JPS61276920A - Production of high tensile steel plate having excellent drop weight characteristic - Google Patents

Abstract

PURPOSE:To produce stably a high tensile steel plate having an excellent drop weight characteristic by subjecting a steel slab contg. a specific ratio each of C, Si, Mn, P, S and Al to heating, rolling and cooling under specific conditions. CONSTITUTION:The steel slab contg., by weight %, 0.03-0.15% C, 0.05-0.80% Si, 0.5-2.0% Mn, <=0.025% P, <=0.007% S and 0.01-0.10% Al and consisting of the balance iron and inevitable impurities is prepd. Such steel slab is heated to <=1,000 deg.C and is subjected to >=50% rolling reduction at <=850 deg.C. The rolling finish temp. is adjusted to (Ar3+20) deg.C to (Ar3-40) deg.C. The steel plate is then cooled at >=10 deg.C/sec cooling rate on the steel plate surface from the temp. region of Ar3 or (Ar3-60) deg.C to an optional temp. of 400-200 deg.C. The high tensile steel plate having about <=-60 deg.C NDT temp., about >=50kgf/mm<2> tensile strength and excellent drop weight characteristic is thus stably obtd.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a method for manufacturing high-strength steel sheets with excellent drop weight properties, and more specifically, the present invention relates to a method for manufacturing high-strength steel sheets with excellent drop weight properties, and more specifically, the NDT'/ in the NRLFi heavy test is 160°C or less. , and the tensile strength is 50kg
The present invention relates to a method for stably manufacturing a high tensile strength steel plate having a strength of f/+nn+" or less.

(Prior art) In recent years, requirements regarding drop weight characteristics have become increasingly strict for various steel structures, especially I and PG tanks, from the standpoint of improving safety against brittle fracture. .

Conventionally, it has been known that it is effective to add a large amount of Ni and to perform quenching and tempering treatment in order to improve the drop weight properties. However, Ni is a rare resource, and adding a large amount of this element significantly increases the price of steel, so the above method is not necessarily preferable.

To this end, a method has recently been proposed to improve the drop weight properties by making the ferrite grain finer by controlled rolling and strengthening the transformation by online cooling immediately after rolling. In order to stably maintain the NDT temperature below -60°C, it is necessary to add approximately 0.6% or more of Ni, so the amount of N1, which is a rare element, must be adjusted accordingly.
There is a desire to reduce this.

(Purpose of the Invention) As a result of intensive research in order to solve the above-mentioned problems in the production of high-strength steel sheets with poor drop weight properties, the present inventors have determined the hot-strength rolling conditions for steel slabs having a predetermined chemical composition. By controlling, it is possible to add substantially no Ni or add N.
Even if the amount of i added is reduced to below the conventional value, t below -60℃
The present invention was achieved by discovering that l') T temperature can be stably ensured.

Therefore, the present invention provides NDT in NRi-drop test.
Temperature is 160℃ or less and tensile strength is 50k
It is an object of the present invention to provide a method for stably manufacturing a high tensile strength steel plate having a drop weight characteristic of GF/mm2 or higher.

(Structure of the Invention) The method for producing a high-strength steel sheet with poor drop weight properties according to the present invention includes, in weight percent, C 0.03 to 0.15%, Si 0.05 to 0.80%, Mn 0.5 to 2. .0%, P 0.025%l or less, S 0.007 or less, Affo, 01~0.10%, balance iron and unavoidable impurities.
Heat to a temperature below 850 °C and 50 °C at a temperature below 850 °C.
% or more of rolling, and the rolling finishing temperature was set to (Ar34-20
) After adjusting from ℃ to (Ar340)'C, Ar3
From the temperature range of (Ar360)"C to 400-200℃
It is characterized by cooling the copper plate surface to an arbitrary temperature at a cooling rate of 10° C./second or more.

In the method of the present invention, a steel slab having a predetermined chemical composition is heated to a temperature of 1000°C or less, and a reduction of 50% or more is applied at a temperature of 850°C or less.

In the present invention, since low-temperature heating is adopted, austenite recrystallized grains are effectively refined, but H<
The reduction in the austenite non-recrystallized region below 850℃ is 5
By setting it to 0% or more, the ferrite grains become finer,
Furthermore, it is possible to improve Charpy characteristics and NRL characteristics. If the reduction amount is less than 50%, even if the subsequent finish rolling and controlled cooling conditions are made appropriate, the crystal grains remain coarse and the drop weight properties cannot be improved. Preferably, a pressure reduction of 50 to 80% is applied at a temperature range of 850 to Ar5 points.

