JPS61174326A - Production of machine structural steel having superior delayed fracture resistance - Google Patents

Abstract

PURPOSE:To obtain a machine structural steel having specified tensile strength or above and superior delayed fracture resistance by restricting the contents of C, Si, Mn, P, S and Cr as principal components and regulating temp. conditions during quenching and tempering after hot rolling. CONSTITUTION:A steel consisting of 0.18-0.35% C, <=0.5% Si, <0.5% Mn, <=0.01% P, <=0.01% S, 0.01-5% Cr and the balance Fe with inevitable impurities is hot rolled, quenched from >=870 deg.C, and tempered at a low temp. of 150-600 deg.C. The machine structural steel having >=125kgf/mm<2> tensile strength and causing no delayed fracture for >=about 5,000hr can be obtd. The steel may further contain 0.005-0.1% Nb and one or more among <=0.3% V, <=0.005% B, <=0.05% Ti and <=0.5% Mo.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to high-tensile bolts and PC steel bars that have a tensile strength of 125 kgf/mm2 or more and excellent delayed fracture resistance.
Furthermore, it relates to a method of manufacturing steel for machine structures such as high-strength steel plates for large machines.

More specifically, the present invention applies to high-strength steel, which is mass-produced steel, which is required to improve economical efficiency by reducing self-weight and reducing cross-section to save material and travel costs as structures become larger. This invention relates to a method for manufacturing strong steel and ultra-strong steel, which are required to withstand high stress and have high specific strength as mechanical parts and the like are improved in performance and reduced in weight.

Conventional technology In recent years, as structures have become larger and automobiles, trucks, civil engineering machinery, etc. have become lighter, there has been a demand for the development of mechanical structural steel with a tensile strength of 125 kgf/mm2 or higher, especially high-tensile bolts and PC steel bars. It has been done.

Conventionally, strong steel for mechanical structures, which generally has a tensile strength of 100 kgf/mm2 or more, is made of, for example, 0.35% C-1,0
%Cr-J I'S- with a composition of 0.2% iAo
SCM431 low alloy steel, 0.31%C-1,8%Cr
- J I S-3NCM4 with a composition of 0.2% Mo
31 low alloy steel, Sarani 0.2% (,0.8%Cr
It is manufactured by quenching and tempering a hot rolled material such as boron steel having a composition of -0,002% B.

However, when these high-strength steels for machine structures are put into practical use, those with a tensile strength of 125 kgf/mm2 or more may suffer delayed fracture during use, so high-tensile bolts and PGI1 rods are not used. Initially, there was a problem of lack of quality stability as an important safety part for automobiles and civil engineering machinery.

Note that delayed fracture is a phenomenon in which steel under a static load suddenly breaks brittle after a certain period of time, and is considered to be a type of hydrogen embrittlement due to hydrogen penetrating into the steel from the external environment.

For this reason, the strength level of the above-mentioned mechanical structural steel is 125 kgf/mm2 in terms of tensile strength.
Currently, high-strength bolts are limited to the following, for example, JIS-B-1186 (1979
F8T (Tensile strength: 80-100
kgf/mm2), FIOT (100~120k
gf/mm2), and FIIT (110 to 130
kgf/mm2), and there are precautions to avoid using FIIT as much as possible.

Further, even in steel plates for civil engineering and construction machinery that require wear resistance, those with a tensile strength exceeding 125 kgf/mm2 have a problem of delayed fracture during use.

On the other hand, for example, 18%Ni-7,5%Co-
There is an 18%Ni maraging steel with a composition of 5%Mo-0.5%Ti-0.1%AI, and this steel can withstand tensile strength of around 150kgf/mm2 without fear of delayed fracture. Although it can be used, since it is an extremely expensive steel, it has only been put to practical use in a few extremely limited applications due to economic efficiency, and it has never been widely used for machine structures. do not have.

