JP3376842B2 - Steel plate excellent in laser cutting property and method for producing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser cutting of a steel sheet, and a steel sheet that enables high-speed cutting while maintaining good cutting quality and a method for manufacturing the same. About. [0002] A large amount of hot-rolled steel sheets are used in shipbuilding, construction, bridges, and the like. The steel sheet here has a thickness of 1.5 to
It is a 25 mm steel plate. In the work of these steel structures, much of the cost and man-hour is occupied by welding and cutting. Recently, automation for welding has progressed considerably to streamline these processes, but development has just begun for streamlining cutting. Conventionally, methods such as shearing, gas cutting, plasma cutting or laser cutting have been used as steel sheet cutting methods, and the suitability and productivity for these automations will be summarized below. Shearing is easy to automate and has high productivity, but it is limited to straight cutting of simple shapes.
Although it is good as a means for preparing a material, it is not suitable for part cutting. Gas cutting is the most commonly used cutting method because there is virtually no upper limit on the cutting plate thickness and the apparatus is simple. However, full automation technology has not been completed in terms of control and monitoring of the combustion flame. In addition, since the cutting width is wide, there are disadvantages such as large thermal deformation and a low cutting speed. Plasma cutting is a cutting method that is becoming popular instead of gas cutting because high-speed cutting is possible when the plate thickness is 50 mm or less. This method is suitable for groove cutting, but has the disadvantage that the accuracy of the bevel angle is poor and it is difficult to ensure the cutting accuracy when many small parts are cut from a large plate. In addition, the life of the torch is only a few hours, and its replacement reduces productivity and is not easy to fully automate. On the other hand, laser cutting is a cutting method that has been put into practical use from about 20 years ago, and has been popularized mainly in the thin plate processing industry with a plate thickness of 3.2 mm or less. As a result, the application is expanding to the cutting of hot-rolled steel sheets having a larger thickness. Except for the fact that laser cutting is relatively expensive and that the upper limit of the target plate thickness is limited to about 20 mm due to the low output of currently available equipment, it is technically ideal. Cutting method. Its features are as follows. First, the cutting width is very narrow.
For this reason, there are advantages such as small thermal deformation due to cutting heat input, common cutting in addition to cut-out cutting, and high-speed cutting for the output of the cutting machine. As a second feature, the cutting quality is good. It is easy to ensure the accuracy of the bevel angle of the cut surface, and it is easy to manage the angular accuracy of the corner portion. Also,
There is little adhesion of slag at the upper edge of the cut, the presence or absence of slag adhesion at the lower edge of the cut is clear, and it is easy to determine the appropriate condition setting. Thirdly, since the laser optical system and cutting torch have a long life and the laser output is easy, it is also a great feature that the cutting operation can be fully automated easily. Therefore, the laser cutting method can be said to be the most powerful method for rationalizing the steel sheet cutting. As described above, although laser cutting is an effective method for rationalizing steel sheet cutting, its spread has not necessarily expanded due to the following problems. That is, the device price is relatively expensive. This needs to be considered as the cutting cost, ie the cost per unit cutting length. To this end, it is necessary to take measures to enable cutting with higher productivity when a laser oscillator that controls the device price is given. Specifically, it is necessary to increase the energy efficiency consumed in the operation of the cutting machine, and particularly to suppress the yield reduction due to the occurrence of defective cutting parts such as notches (partial cutting width expansion). The above problems require improvement of either or both of the cutting device surface and the steel plate surface. However, in the present situation where a considerable amount of the cutting device is already used, improvement of the steel sheet characteristics is effective and strongly demanded. That is, it is an important issue to manufacture a hot-rolled steel sheet capable of laser cutting with higher productivity. In response to such demands, it is known that the surface properties of a steel sheet produced by hot rolling, particularly the scale formed on the surface, plays an important role in improving laser cutting performance. For example, in JP-A-7-48622 and JP-A-7-48623, by controlling the production conditions of the steel sheet, the composition of the scale is mainly magnetite (Fe ₃ O ₄ ), and uniform and excellent adhesion A thin scale is generated to suppress generation of scale flaws during secondary processing of steel sheets and to improve laser cutting performance. However, in these techniques, a method for producing a thin and highly adherent scale is limited to the limited production conditions of the hot-rolled steel sheet, that is, the finish rolling temperature or the steel sheet surface temperature after finish rolling. Since it depends only on control, there exists a problem that the range of the material of the steel plate which can be manufactured will be restricted to the very narrow range. As a result,
The condition for improving the laser cutting performance is that the thickness of the scale to be generated is as thin as 5 μm or less. In JP-A-8-218119,
A method for producing a steel sheet having excellent scale adhesion and laser cutting properties by simultaneously controlling the chemical composition and production conditions of the steel sheet is disclosed. In the present invention, reference is also made to the scale composition and production conditions, and the steel sheet surface temperature after rolling is set to 600 to 700.
