JP3407540B2 - High carbon steel sheet with excellent workability and hardenability - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is typified by a gear that is required to have abrasion resistance and the like and is processed into a complicated shape.
The present invention relates to a high carbon thin steel sheet having excellent workability and hardenability used as a material for transmission parts and the like . [0002] High carbon thin steel sheets used for transmission parts such as gears and clutch covers, and ratchets, etc., are required to have high workability and hardenability. Regarding hardenability, in addition to securing high altitude after quenching, in recent years, cost reduction of quenching work has been required. In order to reduce the cost of the quenching operation, it is effective to lower the heating temperature and shorten the soaking time, and a material that can be sufficiently quenched by holding the material at a low temperature for a short time is desired. When the components are constant, the hardenability is affected by the amount of carbide that forms a solid solution during soaking, and in order to make more carbide form a solid solution during holding for a short time, the size of the carbide must be fine. It is valid. Regarding the improvement of hardenability due to such refinement of carbides, Japanese Patent Publication No. 57-436
21 and JP-A-2-25913. Furthermore, Japanese Patent Application Laid-Open No. 1-25812 discloses a manufacturing method aiming at improving mechanical properties by making the carbide spherical finer. [0003] However, even in the high carbon thin steel sheets manufactured by these manufacturing methods, the elongation (%) before quenching is at most about 15% and has sufficient workability. Not really. Particularly, recently, there has been a movement to simplify the working process, and the demand for workability of a high carbon thin steel sheet has become more severe than ever. [0004] The present invention has been made in view of such circumstances, and has as its object to provide a high carbon steel excellent in both workability and hardenability. SUMMARY OF THE INVENTION Conventionally, hot rolling and cold rolling
Factors that affect the workability of the hot-rolled high carbon steel sheet are the shape and size of the carbide, and as described above, it has been thought that the spherical refinement of the carbide is important, Attempts have been made to reduce the size of carbides with the aim of improving the quality and to make the carbides as spherical as possible to provide workability. However, at present, sufficient workability has not been obtained only by making the carbides spherical and fine. [0006] Therefore, the present inventors have conducted cold rolling and
As a result of intensive studies to improve the workability of high-carbon thin steel sheets with a thickness of several millimeters that are wound into an il shape, it is impossible to improve the workability only by the size and distribution of spherical fine carbides, and It has been found that the ductility is rapidly improved only when the ferrite diameter is within a certain range in the presence of a spherical carbide having the same size as the ferrite particle size and about 1/10 thereof. And it has been found that by adjusting the carbide grain size and the ferrite grain size, a high carbon thin steel sheet excellent in both workability and hardenability can be obtained. [0007] The present invention has been made based on such findings of the present inventors, and comprises a cold-rolled cold-rolled steel sheet.
High carbon steel sheet consisting of 0.2% by weight C
And a mixed structure of ferrite and carbide, having a carbide grain size of 1.3 μm or less and a ferrite grain size of 1% or less.
An object of the present invention is to provide a high carbon steel sheet excellent in workability and hardenability, characterized in that the thickness is more than 4 μm and less than 4 μm. DETAILED DESCRIPTION OF THE INVENTION The high carbon steel of the present invention contains C
, Mainly containing ferrite and carbide, having a carbide particle size of 1.3 μm or less and a ferrite particle size of 1 μm or less.
It is more than μm and 4 μm or less. The reasons for limiting the C content, carbide grain size and ferrite grain size in the present invention will be described below. (1) C content C forms carbides in steel and causes strain distribution to ferrite grain boundaries and ferrite grains, which play an important role in the present invention, or to a ferrite-carbide interface. In addition, it is an important element that imparts hardenability to steel. If the content is less than 0.2% by weight, the amount of carbides is reduced and the hardness after quenching is reduced. On the other hand, when excessively added carbon is excessively hardened during hardening, 2% or less since there is a risk of causing quenching cracks. (2) Carbide Particle Size The carbide particle size greatly affects the hardenability under low-temperature and short-time holding conditions and the strain distribution described later. Regarding the hardenability, it is a condition that the carbide is rapidly dissolved in the austenite within the soaking time so that the hardenability is good under the condition of holding at a low temperature for a short time. In order for the carbide to rapidly dissolve into austenite, it is desirable that the carbide is fine. That is, by precipitating carbides finely in the steel, the surface area with respect to the total volume of the carbides increases, and the solid solution of the carbides is promoted during heating and holding. An experiment for verifying this will be described below. A continuous cast slab having a carbon content of about 0.65%
Heat to 230 ° C, finish temperature 850 ° C, winding temperature 40
Hot rolling is performed under the condition of 0 to 750 ° C.
