JPH0277521A - Production of ultra-high-tension steel sheet for welding having excellent homogeneity in thickness direction - Google Patents

Abstract

PURPOSE:To produce the title ultra-high-tension steel sheet for welding having excellent homogeneity in its thickness direction by successively applying heating, soaking, hot rolling, low-pressure rolling, hardening, and tempering to a steel having specified composition and carbon equivalent under specified conditions. CONSTITUTION:A steel contg., by weight, 0.05-0.16% C, 0.05-0.20% Si, 0.060-1.50% Mn, 0.30-0.80% Cr, 0.20-0.80% Mo, 0.80-3.00% Ni, 0.030-0.100% V, 0.010-0.080% Al, 0.0005-0.0025% B, <=0.0040% N, the balance Fe, and inevitable impurities and having the carbon equivalent Ceq defined by the equation of 0.52-0.60% is heated at 1050-1200 deg.C, and soaked. The steel is hot-rolled at 1000-900 deg.C and at the cumulative draft of >=50%. The rolled sheet is successively rolled at 900-810 deg.C under low pressure at the draft of <10% per pass to the cumulative draft of 10-30%, then immediately hardened, and tempered at a temp. below the Ac1 point.

Description

[Detailed Description of the Invention] <Industrial Field of Application> The present invention relates to a method for manufacturing thick steel plates by direct quenching and tempering treatment, and in particular has excellent homogeneity and weldability in the J7 direction of the plate, and has low tensile anisotropy. The present invention relates to a method of manufacturing an ultra-high tensile strength steel plate having a strength of 100 kgf/mj class.

<Conventional technique j4t> In recent years, in response to the increase in size and performance of pen stocks, pressure vessels, and offshore structures, the tensile strength of the steel plates used has increased to 100 kgf.
The application of thick steel plates with ultra-high tensile strength of /mj class is being considered.

Until now, in the production of this type of ultra-high tensile strength steel, reheating, quenching and tempering treatments have been applied after hot rolling.
In order to satisfy the strength and toughness of the center of the thick steel plate, it was necessary to increase the amount of added alloying elements. Therefore, in the vicinity of the surface layer where the cooling rate during quenching is fast, excessive quenching results in decreased toughness and non-uniformity in the thickness direction has become a problem.

In order to solve this problem, methods have been used such as repeating reheating and quenching two or more times or adding Nb to make the austenite grains finer.

However, in recent years, a direct quenching and tempering treatment method has been developed in which quenching is performed immediately after hot rolling without air cooling to room temperature, followed by tempering. Several proposals have been made regarding l-plate manufacturing methods.

For example, in Japanese Patent Application Laid-Open No. 61-56268, a slab after solution treatment is used, and a
It is hot rolled at a cumulative reduction rate of 40% or more in the low temperature range of ゛C, and then directly quenched and tempered to create a tempered martensite structure near the surface and a mixed structure of tempered martensite and lower bainite in the center. A method for producing a steel plate is disclosed. In addition, in JP-A-62-196326, the hardenability improvement effect of B is maintained by 61% by adding TI,
Furthermore, a method is disclosed in which processing strain is accumulated only in the vicinity of the surface layer by applying light pressure in a low temperature range of 900°C or lower (non-recrystallized region of austenite grains), and the hardenability is reduced to the same level as that in the center. ing.

However, the former requires solution treatment and is disadvantageous in terms of cost, and both focus only on low-temperature rolling. This is expected to be detrimental to manufacturing and productivity. Furthermore, it cannot be said that this technology has been perfected without considering the softening of the welded part and the impact characteristics.

<Problems to be Solved by the Invention> The purpose of the present invention is to produce a steel plate economically by direct quenching and tempering, and to achieve high strength, excellent homogeneity in the thickness direction, and little anisotropy. It is an object of the present invention to provide a method for manufacturing a steel plate that has high toughness with little softening of welded parts.

<Means for Solving the Problems> The present invention provides C: 0.05 to 0.16 t%, Si:
0.05~0.20wt%, Mn: 0.60~
1.50wt%, Cr: 0.30~0.80wt
%, Mo: 0.20-0.80wL%, N!
:0.80-3.00wt%.

