JPH09249915A - Production of high toughness steel small in dispersion in material and excellent in fatigue resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high toughness steel in which restriction in the producing process is eliminated, dispersion in the material is small, and furthermore, both fatigue resistance and weldability are satisfied. SOLUTION: A steel stock having a compsn. contg., by weight, 0.001 to 0.02% C, 1.0 to 3.0% Mn, 0.005 to 0.20%. Ti, 0.0003 to 0.0050% B, 0.5 to 2.0% Cu and <=0.10% Al is heated at the Ac3 to 1350 deg.C, thereafter, rolling in which the draft in the temp. range of >=950 deg.C is regulated to >=20% and the total draft is regulated to >=30% is executed in the temp. range of <650 to under 800 deg.C, and subsequently, cooling is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in the fields of construction, marine structures, pipes, shipbuilding, storage tanks, civil engineering, construction machinery, etc. The present invention relates to a method for producing a steel material such as a shaped steel or a steel bar, particularly a high-toughness steel material with little material variation and excellent fatigue resistance.

[0002]

2. Description of the Related Art As described above, thick steel materials represented by thick steel plates are used in various fields and have been improved in properties such as high strength and high toughness. In recent years, it has been required that these characteristics are uniform in the thickness direction and that the variations among steel materials are small.

[0003] For example, "Iron and steel 74th year (1988) No. 6
Nos. 11 to 21 on page 11 show that as buildings become more tall,
It has been reported that buildings have been designed to absorb vibration energy and prevent collapse due to deformation of buildings in response to huge earthquakes. Specifically, when an earthquake occurs, the framework of the building is collapsed in a predetermined shape, and the collapse of the building is prevented by plasticizing the framework. In other words, it is premised that the framework of the building behaves as intended by the designer at the time of the earthquake, and the designer must fully understand the strength ratio of steel materials such as columns and beams of the building. It is. Therefore, it is indispensable that steel materials such as steel plates and H-section steels used for columns and beams are homogeneous, and variations in strength of steel materials pose a serious problem.

Since high tensile strength and high toughness are required for steel materials used for construction and shipbuilding, this type of steel material is usually manufactured by the controlled rolling control cooling method, so-called TMCP method. Is. However, when a thick steel material is manufactured by the TMCP method, the cooling rate in the cooling process after rolling differs in the thickness direction or between the steel materials, and the structure changes, so that the obtained steel material has a thickness direction or between the steel materials. Material variations occur. Variations in material include those that appear in the thickness direction especially in thick steel plates, those that appear between webs and flanges due to uneven cooling between the webs and flanges in H-section steel, or those that appear between lots, etc. There is.

Therefore, in Japanese Patent Laid-Open No. 4-224623, Nb
Is added, the cooling rate after rolling is set to 3 ° C / s or more, and the upper limit of the cooling stop temperature is set to 500 ° C, so that the structure in the plate thickness direction is a structure in which ferrite and bainite are mixed, It has been proposed to increase the strength of the central portion and reduce the hardness difference in the plate thickness direction. However, the cooling rate must be strictly controlled to 3 ° C./s or more even in the central part of the plate thickness, and if the cooling rate distribution occurs in the plate thickness direction, the material will be dispersed immediately, so the manufacturing process is strict. Therefore, it was not suitable for production on an industrial scale.

On the other hand, in JP-A-62-130215, the strength is secured by Cu precipitation strengthening, and after the hot rolling, the strength is reduced to 0.
It is possible to improve low temperature toughness by cooling to 300 to 700 ° C. at a cooling rate of 5 ° C./s or more, then maintaining the temperature range of 500 to 650 ° C. for a certain period of time, and then cooling to room temperature.
Proposed. However, this technology is aimed at improving low temperature toughness, and by suppressing the variation in materials in the various forms described above, it is possible to satisfy the homogeneity of materials required for recent structural steels. Difficult to do.

