JP2020084276A - Steel for high-strength bolt and method for manufacturing the same - Google Patents

Abstract

To provide a method for producing steel for high-strength bolts having high tensile strength and excellent delayed fracture resistance, with a reduced content of Ni.SOLUTION: The method for producing high-strength steel mainly consisting of a tempered martensite structure is to provide high-strength bolts having a tensile strength of 1,400 MPa or more and a local critical hydrogen concentration measured by the conventional strain rate technique (CSRT method) of 1.5 ppm or more. A steel ingot made of steel having a composition containing, by mass%, C: more than 0.55% and 0.80% or less, Si: more than 1.0% and 2.9% or less, Cr: 0.80% or more and 1.50% or less, Al: 0.010% or more and 0.060% or less, and N: 0.005% or more and 0.030% or less is prepared. The tempered martensite structure is obtained by performing plastic working at a working rate of 50% or more at a processing temperature equal to or lower than a predetermined temperature at which recrystallization does not occur but equal to or higher than the martensitic transformation start temperature, after raising to a heating temperature which is in the austenite zone.SELECTED DRAWING: None

Description

The present invention relates to a high-strength bolt steel having a tensile strength of more than 800 MPa and a manufacturing method thereof, and more particularly to a high-strength bolt steel having a tensile strength of 1400 MPa or more and excellent delayed fracture resistance and a manufacturing method thereof.

High-strength bolts (high-strength bolts) having a tensile strength exceeding 800 MPa are used in the fields of civil engineering and construction. In such high strength bolts, delayed fracture due to hydrogen diffusion is often a problem. In particular, the development of high-strength bolts whose tensile strength has been increased to approximately 1400 MPa or more is required. For high-strength bolts, which are the material of high-strength bolts, it is possible to suppress delayed fracture along with such higher strength Required for steel.

For example, Patent Document 1, with respect to the tensile strength 120~140kgf / mm ² class high strength bolts for steel generally exceeds the tensile strength of 120 kgf / mm ² when the delayed fracture resistance is described to be markedly deteriorated. Here, C: 0.3 to 0.5%, Si: 0.1% or less, Mn: 0.1 to 0.5%, P: 0.015% or less, S: 0.010% or less, Cr : 0.3-1.5%, Mo: 0.1-0.5% of steel is used, and after ingot, forging, and normalizing, the steel is quenched from 850°C and then quenched. Disclosed is a method of manufacturing a high strength bolt steel to be returned. It is said that Si and Mn, which have a strong affinity with oxygen, are reduced to suppress grain boundary oxidation and improve delayed fracture resistance.

Further, Patent Document 2 discloses a manufacturing method for enhancing delayed fracture resistance of high-strength bolt steel having a high tensile strength of 1500 MPa or more. At least for steel containing Ni: 0.8 to 2.4% and V: 0.15 to 0.40%, quenched from a temperature of 1175K (902°C) or higher, at a temperature of 850K (577°C) or higher. Temper. The delayed fracture resistance can be improved by making the V carbides that contribute to the secondary hardening trap hydrogen.

With respect to such high-strength steel having a tensile strength of 1400 MPa or more, research has been conducted on improving delayed fracture resistance.

For example, Non-Patent Document 1 discloses a method for producing high-strength carbon steel using ausforming by rolling. Here, JIS SCM440 steel containing 0.42% C was used, and after the ausforming in which water quenching was performed by rolling at 50% reduction with 810° C. as the finishing temperature and heating to 1050° C., The tensile strength is adjusted to 1450 MPa by tempering. It is said that delayed fracture resistance can be improved by reducing film-like cementite on the former austenite grain boundaries and strengthening the grain boundaries, as compared with the case where ausforming is not performed.

By the way, the fracture is probabilistically evaluated by statistical processing, but the delayed fracture is evaluated by the limit diffuse hydrogen concentration based on the amount of infiltrated hydrogen and the fracture frequency. In addition, in steel where delayed fracture becomes noticeable, resistance to delayed fracture is improved by the local critical hydrogen concentration focusing on the hydrogen concentration at the site that can be the fracture initiation point, instead of the critical diffusible hydrogen concentration that is the average hydrogen content of the entire steel material. It is also proposed to consider. As a method for obtaining the local limit hydrogen concentration, a constant load test (CLT: Constant Load Test), a low strain rate method (SSRT: Slow Strain Rate Technique), a normal rate method (CSRT: Conventional Strain Rate Technique), four-point bending A method (4 Point Bending method) and the like have been proposed (Non-patent documents 2 and 3).