By cooling to ℃, the scale becomes wustite (F
eO) is approaching the isothermal transformation from magnetite (Fe ₃ O ₄ ). However, there is a problem that the addition of chemical components effective for the strength of the steel sheet is suppressed and the manufacturing conditions are limited as described above, so that the range of the steel sheet material that can be manufactured is limited to a very narrow range. On the other hand, much research has been conducted on the structure and composition of the scale layer. For example, the third edition of the Steel Handbook (I) “Rolling Foundation / Steel” (Japan Steel Association, Maruzen) Has been reported as follows. That is, the scale generated at a temperature of 570 ° C. or lower is mostly wustite (FeO) in the inner layer from the iron-iron interface, the upper intermediate layer is magnetite (Fe ₃ O ₄ ), and the outer layer is hematite (Fe ₂ O ₃ ). Wustite (FeO) produced at a high temperature is decomposed into iron (Fe) and magnetite (Fe ₃ O ₄ ) by a eutectoid reaction at a temperature of about 570 ° C. or lower during the cooling process. The fracture strength of iron scale at room temperature is Fe
O, Fe ₂ O ₃ and Fe ₃ O ₄ increase in this order. Thus, in the prior art, the scale composition is particularly preferably magnetite (Fe ₃ O ₄ ) from the viewpoint of adhesion of the scale. A method is disclosed.
However, the quantitative relationship between the laser cutting property and the scale composition itself is not clear, and thus a technique for quantitatively controlling the amount of magnetite produced in a hot-rolled steel sheet has not yet been established. [0020] The present invention relates to a magnetite (Fe) in a scale formed on the surface of a hot-rolled steel sheet.
It is an object of the present invention to provide a steel sheet excellent in laser cutting property, characterized by controlling the amount of ₃ O ₄ ) produced. Therefore, in the production of hot rolled steel sheets,
By controlling the parameters of the overall production conditions, including the final rolling reduction ratio of finish rolling, finish rolling temperature, steel sheet cooling rate immediately after finish rolling, and coiling temperature, it is possible to perform notching during laser cutting within the range of general steel sheet materials. The steel plate which suppressed generation | occurrence | production of this and was excellent in laser cutting property, and its manufacturing method are provided. The inventors have studied the various conditions given to the scale thickness and scale composition of the hot-rolled steel sheet, and as a result, have come up with the following invention. The first invention has a scale layer on the surface of the steel plate, and magnetite (Fe ₃ O ₄ ) is 15 in the scale.
vol% or more, the balance being substantially wustite (Fe
A steel plate excellent in laser cutting property, characterized by being O). According to the present invention, the scale formed on the surface of the steel sheet has high adhesion, and a steel sheet excellent in laser cutting property in which notches are hardly generated during cutting can be obtained. In the second invention, the hot steel slab obtained by continuous casting is subjected to hot rolling directly or after reheating.