V: 0.030-0.100 IIL%, Al
-0,010 to 0.08011t%, B: 0.00
05-0.0025wL%, N: 0.0040w
t% or less, and if necessary, Cu: 1
．． OwL% or less, Nb: 0.050wt% or less,
Ca: Contains one or more types of 0.0100 wt% or less, and the carbon equivalent eq defined by the following formula is 0
．． After heating and soaking steel to 1050 to 1200°C, the cumulative reduction rate is 50% in the temperature range of 1000 to 900°C.
After hot rolling as above, continue to below 900℃ and 810℃
In the above temperature range, after the cumulative reduction rate is set to 10 to 30% by light reduction rolling with a reduction rate of less than 10% per bath, immediately quenching is performed, followed by tempering at a temperature below the C1 point. This is a method for producing ultra-high tensile strength steel plates for welding that are characterized by excellent uniformity in the thickness direction.

Ceq, = C+Mn/6 +Si/24+Ni
/40+Cr15+io/ 4 + V /14 (%
) <Function> 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 metal and weld zone will deteriorate, so it is set in the range of 0.08 to 0.16 wt%.

Si is necessary as a deoxidizing agent during steel manufacturing and to ensure strength through solid solution strengthening, but as shown in Figure 1, the effect of Si on the bond toughness of the weld is remarkable, and the To ensure high toughness, the addition amount range is 0.05 to 0.
．． 20i1t%.

In addition, FIG. 1 shows O, 11wt%C0, 90wt%Mn-
1,504%Ni-0,50wt%Cr-0,50wt
%Mo-0,065wt%■-B system with varying Si addition amount, submerged arc welding part (heat input: 50kJ/
cm).

Mn is 0.6 to improve hardenability and ensure strength.
(1 mt% or more is required, but if it exceeds 1.50 mt%, the toughness of the steel plate and weld will deteriorate, so 0.60 to 1.
The range is 50wt%.

Cr is effective in improving hardenability and increasing strength, and is 0.4
0wt% or more is required, but if it exceeds 0.80wt%, weldability decreases and SR cracking susceptibility increases, so 0.4
The range is 0=0.80wt%.

Mo is effective in improving hardenability, temper softening resistance, and SR cracking resistance, and requires a content of 0.30 wt% or more; however, if it exceeds 0.80 wt%, weldability and toughness deteriorate, and it is also economically disadvantageous. 0.30 to 0.80 wL% because it is disadvantageous
The range shall be .

(2) is an element that improves hardenability and increases resistance to temper softening, and is required in an amount of 0.030 wt% or more to ensure strength, but if it exceeds 0.100 wt%, the toughness of the weld will deteriorate, so The range is .030 to 0.100 wt%.

Ni is effective in improving the strength and toughness of steel plates and welded parts, and 0.60 wL% or more is required, but 3.00 wt
If it exceeds 0.60 to 3.00 wt%, the effect will be saturated and it will be economically disadvantageous.

AI has a deoxidizing effect and is required at 0.010 wt% or more, but if it exceeds 0.080 wt%, the toughness of the steel plate and weld zone will deteriorate, so it should be added at 0.010 to 0.080 wL%.
The range shall be .

B is effective in improving hardenability and ensuring strength and toughness in a small amount, and 0.0005 wt% or more is required, but 0.00
If it exceeds 25 wt%, the toughness of the steel plate and the welded part will deteriorate, so the content should be in the range of o,oos to 0.0025 wt%.

Since N combines with B during rolling and precipitates BN, reducing the hardenability improvement effect of B, it is necessary to keep the content to 0,0040 wt% or less.

Furthermore, in addition to the above-mentioned components, one or more of the following components can be added for the purpose of preventing softening of the steel plate and the welded portion and improving the toughness.

Cu is effective in increasing the strength of steel sheets, but too much Cu reduces hot workability and weldability, so the upper limit is set at 1.0.
Let it be wt%.

Nb is effective in increasing strength and making austenite grains finer and improving toughness, but if too much, the toughness of the weld zone will deteriorate significantly, so the upper limit is set at 0.050 wt%.