Japanese Patent Laid-Open No. 61-117248 JP-A-8-225845

S. Yusa, K. Tsuzaki and T. Takahashi, "Effect of tempering conditions on delayed fracture properties of ausform-treated high-strength carbon steel," CAMP-ISIJ, Japan Iron and Steel Institute, Vol.12. (1999)-1296 "Delayed Fracture Evaluation Guidebook for High Strength Bolts", Japan Steel Structure Association JSSC Technical Report, No. 91 (2010) "Guideline for delayed fracture evaluation of high strength bolts", Japan Steel Structure Association (2014)

With respect to the high-strength steel for bolts having a tensile strength of 1400 MPa or more, delayed fracture resistance was secured after obtaining mechanical strength by adding Ni, as shown in Patent Document 2. However, Ni is expensive as a material, and there is a demand to reduce it.

The present invention has been made in view of the above situation, and an object thereof is to obtain high tensile strength of 1400 MPa or more and excellent delayed fracture resistance while reducing the amount of Ni. It is to provide a steel for bolts and a manufacturing method thereof.

The inventors of the present application have keenly studied the possibility of applying ausforming in order to obtain a high-strength steel for bolts that can obtain high tensile strength and excellent delayed fracture resistance without reducing the amount of Ni or adding Ni. investigated. As a result, it has been found that in steel containing a large amount of C in excess of 0.55 mass %, high strength bolt steel having the above-mentioned properties can be obtained by applying ausforming.

That is, the method for producing a high-strength bolt steel according to the present invention is mainly composed of a tempered martensite structure and has a tensile strength of 1400 MPa or more and a local limit hydrogen concentration of 1.5 ppm measured by a normal velocity method (CSRT method). The method for producing a high-strength steel for bolts as described above, wherein C: more than 0.55% and 0.80% or less, Si: more than 1.0% and 2.9% or less, and Cr in mass %. : 0.80% to 1.50%, Al: 0.010% to 0.060%, N: 0.005% to 0.030%, balance Fe and inevitable impurities A steel ingot made of steel is prepared and subjected to plastic working with a working rate of 50% or more at a working temperature not higher than a predetermined temperature at which the temperature is raised to a heating temperature in an austenite region and recrystallization is not performed and at a martensite transformation start temperature or higher. It is characterized in that the tempered martensite structure is obtained by quenching, tempering and tempering.

According to this invention, a high-strength bolt steel having high tensile strength and excellent delayed fracture resistance can be manufactured without adding Ni.

In the above invention, the heating temperature may be 900° C. or higher, and the predetermined temperature may be 850° C. According to this invention, it is possible to surely strengthen the grain boundary and improve the delayed fracture resistance by working at the working temperature.

In the above invention, the tempering may be held at a temperature of 500° C. or higher. According to this invention, a tempered martensite structure can be reliably obtained, and tensile strength and delayed fracture resistance can be secured and improved.

In the above-mentioned invention, the component composition further contains, by mass %, Mo: 0.80% or more and 1.50% or less, and/or V: 0.05% or more and 0.50% or less. May be Moreover, in the said component composition, in mass %, Mn: 0.80% or less, Nb: 0.10% or less, Ti: 0.10% or less, P: 0.015% or less, S: 0.010% or less. May be included. According to this invention, the tensile strength and/or the delayed fracture resistance can be further improved while ensuring the above-mentioned tensile strength and delayed fracture resistance.

Further, the high-strength bolt steel according to the present invention is a high-strength bolt steel mainly composed of tempered martensite structure and having a tensile strength of 1400 MPa or more, and in mass%, C: exceeds 0.55% and 0 80% or less, Si: more than 1.0% and 2.9% or less, Cr: 0.80% or more and 1.50% or less, Al: 0.010% or more and 0.060% or less, N:0. It is characterized by having a component composition of 005% or more and 0.030% or less, the balance being Fe and unavoidable impurities, and having a local limit hydrogen concentration measured by a normal velocity method (CSRT method) of 1.5 ppm or more. ..