Descaling is performed before and during the rolling, and the finishing rolling temperature is 950 to 650 ° C., the final rolling reduction of the finishing rolling is 5 to 25%, the cooling rate immediately after the finishing rolling is 5 to 60 ° C./s, and Winding temperature is 550-250 ° C
In each range, the final reduction ratio, rolling temperature,
The thickness of the scale to be generated and the amount of magnetite (Fe ₃ O ₄ ) in the scale are controlled by hot rolling under conditions where the cooling rate and the coiling temperature satisfy the following formulas (1) and (2). A method for producing a steel sheet having excellent laser cutting properties is provided. FT × (1−R / 100) × (7.5FT−4.5CR −13.5CR ・ CT + 1800CT) ≦ 6.21 × 10 ⁸ --- (1) 794 × (CT / 100) -230 × (CT / 100) ² +28 .96 × (CT / 100) ³ −1.343 × (CT / 100) ⁴ −1000 ≧ 0 --- (2) where R: Final rolling reduction ratio (%) FT: Finishing rolling temperature (° C) CR : Average cooling rate for 3 seconds immediately after finish rolling (° C./s) CT: Winding temperature (° C.) As described above, finish rolling conditions and the like have a great influence on laser cutting performance. Among the above conditions, in particular, the scale thickness is suppressed to about 15 μm or less, and
The amount of magnetite (Fe ₃ O ₄ ) in the scale is 1
Conditions including 5% or more are desirable, and a steel plate that satisfies these conditions can be manufactured by controlling the rolling conditions within the ranges of the above formulas (1) and (2). DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic concept of the present invention will be described below. The inventors laser cut several types of steel plates manufactured under various manufacturing conditions, and evaluated the cut surfaces. As a result, among the scales which are iron oxide layers mainly composed of wustite (FeO) and magnetite (Fe ₃ O ₄ ) generated on the steel sheet surface, particularly magnetite (Fe
_It has been found that the production amount of ₃ O ₄ ) has a great influence on the laser cutting property, and that the quality of the cut surface in laser cutting improves as the production amount increases. On the other hand, when the thickness of the scale is increased, the scale is partially peeled off due to thermal shock during cutting, so that notches (partial cutting width expansion) are likely to occur.
However, when the composition of the scale is not controlled, the thinner the scale thickness, the better the laser cutting property, and the scale thickness needs to be about 10 μm or less, for example. On the other hand, when the amount of magnetite (Fe ₃ O ₄ ) in the scale is controlled, the limit on the thickness of the scale that does not deteriorate the laser cutting property is relaxed.
For example, it was found that the quality of the cut surface can be ensured if the scale thickness is about 20 μm or less. The details of the present invention and the reasons for limitation will be described below. First, as a result of examining the influence of the production amount of magnetite (Fe ₃ O ₄ ) in the scale on the laser cutting property, when the production amount of magnetite decreases, wustite (FeO) with poor scale adhesion increases, Notches are likely to occur during cutting. Magnetite in scale (Fe ₃ O ₄ )
FIG. 1 shows the relationship between the generation amount (the balance is substantially FeO) and the laser cutting property, expressed as a fraction. Fig. 1 shows that as the magnetite fraction increases, the adhesion of the scale improves, and partial peeling of the scale due to thermal shock during cutting can be prevented.
Since the occurrence of notches (partial cutting width expansion) is suppressed, it is shown that the laser cutting performance is improved. As a result, when the production amount of magnetite (Fe ₃ O ₄ ) in the scale is about 15 vol% or more, the laser cutting property score is 3 or more, and a desirable steel sheet is obtained. On the other hand, as a result of examining the influence of the scale thickness on the laser cutting property, the relationship shown in FIG. 2 was obtained. FIG. 2 shows that when the thickness of the scale is increased, the frequency of notches is increased due to partial peeling of the scale due to thermal shock during cutting, resulting in inferior laser cutting performance. From these results, it is possible to stably manufacture a steel plate having excellent laser cutting ability by simultaneously controlling the thickness of the scale and the amount of magnetite (Fe ₃ O ₄ ) in the scale. FIG. 3 is a graph showing the relationship between the scale thickness, the amount of magnetite produced in the scale, and the laser cutting ability. Here, good laser cutting ability means that the score is 4.
The above is said. From FIG. 3, the scale thickness is 15 μm.
Hereinafter, the amount of magnetite produced in the scale is 15 vol.
