JPH0670248B2 - Manufacturing method of ultra-high-strength steel plate for welding with excellent homogeneity in the thickness direction - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a method for manufacturing a thick steel sheet by direct quenching and tempering treatment, and particularly to a tensile steel sheet having excellent homogeneity and weldability in the sheet thickness direction and little anisotropy. The present invention relates to a method for producing an ultrahigh tensile strength and toughness steel sheet having a strength of 100 kgf / mm ² .

<Prior art> In recent years, the penstock, pressure vessels, marine structures, etc. have become larger and have higher performance, and the steel sheets used have a tensile strength of 100 kgf / mm ²
The application of thick-walled steel sheets having a super high tensile strength has been studied.

Until now, reheating after hot rolling and quenching and tempering have been applied in the production of this type of ultra-high strength steel.
In order to satisfy the strength and toughness of the central part of the thick steel plate, it was necessary to increase the amount of additive alloy elements. Therefore, in the vicinity of the surface layer portion where the cooling rate at the time of quenching is high, excessive quenching occurs and the toughness deteriorates, causing non-uniformity in the plate thickness direction.

In order to solve this problem, reheating and quenching treatment is repeated twice or more, or Nb is added to make austenite grains finer.

However, in recent years, a direct quenching and tempering method has been developed in which quenching is performed immediately after hot rolling without air cooling to room temperature, and then tempering is performed, and there are several methods for producing a homogeneous steel sheet in the thickness direction using this method. Proposals have been made.

For example, in JP-A-61-56268, a slab after solution treatment is used, hot rolling with a cumulative reduction of 40% or more is performed in a low temperature range of 930 to 800 ° C during hot rolling, and then direct quenching. Tempered, tempered martensite structure near the surface layer,
A method is disclosed in which a steel sheet having a mixed structure of tempered martensite and lower bainite in the central portion is used. Further, in JP-A-62-196326, the addition of Ti secures the effect of improving the hardenability of B, and a light reduction is applied in the low temperature region of 900 ° C. or lower (the unrecrystallized region of austenite grains) to form the surface layer. A method is disclosed in which work strain is accumulated only in the vicinity of the portion to reduce the hardenability to the same extent as in the central portion.

However, the former requires solution treatment, which is disadvantageous in terms of cost, and both focus only on low-temperature decompression rolling.Therefore, the anisotropy of the steel sheet is large in the former and stable in the latter. Expected to penalize manufacturing and productivity. Furthermore, it cannot be said that the technology was completed without consideration of softening and impact characteristics of the welded part.

<Problems to be Solved by the Invention> An object of the present invention is to produce an economical steel sheet by direct quenching and tempering treatment, in addition to having high strength, excellent homogeneity in the sheet thickness direction, and little anisotropy. It is an object of the present invention to provide a method of manufacturing a steel sheet having high toughness with less softening of a welded portion.

<Means for Solving the Problems> In the present invention, C: 0.05 to 0.16 wt%, Si: 0.05 to 0.20 wt%, Mn: 0.
60 to 1.50 wt%, Cr: 0.30 to 0.80 wt%, Mo: 0.20 to 0.80 wt%, N
i: 0.80 to 3.00 wt%, V: 0.030 to 0.100 wt%, Al: 0.010 to 0.08
0 wt%, B: 0.0005 to 0.0025 wt%, N: 0.0040 wt% or less, and if necessary Cu: 1.0 wt% or less, Nb: 0.050 wt%
Below, Ca: 0.0100 wt% or less of one or more, the carbon equivalent Ceq. Defined by the following formula is 0.52-0.60%, the balance Fe and unavoidable impurities steel 1050- 12
After heating and soaking to 00 ° C, hot rolling with a volume reduction of 50% or more is performed in the temperature range of 1000 to 900 ° C.
Rolling rate per pass is 10% in the temperature range of 10 ℃ or more
Ultra-high for welding with excellent homogeneity in the plate thickness direction, characterized by immediately quenching after tempering at a cumulative rolling reduction of 10 to 30% by light reduction rolling of less than ₁ and then tempering at a temperature of ₁ point or less of Ac. It is a manufacturing method of a tension steel plate.