According to this invention, a high-strength bolt steel having high tensile strength and excellent delayed fracture resistance can be obtained without adding Ni.

In the above-mentioned invention, the component composition further contains, by mass %, Mo: 0.80% or more and 1.50% or less, and/or V: 0.05% or more and 0.50% or less. May be Moreover, in the said component composition, in mass %, Mn: 0.80% or less, Nb: 0.10% or less, Ti: 0.10% or less, P: 0.015% or less, S: 0.010% or less. May be included. According to this invention, the tensile strength and/or the delayed fracture resistance can be further improved while ensuring the above-mentioned tensile strength and delayed fracture resistance.

It is a table of composition of steel types A to D used in Examples and Comparative Examples. 3 is a list of manufacturing conditions and various test results of Examples and Comparative Examples. It is a side view (a) and a partial enlarged view (b) of a test piece for the CSRT method. It is a side view (a) and a partial enlarged view (b) of a bending delay fracture test piece. It is explanatory drawing of a bending delay fracture test.

A method for manufacturing high-strength bolt steel according to one embodiment of the present invention will be described with reference to FIG.

In the method for manufacturing high strength bolt steel according to the present embodiment, first, a steel ingot having a predetermined composition is prepared. Specifically, the composition of the components is, in mass %, C: more than 0.55% and 0.80% or less, Si: more than 1.0% and 2.9% or less, and Mn: 0.8% or less. , P: 0.015% or less, S: 0.010% or less, Cr: 0.8% or more and 1.5% or less, Al: 0.01% or more and 0.06% or less, N: 0.005% or more Contains 0.03% or less. Steel type A is illustrated in FIG. 1 as a steel having such a composition.

Further, as such a component composition, in mass%, Mo: 0.8% or more and 1.5% or less, V: 0.005% or more and 0.5% or less, Nb: 0.1% or less, Ti: One or more of 0.1% or less may be contained. Steel type B is illustrated in FIG. 1 as a steel having such a composition.

Subsequently, ausforming is performed. Specifically, the above steel ingot is heated to a heating temperature of 900° C. or higher which is an austenite region. In particular, regarding the heating temperature, the carbide before quenching is solid-dissolved to increase the C concentration of the matrix and obtain an austenite single phase structure. Then, plastic working is performed at a working rate of 50% or more at a working temperature within a predetermined range. The working temperature is set to a temperature at which recrystallization is not performed so as to obtain work hardening of austenite, for example, 850° C. or lower, and is set to a martensite transformation start temperature or higher for plastic working in the austenite phase. For example, the heated steel ingot may be air-cooled from the heating temperature described above, and may be processed when the processing temperature is reached. For a rod-shaped body, hot extrusion processing or the like can be used. Cooling after processing shall be quenching such as water quenching.

Then, tempering is performed to obtain a tempered martensite structure. In tempering, for example, the holding temperature may be 500°C or higher. Here, it is preferable to precipitate secondary carbides, which can improve delayed fracture resistance.

According to the manufacturing method as described above, it is possible to obtain a high-strength bolt steel having a tensile strength of 1400 MPa or more and a local limit hydrogen concentration of 1.5 ppm or more and excellent in delayed fracture resistance.

[Manufacturing test]
The results of measuring the tensile strength, the local critical hydrogen concentration, and the bending delayed fracture strength of the high-strength bolt steel manufactured by the above-described manufacturing method will be described.

Using the steel types A to D shown in FIG. 1, test pieces were produced under the production conditions shown in Examples 1 and 2 and Comparative Examples 1 to 6 of FIG.

First, steel ingots of steel types A to D were melted in a 50 kg vacuum melting furnace and hot-forged to form a steel bar having a diameter of 32 mm. Then, as normalizing treatment, heating to 920° C. and holding for 2 hours, then air cooling, and further as spheroidizing annealing, heating to 760° C. and holding for 3 hours, and then to 650° C. at a cooling rate of −15° C./hour. After cooling, it was air-cooled. A material for a test piece having a diameter of 23.7 mm and a length of 48 mm was cut out from the obtained material.

