JPS6254018A - Manufacture of high tensile steel superior in material characteristic after warm working - Google Patents

Abstract

PURPOSE:To obtain steel superior in characteristic after warm working, by respectively specifying compsn. of steel, conditions of hot rolling and warm working in manufacturing high tensile steel by the so-called TMCP. CONSTITUTION:Steel composed of, by weight % 0.03-0.20 C, <=0.6 Si, 0.5-2.0 Mn, 0.005-0.08 Sol Al, if necessary >=one kind among 0.005-0.1 Nb, 0.005-0.15 V, 0.005-0.15 Ti, <=1.0 Cu, <=1.0 Cr, <=3.5 Ni, <=1.0 Mo, 0.0005-0.003 B and the balance iron is prepd. The steel is hot rolled by >=30% cumulative draft at <=900 deg.C. Thereafter, if necessary, it is acceleratedly cooled to temp. where at least transformation is finished by air cooling - 100 deg.C/sec rate. It is further heated to 750-400 deg.C in warm working and worked at 750-250 deg.C immediately or after cooled as it is.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing high-strength steel, and
P (Thermomechanical Control
The purpose of the present invention is to provide a method for producing steel that is produced by a 100% process and exhibits excellent material properties even after warm working.

[Prior art]

Steel used for offshore structures etc. is required to have high strength and toughness.
Previously, this type of steel was produced by standard or hardening-tempering processes. But in recent years.

Plate manufacturing technology such as controlled rolling and accelerated cooling has progressed and is becoming popular as so-called TMCP.

Its application to steel for offshore structures is also increasing. This TMCP
This type of steel has high strength by controlling the transformation from austenite to ferrite by rolling in the austenite low-temperature region or (α + γ) two-phase region, or by accelerated cooling after rolling.

Obtains high toughness.

By the way, steel used for marine structures and the like is often subjected to bending processing and the like during assembly, and cold working is common for thin-walled steel and low-strength steel, and hot working is common for thick steel plates.

However, in the case of TMCP type steel, if it is reheated to the austenite region during hot working, there is a drawback that the material deteriorates more than conventional steel. In addition, although there are no problems with the material quality when cold working, advances in TMCP have made it possible to manufacture high-strength, thick steel, making cold working impossible due to pressing capacity. This problem has arisen.

Taking advantage of this point, direct processing, which involves processing after heating to the α high temperature region or (α + γ) two-phase region, is becoming increasingly used for high-strength or thick-walled TMCP steel.
Although various proposals have been made regarding this technology, no proposal has yet been made that considers the characteristics after warm working.

[Summary of the invention]

The present invention has been made in view of the current state of the prior art described above, and aims to obtain steel with excellent properties after warm working by limiting the composition of steel, hot rolling, and warm working conditions. That is.

That is, according to the method of the present invention, one composition has C: 0.03 to 0.
．． 20'4, si: o, a% or less, Mn: 0.5~
2.0chi, 5oLu: 0.005-0.08chi, limited to steel consisting of balance iron and unavoidable impurities - In addition to the above components, further Nb: 0.005-0.1
%, V: 0.005-0.15chi, Ti: 0.
005-0.15%, cu: 1.0% or less, Cr: 1
．． 0% or less, Ni: a, stlb or less, Mo: 1
．． 0% or less, B: 0.0005 to 0.00396
(7) One or more of these may be added as necessary.

Controlled rolling is then performed on the steel having such components, but the conditions are limited to 900° C. or less and a cumulative reduction rate of 30 − or more. After controlled rolling, it may be left as is or accelerated cooling may be performed. In this case, air cooling or higher ~100℃/3
It is assumed that accelerated cooling is performed at a cooling rate of ee to at least the temperature at which transformation ends.

Thereafter, it is heated to a temperature in the range of 750 to 40011°C, and processed immediately or at a temperature of 750 to 250°C after being allowed to cool.

By the above method, steel with excellent material properties after warm working can be obtained.

Here, the most characteristic warm working conditions of the method of the present invention will be explained first.

The first case is a conventional standard material (marked with ○) and a TMCP material subjected to accelerated cooling after controlled rolling (marked with △), heated to various temperatures from 500 to 750°C, and then warm worked for 10 times at 500°C. This is a plot of each mechanical property when applied. As can be seen from this graph, the TMCP material exhibits superior material quality compared to the standard material at 500 to 650°C, but at 750°C, it becomes the same level as the standard material, which is A.
At heating temperatures below AC1, the effects of controlled rolling and accelerated cooling are maintained and high quality material is obtained;
This is thought to be because the structure changes when the steel is reheated to the two-phase region, and the effects of controlled rolling and accelerated cooling disappear.

