ratio of hydrocarbon gas or carbon oxide gas with another gas (usually a rare gas). In this case, it can be adjusted also by the nozzle height and nozzle diameter, or the type and flow rate of the gas (hydrocarbon-based gas or carbon oxide-based gas) to be used. Further, the strength of the steel material can be adjusted not only by the above-described gas mixing ratio and the like, but also by selecting the interval between the melt-solidified portions.

The steel material to which the present invention is applied is C: 0.00
It is an ultra-low carbon steel material of less than 5 wt%, and such an ultra-low carbon steel material shows excellent press formability before being strengthened by the method of the present invention. Further, Ti, N
b includes all ultra-low carbon steel materials such as steel materials in which interstitial elements are fixed in whole or in part and reinforced by Mn, Si, and P as necessary, and steel materials to which B is added for grain boundary strengthening. . 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.

[0013]

【Example】

[Example 1] The composition of the material steel sheet was C: 0.0026 w
t%, Si: 0.16 wt%, Mn: 0.69 wt%,
Nb: 0.019 wt%, Ti: 0.023 wt%,
B: A 0.0005 wt%, alloyed hot-dip galvanized steel sheet having a thickness of 1.4 mm was subjected to linear laser irradiation using a CO ₂ laser. In this embodiment, as the center gas, a mixing ratio of Ar and CH ₄ ([CH ₄ / (CH ₄
+ Ar)] × 100) were variously changed from 0 to 100%. 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: 5 mm Nozzle height: 7 mm Type of center gas: Ar, Ar + CH ₄ , CH ₄ center gas flow rate: 20 l / min

The micro-Vickers hardness Hv (measuring load 50) of the 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 Ar in the center gas.
And shows the relationship between the mixing ratio of CH ₄ and hardness Hv of the molten-solidified portion. According to this, CH ₄ in the center gas is 0%
(Ar: 100%), the hardness of the melt-solidified portion is Hv
150, which is higher than the base material (hardness Hv: 100) by Hv5.
It just rose by about 0. On the other hand, as the ratio of CH _{4 in} the center gas increases, the hardness of the melt-solidified portion significantly increases. And the mixing ratio of CH ₄ is 1
Even at about 0%, the hardness of the melt-solidified portion increases greatly, and Hv3
It has reached 50.

FIG. 4 and FIG.
The microscopic photographs of the solidified structures in the cases of% Ar (FIG. 4) and 100% CH ₄ (FIG. 5) are shown. According to these photographs, when Ar gas is 100%, the main component is ferrite exhibiting irregular grain boundaries,
Slight bainite is observed. On the other hand, CH ₄ gas 1
In the case of 00%, the melt-solidified portion has a quenched structure composed only of martensite. FIG. 6 shows the relationship between the mixing ratio of Ar and CH ₄ in the center gas and the tensile strength of the JIS No. 5 test piece. According to this, similarly to the experimental result obtained in FIG. 3, as the ratio of CH ₄ in the center gas increases, the tensile strength of the melt-solidified portion increases,
Even when the mixing ratio of CH ₄ is about 10%, the strength is increased by about 20% with respect to the base material.

As described above, by mixing CH ₄ in the center gas or using CH ₄ as the center gas itself, C generated as a result of thermal decomposition of CH ₄ penetrates into the molten and solidified portion, and the solidified structure becomes a martensitic structure. It can be seen that the hardness and tensile strength were increased by As shown in FIGS. 3 and 6, it can be seen that the hardness and tensile strength of the melt-solidified portion can be adjusted by changing the mixing ratio of CH ₄ and the rare gas.

Example 2 Component composition: C: 0.0026
wt%, Si: 0.21 wt%, Mn: 1.37 wt
%, Ti: 0.055 wt%, B: 0.0008 wt%
To 1.6mm cold-rolled steel sheet with A as center gas
with r and C ₄ H _10, respectively it was performed linear laser irradiation by Nd-YAG laser. At this time, the laser beam diameter was changed in the range of 0.4 to 8 mm, test pieces having different aspect ratios (penetration width W / penetration depth H) of the melt-solidified portion were prepared, and their tensile strength, thermal strain and The micro Vickers hardness Hv (measuring load 50 gf) was measured. The tensile test was performed by forming three linear melt-solidified portions as shown in FIG. 2 on a JIS No. 5 test piece. The measurement of thermal strain was performed by forming three melt-solidified portions on a test piece of 300 (l) × 25 (w) mm and measuring the amount of warpage (h) in the longitudinal direction of the test piece.
Was measured and evaluated in h / l × 100 (%). 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 Center gas type: Ar, C ₄ H ₁₀ Center gas flow rate: 20 l / min

FIG. 7 shows the relationship between the tensile strength (rate of increase in the tensile strength relative to the untreated material) and the thermal strain of the test material using C ₄ H ₁₀ as the center gas, and the aspect ratio. According to the figure, in a steel sheet having a surface solidification type melt-solidified portion having an aspect ratio of less than 0.5, the rate of increase in tensile strength relative to the untreated material is relatively small. On the other hand, when the aspect ratio is 0.5 or more, the rate of increase in tensile strength with respect to the untreated material increases. In addition, while deformation due to thermal strain is large in a surface melting type having an aspect ratio of less than 0.5,
In a deep penetration type having an aspect ratio of 0.5 or more, thermal deformation is appropriately suppressed.

