JPS5810442B2 - Manufacturing method for high-toughness, high-strength steel with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing high-toughness, high-strength steel having a tensile strength of 50 kg/mm2 or more and excellent workability.

Class 2 high tensile strength steel with a tensile strength of 50 to 100 kg/mm has a wide range of uses, and good weldability is required when it is applied to structures, shipbuilding, etc.

On the other hand, excellent cold workability and welding materials are required for steel that is processed into predetermined dimensions and then welded as necessary, such as in the industrial machinery field, pipe materials, and bolt materials.

In order to increase the strength of conventional non-heat treated high tensile strength steel and heat treated high tensile strength steel, large amounts of additive elements are used, which inevitably results in the addition of C.
The equivalent weight becomes high, which is not desirable for weldability.

Increasing the strength of such steel has essential problems, including an increase in yield ratio and deterioration of toughness and ductility.

Therefore, high-strength steel is susceptible to cracking during cold working and defective shape freezing due to springback.

The present invention aims to increase the strength of steel by a method different from the conventional methods in order to address such problems.It improves weldability by significantly lowering the C equivalent to obtain the required strength, and furthermore, it achieves extremely low yield. This is a method that achieves the same ratio and provides excellent cold workability.

That is, in the present invention, C: 0.005 to 0.2%, Mn: 0
．． 3 to 2.5%, 1.0% or less of Si, 0.005 to 0.2% of one or both of Nb and V, or further contains 0.1% or less of All, and 0.15% of Ni. % or less, Cr
, Mo.

Cu is 0.7% or less, Ti is 0.05% or less, respectively.
A steel containing one or more of 0.02% or less of Ce and 0.003% or less of Ca, with the balance consisting of iron and unavoidable impurities, is heated to 1000°C to 1300°C, and then heated to at least 980°C. Processing is performed at an area reduction rate of 30% or more in the Ar3 temperature range below, and in the ferrite phase precipitation temperature range during cooling,
There is a method for producing high-toughness, high-strength steel with excellent workability, in which 5 to 60% of the ferrite phase is precipitated and then rapidly cooled to form a two-phase layered structure of ferrite and martensite.

In the present invention, the steel is hot worked at Ar3 or higher.

At that time, 100%
The car is heavy when it is austenitized at 0°C to 1300°C, preferably 1050 to 1200°C.

Temperature range in which austenite grains are refined by hot working and the progress of recrystallization of austenite is significantly delayed, 9
Process at 3 points of As at 80°C or lower with an area reduction rate of 30% or more.

Processing at the relevant temperature introduces a large amount of strain into the austenite, and the hardened austenite becomes
The ferrite phase precipitation temperature range in the CT curve is shifted to the high temperature and short time side.

This is because when the temperature exceeds 980° C., austenite tends to recrystallize, and when the working area reduction rate is 30% or less, no effect of imparting strain to austenite is observed.

After processing, in the ferrite phase precipitation temperature range during cooling,
As the ferrite phase precipitates, C becomes concentrated in the untransformed austenite portion.

After precipitation of 5 to 60% as a ferrite phase, untransformed austenite with a high C concentration is rapidly cooled to obtain a two-phase layered structure of fine ferrite grains and martensite with an apparent high C concentration.

The ferrite grains that precipitate from hardened austenite are fine and are favorable for ductility and toughness, and martensite has extremely high strength due to C enrichment, and as is well known from the example of ausform steel, martensite from hardened austenite is rich in toughness. .

Ferrite by conventional (γ + α) two-phase region heating quenching.
Martensitic dual-phase steel has the disadvantage that it is difficult to refine the ferrite grains, and the ferrite grains are distributed in a dispersed manner surrounded by high-strength martensite, resulting in high stress concentration in the ferrite part, which tends to cause early fracture of the ferrite part. have

The ferrite-martensitic dual-phase layered structure steel according to the present invention has easily fine ferrite grains, and because they are distributed in rows and form a layer with martensite, early fracture of ferrite is prevented, and it has high ductility. , toughness is ensured.

The amount of ferrite precipitated in the ferrite precipitation region before quenching during cooling is 5 to 60%, and if the ferrite amount is less than 5%, the effect of the ferrite-martensite two-phase layered structure is reduced and an extremely low yield ratio cannot be achieved.

If the amount of ferrite is 70% or more, there is little effect in bending to increase the strength of steel.

