JP3343505B2 - High strength bolt steel with excellent cold workability and delayed fracture resistance and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for bolts used for automobiles and various industrial machines, and a method for producing the same. In particular, the present invention has excellent cold workability after hot working. The present invention relates to a bolt steel that provides a high-strength bolt exhibiting a tensile strength of 1000 to 1500 N / mm ² and a high level of delayed fracture resistance by quenching and tempering, and a method for producing the same.

[0002]

^2. Description of the Related Art In general high-strength bolt steel, when the tensile strength exceeds a level of 1000 N / mm ² , delayed fracture tends to occur. Therefore, as a high-strength bolt steel exceeding such a strength level, a medium-carbon steel (for example, SC) having excellent corrosion resistance and capable of being tempered at a relatively high temperature is used.
M435, SCM440, SCr440, etc.).

However, since these steel materials contain a large amount of C and alloying elements, they have high hardenability, and the tensile strength after rolling becomes too high, so that it is difficult to form bolts as they are. For this purpose, the rolled material is first annealed, then subjected to intermediate wire drawing, spheroidizing annealing, and finish wire drawing, then formed into a bolt by cold forging, and finally quenched to a predetermined strength. -An extremely complicated process of tempering was required.

[0004] In view of the above, the bolts manufactured using the above-mentioned medium-carbon low-alloy steel have a total processing cost up to processing into a wire rod in addition to the material cost. And there was a problem that it became considerably expensive compared with the bolt which used normal carbon steel.

[0005] Delayed fracture occurs in a non-corrosive environment.
Some occur in corrosive environments, and the causes are many.
Are intricately intertwined and affect each other.
It is difficult to identify these factors. Improved delayed fracture resistance
Control factors to improve include tempering temperature, microstructure,
Material hardness, grain size, type and content of various alloying elements, etc.
Involvement is recognized for some time, but as mentioned above,
Is not specified.
Although not established, various methods have been tried and tested.
The fact is that it has only been proposed. Also, JP
JP-A-60-114551, JP-A-2-267243,
No. 3-243745, the types of main alloy elements and
By adjusting the content etc., the tensile strength is 140 kg
f / mm ^Two Excellent delayed fracture resistance even at levels above
The disclosed high strength bolt steel is disclosed. However, this
Even with these known techniques, the risk of delayed
Not wiped, their scope was limited
I have.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to reduce the tensile strength of a rolled wire rod while maintaining a high level of steel for bolts. An object of the present invention is to provide a bolt steel having a tensile strength and an enhanced delayed fracture resistance and a method for producing the same.

[0007]

The high-strength bolt steel according to the present invention, which can solve the above-mentioned problems, comprises: C: 0.20% to 0.30% (meaning% by weight, hereinafter the same); Si: 0.10% or less (including 0%), Mn: 0.30% to 0.85%, P: 0.010% or less (including 0%), S: 0.010% or less (including 0%) Cr: 0.30% to 1.50%, Mo: 0.05% to 1.00%, Al: 0.01% to 0.05%, N: 0.010% or less (including 0%), One or more of Ti, Nb, and V: 0.01% to 0.3 in total amount
%, Or B: 0.
003% (not including 0%), the carbon equivalent (C _eq ) represented by the following formula [1] is 75 or less, and the balance substantially consists of Fe and unavoidable impurities. It is characterized by simultaneously improving cold workability and delayed fracture resistance.

C _eq = (C + 1/7 · Si + 1/5 · Mn + 1/9 · Cr + 1/2 · Mo) × 100 [1] Further, the production method of the present invention uses a steel material satisfying the above chemical composition. After hot rolling or hot forging the steel material so that the finish rolling temperature is 750 ° C. or higher,
It is characterized in that it is cooled at a rate of not more than ° C / sec and then left to cool to room temperature, or is further reheated to 680 to 740 ° C and then is simply annealed.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Under the above-mentioned problems, the present inventors have a high level of tensile strength as a steel for bolts while suppressing the tensile strength as a rolled wire rod, and have a high resistance to delayed fracture. Various studies were made on chemical components and production conditions that can improve the properties. As a result, if the chemical composition of the steel material is appropriately adjusted as described above and the cooling rate after rolling and forging is properly controlled, the delayed fracture resistance can be improved without increasing the deformation resistance during cold working. And found the above-mentioned present invention.