It is also important for the steel materials for the above-mentioned applications to have excellent fatigue resistance, that is, to have a high fatigue limit, which is the maximum stress that does not cause fatigue failure even under infinite repeated load. The fatigue limit of this base material is mainly determined by the tensile strength, and in order to raise the fatigue limit, it is effective to increase the amount of additive elements of steel to increase the tensile strength. For example, Japanese Patent Laid-Open No. 2-197525 discloses Nb, Cr, and Mo
It has been shown that the addition of more than one species improves the fatigue resistance by preventing the heat-affected zone of the welded joint from softening, which is also true for the base metal.

However, if the amount of the additional element is increased to increase the strength, the weld crack susceptibility index also rises, and the weldability deteriorates, which is a new problem.

[0009]

DISCLOSURE OF THE INVENTION The present invention solves the above problems, that is, there is no restriction on the cooling rate after rolling, there is little variation in the material in the thickness direction and between steel materials, and fatigue resistance and welding. The purpose of the present invention is to propose a method for producing a high toughness steel material having both good properties.

[0010]

[Means for Solving the Problems] The material variation of thick steel materials, typically thick steel sheets, is caused by a large change in the cooling rate in the thickness direction from the steel sheet surface to the central portion or variation in manufacturing conditions in the cooling process. This is because the change in the cooling rate caused by the change causes the microstructure variation. In order to avoid such a change in the structure, it is important to obtain a homogeneous structure in a wide cooling rate range.

[0011] Therefore, the inventors of the present invention have repeatedly studied the method of obtaining a homogeneous structure even if the manufacturing conditions are changed, returning to the origin, and by changing the cooling rate by newly redesigning the component composition. However, it has been found that a steel sheet having a uniform structure in the thickness direction and having little material variation can be obtained.

That is, by adding an appropriate amount of Mn, Ti and B, the structure was made into a bainite single phase without depending on the cooling rate. In addition, the structure was also strengthened by the structure, and by utilizing it for Cu precipitation and solid solution strengthening, the tensile strength was increased and the fatigue resistance was improved. Further, by rolling at a reduction ratio of 30% or more in the low recrystallization temperature range or the two-phase temperature range, the fatigue limit is increased as shown in FIG.

Here, in FIG. 1, C: 0.008 wt%, Si: 0.
25wt%, Mn: 1.8wt%, Ti: 0.01wt%, B: 0.0015wt
%, Cu: 1.2 wt%, Nb: 0.035 wt% and Al: 0.030 wt
After holding the steel slab of the composition system consisting of 10% at 1150 ° C for 1 h, reduce the pressure by 10 to 50% to each finishing temperature and air cool to 50 mm.
It is a result of manufacturing a thick steel plate and performing a one-way swing fatigue test.

In the present invention, a bainite single-phase structure can be obtained without using rapid cooling, so that heat treatment such as quenching is unnecessary, and low-cost production is possible.

According to the present invention, C: 0.001 to 0.02 wt% and Mn:
1.0-3.0 wt%, Ti: 0.005-0.20 wt%, B: 0.0003-
0.0050wt%, Cu: 1.0 ~2.0 wt % and Al: a steel material comprising the composition containing less 0.10 wt%, after heating to a temperature of A _c3 point to 1350 ° C., the reduction ratio in the temperature range of not lower than 950 ° C. Rolling with 20% or more and total reduction of 30% or more is finished in a temperature range of less than 800 ° C and 650 ° C or more, and then cooled. It is a method of manufacturing a steel material.

Further, according to the present invention, C: 0.001 to 0.02 wt
%, Mn: 1.0 to 3.0 wt%, Ti: 0.005 to 0.20 wt%, B:
0.0003 to 0.0050 wt%, Cu: 1.0 to 2.0 wt% and Al: 0.
A steel material with a composition containing 10 wt% or less, A _c3 point ~ 1350 ℃
After heating to above temperature, the rolling reduction is 20% in the temperature range above 950 ℃.
And rolling with a total reduction of 30% or more
Finish in a temperature range of 0 ° C or higher, then cool to 650 ° C or lower, stop cooling at 650 ° C or lower, and immediately after reheating or cooling stop 1200 to 6000 in the temperature range of 500 to 650 ° C.
It is a method for producing a high-toughness steel material which is characterized by holding for s and has little material variation and excellent fatigue resistance.

