JPH0657367A - Pc steel bar excellent in delayed breakdown resisting property and its production - Google Patents

Abstract

PURPOSE:To produce a PC steel bar excellent in strength and delayed breakdown resisting property by supplying a specific raw material to the central part of a molten steel in a casting mold and making compositions of inner and outer layers of a blank material for the steel bar different from each other when blank material for PC steel bar is produced by a continuous casting. CONSTITUTION:The PC steel bar with about 9mm diameter is produced by sending a steel cylinder 5 in which the casting mold 3 for continuos casting is filled with adding elements consisting essentially of C to the central part of a molten steel 6, hot-rolling an obtained casting piece to form a steel wire bar and subjecting the bar to quenching and tempering. One more kinds from among, in weight, 0.01 to 0.2% C, 0.1 to 2.0% Si, 0.5 to 2.0% Mn, 0.005 to 0.10% Al, 0.0005 to 0.005% B, 0.1 to 2.0% Cr and 0.1 to 2.0% Ni or further 0.01 to 0.1% Ti are incorporated into the outer layer part with 0.2 to 1mm thickness of the steel bar and the part is composed of the mixed organizations of ferrite, bainite and martensite or ferrite and martensite. The inner layer thereof contains 0.15 to 0.5% C, i.e., only C content is higher than the C content of the outer layer, and is composed of martensite or mixed organizations of martensite and bainite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to P having excellent delayed fracture resistance.
The present invention relates to a C steel rod and a manufacturing method thereof.

[0002]

2. Description of the Related Art Among concrete poles and piles, those that give a compressive force to concrete for the purpose of improving rigidity and bending strength and preventing cracking of concrete are called PC poles and piles. Manufactured. First, the auxiliary reinforcements are spirally wound around the PC steel rods arranged on the circumference, and then the intersections of the PC steel rods and the auxiliary reinforcements are fixed, so that the cylindrical reinforcing body shown in FIG. 3 (hereinafter referred to as the reinforcing body). And
Then, this reinforcement is introduced into the mold to form P
Fix both ends of the C steel material and tension it with a stress of about 70% of the tensile strength. After the concrete poured into the mold is solidified, the fixing of both ends of the PC steel material is released, and a compressive force is applied to the concrete to complete the PC pole and pile. In many cases, the assembly of the reinforcing body is automated during this manufacturing process, and the PC steel rod 8 and the auxiliary bar 9 are fixed by spot welding 10. Since the PC steel rod is used in a state where a high tension is applied, there is a risk of sudden brittle fracture, so-called “delayed fracture”, when hydrogen penetrates inside. Conventionally, various measures have been proposed to improve delayed fracture characteristics. For example, a method of limiting the content of P and S, which are inevitable impurities, to an extremely low level (Japanese Patent Laid-Open No. 62-136520) is shown, but there is a drawback that a large amount of cost is required to reduce P and S. there were.

On the other hand, paying attention to the fact that delayed fracture occurs from the surface layer and propagates to the inside, a method of strongly tempering and softening only the surface layer of a steel material to reduce the strength of only the surface layer to reduce the delayed fracture sensitivity (Japanese Patent Laid-Open Publication No. Sho. No. 58-1020) is also proposed. Although it is possible to soften with this method, the components are the same as before tempering, so if the spot welding method is used when braiding the reinforcement, the weld heat affected part will be hardened and it may be a fundamental solution. There wasn't. Further, the method of improving delayed fracture resistance by reducing the carbon content on the surface and forming a ferrite structure (Japanese Patent Laid-Open No. 267420/1987) is reliable because the hardening of the heat-affected zone is suppressed even after welding. This is an excellent method that can be expected to be effective. In manufacturing such a multi-layer steel material having different composition of the surface layer portion and the inside, various multi-layering methods have been disclosed.

For example, the above-mentioned JP-A-62-267420.
In the gazette, decarburization is used. However, the introduction of a decarburized layer having a sufficient thickness so that the thermal effect of spot welding does not reach the inner layer portion requires a decarburization treatment for an extremely long time, which is a drawback that the manufacturing cost is significantly increased. In addition, a multilayer slab can be manufactured by placing an inner layer material in a mold in advance, pouring low-carbon molten steel into the periphery, and solidifying the molten steel. However, it is not practical because special equipment is required to completely bond the interfaces, and the production of very low C molten steel to bring the amount of C in the surface layer to the level of the decarburized layer also has the drawback of increasing costs. there were.

