JPH0140905B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to strong structural steel, and in particular, when using a large member with a thickness of 25 mm or more to increase the strength of the steel, it is possible to obtain a sufficient heat treatment effect up to the center of the large member. It relates to high-strength, tough steel for large structures that has hardenability. Structural tough steel is used after being quenched and tempered to obtain the required strength and toughness depending on the application. Sufficient quenching ensures good mechanical properties. This is the condition for obtaining it. However, the larger the part, the slower the cooling rate during hardening and the more difficult it is to obtain a sufficient hardening effect, so it is necessary to select a steel type with hardenability commensurate with the size of the part. It is said that Conventionally, the steel types commonly used as strong structural steels are those specified by JIS: carbon steel for machine structures, manganese steel for machine structures, chromium-molybdenum steel, and nickel-chromium-molybdenum steel. When compared at the same carbon content level, the hardenability improves in this order. In this way, steel types with good hardenability contain many alloying elements, but especially for large parts, nickel, chromium, and molybdenum are selected, but since nickel and molybdenum are expensive elements, It is expensive compared to other steels, and this is an impediment to its use.Therefore, it is unavoidable that chromium-molybdenum steel is generally used. On the other hand, in recent years, as industrial machinery has become larger, the parts used in it have also become larger, and for such parts, chromium-molybdenum steel lacks hardenability in the center of large parts. Since the strength and toughness are reduced, there is a demand for the development of an economical alternative structural steel. The present invention eliminates the drawbacks of conventional steel, satisfies the above demands, exhibits excellent hardenability, and has toughness commensurate with strength, and is therefore suitable for use in vehicles, industrial machinery, etc. Our goal is to obtain a high-strength steel for large structures that is hardened from the surface to the center, has high strength and toughness, and can be economically applied to large members with a thickness of 25 mm or more. That is the purpose. In the present invention, in order to achieve this objective, we conducted experiments with varying amounts of Si, Mn, Cr, and B, and found that the amounts of Si, Mn, and Cr ranged from 1.0% to 1.0%, respectively.
By keeping the variation between these components within 0.10%, the hardness of the surface and center of large parts is almost the same, resulting in stable high strength and toughness. It was found that the following results could be obtained. In addition to these basic additive elements, Ni, Mo,
It is characterized by adding a small amount of one or more types of Nb to improve hardenability and toughness. That is, the present invention includes (1) a weight ratio of C 0.25 to 0.45% Si 1.00 to 1.50% Mn 1.00 to 1.50% Cr 1.00 to 1.50% B 0.0005 to 0.0030%, with the remainder consisting of Fe and normal impurities;
The ratio of Si, Mn, and Cr is Si:Mn:Cr≒1:1:1
High-strength steel for large structures in which the difference between the highest and lowest values is within 0.10%, and the surface and center hardness of rolled and forged parts with a diameter of 25 mm or more is almost the same. and (2) Ni 0.25-1.00% Mo 0.10-0.20% in addition to the basic additive components of C 0.25-0.45% Si 1.00-1.50% Mn 1.00-1.50% Cr 1.00-1.50% B 0.0005-0.0030% Contains one or more of Nb 0.03 to 0.20%, the balance consists of Fe and normal impurities, and the ratio of Si, Mn, Cr is
Si:Mn:Cr≒1:1:1, and the difference between the highest and lowest values is within 0.10%, and the hardness of the surface and center of rolled and forged parts with a diameter of 25 mm or more is approximately It is characterized by the same high strength and tough steel for large structures. EMBODIMENT OF THE INVENTION Hereinafter, the present invention will be described in detail based on Examples and FIGS. 1 to 4 of the accompanying drawings showing experimental results thereof. First, the steel of the present invention is characterized by having the above-mentioned chemical components, and the reason why the steel is limited to such chemical components will be explained below. C: C is an effective element for improving the strength and hardenability of steel, but if it is less than 0.25%, the strength required for strong steel cannot be obtained, and if it is too high, the toughness decreases and welding becomes difficult. Therefore, the upper limit of the weldable range was set at 0.45%, and 0.25~
It was set at 0.45%. Si: Si dissolves in ferrite to increase strength and improve hardenability, so it is an essential element for large structural members. In addition, the increase in Si has the effect of moving the decomposition of martensite to the high temperature side during tempering, so high strength can be obtained by high temperature tempering beyond the brittle tempering range. The lower limit of Si was set at 1.00% as the amount necessary to obtain the necessary hardenability in balance with other hardenability-improving elements and to enable high-temperature tempering. In addition, increasing Si can increase hardenability and tempering resistance, but it also has the effect of increasing the Ac ₃ transformation point, and the increase becomes steep when it exceeds 1.5%.
