JP2006274373A - Steel for high strength screw having excellent toughness and cold workability and method for producing high strength screw - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a high strength screw, and to provide a method for producing a high strength screw using the same. <P>SOLUTION: The steel for a high strength screw has a composition comprising, by mass, 0.07 to 0.15% C, ≤0.2% Si, 0.5 to 2% Mn, ≤0.015% P, ≤0.015% S, ≤2% (excluding zero) Cr, 0.005 to 0.08% Al and ≤0.01% N, and, if required, comprising at least one kind selected from ≤3.5% Ni, ≤1% Cu, ≤0.3% Mo and 0.0005 to 0.005% B, and/or at least one kind selected from 0.005 to 0.05% Ti and 0.005 to 0.05% Nb, and in which carbon equivalent: Ceq=C+Si/7+Mn/6+Cr/9 is ≤0.50%, and the balance iron with inevitable impurities. In its production method, the steel having the above componential composition is hot-rolled, so as to be a screw stock with a prescribed diameter, and the screw stock is cold-forged, is thereafter formed into a screw shape, and is subjected to quenching/tempering treatment. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel for high-strength screws and a method for producing a high-strength screw using the same, and is particularly subjected to cold forging and quenching and tempering treatment, and has a strength of 800 MPa or more and excellent toughness and cold workability. About things.

  Bolts and small screws specified in JIS-B-1051 with mechanical strength of 8.8 or higher are carbon steels such as S35C to S45C, boron with boron added to S25C to S35C Manufactured using steel, alloy steel such as SMn443, SCM435.

  For example, in SCM435, hot-rolled wire → pickling → spheroidizing annealing → pickling / coating → finish wire drawing or hot-rolling wire → pickling → spheroidizing annealing → pickling / coating → drawing → spheroidizing annealing → Pickling / coating → Finish drawing, forming the head by cold forging, screw rolling, quenching and tempering, and adjusting to a predetermined strength category.

  At that time, in order to avoid the tempering brittle region, the tempering temperature is generally 400 ° C. or higher for the alloy steel, and the minimum tempering temperature of the alloy steel is 425 ° C. in JIS-B-1051.

  Therefore, in order to ensure high strength and toughness, not only will the amount of carbon increase and the use of materials with various alloying elements added will increase the material cost, but such materials When quenching and tempering is performed, the tempering temperature is increased in consideration of toughness and the heat treatment cost is increased.

  In addition, since the strength of such materials increases during wire drawing, it is necessary to perform sufficient soft annealing in consideration of the die life and cold limit cracking during cold forging, leading to further cost increase. .

  For example, Patent Document 1 proposes a steel material containing various alloy elements for the purpose of improving the hardenability, using low carbon steel in consideration of cold workability.

  The purpose of this technology is to perform carburizing and tempering treatments to ensure extremely high surface hardness and ensure tapping properties. However, the manufacturing cost increases due to the carburizing treatment compared to normal quenching and tempering treatments. To do.

Further, from the viewpoint of reducing the heat treatment cost, a steel material for non-tempered bolts that can omit the quenching and tempering treatment has been studied, and for example, proposed in Patent Document 2 and the like.
Japanese Patent Publication No. 5-63452 JP 61-284554 A

  However, since the technique proposed in Patent Document 1 is premised on carburizing quenching and tempering, the manufacturing cost increases as a carburizing treatment cost, and the toughness is impaired because the surface hardness is extremely high.

  Regarding the technique proposed in Patent Document 2, the non-tempered bolt usually secures strength by cold-drawing the material, and the material strength at the time of cold forging is equivalent to the target strength, so deformation resistance As a result, the mold life is extremely short.

  As described above, conventional carbon steels such as S35C to S45C, boron steel obtained by adding boron to S25C to S35C, and alloy steels such as SMn443 and SCM435 have high strength, so soft annealing (including spheroidizing annealing) at the time of wire drawing. Is required, and the tempering temperature is 400 ° C. or higher, which is a heavy heat treatment cost.

  In addition, even after sufficient softening annealing (including spheroidizing annealing), the deformation resistance is high, so cracks occur in the head or flange during cold forging, facilitating wear of the die, and chipping. The increase in manufacturing cost is unavoidable due to the decrease in die life due to, and the decrease in yield due to the deviation of head dimension tolerance.

  The present invention has been made by paying attention to such a situation, and is excellent in cold forgeability, can be tempered at a low temperature in consideration of a reduction in heat treatment cost, has a tensile strength of 800 MPa or more, and is excellent in toughness. An object of the present invention is to provide a steel for high-strength screws and a method for producing a high-strength screw using the same.