[0005]

SUMMARY OF THE INVENTION The present invention provides a P having excellent delayed fracture resistance, the effect of which does not disappear even by spot welding.
The present invention provides a PC steel rod having a reduced amount of C in the outer layer as a C steel rod, and further provides an inexpensive production method for the PC steel rod, which limits the production method of a slab in a continuous casting process.

[0006]

In order to realize a PC steel rod having excellent delayed fracture resistance, the effect of which does not disappear even by spot welding, the present inventors have adopted the structure of a PC steel rod as an outer layer portion. It was examined to form a multilayer structure having a low C layer having a sufficient thickness. Lowering the outer layer structure after QT to a level at which it has a ferrite single phase causes a great increase in cost, but as a result of diligent studies, the surface layer structure is not necessarily ferrite in order to improve delayed fracture resistance. No need, low C
It was found that the martensite of No. 3 has sufficiently excellent delayed fracture resistance. Focusing on this point, only the outer layer has a low-C martensite structure or a mixed structure of martensite, ferrite, and bainite, which is empirically recognized to have low delayed fracture susceptibility, and the inner layer is sufficient as a PC steel bar. The present invention has been completed in consideration of a martensite having a C content capable of achieving strength or a mixed structure of martensite and bainite. That is,

(1) An inner layer portion and an outer layer portion surrounding the inner layer portion. The outer layer portion has a thickness of 0.2 mm or more and 1 mm or less, and the composition of the outer layer portion is C: 0.01% by weight. % To 0.2%, Si: 0.1% to 2.0%, M
n: 0.5% to 2.0%, Al: 0.005% to 0.10%, B: 0.0005% to 0.
005% or less, Cr: 0.1% or more and 2.0% or less, N
i: 0.1% or more and 2.0% or less, containing one or more kinds, and the balance being iron and inevitable impurities, while the composition of the inner layer is C: 0.15% by weight. 0.
5% or less, Si: 0.1% or more and 2.0% or less, Mn:
0.5% to 2.0%, Al: 0.005% to 0.10%, B: 0.0005% to 0.0
05% or less, Cr: 0.1% or more and 2.0% or less, Ni:
0.1% or more and 2.0% or less of one or more kinds, and the balance iron and unavoidable impurities, the C content of the outer layer is lower than the inner layer, and the structure of the outer layer Is a mixed structure of ferrite, bainite and martensite,
Or a mixed structure of ferrite and martensite,
A PC steel rod having excellent delayed fracture resistance, wherein the structure of the inner layer portion is martensite or a mixed structure of martensite and bainite.

(2) An inner layer portion and an outer layer portion surrounding the inner layer portion. The outer layer portion has a thickness of 0.2 mm or more and 1 mm or less, and the composition of the outer layer portion is C: 0.01% by weight. % To 0.2%, Si: 0.1% to 2.0%, M
n: 0.5% to 2.0%, Al: 0.005% to 0.10%, Ti: 0.01% to 0.1%, and B: 0.0005% to 0.1. 005% or less, Cr: 0.1% or more and 2.0% or less, Ni: 0.1%
Of the above 2.0% or less, one or more of them are contained, and the balance is composed of iron and unavoidable impurities. On the other hand, the composition of the inner layer portion is C: 0.15% or more and 0.5% or less by weight. ,
Si: 0.1% to 2.0%, Mn: 0.5% to 2.0%, Al: 0.005% to 0.10%, Ti: 0.01% to 0.1% Below, further B:
0.0005% or more and 0.005% or less, Cr: 0.1%
Or more and 2.0% or less, Ni: 0.1% or more and 2.0% or less, and one or two or more types, and the balance is iron and inevitable impurities, and the C content of the outer layer part is the inner layer part. Lower, the structure of the outer layer is a mixed structure of ferrite, bainite and martensite, or a mixed structure of ferrite and martensite, the structure of the inner layer is martensite or a mixed structure of martensite and bainite PC steel bar with excellent delayed fracture resistance.