Cold rolling was performed at ~ 70%, and annealing was performed at 600 to 740 ° C to produce a 1.2 mm thick high carbon thin steel sheet . Since the hardenability depends on the plate thickness, it is necessary to unify the plate thickness. Therefore, if necessary, the thickness is reduced to 1.2 by grinding.
mm. The steel plate manufactured in this way is 50 × 10
After cutting to a size of 0 mm, the temperature was raised to 750 ° C in a heating furnace,
After holding for 10 seconds, it was quenched into rapeseed oil at about 10 ° C. The heating temperature is specified to be 760 to 820 ° C. in JIS G4401, but in this experiment, it is lower by 10 ° C. than the lower limit specified in JIS in order to clarify the superiority of hardenability. The temperature was maintained. The hardness of the test piece after quenching in this manner was measured on a Rockwell C scale (HRC) to evaluate hardenability. The evaluation of hardenability is ◎, HRC63 or more, taking into account JIS and customer needs, etc.
Less than 63 and 59 or more were evaluated as ○, less than 59 and 55 as Δ, and less than 55 as ×. Table 1 shows the carbide particle size and the hardenability evaluation results at this time. [Table 1] As shown in Table 1, the carbide particle size is about 2
If it is not more than μm, the hardenability is good. In order to obtain more favorable hardenability, the thickness is preferably 0.8 μm or less. (3) Ferrite grain size The ferrite grain size increases under a specific carbide grain size (E
L) is the most important parameter in the present invention that leads to the improvement of L). That is, when the carbide particle size is 1.3 μm or less and the ferrite particle size is more than 1 μm and 4 μm or less, EL is remarkably improved. Although the reason for this is not necessarily clarified, in a structure in which carbides are dispersed in a ferrite ground, the location of a crack at the time of processing changes depending on the grain size of the carbides and ferrite. First, when the carbide exceeds 1.3 μm,
The strain is concentrated at the ferrite-carbide interface, and cracks occur at the interface. At the ferrite-carbide interface, the workability is low because the limit strain at which cracks occur is smaller than at the ferrite grain boundaries or within the ferrite grains. Next, when the carbide grain size is 1.3 μm or less, the strain concentration position changes depending on the ferrite grain size. When the ferrite grain size exceeds 4 μm, the strain concentrates on the ferrite grain boundaries and cracks are generated from the ferrite grain boundaries. When the ferrite grain size is 1 μm or less, the strain is concentrated in the ferrite grains, and cracks are generated from the ferrite grains. In these cases, although the workability is better than when strain is concentrated at the ferrite-carbide interface, the workability is not so much improved. On the other hand, when the ferrite grain size is in the range of more than 1 μm and 4 μm or less, it is considered that the strain is appropriately distributed between the ferrite grain boundary and the inside of the ferrite grain. Is significantly increased as compared with the case where it is concentrated on either the ferrite grain boundary or the inside of the ferrite grain. As a result, the workability is remarkably improved. Hereinafter, experimental results showing this fact will be described. By weight%, C ≒ 0.60%, Si ≒ 0.0
1%, Mn ≒ 0.2%, P ≒ 0.015%, S ≒ 0.0
A continuous cast slab having a composition of 08% is heated to 1150 ° C, a finishing temperature of 850 ° C, and a winding temperature of 400 to 700 ° C.