High temperature heating is required during quenching, causing elongation and
The upper limit was set at 1.5% because it significantly lowers toughness values such as reduction of area and impact value. Mn: Mn is very effective in improving hardenability and also improves toughness value, so
It is an extremely useful element that replaces Ni and Mo.
It is an essential element for large structural members. Therefore, in balance with other hardenability-improving elements, if it is less than 1.0%, the hardenability is insufficient for the purpose of the present invention, so the lower limit is set to 1.0%.
Moreover, if it exceeds 1.5%, the toughness may deteriorate, so the upper limit was set at 1.5%. Cr: Cr improves hardenability, increases core hardness after hardening, and refines crystal grains. Further, since it forms carbides and contributes to an increase in tempering resistance, it is an essential element for large structural members.
Therefore, taking into account other elements that improve hardenability, the lower limit is set at 1.0% as the necessary amount that can expect these effects, and if it exceeds 1.5%, the improvement in the effect will be small and the toughness will decrease. Therefore, the upper limit was set at 1.5%. B: B is an element that significantly improves hardenability in an extremely small amount, and is effective in improving hardenability at 0.0005% or more. However, even if the content is 0.0030% or more, no improvement in hardenability can be expected, so the content was set in the range of 0.0005 to 0.0030%. In addition, in order for B to work effectively to improve hardenability within this range, it is necessary to prevent it from combining with N in the steel, and for this purpose, Ti is added at 0.03% or less to fix N. This method is commonly used. Ni, No, Nb: All of these elements have the effect of making crystal grains finer and improving toughness. However, these elements are ineffective in small amounts and are all expensive elements, so even if too much is added to the basic steel, the effect will be small relative to the proportion of the amount added. ~0.20%, Nb
It was set at 0.03-0.20%. Next, the characteristics of the steel of the present invention will be explained using examples. Table 1 shows the chemical composition of the steel of the present invention melted in an electric furnace and the chromium-molybdenum steel (JIS
The standard chemical composition of the standard SCM440H) and the chemical composition (weight %) of comparative steels 1 and 2 as representative examples are listed. In addition, in Table 1, the invention steel A is:
Inventive steel B is close to the median of the chemical composition limited range, Inventive steel B is also close to the lower limit, Inventive steel C is also close to the upper limit, Inventive steel D has Ni and Ni in addition to the basic additive elements. The steel of the present invention was added with Mo, and the steel of the present invention was also added with Nb.
It shows. Among the test materials shown in Table 1, the invention steels A to D, F
A hardenability test was conducted based on JIS G0561 "Hardenability test method for steel (single quenching method)" and the results are shown in Figure 1. As can be seen from the figure, in each of the steels A to D and F of the present invention, the air-cooled end (distance 50 from the quenched end of the test piece)
Even in the invention steel B, which has a very small decrease in hardness at the point of
It exceeds the upper limit of the hardenability standard for SCM440H, indicating that it is characterized by high hardenability. As described above, since the steel of the present invention has high hardenability, small members can be air hardened to prevent hardening distortion. That is, for the invention steel A and comparative steel 1, the diameter is 20 mm and the length is 500 mm.
In order to give almost the same mechanical properties to round bars, steel A of the present invention was air-cooled (air quenched) from 850°C.
After that, the steel bar was tempered at 625°C for 1 hour.For comparative steel 1, the mechanical properties and the amount of bending of the steel bar due to heat treatment were evaluated when tempering was performed at 600°C for 1 hour after oil quenching from 850°C. The comparison results are shown in Table 2. Figure 2 shows the tempering performance curves of Inventive Steel A and Comparative Steel 1. This curve shows the mechanical properties of a round bar specimen with a diameter of 25 mm that was tempered after oil burning. The figure shows that the steel has almost the same mechanical properties as the comparative steel at the same tempering temperature. Next, Figure 3 compares the quenching hardness of inventive steel A and comparative steel 1 for round steel with a diameter of 150 mm. FIG. 3 is a diagram showing the results of measuring the hardness of a cross section from the surface to the center using a Rockwell hardness tester. From this figure, the steel of the present invention can be used as a round steel with a diameter of 150 mm.
