JPH04110422A - Production of 70kgf/mm2 class steel plate having superior weldability and low yield ratio - Google Patents

Abstract

PURPOSE:To produce a 70kgf/mm<2> class steel plate having superior weldability and low yield ratio by subjecting a slab of a steel with a specific composition to hot rolling and then to heat treatments under respectively specified conditions. CONSTITUTION:A steel plate having an yield ratio as low as <=80% in base material and a base material strength of 70kgf/mm<2> class and also having superior weldability can be produced by subjecting a slab of a steel having a composition which consists of 0.07-0.15% C, 0.05-0.50% Si, 0.30-1.80% Mn, 0.10-1.20% Cr, 0.10-1.00% Mo, 0.01-0.10% Al, 0.02-0.08% V, and the balance Fe with inevitable impurities and in which PcM represented by Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/ 0+5B(%) is regulated to <=0.26% to hot rolling and then to heat treatments consisting of hardening, normalizing, and tempering under the following conditions: hardening temp. between the Ac3 point and 950 deg.C, normalizing temp. between the Ac1 point and <Ac3 point, and tempering treatment of 500 deg.C and <Ac1 point.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to a 70kgf
The present invention relates to a heat-treated high tensile strength steel plate of /mm2 class, and specifically relates to a method for manufacturing a low yield ratio 70 kgf/mm2 class steel plate with excellent weldability.

(Prior art) Heat-treated high-strength steel sheets with a tensile strength of 60 kgf/mm2 or higher have been used for tanks, bridges, penstocks, etc., and have been hardened to create a high-hardness microstructure such as martensite or bainite by quenching and tempering. Since it uses the generation of
The yield ratio (yield strength/tensile strength) is usually as high as 90% or more, and the plastic deformability is insufficient, so it is rarely used for construction purposes.

In recent years, there has been an increasing demand for building structures to be taller and have larger spans, and there has been a growing movement to use stronger 60Jf/mm2 class steel instead of the conventional 50Jf/mm' class steel, and the yield ratio has increased. 60kjgf/m reduced to 80% or less
M' grade steel is now required.

A steel plate that satisfies this requirement must be reheated and quenched (Q) at a temperature of 2 or above Ac+, or directly quenched (DQ) at a temperature of 3 or above Ar, or tempered at a temperature below 01 (T). Unlike the conventional heat treatment method that consists of a combination of quenching and tempering, quenching (
Q') New heat treatment method Q1Q' +T and D
Q1Q'+T method has been developed. According to this method, Q' produces ferrite with low hardness and excellent ductility in the structure, so a low yield ratio can be obtained.

60k with a low yield ratio obtained by such heat treatment
GF/mm' grade steel plates have come to be used for high-rise buildings. Therefore, for the purpose of preventing an increase in the amount of welding work required as buildings become even taller, the use of A, which has even higher strength and can reduce the thickness of steel plates, is being considered. In other words, there is an increasing demand for the development of steel plates with a tensile strength of 70 Jf/mm2 class and a low yield ratio.

However, even with the above-mentioned Q+Q' +T method, 7
In the case of 0 kgf/mm' class steel plate, in order to ensure its high strength, the hardness and fraction of bainite must be set to 60 Jf/m.
It is not easy to obtain a sufficiently low yield ratio of 80% or less because the yield ratio must be higher than that for copper treated with m", and in order to achieve high strength, it is necessary to avoid deterioration in weldability due to increased weight of alloying elements. The problem is that there isn't one.

For example, Materials and Processes Vo1.3, No. 3 (1
990) No. 806 states, "Development of low yield ratio HT70" and reports an example of development using the Q1Q'10T method.Although the plate thickness is relatively thin at 30 mm, P
The mark is 0.27%, and the yield ratio is Si:5%
Therefore, both weldability and yield ratio were insufficient.

(Invention or problem to be solved) As mentioned above, there is no 70 kgf/mm class 2 tempered high tensile strength steel plate that has both weldability and low yield ratio.
The present invention can be applied to heat-treated high-strength steel sheets with a tensile strength of 70 kgf/mm2 class without causing a significant deterioration in weldability.
It is an object of the present invention to provide a method for producing a low yield ratio 70 kgf/mm' class steel plate with excellent weldability and ensuring a sufficiently low yield ratio of 0% or less.