Here, the above steel material further contains Si: 0.60 wt.
% Or less, Cr: 0.2 wt% or less, Ni: 2.0 wt% or less, Mo: 0.
2 wt% or less, W: 0.5 wt% or less, V: 0.005 to 0.20 wt%
And Nb: one or more of 0.20 wt% or less, further Ca: 0.006 wt% or less and REM: 0.02 wt% or less 1
The composition containing one or two kinds is effective for increasing the strength and further improving the toughness of the weld heat affected zone.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reasons for limiting each chemical component of the steel material of the present invention will be explained. C: 0.001 to 0.02 wt% C requires a content of 0.001 wt% or more to secure the strength, but if it exceeds 0.02 wt%, the weldability is significantly impaired, so 0.001 to 0.02 wt% was set. .

Mn: 1.0-3.0 wt% Mn is to obtain a homogeneous bainite structure substantially without depending on the cooling rate and to secure strength by solid solution strengthening to improve fatigue resistance. 1.0 wt% or more is required. On the other hand, the content of more than 3.0 wt% deteriorates the toughness, so the content is made 1.0 to 3.0 wt%.

Ti: 0.005 to 0.20 wt% Ti is required to be 0.005 wt% or more for forming a bainite structure and for improving the HAZ toughness, but the effect is saturated when it exceeds 0.20 wt%. 0.005-0.20wt%
Range.

B: 0.0003 to 0.0050 wt% B is a component that enhances the hardenability of steel and contributes to the formation of bainite structure by the addition of a trace amount, and at least 0.00
03wt% is necessary. On the other hand, if it exceeds 0.0050 wt%, BN
0.0003 to form a B compound, which deteriorates toughness.
~ 0.0050wt% limited.

Cu: 1.0 to 2.0 wt% Cu is required to be 1.0 wt% or more to increase the strength and improve the fatigue resistance by precipitation strengthening and solid solution strengthening. On the other hand, if it exceeds 2.0 wt%, the cost will rise, so
2.0 wt% or less.

Al: 0.10 wt% or less Al needs to be 0.01 wt% or more for deoxidation. However, if added in excess of 0.10 wt%, the cleanliness of the steel will deteriorate, so the upper limit is made 0.10 wt%.

According to the present invention, by adjusting the components to the above basic composition, a homogeneous structure, specifically, a bainite structure having a volume ratio of 90% or more can be obtained with almost no dependence on the cooling rate after rolling. There is a feature in being able to. This feature is apparent from the experiment whose results are shown in FIG.

That is, regarding the steel adjusted to the composition according to the present invention (invention example) and the conventional steel used in building materials (conventional example), the cooling rate in the manufacturing process is 0.1 to
FIG. 2 shows the results of an examination of the tensile strength of the steel sheets obtained by variously changing it at 50 ° C./s. It can be seen from the figure that by adjusting the components according to the present invention, a constant strength can be obtained without depending on the cooling rate. In particular, the YS and TS values show less variation over a wide range of cooling rates that cannot be predicted conventionally. This is where the addition of appropriate amounts of Mn, Ti and B contributes as described above. Therefore, even if the cooling rate changes in the thickness direction of the thick steel sheet, the strength does not change depending on the cooling rate, and a thick steel sheet with less material variation in the thickness direction can be obtained.