On the other hand, as a structural steel that is economical, has high strength, and has excellent delay resistance, for example, JP-A-58-61219, JP-A-58-84960, JP-A-58-11331,
No. 7, JP-A-58-117856 and JP-A-58-1
No. 57921 and other publications propose high-strength steels with various components and methods for producing them.

However, even these steels with a tensile strength exceeding 125 kgf/mm2 cannot completely eliminate the risk of delayed fracture to the extent that they can be used, for example, in high-tensile bolts for bridges, and their range of application is uncertain and It's not enough.

Problems to be Solved by the Invention In order to meet the above-mentioned demands of the industrial world, the present invention
It is an object of the present invention to provide a method for manufacturing steel for mechanical structures having a tensile strength of gf/mm2 or more and excellent delayed fracture resistance.

To explain the purpose of the present invention in more detail, for example, unlike high-tensile bolts for bridges, etc., 125 kgf/ It is an object of the present invention to provide a method for producing mechanical structural steel having a tensile strength of mm2 or more. Such uses include high-tensile steel for various structures, steel for bolts for automobiles, civil engineering machinery, and industrial machinery, and high-tensile steel plates, and by using the steel manufactured by the present invention in these, It is possible to meet the demands of the industry mentioned above.

In other words, the present invention provides that high-tensile bolts for bridges do not have any delayed fracture resistance, but there is no risk of delayed fracture occurring for a predetermined period of time, and therefore, they can be suitably used for parts that require periodic repair or replacement. It is an object of the present invention to provide a method for manufacturing steel for machine structural use having a tensile strength of 125 kgf/mm2 or more.

Means for Solving the Problems In order to achieve the above-mentioned object of the present invention, the inventors of the present invention have conducted extensive experiments and research, and have found that no delayed fracture occurs over a period of 5,000 hours or more, and a tensile strength of 125 kgf/mm2 or more is achieved. To produce steel with strength, low P, low S
In addition to reducing grain boundary segregation and cleaning by reducing Mn, it also improves delayed fracture resistance by reducing Mn, and furthermore, by rapidly cooling from a temperature of 870°C or higher after hot rolling during manufacturing, impurity elements such as P are reduced. It was discovered that it is effective to reduce the segregation of austenite to austenite grain boundaries.

Therefore, according to the present invention, C: 0.18-0.35%, Si: 0.5% or less, Mn: less than 0.5%, P: 0.01% or less, S: 0.01% or less, Cr : 0.01 to 5%, with the remainder consisting of Fe and unavoidable impurities. After hot rolling, the steel is quenched at a temperature of 870°C or higher,
Provided is a method for producing a mechanical structural steel having a tensile strength of 125 kgf7 mm or more and excellent delayed fracture resistance, which is then subjected to low-temperature tempering at a temperature in the range of 150 to 600°C. .

Further, according to the present invention, C: O, 18 to 0.35%, Si: 0.5% or less, Mn: less than 0.5%, P: 0.01% or less, S: 0.01% or less, Contains Cr: 0.01 to 5%, Nb: 0.005 to 0.1%, and further contains V: OJ% or less, B: 0.005% or less, Ti: 0.05% or less, and! , lo: 0.5% or less, and the remainder is Fe and unavoidable impurities. 125 kg characterized by low temperature tempering at a temperature within the range of 600°C
Provided is a method for manufacturing steel for mechanical structures that has a tensile strength of f/mm2 or more and has excellent delayed fracture resistance.

Further, according to a preferred embodiment of the present invention, hot rolling is carried out at 870 mm.
It is preferable to carry out quenching at a finishing temperature of 870° C. or higher and to directly quench the rolled material without cooling it to 870° C. or lower.

Furthermore, according to another aspect of the present invention, the rolled material after quenching is subjected to low-temperature tempering at a temperature within the range of 150 to 600°C.

In this specification, when steel components are expressed in percentages, they are all percentages by weight.

Next, the reason why the component composition and manufacturing conditions employed in the method of the present invention are limited as described above will be explained.

