JPH0598387A - Steel for structural purpose excellent in strength and toughness - Google Patents

Abstract

PURPOSE:To provide steel for structural purposes combining excellent strength and toughness sufficiently satisfiable as connecting rods and shafts and sufficiently low in cost as well as enriched with practicability compared to Ni steel, Mo steel, Ni-Mo steel or the like. CONSTITUTION:The componental compsn. of steel for structural purposes is constituted of a one contg. 0.05 to 0.30% C, 0.10 to 1.00% Si, <=3.00% Mn, 0.010 to 0.100% Al, 0.010 to 0.100% Ti, 0.0005 to 0.0050% B and <=0.0200% N or furthermore contg. one or more kinds among <=5.0% Cr, <=1.0% Cu, <=3.50% Ni, <=3.0% Mo, 0.01 to 0.30% V, 0.010 to 0.100% Nb, <=0.50% Pb and <=0.0100% Ca and the blance Fe with inevitable impurities and in which the content of Al, Ti and N satisfies N(%) <=0.52Al(%)+0.29Ti(%).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to automobiles, construction machines,
The present invention relates to a structural steel having excellent strength and toughness, which is suitable as a material for connecting rods and shafts of machine tools and the like.

[0002]

2. Description of the Related Art Conventionally, for connecting rods and drive shafts of automobiles and construction machines, medium carbon carbon steels (JIS S30C to S50C, etc.) and low alloy steels (JIS) are used.
SMn438, SMnC443, SCr430, SCr440
Etc.) is applied as the material steel, and these are hardened and 400
It was general to carry out a tempering treatment at about 600 ° C to produce a product. This quenching / tempering treatment is carried out in order to impart strength and toughness to the steel, but the treatment conditions are also important for adjusting the balance between strength and toughness.

By the way, in recent years, severe demands for improvement of performance have been rushing to the above-mentioned machines and devices. Therefore, it is desired to apply materials having higher strength mainly to power transmission members of these machines and devices. It started to come. However, the above-mentioned structural steels were not tempered with conventional high temperature tempering (400
It is difficult to obtain the desired level of strength with a material that has been subjected to a temperature of up to 600 ° C. Therefore, when strength is particularly important as a structural material, tempering is not performed, or even if tempering is performed, quenching strain Temperature tempering (150-200 ° C) with almost no effect on strength to prevent
It has been forced to use it in this state by keeping it in a high strength state.

However, the strength and toughness of steel materials are generally inversely related to each other. When the strength is high, the toughness is low, and when the toughness is high, the strength is low. Therefore, it has been pointed out that the steel materials that have been strengthened by "no tempering" or "low temperature tempering" tend to have insufficient toughness and cause unexpected troubles due to cracks or breakage of the material.

However, it has been well known that it is effective to add alloy elements such as Ni and Mo to improve the toughness of steel materials, but these elements are expensive and the alloys are required to secure desired toughness. The addition of elements causes a significant increase in material cost and does not meet the other requirement along with the performance improvement of mechanical devices, that is, the requirement of "cost reduction". Therefore, except for special parts, Ni steel and Mo are excluded. The current situation is that the application of steel, Ni-Mo steel, etc. has been postponed.

From the above, the object of the present invention is to have excellent strength and toughness which are sufficiently satisfactory as connecting rods and drive shafts for machinery, and also Ni steel, Mo steel, Ni- It was to provide a structural steel that is sufficiently low in cost and highly practical compared to Mo steel and the like.

[0006]

Means for Solving the Problems The inventors of the present invention conducted extensive research to achieve the above-mentioned object, and found that "B, which has been conventionally known as a hardenability-improving element for steel, contributes to strength improvement through quenching. Not only that, it also has a considerable effect in improving the toughness of steel, and the steel added with an appropriate amount of B has a small difference in impact value due to the difference in notch shape. " This "the difference in the impact value due to the difference in the notch shape in the B-added steel is small" means that "B has the action of reducing the notch sensitivity of the steel, and when the B-added steel is used as a material, The surface flaws of the product have a small effect on the strength and toughness. "

