JPH0456088B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> This invention is a steel that contains a large amount of carbon but has deep drawability comparable to that of a mild steel plate, can be formed into complex shapes, and can be processed easily. The present invention relates to a method for stably producing a high carbon cold rolled steel sheet that exhibits high hardness and excellent wear resistance without requiring special equipment. <Background technology> Generally, small containers, vehicle parts such as passenger cars, etc.
High carbon steel sheets that require high hardness and wear resistance are often shipped in a spheroidized state, and after being formed into a desired shape, they are heat treated and hardened before use. However, even in the spheroidized state, high carbon steel sheets are considerably harder than mild steel sheets, and their workability is far from comparable to that of mild steel sheets, so cracks may occur when used to form complex shapes. I had the inconvenience of putting it away. However, the poor formability of high carbon steel sheets is not only due to the hardness mentioned above, but also due to plastic anisotropy, which has a low value of 1.0 or less. That is, while the value of general cold-rolled steel sheets is about 1.3 to 2.2, high carbon cold-rolled steel sheets only show a low value of about 0.6 to 1.0. <Problems to be Solved by the Invention> Therefore, the present inventors improved the undesirable formability found in "high carbon steel sheets" in order to meet the above-mentioned demands for hardness, wear resistance, etc. We sought the possibility of stably providing steel sheets on an industrial scale that have "excellent drawability" that allows them to be formed into complex shapes without sacrificing high hardness or wear resistance. We conducted repeated research from various perspectives. By the way, the present inventors have previously stated that ``The presence of a cementite layer is a factor that inhibits the formability of steel materials, but it is possible to
If the Mn content is also restricted, the second phase in α-iron will easily change from cementite to graphite, and only a combination of cold working at a predetermined degree of workability and annealing under specific conditions will result in excellent ductility and workability. Based on the knowledge that ``steel materials with excellent ductility and workability'' having a structure mainly composed of ferrite and graphite phases were proposed and carried out (Japanese Patent Laid-Open No. 1983-1999),
No. 52551), an attempt was made to manufacture a steel plate with high hardness, high wear resistance, and workability by applying this method, but
If the previously proposed method was applied as is, the values of the steel plate obtained were unsatisfactory, and there was a drawback of insufficient workability when performing severe deep drawing or hole enlarging. <Means for solving the problems> Based on the various problems mentioned above, the present inventors have discovered that a high C content is essential in order to ensure high hardness and high wear resistance in steel sheets. From this point of view, we conducted further research to find a means to stably impart excellent drawability (value) to the high carbon steel sheet, and as a result, we obtained the new knowledge shown below. be. In other words, (a) Either the Mn, sol.Al and N contents of high carbon steel are adjusted to a specified range, or a hot rolled steel sheet to which specific elements are added is first subjected to cold rolling at a specified reduction rate and under specific conditions. When annealing is performed, the secondary phase in the ferrite matrix is
The phase becomes a structure consisting mainly of graphite (after hot rolling or in a state where this is annealed as it is, the second phase becomes cementite Fe ₃ C), and a relatively soft and extensible structure is obtained. (b) However, it cannot be said that the drawability (value) will be stably improved as it is, but if the above treatment is followed by one or more cold rolling and annealing under specific conditions. A cold rolling texture develops from the ferrite and graphite structure during cold rolling, and a recrystallized texture favorable for values is formed by subsequent annealing, resulting in high values, low yield stress, and high elongation not found in conventional products. will be obtained. Therefore, through these treatments, it is possible to create a stable steel sheet that has drawability comparable to that of a mild steel sheet even though it contains a large amount of C, and also ensures sufficient hardness through heat treatment after forming and exhibits high wear resistance. It becomes possible to produce. (c) In addition, if the structure before cold rolling is "ferrite + cementite", a texture favorable for the value will not develop, and "ferrite + graphite" will not develop.
It is only when the texture is favorable that a texture with favorable values develops upon cold rolling and annealing, regardless of the C content. This is because cementite is hard and suppresses the deformation of ferrite, but graphite is soft and cannot suppress the deformation of ferrite. Therefore, in the "ferrite + graphite" structure, a texture similar to that of mild steel (low C steel) develops. This is thought to be for the purpose of This invention was made based on the above knowledge, and C: 0.08 to 0.95% (hereinafter, % representing the component ratio).