Various methods have been proposed to improve the toughness of controlled rolled steel sheets that do not undergo a water cooling process after rolling, and such controlled rolled steel sheets generally have good drop weight properties. On the other hand, a steel plate obtained by controlled cooling that includes a water cooling process after rolling generally has a toughness comparable to that of a controlled rolled steel plate, but its drop weight properties are significantly inferior. FIG. 1 shows the relationship between the Charpy fracture surface transition temperature (vTrs) as an index of toughness and the NDT temperature of the controlled rolled steel sheet and the controlled cooled steel sheet. It is clear that even if the same rolling conditions are adopted and vTrs is approximately the same, the weight drop characteristics of the controlled cooling steel sheets that undergo the water cooling process are significantly inferior.

Therefore, the present inventors conducted detailed observations to clarify the differences between control-rolled steel sheets and control-cooled steel sheets. As a result, the Charhy fracture surface of control-rolled steel sheets had 0.
Separations of 0.05n+ or more occur, whereas those of the controlled cooling steel plate are 0.05vbwb or less, and the frequency of separation and NDT
It was found that the temperature decreased. The reason why the separation of the controlled cooled steel sheet is slight compared to the controlled rolled mesh sheet is still not clear, but it is due to the rolling process, especially the rolling in the austenite low temperature range (cooling rate with 1001 texture). This appears to be due to the reduction in ferrite transformation in this range.

Based on the above results, the present inventors manufactured steel plates by variously changing the cooling conditions, and investigated the drop weight characteristics of the steel plates in detail.

That is, steel (Ar 3755°C) having the following chemical components: C0.08%, Si0.2%, Mn 1.48%, P 0.012%, dS 0.003%, Ae 0.037% is heated to 950°C.
In controlled cooling, the finish temperature was 780° C., the cooling rate was 12 to 20° C./sec, and the reduction rate was 60%, and the influence of the cooling start temperature and cooling stop temperature on the NDT temperature was investigated.

As the results are shown in Fig. 2, during controlled cooling after rolling, the cooling start temperature of the steel plate was set at Ar3 to (Ar, 60
)℃, and the cooling stop temperature is 400 to 400℃.
It has been found that by setting the temperature in the range of 200°C, the drop weight characteristics of the steel plate are significantly improved, and the temperature is stably kept at -60°C or less.

Next, separation index and NDT for cooling rate
The temperature relationship is shown in Figure 3. By setting the cooling rate to 10° C./sec or more, it is possible to obtain excellent drop weight characteristics equivalent to or better than air cooling, and this tendency has a good correlation with the frequency of occurrence of separation. Although the upper limit of the cooling rate is not particularly limited, it is usually 50° C./second from the viewpoint of equipment capacity. Further, as shown in the figure below, it is also necessary to employ a heating temperature of 1000°C or lower in order to obtain an ND'F temperature of -60°C or higher.

As described above, it has not been previously known that the drop weight characteristics can be improved by regulating the controlled cooling conditions after rolling, and the present invention was completed based on this new knowledge. be.

Next, the reason for limiting the chemical composition of the steel slab in the present invention will be explained.

C is at least 0 in order to give the steel plate the required strength.
．． It is necessary to add 0.3%. In order to improve the strength, it is advantageous to add polyacids, but if too large amounts are added, the weldability of the steel plate will deteriorate, so the upper limit of the amount added should be set at 0.15%. shall be.

Si is added to deoxidize steel and increase its strength. In order to effectively express this effect, at least 0.05
%, but if added in excess of 0.80%, weldability will be impaired, so the upper limit is set at 0.80%.

Mn has the effect of increasing strength without reducing low-temperature toughness, and in order to effectively obtain this effect, Mn must be at least 0.5
% addition is required. However, when added in excess, weldability deteriorates, so the upper limit of the amount added is set to 2.0%.

P is an element contained in steel as an unavoidable impurity. However, when the content exceeds 0.025%, the low temperature toughness is significantly deteriorated, so the upper limit is set to 0.025%.

Like P, S is an element contained in steel as an unavoidable impurity, but when the content exceeds 0.007%,
Since this significantly reduces the impact absorption energy in the direction perpendicular to the rolling direction, the upper limit is set to 0°007%.

A1 is an extremely effective element for deoxidizing and refining grains, but when added in large amounts exceeding 0.10%, it impairs the cleanliness of the steel and may eventually deteriorate the toughness of the base metal. Since G is connected, the amount added should be in the range of 0.01 to 0.10%.

In addition to the above elements, the steel sheet according to the present invention further contains: Cu0.50% or less, Ni0.60% or less, Cr0.50% or less, MOo, 50% or less, Nb 0.005-0.05%, V 0.01-0.10%, Ti 0.005-0.02%, and B 0.0
003 to 0.010% of at least one element selected from the group consisting of:

Cu and Cr are effective in improving the strength and corrosion resistance of steel, but when added in excess, they deteriorate weldability, so the upper limit of the amount added is set at 0.50 for each element.
%.