Although oil quenching shows uniform hardening hardness from the outer periphery to the center, it can be seen that in Comparative Steel 2, the hardness decreases significantly toward the inside, and no quenching effect can be expected in the inside. In this way, the steel A of the present invention can exhibit an excellent heat treatment effect on large members, and Table 3 shows the results of comparing this effect in terms of mechanical properties after quenching and tempering. ing. Table 3 shows the test results of the mechanical properties of steels A and D of the present invention and comparative steel 2 after quenching and tempering of round steel with diameters of 25 mm and 150 mm. In addition, the quenching is performed at 850°C to austenitize and then oil quenching, and the steels A and D of the present invention are
After tempering, Comparative Steel 2 was tempered at 620°C and 580°C, and then water-cooled. The test pieces were taken at the center for specimens with a diameter of 25 mm, and at the midpoint between the surface and the center of the round steel for specimens with a diameter of 150 mm, in the longitudinal direction. From this table, it can be seen that in the comparison steel, the mechanical properties at a diameter of 150 mm decrease significantly compared to the mechanical properties at a diameter of 25 mm, whereas the inventive steel shows a relatively small decrease in mechanical properties, making it suitable for high-quality materials for large parts. It can be seen that this shows that it is suitable as a strong and tough steel. Furthermore, representative example D of the steel of the present invention containing Ni, Mo or Nb is suitable when it is desired to further increase the yield point and impact value, and this steel, which belongs to the basic steel that does not contain these, is suitable for further increasing the yield point and impact value. A comparison with representative example A of the invention steel is made to clarify its effects. Figure 4 shows the diameter of the invention steel A and comparative steel 2, 150 mm.
This is the hardening depth curve of a specimen of mm in diameter that was subjected to induction hardening and tempering.High frequency conditions were selected to create a deep hardened layer, and tempering was performed at 200℃. Induction hardening is sometimes applied to strong steel for the purpose of improving wear resistance and fatigue strength, but a deep hardened layer is often required for large members. From the figure, it can be seen that the steel of the present invention can obtain a deeper hardened layer than the comparative steel, and in this respect as well, the steel of the present invention can provide advantages when applied to large-sized members. Furthermore, Comparative Steel 1 and Invention Steel A~ shown in Table 1
Regarding D and F, from the hardenability test results shown in Fig. 1 and the hardness distribution diagram from the center to the surface of a round steel with a diameter of 150 mm shown in Fig. Hardened end and 50mm of hardened test
Table 4 shows the hardness difference H _R C between the position and the quenched hardness difference H _R C between the surface part and the center part of the 150 mm round steel. From this table, Si of comparative steel 1,
The variation between Mn and Cr components is 0.81, and the hardness difference between the quenched end and the 50mm position in the one-end quenching test is H _R C15, 26 at the upper and lower limits of the quenching curve, respectively.
Also, the difference in hardness between the surface and center of a 150mm round steel is H _R C21. On the other hand, the variation between the components of the steels A to D and F of the present invention is 0.02 to 0.04
The hardness difference between the quenched end and the 50mm position in the one-end quenching test is H _R C2 ~ 7, The hardness difference between the surface and center of the 150 mm round steel is H _R C5. I understand. That is, the variation in hardness of the steel of the present invention is 1/2 to 1/13 of the variation of the comparative steel. In this way, according to the present invention, by setting the addition ratio of Si, Mn, and Cr to 1:1:1 and suppressing their variation within 0.10%, when a large structural member is heat-treated, it is possible to It can be seen that it is possible to obtain stable hardness down to the middle, almost the same hardness, and high strength and toughness. As mentioned above, as is clear from the examples, the steel of the present invention does not require the addition of expensive elements, unlike conventional steels such as nickel-chromium-molybdenum steel, but is made possible by its unique composition. It improves hardenability and can exhibit high strength and toughness in large parts, so we provide an inexpensive steel that can be used as a high-strength steel for large structures for various uses. This invention can be used for a wide range of purposes and is of great industrial value.

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【table】 [Brief explanation of drawings]

Figure 1 is a diagram showing the hardenability test results of the steel of the present invention and comparative steel, Figure 2 is the tempering performance curve, Figure 3 is the cross-sectional hardness distribution curve after quenching, and Figure 4 is the same. 1 is also a hardening depth diagram by induction hardening.

Claims (1)

[Claims] 1 Contains in weight ratio: C 0.25-0.45% Si 1.00-1.50% Mn 1.00-1.50% Cr 1.00-1.50% B 0.0005-0.0030%, the remainder consisting of Fe and normal impurities,
The ratio of Si, Mn, and Cr is Si:Mn:Cr≒1:1:1
and the difference between the highest value and the lowest value is within 0.10%, and the hardness of the surface and center of the rolled and forged member with a diameter of 25 mm or more is almost the same. High-strength steel for materials. 2 Contains C 0.25-0.45% Si 1.00-1.50% Mn 1.00-1.50% Cr 1.00-1.50% B 0.0005-0.0030%, in addition to this, Ni 0.25-1.00% Mo 0.10-0.20% Nb 0.03- Contains one or more of 0.20% of Si, Mn, and Cr, with the remainder consisting of Fe and normal impurities, and the ratio of Si, Mn, and Cr is
Si:Mn:Cr≒1:1:1 and the difference between the highest and lowest values is within 0.10%, and the hardness of the surface and center of rolled and forged parts with a diameter of 25 mm or more is approximately High-strength steel for large structures characterized by the same characteristics.