JP5814900B2 - High strength bolt steel and high strength bolt with excellent delayed fracture resistance in corrosive environments - Google Patents

Description

  The present invention relates to a steel for bolts used in various industrial machines such as automobiles and transportation machines, and a high-strength bolt obtained by using this steel for bolts. Especially, even if the tensile strength is 1270 MPa or more, it can be used in the atmosphere and wind and rain. The present invention relates to a steel for high-strength bolts and high-strength bolts that exhibit excellent delayed fracture resistance in an environment where they are exposed or in an environment involving corrosion due to salt water or the like.

  As general bolt steels, JIS standard steels such as SCM435 and SCM440 are widely used. However, in these general-purpose steels, when the tensile strength is 1100 MPa or more (particularly 1270 MPa or more), there is a problem that so-called delayed fracture that suddenly causes brittle fracture after use for a certain period is likely to occur. Accordingly, steels for high-strength bolts with improved temper softening resistance have been proposed for the purpose of improving the properties against delayed fracture (delayed fracture resistance).

  For example, in Patent Document 1, the content of Si, Mn, P, S, Al, Cr, Mo, Nb, N, and Sn in a medium C steel of C: more than 0.30% to 0.55% A steel for high-strength bolts with improved delayed fracture resistance has been proposed by appropriately defining the above. This technique also shows that it is effective to contain Mg and Ca at 0.01% or less in order to improve hot workability.

  Patent Document 2 discloses delayed fracture resistance by specifying the content of Mg and Ca in the relationship between Cr and Mn in C: 0.3 to 0.5% medium C steel. A high-strength steel with improved resistance has been proposed.

  However, in these techniques, particularly when the tensile strength is 1270 MPa or more, the environment is exposed to the atmosphere or wind or rain, or is accompanied by corrosion by salt water or the like (hereinafter, sometimes represented by “corrosive environment”). The delayed fracture resistance at is insufficient.

  On the other hand, in Patent Document 3, C: 0.08 to 0.35%, Si: Less than 1.0% in a low C, low Si system, containing Mn, P, S, Cr, Ca, Ti and B Steels for high-strength bolts with improved delayed fracture resistance have been proposed by appropriately defining the amount. However, even in this technique, the tensile strength is 1270 MPa or more, and it cannot be said that the delayed fracture resistance in a corrosive environment is sufficiently exhibited.

JP 2009-293095 A Japanese Patent No. 4476834 JP-A-8-291360

  The present invention has been made by paying attention to the above-described circumstances, and its purpose is that the tensile strength is 1270 MPa or more and the environment is exposed to the atmosphere or wind or rain, or the environment is accompanied by corrosion by salt water or the like. The object is to provide a high-strength bolt steel that can exhibit sufficient delayed fracture resistance, and a high-strength bolt obtained from such a bolt steel.

  The steel for high-strength bolts according to the present invention is C: 0.15-0.21% (meaning mass%, the same applies to the chemical composition), Si: 1.5-2.0%, Mn: 2 0.0% or less (not including 0%), Cr: 2.5 to 5.0%, respectively, and Ca: 0.05% or less (not including 0%) and / or Mg: 0.05 % Or less (excluding 0%), the total content of Ca and Mg is 0.0008% or more, the balance is made of iron and inevitable impurities, and the microstructure is a mixed structure of ferrite and pearlite. It has a gist at a certain point.

  If necessary, the steel for high-strength bolts of the present invention further includes Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), and V: 0.1. It is also useful to contain one or more selected from the group consisting of% or less (not including 0%), and this will further improve delayed fracture resistance.

  In the bolt obtained from the steel for high strength bolts as described above, the tensile strength is 1270 to 1600 MPa, and the microstructure is martensite: 95 area% or more, tempered carbide: 1.0 area% or less (0 area% Not included).