Each material cut out was subjected to ausforming processing. In detail, after holding the material at a predetermined heating temperature for 30 minutes using an electric heating furnace, when it reaches a predetermined processing temperature while air cooling, hot extrusion processing is performed to give a predetermined processing rate, Immediately after knocking out, water quenching was performed. A 600-ton crank press was used for hot extrusion, a processing speed of 80 to 100 mm/s was used, and water-soluble graphite and molybdenum disulfide were used for lubrication with the die. The temperature of the material was measured using a radiation thermometer. Here, the predetermined heating temperature, processing temperature, and processing rate are as shown in FIG. 2 as described above. The processing rate "0" in Comparative Examples 1 and 2 indicates that the ausforming process was not performed. In addition, the processing rate represents the area reduction rate due to extrusion processing.

Furthermore, each material was tempered. The holding temperature is the temperature shown as the tempering temperature in FIG.

From this material, the CSRT method test piece 10 shown in FIG. 3, the bending-delayed fracture test piece 20 and the tensile test piece (not shown) shown in FIG. 4 were cut out. The test piece 10 for the CSRT method was an annular notch test piece having an annular notch 11 with a notch bottom φ6 mm and a notch radius 0.25 mm. The bending-delayed fracture test piece 20 was an annular notch test piece having an annular notch 21 with a notch bottom φ4 mm and a notch radius 0.1 mm. The tensile test piece was a JIS No. 14A smooth tensile test piece in which the diameter of the parallel portion was 6 mm.

The local limiting hydrogen concentration (Hc ^* ) was measured by the CSRT method (A conven- tional strain rate technique) using the test piece 10 for the CSRT method. Hydrogen was allowed to penetrate and diffuse into the test piece by the cathode charge for 120 hours, and a tensile test was immediately performed at a crosshead speed of 1 mm/min to measure the breaking stress. Immediately after the fracture, the amount of diffusible hydrogen was determined using a gas chromatograph for the cut piece obtained by cutting the test piece at a position 10 mm from the fracture surface. The amount of diffusible hydrogen was determined as the amount of hydrogen released up to 300° C. by conducting a temperature programmed desorption hydrogen analysis up to 600° C. at a temperature rise rate of 100° C./h. Taking the logarithm of the obtained breaking stress and diffusible hydrogen amount and linearly approximating the relationship between them, the diffusible hydrogen amount that gives a breaking stress of 0.6 times the breaking stress of the uncharged material that did not penetrate hydrogen was calculated. The local limit hydrogen concentration Hc ^* is shown in FIG.

As shown in FIG. 5, the delayed fracture strength was measured by a bending delayed fracture test. Bending stress due to the weight 32 is applied to the bending-delayed fracture test piece 20 by the moment arm 31, and the bending strength is measured. First, after measuring the static bending strength, 0.1N hydrochloric acid was dropped to apply a stress 0.8 to 0.2 times the static bending strength to determine the fracture time of delayed fracture. In addition, when it did not break, the test termination time was set to 100 h. The delayed fracture strength is shown in FIG. 2 as the 30h bending delayed fracture strength ratio obtained by taking the ratio of the 30h fracture strength and the static bending strength. In addition, when the 30 h bending delay fracture strength ratio was set to 0.6 or more, it was judged as excellent in delayed fracture resistance and passed.

Further, in the tensile test using the tensile test piece described above, the breaking stress is shown in FIG. 2 as the tensile strength.

As shown in FIG. 2, in each of Examples 1 and 2, the tensile strength was 1400 MPa or more, and the local limiting hydrogen concentration measured by the normal velocity method (CSRT method) was 1.5 ppm or more. That is, it is excellent in both tensile strength and delayed fracture resistance.

On the other hand, in Comparative Examples 1 and 2, although the tensile strength is 1400 MPa or more, the similarly measured local limit hydrogen concentration is less than 1.5 ppm, and the bending delay fracture strength ratio is also poor. It is considered that this is because the ausforming process was not performed.

Also in Comparative Examples 3 and 4, the local limit hydrogen concentration is less than 1.5 ppm, and the bending delay fracture strength ratio is also inferior. Since the processing temperature of ausforming was over 850° C., it is considered that recrystallization during processing progressed.