(Means for Solving the Problems) The present inventors have conducted extensive research in order to achieve a low yield ratio of 80% or less and good weldability while ensuring high strength with a tensile strength of 70 kBf/mm2 class. went. As a result, the above Q
+Q" In the +T method, by changing Q' (quenching from the two-phase region) to N' (normalizing in the two-phase region), which is important for achieving a low yield ratio, it is now widely used. It has the same P mark as the 80kgf/mm2 class steel plate, and is 7
The present invention was developed based on the knowledge that it is possible to achieve a strength of 0 kgf/mm2 class and a low yield ratio of 80% or less.

In addition, water cooling is used for quenching, or air cooling is used for normalizing.

The first invention is C・0.07-0.15%, Sl・0.0
5-0.50%, Mn: 0.30-1.80%,
Cr: 0.10-1.20%, M.

0.10-100%, Al:o, 01-0. ]0%, V
: Contains 0.02 to 0.08%, and the following Pcm or 0.2
By hot-rolling a steel billet containing 6% or less Fe and unavoidable impurities, and then subjecting it to the following heat treatment,
Low yield ratio of less than 80% in base material and 70Jf/m
Low yield ratio of 70 with excellent weldability and base metal strength of m' class
This is a method for manufacturing kgf/mm' class steel plate.

Heat treatment method: Quenching, ten-tempering, normalizing, ten-tempering. However, quenching temperature: Ac 3 points or more and 950 degrees Celsius or less Normalizing temperature: Ac+ points or more and Ac less than 3 points Tempering temperature, 500 degrees Celsius
Less than 2 Act points Pcm=C+Si/30+Mn/2
0+Cu/2O-14(i/60+Cr/20+111
0/15+V/10+5B (%) The second invention is C
:0.07~0.15%, Si:0.05~0.50%
, Mn: 0.30-1.80%, C"0.10-1.2
0%, λ(00, 10-100%, Al:o, oI
~0.10%, V:002~0.08%, the following Pcm is 026% or less, and the balance consists of Fe and unavoidable impurities.A steel billet containing Fe and inevitable impurities is hot rolled at a finishing temperature of 950 °C or higher. By performing direct quenching and then applying the following heat treatment, a low yield ratio of 70 kgf with excellent weldability, with a low yield ratio of 80% or less in the base material and a base metal strength of 70 kgf/mm2 class. This is a method for manufacturing /mm2 class steel plate.

Heat treatment method: Normalizing, ten-year-old, but normalizing temperature: Act point to 2 to less than 03 point Tempering temperature: 500℃ or more △c less than 1 point Pcm=C+Si/
30+Mn/20+Cu/20+Ni/60+Cr/2
00Mo/15+~l/10+5B (%) The third invention is C: 0.07~0.15%, Si: 0.05~0,
50%, A1n0.30-1.80%, Cr: 0.1
0-1.20%, M.

0710-100%, A1 area 01-010%, Nb: 0
．． Contains 005-0020% and the following Pcm is 0.26
% or less, a steel piece consisting of the balance Fe and unavoidable impurities is hot-rolled and then subjected to the following heat treatment to achieve a low yield ratio of 80% or less in the base material and a yield ratio of 70 kgf/m
Low yield ratio of 70 with excellent weldability and base metal strength of m2 class
This is a method for producing kgf/mm class 2 steel plate.

Heat treatment method: Quenched Juyaki Nanished Juyaki Chidoshi Pukota
Quenching temperature: Ac3 points or more, 950 degrees Celsius or less Normalizing temperature: Ac1 points or more, Ac2 points or less Tempering temperature: 500
℃ or more △c less than 1 point Pcm=C+Si/30+λin/
20+Cu/20+Ni/60+Cr/20+MO/1
5+V/1. O+5B (%) The fourth invention is Co0.
07-0.15%, Si: 0.05-0.50%, M
n: 0.30-1.80%, Cr: 0.10-1.2
0%, M.