  Moreover, in producing the high-strength bolts as described above, after forming into a bolt shape, the tempering treatment after quenching is performed in a temperature range of 150 to 250 ° C. for 60 seconds or more and 3000 seconds or less. Good.

  In the present invention, a steel for high-strength bolts excellent in delayed fracture resistance can be realized by strictly defining the chemical component composition while making the contents of Ca and Mg appropriate. By heat-treating such high-strength bolt steel under predetermined tempering conditions, sufficient delayed fracture resistance can be exhibited even in a corrosive environment while maintaining the strength as a steel material. Steel for strength bolts is extremely useful as a material for high strength bolts.

FIG. 1 is a schematic view showing the shape of a tensile test specimen. FIG. 2 is a cross-sectional view schematically showing a state when the corrosion pit depth is measured. FIG. 3 is a schematic diagram showing the shape of a delayed fracture test specimen.

  The present inventors have studied from various angles in order to improve delayed fracture resistance of bolt steel. It is known that delayed fracture tends to occur mainly as the strength of the steel material increases, and that the delayed fracture resistance is significantly deteriorated particularly when the tensile strength is 1270 MPa or more. In addition, it is further difficult to exhibit excellent delayed fracture resistance in a corrosive environment.

  The inventors of the present invention have made extensive studies on bolt steel and bolts that can exhibit sufficient delayed fracture resistance even in a corrosive environment while having high strength of 1270 MPa or more. As a result, while suppressing the C content to 0.15 to 0.21% and containing Ca and Mg in appropriate amounts, delayed fracture resistance in a corrosive environment while maintaining the strength of the steel material. Has been improved significantly. In particular, in the bolt after quenching and tempering, the tempered carbide is reduced, and it has been found that if Ca and Mg are adjusted to an appropriate amount, the effect of suppressing corrosion pits is exhibited by these synergistic effects.

  In the steel for bolts of the present invention, as described above, the C content is set to 0.15 to 0.21% and a predetermined amount of Ca and / or Mg is contained, thereby providing delayed fracture resistance in a corrosive environment. Although it is characterized by improving, it is necessary to satisfy the chemical component composition as follows in order to secure other characteristics necessary for the bolt.

[C: 0.15-0.21%]
C is an element necessary for securing the strength of the steel material, but as its content increases, the delayed fracture resistance of the steel material decreases. When the C content is less than 0.15%, it becomes difficult to stabilize the strength of the bolt after tempering. On the other hand, if the C content exceeds 0.21%, the delayed fracture resistance deteriorates due to the deterioration of ductility. The preferable lower limit of the C content is 0.16% or more (more preferably 0.17% or more), and the preferable upper limit is 0.20% or less (more preferably 0.19% or less).

[Si: 1.5-2.0%]
Si is an effective element for securing strength and suppressing tempered carbides, but when its content is excessive, bolt formability (for example, cold forgeability) decreases. From such a viewpoint, the Si content needs to be 1.5% or more and 2.0% or less. A preferred lower limit is 1.6% or more (more preferably 1.7% or more). Moreover, a preferable upper limit is 1.9% or less (more preferably 1.8% or less).

[Mn: 2.0% or less (excluding 0%)]
Mn is a hardenability improving element and is an element useful for achieving high strength. Such an effect increases as the content increases, but when the Mn content becomes excessive, the toughness of the steel material decreases. From such a viewpoint, the Mn content is suppressed to 2.0% or less. In addition, the minimum with preferable Mn content is 0.1% or more (more preferably 0.2% or more), and a preferable upper limit is 1.8% or less (more preferably 1.6% or less).

[Cr: 2.5-5.0%]
Cr has the effect of enhancing the corrosion resistance of steel and is a useful component for suppressing the formation of corrosion pits in a corrosive environment. In order to sufficiently exhibit such effects, it is necessary to contain 2.5% or more of Cr. On the other hand, if the Cr content is excessive, the cold forgeability of the steel material deteriorates, so it is necessary to make it 5.0% or less. In addition, the minimum with preferable Cr content is 2.8% or more (more preferably 3.0% or more), and a preferable upper limit is 4.5% or less (more preferably 4.0% or less).