Therefore, by utilizing B, it is possible to realize a structural steel having sufficient strength and toughness as a "material for connecting rods and drive shafts of automobiles, construction machines, etc., whose demands are so high". While searching for sex
As a result of further research, “B, T was added to steel with a C content of 0.05 to 0.30% (hereinafter,% representing the composition ratio is% by weight).
If the contents of B, Ti and Al and the N content in the steel are strictly controlled according to specific rules at the same time as i and Al are added, only "quenching only" or "quenching / low temperature tempering" can be performed. It is possible to obtain a structural steel that has high toughness, has excellent strength and toughness superior to conventional materials, and has significantly improved fatigue properties. ”

The present invention has been completed on the basis of the above findings and the like. "C: 0.05 to 0.30%, Si: 0.10 to 1.00%,
Mn: 3.00% or less, Al: 0.010 to 0.100%, Ti: 0.010
~ 0.100%, B: 0.0005 to 0.0050%, N: 0.0200% or less, or Cr: 5.0% or less, Cu: 1.0% or less, Ni: 3.
50% or less, Mo: 3.0% or less, V: 0.01 to 0.30
%, Nb: 0.010 to 0.10%, Pb: 0.50% or less,
Ca: Contains 0.0100% or less of one or more, and the balance Fe and unavoidable impurities, and the content of Al, Ti and N is N (%) ≤ 0.52 Al (%) + 0.29Ti (%) By adjusting the structural steel to a chemical composition that satisfies the above requirements, it has both high strength and excellent toughness. "

[0009]

As described above, in the steel according to the present invention, B, Ti
And to add Al and at the same time strictly control the contents of B, Ti, Al and N in the steel, high toughness is exhibited even in the state of "no tempering" or "low temperature tempering (150 to 200 ° C)". The main point of this is that, as in conventional steel, high temperature tempering (40
0-600 ℃) to reduce the strength, Ni or Ni
It is not necessary to secure toughness by adding expensive alloy elements such as Mo.

By the way, the reason why the content range of each chemical component is numerically limited as described above in the present invention is as follows. C C has an effect of imparting a predetermined static strength to steel, but if the content is less than 0.05%, the desired effect due to the above effect cannot be obtained, while if it exceeds 0.30%, it becomes The "effect of improving the toughness by adding B", which is a feature of the invention, is reduced. Therefore, the C content is set to 0.05 to 0.30%.

Si Si is an element necessary for deoxidizing steel, but at the same time has the function of imparting a predetermined static strength to the steel. However, if the content is less than 0.10%, the desired effect due to the above action cannot be obtained, while if Si is contained in excess of 1.00%, segregation at grain boundaries causes significant grain boundary embrittlement, The Si content was set to 0.10 to 1.00%.

[0012] Mn Mn, like Si, is an element necessary for deoxidation of steel, also it is effective in improving the hardenability. However, it was also clarified by the study by the present inventors that it is an element that is likely to segregate at the crystal grain boundaries and cause grain boundary embrittlement as in the case of Si. Then, when the Mn content exceeds 3.00% in particular, the grain boundary embrittlement becomes remarkable, so the Mn content was set to 3.00% or less.

Al Al, like Si and Mn, is an element necessary for deoxidizing steel, and also has an action of forming Al nitrides in the steel to refine the crystal grains. Further, it also has an important function of fixing N in the steel together with Ti to be added and exerting the effect of improving the strength and toughness by the addition of B, which is a feature of the present invention. However, if the Al content is less than 0.010%, the desired effect due to the above action cannot be obtained, while if the Al content is more than 0.100%, a huge Al oxide is generated and the fatigue strength of steel is reduced. Therefore, the Al content was set to 0.010 to 0.100%.

Ti Ti also has an action of forming nitride TiN to refine the crystal grains of the steel, and also fixes N in the steel together with Al, which is a feature of the present invention, “strength and toughness due to addition of B”. It has the effect of exhibiting the “improvement effect”. However, its content is 0.010%
If less than the desired effect due to the above action is not obtained, on the other hand,
If Ti is contained in excess of 0.100%, a huge Ti oxide will be formed and the fatigue strength of steel will be reduced.
It was set at 0.010 to 0.100%.

