JP7543687B2 - Manufacturing method for high strength bolt steel - Google Patents

Description

The present invention relates to a method for manufacturing steel for high-strength bolts that has a tensile strength of more than 1400 MPa while improving resistance to delayed fracture, and in particular, to a method for manufacturing steel for high-strength bolts that is suitable for processing bolts with diameters ranging from small to large.

High-strength bolts made of steel with a tensile strength of over 800 MPa are used in the civil engineering and construction fields. In particular, development of high-strength bolts made of steel with a tensile strength of over 1400 MPa is also underway, but it is generally believed that increasing the tensile strength of steel tends to decrease delayed fracture resistance. Therefore, in the development of steel for high-strength bolts, it is necessary to improve delayed fracture resistance in addition to increasing strength.

For example, Patent Document 1 discloses a steel for high-strength bolts, which is an alloy steel containing 0.35-0.50% C and 0.15-0.40% V along with Ni, Cr, and Mo, to increase the tensile strength to 1500 MPa or more. This steel is made of a tempered martensite structure that is quenched at a temperature of 1175 K (902°C) or higher, and then tempered at a temperature of 850 K (577°C) or higher. Here, it is said that hydrogen can be trapped in the V carbides that contribute to secondary hardening after tempering, improving delayed fracture resistance.

On the other hand, a method for manufacturing high-strength steel with improved resistance to delayed fracture has been proposed, in which the steel is rapidly cooled from the austenite temperature range to the non-recrystallization temperature range, plastically processed, and quenched, known as the ausforming method.

For example, Patent Document 2 discloses a method for manufacturing high-strength structural steel that uses an ausforming method by wire rolling to strengthen prior austenite grain boundaries and improve delayed fracture resistance. The example describes a method in which JIS SCM440 steel containing approximately 0.4% C is used, heated to the austenite temperature range of 1323K (1050°C), then wire-rolled with a 50% reduction in area and a finishing temperature of 1086K (813°C), water quenched, and tempered to adjust the tensile strength to 1450MPa. It is said that the occurrence of delayed fracture can be suppressed by refining the average dimensions of the flat parts of the prior austenite grain boundaries to 5μm or less by ausforming.

Japanese Patent Application Publication No. 8-225845 JP 2001-73064 A

As mentioned above, there is a known method for manufacturing high-strength structural steel that uses the ausforming method to strengthen prior austenite grain boundaries and improve delayed fracture resistance. However, in the processing of large-diameter bolts, there have been cases where the ausforming method has not been able to sufficiently improve delayed fracture resistance. In particular, it has been pointed out that in steels with increased tensile strength due to an increased carbon content, delayed fracture resistance is not sufficiently achieved.

The present invention was made in consideration of the above-mentioned circumstances, and its purpose is to provide a manufacturing method for steel for high-strength bolts that has a high tensile strength of 1400 MPa or more and excellent resistance to delayed fracture, and is suitable for processing bolts with diameters ranging from small to large.

The method for producing high-strength bolt steel according to the present invention is a method for producing high-strength bolt steel mainly made of tempered martensite structure and having a tensile strength of 1400 MPa or more, and is characterized in that a steel ingot made of steel having a composition consisting of, in mass%, C: more than 0.55% and not more than 0.80%, Si: more than 1.00% and not more than 2.90%, Cr: 0.80% to 1.50%, Al: 0.010% to 0.060%, N: 0.005% to 0.030%, the balance being Fe and unavoidable impurities is prepared, and after heating and holding at 1000°C or less and in the austenite single phase region, the steel is subjected to plastic processing at a processing temperature that is less than the recrystallization temperature and greater than the martensitic transformation start temperature, with a processing rate of 50% or more, and then quenched and tempered to adjust the tensile strength, and the local limiting hydrogen concentration measured by the normal rate method (CSRT method) is set to 1.5 ppm or more.

This invention makes it possible to produce steel for high-strength bolts that has high tensile strength of 1400 MPa or more by increasing the carbon content, while at the same time having stable and excellent resistance to delayed fracture when used to process bolts of various diameters, from small to large.