Ceq. = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
(%) <Operation> The reasons for limiting the steel composition and rolling conditions in the present invention will be described below.

C is required to be 0.08 wt% or more in order to obtain the desired strength, but if it exceeds 0.16 wt%, the toughness of the base material and the welded portion deteriorates, so the range is 0.08 to 0.16 wt%.

Si is necessary as a deoxidizing agent during steelmaking and for securing strength by solid solution strengthening, but as shown in Fig. 1, the effect of Si on the bond toughness of the weld is significant, and In order to secure high toughness, the addition amount range is 0.05 to 0.20 wt%.

Fig. 1 shows 0.11wt% C-0.90wt% Mn-1.50wt% Ni-
0.50wt% Cr-0.50wt% Mo-0.065wt% V-B system with different amount of Si added, submerged arc weld (heat input: 50k
J / cm) impact characteristics.

Mn is 0.60wt% to improve hardenability and ensure strength.
The above is required, but if it exceeds 1.50 wt%, the toughness of the steel plate and welds deteriorates, so the range is 0.60 to 1.50 wt%.

Cr has the effect of improving hardenability and strength, and is 0.40 wt%
The above is required, but if it exceeds 0.80 wt%, the weldability decreases and SR crack susceptibility increases, so the range is 0.40 to 0.80 wt%.

Mo has the effect of improving temper softening resistance and SR crack resistance while improving hardenability, and 0.30 wt% or more is required, but 0.80 wt
%, The weldability and toughness deteriorate, and it becomes economically disadvantageous. Therefore, the range is 0.30 to 0.80 wt%.

V is an element that improves hardenability and increases temper softening resistance and is required to be 0.030 wt% or more from the viewpoint of securing strength, but if it exceeds 0.100 wt%, the toughness of the welded portion deteriorates.
The range is 0.030 to 0.100 wt%.

Ni is effective in improving the strength and toughness of steel plates and welds, and is required to be 0.60 wt% or more. However, if it exceeds 3.00 wt%, the effect saturates and it is economically disadvantageous. .
The range is 60 to 3.00 wt%.

Al has a deoxidizing effect and is required to be 0.010 wt% or more.
If it exceeds 80 wt%, the toughness of the steel plate and welded part deteriorates, so the range is 0.010 to 0.080 wt%.

A small amount of B improves the hardenability and is effective in securing the strength and toughness, and is required to be 0.0005 wt% or more, but if it exceeds 0.0025 wt%, the toughness of the steel plate and the welded portion is deteriorated, so 0.0005-
The range is 0.0025wt%.

N combines with B during rolling to precipitate BN and reduces the effect of improving the hardenability of B, so N must be 0.0040 wt% or less.

Further, in addition to the above components, one or more of the following components may be added for the purpose of preventing softening of the steel plate and the weld and improving toughness.

Cu has the effect of increasing the strength of the steel sheet, but if it is too much, the hot workability and weldability deteriorate, so the upper limit is 1.0 wt%.
And

Nb has the effect of increasing the strength and making the austenite grains finer and improving the toughness, but if it is too large, the toughness of the welded portion is significantly deteriorated, so its upper limit is made 0.050 wt%.

Ca has the effect of controlling the morphology (spheroidizing) of sulfides and has the effect of improving anisotropy, but if it is too large, the cleanliness decreases and the toughness deteriorates, so its upper limit is made 0.0100 wt%.

In addition, Ceq.
= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (%) The carbon equivalent Ceq. Should be quantitatively considered so that the carbon equivalent Ceq. Is in the range of 0.52 to 0.60%. . This is because if it is less than 0.52%, the joint strength is insufficient because the softening of the weld is large, and if it exceeds 0.60%, the hardness of the weld increases and the weld crack susceptibility is increased.

