JPS5941503B2 - Hot-rolled steel sheet with excellent carburizing and hardenability and no abnormal structure - Google Patents

Description

[Detailed description of the invention]

The present invention is provided in the form of a hot-rolled steel sheet, and particularly relates to a continuously cast steel sheet that has excellent carburizing and hardenability and does not produce abnormal structures. Some of the steel materials traditionally provided to automobile manufacturers and industrial machinery manufacturers in the form of hot-rolled sheets or cold-rolled sheets have been processed into parts that require surface hardness and wear resistance as product characteristics after processing. There may be cases where For example, various ratchets and sleeves require surface hardness of Vickers hardness of 500 or more. These parts are generally manufactured by forming a steel plate and then subjecting it to carburizing (or carbonitriding) and hardening.
After liquid or gas carburizing (or carbonitriding) in the range of ~900°C, the surface is hardened by quenching in oil at a relatively high temperature (150-180°C). In this way, a relatively easy quenching speed is required to ensure component precision, so the steel plate used as the material is
Rimmed steel sheets or gapped steel sheets are used, which tend to have coarse grains due to carburizing (or carbonitriding) treatment and are hardened by quenching. However, these steel sheets tend to have coarse grains,
Although it is easy to harden, carburizing (or carbonitriding) treatment tends to cause some crystal grains to grow excessively, the entire metal structure tends to become mixed grains, and uneven hardening is likely to occur. Troostite occurs near the product surface and near grain boundaries, reducing the hardness of those areas.
Usually, it tends to cause a phenomenon called abnormal structure, which has many disadvantages in terms of product yield, carburizing (or carbonitriding) treatment process, etc., and it cannot be said to be the best steel plate from the standpoint of quality assurance. The cause of these inconveniences is that rimmed steel sheets and gapped steel sheets do not have precipitates (e.g., precipitates such as aluminum nitrides and titanium carbonitrides) that prevent abnormal growth of crystal grains during heating. and that the oxygen content in the steel is extremely high. In normal uses of steel plates, the inconveniences caused by these two points can be resolved by using aluminum quilted steel plates, but for applications that involve carburizing and quenching, it is recommended to use aluminum killed steel plates. I can't. That is, in a normal aluminium-killed steel sheet, 5OlAl is 0.
．． A portion of this content becomes aluminum nitride, which prevents crystal grain growth during heating of the steel plate. In this sense, it is a favorable phenomenon that does not create mixed grains or extremely coarse grains, but because its grain growth inhibiting effect is too excessive, the temperature used in carburizing treatment (80
(0 to 950°C), no crystal grains grow at all, and unless the quenching speed is rapid such as water cooling, the quenched hardness is reduced to Hv.
It cannot be raised above 500. In addition, since there is a large amount of solid solution Al in the aluminum quilted steel sheet, when carbonitriding is performed, the nitrogen that has entered the steel combines with this solid solution Al, so the nitrided ratio includes the amount of solid solution nitrogen. does not increase, and in this sense as well, it is difficult to obtain sufficient quenched hardness. Therefore, aluminum-killed steel sheets cannot be used as a material in manufacturing processes that involve relatively easy hardening of parts for which precision is important as described above. Furthermore, from the perspective of productivity and production costs in manufacturing steel sheets, it is desirable to adopt continuous casting as the casting method, but in order to prevent casting defects due to blowholes in continuous casting, at least The amount of oxygen in the steel must be below 1100pp, therefore,
No matter how easily the surface of steel is hardened by carburizing and quenching, continuous casting cannot be used for rimmed steel or capped steel, which have an extremely high amount of oxygen in the steel. Note that although aluminum killed steel can of course be continuously cast, aluminum quilted steel sheets cannot be used for carburizing and quenching purposes for the reasons mentioned above. Therefore, from the standpoint of productivity, it is desirable to develop a steel type for carburizing and hardening that can be continuously cast, while at the same time improving the above-mentioned disadvantages in carburizing and hardenability itself. The present inventor paid attention to this point, and as a result of studying in detail the relationship between the Al content, Ti content, and oxygen content in steel and carburizing hardenability, the results shown in FIG. 1 were obtained. That is, FIG. 1 shows ordinary rimmed steel plates, capped steel plates,
Aluminum quilted steel sheets and various steel sheets with different 5olAl and Ti contents were treated at 850°C for 1 hour in a liquid carbonitriding bath with a CN content of 10%, and then heated at 180°C.
The surface hardness of the steel sheet is expressed in correlation with the Al content and Ti content when the steel plate is quenched and held in oil for 15 minutes, then taken out and air cooled, and then tempered at 220°C for 90 minutes. It is. In this case, the points marked R in Figure 1 are from 0.035 to
0.05% carbon, 5o4Al<0.001%, (0)>
The data is for a conventional rimmed steel plate with a concentration of 300 ppm, and the points marked with C in the same figure are 0.03 to 0.045.
% carbon, 5o4Al<0.001%, (0)≧150p
pm data on conventional capped steel sheets, and the points marked with T in the same figure are 002 to 0.06% carbon, 0.0
02 <sol. Al+○, 56Ti≦0043, (Q)<100
Data on ppm Al and Ti added steel sheets, and the points marked A in the same figure indicate 002 to 0.06% carbon, sol, Al > 0.03%,