0, 10-100%, Al: 0.01-0.10%, N
b: Contains 0.005-0°020%, the following Pc
A steel billet consisting of less than 0.26% Fe and unavoidable impurities is hot-rolled at a finishing temperature of 950°C or higher, directly quenched, and then subjected to the following heat treatment. , a low yield ratio of 80% or less in the base material,
This is a method for producing a low yield ratio 7° kgf/mm' class steel plate with excellent weldability and a base metal strength of 70 Jf/+nm2 class.

Heat treatment method, normalizing and tempering, normalizing temperature Ac 1 point or more and less than Ach point Tempering temperature 500℃ or more J: Ac + point Shuman Pcm = C + Si /
30±hi n / 200Cu/20tNi/60+
Cr/20+Mo/15+V/10+5B (%)
The fifth invention is C: 0.07-015%, Si: 0.0
5-0.50%, Mn: 0.30-1.80961, C
"0.10~1.20%, 11100.10~100
%, Al: 0.01-0.10%, V: 0.02-0.
08%, Nb: 0.005 to 0.020%, the following P mark is 026% or less, and the balance consists of Fe and unavoidable impurities. After hot rolling, the steel billet is subjected to the following heat treatment. This is a method for manufacturing a low yield ratio 70 Jf/mm2 class steel plate with excellent weldability, which has a low yield ratio of 80% or less in the matrix and a matrix A strength of 70 kgf/mm' class.

Heat treatment method - Quenching, ten-temperature normalizing, ten-temperature tempering, quenching temperature Aca point or higher and 950℃ or less Normalizing temperature ACl tun-1 - less than Ac11 point Tempering temperature
500℃ or higher, less than 1 point for human CPcm=C+Si/30+
Mn/20+Cu/20+Ni/60+Cr/20+
MO/15+V/10→5B (%) The sixth invention is
C: 0.07-0.15%, Si: 0.05-050%
, Mn: 0.30-1.80%, Cr: 0.10-1
．． 20%, λ(00, lO~100%, A1・0.01
~0.10%, V:0.02~0.08%, Nb0.0
A steel piece containing 0.05 to 0.020%, below 0.026%, and the balance consisting of Fe and unavoidable impurities is 95%
By hot rolling at a finishing temperature of 0℃ or below, directly quenching, and then applying the following heat treatment, we can achieve a low yield ratio of 80% or less in the base material and a yield ratio of 70kgf/mm'.
Low yield ratio of 70 Jf with excellent weldability and base metal strength of
Heat treatment method which is a manufacturing method of /mm2 grade steel plate, Normalizing temperature: 1 AcI point or above A Below C3 point Tempering temperature: 500 ℃ or above 2 Below Act point Pcm=C
+Si/30+Mn/20+Cu/20+Ni/60+
Cr/20+λto/+5+V/10+5B (%)
The seventh invention has Cu: 0.05 to 0.30%, Ni: 0.
20-300%, B: 0.0O03-0.0020%,
T10.003~0.020%, Ca:0.001~
Claims (1), (2(, (3), (4), (5) Matayo (6)) containing one or more selected from 0.01% of Low yield ratio 70kgf/mm2
This is a method for manufacturing grade steel plates.

(for I) The present invention will be explained in more detail below.

First, the reasons for limiting the chemical components in the present invention will be explained.

C is an element necessary to ensure the strength of a high-tensile steel plate, and if the content is less than 0.07%, the tensile strength is 70 J.
It is difficult to obtain f/mm' class strength. Furthermore, if it is added in an amount exceeding 0.15%, the weld cracking resistance will be impaired, which is not preferable. Therefore, the C content should be in the range of 0.07 to 0.15%.

Sl is an element necessary for deoxidation, or the content is 0.05%
If it is less than 0.50%, this effect will be small, and if it is added in excess of 0.50%, weldability and toughness will deteriorate, which is not preferable. Si: content is 005 to 0.5
The range is 0%.

