JP5167616B2 - Metal bolts with excellent delayed fracture resistance - Google Patents

Description

  The present invention mainly relates to high-strength steel for parts for construction, automobiles, and industrial machines, and can be suitably used as steel having excellent delayed fracture resistance, in which currently expensive alloy elements are used. Further, the present invention relates to a high-strength steel excellent in delayed fracture resistance, which has both strength and delayed fracture resistance.

  In recent years, the strength of steel materials has further increased in the automobile and construction fields, and the strength of all members has been increasing. As an example, in the bolt field, even higher strength steels of 1200 MPa class and 1500 MPa class are required even in a region where the tensile strength exceeds 1000 MPa. By the way, when the strength is increased in this way, the most feared is the delayed fracture.

  Delayed fracture is likely to occur in steel materials having a tensile strength of 1200 MPa or more. In particular, for bolts, the upper limit strengths are defined as F10T and F12T in JISB1186 and JISB1051, considering this point. For these steels, SCM or the like is mainly used.

  Further, maraging steel is first known as a material having high strength and excellent delayed fracture. However, since the Ni content is as high as 15 to 20% by mass and is overwhelmingly expensive as compared with the low alloy steel, it cannot be used as a material steel for general member production.

Therefore, steels have been developed for the purpose of having delayed fracture resistance higher than that of low alloy steels with a smaller amount of Ni than maraging steel (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3). .)
Japanese Patent Publication No. 60-14096 Japanese Patent Laid-Open No. 11-80903 JP 2000-144245 A

  However, the steels described in Patent Document 1, Patent Document 2, Patent Document 3 and the like also contain a few mass% of Ni even though it is small, and can be used in large quantities as ordinary high-strength steel. However, there is a drawback that the cost is high.

Accordingly, the object of the present invention is to solve such problems of the prior art and suppress an increase in manufacturing cost due to the addition of a large amount of expensive alloy elements such as Ni, Co, V, etc., and has high strength and delay resistance. The object is to provide a metal bolt excellent in fracture characteristics at low cost.

  As a result of intensive studies to solve the above problems, the inventors have determined that Mo, B, and Ti are within an appropriate range even in the case of a component system that does not contain a large amount of Ni or Co, such as maraging steel. Add and refine the prior austenite grain size after quenching, and then surpass the maraging steel by tempering in the temperature range of 100 ° C to 400 ° C, which is not usually used in the manufacture of bolts etc. The present invention has been completed by obtaining the knowledge that a steel having both excellent tensile strength and high delayed fracture resistance can be obtained.

The present invention has been made based on such findings, and the features thereof are as follows.
(1) By mass%, C: more than 0.30%, 0.50% or less, Si: 1.0% or less, Mn: 1.5% or less, Ti: 0.1% or less, Mo: 0.3 % Or more, 0.5% or less, B: 0.0005% or more, 0.01% or less, the bolt is manufactured using steel consisting of Fe and inevitable impurities , and after quenching the bolt , A metal bolt excellent in delayed fracture resistance, characterized by performing a tempering treatment at 100 ° C. to 400 ° C., and having a steel structure with a martensite fraction of 90% or more and a prior austenite grain size of 10 μm or less.
(2) Steel is further mass%, Al: 1.0% or less, Cr: 2.5% or less, Cu: 1.0% or less, Ni: 2.0% or less, V: 0.5% The metal bolt excellent in delayed fracture resistance according to (1), comprising one or more selected from the following.
(3) The steel further contains one or two kinds selected from the group consisting of W: 1.0% or less and Nb: 0.1% or less by mass% (1) or ( A metal bolt excellent in delayed fracture resistance as described in 2).
(4) The metal bolt excellent in delayed fracture resistance according to any one of (1) to (3), wherein quenching is performed using high-frequency heating.

  According to the present invention, steel having high strength and excellent delayed fracture resistance can be produced at low cost without adding a large amount of expensive alloy elements. Accordingly, it is possible to provide a metal bolt or the like having high strength and excellent delayed fracture resistance at low cost.

  The details of the present invention will be described below.

