JPH032210B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for producing a high carbon steel plate containing 0.30 to 2.50% by weight of C, and particularly provides a method for producing a high carbon steel plate with excellent cutting workability. [Prior art] In general, high carbon steel sheets for machine structures, tools, molds, etc. are JIS standard SC, SMn, SCr, SCM,
Steel grades of SK, SKS, SKH, and SKD series are used. Since these steel types are required to have high hardness and wear resistance when in use, they are often subjected to heat treatment of quenching and tempering, but there is a problem that burn cracks occur during this process. In addition, cracks may also occur during shape cutting, machining, drilling, etc., and the difficulty of cutting has a significant impact on productivity. Due to such problems, it is known to spheroidize carbides to reduce burn cracks, weld cracks, and machinability of high carbon steel, and a process of annealing, processing, quenching, and tempering is employed. According to "Press mold materials and heat treatment" published by Nikkan Kogyo Shimbun on June 30, 1971, carbide spheroidizing annealing is a microstructure improvement as a pre-quenching treatment, and after heating to maintain the temperature just above the transformation point. A method of completely causing cooling transformation by slow cooling or constant temperature treatment in a furnace has been proposed and is well known. Therefore, it was customary for high carbon steel sheets supplied by steel sheet material manufacturers to be cut into shapes by a fusing manufacturer and then annealed.
There is a tendency for fusing manufacturers or processing manufacturers to omit the annealing process, and there is an increasing demand for material manufacturers to supply high carbon steel sheets that are spheroidized. On the other hand, recently there has been a shift from melt cutting to saw cutting in shape cutting, for example, width 1524 mm x
A steel plate with a crop and/or ears that has been rolled to a standard size of 3048 mm may be sawed into strips with a width of 200 mm to 500 mm, or a narrow width may be obtained from the beginning. Keep it
This may be divided into blocks by sawing. This indicates that a cut plate of a predetermined size is more desirable than a conventional rolled steel plate, and the latter narrow width material is, for example, 200 mm to 200 mm wide compared to the conventional standard size.
This is a change in the demand for arbitrary strip sizes such as 500 mm and lengths of 3000 mm x 8000 mm, and as a material manufacturer, we can not only cut irregularly shaped parts at the start and end of rolling, but also mill edges and mid-cuts on both sides in the longitudinal direction. cutting is becoming essential. This cutting is done by fusing on the production equipment of the material manufacturer, but if the hardened layer remains in the supplied state, the material cannot be used for sawing at the customer's site. [Problems to be Solved by the Invention] In view of the above-mentioned demand trends, an object of the present invention is to provide a method for manufacturing a high-carbon steel plate that has particularly excellent cutting workability. [Means for solving the problem] C: After hot rolling a high carbon steel plate with a content of 0.30% or more, it is fused and then subjected to heat treatment, or after hot rolling, it is subjected to primary heat treatment and then fused and cut. This is a method for manufacturing high carbon steel sheets that is subsequently subjected to secondary heat treatment, (1)
C: After hot rolling steel containing 0.30 to 2.50%, it is fused to a specified size, the center of the plate thickness is kept at a temperature of 700 to 900°C for 10 minutes or more, but less than 1 minute per 1 mm of plate thickness, and then left to cool in the atmosphere. (2) After hot rolling steel containing 0.30 to 2.50% C, the center of the plate thickness is heated to 700 to 900°C for 10 minutes or more per mm of plate thickness. Hold the plate for less than 1 minute and let it cool in the atmosphere, then melt-cut it to the specified dimensions, and then heat the center of the plate thickness to 600 to 750℃.
This is a method for producing a high carbon steel sheet, which is characterized by holding the steel sheet at a temperature of 10 minutes or more for 1 minute or less per 1 mm of sheet thickness, and then allowing it to cool in the atmosphere. [Function] As mentioned above, it is necessary to make the material into a spherical shape in order to facilitate machining such as cutting and drilling, and to prevent burn damage. Annealing is suitable for this heat treatment, and there is no problem with small sizes after cutting. I was able to adopt it without any problems. However, in material smokers that handle large quantities of large steel plates, although it is theoretically possible to perform furnace cooling in batch-type heat treatment furnaces, such heat treatment poses a significant production impediment. Even if operational disadvantages are ignored, batch-type heat treatment furnaces cannot produce a flat shape for thin steel plates, so they are limited to thick steel plates. High carbon steel sheets are highly susceptible to cracking and cold straightening is almost impossible, so not being able to obtain a flat shape is fatal, and thin steel sheets must be processed using a continuous heat treatment furnace. Furnace cooling is not possible due to the structure. The present invention aims at spheroidization without furnace cooling, and employs an industrially advantageous heat treatment for both thick and thin steel plates, whether batch or continuous. Specifically, it is heated to a temperature above the transformation point and then allowed to cool in the atmosphere.
Steel types such as SCM are 800-900℃, C: 0.60% or more.
Steel types such as SK, SKS, SKH, SKD are 700℃~800℃