% by weight), Mn: 0.50% or less, sol.Al: 0.20% or less, N: 0.0130% or less, or in addition, B: 0.0005 to 0.0050%, Si: 1.2% or less, Ni: 5.0% The steel containing one or more of the following, Fe and other unavoidable impurities: After hot rolling, the steel is cold rolled at a reduction rate of 20-70% and then heated at 500-750°C. It has excellent drawability by being annealed for more than hours to create a structure mainly composed of ferrite and graphite, and then cold-rolled at a reduction rate of 50-90% and then annealed at 600-850℃. In addition, the present invention is characterized in that it is possible to stably produce a high carbon cold rolled steel sheet that can be imparted with high hardness even by simple heat treatment after forming. Here, in the method for manufacturing a high carbon cold-rolled steel sheet of the present invention, the reason why the content ratio of the raw material steel and the processing conditions are limited as described above will be explained. A Component content ratio of raw material steel () C The C component has the effect of ensuring the hardness and wear resistance of the steel plate, but generally the lower the C content, the better the drawability. If the C content is less than 0.08%, sufficient hardness and wear resistance cannot be obtained, and such steels have high values even without applying the measures specified in the present invention. On the other hand, if C is added in excess of 0.95%, it becomes too hard and cannot be cold rolled.
The content was set at 0.08% to 0.95%. () Mn In addition to imparting the necessary strength to the steel sheet, the Mn component has the effect of ensuring the hot workability of the steel, but if it is contained in excess of 0.50%, the C as a steel component will act as cementite. The Mn content is 0.50% because it becomes stable and difficult to form graphite.
It was determined as follows. () sol.Al It is desirable to add the sol.Al component because the larger the amount, the easier it is to form graphite. but,
If the content exceeds 0.20%, inclusions in the steel will increase and the workability of the steel plate will deteriorate, so the sol.Al content was limited to 0.20% or less. () N N is an impurity element that is unavoidably contained in steel, but if its content exceeds 0.0130% (130 ppm), the elongation value of the steel plate will deteriorate, so the N content should be 0.0130% or less. Limited. () Si and Ni Since Si and Ni have the effect of facilitating the graphite formation of C in steel, one or two types of Si and Ni can be used as needed.
Seeds are the ingredients that are added. Of these, Si is an element that is contained in about 0.2% in ordinary carbon steel, and if it is further increased, graphite formation is likely to occur, but if it is contained in excess of 1.2%, it will lead to a deterioration of toughness. , when adding Si, it was limited to 1.2% or less. On the other hand, in the case of Ni, if the content exceeds 5.0%, the effect of promoting graphite formation will be saturated and the cost will increase significantly, so when Ni is added, it is limited to 5.0% or less. () B B is an element that affects the dispersion of graphite, making the precipitation of graphite finer and facilitating the disappearance of graphite during heat treatment after forming the steel sheet.
This component is added as needed because it has the effect of ensuring high hardness and high wear resistance, but if its content is less than 0.0005%, the desired effect will not be achieved. Not obtained, while 0.0050
If the content exceeds %, cracks in the slab may occur. Therefore, when B is added, its content is determined to be 0.0005 to 0.0050%. Note that it is better to keep the content of P and S, which are unavoidable impurities in steel, as low as possible, but if large amounts of Si or Ni are added, graphitization will occur even at normal P and S content levels. For the purpose, there is no need to limit the amount of these impurities. In addition to the above ingredients, we also aim to improve strength, etc., and add
It goes without saying that the conventional means of adding trace amounts of Cu, Nb, Zr, Ti, Ca, etc. may be applied. B Processing Conditions In the method of the present invention, after the above-mentioned plastic steel is subjected to normal hot rolling and pickling (however, the lower the coiling temperature during hot rolling, the better, and after pickling, annealing treatment is performed. (This preliminary annealing is effective because cold rolling is difficult in hard steel with a high C content.), first cold rolling and annealing,
Second cold rolling and annealing are performed, but the processing conditions are limited for the following reason. () Reduction ratio of the first cold rolling The aim of the first cold rolling is to destroy the cementite in the hot rolled sheet. This cementite destruction creates cavities in the steel sheet structure,
Preparation is made for graphite formation. However, if the rolling reduction rate is less than 10%, the formation of the cavities is not sufficient, while if the rolling reduction rate exceeds 70%, the cavity formation effect is saturated and the rolling load becomes too large. The rolling reduction ratio is 20
Limited to ~70%. () First annealing conditions The first annealing carried out following the first cold rolling is for forming graphite, but the annealing temperature is less than 500 ° C. 750
If the annealing time exceeds .degree. C., graphitization is difficult to proceed, and if the annealing time is less than 2 hours, sufficient graphitization cannot be achieved. Therefore, to ensure uniform graphite formation, the first annealing is
It was specified that the test should be carried out at a temperature of 500 to 750°C for 2 hours or more. () Reduction rate of second cold rolling The aim of the second cold rolling is to form a cold rolling texture. And the reduction rate at this time is 50%
If the reduction rate is less than 90%, the texture formation will be insufficient and the value of the steel sheet product will decrease.On the other hand, even if the reduction rate exceeds 90%, the value will not improve any more but the load will increase. The reduction ratio of cold rolling was limited to 50 to 90%. The intermediate material subjected to this second cold rolling has a steel structure of "ferrite+" due to the above treatment.