Ni has the effect of improving the strength and toughness of steel, as well as improving the drop weight characteristics. In the method of the present invention, the drop weight characteristics can be significantly improved without particularly adding Ni to the steel, so 14.'.

Usually, it is not necessary to add it. However, the guaranteed value of the drop weight characteristic, that is, NDT? It can be added when IJ and degrees are on the very low side to satisfy the guaranteed value. Even in this case, the upper limit of the amount added is set at 0.60% in consideration of economic efficiency.

MO has the effect of increasing strength without deteriorating low-temperature toughness, but when added in excess of 0.50%, it significantly deteriorates weldability.
shall be.

Nb is effective in improving strength and toughness through precipitation strengthening through grain refinement, and in order to effectively obtain this effect, it is necessary to add 0.005% or more. However, addition of an excessive amount exceeding 0.05% will deteriorate the toughness of the weld heat affected zone, so the upper limit of the addition amount should be set at 0.05%.
shall be.

(2) contributes to increasing the strength of steel by 1 through precipitation hardening.

In order to effectively obtain such an effect, it is necessary to add 0.01% or more. However, if added in excess of 0.10%, weldability deteriorates, so the upper limit of the amount added is 0.10%.

Ti is an element effective in improving the toughness of the weld heat affected zone,
In order to effectively exhibit this effect, it is necessary to add 0.005% or more. However, when adding a large amount exceeding 0.02%, not only the effect described in 1-1 cannot be obtained, but also the toughness deteriorates, so the upper limit is set to 0.
．． 02%.

B is an element that increases the strength of the steel plate through transformation strengthening,
This effect can be effectively obtained by adding 0.03% or more. However, 0.01
If it is added in excess of 0.01%, the above effect will not only be saturated, but also the weldability will be deteriorated, so the upper limit of the amount added is 0.010%.

In the present invention, at least one element selected from the group consisting of REM and Ca can be further added.

REM and Ca are effective in controlling the morphology of sulfides, and are added to increase the impact absorption energy in the direction perpendicular to the rolling direction. In order to effectively obtain these effects, it is necessary to add 0°001% or more of REM and 0.0005% or more of Ca. However, when adding too much, it causes many surface and internal defects in the steel sheet, so the upper limit of the addition amount is set to 0°01% for REM and 0.01% for Ca.

(Effect of the invention) By regulating the controlled cooling conditions of a steel slab having a predetermined chemical composition, in particular, in addition to heating the steel slab to a predetermined temperature, in the controlled cooling after rolling, the cooling start temperature and cooling temperature of the steel plate can be adjusted. By regulating the stopping temperature within a predetermined temperature range and regulating the cooling rate, the drop weight characteristics of the steel plate are significantly improved and the NDT @ degree is stably maintained at -60℃ or less. A high tensile strength steel plate having a tensile strength of 50 kgf/mm2 or more can be obtained.

In particular, according to the method of the invention, without substantially adding Ni,
Alternatively, even if the amount of Ni added is reduced to more than the conventional value, it still has the specified drop weight characteristics and has a tensile strength of 50 kgf/mm.
It is possible to obtain a high tensile strength steel plate of the above.

(Examples) The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way.

Examples Slabs made of inventive steel and comparative steel having chemical compositions not shown in Table 1 are heated to a predetermined temperature and subjected to finish rolling at a predetermined temperature, and then subjected to a predetermined rolling process as shown in Table 2. A high tensile strength steel plate according to the present invention was obtained by controlled cooling under certain conditions. Its mechanical properties and Nr)T temperatures are shown in Table 2.

Compared to Inventive Steel A, Comparative Steel H has a higher cooling stop temperature and is therefore inferior in NRL properties. Compared to the invention steel B, comparative steel I
Since the cooling rate is slow, the NRL characteristics are inferior. Similarly, compared to the invention steel C, the comparative steel ■ has a lower cooling stop temperature.

Compared to the steel of the present invention, the finishing temperature and cooling start temperature of the comparative steel are lower. Compared to the invention fiE, the comparative steel I
No. 7 has a high finishing temperature and a high cooling start temperature. Compared to the invention steel F, the comparative steel M has a higher cooling stop temperature, and
Cooling speed is slow. Compared to the present invention@C, the heating temperature of the comparative steel 1 is higher. Therefore, all of these comparative steels have inferior NRL characteristics compared to the present invention network.