Ca has the effect of controlling the shape of sulfide (spheroidization) and improving anisotropy, but if it is too large, the cleanliness decreases and the toughness deteriorates, so the upper limit is set to 0.0100 wt%.

Furthermore, in order to prevent softening of the weld and ensure toughness, Ce
q, = C+Mn/6 +Si/24+Ni/40
These various additive elements must be taken into consideration quantitatively so that the carbon equivalent Ceq, expressed as +Cr15 +Mo/4+V/14 (%), is in the range of 0.52 to 0.60%.

This is because if it is less than 0.52%, the weld zone will soften to a large extent, resulting in insufficient joint strength, and if it exceeds 0.60%, the hardness of the weld zone will increase, increasing the susceptibility to weld cracking.

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. The reasons for limiting the acceleration temperature are as follows. In other words, below 1050°C, hot rolling as described below becomes difficult, and above 1200°C, austenite grains grow and coarsen significantly, and to eliminate this (grain refinement), it is necessary to increase the number of rolling baths. This is undesirable as productivity is hindered.

Next, rolling in a high temperature recrystallization zone in a temperature range of 1000 to 900°C is very effective in isotropically refining austenite grains by repeating rolling and recrystallization. However, if the cumulative rolling reduction is less than 50%, the effect will not be sufficient, and subsequent rolling will result in mixed grains and expanded austenite grains, which will easily cause anisotropy in the steel sheet, so a cumulative rolling reduction of 50% or more is required. . Figure 2 shows 0.11wt%CO, 90wt%
Mn-0,12-mu%5i-1,504%Ni-0,
50Ht%CrO, 50wt%Mo-0,065
From Figure 0, which is the result of investigating the difference in properties in the C direction when only the rolling reduction amount in the temperature range of 1000 to 900°C was changed using wt% V-B series steel, the cumulative rolling reduction rate was calculated. By setting it to 50% or more, anisotropy (L.

It can be seen that the change in material properties due to direction C) is reduced.

Furthermore, the reason for limiting the rolling conditions in the low temperature range of less than 900°C and more than 810°C is as follows. In other words, light reduction rolling with a reduction rate of 10% or less per bath results in austenite grains that are more finely elongated near the surface layer than in the center, and therefore the hardenability of the surface layer is suppressed. . However, if the cumulative reduction rate at this time is less than 10%, the effect will be small, and if it exceeds 30%, the deformation zone will be too large and the hardenability will be too low, and the hardenability effect of B cannot be expected, and the center Since the strength and toughness of the steel become poor and the anisotropy becomes significant, it is necessary to set the cumulative reduction ratio in the range of 10 to 30%.

Furthermore, the temperature range in which this rolling effect is more effectively developed is 9
The temperature is less than 00°C and 810°C or higher. That is, 900℃
At higher temperatures, fine elongation of austenite grains cannot be expected because the austenite grains are in the recrystallization region, and over-quenching occurs. Moreover, if it is less than 810"C, the hardenability will be too low and the desired strength will not be achieved in both the vicinity of the surface layer and the center.

Toughness cannot be ensured.

After the above hot rolling, immediately quenching is performed to provide the desired performance. Note that, due to the above-mentioned rolling conditions and quenching treatment, the microstructure of the steel sheet is a martensite + lower bainite mixture both near the surface layer and in the center, and the austenite grain shape is fine in the center.

The grains become elongated, and the vicinity of the surface layer is more elongated than the center, and many deformed bands are introduced.

Thereafter, it is necessary to perform tempering treatment to an Ac point of 1 or less, with the main purpose of further homogenizing the plate in the thickness direction and increasing its toughness.

The gist of the present invention can be summarized as follows (i.e., using a composition system that takes into account prevention of softening of the welded joint and ensuring high toughness, martensite + lower bainite at each position in the plate thickness direction). Rolling conditions are set to obtain a mixed structure.

That is, by securing a sufficient rolling reduction rate in the high temperature range to obtain finely sized austenite grains, and further applying light reduction rolling in the low temperature range,
Suppresses hardenability near the surface layer and ensures uniformity in the plate thickness direction.