[Ca: 0.05% or less (not including 0%) and / or Mg: 0.05% or less (not including 0%), provided that the total content of Ca and Mg: 0.0008% or more]
Ca and Mg are effective elements for suppressing the formation of corrosion pits. By containing one or two of these elements, delayed fracture resistance in a corrosive environment can be improved. . In order to exert such effects, the total content of Ca and Mg needs to be 0.0008% or more. On the other hand, if these contents are excessive, inclusions increase, which becomes the starting point of destruction and deteriorates delayed fracture resistance, so both of them must be 0.05% or less. The preferable lower limit of the total content of Ca and Mg is 0.001% or more (more preferably 0.01% or more). The upper limit with preferable Ca and Mg is 0.04% or less (more preferably 0.03% or less).

  The basic components in the steel for high-strength bolts according to the present invention are as described above, and the balance is iron and unavoidable impurities (P, S, Al, N, etc.). As the unavoidable impurities, raw materials, materials, The mixing of elements brought in depending on the situation of the manufacturing equipment or the like can be allowed. Moreover, in the high carbon steel wire rod of the present invention, if necessary, Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), and V: 0.0. It is also useful to contain one or more selected from the group consisting of 1% or less (not including 0%).

[Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), and V: 0.1% or less (not including 0%) One or more
Ti, Nb and V are useful elements for further improving delayed fracture resistance. In order to exert such an effect, it is preferable to contain 0.001% or more of any one or more of them. However, if these contents are excessive, the cold workability deteriorates, so it is preferable that both be 0.1% or less. In addition, the more preferable lower limit of these elements is 0.005% or more (more preferably 0.01% or more), and the preferable upper limit is 0.08% or less (more preferably 0.05% or less).

  The steel for bolts of the present invention is basically composed of a mixed structure of ferrite and pearlite (hereinafter sometimes referred to as “ferrite / pearlite”) in order to ensure workability during bolt forming. However, it is permissible to include other structures (for example, bainite) to such an extent that the workability is not hindered (5 area% or less).

  In order to obtain a microstructure as described above in bolt steel, it is recommended to cool the steel for some time after hot rolling. Specifically, after winding in a temperature range of 750 to 900 ° C., cool to a temperature range of 500 to 600 ° C. within 60 seconds, hold in the temperature range for 60 seconds or more, and then cool to less than 500 ° C. The method of doing is illustrated. If necessary, spheroidizing annealing may be performed.

  The steel for bolts having the chemical composition of the present invention has potentially excellent strength characteristics and delayed fracture resistance, but with this steel material, high strength characteristics and delayed fracture resistance are sufficiently excellent. In order to obtain a strength bolt, it is recommended to perform tempering after quenching under appropriate conditions after processing into a predetermined bolt shape.

  From such a viewpoint, it is recommended that the tempering treatment after quenching is performed in a relatively low temperature range of 150 to 250 ° C. for 60 seconds or more and 3000 seconds or less. When the tempering temperature is lower than 150 ° C., strain and residual stress introduced during quenching remain, and the delayed fracture resistance deteriorates. The tempering temperature is preferably 170 ° C. or higher (more preferably 180 ° C. or higher). On the other hand, when the tempering temperature exceeds 250 ° C., a predetermined strength (1270 to 1600 MPa) cannot be obtained. The tempering temperature is preferably 230 ° C. or lower (more preferably 210 ° C. or lower). The above temperatures are all controlled by the surface temperature of the steel material.

  If the tempering time is short, the uniformity of the material structure becomes insufficient. From such a viewpoint, the tempering time is preferably at least 60 seconds. The tempering time is preferably 120 seconds or more, more preferably 180 seconds or more. However, if the tempering time is too long, the cost increases, so it is preferable to set it to 3000 seconds or less (preferably 1000 seconds or less, more preferably 500 seconds or less).