°C and hold at this temperature for at least 10 minutes. In this heat treatment, reticular cementite is precipitated at the prior austenite grain boundaries when the product is hot-rolled, and the purpose is to break this up and make it spheroidized.The longer the holding time is, the better, and it is necessary to hold it for at least 10 minutes. On the other hand, as for the upper limit, according to experiments conducted by the present inventors, the progress of spheroidization that is commensurate with long-term holding is slow, so it is preferable to keep the holding for 1 minute per 1 mm of plate thickness. If the heating temperature is lower than 700°C, the network cementite will not be sufficiently divided, and if the heating temperature is higher than 900°C, grain boundary cracking may occur. The degree of spheroidization is related to the amount of C and the holding time; for example, in the case of a steel with a C content of about 0.50% or more, more sufficient spheroidization can be achieved by heat-treating it once more. That is, when cementite is to be further spheroidized, it is preferable to perform a second heat treatment after the above-mentioned first heat treatment. This secondary heat treatment is also allowed to cool in the atmosphere without furnace cooling, but the holding time is still required to be 10 minutes or more, and 1 minute or less per 1 mm of plate thickness. However, the heating temperature at the center of the thickness of the plate is specifically set to 600 to 750°C, since the reticulated cementite has been fragmented by the previous heat treatment and has become quite spheroidal. In other words, at temperatures below 600°C, the holding time becomes too long, which is disadvantageous, and even at temperatures above 750°C, the holding time cannot be shortened to 10 minutes or less, and even at high temperatures, the spheroidizing effect is small, so the upper limit is 750
℃. However, in the present invention, fusing is performed to a predetermined size before the above-mentioned first heat treatment or before the latter second heat treatment, but this fusing may be carried out following the previous process as long as the schedule between processes allows. is preferred. That is, the material is melt-cut early after hot rolling and then subjected to heat treatment, or alternatively, the material is melt-cut early after hot rolling and primary heat treatment, and then subjected to secondary heat treatment. This melts the steel plate under warm conditions and is effective in preventing bending, twisting, and cracking. As a measure to prevent bending, twisting, and cracking, it is more effective to perform post-fusion heat treatment early after fusing, and such a series of early treatments is also preferable in terms of shortening the manufacturing period. If it is not possible to make early inter-process adjustments in actual operation, the steel plate may be preheated using a burner or placed in a furnace before or after fusing. In particular, what is more effective against bending and twisting when cutting strips by melting is to cut the steel sheet so that both ends in the longitudinal direction are left partially uncut. This type of form is suitable for charging/extracting into large batch heat treatment furnaces such as those used in plate factories in steel mills,
Threading in a continuous heat treatment furnace can be handled in the same way as ordinary steel sheets before strip cutting, resulting in extremely high productivity in terms of handling. The uncut portions at both ends in the longitudinal direction are fused in the final step, but in the case of sawing only in the width direction, there is usually no problem even if there are partially fused and hardened portions at both ends. However, if it is not acceptable for the hardened part to remain even at both ends, a pre-heating or post-heating crater may be installed separately from the fusion crater and run parallel to the fusion crater. good. The reason for limiting the C content in the present invention will be described.
C: If it is less than 0.30%, there will be almost no troubles such as quench cracking, welding cracks, or cutting cracks, but if it is more than 0.30%, these troubles will occur. C: Exceeding 2.5% actually increases manufacturing costs, so the upper limit is
The rate shall be 2.5%. Next, the form of the steel plate according to the process flow of the present invention will be explained with reference to the drawings. First, FIG. 9 shows a conventional supply form of high carbon steel sheet 1. A has a unique form because it is a high carbon steel plate, and it remains in its rolled shape.
It is customary for fusing manufacturers to cut the material into the shape shown by the broken line in the figure, and then annealing it after fusing. B fuses only the crop at the rolling head and tail,
Mille Tsuji is a so-called steel plate with ears as it is, and since both ends are still fused, a hardened layer 2 remains due to the rapid cooling of the high carbon steel, and when cutting as shown by the broken line in the figure, the saw is not sharp, and the fused cut is also cracked. There is a risk of On the other hand, in the present invention, as shown in FIG. 1, the shape of the hole a is the same as that in FIG. Cutting as shown by the broken line and cutting in the width direction as shown in Ho-b can also be easily performed. In addition, as shown in Fig. 2, when heat treatment is performed twice after rolling, the first heat treatment is followed by fusing into strips (C), and then the second heat treatment (D) is performed in the width direction and longitudinal direction as shown by the broken line in E. Any sawing can be done with both. Figure 3 shows a case where the band-shaped fusing is even narrower, and so-called strip cutting is performed, but both ends are left uncut to prevent bending and twisting during fusing and to improve subsequent handling efficiency. It is then melted and cut in the final step after heat treatment. Since there is no problem with cutting in the width direction as shown by the broken line in F, the fusing portions 2 at both ends can be left as they are, but if this is inconvenient, the hardened layer may be softened by post-heating. Examples of the present invention are listed below. [Example 1] Ladle analysis values are C: 0.31%, Si: 0.24%,
SCr430 with Mn: 0.77% and Cr: 0.92% is hot-rolled to a thickness of 80 mm, and the crop at the rolling start and end ends is removed by gas cutting at a surface temperature of approximately 200°C. I made it to a standard size of 3048 x 3048 with ears. Next, the center of the plate thickness was heated to 60°C at a temperature of 870°C.
After heating for 1 minute, the mixture was allowed to cool in the atmosphere. The mechanical properties of this steel plate are shown in Table 1. The steel plate of the present invention has the rolling start and end parts removed, and the cut part has no hardened layer due to heat treatment, so it can be sawed in the longitudinal direction and has high toughness even in the subsequent cutting process. No cracks occur during processing.