111n is an element necessary to improve hardenability and ensure strength inside the plate thickness.If the content is less than 0.30%, this effect cannot be sufficiently obtained.
If it is added in excess of more than %, weldability and toughness will deteriorate, which is undesirable. Because of this, the Mn content is 0.3
The range is 0 to 1.80%.

Is Cr an effective element for improving hardenability?Is the content 0?
．． If it is less than 10%, this effect will not be fully demonstrated,
Moreover, if added in excess of 1.20%, weldability will be impaired. Therefore, the Cr content is in the range of 0.10 to 1.20%.

Mo is an element that improves hardenability and increases temper softening resistance. If the content is ○, less than 10%, sufficient effect cannot be obtained, and if it is added in excess of 00%,
Since it deteriorates weldability and increases cost, the Mo content should be in the range of 0.10 to 100%. ■■ is an element that increases hardenability and resistance to temper softening when added in small amounts. To obtain the effect, use 0.02% or more.
It is necessary to add more than 0.08%, and addition of more than 0.08% impairs weldability. I wanted to, ■Content is 0.0
The range is 2% to 0.08%.

Nb is an element that has a crystal grain refining effect and also brings about a precipitation strengthening effect when directly quenching and tempering is performed. To obtain this effect, it is necessary to add 0.005% or more, and addition of more than 0.020% deteriorates weldability and toughness. Because I wanted to, Nb content ■ is 0
．． The range is 0.005% to 0.020%.

A1 is a deoxidizing element, and if the content is less than 0.01%, such effect will be small, and if it is added in excess of 0.10%, it will cause deterioration of toughness. Therefore, the Al content is in the range of 0.01 to 0.10%.

In addition, one or two of Co, Ni, B, Ti, Ca, etc. are added depending on the sheet thickness and target toughness.

Cu is an element that is effective in increasing strength through solid solution strengthening and precipitation strengthening, but if the content is less than 0.105%, such effects cannot be fully exhibited, and if the content is less than 0.30%, Cu is an element that is effective in increasing strength. As a result, hot workability deteriorates and cracks are likely to occur on the surface of the steel sheet. The Cu content is 0.05 to 0.3.
The range is 0%.

Does Ni have the effect of improving toughness?Is the content 0.
If it is less than 20%, the sufficient effect cannot be obtained, and 3.
If it is added in excess of 0.00%, scale defects are likely to occur and the cost will increase. Therefore, the N1 content is set in the range of 0.20 to 3.00%.

B is an element that improves hardenability in trace amounts; if the content is less than 0.0003%, this effect cannot be obtained, and if it is added in excess of 0.0020%, the toughness deteriorates. Therefore, the B content is 0.0003 to 0.0020
% range.

T1 improves the toughness of the weld heat affected zone as a fixed element of N,
It is an effective element for improving the hardenability of B. If the content is less than 0.003%, sufficient effects cannot be obtained, and if it is added in excess of 0.020%, the toughness of the base material will be impaired. The Ti content is 0.003 to 0.02.
The range is 0%.

Ca has a spheroidizing effect on nonmetallic inclusions and is effective in reducing anisotropy.If the content is less than 0.001%, the sufficient effect cannot be obtained, and if the content is less than 0.010% If added in excess of this amount, the toughness will deteriorate due to an increase in inclusions. Therefore, the Ca content should be in the range of 0.00% to 0.010%.

Further, Pcm is limited to 0.26% or less, assuming a certain degree of preheating, and in order to ensure weldability equivalent to that of 80 Jf/mm 2 class high tensile strength steel plate, which is still widely used.

Next, manufacturing conditions in the present invention will be explained.

First, the reason for limiting the heat treatment method will be explained.

The present inventors have developed the current 80 kgf/mm' shown in Table 1.
Using steel with a P content of 0.25%, which is equivalent to grade high-strength steel plate, various heat treatments were applied to this steel, and the effects of heat treatment methods on strength and yield ratio were investigated. In addition, the heat treatment method is Q1Q'
+T, Q+N'-1-T, N0Q'-to-T 3
It is a kind.