[0015] B B is known to be an effective element to increase the static strength of the hardenability is improved in the steel, the studies of the present inventors, and at the same time greatly improve the toughness It was found that it also has the effect of reducing notch sensitivity.
However, if the content is less than 0.0005%, the desired effect due to the above-mentioned action cannot be obtained, while if B is added in excess of 0.0050%, coarsening of crystal grains of the steel is caused and conversely the toughness is deteriorated. Therefore, the B content was set to 0.0005 to 0.0050%.

N N has an action of forming a nitride by combining with Ti, Al, etc., and refining the crystal grains of steel. However, the characteristic feature of the present invention, “strength and toughness due to addition of B” In order to secure the "improvement effect", it is necessary to fix N in steel with Ti and Al. However, N
If the content exceeds 0.0200%, it becomes difficult to fix N with Ti and Al, and the effect of adding B is reduced, so the upper limit of the N content was set to 0.0200%. N is an element that easily bonds with B, but it is necessary to stop B in the steel in a free state in order to fully exert the above-described effect of adding B. For that purpose, the contents of N, Al and Ti must be adjusted so as to satisfy the formula N (%) ≤ 0.52Al (%) + 0.29Ti (%), which does not satisfy this condition. Therefore, it becomes difficult to realize steel having desired properties.

Cr, Cu, Ni, Mo, V and Nb All of these elements have the effect of further improving the strength and toughness of steel, and therefore one or more kinds of them are added if necessary. The reasons for limiting the content of each are explained together with the main action.

A) Cr is an element that is particularly effective for improving the hardenability of steel and improving the static strength, but if its content exceeds 5.0%, Cr carbides are formed during tempering to form crystals. Segregation occurs at the grain boundaries, which causes embrittlement at the grain boundaries, resulting in a decrease in toughness. Therefore, the upper limit of Cr content is set to 5.0%.

B) Cu is an element which is particularly effective for improving the static strength of steel, but if it is contained in an amount exceeding 1.0%, hot workability is deteriorated. Therefore, the upper limit of Cu content was set at 1.0%.

C) Ni is an element which is particularly effective for improving the hardenability of steel and improving the static strength and toughness,
Even if added over 3.50%, the effect is saturated and the economic efficiency is impaired, so the upper limit of Ni content was set at 3.50%.

D) Mo, like Ni, is an element effective for improving the hardenability of steel and improving the static strength and toughness, but even if added in excess of 3.0%, its effect is Since it saturates and impairs economic efficiency, the upper limit of Mo content is set.
It was set at 3.0%.

E) V has an effect of particularly improving the high temperature strength of steel, but if its content is less than 0.01%, the desired effect due to the above effect cannot be obtained, while on the other hand, it exceeds 0.30%. Content of V causes deterioration of hot workability, so the V content was set to 0.01 to 0.30%.

F) Nb exerts an effect of improving the toughness of steel by forming nitrides and refining crystal grains, but if the content is less than 0.010%, the desired effect due to the above effect is exerted. On the other hand, when the content exceeds 0.100%, not only the effect is saturated but also the hot workability is deteriorated, so the Nb content is 0.010 to 0.100.
Defined as%.

Pb and Ca Pb and Ca both have the effect of improving the machinability of steel, so one or two kinds are added if necessary, but in the case of Pb, 0.5% If it is contained in excess of 0.05%, the fatigue properties will be significantly reduced, and in the case of Ca, if it is contained in excess of 0.0100%, the toughness will be significantly reduced, so the upper limit of Pb content is 0.5%, and the Ca content of The upper limit was set to 0.0100%.

Next, the effects of the present invention will be described more specifically by way of examples.

EXAMPLES First, steel (150 kg) having the chemical composition shown in Tables 1 and 2 was melted in a vacuum melting furnace, heated at 1250 ° C. for 1 hour, and then formed into a round bar having a diameter of 20 mm. Forging, further quenching (930 ° C. × 1 hr → water cooling) and tempering (200 ° C. × 1 hr → air cooling) were performed to obtain test materials.

[0026]

[Table 1]

[0027]

[Table 2]

Next, a JIS No. 4 tensile test piece, a JIS No. 3 U-notch Charpy impact test piece and a rotary bending fatigue test piece were sampled from each of the test materials, and subjected to a tensile test, an impact test (normal temperature) and a rotary bending fatigue test. Was carried out. The rotary bending fatigue test piece had the shape and dimensions shown in FIG. The results of these tests are shown in Table 3.

