JP2008156678A - High-strength bolt excellent in delayed fracture resistance and corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively provide a high-strength bolt that suppresses an increase in a production cost, which is caused by adding a large amount of expensive alloying elements such as Mo, Co and V, has excellent delayed fracture resistance and corrosion resistance, is easily worked, and besides, has high strength. <P>SOLUTION: The high-strength bolt having excellent delayed fracture resistance and corrosion resistance is manufactured by preparing a steel comprising, by mass%, more than 0.15 to 0.30% C, 1.0% or less Si, 1.5% or less Mn, 0.1% or less Ti, 0.3 to 0.5% Mo, and 0.0005 to 0.01% B and the balance Fe with unavoidable impurities, quenching the steel, and then tempering the steel at 100°C to 400°C; thereby acquires a steel structure in which the average diameter of former austenite grains is 10 μm or smaller after having been quenched; and can show a strength in a range of about 1,200 to 1,800 MPa, high workability, high delayed fracture resistance, and corrosion resistance. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention mainly relates to high-strength bolts for parts related to construction, automobiles, and industrial machines, and can be suitably used particularly as bolts having excellent delayed fracture resistance in which currently expensive alloy elements are used. The present invention relates to a bolt having both strength, delayed fracture resistance and corrosion 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, it is overwhelmingly expensive compared to low alloy steel, and the strength level is very high such that it exceeds 2000 MPa. It is not used as a material steel for manufacturing a high-strength member of about 1600 MPa.

  Therefore, steels aiming at delayed fracture resistance more than low alloy steel with less Ni content than maraging steel have been developed as high strength steel of about 1200 to 1600 MPa with characteristics higher than low alloy steel (for example, patents) Reference 1 and Patent Reference 2).

  In addition, a technology for high-strength bolts in the above-described strength range has been announced. Known are those in which characteristics are obtained by dispersing a large number of fine precipitates in tempered martensite and those in which a large number of Ti carbonitrides are dispersed (see, for example, Patent Document 3 and Patent Document 4). ).

In addition, as a technology to improve delayed fracture resistance, the steel structure is austenite grain number by adjusting the heating conditions, rolling temperature conditions, and tempering conditions when hot-rolling to steel rods with adjusted chemical composition. A technique for obtaining a PC bar wire having a tempered martensite structure refined to 10th or more and excellent delayed fracture resistance is known (for example, see Patent Document 5).
JP 2000-8137 A JP 2000-144245 A JP 2003-321743 A Japanese Patent No. 3426495 JP-A-6-336648

  However, the steels described in Patent Document 1, Patent Document 2 and the like also contain a few mass% of Ni even though it is small, and it is still expensive to use in large quantities as ordinary high-strength steel. There is a drawback.

  Further, the steels described in Patent Document 3 and Patent Document 4 require a large number of precipitates, and a certain amount of alloy components are required to produce them. Although they are not as expensive as Ni described above, they are increased amounts of C, Ti, and N, and this causes a problem that the formability (cold forgeability) is impaired.

  Furthermore, since the steel described in Patent Document 5 is intended for the production of PC steel rods, there is a problem in cold forgeability when forming into bolts when applying to the production of high-strength bolts. And cannot be applied as is.

  Therefore, the object of the present invention is to solve such problems of the prior art, suppress an increase in manufacturing cost due to the addition of a large amount of expensive alloy elements such as Mo, Co, V, etc. The purpose is to provide a high-strength bolt with high strength and excellent delayed fracture resistance and corrosion resistance at low cost.

  As a result of diligent studies to solve the above problems, the inventors added C, Mo, B, and Ti within an appropriate range, even in the case of a component system that does not contain a large amount of Ni or Co, and quenching. By finely refining the prior austenite grain size later, and then tempering in a temperature range of 100 ° C. to 400 ° C., which is not usually used in the manufacture of bolts and the like, the strength range is high at about 1200 to 1800 MPa and high The present invention has been completed by obtaining the knowledge that processability, high delayed fracture resistance, and corrosion resistance can be expressed.