A characteristic of the PC steel rod according to the present invention is that the C content in the outer layer portion is smaller than that in the inner layer portion. In addition, when an element that improves the hardenability and strength is added only to the inner layer, the hardenability and strength of the inner layer are significantly improved compared to the outer layer, so cooling after quenching and solutionizing is performed at a relatively low cooling rate. However, the strength of the inner layer portion can be sufficiently secured. Further, since the hardenability of the outer layer portion remains low, it is possible to suppress hardening of the heat-affected portion even if spot welding is performed during braiding of the reinforcing material. As described above, the effect of containing more elements other than C in the inner layer portion is larger than that in the outer layer portion. When producing a PC steel rod having different composition of the outer layer portion and the inner layer portion, a PC steel material excellent in delayed fracture resistance can be easily produced by the method of the present invention by specifying the billet production method in the continuous casting process. can do. That is,

(3) C or C and S filled in a steel cylinder at the center of the molten steel in the mold in continuous casting
One or two or more of i, Mn, B, Cr, and Ni are continuously supplied, and the component structure of the inner layer portion is C: 0.1% by weight.
5% or more and 0.5% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.5% or more and 2.0% or less, Al: 0.00
5% or more and 0.10% or less, further B: 0.0005% or more and 0.005% or less, Cr: 0.1% or more and 2.0% or less, Ni: 0.1% or more and 2.0% or less One or two or more are contained, the balance is iron and unavoidable impurities, and the composition of the outer layer is C: 0.01% to 0.2% by weight.
Below, Si: 0.1% to 2.0%, Mn: 0.5
% To 2.0%, Al: 0.005% to 0.10
% Or less, B: 0.0005% or more and 0.005% or less, Cr: 0.1% or more and 2.0% or less, Ni: 0.1%
A multi-layered slab containing one or more of 2.0% or less and a balance of iron and unavoidable impurities, and quenching or quenching and tempering a wire rod obtained by hot rolling the slab. A method for producing a PC steel rod excellent in delayed fracture resistance, characterized by:

(4) C or C and Si filled in a steel cylinder in the central portion of the molten steel in the mold in continuous casting,
One or more of Mn, B, Cr, and Ni are continuously supplied, and the component structure of the inner layer portion is C: 0.15% by weight.
Or more and 0.5% or less, Si: 0.1% or more and 2.0% or less,
Mn: 0.5% to 2.0%, Al: 0.005%
Or more and 0.10% or less, Ti: 0.01% or more and 0.1% or less, further B: 0.0005% or more and 0.005% or less,
Cr: 0.1% or more and 2.0% or less, Ni: 0.1% or more and 2.0% or less, and one or more types are contained, and the balance is iron and unavoidable impurities. weight%
C: 0.01% or more and 0.2% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.5% or more and 2.0% or less, A
l: 0.005% or more and 0.10% or less, Ti: 0.01
% Or more and 0.1% or less, B: 0.0005% or more and 0.005% or less, Cr: 0.1% or more and 2.0% or less,
Ni: A multilayer cast product containing one or two or more of 0.1% or more and 2.0% or less and a balance of iron and inevitable impurities, and a wire rod obtained by hot rolling the cast product. It is a method for producing a PC steel rod having excellent delayed fracture resistance, which is characterized by quenching or quenching and tempering.

Next, the reasons for limiting the conditions of the present invention will be described. If the C content of the outer layer is less than 0.01%, the delayed fracture resistance does not deteriorate, but the decarburization cost increases, so 0.01% is used.
% Was the lower limit. However, if added over 0.2%,
Since the strength after quenching and tempering becomes too high and the delayed fracture resistance deteriorates, 0.2% was made the upper limit. C of the inner layer is
If less than 0.15%, sufficient strength cannot be obtained, so 0.1
The lower limit is 5%, and if added in excess of 0.5%, quench cracking occurs, so 0.5% was made the upper limit. Si contributes to the improvement of strength as a solid solution strengthening element and is also useful as a deoxidizing agent, so 0.1% or more is added to the inner layer portion and the outer layer portion.
However, if added in excess of 2.0%, the hot workability deteriorates significantly, so 2.0% was made the upper limit.