[Example 2]

Ladle analysis value is C: 0.53%, Si: 0.25%,
Hot rolled S53C with Mn: 0.83% to a thickness of 175mm,
The steel plate was cut into strips at a surface temperature of approximately 200°C.
Table 2 shows the steel plate dimensions.

[Table] Next, the center of the plate thickness was heated at a temperature of 830°C for 30 minutes, and then allowed to cool in the atmosphere. Table 3 shows the measurement results of the amount of lateral bending (camber) in the gas-off state and after heat treatment.

[Example 3]

Ladle analysis value is C: 0.96%, Si: 0.23%,
After hot rolling SK4 with Mn: 0.41% to a thickness of 60 mm, the center of the thickness was heated at a temperature of 740°C for 1 hour, and when the surface temperature of the steel plate decreased to about 200°C by cooling in the atmosphere, the sixth The gas was cut to the dimensions shown in Figure A. Next, the center of the steel plate was heated at a temperature of 670°C for 1 hour, and then allowed to cool in the atmosphere until the surface temperature of the steel plate reached approximately 200°C.
When the temperature dropped to ℃, the gas was cut off to separate it as shown in B, and 5 strips with dimensions of 405 mm in width and 5000 mm in length were collected. FIG. 7 shows the hardness measurement results of the longitudinal gas-cut surface (a) (heat treated after degassing) and the width-direction gas-cut surface (b) (as is after degassing). FIG. 8 shows the results of microscopic observation of A and B after the primary and secondary heat treatments, respectively. Since the steel plate of the present invention is heat-treated after degassing, the cut surface is softened and easy to saw. However, for comparison, when sawing was attempted on the surface as shown in Figure 6 (b), the serrations were completely removed. It was impossible to cut without entering. Although the heat treatment of the present invention is performed by cooling in the air, even in the first heat treatment, spheroidization occurs to the extent shown in Figure 8A.
When the next heat treatment is applied, a sufficiently spheroidized structure is obtained as shown in FIG. 8B. [Effects of the Invention] According to the present invention, there is an advantage that the sawing and machining properties are excellent, and the annealing treatment, which was essential due to the conventional shape cutting by fusing, is unnecessary. Furthermore, on the production side of raw steel sheets, highly efficient production can be achieved using a continuous heat treatment furnace.

[Brief explanation of the drawing]

Fig. 1, Fig. 2, and Fig. 3 show the processing forms of steel plates according to the method of the present invention, Fig. 1 shows cutting into small pieces after heat treatment, and Fig. 2 shows cutting into strips after rolling or primary heat treatment, and then heat treatment. FIG. 3 shows a processing mode for preventing bending and twisting of the thread to be cut and efficiently handling the thread. FIG. 4 is a graph showing the results of the Jyominy test in Example 1. FIG. 5 is a graph showing the cross-sectional hardness after heat treatment in Example 2. FIG. 6 is an explanatory diagram showing a gas cutting mode in Example 3. FIG. 7 is an explanatory diagram showing hardness with and without heat treatment in Example 3. FIG. 8 is a photograph in place of a drawing of the metallographic structure of the steel plate of Example 3, and FIG. 8A shows the metallographic structure after the primary heat treatment, and FIG. 8B shows the metallographic structure after the secondary heat treatment. FIG. 9 is an explanatory diagram showing a conventional supply form of high carbon steel sheets.

Claims (1)

[Claims] 1 Steel containing 0.30 to 2.50% C is hot-rolled and then fused to a predetermined size, and the central part of the plate is kept at a temperature of 700 to 900°C for 10 minutes or more and 1 minute or less per 1 mm of plate thickness. A method for manufacturing a high carbon steel sheet, which comprises cooling the steel sheet in the atmosphere. 2 After hot-rolling steel containing 0.30 to 2.50% C, the center of the plate is kept at a temperature of 700 to 900°C for 10 minutes or more and 1 minute or less per 1 mm of plate thickness, then allowed to cool in the atmosphere, and then rolled to the specified dimensions. After that, the thickness of the center part is 600 to 750.
A method for manufacturing a high carbon steel plate, which comprises holding the steel plate at a temperature of 10°C for 10 minutes or more and 1 minute or less per 1 mm of plate thickness, and then allowing it to cool in the atmosphere.