[0010] In particular, if the C content is appropriately controlled in order to secure the cold forging property without deteriorating the hardenability, and if the C content is kept smaller than before, it is possible to improve both toughness and delayed fracture resistance. Knew. Further, conventionally, it has been considered that tempering at a high temperature is effective for improving delayed fracture resistance. However, while containing appropriate amounts of Cr and Mo in steel,
If one or more of i, Nb, and V are contained in an appropriate amount and the contents of Mn, P, and S, which are grain boundary embrittlement elements, are reduced, delayed fracture resistance can be obtained even when tempering at a low temperature is employed. Was found to be able to improve.

Hereinafter, the reasons for determining the chemical composition of the steel material in the present invention will be clarified.

C: 0.20 to 0.30% C is an element essential for improving the hardenability of steel and ensuring high strength, and at least 0.20 C for effectively exhibiting these effects. %, More preferably 0.23% or more, even more preferably 0.24% or more. However, if the amount is too large, not only the toughness is deteriorated, the delayed fracture resistance is deteriorated, but also the cold workability is deteriorated, and the simplification or omission of the soft annealing step cannot be achieved. And more preferably 0.28% or less, more preferably 0.26% or less.

Si: 0.10% or less (including 0%) Si effectively acts as a deacidifying element, but as its content increases, the cold workability decreases, and at the time of heat treatment such as quenching. Since it promotes grain boundary oxidation and deteriorates delayed fracture resistance, the Si content is 0.10% or less.
More preferably 0.05% or less, further preferably 0.0%
Should be kept below 3%.

Mn: 0.30 to 0.85% Mn is an element for improving hardenability, and when a proper amount of Mn is contained, it is easy to secure high strength. In order to effectively exhibit such effects, the content must be at least 0.30% or more, preferably 0.40% or more, and more preferably 0.45% or more. However, if too much, the transformation of the structure at the time of cooling after rolling is promoted, and the cold workability deteriorates, and the simplification or omission of the soft annealing process cannot be achieved. Therefore, the content must be suppressed to 0.85% or less. It is more preferable that the content be suppressed to 0.75% or less.

P: 0.010% or less (including 0%) Since P causes grain boundary segregation and deteriorates delayed fracture resistance, it must be suppressed to 0.010% or less, and more preferably 0% or less. 0.007% or less, more preferably 0.005%
% Or less.

S: 0.010% or less (including 0%) MnS is generated in steel, and when stress is applied, it becomes a stress concentration point and causes an increase in delayed fracture.
It must be suppressed to 0.010% or less, more preferably 0.007% or less, and still more preferably 0.005% or less.

Cr: 0.30 to 1.50% Cr does not significantly reduce the deformability during cold working, contributes to an increase in hardenability and contributes to an increase in strength, and furthermore, to a delayed fracture due to an improvement in corrosion resistance. It also has the effect of enhancing the properties, and in order to exhibit these effects effectively, it should be contained at least 0.30%, preferably 0.70%
%, More preferably 0.85% or more. However, if it is too large, it will stabilize the carbides and adversely affect the cold workability, so it must be suppressed to 1.50% or less, more preferably 1.10% or less.
More preferably, the content is controlled to 1.00% or less.

Mo: 0.05 to 1.00% Mo is a useful element for improving delayed fracture resistance by quenching property and grain boundary strengthening action, and its effect is 0.05% or more, more preferably 0%. Effectively exhibited by containing at least .15%, more preferably at least 0.20%. However, if the content is too large, fine carbides are precipitated and the cold workability is impaired. Therefore, the content must be suppressed to 1.00% or less, more preferably 0.8% or less, and still more preferably 0.6% or less. Must.

Al: 0.01 to 0.05% Al captures N in steel to form AlN and refines crystal grains, thereby contributing to an improvement in delayed fracture resistance. Such an effect is 0.01% or more, more preferably 0.0% or more.
Effectively exhibited by containing 25% or more.
However, if the content is too large, the delayed fracture resistance is deteriorated due to the formation of oxide-based inclusions. Therefore, the content must be suppressed to 0.05% or less, more preferably 0.35% or less.

N: 0.010% or less (including 0%) N combines with Al and Ti to form AlN and TiN,
The crystal grains are refined to contribute to the improvement of delayed fracture resistance,
If it is too large, it cannot be captured by Al or Ti, and the delayed fracture resistance deteriorates due to an increase in the amount of solute N. Therefore, in order to effectively exhibit the above-mentioned effect due to the formation of AlN or TiN without causing any trouble due to solid solution N, N is preferably 0.002% or more, more preferably 0.0035% or more and 0.010% or more. % Or less, more preferably 0.007%
%, More preferably 0.005% or less.

