JPH0741842A - Method for strengthening steel product - Google Patents

Abstract

PURPOSE:To highly strengthen steel products without generating problems, such as thermal strains. CONSTITUTION:While the steel products 3 are irradiated with a laser 2, high- carbon filler wires 6 are supplied to the part irradiated with the laser, by which carbon is added to a molten part 7 formed by the irradiation with the laser to form a bead-shaped molten and solidified part where carbon is enriched to a hardened structure. The aspect ratio (penetration depth H/penetration width W) of the molten part 7 is preferably specified to >=0.5 in order to effectively enhance the strength and to suppress the generation of the thermal strains.

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 problems such as thermal strain. Here, the steel material includes a steel sheet and other unprocessed materials and processed materials obtained by processing these with a press or the like.

2. Description of the Related Art In recent years, there has been an increasing demand for higher strength and lighter weight of press-formed products. However, high-strength materials cause problems in terms of shape fixability, etc. There is a limit. The strength of the press-formed product can be improved by pressing a thin steel plate into a complicated shape. However, in order to press the carbon steel sheet into a complicated shape that can obtain the required strength, it is necessary to use high energy velocity processing methods such as impact hydraulic forming method and explosion forming method. It has the disadvantage of low productivity and high cost.

On the other hand, as a technique for obtaining a press-formed product that is lightweight and has high strength, 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. The technology for improving the strength of press-formed products by forming a structure (quenching and hardening part)
It is proposed as JP-A-4-72010. This technology can be applied to materials with high quench-hardenability, that is, materials with a carbon content of usually 0.05 wt% or more. It is possible to obtain a press-formed product that has high strength, is lightweight, and has high strength.

[0003]

The quench hardenability of steel materials is generally determined by the carbon equivalent (for example, Ceq = C + Si / 24).
+ Mn / 6), but the increase in strength of the above laser-treated material is controlled by the carbon equivalent as well as the quench-hardenability. According to the above-mentioned JP-A-4-72010, for example, a steel material having a carbon content of 0.05 wt% is used to form three laser quench-hardened parts in parallel with the lengthwise direction J.
The tensile strength of the IS5 test piece is increased by about 20% as compared with the untreated test piece. 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%, the JIS in which the laser quench hardening portion similar to the above is formed
The No. 5 test piece has a strength increase of only about 2% as compared with the untreated test piece. As described above,
The technique of No. 72010 has a problem that it cannot be practically applied to a material having a relatively low carbon content because the rate of increase in strength is low. In addition, the strength increase rate of steel materials having a high carbon content is at most about 20%, and higher strength cannot be achieved.

The present invention has been made in view of such conventional problems, and it is possible to increase the strength of a steel material according to its carbon content and without causing problems such as thermal strain.
It is intended to provide a method that makes it possible to obtain a steel material that is lighter and has higher strength than ever before.

In order to achieve such an object, the present invention is obtained when carbon steel or high carbon steel is quenched by externally adding carbon to the molten portion of the steel material by laser irradiation. In order to increase the strength of the steel material, a melt-solidified portion having the same quenched structure as that described above is formed.

That is, according to the present invention, by irradiating a steel material with a laser and supplying a high carbon filler wire to the laser irradiation portion, carbon is added to the melting portion formed by the laser irradiation to enrich the carbon. A method for strengthening a steel material, which comprises forming a bead-shaped melt-solidified portion. The laser irradiation in the method of the present invention is carried out so that the molten shape has a relatively deep penetration in order to effectively increase the strength and suppress the occurrence of thermal strain, and in particular, the aspect ratio (melting depth It is preferable to carry out so that H / melting width W) becomes 0.5 or more.

As the laser, any laser system that can be used for thermal processing, such as CO ₂ laser, CO laser, Nd-YAG laser, glass laser and excimer laser, can be applied. The carbon content of the high carbon filler wire supplied to the laser irradiation part is appropriately adjusted according to the required hardness of the melting and solidifying part and the strength of the material. Moreover, as the shield gas, a rare gas such as Ar or He that is usually used can be used.

[0007]

The operation of the present invention will be described with reference to FIG. FIG. 1 shows an example in which a high carbon filler wire is fed to a laser irradiation section and is melted by a laser beam to form a melting section. Laser beam 2 (normally energy density: 10 ⁴ to 1) condensed by a condenser lens 1 (for example, ZnSe lens)
( ⁷ W / cm ² ) is applied to the steel material 3, and the high carbon filler wire 6 is laterally supplied to this irradiation part. The laser irradiation part of the steel material 3 and the high carbon filler wire 6 are melted and mixed to form the keyhole 4. A so-called melting hole is formed, and carbon of the high carbon filler wire penetrates into the melting portion 7. Usually, a rare gas such as Ar or He is used as the shield gas.