  In addition, what is necessary is just to follow the conditions in the case of quenching, for example, after heating to about 880-920 degreeC, and performing oil cooling or water cooling.

  The bolt obtained as described above has a tensile strength of 1270 to 1600 MPa and a microstructure of martensite: 95 area% or more. A predetermined strength can be secured by making the organization a mantensite. If the tempered carbide is reduced to 1.0 area% or less, corrosion pits can be suppressed, and the delayed fracture resistance can be improved in cooperation with the corrosion pit suppression effect by Ca or Mg. The area ratio of martensite is preferably 96 area% or more, more preferably 97 area% or more. The area ratio of tempered carbide is preferably 0.8 area% or less, more preferably 0.6 area% or less. The balance of the microstructure does not affect the steel properties, but is usually at least one of bainite and retained austenite.

  When tempering is performed at an extremely low temperature, tempered carbide may not be precipitated at all. In such a low temperature tempered steel material (steel material having no tempered carbide), the toughness is reduced instead. Therefore, the area ratio of tempered carbide is, for example, more than 0 area%, preferably 0.05 area% or more (more preferably 0.1 area% or more).

Tempered carbide is produced during tempering (mainly Fe ₃ C), and can be distinguished from other carbides (for example, carbide existing before tempering) by a scanning electron microscope. That is, the tempered carbide is formed into a needle shape at a low temperature (for example, 150 ° C. to 250 ° C.), and the amount thereof increases as the temperature is tempered at a high temperature. When the temperature is further increased (for example, a temperature exceeding 300 ° C.), a film is formed at the crystal grain boundary, and the delayed fracture resistance is lowered. On the other hand, other carbides have different shapes and distribution forms, for example, TiC has a cubic shape and is randomly present in the steel.

  In the present invention, the production conditions other than those described above are not limited. For example, after hot rolling, if necessary, the steel material satisfying the chemical composition described above is subjected to spheroidizing annealing and then drawn. It can be made into a bolt shape by performing cold working such as cold forging.

  More specifically, high-strength bolts having a tensile strength of 1270 to 1600 MPa are usually formed after spheroidizing annealing, but in the present invention, the C content is 0.21% or less. Thus, it becomes possible to form a bolt shape without performing spheroidizing annealing. After forming into a bolt shape, prior to the quenching / tempering treatment as described above, threading is performed by rolling.

  The high-strength bolt of the present invention exhibits excellent corrosion resistance. It also exhibits excellent delayed fracture resistance in a corrosive environment. A preferable range of the corrosion resistance (μm) measured under the conditions of Examples described later is, for example, 40 μm or less, more preferably 30 μm or less (for example, about 20 to 30 μm). Moreover, the preferable range of delayed fracture resistance (breaking stress ratio) measured under the conditions of Examples described later is, for example, 0.60 or more, more preferably 0.7 or more (for example, about 0.7 to 0.9). is there.

  The high-strength bolt of the present invention can be applied to high-tension bolts, torcia-type bolts, hot-dip galvanized high-strength bolts, rust-proof high-strength bolts, refractory steel high-strength bolts, etc. It is optimal as a bolt with high strength and excellent delayed fracture resistance in corrosive environments.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel materials (steel types A to W, A1 to N1) having chemical composition shown in Tables 1 and 2 below were melted and hot-rolled to form a wire with a diameter of 12 mm. After hot rolling, after winding at 700 ° C., it was cooled to 600 ° C. within 30 seconds, kept in that temperature region for 60 seconds or more, and then cooled to room temperature (25 ° C.). The material structure (microstructure as steel for bolts) after hot rolling and cooling was evaluated by observing the wire cross section. Specifically, the surface which appeared by etching the crossing sample from the surface to the depth of 2 mm in the axial direction with 5% nital was observed with a scanning electron microscope (SEM). The results are also shown in Tables 1 and 2 below. In Tables 1 and 2, the “-” column means that no addition was made.