The inventors of the present invention have made extensive studies on the above problems and have obtained the following knowledge.
(1) The decrease in toughness when the tempering temperature is low is mainly due to precipitation of cementite at grain boundaries, and in order to avoid it, it is important to reduce the amount of cementite that precipitates.
(2) If the amount of carbon is 0.15% or less by mass, the amount of precipitated cementite can be reduced, embrittlement can be mitigated, and the amount of each element added can be controlled by Ceq. Linearity and cold forgeability can be improved. In the present invention, the screw includes a bolt, a small screw, and nuts.

The present invention has been further studied based on the above findings, that is, the present invention is 1% by mass of C: 0.07 to 0.15%, Si: 0.2% or less, Mn: 0.5 to 2%, P: 0.015% or less, S: 0.015% or less, Cr: 2% or less (excluding 0), Al: 0.005 to 0.08%, N: 0.01% or less, and carbon equivalent: Ceq is Ceq = C + Si / 7 + Mn / 6 + Cr / 9 is 0.50% or less. For high-strength screws excellent in toughness and cold forgeability consisting of the remaining iron and inevitable impurities steel.

  2 Further, as a steel component, at least one of Ni: 3.5% or less, Cu: 1% or less, Mo: 0.3% or less, B: 0.0005 to 0.005% in mass%, and / or Ti The steel for high-strength screws excellent in toughness and cold forgeability according to 1, containing at least one of 0.005 to 0.05% and Nb: 0.005 to 0.05%.

  31 A toughness characterized by hot-rolling a steel having a component composition according to 1 or 2 and cold forging a screw material having a predetermined diameter, then forming into a screw shape, and performing a quenching and tempering process. A method for manufacturing high-strength screws with excellent cold forgeability.

  4. A method for producing a high-strength screw excellent in toughness and cold forgeability according to 3, characterized in that, after hot rolling, the screw material is subjected to finish drawing without cold annealing and cold forging of a screw material having a predetermined diameter. .

  According to the present invention, it is possible to provide a high-strength screw steel having a tensile strength of 800 MPa or more and a high-strength screw excellent in toughness and cold workability, which is extremely useful industrially.

The reason for limitation of the component of this invention is demonstrated. % Means mass%.
C: 0.07 to 0.15%
C is an important element for securing the strength of steel, and if it is less than 0.07%, a desired strength cannot be obtained. On the other hand, if it exceeds 0.15%, the amount of cementite precipitated during tempering increases and the toughness decreases. Furthermore, the wire drawing property deteriorates, the deformation resistance during cold forging increases, and the cold forging property decreases. Therefore, the C content is limited to a range of 0.05 to 0.15%.

Si: 0.2% or less Si is an important element as a deoxidizing material for steel, but if added in a large amount, cold forgeability is lowered.
Therefore, the Si content is set to 0.2% or less.

Mn: 0.5-2%
Mn is an important element for enhancing the hardenability of steel, making the structure after quenching finer, increasing the proportion of martensite in the structure, and ensuring toughness. For this purpose, Mn needs to be added in an amount of 0.5% or more.

  On the other hand, if there is too much Mn, the deformation resistance increases and the cold workability decreases. Therefore, the Mn content is limited to a range of 0.5 to 2.0%.

P: 0.015% or less P segregates at austenite grain boundaries, weakens grain boundary strength, and causes toughness reduction. Moreover, it dissolves in ferrite and lowers the deformability of the steel. Therefore, the content is made 0.015% or less.

S: 0.015% or less S, like P, segregates at the austenite grain boundaries, weakens the grain boundary strength, and causes a decrease in toughness. Furthermore, MnS is formed to lower the deformability of the steel, and it becomes a starting point of crack generation. Therefore, the content is made 0.015% or less.

Cr: 2% or less Since Cr can improve hardenability, it acts as an element useful in the present invention. However, if it exceeds 2%, the deformation resistance increases, the wire drawing property and the cold forgeability deteriorate, and the cold workability decreases. Therefore, the Cr content is set to 2% or less.

sol. Al: 0.005 to 0.08%
Al is not only an element necessary as a deoxidizing material, but also has an effect of fixing N segregated at the grain boundary as AlN to increase the grain boundary strength and alleviate the decrease in toughness. Further, solid solution N is fixed as AlN to suppress strain age hardening. In order to exert such effects by Al, sol. An amount of 0.005% or more is required as Al (acid-soluble Al).

However, sol. If Al exceeds 0.08%, an agglomerate of Al ₂ O ₃ is formed during continuous casting of the slab, causing nozzle clogging and making the casting operation difficult. Therefore, sol. The Al content is limited to a range of 0.005 to 0.08%.

N: 0.01% or less N also segregates at austenite grain boundaries to weaken the grain boundary strength and cause toughness reduction. In addition, strain age hardening is caused during cold forging and screw rolling to reduce the cold workability of the steel. Thus, since N is an impurity element in the present invention, its content is set to 0.01% or less.