Hot rolling is performed under the following conditions, followed by primary cold rolling and 68
After annealing for 0 × 40 hours, secondary cold rolling was performed at a cold rolling reduction of 10 to 95%, and annealing was performed at 500 to 740 ° C. to produce a high carbon thin steel sheet having a thickness of 1.5 mm. The ferrite grain size and the carbide grain size of the steel sheet thus manufactured were measured, and the EL was measured by a tensile test in order to evaluate the workability. JIS Z for tensile test
The test piece used was a No. 5 test piece of JIS Z 2201 in accordance with 2241. FIG. 1 shows the evaluation results. In the figure, the black circles indicate EL <15%, and 90% or more of the cracks were ferrite.
Those generated at the carbide interface, black square: 15% ≦ EL <2
0%, 90% or more of cracks occurred in ferrite grains, black diamonds: 20% ≦ EL <25%, 9% of cracks
0% or more occurred at the ferrite grain boundary.
% ≦ EL, where 60 to 80% of the cracks are formed in the ferrite grain boundaries and 30 to 20% are formed in the ferrite grains. From this figure, it can be seen that when the carbide particle size exceeds 1.3 μm, cracks occur at the ferrite-carbide grain boundary, and EL
Is low. Also, the carbide particle size is 1.3 μm
If the ferrite grain size is 1 μm or less, the ferrite grain fracture occurs, and if the ferrite grain size exceeds 4 μm, the ferrite grain boundary fracture occurs, which also leads to a low EL. On the contrary,
Carbide particle size is 1.3μm or less and ferrite particle size is 1μ
In the range of more than m and 4 μm or less, cracks occur in both ferrite grain boundaries and in the grains, and it was confirmed that EL was improved. As described above, the carbide particle size may be about 2 μm or less from the viewpoint of hardenability alone. However, in consideration of the above experimental results, the present invention is excellent in both hardenability and workability. The range was specified as a carbide particle diameter of 1.3 μm or less and a ferrite particle diameter of more than 1 μm and 4 μm or less. In the present invention, the structure of the steel is a mixed structure of ferrite and carbide, but the ferrite grain boundary may not be clearly observed due to the carbide, and the actual ferrite grain size may not be clear. Therefore, in the present invention, the ferrite grain size is defined as follows. (1) In the case where ferrite grain boundaries are observed without being interrupted by carbide, the range surrounded by the grain boundaries is defined as the grain size of ferrite grains. (2) When no ferrite grain boundaries are observed due to carbides, the area surrounded by the ferrite grain boundaries and the carbide grain boundaries is defined as the area occupied by ferrite grains as shown in FIG. Is defined as the ferrite grain size. (3) As shown in FIG. 3, when carbide is present in the ferrite grains, the area of the carbide is not included in the area occupied by the ferrite grains. (4) As shown in FIG. 4, when the carbide traverses the ferrite grains and is in contact with the ferrite grain boundaries, the ferrite grains divided by the carbides are each independent ferrite grains. Similarly, the diameter of a circle having an equivalent area is defined as the carbide particle size. The method for measuring the ferrite grain size and the carbide grain size is not particularly limited. However, after the thickness section of the sample is polished and corroded, a scanning electron microscope photograph of a magnification of 1500 to 5000 times is taken. It is desirable to measure the ferrite grain size and the carbide grain size from the photograph. When actually determining the ferrite grain size of a sample, the average of the grain sizes of the grains that are photographed as a whole is taken as the grain size of the field of view, and after performing this for two or more fields, the average of those is further taken. Is the ferrite grain size of the sample. The particle size of the carbide is determined in the same manner. It is preferable that the photographing is performed so that the magnification is such that at least 30 grains exist. In addition, as a corrosive liquid, when measuring ferrite particle size, a nital corrosive liquid is used.
When measuring the carbide particle size, it is preferable to use a picral etchant. FIG. 5 shows an example of the measurement result. FIG. 5 shows the result of corrosion by the nital etchant. In this figure, the ferrite particle size is 1.94 μm, and the carbide particle size is 1.94 μm.
04 μm. It should be noted that the present invention exerts the desired effect as long as it contains 0.2% or more of C by weight and has a grain size in the above specific range and is mainly composed of ferrite and carbide. There is no need to particularly define other components, and there is no problem even if elements such as Mn, Si, P, S, Al, and N are contained in a normal range. However, Mn is desirably 2% or less because it tends to cause a decrease in hardenability due to an effect of suppressing solid solution of carbide. Further, the content of Si is desirably 2% or less because carbides tend to be coarsened and hardenability tends to be impaired. As for P and S, if they are contained excessively, the ductility is reduced.