  The wire after hot rolling and cooling is heated to a temperature of 870 ° C. and then oil quenching is performed. After tempering within a range in which a tensile strength of 1270 MPa can be secured, the tensile strength is measured and the structure is observed. The steel material characteristics were confirmed and subjected to a corrosion resistance test and a delayed fracture test. Tables 3 and 4 below show the tempering conditions, tensile strength, and structure of each steel material. As a comparative example, a wire rod having a tensile strength lower than 1270 MPa was also prepared (Test Nos. 24 and 25).

  At this time, the tensile test was performed by preparing a test piece for a tensile test shown in FIG. In the structure evaluation, the surface appearing by etching the cross section of the sample in the axial direction to a depth of 0.1 mm with 5% nital was observed by SEM, and the martensite fraction was measured. A sample etched separately with sodium picrate was prepared, and the area ratio of tempered carbide was measured by SEM observation. Cubic TiC (mixed with TiN) was confirmed as a carbide other than tempered carbide, but TiC was excluded from the tempered carbide area ratio measurement. These results are also shown in Tables 3 and 4 below.

  Each wire rod quenched and tempered (hereinafter sometimes referred to as “quenched / tempered”) was evaluated for corrosion resistance and delayed fracture resistance by the following methods.

[Evaluation of corrosion resistance]
In the corrosion resistance test, a test piece cut into a cylindrical shape (diameter: 4 mm × length: 100 mm) was used. A salt dry and wet combined cycle tester (“CYP-90” (trade name) manufactured by Suga Test Instruments Co., Ltd.) was used, and the test piece was placed vertically on the tester. And (1) salt water spraying step (35 ° C. 5% NaCl aqueous solution spraying × 2 hours) → (2) drying step (60 ° C., relative humidity: 20-30% for 4 hours) → (3) wetting step (50 A corrosion test (CCT test) was performed by repeating this for 30 cycles at 1 ° C. and relative humidity: 95% or more for 2 hours.

  After the corrosion test, the cross section at a position 20 mm from the upper end of the test piece was observed with an optical microscope, and the depth of the corrosion pit was measured. The state when measuring the depth of the corrosion pit is shown in FIG. 2 (schematic cross-sectional view). For the pit depth, in the observation field of view of the sample (the length in the circumferential direction is about 1 mm), the straight line A connecting the convex portion and the convex portion and the distance L to the deepest portion of the concave portion are obtained over the entire circumference, and the maximum value ( The maximum pit depth of the steel type was defined as the maximum value in the entire circumference. The test was accepted when the maximum pit depth in the entire circumference of the test piece was less than 50 μm.

[Evaluation of delayed fracture resistance]
A tensile test piece with a notch shown in FIG. 3 (the unit in the figure is mm) was prepared by cutting and subjected to a delayed fracture test. It is known that the delayed fracture test result with this test piece has a good correlation with the delayed fracture test result with actual bolts. In the delayed fracture test, the delayed fracture test piece was placed in a vertical position on the test machine using a salt-wet combined cycle tester (“CYP-90” (trade name) manufactured by Suga Test Instruments Co., Ltd.). . And (1) salt water spraying step (35 ° C. 5% NaCl aqueous solution spraying × 2 hours) → (2) drying step (60 ° C., relative humidity: 20-30% for 4 hours) → (3) wetting step (50 A CCT test was conducted in which one cycle was set at 95 ° C. and relative humidity: 95% or more for 2 hours, and this was repeated for 30 cycles.