  The properties required for the high strength steel of the present invention are a tensile strength of 1500 MPa or more and a yield ratio of 0.9 or more. For example, as a high-strength bolt, in order to achieve F15T or higher, it is important that the tensile strength is 1500 MPa or higher and that the yield ratio is high. In order to prevent the tightened torque from being removed, the yield ratio (yield strength / tensile strength) needs to be 0.9 or more. Moreover, it is desirable to have delayed fracture resistance equivalent to or better than maraging steel and cold forgeability. Such a high-strength steel having a tensile strength of 1500 MPa or more and excellent delayed fracture resistance can be manufactured by the following manufacturing method using a steel having the following component composition and having the following structure. it can.

  First, the reason for limiting the steel composition in the present invention will be described. In the following description, the content% of component elements means mass%.

C: Over 0.30% and 0.50% or less.
C is an essential element for securing the necessary strength, and it is difficult to secure a predetermined strength at 0.30% or less. On the other hand, if it exceeds 0.50%, huge carbides are formed in the steel structure, and the delayed fracture property is lowered, so 0.5% was made the upper limit.

Si: 1.0% or less.
Since Si acts as a deoxidizer during the melting of steel, it can be contained. However, if it exceeds 1.0%, the cold forgeability of the steel is significantly reduced, so the upper limit was made 1.0%.

Mn: 1.5% or less.
Since Mn has an action as a deoxidizing agent when melting steel, it can be contained. However, if it exceeds 1.5%, the cold forgeability of the steel is significantly reduced, so the upper limit was made 1.5%.

Mo: 0.3% or more and 0.5% or less.
Mo is a particularly important element in the present invention. Mo improves the strength without greatly impairing the ductility. Addition of 0.3% or more is essential to achieve the effect. On the other hand, even if added over 0.5%, the strength is not further improved and the cost is increased. Moreover, since the cold forgeability tends to decrease when added in excess, the upper limit was made 0.5%.

B: 0.0005% or more and 0.01% or less.
B is the most important element that concentrates at the grain boundary portion and contributes to the improvement of the grain boundary strength. Delayed fracture occurs mainly at austenite grain boundaries, and strengthening the grain boundaries greatly contributes to the improvement of delayed fracture resistance. For that purpose, the content of 0.0005% or more is necessary. However, even if contained over 0.01%, the effect is saturated, so it was limited to the above range.

Ti: 0.1% or less.
Ti binds to N mixed as an inevitable impurity, thereby preventing B from forming BN and losing the effect of B. In order to obtain this effect, it is preferable to contain 0.005% or more, but even if added over 0.1%, a large amount of TiN is formed, leading to a decrease in strength and fatigue strength, so the upper limit is made 0.1% .

  The above is the basic component in the present invention. Next, the structure of the high-strength steel of the present invention will be described. In the present invention, the following conditions are required.

The steel structure is a martensite structure having a fraction of 90% or more.
Martensite is an essential structure for obtaining strength. In the case of the present invention, a martensite structure having a fraction of 90% or more exhibits excellent strength characteristics, so the lower limit of the martensite structure fraction of the steel structure is limited to 90%. If the martensite fraction is less than 90%, the amount of precipitates such as untransformed phases such as retained austenite phase and carbides that do not contribute to the increase in strength is excessive, and it is difficult to achieve high strength. Become.

The prior austenite grain size of the steel structure is 10 μm or less.
In the present invention, the adjustment of the prior austenite particle size is also important. By refining the prior austenite grain size, precipitation of film-like carbides that precipitate at the grain boundaries and lower the delayed fracture characteristics is suppressed, and the grain boundary strength is improved. For this purpose, the particle size needs to be 10 μm or less. More preferably, the particle diameter is 7 μm or less. If the particle size is 7 μm or less, there is an effect of further improving the delayed fracture resistance.

  In this invention, you may contain 1 type, or 2 or more types selected from Al, Cr, Cu, Ni, and V shown below.

Al: 1.0% or less.
Al is an element effective for deoxidation. In addition, it is an element effective in maintaining strength by suppressing austenite grain growth during quenching. However, even if the content exceeds 1.0%, the effect is saturated, not only causing disadvantages that increase costs, but also cold forgeability is reduced. Therefore, when adding Al, it is 1.0% or less.