In the invention examples, C: 0.005 wt% and Mn: 1.
80wt%, Ti: 0.01wt%, B: 0.0013wt%, Cu: 1.1wt%
And Al: 0.035 wt%, the composition is such that the balance is iron and inevitable impurities, while the conventional example is C: 0.14
wt%, Si: 0.4 wt%, Mn: 1.31 wt%, Al: 0.024 wt%,
Nb: 0.015 wt% and Ti: 0.013 wt%. Then, in the same manufacturing process, the cooling rate was changed to obtain a thickness of 50
A large number of mm thick steel plates were manufactured, and the tensile strength was measured using test pieces taken from each thick steel plate.

In the present invention, the strength and toughness level can be freely controlled by adding a predetermined chemical component to the basic component. At this time, since the already obtained homogeneous structure is less affected by the addition of a new component, a thick steel plate having high strength and / or high toughness with little material variation can be easily obtained.

First, in order to improve the strength, as a precipitation strengthening component, Si: 0.60 wt% or less, Cr: 0.2 wt% or less, N:
One or more of i: 2.0 wt% or less, Mo: 0.2 wt% or less, W: 0.5 wt% or less, V: 0.005 to 0.20 wt%, Nb: 0.20 wt% or less can be added. When these precipitation strengthening components are added, it is possible to further strengthen them by performing a precipitation strengthening treatment described later. Since these components are effective even in a small amount, the lower limits other than V can be appropriately set.

Si: 0.60 wt% or less Si is preferably added in an amount of 0.05 wt% or more to increase the strength, but if added in excess of 0.60 wt%, weldability is impaired, so the upper limit is made 0.60 wt%. .

Cr: 0.2 wt% or less Cr is effective in increasing the strength of the base metal and the welded portion, but if added over 0.2 wt%, the weldability and the toughness of the heat affected zone (HAZ) deteriorate. Therefore, it is added in the range of 0.2 wt% or less.

Ni: 2.0 wt% or less Ni is effective in improving strength and toughness and preventing Cu cracking during rolling when Cu is added, but is expensive and excessively added. Even so, the effect is saturated,
Add within 2.0 wt% or less. If the addition amount is less than 0.05 wt%, the above effect is insufficient, so the addition amount is 0.05 wt%.
% Is preferable.

Mo: 0.2 wt% or less Mo has the effect of increasing the strength at normal temperature and high temperature, but if it exceeds 0.2 wt%, the weldability deteriorates.
Add in the range of wt% or less.

W: 0.5 wt% or less W has the effect of increasing the high temperature strength, but it is expensive, and if it exceeds 0.5 wt%, the toughness deteriorates.
Add in the range of wt% or less.

V: 0.005-0.20 wt% V is added in an amount of 0.005 wt% or more for precipitation strengthening, but even if added over 0.20 wt%, the effect is saturated, so 0.20 wt% is added. The upper limit.

Nb: 0.20 wt% or less Nb is added for precipitation strengthening and formation of a granular bainitic ferrite structure, but if it exceeds 0.20 wt%, the effect is saturated, so it is made 0.20 wt% or less. .

In order to improve the toughness of HAZ,
At least one selected from Ca and REM can be added. Ca: 0.006 wt% or less Ca is effective in controlling the morphology of sulfide-based inclusions and improving the toughness of HAZ, but if it exceeds 0.006 wt%, inclusions in the steel are formed and the properties of steel are improved. Since it deteriorates, it is 0.006 wt% or less.

REM: 0.02 wt% or less REM is useful for forming ferrite precipitation nuclei and serves as oxysulfide to suppress the grain growth of austenite grains to improve the toughness of HAZ, but REM is added in excess of 0.02 wt%. Then, the cleanliness of the steel is impaired, so the content is made 0.02 wt% or less.

If Ca and REM are added in an amount of less than 0.001 wt%, the above HAZ toughness improving effect is insufficient, so it is preferable that the addition amount be 0.001 wt% or more.

A steel sheet having the above-mentioned composition has a uniform structure by adjusting the composition to the above-mentioned basic composition, so that it is not necessary to strictly control the production conditions, and a steel sheet of this kind is produced. Although it may be manufactured according to a usual method, the following manufacturing process is advantageously adapted to secure fatigue resistance and weldability together with suppression of material variation.