Here, Q: Reheating and quenching from a temperature of 3 Ac points or higher Q" Reheating and quenching from a two-phase region temperature (above Ac+ point and below 3 points) N: Normalizing at a temperature of 1 Ac or higher ', Normalizing at a temperature in the two-phase region T: Tempering at a temperature below the +tct point The results are shown in Table 2.

(Left below) As is clear from Table 2, only in the case of the Q+N' +T method, Pcm: 0.25% component, 70kgf/mm
” class strength and a yield ratio of less than 80% can be obtained.In the case of other heat treatment methods, 70 kgf/mm
Although class 2 strength is obtained, the yield ratio is not sufficiently reduced. Therefore, the heat treatment method is Q1N'+T method. Regarding the Q treatment, direct quenching (DQ) after rolling, which is equivalent in the sense of quenching from the complete austenite region, may also be used.Next, the limited range of temperature ranges in each heat treatment described in 2 will be explained.

Regarding the quenching (Q or DQ) temperature, a high hardness microstructure such as martensite or bainite is generated,
In order to ensure sufficient strength, it is necessary to have a complete austenite region, and the Ac point should be 3 or higher. However, if the temperature is too high, the structure will become coarse and the ductility and toughness will deteriorate, so the temperature is set at 950°C or lower.

Regarding the annealing temperature (N'), in order to generate ferrite 1 and obtain a low yield ratio, the tempering temperature is set to two range temperatures, that is, between the Ac1 point and 12△C3 point. In this case, the cooling method may be air cooling instead of water cooling in order to reduce the cooling rate, lower the fraction of high hardness microstructures such as martensitic (martensis) and bainite, and reduce the yield ratio. As already mentioned.

Regarding the temperature, in order to reduce the residual stress in the steel plate caused by the heat treatment in the previous stage and ensure the safety of the structure, a temperature that is too low is not desirable, so it should be 500'C or higher. shall be.

On the other hand, if it exceeds the Ac+ point, the strength will decrease, so
The upper limit is set below the Act point.

(Example) Low yield ratio of 70 kgf/m with excellent weldability according to the present invention
An example of the method for manufacturing M2 grade steel plate will be described, but the present invention is not limited to this example.

The test steel plate was a steel piece having the chemical composition shown in Table 3-1.
After rolling to a plate thickness of 30 to 50 mm at the rolling finishing temperatures shown in the same table, the sheets were heat treated under the heat treatment conditions shown in Table 3-2. Test pieces were taken from these steel plates, and a tensile test was conducted on the base metal. The results are listed together with the heat treatment conditions in Table 3-2.

Table 3-1 shows the chemical components, plate thicknesses, and rolling conditions of the present invention methods A-G and Comparative Examples I to N, and Table 3-2 shows the heat treatment conditions and tensile properties of the base material, respectively.

(Left below) (mDQ Opening 11!, 7, and Table 3-2. As is clear from Table 3-2, all of the methods A to H of the present invention have a tensile strength of 70 Jf/mm' or more and a stable tensile strength of less than 80%. Has a low yield ratio.

On the other hand, in Comparative Examples 1 to N, the heat treatment method was Q+N'
+T method or DQ-1-N' Since it is not a +T method, the yield ratio is too high.

(Effects of the Invention) As explained above, the present invention controls the chemical components, and after rolling and quenching (Q or DQ), performs annealing N′) at a temperature in the two-phase region, and then Because the heat treatment is performed to temper T), it is possible to produce a 70kgf/mm2 class steel plate with excellent weldability and a yield ratio of 80% or less of the base material, which has an excellent effect. be.