The test piece was cooled to liquid nitrogen temperature after the CCT test, and a low strain rate tensile test (SSRT test) was performed. In the SSRT test, a uniaxial tensile test was performed at a strain rate of 10 ⁻⁶ / sec using the thread portion of the test piece as a chuck portion, and the breaking stress was measured. And when the breaking stress ratio X shown by the following (1) formula was larger than 0.5 (X> 0.5), it evaluated that it was excellent in delayed fracture resistance.
X = σH / TS (1)
However, TS: Tensile strength of material before CCT / SSRT test, σH: Breaking stress after SSRT test

  These results are shown in Tables 5 and 6 below. In Tables 5 and 6 below, regarding the material property evaluation, when both the corrosion resistance and delayed fracture resistance are good, it is acceptable (indicated by “◯”), at least one of corrosion resistance and delayed fracture resistance Was bad (indicated by “x”).

  From these results, we can consider as follows. Test No. Nos. 1 to 23 satisfy the requirements specified in the present invention, exhibit high strength of 1270 MPa or more, and are excellent in delayed fracture resistance in a corrosive environment. In contrast, test no. 24 to 37 are examples that do not satisfy any of the requirements defined in the present invention, and any of the characteristics is deteriorated.

  Test No. No. 24 cannot secure a tensile strength of 1270 MPa or more because 25 has a low C content (other properties are not evaluated). Test No. Since No. 26 has too much C content, the strength becomes too high and the delayed fracture resistance is deteriorated.

  In Test No. 27, since the C content was too large, the tempering temperature was set in order to obtain an appropriate strength, but the tempered carbide was precipitated excessively, so that the delayed fracture resistance was deteriorated. Test No. 28 has a low Si content, so the effect of suppressing the precipitation of carbides during quenching is insufficient, and even if tempering is performed at an appropriate temperature, tempered carbides precipitate excessively, so delayed fracture resistance The quality has deteriorated.

  Test No. In No. 29, since the Mn content was excessive, the delayed fracture resistance deteriorated due to the decrease in the toughness of the steel material. Test No. Since No. 30 has a small Cr content, the corrosion resistance is deteriorated and the corrosion pits cannot be suppressed.

  Test No. 31 to 33 are states in which corrosion pits cannot be suppressed because the total content of Ca and Mg is insufficient. Test No. In No. 34, since the Ca content was excessive, the Ca oxide became an excessive inclusion, and the delayed fracture resistance deteriorated. Test No. In No. 35, since the Mg content was excessive, the Mg oxide became an excessive inclusion, and the delayed fracture resistance was deteriorated.

  Test No. No. 36 has an appropriate chemical composition of the steel material, but the tempering temperature is too low, so that the strength is too high and the delayed fracture resistance is deteriorated. Test No. In No. 37, the chemical composition of the steel material was appropriate, but the tempering temperature was too high, so that tempered carbides excessively precipitated and delayed fracture resistance deteriorated.

Claims (4)

C: 0.15-0.21% (meaning of mass%, the same applies to the chemical composition)
Si: 1.5-2.0%,
Mn: 2.0% or less (excluding 0%),
Cr: 2.5 to 5.0% respectively,
Ca: 0.05% or less (not including 0%) and / or Mg: 0.05% or less (not including 0%), and the total content of Ca and Mg being 0.0008% or more A high-strength bolt steel with excellent delayed fracture resistance in a corrosive environment, characterized in that the balance is composed of iron and inevitable impurities, and the microstructure is a mixed structure of ferrite and pearlite.   Further, Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (not including 0%), and V: 0.1% or less (not including 0%) The steel for high-strength bolts according to claim 1, comprising one or more selected.   A bolt obtained from the steel for high-strength bolts according to claim 1 or 2, wherein the tensile strength is 1270 to 1600 MPa, and the microstructure is martensite: 95 area% or more, tempered carbide: 1.0 area% A high-strength bolt excellent in delayed fracture resistance in a corrosive environment characterized by the following (excluding 0 area%).   In producing the high-strength bolt according to claim 3, after forming into a bolt shape, tempering after quenching is performed in a temperature range of 150 to 250 ° C. for 60 seconds or more and 3000 seconds or less. To manufacture high-strength bolts.

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