%, A steel sheet having a laser cutting score of 4 or more can be obtained. Accordingly, in order to simultaneously control the thickness of the scale and the amount of magnetite formed in the scale in the hot rolling, the hot steel slab obtained by continuous casting is hot-rolled directly or reheated. In addition, the descaling before and during rolling not only removes the scale generated during heating and during rolling, but also controls the generation, growth and composition of the scale generated during rolling and during cooling after rolling. There is a need to. As a result of examining the final reduction ratio of finish rolling in hot rolling, the scale generated between each pass of finish rolling is more difficult to peel off as the final reduction ratio of finish rolling is higher. In addition, the scale thickness decreases as the final rolling reduction increases. By utilizing this, it is not necessary to cause an excessive decrease in the finishing temperature, so that hot-rolled steel sheets of a wider range of materials can be manufactured. However, the range of 5-25% is desirable from the capacity of the rolling mill and the shape of the manufactured steel sheet. As a result of examining the finish rolling temperature, the higher the finish rolling temperature, the faster the growth rate of the scale, and the generated scale is easily peeled off, and it is difficult to reduce the thickness of the scale. Therefore, as long as other materials such as mechanical properties are secured, a lower finish rolling temperature is more advantageous for thinning the scale. However, the range of 950-650 degreeC is desirable from the capability of a rolling mill, and the shape of the manufactured steel plate. As a result of examining the cooling rate immediately after finish rolling, the scale that is susceptible to thermal shock falls off as the cooling rate increases, and the scale that easily generates notches during cutting is excluded. Furthermore, scale growth is suppressed and the scale thickness is reduced. Therefore, it is advantageous for the laser cutting property that the cooling rate immediately after rolling is large. However, the range of 5-60 degreeC / s is desirable from the point of controllability of cooling capacity and cooling rate. As a result of examining the coiling temperature,
The lower the coiling temperature, the smaller the growth of the scale, and the more magnetite is produced, resulting in the production of a thin scale with high adhesion. However, from the viewpoint of cooling capacity and controllability, 5
The range of 50 to 250 ° C is desirable. Furthermore, in order to quantitatively control the scale thickness of the hot-rolled steel sheet and the amount of magnetite (Fe ₃ O ₄ ) in the scale, the following formulas (1) and (2) are used. It is necessary to comprehensively control the conditions. These equations are obtained by multiple regressions of the relationship between the thickness of the scale and the amount of magnetite (Fe ₃ O ₄ ) in the scale and the parameters of the rolling conditions. FT × (1−R / 100) × (7.5FT−4.5CR−13.5CR · CT + 1800CT) ≦ 6.21 × 10 ⁸ --- (1) 794 × (CT / 100) −230 × (CT / 100 ) ² +28.96 × (CT / 100) ³ −1.343 × (CT / 100) ⁴ −1000 ≧ 0 --- (2) where R: Final rolling reduction ratio (%) FT: Finishing rolling temperature (%) C: CR: Average cooling rate for 3 seconds immediately after finish rolling (° C./s) CT: Winding temperature (° C.) FIG. 4 shows the relationship between the left side of equation (1) and the thickness of the scale produced. Show. The relationship between the two is a linear relationship. When the left side of this equation is 6.21 × 10 ⁸ or less, the thickness of the scale is 15 μm or less. Also, the relationship between the left side of equation (2) and the amount of magnetite in the scale to be generated is
As shown in FIG. The relationship between the two is linear, and if the left side is zero (zero) or more, the amount of magnetite (Fe ₃ O ₄ ) produced is 15 vol% or more. In the present invention, the influence of the chemical composition of the steel sheet on the thickness of the scale and the amount of magnetite (Fe ₃ O ₄ ) in the scale was examined in advance.
That is, steel having several kinds of chemical components (the ranges of chemical components are C: 0.02-0.15%, Si: 0.01-0.30%, Mn: 0.25-1.25%, P : 0.0
05-0.030%, S: 0.001-0.030%,
Sol. Al: 0.005 to 0.050%) was melted in a laboratory, hot-rolled to a plate thickness of 10 mm, heat-treated, and various characteristics of the scale were measured. As a result, no difference was observed between the scale thickness and the amount of magnetite produced in any of the steel components. Therefore, in the steel of the present invention, the chemical composition is not particularly limited. For example, G3101 and 310 specified in JIS standards.
6, 3113, 3114, 3125, 3131, 313
It can be applied to various hot rolled steel materials such as 2. EXAMPLE Steels having the composition shown in Table 1 were melted and then made into slabs by continuous casting, which were immediately or after reheating and hot-rolled to produce steel plates having a thickness of 5 to 15 mm. The hot rolling conditions at this time were changed in Table 2 shown in FIG. [Table 1] After manufacturing by hot rolling and cooling to room temperature, a sample was taken from each steel plate and the thickness of the scale was measured using an optical microscope or a scanning electron microscope. The fraction of magnetite (Fe ₃ O ₄ ) in the scale was measured from X-ray diffraction. Furthermore, 50% for each steel plate.