[Brief explanation of the drawing]

Figure 1 is a graph showing the NRI and characteristics of a controlled rolled steel sheet and a controlled cooling copper plate. Figure 2 is a graph showing the effect of temperature on the cooling start temperature and cooling stop after rolling on the NDT A degree. FIG. 3 is a graph showing the influence of the cooling rate after rolling on the NDT temperature. ? ■ Procedural amendment (voluntary) May 10, 1985 1. Patent Application No. 118585 of 1985 2. Name of the invention Method for producing high-strength steel sheets with excellent drop weight properties 3. Relationship with the person making the amendment case Patent applicant address: 1-3-18 Wakihama-cho, Chuo-ku, Kobe City Name:
Kobe Steel, Ltd. 4, Agent Address: Shinmachi Shichifuku Building 5, 1-8-3 Shinmachi, Nishi-ku, Osaka City, Date of Amendment Order: Month, Day, 1920 (Date of Shipment: Month, Day, 1920) 6. Inventions Increased by Amendment The number of ＼,,! 4'"7 Contents of correction (3) rAr: +755°C on page 11, line 3 of the specification is corrected to rAr3775°C." I-F

Claims (4)

[Claims] (1) C0.03-0.15% by weight, Si0.05-0.80%, Mn0.5-2.0%, P0.025% or less, S0.007% or less, Al0.01-0 .10% steel slab with the balance iron and unavoidable impurities
℃ or less, apply a rolling reduction of 50% or more at a temperature of 850℃ or less, and reduce the finishing temperature to (Ar_3+20)℃.
After adjusting from (Ar_3-40) °C to (Ar_3-40) °C, the steel plate surface is cooled at a cooling rate of 10 °C / second or more from the temperature range of Ar_3 to (Ar_3-60) °C to an arbitrary temperature of 400 to 200 °C. A method for manufacturing high-strength steel sheets with excellent drop weight characteristics. (2) In weight% (a) C0.03-0.15%, Si0.05-0.80%, Mn0.5-2.0%, P0.025% or less, S0.007% or less, and Al0 .01 to 0.10%, and (b) C
u0.50% or less, Ni0.60% or less, Cr0.50% or less, Mo0.50% or less, Nb0.005-0.05%, V0.01-0.10%, Ti0.005-0.02% , and B0.0003 to 0.010%, and the balance is iron and unavoidable impurities.
℃ or less, apply a rolling reduction of 50% or more at a temperature of 850℃ or less, and reduce the finishing temperature to (Ar_3+20)℃.
After adjusting the temperature from Ar_3 to (Ar_3-60)°C to any temperature between 400 and 200°C, the surface of the copper plate is cooled at a cooling rate of 10°C/second or more. A method for manufacturing high-strength steel sheets with excellent drop weight characteristics. (3) In weight% (a) C0.03-0.15%, Si0.05-0.80%, Mn0.5-2.0%, P0.025% or less, S0.007% or less, and Al0 .01 to 0.10%, and (b) R
EM0.001~0.01% or less, and Ca0.000
1,000 steel slabs containing at least one element selected from the group consisting of 5 to 0.01%, with the balance consisting of iron and unavoidable impurities.
℃ or less, apply a reduction of 50% or more at a temperature of 850℃ or less, and set the rolling finishing temperature to (Ar_3+20)
After adjusting from ℃ to (Ar_3-40)℃, from the temperature range of Ar_3 to (Ar_3-60)℃ to 400-200℃
A method for producing a high-strength steel sheet with excellent drop weight properties, characterized by cooling the copper sheet surface to an arbitrary temperature at a cooling rate of 10° C./second or more. (4) In weight% (a) C0.03-0.15%, Si0.05-0.80%, Mn0.5-2.0%, P0.025% or less, S0.007% or less, and Al0 .01 to 0.10%, and (b) C
u0.50% or less, Ni0.60% or less, Cr0.50% or less, Mo0.50% or less, Nb0.005-0.05%, V0.01-0.10%, Ti0.005-0.02% , and at least one element selected from the group consisting of B0.0003-0.010%, (c) REM0.001-0.01% or less, and Ca0
．． 0005 to 0.01% of at least one element selected from the group consisting of 0.0005 to 0.01%, with the balance consisting of iron and unavoidable impurities.
℃ or less, apply a rolling reduction of 50% or more at a temperature of 850℃ or less, and reduce the finishing temperature to (Ar_3+20)℃.
After adjusting from (Ar_3-40) °C to (Ar_3-40) °C, the steel plate surface is cooled at a cooling rate of 10 °C / second or more from the temperature range of Ar_3 to (Ar_3-60) °C to an arbitrary temperature of 400 to 200 °C. A method for manufacturing high-strength steel sheets with excellent drop weight characteristics.