The following is desirable. If Al is contained excessively, the hardenability decreases, so 0.08% or less is desirable. N
Also, if contained excessively, the ductility is reduced, so that the content is preferably 0.01% or less. Depending on the purpose, B, Cr, Cu, Ni, M
Various elements such as o, Ti, Nb, W, V, and Zr may be added. These elements have no effect on the effects of the present invention. Further, various elements such as Sn and Pb may be mixed as impurities in the manufacturing process, but such impurities do not affect the effect of the present invention at all. Next, a preferred method for producing the high carbon steel according to the present invention will be described. First, molten steel adjusted to the composition within the range of the present invention is made into a slab by ingot slab treatment or continuous casting. Next, hot rolling is performed. The slab heating temperature at that time is appropriately 1250 ° C. or less from the viewpoint of change in the surface state due to scale generation, and the finishing temperature is Ar ₃ or more from the viewpoint of workability. It is desirable to do. The winding temperature is preferably set to 600 ° C. or lower for fine precipitation of carbides, and is preferably set to 400 ° C. or higher from the viewpoint of cold pressure load. When used as a cold rolled sheet,
Cold rolling is performed, and the cold pressure ratio at that time is preferably 20% or more in order to refine carbide during annealing. However, if the rolling ratio is excessive, ferrite grains after rolling become fine. 85% or less is preferable because it becomes too much. In the subsequent annealing, the temperature must be 600 ° C. or higher in order to grow ferrite grains appropriately, but in order to suppress excessive growth of carbide (Ac ₃
-30) C. or less. Regarding the annealing method, there is no problem whether continuous annealing or box annealing is performed. In addition, you may combine cold rolling and annealing twice or more. Thereafter, temper rolling is performed if necessary.
Since the temper rolling does not affect the hardenability, the conditions are not limited. In addition, both the converter and the electric furnace can be used for adjusting the composition of the steel of the present invention, and there is no problem at all even if the rough rolling is omitted during the hot rolling and the finish rolling is performed. Further, the direct casting rolling may be performed by rolling the continuous casting slab as it is or by performing a heat retaining treatment for the purpose of suppressing a temperature decrease. Further, the steel of the present invention may be a hot-rolled steel sheet or a cold-rolled steel sheet, and in each case, the effects of the present invention can be similarly exhibited. Embodiments of the present invention will be described below. Example 1 A continuously cast slab having a composition shown in Table 2 and having a carbon content of about 0.5% was heated to 1080 ° C., and finished at a temperature of 870 ° C. and a coiling temperature of 3.
Hot rolling is performed at 50 to 700 ° C, cold rolling is performed at a cold rolling rate of 10 to 95% after pickling, and 55 to 350 ° C to 740 ° C.
By performing box annealing for a long time, a high carbon thin steel sheet having a sheet thickness of 1 mm was produced. A tensile test and a quenching test were performed on each of the thin steel sheets to evaluate workability and hardenability. The test conditions are shown below. <Tensile test> The tensile test was conducted according to JIS Z22.
The test was performed in accordance with No. 41, and the test piece used was a No. 5 test piece of JIS Z 2201. Evaluation is performed by EL, EL is 25%
Above: ○, less than 25% 15% or more △, less than 15% ×
And <Hardenability test> The above steel sheet was 50 × 100 m
m, cut to 800 ° C in a heating furnace,
After holding for 2 seconds, it was quenched into rapeseed oil at about 10 ° C. The hardness of the test piece after quenching on the plate surface was measured on a Rockwell C scale (HRC) to evaluate hardenability. Evaluation is H
RC 57 or more: ◎, less than 57 53 or more: ○, less than 53 4
9 or more was evaluated as Δ, and less than 49 was evaluated as ×. The results of the tensile test and the hardenability test are shown in Table 3. [Table 2] [Table 3] As shown in Table 3, the carbide particle size was 1.3 μm.