Mn is an element for improving hardenability and is an element essential for improving strength.
Add 5% or more. However, if added over 2.0%, the effect is saturated and the hot workability is significantly deteriorated, so 2.0% was made the upper limit. Al of the inner layer portion and the outer layer portion,
If it is less than 0.005%, deoxidation is insufficient and the cleanliness of the steel deteriorates, so 0.005% is the lower limit, and if it exceeds 0.10%, the hot workability deteriorates, so 0.10% is the upper limit. did. Further, Ti can be added in order to fix N in the steel and significantly improve the yield of B added to improve the hardenability. In this case, if less than 0.01%, there is no N fixing effect, so 0.01% is the lower limit, and 0.10%
If it exceeds 0.1%, coarse inclusions are likely to occur, so 0.10% was made the upper limit.

If B is less than 0.0005%, the hardenability is not sufficiently improved, so 0.0005% is the lower limit, and
Even if added over 005%, the effect is saturated and it is economically disadvantageous, so 0.005% was made the upper limit. Cr increases the strength of steel by solid solution strengthening and improvement of hardenability, but if the content is less than 0.1%, the effect is insufficient, so 0.1% is the lower limit, and Cr is added in excess of 2.0%. However, the effect is saturated, so 2.0% was made the upper limit. Ni is an element that improves corrosion resistance and hardenability, but if the content is less than 0.1%, the effect is insufficient. Therefore, the lower limit is 0.1%, and the effect is saturated even if added in excess of 2.0%. Since it is economically disadvantageous, the upper limit is 2.0%.

When the reinforcing body is assembled by spot welding, the heat-affected zone extends to a depth of at least 0.2 mm from the surface if the heat input amount required to obtain sufficient joint strength is applied. Therefore, if the thickness of the outer layer portion is less than 0.2 mm, the inner layer portion of high C is hardened by the heat effect of the spot welding and the delayed fracture resistance deteriorates. However, if the thickness of the outer layer portion with low C and low strength exceeds 1 mm, the overall strength will be P
It is difficult to maintain the level required for C steel material, or it is necessary to add a large amount of special elements, so the upper limit was 1 mm.

The structure of the inner layer is limited to martensite or a mixed structure of martensite and bainite for the following reason. Heat treatment, wire drawing strengthening, and a combination thereof are conceivable as a method of obtaining the strength required for a PC steel rod with a steel material having a C content limited to the inner layer portion. If the structure is aimed to be martensite or a mixed structure of martensite and bainite, the strength can be achieved only by heat treatment, but for other structures, combined use of work strengthening is indispensable and the manufacturing cost increases.

Since the outer layer portion is low carbon, it has sufficient delayed fracture resistance even if the structure becomes martensite. Bainite is generally superior in delayed fracture resistance to martensite, and even if mixed with martensite, delayed fracture resistance is not deteriorated. Further, an unavoidable decarburized layer exists on the surface layer of the PC steel rod, but this also does not deteriorate the delayed fracture resistance. Therefore, the structure of the outer layer portion is a mixed structure of ferrite, bainite and martensite, or a mixed structure of ferrite and martensite.

[0018]

The PC steel rod according to the present invention has low delayed fracture susceptibility due to the low C in the outer layer portion, and is excellent in delayed fracture resistance because crack generation and propagation from the surface layer are suppressed. Inner layer is C
It is high in strength and high in strength, and maintains excellent properties as a PC steel material. Further, the low C outer layer has a sufficient thickness, and even if the spot welding method is used for braiding the reinforcing material, the heat-affected zone of welding does not reach the inner layer, so that delayed fracture resistance does not deteriorate.
As described above, the PC steel rod according to the present invention has the features of high strength and excellent delayed fracture resistance.

Further, according to the method of the present invention, there is no need for long-term decarburization treatment and costly low C molten steel, and delayed fracture resistance can be achieved by simply adding a simple element addition device to conventional manufacturing equipment. A PC steel rod having excellent properties can be easily manufactured. When a PC steel rod is manufactured from the cast piece produced by this apparatus, the manufacturing process after slabbing is the same as before, so the conventional manufacturing equipment can be used as it is. FIG. 1 shows an example of a manufacturing apparatus embodied by the present inventors. One or more steel cylinders 5 filled with C or another alloying element are prepared by using a cylinder supply device 4 installed in a conventional continuous casting facility composed of a tundish 1, a nozzle 2 and a mold 3. , Is continuously inserted into the central portion of the molten steel 6 in the mold. Since the steel cylinder is melted, C or C and B are concentrated inside the slab.