One or more of Ti, Nb and V: 0.0 in total
1 to 0.3% These elements combine with N and C in steel to form carbon / nitride and contribute to improvement in delayed fracture resistance. In addition, the generated nitrides and carbides also have a crystal grain refining effect,
This also contributes to the improvement in delayed fracture resistance. In order to effectively exert such effects, the content should be 0.01% or more, more preferably 0.02% or more, and still more preferably 0.03% or more. However, if the content of these elements is too large, Not only does the amount of carbon and nitride become too large, which not only impairs delayed fracture, but also adversely affects toughness, so the total sum of these elements must be suppressed to 0.3% or less, which is more preferable. Is preferably suppressed to 0.15% or less.

The essential constituent elements of the steel for bolts according to the present invention are as described above, and the balance is substantially Fe, but inevitable impurities can be mixed in a range not to impair the effects of the present invention. If necessary, it is also effective to improve the performance by adding an appropriate amount of B as shown below.

B: 0.003% or less (excluding 0%) B is an effective element for improving the hardenability of steel and enhancing the strength, and such an effect is exhibited by adding a very small amount. However, in order to significantly exhibit the effect of the addition on a practical scale, it is desirable to contain 0.0005% or more. However, if the B content is too large, a significant adverse effect on toughness appears, so that the content is 0.003% or less, more preferably 0.002% or less.
It must be kept below 5%.

The constituent elements of the steel for bolts according to the present invention are as described above. In addition to the content of these constituent elements, in order to ensure excellent cold workability as a rolled material and a simple annealed material. It is important that the C equivalent (C _eq ) represented by the formula [1] be suppressed to 75 or less.

That is, as the C _eq value increases, the strength of the rolled material increases, and the cold workability deteriorates.
If it exceeds 5, the strength after rolling cannot be sufficiently reduced even if the cooling rate after rolling is reduced and simple annealing treatment is performed. Therefore, in order to suppress the strength as a rolled material to a low level and to ensure excellent cold workability, the C _eq value is 75 or less,
It is necessary to adjust the content of each of the above elements so as to be more preferably 65 or less, and still more preferably 60 or less. In the case where the C _eq value is in the range of more than 60 to 75 or less, it is desirable to increase the cold workability by performing simple annealing after rolling, but the steel material having the C _eq value adjusted to 60 or less is as-rolled. Excellent workability even in the state.

Although the steel for bolts of the present invention satisfies the above-mentioned chemical composition, in order to obtain a high-strength steel for bolts having both excellent cold workability and delayed fracture resistance by using this steel, When hot rolling or hot forging is performed using the above steel material, the finish rolling temperature is controlled to be 750 ° C. or more, and the temperature from the finish rolling temperature to 600 ° C. is 1.0 ° C./s.
It is preferable to perform cooling at a speed of ec or less.

The reason why the finish rolling temperature is set to 750 ° C. or higher is that if the temperature is lower than this, the working resistance increases, and it becomes difficult to roll or forge into an appropriate shape. A more preferable finish rolling temperature for performing rolling or forging more smoothly is 800 ° C. or more. The reason why the cooling rate from the finish rolling temperature to 600 ° C. is set to 1.0 ° C./sec or less is to enhance the cold workability by changing the metal structure of the finished rolled steel material to a mixed structure of ferrite and pearlite. Speed is 1.0 ° C / s
If it exceeds ec, bainite or martensite structure appears partially, and the cold workability deteriorates. More preferable cooling rate for obtaining excellent cold workability is 0.5
° C / sec or less, more preferably 0.2 ° C / sec or less, and even more preferably 0.1 ° C / sec or less.

In the case of the batch method, the cooling rate may be adjusted by slow cooling by furnace cooling. However, since the productivity is greatly reduced by prolonging the cooling time, it is preferable to use hot rolling or hot rolling. It is desirable to adopt a method in which the continuous rolling from the forging to the finish rolling is performed, and the cooling zone after the finish rolling is lengthened and gradually cooled.

Since the cooling rate from 600 ° C. to room temperature is not particularly limited, normal cooling may be performed.

