JPH04180544A - Production of high strength steel excellent in delayed fracture resistance and machine parts using the same - Google Patents

Abstract

PURPOSE:To obtain a high strength steel excellent in corrosion resistance and delayed fracture resistance by specifying a composition consisting of C, Si, Mn, Cr, Mo, V, P, S, and Fe. CONSTITUTION:This steel is a high strength steel having a composition which consists of 0.15-0.40% C, <=0.30% Si, <=0.60% Mn, 2.0-14.0% Cr, 0.5-2.0% Mo, 0.05-1.0% V, and the balance essentially Fe and in which P and S contents are limited to <=0.020% and <=0.010%, respectively, and further, if necessary, <=4.0% Ni, further one or more kinds among <=0.20% Nb, <=0.10% Ti, and <=0.10% Zr, and further <=0.010% B and/or <=0.010% Ca are added and also having superior corrosion resistance and delayed fracture resistance. Moreover, this steel can be used under relatively severe conditions without corrosion preventing treatment. The above high strength steel can be formed into machine parts, in which tensile strength is regulated to <=1200MPa, by means of forming, hardening, and tempering at a temp. between 500 deg.C and the A1 point.

Description

[Detailed description of the invention]

[Industrial Field of Application 1] The present invention relates to high-strength steel with excellent delayed fracture resistance. This steel also has good corrosion resistance, so it can be suitably used for bolts for automobile suspensions, for example, without the need for anti-corrosion measures such as galvanizing. [Prior Art] In order to meet the demand for lighter weight automobiles and other machines, parts must be made smaller and have higher strength, and fastening bolts and the like must also be more sophisticated. However, it is known that high-strength bolts with a tensile strength of 1200 MPa or more have deteriorated delayed fracture resistance.
On the other hand, many parts that are used in harsh environments, such as automobile suspension parts, are galvanized to provide anti-corrosion properties. Significantly reduces destructiveness. Therefore, galvanizing bolts with a tensile strength of 1200 MPa or more should be avoided as much as possible. Up until now, examples of this kind of high-strength Hole I ~ being galvanized and used are:
I can hardly find it. [Problem to be Solved by the Invention 1] The purpose of the present invention is to achieve both corrosion resistance and delayed fracture resistance in high-strength steel, which is typically used for high-strength bolts, as described above, and to improve galvanized steel. The object of the present invention is to provide a material that is resistant to rust even without such anti-corrosion treatment and has excellent delayed fracture resistance. Objects of the invention also include providing a method for manufacturing mechanical parts using such high strength steel. [Means for Solving the Problems 1] The high-strength steel with excellent delayed fracture resistance of the present invention has a C:0.
15-0.40%, Si: 0-30% or less, Mn
: 0.60% or less, Cr: 2. O~14゜0%, Mo
2. Contains 0.5-2.0% and V: 0.05-1.0%, P: 0.020% or less, S: 0.010% or less, and the remainder substantially consists of Fe. It has an alloy composition. The above alloy composition is basic, and one, two or three of the following components may be added as desired. I>Ni: 4°0% or less n) One or more of Nb: 0.20% or less, Ti: 0.10% or less, and Zr: 0.10% or more I[[)B: 0.010 % or less and Ca: 0.010
% or less Type 1 or Type 2 The method for manufacturing mechanical parts of the present invention is to form high-strength steel having the above-mentioned basic composition or a composition in which one or two optional additives are added into a mechanical part, and after quenching, By tempering at a tempering temperature of 500℃ or higher and lower than A1 point,
It consists of refining the tensile strength to 1200 MPa or more.

[For use]