The present invention has been made based on such findings, and the features thereof are as follows.
(1) By mass%, C: more than 0.15%, 0.30% 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% As mentioned above, steel containing 0.01% or less, the balance being Fe and inevitable impurities, tempering after quenching at 100 ° C to 400 ° C, and the average prior austenite grain size after quenching is 10 µm or less A high-strength bolt with excellent delayed fracture resistance and corrosion resistance that is characterized by its structure.
(2) The steel is further selected by mass% from 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, or The high-strength bolt excellent in delayed fracture resistance and corrosion resistance as described in (1), comprising two or more types.
(3) According to (1) or (2), the steel further contains one or two kinds selected from W: 0.1% or less and Nb: 0.1% or less by mass%. High strength bolt with excellent delayed fracture resistance and corrosion resistance as described.
(4) The high strength bolt excellent in delayed fracture resistance and corrosion resistance according to any one of (1) to (3), wherein quenching is performed using high frequency heating.

  According to the present invention, a bolt having high strength and delayed fracture resistance, corrosion resistance, and cold forgeability can be manufactured at low cost without adding a large amount of expensive alloy elements.

  The details of the present invention will be described below.

  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 0.15% and 0.30% or less.
C is an essential element for securing the necessary strength, and it is difficult to secure a predetermined strength at 0.15% or less. On the other hand, if it exceeds 0.30%, the strength is increased too much, the delayed fracture property is lowered, and the cold forgeability is also lowered, so 0.3% 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. It is also an element necessary for maintaining corrosion resistance. 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 bolt of the present invention will be described.

The prior austenite grain size of the steel structure of the high-strength bolt is set to 10 μm or less.
In the present invention, it is important to adjust the prior austenite particle size. 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%, cracks occur during hot working such as 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.

W: 0.1% or less.
W forms a stable carbide and is effective as a strengthening element. On the other hand, if adding over 0.1%, the cold forgeability is lowered, so when adding W, the content is made 0.1% or less.

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.

  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 bolt of this invention is demonstrated. The high-strength bolt of the present invention is manufactured by quenching and tempering a material having a predetermined shape using steel having the above-described component composition.

  The steel containing the above-mentioned components can be either manufactured by melting in a converter or manufactured by vacuum melting. The steel ingot or continuous cast slab is heated and hot-rolled, pickled and scale-removed, and then cold-rolled or cold-forged to give a bolt shape. And in order to give predetermined intensity | strength, quenching and tempering are given and it is set as a martensitic structure.

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. That is, the tempering temperature of 100 ° C. to 400 ° C. is a temperature range that is not generally used in ordinary steel for bolts. However, in the present invention, by setting the temperature range, unnecessary carbides do not precipitate. If the tempering temperature is higher than this range, carbides are precipitated. When carbide precipitates, a local battery is generated between the carbide and the matrix in a low pH (approximately less than pH 2), and the weight loss due to corrosion of the steel itself increases. Therefore, the temperature range is set to prevent unnecessary precipitation of carbides. Further, the contained B does not diffuse or cause unnecessary precipitation, but concentrates at the grain boundary and contributes to strengthening of the grain boundary. And since the tempering temperature is not high, the strength level above a certain level and the delayed fracture resistance are maintained by superimposition with the fine grain effect. The tempering temperature is more preferably 100 ° C to 250 ° C.

  Thus, 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 achieve both conflicting properties such as high strength and delayed fracture resistance.

  The bolts thus obtained have high strength, excellent delayed fracture resistance, corrosion resistance, and forgeability even though they can be manufactured at low cost. High-strength bolts for automobiles and buildings that require high strength. Can be used for bolts.

  Steels of symbols 1 to 17 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 1/4 d position of the material round bar. This test piece is heated to 1050 ° C. at a heating rate of 400 ° C./s by induction heating and then immediately quenched, and then heated to 1050 ° C. by high frequency heating at the same 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. Those having a tensile strength of 1200 MPa or more were evaluated as high strength.

  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, the test piece is immersed in a 5 mass% 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. It was done. 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 / tensile strength of 0.8 or more, which is the ratio of the lower limit stress to the tensile strength, were evaluated as having good delayed fracture resistance.