(A) Composition arc: C has the effect of imparting strength to steel, but its content is 0.
．． If it is less than 18%, the desired strength cannot be secured;
On the other hand, if the content exceeds 0.35%, the toughness will deteriorate in relation to other alloy components, so the content was set at 0.18 to 0.35%.

b) Si: Si is an element necessary for deoxidizing steel, but if its content exceeds 0.5%, the steel will become significantly brittle.
The upper limit was set at 0.5%.

C) Mn: Mn is an effective element for deoxidizing and improving hardenability, but when added in large amounts, Mn oxides or carbides are formed at grain boundaries, causing grain boundary embrittlement. Promotes the occurrence of delayed fracture. Furthermore, since Mn combines with S and becomes a starting point for cracking, its content must be reduced as much as possible in order to improve delayed fracture resistance. Therefore, in the present invention, which aims to improve delayed fracture resistance, the Mn content is reduced to 0.
．． It was set to less than 5%. In this way, the Mn content is limited,
By adjusting other alloy components and heat treatment conditions, it is possible to manufacture a mechanical structural steel having a tensile strength of 125 kgf/mm" or more and excellent delayed fracture resistance.

d) P: P cannot completely eliminate its grain boundary segregation no matter what heat treatment is applied, and also reduces grain boundary strength and deteriorates delayed fracture resistance, so the upper limit is set at 0.01%. And so.

e) S: As mentioned above, S combines with Mn and becomes the starting point of cracking.
Furthermore, even if it is used alone, it segregates at grain boundaries and promotes embrittlement, so it is necessary to limit its content as low as possible. Therefore, in the present invention, S is set to 0.01% or less.

f) Cr: Cr has the effect of improving the hardenability of steel and imparting temper softening resistance to the steel, but if its content is less than 0.01%, the desired effect cannot be obtained. On the other hand, since Cr is an expensive alloying element, its content was reduced to 0 in consideration of economic efficiency.
．． 01 to 5%.

g) Nb: When added to clean steel such as the steel of the present invention, Nb significantly improves delayed fracture resistance. In order to ensure this effect, it is necessary to add 0.005% or more. On the other hand, if more than 0.1% is added, the effect will be saturated and the cost will be high, so the range should be limited to 0.005 to 0.1
%.

h) V: (1) has the effect of making the steel grain finer and further improving the strength of the steel by precipitation hardening, so it is added as necessary when higher strength is required, but 0.3% is added. If the content exceeds 0.3%, the effect of addition will be saturated and further strength-improving hardening cannot be obtained, so 0.3% was set as the upper limit.

1) BSTi and Mo: These components have the effect of further improving the hardenability of steel, so they are added as necessary to ensure high strength, especially when the steel size is large. If Ti is contained in excess of 0.003% B and 0.05% Ti, the toughness of the steel will deteriorate, and the machinability of Ti will also deteriorate. B: 0.003% or less, Tl: 0.05% or less. Furthermore, even if MO is added in excess of 0.5%,
Since the effect reaches saturation and only increases costs, the upper limit was set at 0.5%.

(B) Heat treatment conditions a) Quenching temperature Tough steel with a tensile strength exceeding 125 kgf/mm2 is a hot rolled steel bar or hot rolled steel plate of ordinary low alloy steel that is reheated to Ac3 point or higher and then quenched. , produced by tempering at temperatures below the point. C content is 0.18~
For 0.35% steel, quenching is performed at 870°C or higher, so the quenching temperature was set at 870°C or higher.

In addition, there are two methods of quenching: direct quenching after rolling and reheating after rolling, both of which are effective, but the former results in less segregation of P, etc. at the γ grain boundaries.
more effective.

b) Tempering temperature Generally, as-quenched steel has a low yield point, and when used as mechanical structural steel, stress relaxation increases during use, and furthermore, as-quenched steel has poor toughness, workability, etc. There is. Therefore, in order to impart predetermined strength and toughness to steel, it is necessary to perform a tempering treatment after quenching. Generally, tempering of steel is carried out at a temperature below the Ac point, but generally in a temperature range of 150 to 600°C. However, in the range of 300 to 400°C, low-temperature tempering tends to cause embrittlement and deteriorate delayed fracture resistance, so it is better to avoid tempering in this range. On the other hand, care should be taken to ensure that the tempering temperature for obtaining the required strength does not fall within this range.