That is, after heating the steel slab whose composition has been adjusted to the above-mentioned basic composition to a temperature of Ac ₃ point to 1350 ° C., it is heated to 950 ° C.
The rolling reduction in the above temperature range is 20% or more and the total rolling reduction is 30%.
% Or more of rolling is finished in a temperature range of less than 800 ° C and 650 ° C or more, and then cooled, which is recommended as a method for improving fatigue resistance and weldability.

Here, the heating temperature is set to the A _c3 point to 1350 ° C. so that the structure is once austenite and the bainite structure is obtained in the subsequent rolling step.

Next, in the hot rolling, the rolling reduction in the temperature range of 950 ° C. or higher is set to 20% or more to obtain γ
The grain size of the structure is refined to make the transformed structure finer, the strength is enhanced to improve the fatigue resistance, and at the same time the weldability is secured by the finer structure.

Further, in hot rolling, the total rolling reduction is 30%.
Finish above the temperature range of less than 800 ° C and 650 ° C or more. Because the rolling finishing temperature is less than 800 ℃,
This is because the texture is developed to increase the fatigue crack growth resistance. That is, the texture prevents the development of fatigue cracks. In addition, rolling finish temperature is 800 ℃
If it is less than this, as shown in FIG. 3, the effect of ensuring high toughness of _v T _rs ≦ −40 ° C. can be expected. Note that FIG.
The No. 20 steel in Table 1 was heated to 1150 ° C, held for 1 hour, rolled by 30% up to 950 ° C, and then rolled by 77% between finishing temperature + 50 ° C and finishing temperature. Rolling,
It was conducted under the condition of air cooling. A Charpy test piece was sampled from the thickness center of the obtained steel sheet, and its vTrs was investigated.

On the other hand, when the rolling finishing temperature is less than 650 ° C., the deformation resistance becomes so large that rolling becomes impossible. Further, the total rolling reduction of hot rolling is set to 30% or more, because if the total rolling reduction is less than 30%, the texture does not develop and the fatigue resistance cannot be sufficiently improved. . The cooling rate after that is not particularly limited, but to secure toughness,
It is preferable not to exceed 30 ° C / s.

Note that, with the intention of strengthening Cu precipitation, after rolling,
Cooling to 650 ° C or lower, stopping cooling at 650 ° C or lower, and holding for 1200 to 6000s in the temperature range of 500 to 650 ° C immediately after reheating or cooling stop secures high strength and improves fatigue resistance. It is effective in improving.

That is, if the temperature range is 500 ° C. or higher, sufficient diffusion of Cu does not take place, and conversely, if it exceeds 650 ° C., Cu does not diffuse.
Does not strengthen precipitation. Therefore, the holding temperature should be 500 ° C to 650 ° C. Also, during or after cooling, if it is not reheated for more than 1200s, Cu precipitation will not be sufficient, and if it is held for more than 6000s, overaging will occur and the strength will decrease.
The following is assumed.

[0047]

【Example】

Example 1 A steel plate having a thickness of 50 mm was manufactured according to the conditions shown in Table 2 by using the steel slabs adjusted to various component compositions shown in Table 1.

Each of the thick steel plates thus obtained was subjected to a tensile test and a Charpy test to investigate its mechanical properties and to evaluate the variation in strength in the thickness direction. The hardness distribution in the plate thickness direction was investigated by measuring the pitch. In addition, in order to evaluate the toughness of HAZ, a peak temperature of 1400 ° C was measured from 800 ° C to 500 ° C.
After a thermal cycle of cooling to ℃ in 10s (corresponding to the thermal history of CGHAZ when welding with a heat input of 50kJ / cm),
A Charpy test piece was collected and the Charpy absorbed energy at 0 ° C was measured. Furthermore, in order to evaluate the fatigue resistance, a fatigue test of unidirectional tension with a frequency of 10 Hz was performed. The surface of the test piece was ground.