Graphite" and is soft, so it has rollability comparable to that of ordinary mild cold-rolled steel sheets. () Secondary annealing conditions Secondary annealing carried out after the second cold rolling. recrystallizes the cold-rolled structure to soften it,
It is applied for the purpose of forming a recrystallized texture favorable to the value. However, in this case, if the annealing temperature is less than 600℃, recrystallization will not occur;
If the annealing temperature exceeds 850℃, γ phase appears and the value decreases, so the temperature of the second annealing is 600 to 850℃.
It was determined that In this case, the soaking time is not particularly important, and either box annealing or continuous annealing may be employed. It should be noted that the processes corresponding to the second cold rolling and second annealing described above may be repeated two or more times, but in the present invention, they are carried out only once in consideration of cost. did. Furthermore, it goes without saying that during the rolling and annealing steps described above, treatments for surface conditioning, measures to prevent buckling, etc. may be carried out as necessary. Then, the steel plate subjected to the second annealing is subjected to skin pass rolling or the like as usual before being shipped. By the way, the thin plate products manufactured as described above are usually subjected to forming processing such as drawing to form products into a predetermined shape, but if they are then hardened by heat treatment, they have excellent strength and wear resistance. will be able to demonstrate this. The heat treatment temperature at this time is preferably 750 to 1100°C, and after holding the heat, it is preferable to rapidly cool it with oil cooling or water cooling to form a low-temperature transformed structure and ensure hardness. In this case, it is recommended that tempering be performed subsequently at 100-600°C. The molded parts manufactured in this way have high hardness and excellent wear resistance, so their application fields are extremely wide. Next, the present invention will be specifically explained using Examples and in comparison with Comparative Examples. <Example> Example 1 First, C: 0.65%, Si: 0.20%,
Mn: 0.20%, P: 0.009%, S: 0.006%, sol.
Steel containing Al: 0.03%, N: 0.0048%, and the remainder substantially consisting of Fe is melted into a slab, which is then heated to 1100℃, finishing at a temperature of approximately 820℃, A hot-rolled steel sheet was obtained by hot rolling at a coiling temperature of 600°C. Next, the pickled hot-rolled steel sheet was cold rolled at various reduction rates up to 70%, except for some parts, and then batch annealed at 650°C for 15 hours, and further reduced. After cold rolling again at a rate of 15% to 90%, continuous annealing at 820℃ for 1 minute or 690℃
Batch annealing was performed at ℃ for 16 hours. Next, from each steel plate obtained in this way, JIS5
A No. 1 tensile test piece was taken, and tensile tests were conducted in three directions of the steel plate to investigate the values. The investigation results of this value are shown in Figs. 1 and 2. Furthermore, Figure 1 shows the influence of the primary cold reduction ratio on the value after secondary annealing when the secondary cold reduction ratio is kept constant at 66%, while Figure 2 shows the effect of the primary cold reduction ratio on the value after the secondary annealing. Similar results are shown when the cold compression ratio is kept constant at 55%. Further, the △ mark in the figure indicates the value obtained by continuous annealing at 820°C for 1 minute, and the ◯ mark indicates the value obtained by batch annealing at 690°C for 16 hours. From the results shown in FIG. 1, it can be confirmed that the primary cold compression ratio within the range of 20% to 70% shows a high value. Fig. 1 shows data for secondary annealing temperatures of both 820°C (△ mark) and 690°C (○ mark), but the annealing temperature is within the specified range of the present invention. No particular influence on the value due to the difference is observed. Moreover, from the results shown in FIG. 2, it can be confirmed that an excellent value of 1.2 or more can be obtained at a secondary cold compression ratio of 50 to 90%. Example 2 Steel having the composition shown in Table 1 was melted by a conventional method and subjected to the treatments shown in Table 2 to obtain a cold rolled steel plate. Next, JIS No. 5 tensile test pieces were taken from the obtained cold-rolled steel sheet in three directions and subjected to a tensile test. Table 2 shows yield strength, tensile strength, elongation, and average values in three directions. From the results shown in Table 2, all steel plates manufactured according to the conditions specified in the present invention have a yield strength of around 20 Kgf/mm ² and a tensile strength of 30 to 35 Kgf when the Si content is low. /mm ² , elongation is 40% or more,

【table】

[Table] (Note) * indicates that the conditions are outside the conditions of the present invention.