In Comparative Examples 5 and 6, although the local limit hydrogen concentration and the bending delay fracture strength ratio were excellent, the tensile strength was less than 1400 MPa. It is considered that this is because the steel type C having a low Si content and the steel type D having a low C content (see FIG. 1) are used, respectively. That is, it is considered that steel type C or steel type D is not suitable as high strength bolt steel.

Meanwhile, local invasion of hydrogen concentration by the hydrogen entering from the outside to the bolt (H E ^_*) is generally at most 1ppm or so, the probability ^* local limit hydrogen concentration Hc is generated delayed fracture if more 1.5ppm is considerably less It will be. In this respect, as described above, in Examples 1 and 2, it was found that the local limit hydrogen concentration could be 1.5 ppm or more, which was a good result.

As described above, according to the manufacturing methods of Examples 1 and 2, it is possible to obtain high-strength bolt steel having a tensile strength of 1400 MPa or more and excellent delayed fracture resistance.

By the way, the composition range of the steel which can give the tensile strength and the local limit hydrogen concentration almost equivalent to the steel for high strength bolts excellent in delayed fracture resistance including the above-mentioned examples is defined as follows.

First, the essential additive element will be described.

C is necessary to increase the hardness of steel after quenching and tempering and to secure a tensile strength of 1400 MPa or more. On the other hand, if it is contained excessively, the ductility and toughness are lowered, the delayed fracture resistance is lowered, and the manufacturability such as forgeability and rolling property at the time of bolt forming is also lowered. Taking these into consideration, C is in the range of more than 0.55% and 0.80% or less by mass %.

Si is used as a deoxidizing agent when producing steel, and improves the mechanical strength of steel by solid solution strengthening. On the other hand, if added excessively, it promotes intergranular oxidation to reduce delayed fracture resistance and hot workability. Considering these, Si is in the range of more than 1.0% and 2.9% or less by mass %.

Cr is an element effective for enhancing the hardenability and obtaining the martensite structure, thus enhancing the softening resistance during the tempering treatment, and for lowering the transformation temperature of the pearlite structure and the bainite structure to enhance the mechanical strength. On the other hand, when it is contained excessively, workability and toughness may be deteriorated, which accelerates the grain boundary oxidation and deteriorates the delayed fracture strength. Considering these, Cr is in the range of 0.80% to 1.50% in mass %.

Al is used as a deoxidizing agent for steel, forms an oxide or a nitride, contributes to the refinement of crystal grains, improves toughness, and can suppress a decrease in delayed fracture resistance. On the other hand, if it is contained excessively, hard non-metallic inclusions are generated, which becomes the starting point of fatigue fracture and reduces the fatigue life. Considering these, Al is in the range of 0.010% to 0.060% by mass.

N can form a nitride or carbonitride with Al, contribute to the refinement of crystal grains, and improve mechanical strength. In addition, the delayed fracture resistance can be improved by contributing to the formation of diffusible hydrogen trap sites. On the other hand, if it is contained excessively, non-metallic inclusions are generated, which becomes the starting point of fatigue fracture and reduces the fatigue life. Considering these, N is in the range of 0.005% or more and 0.030% or less by mass %.

Next, the optional additional element will be described.

Mo improves the hardenability of steel by forming and precipitating carbides to improve the mechanical strength and suppresses the decrease in hardness during tempering. Further, the interface of the precipitate can be used as a diffusible hydrogen trap site to improve delayed fracture resistance. Therefore, it may be added arbitrarily. On the other hand, if it is contained excessively, the material cost is increased and the hot workability and the machinability are deteriorated. When these are taken into consideration, Mo is in the range of 0.80% or more and 1.50% or less by mass%.

V precipitates as fine carbides to increase mechanical strength and form diffusible hydrogen trap sites, which can improve delayed fracture resistance. Therefore, it may be added arbitrarily. On the other hand, if it is contained excessively, the material cost is increased and the hot workability and the machinability are deteriorated. When these are taken into consideration, V is in the range of 0.05% or more and 0.50% or less in mass%.

Mn can be optionally added to improve hardenability and ensure mechanical strength and toughness. However, if it is contained excessively, manufacturability such as turning machinability is deteriorated due to excessive increase in mechanical strength, and Mn is This causes a decrease in toughness due to an increase in segregation. Considering these, Mn is in the range of 0.80% or less in mass %.