[0029]

[Table 3]

FIG. 2 shows the "relationship between tensile strength and Charpy absorbed energy" prepared based on the above test results.
Is a graph relating to. As is clear from Table 3 and FIG. 2, the steel of the present invention has an impact value (Charpy absorption energy) twice or more that of the comparative steel when compared at the same tensile strength, and It can be seen that although the tensile strength is 1.5 to 1.8 times as high as that of the conventional steel (400 to 600 ° C. tempered material), it exhibits toughness equivalent to this.

FIG. 3 is a graph relating to the "relationship between tensile strength and fatigue strength" prepared based on the above test results. As is clear from Table 3 and FIG.
It can be seen that the fatigue strength of the steels of the present invention shows an excellent value of 1.2 to 1.6 times that of the comparative steels, even when the steels having the same tensile strength are compared.

[0032]

[Summary of Effects] As described above, according to the present invention, it becomes possible to provide structural steel having high strength and high toughness and excellent fatigue properties at a low cost, and automobiles and construction machinery. , It will be possible to achieve further improvement in performance by applying it to connecting rods, drive shafts, etc. of machine tools, which will bring extremely useful effects in industry.

[Brief description of drawings]

FIG. 1 Shape and shape of a rotating bending fatigue test piece prepared in Examples
It is explanatory drawing regarding a dimension.

FIG. 2 is a graph relating to “relationship between tensile strength and Charpy absorbed energy” created based on the test results in the examples.

FIG. 3 is a graph relating to the “relationship between tensile strength and fatigue strength” created based on the test results in the examples.

Claims (4)

[Claims] 1. By weight ratio, C: 0.05 to 0.30%, Si: 0.10 to 1.00%, M
n: 3.00% or less, Al: 0.010 to 0.100%, Ti: 0.010
~ 0.100%, B: 0.0005 to 0.0050%, N: 0.0200% or less, the balance consisting of Fe and unavoidable impurities, and the content of Al, Ti and N is expressed by the formula N (%) ≤ 0.52Al (%) Structural steel with excellent strength and toughness, characterized by satisfying + 0.29Ti (%). 2. A weight ratio of C: 0.05 to 0.30%, Si: 0.10 to 1.00%, M
n: 3.00% or less, Al: 0.010 to 0.100%, Ti: 0.010
~ 0.100%, B: 0.0005 to 0.0050%, N: 0.0200% or less, Cr: 5.0% or less, Cu: 1.0% or less, Ni: 3.
50% or less, Mo: 3.0% or less, V: 0.01 to 0.30
%, Nb: 0.010 to 0.10%, and the balance is Fe and unavoidable impurities, and the content of Al, Ti and N is N (%) ≤ 0.52Al (%) + 0.29Ti. Structural steel with excellent strength and toughness, characterized by satisfying (%). 3. A weight ratio of C: 0.05 to 0.30%, Si: 0.10 to 1.00%, M
n: 3.00% or less, Al: 0.010 to 0.100%, Ti: 0.010
~ 0.100%, B: 0.0005 to 0.0050%, N: 0.0200% or less, Pb: 0.50% or less, Ca: 0.0100% or less, and the balance Fe and inevitable impurities. A structural steel having excellent strength and toughness, characterized in that the contents of Al, Ti and N satisfy the formula N (%) ≤ 0.52Al (%) + 0.29Ti (%). 4. A weight ratio of C: 0.05 to 0.30%, Si: 0.10 to 1.00%, M
n: 3.00% or less, Al: 0.010 to 0.100%, Ti: 0.010
~ 0.100%, B: 0.0005 to 0.0050%, N: 0.0200% or less, Cr: 5.0% or less, Cu: 1.0% or less, Ni: 3.
50% or less, Mo: 3.0% or less, V: 0.01 to 0.30
%, Nb: 0.010 to 0.10%, one or more, and Pb: 0.50% or less, Ca: 0.0100% or less, and the balance consisting of Fe and unavoidable impurities, and Al, Ti, and N. Structural steel with excellent strength and toughness, characterized in that the content satisfies the formula N (%) ≤ 0.52Al (%) + 0.29Ti (%).