Next, Figure 2 shows the relationship between processing temperature and material quality.

The above-mentioned standard materials (marked with ○) and TMCP materials (marked with Δ).

0 mark) was heated to 650°C and held for 1 hour.

Processed at various temperatures. From this figure, TMCP
It can be seen that the type steel materials (△9 mouths 0) are superior in toughness compared to the reference material (O mark), and the Nb-added steel (△ mark) also has a high YS.

Even when the processing temperature is as low as 400 to 250°C, a good material is obtained and no cracks are observed.

The compositions of the reference material (○ mark) and the TMCP material (Δ2 0) are shown below.

From the above results, it is possible to manufacture steel of extremely superior quality that cannot be obtained with conventional materials by warm working steel manufactured under appropriate controlled rolling or accelerated cooling conditions under appropriate conditions. This led to the knowledge that, in the method of the present invention, the heating temperature is set to 750 to 400°C as warm processing conditions, preferably A
c1~400℃, processing temperature 750~250℃
, preferably limited to Ael to 400°C. The reason for limiting the upper limit of the temperature here is as described above, but the lower limit is because when the heating temperature is 400° C. or less, the processing temperature decreases and the merits of warm processing are reduced. Regarding the processing temperature, if it is less than 250°C, the processing temperature will drop and the benefits of warm processing will be reduced, so this is the lower limit, and it is preferably 400°C to avoid the back heat brittle region.
The lower limit is ℃.

In addition, after heating to the above temperature, the warm working may be performed immediately or after being allowed to cool, and the effects of the present invention can be obtained as long as it is performed within the above limited temperature. Further, the cooling rate after warm working is not particularly limited since it has little effect on the material quality.

Next, the reason for limiting the components in the method of the present invention and other manufacturing conditions will be explained.

C: 0.03% is necessary to effectively impart strength to this basic steel at the lowest cost, but if it exceeds 0.2%, weldability will be significantly impaired, so 0.03% to 0.2% is necessary. limited to.

si: ST is effective for high strength reaction through solid solution strengthening, but addition of a large amount impairs weldability, so limit it to 0.6% or less. ◎ Mn: Mn is a steel additive that is effective for improving the strength and toughness of copper. It is added as a basic element, but if it is less than 0.5%, its effect will be small, and if it exceeds 2,0 -, the weldability will be significantly impaired, so it is set at 0.5 to 2.0%.

SolAl: 5OtAt of minimum o, oos% for deoxidizing steel
This effect is saturated when the required Somt exceeds 0.08, so o, o o s ~ 0.08
%◎ In addition to the above basic ingredients, the following elements may be added as needed:
It is possible to add a species or two or more species.

Cu, Cr, Ni, Mo: By adding these, it is possible to strengthen the steel without impairing toughness through solid solution strengthening and structural changes based on increased hardenability of the steel, but from the viewpoint of weldability and economic efficiency. cu, Cr, M
The upper limit was 1.0 inch for O, and 3.5 inch for Ni.

Nb, V, Ti: These elements have a remarkable effect on increasing strength through precipitation strengthening and improving low-temperature toughness, and are added as necessary, but in order to achieve this effect, 0.005% or more of any one of them is required. It is necessary to add

This was taken as the lower limit. Further, since any of these impairs weldability when added in large amounts, the upper limit for Nb was set at 0.10%, and the upper limit for v and 'rt was set at 0.15% each.

B: B increases the hardenability of steel and has a great effect on increasing strength, but if it is less than 0.05% O,000, this effect is small, and if it exceeds 0.003%, it impairs weldability, so it is limited to this range. .

In the method of the present invention, steel whose composition has been adjusted as described above is heated to 900°C.
Hot rolling is performed so that the following cumulative rolling reduction ratio becomes 30% or more. This is because if the rolling reduction ratio is lower than this, a sufficient controlled rolling effect cannot be obtained, resulting in insufficient strength and toughness. That is.

During controlled rolling of practical steel, it is important to apply rolling in the austenite non-recrystallized region to refine the transformation composition, and in steels containing Nb, V, Ti, etc., the upper limit of the austenite non-recrystallized region Since the temperature was 900°C, this temperature was set as the upper limit. In steels that do not contain Nb, V, TL, etc., the upper limit temperature of this unrecrystallized region is 90
Although it is about 50°C lower than 0°C, in actual operation it is 900°C.
Since there is little difference even if it is below ℃, this was set as the upper limit. After hot rolling, the material may be left as it is, or it may be subjected to accelerated cooling.The accelerated cooling conditions after hot rolling are effective in improving the material quality by cooling the transformation region faster than when cooling is done. The cooling temperature range should be at least up to the temperature range where transformation ends, and the lower limit of the cooling rate should be faster than air cooling, and the upper limit should be 100°C, which is allowed by the equipment.
/sec.