EXAMPLES Next, the present invention will be explained by examples in comparison with comparative examples.

Steel having the chemical composition shown in Table 1 is melted using a normal melting method, formed into a billet with dimensions of 500 mm in diameter x 1 m in length, and then the billet is soaked at 1200°C for 1 hour, and the finishing temperature is Hot rolling was carried out at a temperature of 870° C. or higher to produce a steel bar of 25 mmφ.

In order to obtain a strength of 125 kgf/mm2 or more, the heat treatment used was a direct quenching method in which quenching is performed immediately after hot rolling, and a conventional method in which quenching is performed after reheating to a temperature of 870° C. or higher. In addition, the tempering temperature is such that the tensile strength is 125 kgf.
/mm2 was confirmed in a preliminary experiment, and each was selected.

On the other hand, the presence or absence of delayed fracture was confirmed by the wedge insertion type delayed fracture test method shown in FIG.

That is, the notch part (shown in Fig. 1 et al.) of the test piece having the shape and dimensions as shown in Fig. 1rIIJ (a) was
) A static load was applied by inserting a wedge as shown in (), and the wedge was placed in hot water maintained at 55° C., and the time required for cracking to occur was observed.

The reason why 5,000 hours was used as one of the criteria for determining delayed fracture resistance was because it was assumed that the equipment would be regularly repaired or inspected for three months, and an error of approximately twice that was estimated. As a test environment, 55°C warm water corresponds to the most severe environment in actual use. Therefore, the obtained delayed fracture time is considered to correspond to the delayed fracture occurrence time in the harshest environment in actual use.

Steel Nos. 1 to 7 in Table 1 are the steels of the present invention, and Steel No. 8
~lO is the comparison steel. As can be seen from the test results shown in Table 1, good delayed fracture resistance was demonstrated in N011 steel with low Mn and low P, and from this result, the significance of lowering Mn and P was understood. can. Furthermore, steels No. 2 and No. 2 with added Nb have the same delayed fracture resistance even if the strength is further increased.

However, the tensile strength of N011 steel is 149.5 kgf/
In the case of mm2, the delayed fracture occurrence time is 3500 hours, whereas the tensile strength of Nb-added steel (N[L2 steel) is 151.5 hours.
kgf/mm", the delayed fracture occurrence time was more than 5000 hours, indicating the effect of improving delayed fracture resistance by adding Nb.

On the other hand, in steel with Nα1, the delayed fracture occurrence time is 35
00 hours is short when the tensile strength is set to a high strength level of 145 kgf/mm2 or more. For a steel composition of Nα1, delayed fracture resistance of 5000 hours or more, which is the objective of the present invention, can be reliably achieved when the tensile strength is 125 kgf/mm2, or at most less than 140 kgf/n+n+2.

Even if C is increased to 0.33% like the steel No. 3, or T1 like the steel No. 3.4.6.7.
It can be seen that the delayed fracture resistance does not deteriorate even when B is added, or Mo is added as in steel No. 4, or V is added as in steel No. 5. In this way, low Mn and low P are important points for improving delayed fracture resistance, and N
It can be seen that addition of b is even more effective.

On the other hand, the comparison steel No. 3 has a high Mn content of 1%, and the No. 9 steel has a high C content of 0.38%.
Steel No. 10 has a high C content of 0.39% and a P content of 0.030.
It is presumed that the delayed fracture resistance deteriorated due to the high %.