As shown in Table 3 of the results of each of these investigations, the thick steel sheet obtained according to the present invention has a tensile strength of 500 MPa or more and a uniform structure, so that the hardness in the thickness direction is It can be seen that the variation is extremely smaller than that of the steel material 21, and the difference between the maximum hardness value and the minimum hardness value is less than 20 in H _v . Further, the steel materials 1 to 20 (excluding the steel materials 9 and 10) that have been subjected to the precipitation treatment by adding the precipitation strengthening component have higher strength than the steel material 9 that does not contain the precipitation strengthening component.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

EFFECTS OF THE INVENTION The steel material obtained by the present invention does not vary with the cooling rate used in the cooling step in the production on an industrial scale. Therefore, it is possible to industrially stably supply a thick steel plate, which is expected to have an increased demand in the future and in which the material variation in the thickness direction is extremely small. The invention is also advantageously adapted to the field of shaped steel.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a finishing temperature and a 2 × 10 ⁶ times fatigue strength of a thick steel plate.

FIG. 2 is a diagram showing a relationship between a cooling rate and strength of a thick steel plate.

FIG. 3 is a diagram showing a relationship between rolling finish temperature and toughness.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masanori Nishimori, 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (no address) Inside the Mizushima Works, Kawasaki Steel Co., Ltd. (72) Fumaru Kawabata, Mizushima-kawasaki-dori, Kurashiki-shi, Okayama Prefecture 1-chome (without street number) Inside Kawashima Steel Co., Ltd. Mizushima Steel Works (72) Inventor Shinichi Amano 1-chome (without street number) Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture Inside Kawashima Steel Co., Ltd. Mizushima Steel Works

Claims (4)

[Claims] 1. C: 0.001 to 0.02 wt%, Mn: 1.0 to 3.0 wt%, Ti: 0.005 to 0.20 wt%, B: 0.0003 to 0.0050 wt%, Cu: 1.0 to 2.0 wt% and Al: 0.10 wt%. After heating a steel material having a composition including the following to a temperature of A _c3 point to 1350 ° C, 950
In the temperature range above ℃, the rolling reduction is 20% or more and the total rolling reduction is
A method for producing a high-toughness steel material having less variation in material and excellent in fatigue resistance, which comprises rolling at least 30% of rolling in a temperature range of less than 800 ° C and 650 ° C or more, and then cooling. 2. C: 0.001 to 0.02 wt%, Mn: 1.0 to 3.0 wt%, Ti: 0.005 to 0.20 wt%, B: 0.0003 to 0.0050 wt%, Cu: 1.0 to 2.0 wt% and Al: 0.10 wt%. After heating a steel material having a composition including the following to a temperature of A _c3 point to 1350 ° C, 950
In the temperature range above ℃, the rolling reduction is 20% or more and the total rolling reduction is
Rolling of 30% or more is completed within a temperature range of less than 800 ° C and 650 ° C or more, then cools to 650 ° C or less, stops cooling at 650 ° C or less, and immediately after reheating or cooling, 500 ~
A method for producing a high-toughness steel material having little material variation and excellent fatigue resistance, which is characterized by holding for 1200 to 6000 seconds in a temperature range of 650 ° C. 3. The steel material according to claim 1, further comprising: Si: 0.6 wt% or less, Cr: 0.2 wt% or less, Ni: 2.0 wt% or less, Mo: 0.2 wt% or less, W: 0.5 wt% or less. , V: 0.005 to 0.20 wt% and Nb: 0.20 wt% or less. A method for producing a high-toughness steel material having excellent fatigue resistance and having little composition variation in a composition containing one or more. 4. The steel material according to claim 1, 2 or 3,
Furthermore, a method for producing a high toughness steel material having a composition containing one or two kinds of Ca: 0.006 wt% or less and REM: 0.02 wt% or less and having excellent fatigue resistance with little material variation.