Cr: 2.5% or less.
Cr is effective for improving the hardenability and is useful for securing the hardening depth. However, if contained excessively, the formation of residual carbides is promoted by the carbide stabilizing effect, resulting in a decrease in strength. Therefore, it is desirable to reduce the Cr content as much as possible, but up to 2.5% is acceptable. In addition, in order to express the effect | action which improves hardenability, it is preferable to make it contain 0.2% or more.

Cu: 1.0% or less.
Cu is effective in improving the hardenability, and improves the strength by solid solution in ferrite. However, if it exceeds 1.0%, cracking occurs during hot rolling. Therefore, when adding Cu, the content is made 1.0% or less. In order to develop the effect of improving hardenability and strength, it is preferable to contain 0.2% or more.

Ni: 2.0% or less.
Ni is effective in improving hardenability, and in order to suppress the formation of carbides, it contributes to the improvement of strength and delayed fracture characteristics by suppressing the formation of film-like carbides at the grain boundaries and increasing the grain boundary strength. To do. However, Ni is a very expensive element, and if it exceeds 2.0%, the cost of the steel material is significantly increased. Therefore, when adding Ni, the content is made 2.0% or less. In order to exhibit the effect of improving hardenability, strength, and delayed fracture characteristics, it is preferable to contain 0.5% or more.

V: 0.5% or less.
V binds to C in steel and is expected to act as a strengthening element. It also has the effect of improving resistance to temper softening and contributes to strength improvement. However, since the effect is saturated even if it contains exceeding 0.5%, when adding V, it is made 0.5% or less. In addition, in order to express the effect | action which improves an intensity | strength, it is preferable to make it contain 0.1% or more.

  Furthermore, in this invention, 1 type or 2 types selected from W and Nb shown below can be contained.

Nb: 0.1% or less.
Nb contributes to the improvement of strength and toughness as a precipitation strengthening element in addition to the effect of improving hardenability. In order to exhibit this effect, it is preferable to contain 0.005% or more. However, even if the content exceeds 0.1%, the effect is saturated, so when Nb is added, the content is made 0.1% or less.

W: 1.0% or less.
W forms a stable carbide and is effective as a strengthening element. On the other hand, when adding over 1.0%, the cold forgeability is lowered, so when adding W, the content is made 1.0% or less.

  The balance other than the elements described above is Fe and inevitable impurities. The main inevitable impurities include S, P, N, and O. These elements are acceptable if S: 0.05% or less, P: 0.05% or less, N: 0.01% or less, and O: 0.01% or less.

  Next, the manufacturing method of the high strength steel excellent in the delayed fracture resistance of the present invention will be described. The high-strength steel of the present invention is manufactured by quenching and tempering a material having a predetermined shape using steel having the above component composition.

  The steel containing the above-mentioned components can be either manufactured by melting in a converter or manufactured by vacuum melting. The ingot or continuous cast slab is heated and hot-rolled, pickled and scaled, and then cold-rolled to a predetermined thickness. Then, after being formed into a predetermined part shape by forging, rolling or the like, quenching and tempering is performed in order to obtain a martensite structure, for example, bolts are manufactured.

Quenching treatment: It is preferable to perform induction hardening.
In the quenching process, by using high-frequency heating, it is possible to quench immediately after reaching a necessary temperature range, and a fine crystal grain structure can be obtained while avoiding unnecessary coarsening of crystal grains. For this purpose, in induction hardening, a method of heating to a maximum temperature of 800 ° C. to 1100 ° C. at a temperature rising rate of 100 ° C./s or more and quenching immediately after reaching is effective.

Tempering temperature: 100 to 400 ° C.
This condition is the most important part in the present invention. A tempering temperature of 100 ° C. to 400 ° C. is a temperature range that is not generally used. However, in the case of the present invention, by setting to this temperature range, the contained B does not diffuse or cause unnecessary precipitation, but concentrates on the grain boundary and contributes to the strengthening of the grain boundary. . And by superimposing with the fine grain effect, a certain level of strength level and delayed fracture resistance are maintained. However, if the tempering temperature is less than 100 ° C., there is no tempering effect, and particularly the cold forgeability is lowered. Moreover, when it exceeds 400 degreeC, a big strength fall will arise. Therefore, it is limited to the above range. The tempering temperature is preferably 150 to 400 ° C.