[0]≦100pp
This is data regarding conventional aluminum killed steel of m. Note that Al and Ti originally exhibit the same effect if the number of atoms contained in the steel is the same, so the surface hardness after quenching is a function of the A-4 atomic weight and the Ti atomic weight in the steel. Although it is related to the total sum, the alloying elements in steel are generally expressed in weight percentages, so the content is 5olAl(%;
)+o, s 6T i (%) are shown as parameters in FIG. As is clear from the figure, 5olAl (%) + 0.56T
Carburizing and quenching can be achieved by adding one or both of Al and Ti to the steel so that i (%) is more than 0.005% and less than 0.002%, and the oxygen content in the steel is 1100pp or less. It was discovered that this steel sheet has good properties and does not produce abnormal structures, making it ideal for carburizing and quenching. That is, the present invention is a steel plate cast by a continuous casting method, and the alloying elements include co, oi~02%, Mn0.
1 to 0.5%, Si 0.06% or less, Po 0.03% or less, 80.03% or less and further sol A10.005
% more than 0.02% and less than 0.01% of Ti 5ol
Al (%) + 〇, 56Ti (%) exceeds 0.005%00
The content is within the range of 2% and the amount of oxygen in the steel is 1.
00 ppm or less, with the remainder consisting of iron and unavoidable impurities.This hot rolled steel sheet has excellent carburizing and hardenability and does not produce abnormal structures. Unlike conventional aluminum killed steel with 5olA of 10.03% or more, steel sheets with such a composition do not have excessive precipitates such as AlN that inhibit grain growth in the steel.
Carburizing allows grains to grow to an appropriate size for subsequent quenching, and unlike conventional rimmed steel sheets and capped steel sheets, some grains may grow abnormally or have extremely mixed grains. Moreover, since the amount of oxygen in the steel is low, there is no concern that an abnormal structure will occur. The present invention will be explained in detail below. Regarding the amount of C in the steel sheet of the present invention, the range is 001 to 0.
．． It is necessary that it be 2%. If it exceeds 0.2%, the material will be hard to withstand punching, squeezing, etc., and the material will be hard. Therefore, rather than using a manufacturing process that includes carburizing, it is preferable to directly quench it without carburizing. Choosing is more advantageous in terms of production efficiency and production costs. On the other hand, if it is less than 0.01%, the steel plate becomes too soft,
This is because it is disadvantageous for processing such as punching, and if it is assumed that the material will be carburized (or carbonitrided) later, there is no point in decarburizing the material any further. The amount of Mn is indispensable to improve hardenability, but the reason why the range is 01 to 05% is 05%.
If Mn is present in an amount exceeding %, selective oxidation of Mn will proceed due to oxygen in the atmosphere during carburizing (or carbonitriding), and there will be a strong tendency for Mn acid and arsenate to precipitate at grain boundaries. This is because not only is it disadvantageous to mechanical properties, but also it becomes a cause of a mixed grain structure and an abnormal structure. Moreover, if it is less than 0.1%, the hardenability after carburizing will be poor, and sufficient surface hardness will not be obtained under relatively easy hardening conditions such as oil hardening. Although the Si content does not originally have a large effect on the carburizing J-hardenability, as the Si content increases, the number of B-based inclusions increases rapidly, which is disadvantageous to the mechanical properties. However, since Si is indispensable as a preliminary deoxidizing agent in the steel manufacturing process, it is unavoidable that some amount of Si remains in the steel, and in that sense it was set at 0.06% or less. Since the S and P contents are elements that have a negative effect on mechanical properties, it is desirable that they be as low as possible, but the upper limit of each was set to 0.030% or less within the range possible due to current steel manufacturing technology problems. Regarding 5olAl and Ti contents, 5olAl (%) 10 and 56Ti (capital) were set to more than 0.005% and less than 0.02% because, as is clear from the example in Figure 1, if it exceeds 0.002%, crystals This is because the effect of inhibiting grain growth becomes excessive, making it impossible to obtain sufficient quenched hardness.Also, with current steelmaking technology, if 0002% or more 5ol Al is contained in steel, the oxygen content can be reduced to less than 100 ppmυ. Although it is possible, the most stable means of production is to convert 5olAl into 0.