Conventionally, surface modification techniques using laser irradiation for the purpose of improving the mechanical properties and heat resistance of metal materials have been known, but all of these techniques are surface melting type laser treatments. The aspect ratio of the laser processing layer is less than 0.5. According to the above test results, although the surface melting type laser treatment having an aspect ratio of less than 0.5 can provide a tentative effect as the method of the present invention,
It can be seen that increasing the strength of the steel material and suppressing the thermal strain are not always sufficient, and it is found that the aspect ratio is preferably 0.5 or more in order to effectively achieve the increase in the strength and the suppression of the thermal strain.

Table 1 shows the results of thermal strain, micro Vickers hardness Hv, and rate of increase in tensile strength for a typical treatment example and a transformation quenching treatment example in this embodiment. According to the same table, when C ₄ H ₁₀ gas was used as the center gas, a significant increase in hardness was observed in both the deep penetration melting and the surface melting type. Since it is not sufficient, the rate of increase in tensile strength is smaller than that of the deep penetration melting type, and the thermal strain is also large.

[0021]

[Table 1]

Example 3 Component composition: C: 0.0023
wt%, Si: 0.15 wt%, Mn: 1.40 wt
%, Nb: 0.015 wt%, Ti: 0.055 wt%
, A linear laser irradiation was performed with a CO ₂ laser on a cold-rolled steel sheet having a thickness of 1.4 mm. In this embodiment, Ar and C
Three types of center gases were used in which the mixing ratio with O [CO / (CO + Ar)] × 100% was 0%, 50%, and 100%. 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: 1.2 Nozzle diameter: 3 mm Nozzle height: 5 mm Center gas flow rate: 20 l / min

The micro Vickers hardness Hv (measurement load 50) of the bead-shaped melt-solidified portion obtained by laser irradiation
gf) was measured. In addition, three bead-shaped melt-solidified parts were formed parallel to the tensile direction on a JIS No. 5 test piece as shown in FIG. 2, and the strength of the untreated material was increased from the tensile strength of each test piece. The rate was measured. Table 2 shows the results. According to the table, by supplying CO to the laser irradiation part, the hardness of the melt-solidified part and the strength of the material are significantly increased as compared with the case of using Ar gas alone, and the mixing ratio of CO in the center gas is increased. It can be seen that the above characteristics are improved by increasing the value.

[0024]

[Table 2]

[0025]

According to the present invention described above, it is possible to effectively increase the strength of an ultra-low carbon steel material excellent in press formability, and obtain a lighter and higher strength steel material than ever before. Becomes possible. In addition, by setting the laser irradiation conditions such that a deep penetration fusion part having an aspect ratio of 0.5 or more is formed, the strength can be more effectively increased, and the heat causing a shape defect and the like can be improved. The generation of distortion can be effectively suppressed, and a lighter, higher strength, and high dimensional accuracy steel material 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 is a graph showing a relationship between a mixing ratio of CH ₄ in a gas supplied to a laser irradiation part and a hardness Hv of a melt-solidified part.

FIG. 4 is a microscope enlarged photograph showing a metal structure of a melt-solidified portion when 100% Ar is used as a center gas.

FIG. 5 is a microscope enlarged photograph showing the metal structure of the melt-solidified portion when 100% CH ₄ is used as the center gas.

FIG. 6 is a graph showing a relationship between a mixing ratio of CH ₄ in a gas supplied to a laser irradiation unit and a tensile strength of a steel sheet.

FIG. 7 is a graph showing the relationship between the aspect ratio of a fusion zone by laser irradiation, the tensile strength of a steel sheet (increase rate of tensile strength with respect to an untreated material), and thermal strain.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Condensing lens, 2 ... Laser beam, 3 ... Ultra low carbon steel material, 4 ... Keyhole, 5 ... Center gas, 6 ... Fused part

Continuing on the front page (72) Inventor: Satoshi Kaizu 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Makoto Kabazawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Seiji Tsuyama 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Hiroyuki Tsunoda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-56-116820 (JP, A) JP-A-57-171618 (JP, A) JP-A-4-52265 (JP, A) JP-A-59-179776 (JP, A) (58) Survey Field (Int. Cl. ⁷ , DB name) C21D 1 / 09,1 / 34,6 / 00 C23C 8/22 B23K 26 / 00,26 / 18

Claims (2)

(57) [Claims] 1. An ultra-low carbon steel material containing less than 0.005 wt% of C: linearly irradiating a laser at appropriate intervals .
By supplying a gas containing one or more selected from the group consisting of a hydrocarbon-based gas and a carbon oxide-based gas to the laser irradiation part, the gas component is supplied to a melting part formed by laser irradiation. A method for strengthening an ultra-low carbon steel material, comprising adding carbon therein to form a bead-like melt-solidified portion enriched with carbon in a linear shape at appropriate intervals . 2. The method according to claim 1, wherein the laser beam is melted by an aspect ratio.
Characterized by a deep penetration fusion zone with a ratio of 0.5 or more
The method for strengthening an ultra-low carbon steel material according to claim 1.