Next, the reasons for limiting the heating and rolling conditions will be described.

The above composition steel is made into a slab steel ingot by a continuous casting method or an ingot making method, heated to 1050 to 1200 ° C, soaked, and then hot rolled.
Here, the reason for limiting the acceleration temperature is as follows. That is, if the temperature is lower than 1050 ° C, the hot rolling described later becomes difficult, and if the temperature exceeds 1200 ° C, the growth and coarsening of austenite grains are remarkable, and it is necessary to increase the number of rolling passes to eliminate them (fine grain). And productivity is hindered, which is not preferable.

Next, rolling in the high temperature recrystallization region in the temperature range of 1000 to 900 ° C. is very effective for isotropic refining of austenite grains by repeating rolling-recrystallization. However, if the cumulative rolling reduction is less than 50%, the effect is not sufficient, and subsequent rolling causes mixed grains and expanded austenite grains to easily cause anisotropy of the steel sheet, so a cumulative rolling reduction of 50% or more is required. . Fig.2 shows 0.11wt% C-0.90wt% Mn-0.12wt% Si-1.50wt
% Ni-0.50wt% Cr-0.50wt% Mo-0.065wt% Using V-B steel, the characteristics in the L and C directions when only the amount of reduction in the temperature range of 1000 to 900 ° C is changed It is the result of examining the difference. From the figure, by setting the cumulative reduction rate to 50% or more,
It can be seen that anisotropy (material change due to L and C directions) is reduced.

Further, the reasons for limiting the rolling conditions in the low temperature range of less than 900 ° C and 810 ° C or more are as follows. That is, in the light reduction rolling with a reduction rate of 10% or less per pass, the austenite grains in the vicinity of the surface layer are more finely expanded than in the central portion, so that the hardenability of the surface layer is suppressed. . However, if the cumulative reduction ratio at this time is less than 10%, the effect is small, and if it exceeds 30%, the deformation zone is excessive and the hardenability deteriorates too much, and the hardenability effect due to B cannot be expected.
Since the strength and toughness of the central part becomes poor and the anisotropy becomes remarkable, the cumulative rolling reduction must be within the range of 10 to 30%.

Furthermore, the temperature range where this rolling effect is more effectively exhibited is 90.
Below 0 ℃ and above 810 ℃. That is, at a high temperature of 900 ° C. or higher, since it is in the recrystallization region, fine expansion of austenite grains cannot be expected and quenching becomes excessive. Also, 810 ℃
If the amount is less than the above value, the hardenability becomes too low and desired strength and toughness cannot be secured in the vicinity of the surface layer and the center.

Immediately after the above hot rolling, quenching is performed so as to have predetermined performance. Note that the microstructure of the steel sheet is a martensite + lower bainite mixed microstructure in the vicinity of the surface layer portion and the center portion due to the above-described rolling conditions and quenching treatment, and the austenite grain shape is fine in the center portion and expanded grain, and the vicinity of the surface layer portion It is more expanded than that of the central part and many deformation zones are introduced.

After that, it is necessary to carry out a tempering treatment at a point of Ac ₁ or less with the main purpose of further homogenizing and toughening in the plate thickness direction.

In other words, the gist of the present invention is as follows. That is, the rolling conditions are set so that a mixed structure of martensite and lower bainite is formed at each position in the plate thickness direction by using a component system that takes into consideration softening prevention of the welded joint and ensuring high toughness.
That is, to obtain a finely sized austenite grains with a sufficient reduction ratio in the high temperature region, and further subject to light reduction rolling in the low temperature region,
Suppresses hardenability near the surface layer and ensures homogeneity in the plate thickness direction.

The steel of the present invention can be melted in a converter or an electric furnace, made into a slab or a steel ingot by a continuous casting method or an ingot making method, and then subjected to slab rolling or thick plate rolling as it is to obtain a steel sheet.