The above-mentioned invention may be characterized in that after heating and holding at 900°C or higher, the steel is air-cooled and the plastic processing is performed at a processing temperature of 850°C or lower. According to this invention, it is possible to stably manufacture high-strength bolt steel that has excellent delayed fracture resistance in bolt processing from small diameter to large diameter while increasing the carbon content and imparting high tensile strength of 1400 MPa or higher.

In the above-mentioned invention, the tempering may be performed at a temperature of 400°C or higher. According to this invention, it is possible to stably manufacture high-strength bolt steel that has excellent delayed fracture resistance in bolt processing from small diameter to large diameter.

In the above invention, the composition may further include, by mass%, Mo: 0.80% to 1.50%, and/or V: 0.05% to 0.50%. The composition may also include, by mass%, one or more of Mn: 0.80% or less, Nb: 0.10% or less, Ti: 0.10% or less, P: 0.015% or less, and S: 0.010% or less. According to this invention, it is possible to stably manufacture high-strength bolt steel that has excellent delayed fracture resistance in bolt processing from small diameter to large diameter.

1 is a table showing the composition of steel types A to D used in examples and comparative examples. 1 is a table showing the manufacturing conditions and various test results of the examples and comparative examples. 1A is a side view of a test piece for the CSRT method, and FIG. 1B is a partially enlarged view thereof. 1A is a side view of a delayed fracture bending test piece, and FIG. 1B is a partially enlarged view thereof. FIG. 2 is an explanatory diagram of a bending delayed fracture test.

The manufacturing method of steel for high-strength bolts as one embodiment of the present invention will be described with reference to Figure 1.

First, prepare a steel ingot with a composition that is relatively high in carbon, i.e., C: more than 0.55% and not more than 0.80%, Si: more than 1.00% and not more than 2.90%, Mn: 0.8% or less, P: 0.015% or less, S: 0.010% or less, Cr: 0.8% or more and not more than 1.5%, Al: 0.01% or more and not more than 0.06%, and N: 0.005% or more and not more than 0.03% (see Figure 1, steel type A).

It may also contain, by mass%, one or more of the following: Mo: 0.8% to 1.5%, V: 0.005% to 0.5%, Nb: 0.1% or less, Ti: 0.1% or less (see Figure 1, steel type B).

Next, ausforming is performed by heating to the austenite single phase region and plastically processing. Here, by using steel with a high C content, the resulting bolt has high tensile strength. On the other hand, for example, in bolts with a relatively large diameter exceeding M16, the delayed fracture resistance was not improved by ausforming as much as in bolts with a small diameter. In steels with a high C content that are highly susceptible to delayed fracture, heat treatment corresponding to the material size is performed prior to ausforming to fully dissolve carbides and to sufficiently homogenize the material to alleviate the causes of stress concentration. At this time, in bolts with a relatively large diameter, the crystal grains become coarse, and as a result, the effect of ausforming cannot be fully obtained.

First, the steel ingot with the above-mentioned composition is heated to a temperature of 1000°C or less and in the austenite single phase region and held at that temperature. The holding temperature in this heat treatment is hereinafter referred to as the heating temperature. At the heating temperature, the C that formed carbides is dissolved in the parent phase, increasing the C concentration and forming an austenite single phase structure. This heating temperature is preferably 900°C or more. In addition, in order to fully advance the solid solution, it is necessary to heat the material evenly to the inside, and the holding time at the heating temperature is set according to the dimensions of the material. For example, if the material diameter is 30 mm or more, it is preferable to hold for 40 minutes or more. On the other hand, a high heating temperature and a long holding time promote coarsening of the crystal grains, and in materials with high delayed fracture susceptibility, the effect of improving delayed fracture resistance by ausforming cannot be fully obtained. Therefore, the heating temperature is set to 1000°C or less to suppress the coarsening of the crystal grains.

Then, plastic processing is performed at a processing temperature within a specified range with a processing rate of 50% or more. At this time, the processing temperature is set to be equal to or higher than the martensite transformation start temperature in order to obtain plastic processing in the austenite phase. In addition, in order to obtain sufficient work hardening of austenite, the processing temperature is set to be equal to or lower than the recrystallization temperature, for example, 850°C or lower. In addition, cooling from the heating temperature to the processing temperature can be performed, for example, by air cooling. If air cooling is performed, the material can be removed from the heating furnace and wait while preparing for processing, and processing can be started when the processing temperature is reached. Then, processing is completed at a temperature equal to or higher than the martensite transformation start temperature. If it is a rod-shaped body, for example, hot extrusion processing can be used, which can be processed in a short time, so that the temperature drop until processing is completed can be suppressed. After processing, the material is quenched.