[Table] (Note) * indicates that the conditions are outside the conditions of the present invention.
The value is 1.2 or more, indicating that the material has excellent moldability, and it can be confirmed that it has good moldability, although the properties are somewhat degraded when the Si content is high. On the other hand, in Comparative Method 13, the soaking time in the first annealing is short, so graphite formation is not sufficient.
Only cold-rolled steel sheets with low values have been obtained. Furthermore, in Comparative Method 14, the r value is even lower because primary cold rolling is not performed. On the other hand, in Comparative Methods 15 and 16, the graphite phase does not develop during primary annealing due to the high Mn content of the steel material, and no graphite phase is observed even after secondary annealing. Therefore, the obtained cold-rolled steel sheet is not only hard but also has a low r value. Although the steel plate obtained by Comparative Method 17 has specifications equivalent to S45C specified by JIS, it is clear that it does not exhibit sufficient formability. Reference Example 1 Among the cold-rolled steel sheets obtained by the methods shown in Table 2 above, those obtained in Invention Examples 1 and 11 and Comparative Example 17 were each kept at 845°C for 15 minutes and then oiled. The hardness was measured after cooling and tempering by holding at 200°C for 30 minutes and cooling in air. The results are shown in Table 3.

[Table] As is clear from the results shown in Table 3,
The hardness of the cold-rolled steel sheet after quenching and tempering is the highest in the JIS standard product obtained by Comparative Method 17, but the hardness obtained by Inventive Method 11 is slightly lower than that by Inventive Method 1. It can be seen that even if there is a hardness, it shows sufficiently high hardness equivalent to the JIS standard product mentioned above. <Summary of Effects> As explained above, according to the present invention, a high-carbon cold-rolled material that not only has excellent drawability comparable to that of mild steel sheet but also exhibits high hardness and excellent wear resistance through simple heat treatment. Industrially useful effects are brought about, such as making it possible to produce steel plates stably and reliably and further expanding the fields of application of steel plates.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the first cold rolling reduction ratio and the value of the finished steel sheet. Figure 2 shows
It is a graph which shows the relationship between the secondary cold rolling reduction ratio and the value of a finished steel plate.

Claims (1)

[Claims] 1 Steel consisting of C: 0.08 to 0.95%, Mn: 0.50% or less, sol.Al: 0.20% or less, N: 0.0130% or less, Fe and other unavoidable impurities: the remainder After hot rolling, it is cold rolled at a reduction rate of 20 to 70%, then annealed at 500 to 750°C for 2 hours or more to create a structure mainly composed of ferrite and graphite, and then further rolled at a reduction rate of 50 to 70%. A method for producing a high carbon cold rolled steel sheet with good drawability, which is characterized by cold rolling at 90% and then annealing at 600 to 850°C. 2 Contains C: 0.08 to 0.95%, Mn: 0.50% or less, sol.Al: 0.20% or less, N: 0.0130% or less, Si: 1.2% or less, Ni: 5.0% or less in terms of weight percentage. Fe and other unavoidable impurities: After hot rolling, the steel is cold rolled at a reduction rate of 20 to 70% and then annealed at 500 to 750°C for 2 hours or more. A high-carbon cold-rolled steel sheet with good drawability, which is characterized by being processed to form a structure consisting mainly of ferrite and graphite, and then cold-rolled at a reduction rate of 50-90% and then annealed at 600-850°C. manufacturing method. 3 Consists of weight percentage: C: 0.08-0.95%, Mn: 0.50% or less, sol.Al: 0.20% or less, B: 0.0005-0.0050%, N: 0.0130% or less, Fe and other unavoidable impurities: the remainder After hot rolling the steel, it is cold rolled at a reduction rate of 20 to 70%, then annealed at 500 to 750°C for 2 hours or more to create a structure consisting mainly of ferrite and graphite, and then further reduced to 50%. A method for producing a high carbon cold rolled steel sheet with good drawability, characterized by cold rolling at ~90% and then annealing at 600~850°C. 4 Contains C: 0.08-0.95%, Mn: 0.50% or less, sol.Al: 0.20% or less, B: 0.0005-0.0050%, N: 0.0130% or less, Si: 1.2% or less, Ni: 5.0% or less of Fe and other unavoidable impurities: After hot-rolling the steel, cold-rolling at a reduction rate of 20-70% and then heating at 500-750°C. It is characterized by being annealed for 2 hours or more to form a structure consisting mainly of ferrite and graphite, and then cold rolled at a reduction rate of 50 to 90% and then annealed at 600 to 850 °C. A method for producing good high carbon cold rolled steel sheets.