Additionally, upper bainite tends to form during rapid cooling, which induces deterioration in the toughness of the steel.

Martensite is generated from untransformed austenite by quenching after precipitation of 5 to 60% ferrite, but a small amount (less than 10 to 20%) of ferrite is still observed to precipitate depending on the low C steel and the degree of quenching. It will be done.

As mentioned above, the ferrite crystal grains are fine and are favorable for ductility and toughness.

For cooling after hot working to adjust the amount of ferrite, it is desirable to rapidly cool from about 780 to 680°C after working.

Rapid cooling requires water cooling or forced air cooling to cool the vehicle below 400°C, which is heavy.

The cooling rate in this case is 5-b or more.

Next, the reasons for limiting the composition of steel in the method of the present invention are as follows.

Melting C at 0.005% or less is expensive and has little effect on increasing strength.

Since a content of 0.2% or more significantly deteriorates weldability, the upper limit is set at 0.2%.

Dove is necessary for toughening, and if the content is less than 0.3%, the effect will be small, and if the content exceeds 2.5%, the effect will be saturated and upper bainite will be likely to be generated.

Nb. (3) produces fine carbonitrides, which has the effect of widening the recrystallization retardation temperature range of austenite, and helps the present invention to exhibit its effectiveness; however, if it is less than 0.005%, it is ineffective;
If the content exceeds 0.02%, the effect will be saturated.

Although Si is effective in increasing strength through its solid solution strengthening effect, its content exceeding 1.0% causes significant deterioration of toughness, so it is limited to 1.0% or less.

Small amounts of Cr, Mo, and Cu are good for increasing strength, but 0
．． If the content exceeds 7%, it becomes difficult to adjust the amount of ferrite phase precipitation, and the toughness and weldability are impaired.
7% or less.

Ni has the effect of increasing toughness, but if the content exceeds 1.5%, the effect will be saturated, so it was set to 1.5% or less.

A/ is added for the purpose of stabilizing deoxidation and refining crystal grains, but if the content exceeds 0.1%, these effects will be saturated.
Since inclusions increase rapidly and impair ductility, the content was set at 0.1% or less.

Ti has the effect of increasing strength and improving weld toughness, but 0
．． If the content exceeds 0.05%, the effect will be saturated.

Ce and Ca are used for the purpose of inclusion adjustment, but Ce is 0.
If the content exceeds 0.02% and Ca content exceeds 0.003%, the effect is saturated.

In addition, in the present invention, in order to obtain excellent weldability, the C equivalent (=
C+Si/24+Mn/6+Ni/40+Cr15+M
o/4+V/14) is 0.40% or less because it makes the car heavier.

Next, examples of the present invention will be shown together with comparative examples.

Example 1 Table 1 shows the chemical composition of the sample materials.

Test materials A to D steels are steels applicable to the present invention, and E steel is 80k.
G1ma class non-heat treated high tensile strength steel, F steel: 100 kg/m
It is a class A tempered high tensile strength steel.

For steel samples A and D, the austenitizing temperature was 1.
It was heated to 150°C and controlled rolling was performed at a reduction in area of 70% from 980°C and a total reduction of area of 85%.

The rolling finishing temperature is 830°C.

Steel A was manufactured by water-cooling from 770°C with air cooling for 44 seconds after rolling (A1 steel) and water-cooling from 720°C with air cooling for 110 seconds after rolling (A2 steel).

A1. The ferrite fraction of A2 steel is 18% and 58, respectively.
%Met.

Rani A1. A2 steel was tempered at 400°C (referred to as A11 steel and A21 steel, respectively) and one tempered at 600°C (referred to as A12 steel and A22 steel, respectively) was obtained.

After rolling, the BD steel was air cooled and then water cooled from 720°C.

The ferrite fraction was 59% in B steel, 55% in C steel, and 33% in D steel.

Comparative steel E has a bainitic structure as hot rolled, and comparative steel F has a bainitic structure.
The steel was hot-rolled and then quenched and tempered, and has a tempered martensitic structure.

Table 2 shows the mechanical properties of these materials.

As can be seen from Table 2, the steel produced by the method of the present invention has a significantly lower C equivalent and a tensile strength of 9' for the hardened material compared to the ordinary 80-100 Kp/- edge high strength steel.
It is characterized by a yield ratio of about 70% or less, an elongation of 22% or more, and a particularly high average elongation.

