JPH01242761A - Ultra high strength steel having low yield ratio and its manufacture - Google Patents

Abstract

PURPOSE:To obtain the title steel having high safety by subjecting a low alloy thermal refined steel having specific componental compsn. to quenching and tempering under specific conditions and depositing a Cu phase into a martensitic structure. CONSTITUTION:The steel constituted of, by weight, 0.20-0.50% C, <=0.40% Si, <=1.20% Mn, 0.50-4.50% Ni, 0.50-3.50% Cr, 0.30-1.50% Mo, 0.05-0.30% V, 0.30-2.50% Cu, <=0.010% P, <=0.005% S and the balance consisting of inevitable impurities with Fe is refined. The steel ingot is rolled and is quenched from the temp. range of Ac3 transformation point or above into a martensitic structure. The steel is thereafter tempered at the temp. range of 400-650 deg.C to deposit a Cu phase into the martensitic structure. By this method, the ultra high strength steel suitable for a machine structure, etc., and having about <=95% yield ratio can be obtd.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is applicable to mechanical structures with a tensile strength of 100
The present invention relates to an ultra-high tensile strength steel having a yield ratio (yield point/tensile strength) of 95% or less and a method for producing the same.

<Prior Art> Generally, the higher the yield ratio, the lower the plastic deformability and the more likely brittle fracture will occur. Ultra-high tensile strength steel used for structural steel materials for i-machines is required to have a low yield ratio from the viewpoint of safety during use, ie, prevention of brittle fracture.

However, with high-strength steel, the yield ratio tends to increase with increasing strength, so the tensile strength of 100 kg
The yield ratio of ultra-high tensile strength steel of f/- or higher was as high as over 95%.

By the way, there are three conventional methods for obtaining a lower yield ratio.

■ Hardenability is reduced by decreasing the quenching cooling rate or reducing the amount of alloying elements to create a bainite composite fiber.

■ Use as quenched without tempering.

■ Adopt α + γ two-phase quenching.

However, the method (■) has a low yield ratio (Y, R).

), but due to incomplete quenching, strength and
Method (2) attempts to obtain low Y, R, and hardness by leaving it as quenched, but although high strength is obtained, the toughness is low and significant residual stress does not disappear, etc. There are drawbacks.

Method (3) is based on two-phase quenching, for example, JP-A No. 55
As shown in No. 131130, it is effective in reducing Y and R, but it is difficult to obtain high strength.<100 kgf/d or more tensile strength cannot be obtained.

In other words, either method is insufficient for achieving both high tensile strength and low yield ratio.

<Problems to be Solved by the Invention> The present invention achieves a tensile strength of 100K with a yield ratio of 95% or less without causing the deterioration in strength and toughness seen in the above-mentioned prior art.
The present invention provides an ultra-high tensile strength steel having a tensile strength of gf/- or more and a method for manufacturing the same.

Means for Solving the Problems〉 As a result of repeated research on chemical composition systems that can obtain ultra-high tensile strength and low yield ratio, the present inventors found that ■ By incorporating an appropriate amount of alloy components, a martensitic structure is created. This ensures high strength and toughness. ■By further containing Cu,
The present invention was completed based on the discovery that only the yield point is lowered and Y and R can be lowered without lowering the strength and toughness due to the precipitation of soft Cu. That is, the gist of the present invention is that the weight ratio of C: 0.20 to 0.
50%, Si: 0.40% or less, Mn: 1
．． 20% or less, Ni: 0.50-4.50%, C
r: '0.50-3.50%, Mo: 0
．． 30-1.50%, V: 0.05-0.3
0%, Cu: 0.30-2.50%1Pj
0.010% or less, S: 0.005% or less, the remainder consists of unavoidable impurities and re, and is an ultra-high strength steel with a low yield ratio that has a martensitic structure containing a Cu precipitate phase, and After quenching the steel with the above composition at a temperature range above the transformation point to form a martensitic structure,
This is a method for producing ultra-high tensile strength steel with a low yield ratio, which is characterized by performing tempering in a temperature range of ~650°C to precipitate a Cu phase in a martensitic structure.

Function> First, the reason for limiting the chemical composition in the present invention will be explained.

C is an effective element for improving strength, and at least 0.20% (wt%, same hereinafter) is required to ensure the required strength, but if it exceeds 0.50%, toughness decreases, so It is limited to a range of .20 to 0.50%.

St is an element that, together with P, significantly increases the susceptibility to tempering embrittlement, and in order to prevent embrittlement during low-temperature tempering, it is desirable that its content be low, but the present invention regulates P: 0.010%. below.

When Mo: 0.30% or more, St does not cause embrittlement if it is 0.40% or less, so it was limited to 0.40% or less.

Mn is an effective element that improves hardenability, strength, and toughness, but if added in excess of 1.20%, it not only reduces toughness but also increases susceptibility to tempering embrittlement. Limited to 20% or less.

Both Ni and Cr are effective elements for improving hardenability, strength, and toughness like Mn, and in order to expect this effect, they must be at least 0.50% old.

Cr: 0.50% or more is required. However, Ni is 4
．． 50%.

If Cr is added in excess of 3.50%, it not only reduces the toughness but also increases the susceptibility to tempering embrittlement, so this is the upper limit. Ni: 0.50 to 4.50%, C
r: limited to a range of 0.50 to 3.50%.