Then, tempering is performed to obtain a tempered martensite structure and adjust the tensile strength to 1400 MPa or more. In tempering, the holding temperature can be 400°C or more, for example. Here, it is preferable to precipitate secondary carbides, which can improve delayed fracture resistance.

The above manufacturing method allows us to obtain high-strength bolt steel that has high tensile strength of 1400 MPa or more by increasing the carbon content, and has stable and excellent delayed fracture resistance when processing bolts with small diameters to large diameters exceeding M16, for example.

[Production testing]
The results of measurements of the tensile strength, local limit hydrogen concentration, and delayed fracture strength of the high-strength bolt steel produced by the above-mentioned production method will be described below.

Test pieces were prepared using the steel types A to D shown in Figure 1 under the manufacturing conditions shown in Examples 1 and 2 and Comparative Examples 1 to 6 in Figure 2.

First, steel ingots of steel types A to D were melted in a 50 kg vacuum melting furnace and hot forged into steel bars with a diameter of 32 mm. Next, as a normalizing treatment, they were heated to 920°C and held for 2 hours, then air-cooled, and further, as a spheroidizing annealing treatment, they were heated to 760°C and held for 3 hours, then cooled to 650°C at a cooling rate of -15°C/hour, and then air-cooled. From the obtained material, material for test pieces with a diameter of 23.7 mm and a length of 48 mm was cut out.

Each cut material was ausformed to produce a large-diameter, high-strength bolt of M24. When producing a large-diameter bolt of M24, the diameter of the material must be 50 mm or more, and the heating time must be 60 minutes or more to heat the material evenly to the inside. In this test, the material for the test piece described above was held at a predetermined heating temperature for 60 minutes using an electric heating furnace, and then air-cooled to reach a predetermined processing temperature, hot extrusion was performed to give a predetermined processing rate, and water quenching was performed immediately after knocking out from the die. A 600-ton crank press was used for hot extrusion, with a processing speed of 80 to 100 mm/s, and water-soluble graphite and molybdenum disulfide were used for lubrication between the die and the material. The temperature of the material was measured using a radiation thermometer. The predetermined heating temperature, processing temperature, and processing rate are shown in Figure 2. The processing rate represents the area reduction rate due to extrusion.

Furthermore, each material was tempered at 500°C.

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

Using the CSRT test piece 10, the local limit hydrogen concentration (Hc ^* ) was measured by the CSRT method (Conventional Strain Rate Test). Hydrogen was infiltrated and diffused into the test piece by cathodic charging for 120 hours, and immediately a tensile test was performed at a crosshead speed of 1 mm/min to measure the fracture stress. 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 immediately after fracture. As the amount of diffusible hydrogen, a temperature rise rate of 100°C/h was used to perform a temperature rise desorption hydrogen analysis up to 600°C, and the amount of hydrogen released up to 300°C was determined. The logarithms of the obtained fracture stress and the amount of diffusible hydrogen were taken, and the relationship between the two was linearly approximated, and the amount of diffusible hydrogen that is 0.6 times the fracture stress of the uncharged material in which hydrogen was not infiltrated was shown in Figure 2 as the local limit hydrogen concentration Hc ^* .

As shown in Figure 5, the delayed fracture strength was measured by a delayed fracture bending test. A bending stress was applied to the delayed fracture bending test piece 20 by a weight 32 using a moment arm 31, and the bending strength was measured. First, the static bending strength was measured, and then 0.1 N hydrochloric acid was dropped into the notch to apply a stress of 0.8 to 0.2 times the static bending strength, and the fracture time of delayed fracture was obtained. If no fracture occurred, the test was cut off at 100 hours. The delayed fracture strength was calculated by taking the ratio of the 30-hour fracture strength to the static bending strength, and is shown in Figure 2 as the 30-hour delayed fracture strength ratio. A 30-hour delayed fracture strength ratio of 0.6 or more was deemed to be excellent in delayed fracture resistance and to pass the test.