If the controlled rolling as-is or warm working after accelerated cooling is performed under the above-mentioned conditions, steel with excellent material properties can be obtained.

〔Example〕

Steels 1 to 8 having the components shown in Table 1 were hot rolled under the conditions shown in Table 2, warm worked under the conditions shown in Table 3, and their mechanical properties were investigated. The results are shown in Table 3.

Steel 1 and Steel 6 were standard processed without controlled rolling. Steels 2, 3.5, and 8 were accelerated cooled after controlled rolling, and steels 4 and 7 were as-controlled rolled.

As can be seen from Table 3, all the products subjected to warm working under the conditions of the present invention after controlled rolling have excellent material properties.
Especially in terms of toughness, it shows a value superior to that of steels 1 and 6, which are standard materials. Comparative material (■) was subjected to accelerated cooling after controlled rolling, but since the warm working temperature was outside the range of the present invention, the toughness was significantly deteriorated.

Table 3 also shows the results of investigating the influence of the amount of strain during warm working on the material quality and the influence of stress relief annealing (SR) after warm working. These results show that the amount of distortion up to 10 inches, which is the normal processing rate, and the SR processing are
This indicates that it does not have a major effect on the properties of P steel after warm working.

[Brief explanation of the drawing]

FIG. 1 is a graph showing changes in material properties depending on heating temperature of TMCP material and reference material, and FIG. 2 is a graph showing the relationship between processing temperature and material properties.

Claims (1)

[Claims] 1. C: 0.03 to 0.20%, Si: 0.6% or less,
Mn: 0.5-2.0%, SolAl: 0.005-0
．． 08%, the balance being iron and unavoidable impurities, hot rolling is carried out at a cumulative reduction rate of at least 30% at 900°C or less, and further heated to a range of 750 to 400°C during warm working,
A method for producing high-strength steel having excellent material properties after warm working, characterized by working immediately or at a temperature of 750 to 250°C after cooling. 2, C: 0.03 to 0.20%, Si: 0.6% or less,
Mn: 0.5-2.0%, SolAl: 0.005-0
．． 08%, further Nb: 0.005 to 0.1%, V: 0.
005-0.15%, Ti: 0.005-0.15%,
Cu: 1.0% or less, Cr: 1.0% or less, Ni: 3.
5% or less, Mo: 1.0% or less, B: 0.0005 to 0
．． Steel containing one or more of 0.003% and the balance consisting of iron and unavoidable impurities is subjected to hot rolling at a temperature of 900°C or less with a cumulative reduction rate of at least 30%, and further during warm working. A method for producing high-strength steel with excellent material properties after warm working, characterized by heating to a range of 750 to 400°C and processing immediately or at a temperature of 750 to 250°C after cooling. 3, C: 0.03 to 0.20%, Si: 0.6% or less,
Mn: 0.5-2.0%, SolAl: 0.005-0
．． 08%, the balance iron and unavoidable impurities is subjected to hot rolling at a cumulative reduction rate of at least 30% at 900°C or lower, and then undergoes at least transformation at a cooling rate of at least air cooling to 100°C/sec. A material after warm processing, characterized by accelerated cooling to a temperature at which the temperature of A method of manufacturing high-strength steel with excellent properties. 4, C: 0.03 to 0.20%, Si: 0.6% or less,
Mn: 0.5-2.0%, SolAl: 0.005-0
．． 08%, further Nb: 0.005 to 0.1%, V: 0.
005-0.15%, Ti: 0.005-0.15%,
Cu: 1.0% or less, Cr: 1.0% or less, Ni: 3.
5% or less, Mo: 1.0% or less, B: 0.0005 to 0
．． Steel containing one or more of 0.003% and the balance consisting of iron and unavoidable impurities is subjected to hot rolling at a cumulative reduction rate of at least 30% at 900°C or less, and then air-cooled or more. Accelerate cooling at a cooling rate of 100°C/sec to at least the temperature at which transformation ends, further heat to a range of 750 to 400°C during warm processing, and process at 750 to 250°C immediately or after cooling. A method for manufacturing high-strength steel with excellent material properties after warm working.