As described above, carbon is enriched in the melted portion by laser irradiation and the melted portion is rapidly solidified and cooled. Therefore, a quenching structure similar to that obtained when quenching carbon steel or high carbon steel is obtained. A melted and solidified portion having the same is formed, and the hardness and strength of the melted and solidified portion are significantly increased as compared with the base material. Therefore, if the laser irradiation is linearly performed at an appropriate interval for a portion of the steel material requiring at least strength, a linear melt-solidified portion having a quenched structure is formed at the portion, and the portion is formed. The strength of can be significantly increased. Needless to say, the strength of the entire steel material can be increased by linearly irradiating the entire steel material at appropriate intervals. Usually, the melt-solidified portions are formed in a stripe shape or a lattice shape at appropriate intervals.

By setting the aspect ratio (penetration depth H / penetration width W) of the fusion zone to 0.5 or more,
It is possible to increase the strength more effectively and to effectively suppress the occurrence of thermal strain. The hardness of the molten and solidified portion and the strength of the steel material can be controlled by adjusting the carbon content of the high carbon filler wire. The strength of the steel material can also be adjusted by selecting the distance between the melt-solidified portions.

The steel materials targeted by the present invention include general carbon steel materials and ultra-low carbon steel materials, and if necessary, Mn,
It includes all kinds of steel materials such as steel materials strengthened with Si and P, steel materials with B added for grain boundary strengthening, and ultra-low carbon steel materials with at least some of the interstitial elements fixed with Ti and Nb. Further, the types of steel materials (worked materials and unprocessed materials) are not limited to steel plates, and can be applied to all kinds of materials such as pipes, strips, and wire rods. Further, the present invention can be applied to steel materials such as steel plates whose surfaces are plated (electroplating or hot dipping), and the type of plating does not matter.

[0011]

【Example】

[Example 1] The composition of the raw steel sheet was C: 0.01 to 0.
20 wt%, Si: 0.02 wt%, Mn: 0.69w
At t%, a linear laser irradiation was performed using a CO ₂ laser on a galvannealed steel sheet having a plate thickness of 1.4 mm. In the example of the present invention, the high-carbon filler wire was supplied to the laser irradiation portion to perform the irradiation, while in the comparative example, the laser irradiation was performed without supplying the high-carbon filler wire. The supplied filler wire had a diameter of 0.8 mm and had a chemical composition of C: 0.
50wt%, Si: 0.50wt%, Mn: 1.40w
t%. The laser irradiation conditions are as follows. Laser output: 3.0 kW Processing speed: 3 m / min Focal length of condenser lens: 254 mm Focal position: -0.5 mm Aspect ratio: 0.7 Nozzle diameter: 3 mm Nozzle height: 5 mm Center gas type: Ar Center gas Flow rate: 10 l / min High carbon filler wire supply: 2 g / min

The micro Vickers hardness Hv (measurement load 50) of the bead-like melted and solidified portion obtained by laser irradiation.
gf) was measured. Further, according to each test condition, a JIS No. 5 test piece as shown in FIG. 2 having three bead-shaped melt-solidified portions formed in parallel with the tensile direction was prepared, and the tensile strength of each test piece was measured. . FIG. 3 shows the relationship between the carbon content of the steel plate (base material) and the micro Vickers hardness Hv of the melt-solidified portion. According to this, the micro-Vickers hardness of the molten and solidified portion becomes higher as the carbon content of the base material increases, regardless of whether or not the high-carbon filler wire is supplied, but when the high-carbon filler wire is supplied, the high-carbon filler wire is The micro Vickers hardness is about 200 higher than when not supplied.

FIG. 4 shows the relationship between the carbon content of the base material and the carbon content of the melt-solidified part obtained by supplying the high carbon filler wire. According to this, the carbon content C _L of the melt-solidified portion is
Between the carbon content C _{O of the} base material and the carbon content C _W (0.5 wt%) of the filler wire, C _L = (C _W +
The relationship of C _O ) / 2 [wt%] is recognized. This is because the melt-solidification part has a dilution ratio of the filler wire and the base material of 5
It means 0%.

FIG. 5 shows the carbon content of the steel sheet (base material) and JIS.
The relationship between the tensile strength of the No. 5 test piece and the strength increase rate with respect to the untreated material is shown. According to this, similar to the result shown in FIG. 3, the tensile strength increases as the carbon content of the base material increases, regardless of whether or not the high carbon filler wire is supplied.
When the high carbon filler wire is supplied, the strength increase rate is higher than when the high carbon filler wire is not supplied. In the steel sheet with a carbon content of 0.03 wt%, the strength increase rate is only about 2% when the high carbon filler wire is not supplied, but the strength increase rate is about 23% when the high carbon filler wire is supplied. Is. Further, when a high carbon filler wire is supplied to a steel sheet having a carbon content of 0.05 wt%, the strength increase rate reaches about 30%. It can be seen that by supplying the high-carbon filler wire in this way, carbon penetrates into the molten and solidified portion, and the solidification structure becomes a high-carbon martensite structure as compared with the base material, thereby increasing hardness and tensile strength.

[Example 2] Component composition C: 0.08 wt
%, Si: 0.15 wt%, Mn: 1.70 wt%, and a cold-rolled steel sheet having a thickness of 1.6 mm was subjected to linear laser irradiation by an Nd-YAG laser using Ar as a shield gas. In the example of the present invention, laser irradiation was performed by supplying a high carbon filler wire, while in the comparative example, laser irradiation was performed without supplying a high carbon filler wire.