In addition, in the tensile test using the above-mentioned tensile test specimen, the breaking stress is shown as the tensile strength in Figure 2.

As shown in Figure 2, the tensile strength of both Examples 1 and 2 was 1400 MPa or more, and the local limiting hydrogen concentration measured by the normal rate test method (CSRT method) was 1.5 ppm or more. In other words, both the tensile strength and delayed fracture resistance were excellent.

In contrast, in Comparative Examples 1 and 2, although the tensile strength was 1400 MPa or more, the local limiting hydrogen concentration measured in the same manner was below 1.5 ppm, and the bending delayed fracture strength ratio was also poor. This is thought to be because the heating temperature during ausforming was set to more than 1000°C, which resulted in coarsening of the crystal grains and prevented the full effect of ausforming.

In Comparative Examples 3 and 4, the local limit hydrogen concentration was below 1.5 ppm, and the bending delayed fracture strength ratio was also poor. This is thought to be due to the ausforming processing temperature being set to above 850°C, which promoted recrystallization during processing.

In Comparative Examples 5 and 6, the tensile strength was less than 1400 MPa. This is thought to be because steel type C, which has a low C content, and steel type D, which has a low Si content (see Figure 1), were used, respectively. In other words, steel type C and steel type D are thought to be unsuitable for the high-strength bolt steel to be obtained in this example.

Incidentally, the local hydrogen concentration (H _E ^* ) due to hydrogen penetrating into a bolt from the outside is generally at most about 1 ppm, and if the local limit hydrogen concentration Hc ^* is 1.5 ppm or more, the probability of delayed fracture occurring is significantly low. In this regard, as described above, in Examples 1 and 2, the local limit hydrogen concentration could be set to 1.5 ppm or more, and it is clear that good results were obtained.

As described above, the manufacturing methods of Examples 1 and 2 can produce high-strength bolt steel that has a tensile strength of 1400 MPa or more, has excellent resistance to delayed fracture, and is suitable as a material for large-diameter bolts, such as M24, that exceed M16.

The composition range of steel that can provide tensile strength and local limit hydrogen concentration almost equivalent to those of the high-strength bolt steel with excellent delayed fracture resistance, including the above-mentioned examples, is determined as follows.

First, let's explain the essential additive elements.

C is necessary to increase the hardness of the steel after quenching and tempering, and ensure a tensile strength of 1400 MPa or more. On the other hand, excessive inclusion of C reduces ductility and toughness, reducing manufacturability such as forgeability and rollability when forming bolts, as well as reducing resistance to delayed fracture. Taking these factors into consideration, C is set within the range of more than 0.55% and not more than 0.80% by mass.

Si is used as a deoxidizer when steel is produced, and improves the mechanical strength of steel through solid solution strengthening. On the other hand, adding too much of it promotes grain boundary oxidation, reducing delayed fracture resistance and hot workability. Taking these factors into consideration, the Si content is set within the range of more than 1.00% and not more than 2.90% by mass.

Cr is an effective element for improving hardenability to obtain a martensite structure, and for increasing softening resistance during tempering and improving mechanical strength. On the other hand, excessive inclusion can reduce workability and toughness, and promote grain boundary oxidation, reducing delayed fracture strength. Taking these factors into consideration, Cr is in the range of 0.80% to 1.50% by mass.

Aluminum is used as a deoxidizer for steel, and by forming oxides and nitrides, it contributes to the refinement of crystal grains, improving toughness and preventing a decrease in delayed fracture resistance. On the other hand, excessive content of aluminum generates hard nonmetallic inclusions that become the starting point of fatigue fracture and reduce fatigue life. Taking these factors into consideration, the aluminum content is within the range of 0.010% to 0.060% by mass.

N forms nitrides and carbonitrides together with Al, which contributes to the refinement of crystal grains and can improve toughness. It can also contribute to the formation of trap sites for diffusible hydrogen, improving resistance to delayed fracture. On the other hand, excessive inclusion of N generates nonmetallic inclusions of oxides and nitrides, which become the starting point of fatigue fracture and reduce fatigue life. Taking these factors into consideration, N is within the range of 0.005% to 0.030% by mass.