  In this way, by properly combining the three conditions of the composition range that strengthens the grain boundary, the fine grain structure, and the tempering temperature, it is possible to produce steel that has both conflicting properties such as high strength and delayed fracture resistance. It becomes.

  The steel material thus obtained has strength and delayed fracture resistance comparable to maraging steel, although it can be manufactured at low cost, and is used for high-strength bolts for automobiles and buildings that require high strength. It can be used for a power transmission metal belt for a PC bar, a continuously variable transmission (CVT) which is a transmission of an automobile, and the like.

  When manufacturing a bolt, the steel having the above composition is used. For example, after forming into a bolt by cold forging and rolling, the bolt is quenched and tempered at 100 to 400 ° C. To produce a metal bolt having a steel structure with a martensite fraction of 90% or more and a prior austenite grain size of 10 μm or less.

  Steels of symbols 1 to 18 shown in Table 1 were manufactured by vacuum melting. These steels were heated to 1100 ° C. and hot forged to obtain round bars having a diameter of 60 mm (φ60 mm). Thereafter, a normal treatment was performed at 850 ° C. for 1 hour to obtain a raw material, which was subjected to the following heat treatment, and subjected to a tensile test, delayed fracture evaluation, structure observation, and cold forgeability evaluation.

  The shape of the tensile test piece (JIS No. 5) was cut out from the position 1 / 4d of the material round bar. The test piece is heated to 1050 ° C. at a heating rate of 400 ° C./s by high-frequency heating and then immediately quenched, and then immediately heated to 950 ° C. by high-frequency heating at a heating rate of 400 ° C./s. Step induction hardening was performed. Thereafter, tempering was performed at 180 ° C. for 30 minutes, and a tensile test was performed.

  Delayed fracture was evaluated according to the following procedure. A test piece as shown in FIG. 1 was cut out from the 1 / 4d position of the round bar material. The quenching and tempering conditions were the same as for the tensile specimen. Using this test piece, the delayed fracture property was evaluated by conducting a constant load type test. In the constant load type test, a test piece is immersed in a 5% NaCl solution adjusted to pH 1.5 using acetic acid, a certain load is applied to the test piece, and the time until the test piece breaks is measured. I did it. When the test time exceeded 200 hours and the specimen did not break, the test was interrupted and evaluated as not broken. By performing the test while changing the load, a curve indicating the relationship between the breakage time and the load can be obtained. Therefore, the lower limit stress was obtained from the load at which the fracture does not occur, and the delayed fracture was evaluated based on the magnitude of this value. Those having a lower limit stress of 550 Mpa or more, which is the lower limit stress of maraging steel, were evaluated as having good delayed fracture resistance.

  The fraction of the martensite structure was measured from the area ratio of the martensite in the visual field by observing a certain area with an optical microscope after corrosion with 3% nitrate alcohol.

  The prior austenite particle size is obtained by adding 11 g of sodium dodecylbenzenesulfonate: 1 g of ferrous chloride: 1 g and oxalic acid: 1.5 g to a picric acid aqueous solution in which 50 g of picric acid is dissolved in 500 g of water. After acting as a corrosive solution and revealing prior austenite grain boundaries by corrosion, the film was observed and photographed at a magnification of 1000 times, and determined from the obtained image by a cutting method.

  For the evaluation of the cold forgeability, a tablet test piece 1 having a diameter of 15 mm and a height of 22.5 mm as shown in FIG. 2A is cut out from the 1 / 4d position of the bar so as to coincide with the rolling direction. It was. In the forging test, 10 test pieces (n = 10) were compressed at various compression ratios, and judged by the presence or absence of cracks. The arrow shown in FIG. 2 is the compression direction. Crack 2 occurred as shown in FIG. The relationship between the crack occurrence rate and the compression rate at each compression rate was plotted on a graph, and the compression rate at which 50% (5 pieces) of the test piece was broken was used as the forgeability evaluation value. The larger this value, the better the forgeability, and the forgeability evaluation value of 80% or more was evaluated as having good cold forgeability equivalent to or better than maraging steel.