Ceq: 0.50% or less If the carbon equivalent (Ceq) of the steel exceeds 0.50%, the deformation resistance increases, cracking occurs during wire drawing and cold forging, and cold workability is impaired. Ceq is made 0.50% or less because toughness is lowered. More preferably, if Ceq is less than 0.45%, the drawability and the cold forgeability are not lowered even if the annealing at the time of drawing is omitted. Here, Ceq = C + Si / 7 + Mn / 6 + Cr / 9 + Mo / 4 + Cu / 7 + Ni / 40, and C, Si, Mn, Cr and Mo represent the content (%) of each element in steel.

  The above is the basic component composition of the present invention. When further improving the characteristics, one or more of Ni, Cu, Mo, B, Ti and Nb should be added at Ceq: 0.50% or less. Is possible.

Ni: 3.5% or less Ni is an element effective for imparting hardenability to steel and increasing static strength. Moreover, it has an effect of improving toughness during low-temperature tempering, so it is an effective element for ensuring hardenability and toughness. However, when added in a large amount, the deformation resistance is increased, the wire drawing property and the cold forgeability are deteriorated, and it is a very expensive element. Therefore, when added, the upper limit is made 3.5%.

Cu: 1% or less Cu is also an effective element for imparting hardenability to steel and increasing static strength. Adding an appropriate amount is effective for improving the tensile strength, but if it is added too much, surface flaws are likely to occur during hot rolling, resulting in cold forging failure. To do.

Mo: 0.3% or less Mo is an element effective in preventing segregation of P to grain boundaries and increasing grain boundary strength. However, when added in a large amount, Mo, like Cr, has an increased deformation resistance and deteriorates the drawability and cold workability. On the other hand, Mo is an expensive element, and adding a large amount impairs economy. Therefore, when added, the Mo content is set to 0.3% or less.

B: 0.0005 to 0.005%
B has the effect of improving hardenability by adding a small amount. In addition, BN is formed to prevent N grain boundary segregation. By adding B, the Mn, Cr, and Mo contents can be reduced, and the cold workability of the steel is not impaired. In order to exhibit such an effect by B, it is necessary to add 0.0005% or more.

  However, if added over 0.005%, boron cementite is precipitated, grain boundary strength is weakened, and toughness is reduced. Therefore, when adding, B content rate is limited in the range of 0.0005 to 0.005%.

Ti: 0.005 to 0.05%
Ti has a crystal grain refinement effect. However, if it is less than 0.005%, the effect is small, and the effect of fixing N as TiN is also small. On the other hand, even if added over 0.05%, these effects are not only saturated, but if Ti is too high, a large number of hard TiN and TiC are formed, and toughness and cold forgeability are reduced. Also, the alloy costs. Therefore, when adding, Ti content rate is limited in the range of 0.005 to 0.05%.

Nb: 0.005 to 0.05%
Nb, like Ti, has a crystal grain refinement effect. And, for the same reason as Ti, the Nb content is also limited to a range of 0.005 to 0.05% when added.

  The high-strength screw according to the present invention hot-rolls steel having the above-described component composition to produce a screw material having a predetermined diameter, and after cold forging, is formed into a screw shape. Note that annealing such as spheroidization after hot rolling may or may not be performed.

  Thereafter, quenching and tempering are performed for manufacturing. Conditions in each step can be appropriately selected according to desired dimensions and characteristics. Under normal conditions, a material having a tensile strength of 800 MPa or more can be obtained.

Hereinafter, the present invention will be described based on examples.
A steel material containing the chemical components shown in Table 1 is melted in a vacuum melting furnace at 150 kg / ch, hot forged into a 116-square billet, and then turned into a 7.0 mmφ wire rod by hot rolling. Cold drawing was carried out at 2, and after cold forging and thread rolling, quenching and tempering treatment was performed to produce a hexagonal bolt with a cross hole of M6. FIG. 1 shows the bolt dimensions.

Step 1: Hot-rolled wire → Pickling → Spheroidizing annealing → Pickling / coating → Finish wire drawing Process 2: Hot-rolled wire → Pickling / coating → Finish wire drawing Note that quenching (T1 × 60 minutes → oil cooling) ) · Tempering (T2 × 60 minutes → water cooling) was adjusted such that the bolt hardness was HRc: 24-26.

  Table 2 shows the evaluation results of cold forgeability and the characteristics of the obtained bolts.

[Evaluation of cold forgeability]
Evaluation of cold forgeability was performed immediately after cold forging (before thread rolling). The evaluation is based on visual observation of the presence or absence of cracks in the head, and whether or not the tolerance of the two-sided width dimension of the head is within the tolerance of the two-sided width dimension described in Item 4 of Appendix 1 of JIS-B-1021. Judged by. The wire drawing step was performed in the above steps 1 and 2, respectively. In addition, the process 2 was implemented about some steel.