The steel material used has a C _eq value of 6
0 or less shows excellent cold-forging property in the state of being left to cool, but when a steel material having a C _eq value in the range of more than 60 to 75 or less is used, the steel is slightly cold-worked while being left to cool. In this case, after cooling, 680 to 740
It is better to perform a simple annealing treatment by reheating to ℃.
Thereby, steel for bolts having excellent cold forging properties can be obtained. In the ordinary softening treatment, after holding at a temperature of 740 ° C. or more for a long time, slow cooling also requires a long time,
The simple annealing employed in the present invention requires only short-time heating to a relatively low temperature as described above, so that the heat treatment can be performed very easily.

The bolt steel of the present invention thus obtained is
Deformation resistance at the time of cold working is low as it is, showing excellent workability, and after working into a bolt shape, for example, 840
Quenching after heating to a temperature of ~ 900 ° C, then 35
By performing a tempering treatment at a temperature of about 0 to 550 ° C., a high-strength bolt having a high tensile strength of 1200 N / mm ² level or more and having a high delayed fracture resistance exceeding 2000 N / mm ² level can be obtained. It is now possible to provide steel for bolts to be given.

[0032]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and may be appropriately performed within a range that can conform to the purpose of the preceding and following descriptions. Of course, the present invention can be embodied with modifications, and all of them are included in the technical scope of the present invention.

Example A test steel having the chemical composition (% by mass) shown in Tables 1 and 2 below was used, and hot-rolled to a diameter of 11 mm under the conditions shown in Table 2, and then a part was subjected to simple annealing. And the diameter 1
Drawing was performed to 0.4 mm (drawing ratio: about 10.6%).

The cold workability (deformation resistance) of each of the obtained drawn wires was examined, and the tensile strength of each drawn wire was set to 120.
After quenching and tempering so as to be 0 to 1500 N / mm ² , a delayed fracture test piece having the dimensions and shape shown in FIG. 1 was prepared, and the delayed fracture characteristics were examined.

At this time, the cold workability was evaluated by the deformation resistance at the time of a rolling reduction of 70% by adopting the end face constraint compression test method.
HCl), washed with water and dried, and then applied with a load in the air, and evaluated by the delayed fracture strength after 100 hours. Table 3 shows the results of the evaluation of the cold workability, and Table 4 shows the results of the evaluation of the delayed fracture.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

The test materials A to L are examples in which the conditions of the test steel composition, the finishing temperature, the cooling rate and the like all satisfy the specified requirements of the present invention, and the chemical components of the steel material are all specified including the carbon equivalent. Since it satisfies the requirements, and the finishing rolling temperature and the cooling rate to 600 ° C. thereafter also satisfies the preferable requirements, it has low deformation resistance, excellent workability, and tensile strength after quenching and tempering. Excellent delayed fracture resistance.

On the other hand, in the test materials M to AK, since the chemical composition of the steel material or the cooling conditions after hot rolling were out of the specified requirements as described below, the cold workability and the mechanical properties after quenching and tempering were One of the mechanical properties (especially delayed fracture resistance) is insufficient.

Specimen M: The chemical composition of the steel material is appropriate, but the cooling rate after hot rolling is too high, so that the deformation resistance during cold working is large and the workability is lacking.

Specimen N: The chemical composition of the steel is appropriate, but the cooling rate after hot rolling is too fast, so that even if simple annealing is performed after hot rolling, the deformation resistance does not sufficiently decrease. , Lacks workability.

Specimen O: Since the C content of the steel is insufficient, the deformation resistance is low and the cold workability is good, but the tensile strength after tempering is insufficient and the delayed fracture resistance is poor.

Specimen P: Since the C content of the steel material is too high and the carbon equivalent is too high, the deformation resistance is high, the cold workability is poor, and the delayed fracture resistance is insufficient.

Specimen Q: Since the Si content of the steel exceeds the specified range, the deformation resistance during cold working is not sufficiently reduced.

Specimen R: The delayed fracture resistance is poor because the Mn content of the steel is insufficient.

Test material S: Since the Mn content of the steel material is too high, the deformation resistance during cold working is high, and the mechanical properties after tempering are not sufficient.

Specimens T, U: The steel material lacks the delayed fracture resistance due to excessive P or S content.

Specimen V: Since the Cr content of the steel is insufficient, the tensile strength after tempering and the delayed fracture resistance are insufficient.

Specimens W, X: Since the Cr or Mo content of the steel material is too large and the carbon equivalent is too high, the deformation resistance during cold working is high.

Specimens Y, Z: Since the Al or N content of the steel material is too large, the delayed fracture resistance is poor.