Effects of the invention From the above examples, C: 0.18 to 0.35%, Si: 0
．． 5% or less, Mn: less than 0.5%, P: 0.01 or less,
Steel containing 0.01 to 5% Cr and the remainder consisting of Fe and unavoidable impurities, or steel containing 0.0% Nb in the above composition system.
05 to 0.1%, further V: 0.3% or less, B
: 0,005% or less, and one or more of Ti: 0.05% or less and Mo: C 1,5% or less, with the balance consisting of Fe and inevitable impurities,
After hot rolling, it is rapidly cooled from a finishing temperature of 870°C or higher to 150°C.
Perform low temperature tempering treatment within the temperature range of ~600℃, or perform reheating and quenching treatment at a temperature of 870℃ or higher after hot rolling, and perform low temperature tempering treatment within the temperature range of 150~600℃. It has been found that by doing so, it is possible to produce a mechanical structural steel having a tensile strength of 125 kgf/mm2 or more and excellent delayed fracture resistance.

That is, according to the method of the present invention, 125 kgf/mm
It is possible to use mechanical structural steel that has a tensile strength of 5,000 hours or more and does not cause delayed fracture for a period of 5,000 hours or more. The mechanical structural steel produced by the method of the present invention can be used in a wide range of steel materials for applications where the degree of fracture resistance is clear.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the shapes and dimensions of the test piece and wedge used in the delayed fracture test conducted in this example. Figure 1 (
Figure 1 (a) shows a test piece, Figure 1 et al.) shows details of the notch part of the test piece, and Figure 1 (C) shows a wedge to be inserted into the notch part of the test piece to apply a load. Note that in the figures, numbers indicate lengths in mm.

Claims (8)

[Claims] (1) C: 0.18-0.35%, Si: 0.5% or less, Mn: less than 0.5%, P: 0.01% or less, S: 0.01% or less, Cr: 0. After hot rolling a steel containing 01 to 5%, with the remainder consisting of Fe and unavoidable impurities, quenching is performed at a temperature of 870°C or higher,
A method for manufacturing a mechanical structural steel having a tensile strength of 125 kgf/mm^2 or more and excellent delayed fracture resistance, which is then subjected to low-temperature tempering at a temperature in the range of 150 to 600°C. (2) The above hot rolling is carried out at a finishing temperature of 870°C or higher,
2. The method for producing steel for machine structures according to claim 1, wherein the rolled material is directly quenched without being cooled to 870° C. or lower. (3) The method for manufacturing steel for machine structures according to claim 1, characterized in that after the hot rolling, the rolled material is reheated to a temperature of 870° C. or higher and quenched. (4) Claims 1 to 3, characterized in that the low-temperature tempering is performed at a temperature within the range of 150 to 300°C.
A method for manufacturing steel for machine structural use according to any of paragraphs. (5) C: 0.18-0.35%, Si: 0.5% or less, Mn: less than 0.5%, P: 0.01% or less, S: 0.01% or less, Cr: 0. 01 to 5%, Nb: 0.005 to 0.1%, and further contains V: 0.3% or less, B: 0.005% or less, Ti: 0.05% or less, and Mo: 0.5. % or less, with the remainder consisting of Fe and unavoidable impurities. After hot rolling, the steel is quenched at a temperature of 870°C or higher, and then at a temperature within the range of 150 to 600°C. 125kgf/
A method for producing mechanical structural steel having a tensile strength of mm^2 or more and excellent delayed fracture resistance. (6) Performing the above hot rolling at a finishing temperature of 870°C or higher,
6. The method for producing steel for machine structures according to claim 5, wherein the rolled material is directly quenched without being cooled to 870° C. or lower. (7) The method for manufacturing steel for machine structures according to claim 5, characterized in that after the hot rolling, the rolled material is reheated to a temperature of 870° C. or higher and quenched. (8) Claims 5 to 7, characterized in that the low-temperature tempering is performed at a temperature within the range of 150 to 600°C.
A method for manufacturing steel for machine structural use according to any of paragraphs.