  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 generation rate and the compression rate at each compression rate was plotted on a graph, and the cold forgeability was determined with the compression rate at which 50% (5 pieces) of the test piece was broken. The larger this value, the better the forgeability, and the cold forgeability of 70% or more was evaluated as having good cold forgeability.

  The evaluation of corrosivity was performed as follows. A round bar of φ10 × 60 mm was cut out from a position 1 / 4d of the φ60 mm forged material. After induction hardening and tempering, a tablet of φ10 × 30 mm was cut out from the center of the length of the round bar, and the entire surface was polished with a surface roughness of a triangle symbol to be ▽▽▽. After measuring the mass of this test piece, it was immersed in a 5 mass% NaCl aqueous solution adjusted to pH with acetic acid so that the pH was 1.5. After dipping for 200 hours, the sample was pulled up and rusted with an aqueous solution of ammonium hydrogen citrate II, and the mass was measured. The weight loss (g) before and after immersion was taken as the corrosive evaluation value. Those having a weight loss of 0.30 g or less were evaluated as having excellent corrosion resistance.

  Table 1 also shows the measurement results of prior austenite grain size, tensile strength, delayed fracture (lower limit stress), cold forgeability, and corrosivity. From Table 1, the steel whose chemical composition and structure are within the scope of the present invention has a strength of 1200 MPa or more, and with respect to delayed fracture, high delayed fracture resistance with a “lower limit stress / tensile strength” of 0.8 or more. It was found that both forgeability and corrosivity were excellent.

  In this example, an experiment for examining the influence of the structure was performed on the steel having the component of symbol 3 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 materials of symbols 18 and 19 were manufactured by changing the induction heating temperature, which is the second stage quenching temperature, from 1050 to 1100 and 1150 ° 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 “lower limit stress / tensile strength” indicating delayed fracture resistance is significantly reduced.

  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 20 to 33) was manufactured by vacuum melting, and in the same manner as in Example 1, tensile test and delayed fracture evaluation, structure observation, cold forgeability, corrosiveness Evaluation was performed. The results are also shown in Table 3.

  It has been found that when Cr, Al, and W are excessively contained, the cold forgeability is lowered, and the effects of Ni, V, and Nb are saturated. Further, in steel symbol 25 with Cu 1.3%, partial cracks were observed in the round bar after hot forging.

  In this example, an experiment for examining the influence of the tempering temperature was performed on the steel having the component of symbol 3 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 75 to 450 ° C. to produce steel materials of symbols 34 to 38. 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, cold forgeability, and corrosivity are obtained.

  In this example, delayed fracture resistance when actually producing bolts was evaluated. A forged round bar was produced in the same manner as in Example 1 for steels of symbol 3 (chemical component is within the scope of the present invention) and 10 (Mo is outside the scope of the present invention) shown in Table 1. A specimen having a predetermined size was cut from the / 4d position, formed into a M22 bolt by cold forging and rolling, and induction-hardened and tempered 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 bolt of the symbol 3 which is the bolt of the present invention shows good characteristics that hardly break, but the bolt using the steel of the symbol 10 broke 90%.

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 0.15%, 0.30% 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% A steel structure containing the following, with the balance being Fe and inevitable impurities, tempering after quenching at 100 ° C to 400 ° C, and having an average prior austenite grain size after quenching of 10 µm or less High strength bolt with excellent delayed fracture resistance and corrosion resistance.   In addition, one or more steels selected from the group consisting of Al: 1.0% or less, Cr: 2.5% or less, Cu: 1.0% or less, Ni: 2.0% or less, and V: 0.5% or less in terms of mass% The high-strength bolt excellent in delayed fracture resistance and corrosion resistance according to claim 1, which is contained.   3. The delay resistance according to claim 1, wherein the steel further contains one or two kinds selected from W: 0.1% or less and Nb: 0.1% or less in terms of mass%. High strength bolt with excellent fracture characteristics and corrosion resistance.   The high strength bolt excellent in delayed fracture resistance and corrosion resistance according to any one of claims 1 to 3, wherein quenching is performed using high frequency heating.

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