Laser cutting is performed in units of m, and defective parts (number of notches)
The laser cutting property was evaluated by measuring the number. The evaluation criteria are as follows. Grade 1: 31 or more notches Grade 2: 〃 11-30 pieces Score 3: ３ 3-10 pieces Grade 4: 〃 1-2 pieces Grade 5: 〃 None Here, the notch is a cutting width of 50% This refers to the enlarged part. These results are also shown in Table 2 shown in FIG. As shown in Table 2 shown in FIG.
The steel sheet produced under the production conditions of the present invention by controlling the final rolling reduction ratio of finish rolling, the finish rolling temperature, the cooling rate immediately after the finish rolling, and the coiling temperature within appropriate ranges has a scale thickness of 1.
It is about 5 μm or less, and the amount of magnetite (Fe ₃ O ₄ ) in the scale is 15 vol% or more, indicating that the laser cutting property is good. On the other hand, in the comparative example, since the hot rolling conditions are not appropriate, the scale is easily peeled at the time of cutting, and the amount of magnetite produced is not sufficient, so that the laser cutting property is inferior. As described above, according to the steel plate excellent in laser cutting property and the manufacturing method thereof according to the present invention, a steel plate having a scale having excellent adhesion can be manufactured stably and reliably. Therefore, laser cutting having various advantages can be applied more than ever. That is, according to the first aspect of the invention, a steel sheet having a highly adhesive scale having excellent laser cutting properties can be obtained. Further, according to the second invention, a more excellent steel sheet having a scale thickness of 15 μm or less can be produced. Also, according to this method, the plate thickness that can be laser cut industrially is limited to about 20 mm, but this is about 2 mm.
It becomes possible to expand to about 5 mm, and laser cutting can be applied to a wide range of hot-rolled steel sheets.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the amount of magnetite produced and the laser cutting ability. FIG. 2 is a graph showing the relationship between scale thickness and laser cutting ability. FIG. 3 is a diagram showing the relationship between the scale thickness, the amount of magnetite produced, and the laser cutting ability. FIG. 4 is a diagram showing the relationship between the value on the left side of equation (1) and the scale thickness. FIG. 5 is a diagram showing the relationship between the value on the left side of equation (2) and the amount of magnetite produced. FIG. 6 is a diagram showing the relationship between hot rolling conditions, scale thickness, magnetite production, and laser cutting ability.

──────────────────────────────────────────────────── ─── Continuation of Front Page (72) Inventor Kaoru Kaizu 1-2 1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe Co., Ltd. (72) Inventor Makoto Serizawa 1-2 1-2 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (56) References JP-A-8-218119 (JP, A) JP-A-6-73504 (JP, A) JP-A-5-195055 (JP, A) (58) Fields surveyed ( Int.Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (1)

(57) [Claims] [Claim 1] When hot rolling a hot steel slab obtained by continuous casting directly or after reheating, descaling is performed before and during the rolling, and Finish rolling temperature is 950 ~ 650 ℃, final rolling reduction rate is 5 ~ 25
%, Average cooling rate for 3 seconds immediately after finish rolling is 5-60 ° C /
s and the coiling temperature in the respective ranges of 550 to 250 ° C., and further under conditions where the final shrinkage ratio, rolling temperature, cooling rate, and coiling temperature satisfy the following formulas (1) and (2): A method for producing a steel sheet excellent in laser cutting property, characterized by controlling the thickness of the scale to be produced and the amount of magnetite (Fe304) produced in the scale by rolling. FT × (1-R / 100) × (7.5FT−4.5CR-13.5CR · CT + 1800CT) ≦ 6.21 × 10 ⁸ −−−− (1) 794 × (CT / 100) −230 × (CT / 100) ² + 28.96 × (CT / 100) ³ −1.343 × (CT / 100) ⁴ −1000 ≧ 0 −−−− (2) where R: Final rolling reduction ratio (%) FT: Finishing rolling temperature (° C. ) CR: Average cooling rate for 3 seconds immediately after finish rolling (℃ / s) CT: Winding temperature (℃)