The fracture surface transition temperature (vTrs) is -90°C or lower, which is good.

After tempering, the tensile strength decreases and the yield strength does not show a nearly constant value, so the yield ratio is higher but lower than the comparative steel.

Figure 1 shows the balance between strength and elongation.

When compared at the same strength level, the steel according to the invention has a low C equivalent and excellent ductility.

Example 2 Table 3 shows the chemical composition of the sample materials.

Sample material G is a steel applied to the present invention for cold forging bolts and the like, and sample material H is a comparative steel.

For the specimen G steel, the austenitizing temperature was 1150°C.
heating to 980°C, area reduction rate 64%, total area reduction rate 8
Control rolling was performed at 8% to obtain a wire rod of 13.5 mmφ.

The rolling finishing temperature is 830°C. 800 after rolling for G steel
Two types were manufactured: one water-cooled from 740°C (referred to as G1 steel) and one water-cooled from 740°C (referred to as G2 steel).

On the other hand, H steel was heated to an austenitizing temperature of 1150°C, finished rolling at 950°C, and cooled.

These G1. 18 more for G2 and H steel each
% wire drawing processing (Gll l G21 l H1 steel); and further heat treatment at 300°C x 2 minutes (G1□, G2
□.

H2) was carried out.

The mechanical properties of these materials are shown in Table 4.

As can be seen from Table 4, the steel rolled material manufactured by the method of the present invention has an extremely low yield ratio compared to the comparative example, and the load on processing tools during cold working such as expansion and contraction is small. Also, since it has high strength, it is suitable as a material for non-tempered bolts.

Example 3 Table 5 shows the chemical components of the test materials.

Test material J is a steel applied to the present invention as a high-strength steel wire such as a PC steel wire, and test material K is a steel for high carbon steel wire.

For steel specimens, the austenitizing temperature is 1150°.
C and controlled rolling at a reduction in area of 64% from 980°C and a total reduction of area of 88% to obtain a wire rod of 13.5 mmφ.

The rolling finishing temperature is 830°C. 800 after rolling for J steel
℃, 770°C and 740°C (designated as Jl, J2 and J3 steels, respectively) were produced.

These Jl, J2. J3 steel was subjected to 2% stretch processing and heat treated at 300°C for 2 minutes.

(Represented as Jll, J21, and J31, respectively) On the one hand, the steel was conventionally rolled, then lead patented, and further subjected to 15-inch wire drawing and heat treatment at 430° C. for 3 minutes.

The mechanical properties of these materials are shown in Table 6. As is known from Table 6, the rolled steel material manufactured by the method of the present invention has high strength and low yield ratio, is easy to process, and can be made to have a yield ratio of 0.9 or more by heat treatment. , P
It is a suitable material for C steel wire (bar).

[Brief explanation of the drawing]

Figure 1 shows the relationship between tensile strength and elongation of high tensile strength steel.

Claims (1)

[Claims] 1C: 0.005-0.2%, Mrl: 03-2.
5%. Si: 1.0% or less, one or both of Nb and V 0.0%
A steel containing 05 to 0.2% with the balance consisting of iron and unavoidable impurities is heated to 1000°C to 1300°C, processed at a temperature range of at least 980°C or less with an area reduction of 30% or more, and then cooled. In the intermediate ferrite phase precipitation temperature range,
A method for producing high-toughness, high-strength steel with excellent workability, in which 5 to 60% of ferrite phase is precipitated and then rapidly cooled to form a two-phase layered structure of ferrite and martensite. 2C: 0.005-0.2%, Mn: 0.3-2.5
%. Si 1.0% or less, one or both of Nb and V 0.00%
Contains 5 to 0.2%, and further contains AIo, 1% or less, Ni1
．． 5% or less, Cr, Mo, Cu each 0.7% or less,
TiO, 05%. Hereinafter, steel containing one or more of CeO, 0.02% or less, Ca 0.03% or less, and the balance consisting of iron and unavoidable impurities, is heated to 1000°C to 1300°C, and Ar3 is heated to at least 980°C or less. Machining with an area reduction rate of 30% or more in a temperature range, and in the ferrite phase precipitation temperature range during cooling,
A method for producing high-toughness, high-strength steel with excellent workability, in which 5 to 60% of the ferrite phase is precipitated and then rapidly cooled to form a two-phase layered structure of ferrite and martensite.