[0020]

EXAMPLES Next, the production results of PC steel rods according to the present invention will be described. FIG. 2 shows a structural example of a steel cylinder. The steel cylinder 5 used in this example has an outer diameter of 7.0 mm and an inner diameter of 5.4 m.
At m, the additional element 7 consisting of C (graphite) or C (graphite) and B powder was filled inside. This cylinder was fed into molten steel having the composition shown in Table 1 to produce a multi-layer cast slab having an inner layer portion enriched with C and B, and a wire rod having a diameter of 9 mm was formed by a normal hot rolling process. did. Using a PC steel rod manufactured by subjecting this wire to heat treatment such as quenching or quenching and tempering,
A PC pole reinforcement was assembled by spot welding. afterwards,
PC steel bar with heat-affected zone by spot welding,
Delayed fracture resistance was evaluated by applying a stress of 100 kgf / mm ^{2 in} a 0 wt% NH ₄ SCN aqueous solution and measuring the time until fracture. Further, the chemical composition of the inner layer portion and the outer layer portion of the PC steel rod was examined in order to confirm the addition state of C and B. Avoid the unavoidable decarburization of the very surface layer, and use the cutting chips that have been peripherally cut within a distance of 0.1 mm to 0.2 mm from the surface as the analysis sample of the outer layer, and the 1 / 4d part with a diameter of 1.5 mm. The shavings perforated in step 1 were provided as analytical samples for the inner layer. Regarding the above results, examples of the present invention are shown in Table 2 and comparative examples are shown in Table 3. The PC steel rods A to N of the present invention showed good delayed fracture resistance and had sufficient mechanical properties as a PC steel rod. The amounts of C and B in the inner layer portion are surely increased as compared with the outer layer portion, and it is understood that the method of the present invention is suitable for the production of such a steel material having different components in the inner layer portion and the outer layer portion. In Comparative Example O, since the thickness of the outer layer portion was insufficient,
Comparative Example P was also inferior in delayed fracture resistance due to the large amount of C in the outer layer. In Comparative Example R, the thickness of the outer layer portion was too large, and in Comparative Example Q, the amount of C in the inner layer portion was too small, so that the strength was insufficient. In Comparative Example S, quench cracking occurred because the amount of C in the inner layer portion was excessive.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

As is clear from the above examples, the PC steel rod of the present invention has an excellent delayed fracture resistance that does not deteriorate even by spot welding. Therefore, it can be applied to the braiding of the reinforcing material of the PC structure by the spot welding method with high production efficiency, the manufacturing cost of the PC structure can be significantly reduced, and the industrial effect is extremely large.

[Brief description of drawings]

FIG. 1 is a C and B addition device 1 installed in a continuous casting device.
Figure showing an example,

FIG. 2 is a diagram showing a structural example of a steel cylinder,

FIG. 3 is a diagram showing an example of a PC pole reinforcement.

[Explanation of symbols]

 1 Tundish 2 Nozzle 3 Mold 4 Cylinder Supply Device 5 Steel Cylinder 6 Molten Steel 7 Additive Element 8 PC Steel Rod 9 Spot Welding Point 10 Auxiliary Muscle

Claims (4)