Next, we will explain optional added elements.

Mo improves the hardenability of steel and increases its mechanical strength. In addition, Mo dissolved in the parent phase suppresses the decrease in hardness during tempering due to secondary hardening, which generates fine carbides during tempering, and can contribute to improving mechanical strength. In addition, the interface of the generated carbides can serve as a trap site for diffusible hydrogen, improving delayed fracture resistance. Therefore, Mo may be added at will. On the other hand, excessive inclusion increases material costs and reduces hot workability and machinability. Taking these factors into consideration, when added, Mo is within the range of 0.80% to 1.50% by mass.

V precipitates as fine carbides, which increases mechanical strength and forms trap sites for diffusible hydrogen, improving resistance to delayed fracture. Therefore, it may be added at will. However, excessive inclusion increases material costs and reduces hot workability and machinability. Taking these factors into consideration, when added, V is within the range of 0.05% to 0.50% by mass.

Mn is used as a deoxidizer when producing steel, and can be added as needed to improve hardenability and ensure mechanical strength and toughness. However, excessive content of Mn leads to an excessive increase in mechanical strength, which reduces manufacturability such as machinability. Taking these factors into consideration, Mn is limited to a range of 0.80% or less by mass percent.

Nb forms carbonitrides together with V and Ti or on its own, contributing to precipitation strengthening, and can improve delayed fracture resistance by making these carbonitrides trap sites for diffusible hydrogen. However, even if added in excess, the effect becomes saturated. Taking these factors into consideration, Nb is limited to a range of 0.10% or less by mass percent.

Ti forms carbonitrides together with V and Nb or alone, contributing to precipitation strengthening, and can improve delayed fracture resistance by making these carbonitrides trap sites for diffusible hydrogen. On the other hand, if added in excess, it forms nitrides that act as nonmetallic inclusions and become the starting point of fatigue fracture, reducing fatigue life. Taking these factors into consideration, Ti is limited to a range of 0.10% or less by mass percent.

Since P segregates at austenite grain boundaries and reduces grain boundary strength, leading to reduced toughness and resistance to delayed fracture, it is preferable to reduce the content. On the other hand, excessive refining leads to increased costs. Taking these factors into consideration, P is limited to a range of 0.015% or less by mass percent.

S impairs hot workability and combines with Mn and other elements to form nonmetallic inclusions that reduce toughness and delayed fracture resistance, so it is preferable to reduce the content. On the other hand, it has the effect of improving machinability, and excessive refining leads to increased costs. Taking these factors into consideration, S is limited to a range of 0.010% or less by mass percent.

Although 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 will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims.

Claims (5)

A method for producing a steel for high strength bolts having a tempered martensite structure and a tensile strength of 1400 MPa or more,
In mass percent,
C: more than 0.55% and not more than 0.80%;
Si: more than 1.00% and not more than 2.90%;
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,
preparing a steel ingot having a composition including the remainder Fe and unavoidable impurities;
A manufacturing method of steel for high strength bolts, characterized in that after heating and holding at 1000°C or less in the austenite single phase region, the steel is subjected to plastic working at a working temperature of 50% or more at a working temperature of 1000°C or less and 100% or more above the martensitic transformation start temperature, and then quenched and tempered to adjust the tensile strength, so that the local limit hydrogen concentration measured by the normal rate test method (CSRT method) is 1.5 ppm or more. The method for manufacturing high-strength bolt steel according to claim 1, characterized in that the steel is heated and held at 900°C or higher, then air-cooled, and the plastic processing is performed at a processing temperature of 850°C or lower. The method for manufacturing high-strength bolt steel according to claim 1 or 2, characterized in that the tempering is performed at a temperature of 400°C or higher. In the above-mentioned 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 a steel for high strength bolts according to any one of claims 1 to 3, further comprising: The method for manufacturing steel for high-strength bolts according to any one of claims 1 to 4, characterized in that the composition contains, in mass%, one or more of the following: Mn: 0.80% or less, Nb: 0.10% or less, Ti: 0.10% or less, P: 0.015% or less, and S: 0.010% or less.

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