It was confirmed that a high carbon steel excellent in both workability and hardenability can be obtained in the range of m or less and the ferrite particle size in the range of more than 1 μm and 4 μm or less. (Second Example) A continuously cast slab having a composition shown in Table 4 and having a carbon content of about 0.35% was heated to 1180 ° C, and hot-worked at a finishing temperature of 845 ° C and a winding temperature of 400 to 700 ° C. Rolling was performed. After pickling, primary cold rolling and box annealing at 640 ° C are performed, and secondary cold rolling is performed at a cold rolling reduction of 10 to 95%, and 500 to 770 ° C.
To perform a continuous annealing to produce a high carbon thin steel sheet having a thickness of 2 mm. A tensile test and a quenching test were performed on each of the thin steel sheets to evaluate workability and hardenability. These test conditions were basically the same as those of the first embodiment. However, since the component system was different from that of the first embodiment, the evaluation criteria were different from those of the first embodiment.
で at 0% or more, Δ at less than 30% 20% or more, × at less than 20%
Less than 46 or less than 0 was evaluated as ○, less than 42 or more as Δ, and less than 42 as ×. Table 5 shows the results of the tensile test and the hardenability test. [Table 4] [Table 5] From Table 5, it is confirmed that a high carbon steel excellent in both workability and hardenability can be obtained when the carbide particle size is 1.3 μm or less and the ferrite particle size is more than 1 μm and 4 μm or less. Was. (Third Example) A continuously cast slab having a composition shown in Table 6 and having a carbon content of about 0.65% was heated to 1200 ° C, and hot-rolled at a finishing temperature of 845 ° C and a winding temperature of 350 to 700 ° C. Rolling was performed. After pickling, primary cold rolling and box annealing are performed at 640 ° C., and secondary cold rolling is performed at a cold rolling reduction of 15 to 95%, and 550 to 740 ° C.
To produce a high carbon thin steel sheet having a thickness of 1.6 mm. A tensile test and a quenching test were performed on each of the thin steel sheets to evaluate workability and hardenability. The test conditions were basically the same as in the first embodiment, and the evaluation criteria for the tensile test were the same as those in the first embodiment. However, the evaluation criteria for the hardenability test were different from those in the first embodiment. H
RC 63 or more: 、, less than 63 59 or more: ○, less than 59 5
5 or more was evaluated as Δ, and less than 55 was evaluated as ×. Table 7 shows the results of the tensile test and hardenability test. [Table 6] [Table 7] From Table 7, it is confirmed that a high carbon steel excellent in both workability and hardenability can be obtained when the carbide particle size is 1.3 μm or less and the ferrite particle size is more than 1 μm and 4 μm or less. Was. As described above, according to the present invention,
The ferrite grain size is controlled to more than 1 μm and 4 μm or less by focusing not only on hardenability but also on making carbide finer and also on workability (ductility) being greatly affected by ferrite grains.
In addition, since the carbide particle size is controlled to 1.3 μm or less, a high carbon thin steel sheet excellent in both workability and hardenability can be provided. Thus, the high carbon thin steel according to the present invention
Since the plate is excellent in workability and hardenability, transmission parts and the like represented by gears can be manufactured at low cost and with stable quality.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the influence of carbide particle size and ferrite particle size on EL. FIG. 2 is a schematic diagram for explaining ferrite grain size according to the present invention. FIG. 3 is a schematic diagram for explaining a ferrite grain size according to the present invention. FIG. 4 is a schematic diagram for explaining a ferrite grain size according to the present invention. FIG. 5 is a micrograph showing an example of a measurement result of a ferrite grain size and a carbide grain size.

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-5-202445 (JP, A) JP-A-5-271861 (JP, A) JP-A-8-73985 (JP, A) 322479 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/10 C21D 9/46-9/48

Claims (1)

(57) [Claims] [Claim 1] High carbon made of cold-rolled cold-rolled steel sheet
A thin steel sheet containing 0.2% or more and 2% or less by weight of C, having a mixed structure of ferrite and carbide, having a carbide particle diameter of 1.3 μm or less and a ferrite particle diameter of more than 1 μm and 4 μm or less. High carbon thin steel sheet with excellent workability and hardenability.