In this embodiment, the laser beam diameter is 0.4 to 8
The test pieces having different aspect ratios (penetration depth H / penetration width W) of the molten and solidified portion are prepared by changing the range of mm, and their tensile strength, thermal strain and micro Vickers hardness H
v (measurement load 50 gf) was measured. The tensile test is JI
Three linear melt-solidified portions as shown in FIG. 2 were formed on the S5 test piece. Also, the measurement of thermal strain is 300 (l)
The amount of warpage (h) in the longitudinal direction of the test piece was measured by forming three melt-solidified portions on a test piece of × 25 (w) mm, and h / l
It evaluated by x100 (%). The laser irradiation conditions are as follows. Laser output: 2.5 kW Processing speed: 3 m / min Focal length of condenser lens: 127 mm Focal position: 0-20 mm Laser beam diameter on steel plate: 0.4-8 mm Nozzle diameter: 5 mm Nozzle height: 7-27 mm Type of center gas: Ar Center gas flow rate: 20 l / min Carbon content of high carbon filler wire: 0.4 wt% Supply amount of high carbon filler wire: 4 g / min

FIG. 6 shows the relationship between the rate of increase in strength, the thermal strain, and the aspect ratio of the test material supplied with the high carbon filler wire with respect to the untreated material. According to this, the aspect ratio is 0.
A steel sheet having a surface-melting type melt-solidified portion of less than 5 has a relatively small increase rate of strength, whereas an aspect ratio of 0.5 or more increases the increase rate of strength. Further, in the surface melting type having an aspect ratio of less than 0.5, deformation due to thermal strain is large, whereas in the deep penetration type having an aspect ratio of 0.5 or more, thermal deformation is appropriately suppressed.

Conventionally, surface modification techniques utilizing laser irradiation have been known for the purpose of improving the mechanical properties and heat resistance of metal materials. In these techniques, the metal material is heated to a temperature just above the Ac ₃ point, or Melting. However, in all of these conventional techniques, the vicinity of the surface of the metal material is only thinly processed, and even when melting is performed, the surface melting type laser processing is performed, and the aspect ratio of the laser processing layer is less than 0.5. . According to the above test results, although such a surface melting type laser treatment with an aspect ratio of less than 0.5 can provide a tentative effect as the method of the present invention, it is not always possible to increase the strength of a steel material and suppress thermal strain. It is not sufficient, and it is understood that the aspect ratio is preferably 0.5 or more in order to effectively achieve high strength and suppression of thermal strain.

Table 1 shows the results of thermal strain, micro-Vickers hardness Hv, and strength increase rate for the representative example of treatment in this example and the example for which transformation and quenching treatment was performed. According to the same table, when a high carbon filler wire is supplied, a significant increase in hardness is observed in both deep penetration melting and surface melting types. Compared to the penetration-melt type, the strength increase rate is small and the thermal strain is large.

[0020]

[Table 1]

[0021]

According to the present invention described above, a carbon steel material,
It is possible to effectively increase the strength of the steel material regardless of whether it is an ultra-low carbon steel material, and it is possible to obtain a lighter and higher strength steel material than ever before. Also, especially when the aspect ratio is 0.5
By setting the laser irradiation conditions so that the deep-penetration molten portion is formed, the strength can be more effectively increased, and the occurrence of thermal strain that causes a defective shape or the like can be effectively suppressed. It is possible to obtain a steel material having higher strength and higher dimensional accuracy.

[Brief description of drawings]

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

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

FIG. 3 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 with and without the supply of the high-carbon filler wire.

FIG. 4 is a graph showing the relationship between the carbon content of the base metal and the carbon content of the melt-solidified part obtained by supplying the high carbon filler wire.

FIG. 5 is a graph showing the relationship between the carbon content of the base material, the tensile strength of the test piece, and the strength increase rate with respect to the untreated material, with and without the supply of the high carbon filler wire.

FIG. 6 is a graph showing the relationship between the aspect ratio of the melted portion due to laser irradiation, the strength increase rate of the test piece with respect to the untreated material, and the thermal strain.

[Explanation of symbols]

1 ... Focusing lens, 2 ... Laser beam, 3 ... Steel material, 4 ... Keyhole, 5 ... Shield gas, 6 ... High carbon filler wire, 7 ... Melting part

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinji Kabazawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan KK (72) Inventor Yukio Maho 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd. (72) Inventor Aotsu Tsuyama, 1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Tube Co., Ltd. (72) Inventor, Hiroyuki Tsunoda 1-2, Marunouchi, Chiyoda-ku, Tokyo Nihon Inside Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A steel material is irradiated with a laser and a high carbon filler wire is supplied to the laser irradiation portion to add carbon to a molten portion formed by the laser irradiation to form a bead-shaped carbon-enriched material. A method for strengthening a steel material, which comprises forming a melt-solidified portion. 2. The method for strengthening a steel material according to claim 1, wherein the aspect ratio of the molten portion by laser irradiation is 0.5 or more.