<Example> A 5 ton steel ingot of each composition shown in Table 1 was melted by the ingot making method, and Table 2
After manufacturing a steel plate with a thickness of 50 to 75 mm under the respective manufacturing conditions shown in, the mechanical properties were investigated.

The results obtained are shown in Table 3. It can be seen from Table 3 that by applying rolling conditions within the scope of the present invention, an ultra-high-strength steel sheet with little anisotropy and excellent homogeneity can be stably produced. However, it is understood that when the added alloy amount is small (A steel), the strength is insufficient, and when it is large (E class), it is difficult to secure the toughness under the surface layer. In addition, as shown in Table 4, from the results of hardness measurement of the welded joint, the A class is softened at the welded part, and the E class is concerned that the hardness of the welded part is high and weld cracking may occur. B,
The maximum hardness of welded parts of C and D is similar to HT-80 steel, and it is clear that they have sufficient weldability.

<Effects of the Invention> According to the present invention, as a penstock, a pressure vessel, and a steel for offshore structures, an ultra-high-strength steel sheet that has the same weldability as HT-80 kg steel and is excellent in the homogeneity in the plate thickness direction is inexpensive. It can be provided, and its significance is great.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the effect of the amount of Si added on the impact properties of welds, and FIG. 2 is a graph showing the effect of the amount of reduction at high temperatures on the anisotropy of steel materials. is there.

Claims (2)

[Claims] 1. C: 0.05 to 0.16 wt%, Si: 0.05 to 0.20 wt%, Mn:
0.60 to 1.50 wt%, Cr: 0.30 to 0.80 wt%, Mo: 0.20 to 0.80 wt
%, Ni: 0.80 to 3.00 wt%, V: 0.030 to 0.100 wt%, Al: 0.010 to
0.080 wt%, B: 0.0005 to 0.0025 wt%, N: contains 0.0040 wt% or less, the carbon equivalent Ceq. Defined by the following formula is 0.52 to 0.
Steel containing 60% and balance Fe and unavoidable impurities,
After heating and soaking to 1050 to 1200 ℃, hot rolling with cumulative rolling reduction of 50% or more is performed in the temperature range of 1000 to 900 ℃, and then 90
Cumulative reduction of 10 to 30% by light reduction rolling with a reduction of less than 10% per pass in the temperature range of less than 0 ℃ and 810 ℃ or more.
After that, quenching is performed immediately, and then tempering is performed at a temperature of ₁ point or less of Ac, and a method for producing an ultra-high-strength steel plate for welding having excellent homogeneity in the plate thickness direction. Ceq. = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
(%) 2. C: 0.05 to 0.16 wt%, Si: 0.05 to 0.20 wt%, Mn:
0.60 to 1.50 wt%, Cr: 0.30 to 0.80 wt%, Mo: 0.20 to 0.80 wt
%, Ni: 0.80 to 3.00 wt%, V: 0.030 to 0.100 wt%, Al: 0.010 to
0.080 wt%, B: 0.0005 to 0.0025 wt%, N: 0.0040 wt% or less, further Cu: 1.0 wt% or less, Nb: 0.050 wt% or less, Ca:
It contains one or more of 0.0100 wt% or less, the carbon equivalent Ceq. Defined by the following formula is 0.52 to 0.60%, and the balance Fe
And steel made of unavoidable impurities is heated to 1050 to 1200 ℃ and soaked, and then the cumulative rolling reduction is 50% in the temperature range of 1000 to 900 ℃.
After the above hot rolling, and subsequently in the temperature range of less than 900 ° C and 810 ° C or more, the reduction ratio per pass is less than 10% by light reduction rolling to make the cumulative reduction ratio 10 to 30%, and then immediately quenching. And then tempering at a temperature not higher than the Ac ₁ point, which is a method for producing a super-high-strength steel plate for welding having excellent homogeneity in the plate thickness direction. Ceq. = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
(%)