Test material AA: Since the steel material does not substantially contain any of Ti, Nb and V, the delayed fracture resistance is insufficient.

[0054]

Specimen AB: Since the C, Si, and P contents of the steel material exceeded the specified requirements and did not substantially contain any of Ti, Nb and V, a simple annealing was performed. However, the deformation resistance during cold working is not sufficiently reduced, and the delayed fracture resistance is also poor.

Specimen AC: The C, Si, P, S contents of the steel exceeded the specified requirements, the carbon equivalent was high, and T
Since i, Nb, and V are not substantially contained, deformation resistance during cold working is not sufficiently reduced even when simple annealing is performed, and delayed fracture resistance is poor.

Specimen AD: Since the C and Si contents of the steel material exceeded the specified requirements and the carbon equivalent was high, the deformation resistance during cold working was not sufficiently reduced even after simple annealing. In addition, the delayed fracture resistance is poor.

Test material AE: The content of Si, P, S, and N in the steel material exceeds the specified requirements, and Ti, Nb, and V are not substantially contained. Therefore, after the quenching and tempering, Both the tensile strength and the delayed fracture resistance are insufficient.

Specimen AF: Steel material has a high Mn content and does not contain Mo. Deformation resistance during cold working is reduced by simple annealing, but tensile strength after quenching and tempering. And the delayed fracture resistance are all insufficient.

Test materials AG, AH, AI: All were Mo in steel
, The tensile strength after quenching and tempering and the delayed fracture resistance are all low.

Specimens AJ, AK: The specimens satisfy the requirements for the elements contained in each steel, but have too high carbon equivalents.
Even when simple annealing is performed, the deformation resistance during cold working is not sufficiently reduced, and workability is poor.

[0062]

The present invention is constituted as described above. By defining the elements contained in the steel material and specifying the carbon equivalent, it exhibits low deformation resistance during cold working and excellent workability. Further, it is possible to provide a high-strength bolt steel having a high level of tensile properties after quenching and tempering and exhibiting delayed fracture resistance of 2000 N / mm ² level or more. Also, by using this steel material, by suppressing the cooling rate after hot rolling or hot forging as low as possible,
High-strength steel for bolts having a high level of mechanical properties under excellent cold workability can be reliably obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing dimensions and shapes of a test piece for evaluating delayed fracture property adopted in Examples.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Nobuhiko Ibaraki 2 Nadahama-Higashi-cho, Nada-ku, Kobe Kobe Steel Works, Ltd. Inside Kobe Works (56) References JP-A-4-263047 (JP, A) JP-A Heihei 8-60245 (JP, A) JP-A-5-255738 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 C22C 38/28 C21D 8/06

Claims (4)

(57) [Claims] 1. C: 0.20% to 0.30% (meaning% by weight, the same applies hereinafter), Si: 0.10% or less (including 0%), Mn: 0.30% to 0.85 % , P: 0.010% or less (including 0%), S: 0.010% or less (including 0%), Cr: 0.30% to 1.50%, Mo: 0.05% to 1.00 %, Al: 0.01% to 0.05%, N: 0.010% or less (including 0%), one or more of Ti, Nb, V: 0.01% to 0.3 in total amount
%, And the carbon equivalent (C
_eq ) is 75 or less, and the balance substantially consists of Fe and unavoidable impurities. A high-strength bolt steel excellent in cold workability and delayed fracture resistance. C _eq = (C + ／ · Si + ／ · Mn + ／ · 9 · C
(r + ／ · Mo) × 100 2. B: 0.003% as another component
The steel for high-strength bolts according to claim 1, comprising the following (not including 0%). 3. A steel material satisfying the chemical composition according to claim 1 or 2, wherein the steel material has a finish rolling temperature of 750 ° C.
After hot rolling or hot forging as described above, 600
A method for producing a high-strength bolt steel excellent in cold workability and delayed fracture resistance, characterized in that the steel is cooled to 1.0 ° C at a rate of 1.0 ° C / sec or less and subsequently cooled to room temperature. 4. A steel material satisfying the chemical composition according to claim 1 or 2, wherein the steel material has a finish rolling temperature of 750 ° C.
After hot rolling or hot forging as described above, 600
C. at a rate of 1.0 ° C./sec or less, subsequently allowed to cool to room temperature, and then re-heated to 680-740 ° C., followed by simple annealing. A method for producing high-strength bolt steel with excellent delayed fracture resistance.