No is an element effective in suppressing temper embrittlement, and is required to be at least 0.30% in order to suppress temper embrittlement during low-temperature tempering. However, excessive addition exceeding 1.50% will reduce toughness, so
% range.

■ is an element that improves strength, and for that purpose a small amount (
Both are required in an amount of 0.05% or more, but if it exceeds 0.30%, toughness decreases, so it is limited to a range of 0.05 to 0.30%.

Since P significantly increases the susceptibility to tempering embrittlement, it is desirable to have a small amount, but if it is 0.010% or less, embrittlement will not occur, so it is limited to 0.010% or less.

S is also an element that reduces toughness, and it is desirable to have less S, but if it is less than o, oos%, there is little effect of reducing toughness, so it is limited to less than o, oos%.

Cu is an important element for achieving a low yield ratio, which is a feature of the present invention. The results of experiments conducted by the present inventors regarding the influence of Cu on the yield ratio will be explained.

Steels with different Cu contents shown in Table 1 were quenched at 900'C above the Ac3 transformation point, tempered at 580'C, and their tensile properties were investigated.The results are shown in Table 1.
Yield strength (Y, S,) for Cu-free material: 152 kg
f/lj, tensile strength (T, S, ):
Yield ratio (yield strength/tensile strength) at 159 kgf/d (
Y, R, ) is as high as 96%, whereas when the Cu content is increased, T.

Although S hardly decreases, only Y and S decrease significantly, and Y and R can be decreased to 95% or less.

In this way, Cu has a remarkable effect on reducing Y and R, but for this purpose a small amount (0.30% or more of both is required, but if it exceeds 2.50%, hot workability decreases). Therefore, it is limited to a range of 0.30 to 2.50%.

Next, the refining conditions for the limited composition steel will be explained.

In the present invention, after quenching from a temperature above the Acs point,
Tempering treatment at a temperature range of ~650°C.

Since sufficient strength cannot be obtained at a quenching temperature below the Acs point, it is necessary to form a quenched martensitic structure at a temperature above the Ac3 point. The tempering temperature range is 400
Below 650°C, the tempering effect is not sufficient and the toughness is low, and if it is soft, the precipitated phase cannot be obtained, making it difficult to obtain a low yield ratio. On the other hand, excessive tempering above 650°C will not provide sufficient strength. , since the solubility of Cu also increases, C
The amount of precipitation of u is reduced, and the effect of lowering the yield ratio is small (therefore, the tempering temperature range is limited to 400 to 650°C).

<Example> 15 types of tlii of the present invention and 1115 types of comparison having the compositions shown in Table 2 were used as test materials. The comparison steel with Nα6 and 7.8 is conventional N[-Cr-Mo steel (fls
G4103 SNCM), and none of them contain Cu; Comparative Steel 9 has C, Cr, and Mo removed, and Comparative Steel 10 does not contain Cu.
St, P.

The Cr content does not satisfy the compositional conditions of the present invention.

These test materials were melted in a vacuum melting furnace, and the steel ingots were rolled into 40mm thick steel plates, then quenched and tempered, and then subjected to tensile tests and Charby impact tests. All 0 structures were martensite. It was an organization. The results are shown in Table 3. Comparative steel 8116.7, which is currently in use, has a yield ratio of 9.
It can be seen that the yield ratio is as high as 7%, whereas the yield ratio of grades 1 to 5 of the steel of the present invention is as low as 85 to 92%. Similar to Na6,7m, Comparative Steel 8, which does not contain Cu, has a high yield ratio of 96%, and like Comparative Steel N119,10, it contains C, Cr, M.
If the content of o or Si, P, and Cr is out of the component range of the present invention, high strength and high toughness cannot be obtained, and furthermore, if the tempering temperature is outside the conditions of the present invention and is as low as 350 ° C.
The effect of lowering the yield ratio due to Cu precipitation is not fully demonstrated
,R.

is as high as 96%, and the toughness is also low.Conversely, it is clear that high strength cannot be obtained even if the tempering temperature is too high as 680°C.

<Effects of the Invention> As is clear from the above examples, the present invention limits the components of the low-alloy heat-treated steel, and in particular contains an appropriate amount of Cu, thereby creating a yield ratio with high safety and suitable for mechanical structures. is 95%
We were able to obtain the following ultra-high tensile strength steel.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of Cu content on tensile properties. Patent applicant Kawasaki Steel Corporation Figure 1 Cu content (wt%)

Claims (1)

[Claims] 1. By weight: C: 0.20-0.50% Si: 0.40% or less Mn: 1.20% or less Ni: 0.50-4.50% Cr: 0. 50 to 3.50% Mo: 0.30 to 1.50% Cu: 0.30 to 2.50% P: 0.010% or less S: Contains 0.005% or less, the remainder being unavoidable impurities. Consists of Fe, Cu
Ultra-high tensile strength steel with a low yield ratio characterized by a martensitic structure containing a precipitated phase. 2. By weight: C: 0.20-0.50% Si: 0.40% or less Mn: 1.20% or less Ni: 0.50-4.50% Cr: 0.50-3.50% Mo: 0.30-1.50% V: 0.05-0.30% Cu: 0.30-2.50% P: 0.010% or less S: 0.005% or less, the balance is A steel consisting of unavoidable impurities and Fe
Ultra-high tensile strength with a low yield ratio characterized by quenching at a temperature range above the c_3 transformation point to form a martensitic structure, followed by tempering at a temperature range of 400 to 650°C to precipitate a Cu phase in the martensitic structure. Method of manufacturing steel.