[Claims] 1. An inner layer portion and an outer layer portion surrounding the inner layer portion,
The thickness of the outer layer portion is 0.2 mm or more and 1 mm or less, and the component composition of the outer layer portion is C: 0.01% or more by weight%.
2% or less, Si: 0.1% or more and 2.0% or less, Mn:
0.5% to 2.0%, Al: 0.005% to 0.10%, B: 0.0005% to 0.0
05% or less, Cr: 0.1% or more and 2.0% or less, Ni:
0.1% or more and 2.0% or less of one or two or more, and the balance is iron and unavoidable impurities, while the composition of the inner layer portion is C: 0.15% or more by weight% 0 .5%
Below, Si: 0.1% to 2.0%, Mn: 0.5
% To 2.0%, Al: 0.005% to 0.10
% Or less, B: 0.0005% or more and 0.005% or less, Cr: 0.1% or more and 2.0% or less, Ni: 0.1%
Contains one or more than 2.0% or less,
The balance consists of iron and unavoidable impurities, the C content of the outer layer is lower than the inner layer, and the structure of the outer layer is a mixed structure of ferrite, bainite, and martensite, or a mixed structure of ferrite and martensite, A PC steel rod having excellent delayed fracture resistance, wherein the structure of the inner layer portion is martensite or a mixed structure of martensite and bainite. 2. An inner layer portion and an outer layer portion surrounding the inner layer portion,
The thickness of the outer layer portion is 0.2 mm or more and 1 mm or less, and the component composition of the outer layer portion is C: 0.01% or more by weight%.
2% or less, Si: 0.1% or more and 2.0% or less, Mn:
0.5% to 2.0%, Al: 0.005% to 0.10%, Ti: 0.01% to 0.1%,
Further, B: 0.0005% or more and 0.005% or less, C
r: 0.1% or more and 2.0% or less, Ni: 0.1% or more and 2.0% or less, and one or more of them, and the balance is iron and unavoidable impurities. The composition of the components in% by weight is C: 0.15% or more and 0.5% or less, S
i: 0.1% to 2.0%, Mn: 0.5% to 2.0%, Al: 0.005% to 0.10%, Ti: 0.01% to 0.1% Below, further B:
0.0005% or more and 0.005% or less, Cr: 0.1%
Or more and 2.0% or less, Ni: 0.1% or more and 2.0% or less, and one or two or more types, and the balance is iron and inevitable impurities, and the C content of the outer layer part is the inner layer part. Lower, the structure of the outer layer is a mixed structure of ferrite, bainite and martensite, or a mixed structure of ferrite and martensite, the structure of the inner layer is martensite or a mixed structure of martensite and bainite PC steel bar with excellent delayed fracture resistance characterized by. 3. C or C and Si, M filled in a steel cylinder at the center of molten steel in a mold in continuous casting.
One or two or more of n, B, Cr, and Ni are continuously supplied, and the component composition of the inner layer portion is C: 0.15% or more and 0.5% or less and Si: 0.1% or more by weight%. 2.0% or less, M
n: 0.5% to 2.0%, Al: 0.005% to 0.10%, B: 0.0005% to 0.
005% or less, Cr: 0.1% or more and 2.0% or less, N
i: One or more of 0.1% or more and 2.0% or less, with the balance being iron and unavoidable impurities, and the composition of the outer layer portion in% by weight C: 0.01% or more and 0.2 %Less than,
Si: 0.1% to 2.0%, Mn: 0.5% to 2.0%, Al: 0.005% to 0.10%, B: 0.0005% to 0.005 %Less than,
Cr: 0.1% or more and 2.0% or less, Ni: 0.1% or more and 2.0% or less, and one or two or more types of multi-layer casting with a composition range consisting of the balance iron and inevitable impurities. A method for producing a PC steel rod having excellent delayed fracture resistance, which comprises quenching or quenching and tempering a wire rod obtained by hot rolling the slab. 4. C, or C and Si, M filled in a steel cylinder at the center of the molten steel in the mold in continuous casting.
One or two or more of n, B, Cr, and Ni are continuously supplied, and the component composition of the inner layer portion is C: 0.15% or more and 0.5% or less and Si: 0.1% or more by weight%. 2.0% or less, M
n: 0.5% to 2.0%, Al: 0.005% to 0.10%, Ti: 0.01% to 0.1%, further B: 0.0005% to 0.005 %Less than,
Cr: 0.1% or more and 2.0% or less, Ni: 0.1% or more and 2.0% or less, and one or more types are contained, and the balance is iron and unavoidable impurities. weight%
C: 0.01% or more and 0.2% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.5% or more and 2.0% or less, A
l: 0.005% or more and 0.10% or less, Ti: 0.01
% Or more and 0.1% or less, B: 0.0005% or more and 0.005% or less, Cr: 0.1% or more and 2.0% or less,
Ni: A multi-layer cast product containing one or two or more of 0.1% or more and 2.0% or less and the balance of iron and inevitable impurities, and a wire rod obtained by hot rolling the cast product. A method for producing a PC steel rod excellent in delayed fracture resistance, characterized by quenching or quenching and tempering.