Since Al is added so that the content exceeds 0.005%, and Ti is added at the same time, the lower limit is set to exceed 0.0005%. Ti has the same effect as Al when it comes to bonding with oxygen, and the effect of suppressing abnormal growth of crystal grains during hardening is much more stable when Ti is added at the same time. However, addition of more than 0.01% of Tl significantly increases scratches on the surface of the steel sheet, so the range of TI content is limited to 0.01%.
It was set to 1% or less. Furthermore, in the current continuous casting technology, when the oxygen content exceeds 1100 pp, blowholes occur significantly inside the slab, which significantly reduces the casting yield. Therefore, continuous casting cannot be applied to steel with an oxygen content exceeding 100 ppm. Since the present invention is a steel intended for continuous casting, the oxygen content is set to 100 ppm or less. In manufacturing the steel plate of the present invention, the molten steel adjusted to be within the scope of the present invention is made into a slab by continuous casting, and then hot rolled to a plate thickness of 10 mm or less and carburizing hardenability of 1 mm or more. A hot-rolled steel sheet with excellent quality and no abnormal structure. Next, the effects of the present invention will be illustrated in more detail with reference to Examples. Examples Steel sheets according to the present invention and comparative materials (both 5 mm in thickness) having chemical compositions as shown in Table 1 were carbonitrided at 860° CXIhr in a carbonitriding bath containing 10% CN, and then heated at 180° C. It was quenched in oil and held for 15 minutes, then taken out and cooled in the air. After that, after completely wiping off the oil, it is heated in a tube furnace for 2 hours.
Tempering treatment was performed at 20°C for 90 minutes. Note that the above steps are standard steps for carburizing and hardening various ratchets, which are the main uses of the steel plate of the present invention. Table 2 shows the observation results of the surface hardness, hardening depth, and metal structure near the surface of the product that underwent the above steps. As is clear from this table, the steel sheet of the present invention completely eliminates the problems of abnormal structure and extreme grain aggregation that were disadvantageous to conventional rimmed steel sheets, and also lacks hardness like conventional aluminum quilted steel sheets. It can be seen that this steel plate is most suitable for carburizing and quenching. In addition, Fig. 2 A, B, and C are steel plate points 1 of the present invention in Table 1,
Ordinary aluminum killed steel plate 56 and ordinary rimmed steel plate A
9 are micrographs of cross sections of products obtained by carbonitriding and quenching under the above conditions, clearly illustrating the above facts, and show that the steel sheet of the present invention is superior to conventional aluminum-killed steel sheets, rimmed steel sheets, etc. It is clear that they are completely different materials. The steel plates of the present invention in Table 1 are all manufactured by continuous casting, and the oxygen content is kept below 1100 pp, so there is no reduction in yield due to blowholes during casting. I was not able to admit.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the amounts of 5olAl and Ti in steel and the surface hardness of the steel plate after carburizing and quenching, and Figure 2 is a photograph comparing the surface structures of the steel plate of the present invention and conventional materials after carburizing and quenching. be. A: Steel plate of the present invention, B: Conventional aluminum killed steel plate, C: Conventional rimmed steel plate.

Claims (1)

[Claims] 1 A steel plate manufactured by a continuous casting method, containing 1 to 02% COO, 0.1 to 0.5% Mn as alloying elements,
Si 0.06% or less, Po 03% or less, 8003% or less, and 5olA more than 10.005% 002% or less, T
i 0.01% or less both so # Al opening 0.56Ti
c%) is in the range of more than 0005% and less than 002%, and the amount of oxygen in the steel is 100pp or less, with the remainder consisting of iron and unavoidable impurities.It has excellent carburizing and hardenability and has an abnormality A hot-rolled steel plate that does not have a structure.