The steel of the present invention can be melted in a converter or electric furnace, made into a slab or steel ingot by a continuous casting method or an ingot-forming method, and then made into a steel plate by blooming rolling or plate rolling as it is.

<Example> 5-ton steel ingots with the respective compositions shown in Table 1 were melted by the ingot making method, and the compositions shown in Table 2 were melted.
After manufacturing 50 to 75 r1m thick steel plates under each manufacturing condition shown below, their mechanical properties were investigated.

The obtained results are shown in Table 3. From Table 3, it can be seen that by applying the rolling conditions within the range of the present invention, it is possible to stably produce ultra-high tensile steel sheets with less anisotropy and excellent homogeneity. However, it can be seen that when the amount of added alloy is small (A steel), the strength is insufficient, and when it is large (E steel), it becomes difficult to ensure the toughness under the surface layer. In addition, as shown in Table 4, the hardness measurement results of the welded joint show that steel A has a softened weld, and steel E has a high hardness of the weld, which raises concerns about weld cracking. B
, C, and D have the maximum hardness of the welded parts comparable to that of HT-80 steel, indicating that they have sufficient weldability.

<Effects of the Invention> The present invention provides an inexpensive ultra-high-strength steel plate that has weldability equivalent to that of HT-80kg steel and excellent homogeneity in the thickness direction, as steel for penstocks, pressure vessels, and offshore structures. It is of great significance that it is now possible.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of the amount of Si added on the impact characteristics of the weld, and FIG. 2 is a graph showing the influence of the amount of reduction in the high temperature range on the anisotropy of the steel material. Patent applicant Kawasaki Steel Corporation Figure 1 Si content (int%) Figure 2 Cumulative reduction rate (%)

Claims (1)

[Claims] 1. C: 0.05-0.16wt%, Si: 0.05-0.05%
0.20wt%, Mn: 0.60-1.50wt%, C
r: 0.30-0.80 wt%, Mo: 0.20-0.
80wt%, Ni: 0.80-3.00wt%, V: 0
．． 030-0.100 wt%, Al: 0.010-0.
080wt%, B: 0.0005-0.0025wt%
, N: 0.0040 wt% or less, and has a carbon equivalent Ceq. defined by the following formula. is 0.52 to 0.60%, and the balance is Fe and unavoidable impurities.
After heating and soaking at 0 to 1200°C, hot rolling is performed at a temperature range of 1000 to 900°C with a cumulative reduction rate of 50% or more,
1 in a temperature range of below 900℃ and above 810℃.
Homogeneity in the thickness direction, characterized by the fact that after the cumulative reduction rate is set to 10 to 30% by light reduction rolling with a reduction rate of less than 10% per bath, it is immediately quenched and then tempered at a temperature of Ac_1 point or less. A method for producing ultra-high tensile strength steel plates for welding with excellent properties. Ceq. =C+Mn/6+Si/24+Ni/40+C
r/5+Mo/4+V/14 (%) 2, C: 0.05~0.16wt%, Si: 0.05~
0.20wt%, Mn: 0.60-1.50wt%, C
r: 0.30-0.80 wt%, Mo: 0.20-0.
80wt%, Ni: 0.80-3.00wt%, V: 0
．． 030-0.100wt%, M: 0.010-0.0
80wt%, B: 0.0005-0.0025wt%,
Contains N: 0.0040 wt% or less, and Cu: 1
．． 0wt% or less, Nb: 0.050wt% or less, Ca:
Carbon equivalent Ceq. is 0.52 to 0.
After heating and soaking the steel to 1050-1200°C, the balance being Fe and unavoidable impurities.
Hot rolling with a cumulative reduction rate of 50% or more in a temperature range of 900°C, followed by light reduction rolling with a rolling reduction rate of less than 10% per bath in a temperature range of less than 900°C and 810°C or more. A method for producing an ultra-high tensile strength steel plate for welding with excellent uniformity in the thickness direction, characterized by immediately quenching after setting the ratio to 10 to 30%, and then tempering at a temperature of Ac_1 point or lower. Ceq. =C+Mn/6+Si/24+Ni/40+C
r/5+Mo/4+V/14(%)