50 heads were respectively formed, and when the number of crack occurrences and dimensional tolerances was 1 to 4 respectively, Δ was determined, and when 5 or more, × was determined.
[Evaluation of bolt toughness]
The toughness of the bolt was evaluated by a head impact test (JIS-B-1051). The head impact test was performed 10 times for each head, and even if one head jump or crack occurred, the angle at the head impact test was evaluated as bad (×) and was 60 °.

  Steel No. 1 to 20 are within the scope of the present invention, and the toughness is sufficiently ensured, and further, the ductility before cold forging is sufficient, so there is no cracking during cold forging, and the two-sided width dimension of the head is also within tolerance. Good inside. Furthermore, steel no. 3, 6, 8, and 13 were good in cold workability even in the wire drawing step 2 as in the wire drawing step 1.

  Steel No. Since 21 has a high carbon content of 0.31%, the deformation resistance is high, and cracking of the head and deviation in dimensional tolerance occur. Furthermore, cracks have also occurred in the head hitting test of the bolt.

  Steel No. No. 22 has low deformation forging due to high deformation resistance because Si is added exceeding the upper limit of the present invention. Yes. Furthermore, the toughness is low and cracks occur in the head impact test.

  Steel No. No. 23 had low toughness due to low Mn, and cracking occurred in the head hitting test.

  Steel No. In No. 24, since Mn is added in excess of the upper limit of the present invention, the deformation resistance is high, the flange cracking of the head is generated, and the dimension of the head is also out of tolerance.

  Steel No. Nos. 25 and 26 were high in impurities P and S, so that the workability was lowered, and the flange cracking occurred during head forming. In addition, cracking occurred in the head impact test due to a decrease in toughness.

  Steel No. In No. 27, since Cr is added exceeding the upper limit of the present invention, the deformation resistance becomes high, the cold forgeability is poor, the head flange cracks occur, and the head dimension is also out of tolerance. Cracks also occurred in the head impact test.

  Steel No. In No. 28, since Al is low, deoxidation is insufficient, and further N concentration suppression to the grain boundary is insufficient, so that toughness is lowered and cracking occurs in the head impact test. Further, since N fixation with Al was insufficient, strain age hardening was large and deformation resistance was high, and cold forgeability was deteriorated. As a result, a dimensional tolerance was deviated and a crack was generated in the head.

  Steel No. In No. 29, N exceeds the upper limit of the present invention, and therefore segregates at the grain boundary, the toughness is lowered, and cracking occurs in the head impact test. In the cold forgeability, cracks occurred in the head due to an increase in deformation resistance due to strain age hardening of N.

  Steel No. In No. 30, since Ni is added exceeding the upper limit of the present invention, the deformation resistance is high, the cold forgeability is poor, the head flange is cracked, and the head dimensions are also out of tolerance.

  Steel No. In No. 31, since Ti was higher than appropriate, hard inclusions such as TiC and TiN inhibited toughness and cold forgeability, cracking occurred in the head impact test, and cracking occurred during head molding.

  Steel No. In No. 32, since B is added exceeding the upper limit of the present invention, the toughness is lowered and cracking occurs in the head impact test.

  Steel No. No. 33 had a high carbon equivalent exceeding the upper limit of 0.50% of the present invention, so the deformation resistance was high, cracking occurred during head molding, and the dimensional tolerance was also off. Furthermore, cracks have also occurred in the head impact test.

  Steel No. 34 is JIS-SCM435H, and C, Si, and carbon equivalent are out of the scope of the present invention, the deformation resistance is high, and the cold workability is inferior.

The figure which shows a bolt dimension (Example).

Claims (4)

  C: 0.07 to 0.15% by mass, Si: 0.2% or less, Mn: 0.5-2%, P: 0.015% or less, S: 0.015% or less, Cr: 2 % Or less (not including 0), Al: 0.005 to 0.08%, N: 0.01% or less, and carbon equivalent: Ceq is Ceq = C + Si / 7 + Mn / 6 + Cr / 9 is 0.50% or less. Steel for high-strength screws excellent in toughness and cold forgeability, consisting of iron and inevitable impurities.   Further, as a steel component, at least mass of Ni: 3.5% or less, Cu: 1% or less, Mo: 0.3% or less, B: 0.0005 to 0.005%, and / or Ti: The steel for high-strength screws excellent in toughness and cold forgeability according to claim 1, containing at least one of 0.005 to 0.05% and Nb: 0.005 to 0.05%.   A toughness characterized by hot-rolling the steel having the component composition according to claim 1 or 2 and cold forging a screw material having a predetermined diameter, and thereafter forming into a screw shape, followed by quenching and tempering. And a method for producing a high-strength screw excellent in cold forgeability.   4. The production of a high-strength screw excellent in toughness and cold forgeability according to claim 3, characterized in that after hot rolling, a finish-drawn wire material is cold-forged without being annealed and cold forged. Method.

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