Nb forms a carbonitride together with V or Ti or alone and contributes to precipitation strengthening, and by using such a carbonitride as a diffusible hydrogen trap site, delayed fracture resistance can be improved. On the other hand, even if added excessively, the effect will be saturated. Considering these, Nb is in the range of 0.10% or less in mass %.

Ti forms carbonitrides alone or together with V and Nb to contribute to precipitation strengthening, and by using such carbonitrides as trap sites for diffusible hydrogen, delayed fracture resistance can be improved. On the other hand, if added excessively, it forms a nitride and becomes a starting point of fatigue fracture as a non-metallic inclusion to reduce the fatigue life. Considering these, Ti is in the range of 0.10% or less in mass %.

P is preferably segregated at the austenite grain boundaries, embrittles the crystal grain boundaries, and lowers the mechanical strength, so the content is preferably reduced. On the other hand, excessive refining leads to increased costs. Considering these, P is in the range of 0.015% or less by mass %.

Since S impairs the hot workability and combines with Mn and the like to form non-metallic inclusions to reduce toughness and delayed fracture resistance, it is preferable to reduce the content. On the other hand, it has an effect of improving machinability and excessive refining leads to an increase in cost. Considering these, S is in the range of 0.010% or less by mass %.

Although the typical embodiments of the present invention have been described above, the present invention is not necessarily limited to these, and a person skilled in the art can deviate from the gist of the present invention or the scope of the appended claims. , It will be possible to find various alternatives and modifications.

Claims (8)

A method for producing high-strength steel for bolts, which mainly comprises tempered martensite structure and has a tensile strength of 1400 MPa or more and a local limit hydrogen concentration of 1.5 ppm or more measured by a normal velocity method (CSRT method),
In mass %,
C: more than 0.55% and 0.80% or less,
Si: more than 1.0% and 2.9% or less,
Cr: 0.80% or more and 1.50% or less,
Al: 0.010% or more and 0.060% or less,
N: 0.005% or more and 0.030% or less,
A steel ingot made of steel having a composition consisting of the balance Fe and unavoidable impurities is prepared,
After heating to a heating temperature in the austenite region, and subjecting it to plastic working with a working rate of 50% or more at a working temperature below a predetermined temperature at which recrystallization does not occur and above the martensite transformation start temperature, quenching and further tempering are performed. The tempered martensite structure is obtained by doing so. The method for producing high-strength bolt steel according to claim 1, wherein the heating temperature is 900°C or higher and the predetermined temperature is 850°C. The method for producing high-strength bolt steel according to claim 1 or 2, wherein the tempering is performed at a temperature of 500°C or higher. In the above component composition, in mass%,
Mo: 0.80% or more and 1.50% or less, and/or
V: 0.05% or more and 0.50% or less,
The method for producing high-strength bolt steel according to any one of claims 1 to 3, further comprising: In the above component composition, in mass%, Mn: 0.80% or less, Nb: 0.10% or less, Ti: 0.10% or less, P: 0.015% or less, S: 0.010% or less The method for producing a high-strength steel for bolts according to any one of claims 1 to 4, which is obtained. A high-strength bolt steel mainly comprising a tempered martensite structure and having a tensile strength of 1400 MPa or more,
In mass %,
C: more than 0.55% and 0.80% or less,
Si: more than 1.0% and 2.9% or less,
Cr: 0.80% or more and 1.50% or less,
Al: 0.010% or more and 0.060% or less,
N: 0.005% or more and 0.030% or less,
It has a composition of the balance Fe and unavoidable impurities,
Steel for high-strength bolts, which has a local limit hydrogen concentration of 1.5 ppm or more measured by the normal velocity method (CSRT method). In the above component composition, in mass%,
Mo: 0.80% or more and 1.50% or less, and/or
V: 0.05% or more and 0.50% or less,
The steel for high-strength bolts according to claim 6, further comprising: In the above component composition, in mass%, Mn: 0.80% or less, Nb: 0.10% or less, Ti: 0.10% or less, P: 0.015% or less, S: 0.010% or less The steel for high-strength bolts according to claim 6 or 7, which is obtained.

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