  The steel of symbol 1 shown in Table 1 is a comparative maraging steel (Fe-18% Ni-5% Mo-10% Co). After cutting out the test piece having the same shape as described above, after heating at 820 ° C., quenching was performed by air cooling, aging treatment was performed by heating at 520 ° C., and the samples were subjected to various evaluations.

  Table 1 also shows the measurement results of the martensite structure fraction, prior austenite grain size, tensile strength, yield ratio, delayed fracture (lower limit stress), and cold forgeability. From Table 1, it was found that the steel having the chemical composition and the structure within the range of the present invention has higher strength characteristics and delayed fracture resistance than the maraging steel.

  In this example, an experiment for examining the influence of the structure was performed on the steel having the component 4 shown in Table 1. All experimental methods are the same as in Example 1. However, in order to investigate the influence of the prior austenite grain size, the steel material of symbols 19 to 21 was manufactured by changing the high-frequency heating temperature, which is the second stage quenching temperature, at 850 to 1050 ° C. The measurement results are shown in Table 2.

  It has been found that when the austenite grain size is larger than 10 μm, the delayed fracture resistance is remarkably lowered.

  In this example, an experiment was conducted to examine the effects of other components other than the basic component. Steel having the composition shown in Table 3 (symbols 22 to 35) was manufactured by vacuum melting, and in the same manner as in Example 1, a tensile test and delayed fracture evaluation, structure observation, and cold forgeability evaluation were performed. It was. The results are also shown in Table 3.

  It has been found that when Cr and Al are contained excessively, the cold forgeability is lowered, and the effects of Ni, Cu, V, W, and Nb are saturated.

  In this example, an experiment for examining the influence of the tempering temperature was performed on the steel having the component 4 shown in Table 1. In the same manner as in Example 1, quenching was performed and the tempering temperature of 180 ° C. was changed at 70 to 450 ° C. to produce steel materials of symbols 36 to 40. Table 4 shows the measurement results.

  It was found that when the tempering temperature is in the range of 100 to 400 ° C., high strength, excellent delayed fracture resistance, and cold forgeability are compatible.

  In this example, delayed fracture resistance when actually producing bolts was evaluated. For the steels of symbol 1 (maraging steel), 4 and 11 shown in Table 1, a forged round bar is produced in the same manner as in Example 1, and a specimen having a predetermined size from the 1 / 4d position of the forged round bar. Was cut into a M22 bolt by cold forging and rolling, induction hardening and tempering at 180 ° C. Thirty bolts were prepared from each test material, tightened to the maximum load on the steel plate (SS400), and 3.5 mass% saline solution spraying and drying were repeated for 5 months. Thereafter, evaluation was performed with the number of broken bolts in 30 pieces.

  The results are shown in Table 5.

  The steel of symbol 4 which is the steel of the present invention shows characteristics superior to that of the maraging steel of symbol 1, but 80% of the bolts using the steel of symbol 11 which is a comparative steel broke.

Explanatory drawing of specimen for delayed fracture property evaluation test Explanatory drawing of the evaluation test of cold forgeability. (A) Specimen shape before starting test, (b) State where compression cracking occurred

Explanation of symbols

1 Test piece 2 Crack

Claims (4)

In mass%, C: more than 0.30%, 0.50% or less, Si: 1.0% or less, Mn: 1.5% or less, Ti: 0.1% or less, Mo: 0.3% or more, 0.5% or less, B: 0.0005% or more, 0.01% or less is contained, the bolt is manufactured using steel consisting of Fe and inevitable impurities , and after quenching the bolt, A metal bolt excellent in delayed fracture resistance, which is tempered at 400 ° C. and has a martensite fraction of 90% or more and a prior austenite grain size of 10 μm or less. Steel is further in mass%, Al: 1.0% or less, Cr: 2.5% or less, Cu: 1.0% or less, Ni: 2.0% or less, V: 0.5% or less The metal bolt excellent in delayed fracture resistance according to claim 1, comprising one or more selected from the group consisting of: The steel according to claim 1 or 2, further comprising one or two kinds selected from W: 1.0% or less and Nb: 0.1% or less in mass%. Metal bolts with excellent delayed fracture resistance as described. The metal bolt excellent in delayed fracture resistance according to any one of claims 1 to 3, wherein quenching is performed using high-frequency heating.

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