The alloy composition of the high strength steel of the present invention was determined based on the following reasons. C: 0.15 to 0.40% Is it essential to contain 0.15% or more to ensure strength? If it exceeds 0.40%, is it necessary to reduce the firing temperature to prevent alpha cracking? inferior and defeats the purpose. Si: 0.30% or less Since grain boundary oxidation occurs on the surface during hardening, which impairs delayed fracture resistance and reduces workability, the lower the Si content, the better. 0°30% is an acceptable limit. ~In: 0.60% or less Promotes grain boundary segregation of P during quenching, resulting in poor delayed fracture resistance and lower workability. Therefore, like Si, Mn should be as small as possible, and 0.60
The content should be kept within %. Cr:2. O~14.0% Improves hardenability and prevents rust. This effect is observed when 2.0% or more is added, and increases gradually as the amount is increased, but this has a negative effect on workability and increases cost, so 14.0% is set as the upper limit. Mo2: 0.5 to 2.0% and V: 0.05 to 1.0% both contribute to high strength through secondary hardening, so they are used together in amounts equal to or higher than the above lower limit. The above limit was set because if the amount is too large, the effect will be saturated, and giant primary carbides will crystallize and remain until quenching, resulting in a decrease in toughness and hardness. Both P: 0.020% or less and S: 0.010% or less are undesirable impurities, which segregate at grain boundaries during hardening and impair delayed fracture resistance. S reduces toughness due to the formation of MnS. Permissible limits from these viewpoints were determined as described above. The functions and reasons for limiting the composition of alloy components added as desired are as follows. Ni: 4.0% or less It is useful for improving toughness, so it is best to add within the above limits. The value of 4.0% was determined because the effect would reach saturation and the cost would increase. One or more of the following elements: Nb: 0.20% or less, Ti: 0.10% or less, and Zr: 0.10% or less.These elements form fine carbonitrides to make crystal grains fine ( Helps improve toughness and delayed fracture resistance. Excessive addition of 7JO will reduce boT properties. Both B: 0.010% or less and Ca: Q, 0.010% or less have hot workability. The above-mentioned limits were set because the hot workability deteriorates if the amount is added to improve or if the amount is too large. Typically, a temperature of about 950'C is preferable. If hardening is performed at a temperature lower than 900'C, it is necessary to lower the tempering temperature to adjust the strength, but the requirement that the tempering temperature be 500°C or higher The reason why the tempering temperature is set at a high temperature of 500°C or higher is to increase strength by ensuring secondary hardening and improve delayed fracture resistance by spheroidizing grain boundary precipitated carbides. .In addition, A1
Needless to say, the reason for setting the upper limit at this point is that if the structure becomes austenite, the strength cannot be ensured. (Example 1 Steel having the alloy composition shown in Table 1 was melted, rolled, and processed into a wire rod with a diameter of 8M. The wire rod was annealed and processed into a test piece, and a 950'CX30
After quenching for minutes → oil cooling, tempering at various temperatures (440 to 650'C) shown in Table 2 results in a tensile strength of 1.
It was tempered to 100 to 1500 MPa. A test was conducted on each test piece. In addition, the comparative example was galvanized and then subjected to a delayed fracture test and a corrosion test. (Tensile test) Reduced JIS No. 4 test piece (delayed fracture test) The accelerated bending method test piece is a 0.1R round bar with a diameter of 6 shoes and a length of 40 m.
It has a shape that is narrowed to a diameter of 4IIIn by providing a notch. A bending stress was applied to this while dropping 0.1N-HC, l), and a delayed fracture curve was drawn by examining the relationship between the bending stress and the rupture time. (30 hour strength)
/(static bending stress) is defined as "delayed fracture strength ratio", and delayed fracture resistance is evaluated based on the magnitude of the value. (Corrosion test) Condition 1 (assuming environment up to parts assembly): After tempering, #1
Polish the surface with 200 emery paper (after galvanizing it in the comparative example) and 5% NaCf1 at 20°C.
The corrosion weight loss is measured by immersing it in an aqueous solution. Condition 2 (assuming the environment when parts are used): After tempering, the surface is polished with #400 emery paper. In the comparative example, zinc plating is applied on top of that, and further emery polishing is performed to scratch the plating), 40 Measurement of immersion in 5% NaC,Q aqueous solution at ℃. The evaluation of the corrosion test was based on the following criteria. Rank @ Destruction @ Evaluation A 0.
19'm3-11r [T Usage (1) There is no m between B O, 1 or more and less than 1.0 C suitable for use
1.0 or more and less than 3.0 Not suitable for use D 3.0 or more and less than 10.0 Not usable E
Test results of 10.0 or higher, unusable or higher, are summarized in Table 2. For comparative examples, the delayed fracture strength ratio without galvanizing is also listed. In the comparative example, the galvanized one had a tensile strength of 1
It is clear that when tempered to a higher pressure than 200 MPa, the delayed fracture strength ratio decreases markedly, and on the other hand, those with scratches on the galvanized plating have poor corrosion resistance. In contrast, the present invention has an intensity level of 1200 to 1500.
Has an excellent delayed fracture strength ratio over MPa,
Corrosion resistance is also good. [Effect of the Invention 1] The present invention has realized a high-strength steel with improved delayed fracture resistance, particularly a steel with a tensile strength of 1200 MPa or more. Since this steel exhibits good corrosion resistance, it can be used without the need for anti-corrosion treatments such as galvanizing, which is conventionally applied to parts, and reduces the delayed fracture resistance that is unavoidable with galvanizing. You can escape from this dilemma. Therefore, the present invention shows its significance particularly when applied to the production of mechanical parts used under relatively harsh conditions, including the bolts for the automobile tiers cited as an example. Patent applicant Daido Steel Co., Ltd. Agent Patent attorney Souo Suga

Claims (5)

[Claims] (1) C: 0.15-0.40%, Si: 0.30% or less, Mn: 0.60% or less, Cr: 2.0-14.0%
, Mo: 0.5-2.0% and V: 0.05-1.0
%, P: 0.020% or less, S: 0.010%
A high-strength steel having excellent delayed fracture resistance and having a composition in which the balance is substantially Fe. (2) In addition to the components described in claim 1, further Ni:4
．． A high-strength steel with excellent delayed fracture resistance and a composition containing 0% or less of additives. (3) In addition to the components according to claim 1 or 2, further Nb: 0.20% or less, Ti: 0.10% or less, and Z
r: High-strength steel with excellent delayed fracture resistance and having a composition containing one or more of 0.10% or less. (4) In addition to the components according to any one of claims 1 to 3, B: 0.010% or less and Ca: 0.01%
A high-strength steel with excellent delayed fracture resistance that has a composition containing 0% or less of one or two types. (5) The high-strength steel according to any one of claims 1 to 4 is formed into a mechanical part, and after quenching, the temperature is 500°C or higher A_1
1. A method for manufacturing mechanical parts, which comprises tempering the mechanical parts to a tensile strength of 1200 MPa or higher by tempering at a tempering temperature below 1200 MPa.