Claims (7)

[Claims] (1) C: 0.07-0.15%, Si: 0.05-0
．． 50%, Mn: 0.30-1.80%, Cr: 0.1
0-1.20%, Mo: 0.10-1.00%, Al:
After hot rolling a steel billet containing 0.01 to 0.10%, V: 0.02 to 0.08%, the following Pcm being 0.26% or less, and the balance consisting of Fe and unavoidable impurities, the following By applying the heat treatment of
/mm^Method for manufacturing class 2 steel plate. Heat treatment method: Quenching + normalizing + tempering However, Quenching temperature: Ac_3 points or more and 950°C or less Normalizing temperature: Ac_1 point or more and Ac_less than 3 points Tempering temperature: 500°C or more and Ac_1 point or less Pcm=C+S
i/30+Mn/20+Cu/20+Ni/60+Cr
/20+Mo/15+V/10+5B(%) (2) C: 0.07-0.15%, Si: 0.05-0
．． 50%, Mn: 0.30-1.80%, Cr: 0.1
0-1.20%, Mo: 0.10-1.00%, Al:
A steel billet containing 0.01 to 0.10%, V: 0.02 to 0.08%, the following Pcm being 0.26% or less, and the balance consisting of Fe and unavoidable impurities is rolled at a finishing temperature of 950°C or higher. It is characterized by having a low yield ratio of 80% or less in the base material and a base material strength of 70 kgf/mm^2 class by hot rolling it, directly quenching it, and then subjecting it to the following heat treatment. Low yield ratio 70kgf/ with excellent weldability
Method for producing mm^2 grade steel plate. Heat treatment method: Normalizing + tempering However, Normalizing temperature: Ac_1 point or more, Ac_3 points or less Tempering temperature: 500℃ or more, Ac_1 point or less Pcm=C+S
i/30+Mn/20+Cu/20+Ni/60+Cr
/20+Mo/15+V/10+5B(%) (3) C: 0.07-0.15%, Si: 0.05-0
．． 50%, Mn: 0.30-1.80%, Cr: 0.1
0-1.20%, Mo: 0.10-1.00%, Al:
0.01-0.10%, Nb: 0.005-0.020
%, the following Pcm is 0.26% or less, and the balance is Fe
By hot-rolling a steel billet containing unavoidable impurities and then subjecting it to the following heat treatment, the base material has a low yield ratio of 80% or less and a base material strength of 70 kgf/mm^2 class. Low yield ratio of 70 with excellent weldability
Method for manufacturing kgf/mm^2 class steel plate. Heat treatment method: Quenching + normalizing + tempering However, Quenching temperature: Ac_3 points or more and 950°C or less Normalizing temperature: Ac_1 point or more and Ac_less than 3 points Tempering temperature: 500°C or more and Ac_1 point or less Pcm=C+S
i/30+Mn/20+Cu/20+Ni/60+Cr
/20+Mo/15+V/10+5B(%) (4) C: 0.07-0.15%, Si: 0.05-0
．． 50%, Mn: 0.30-1.80%, Cr: 0.1
0-1.20%, Mo: 0.10-1.00%, Al:
0.01-0.10%, Nb: 0.005-0.020
%, the following Pcm is 0.26% or less, and the balance is Fe
By hot-rolling a steel billet containing unavoidable impurities at a finishing temperature of 950°C or higher, directly quenching it, and then subjecting it to the heat treatment described below, a yield ratio as low as 80% or less in the base material and a yield ratio of 70kgf/ Low yield ratio 70k with excellent weldability characterized by base metal strength of mm^2 class
Method for producing gf/mm^2 grade steel plate. Heat treatment method: Normalizing + tempering However, Normalizing temperature: Ac_1 point or more, Ac_3 points or less Tempering temperature: 500℃ or more, Ac_1 point or less Pcm=C+S
i/30+Mn/20+Cu/20+Ni/60+Cr
/20+Mo/15+V/10+5B(%) (5) C: 0.07-0.15%, Si: 0.05-0
．． 50%, Mn: 0.30-1.80%, Cr: 0.1
0-1.20%, Mo: 0.10-1.00%, Al:
0.01-0.10%, V: 0.02-0.08%, N
b: 0.005 to 0.020%, the following Pcm is 0.26% or less, and the balance consists of Fe and unavoidable impurities. After hot rolling, the following heat treatment is applied to the base material. Low yield ratio of less than 80% at 70k
Manufacturing method of low yield ratio 70 kgf/mm^2 class steel plate with excellent weldability characterized by base metal strength of gf/mm^2 class Heat treatment method: Quenching + normalizing + tempering However, Quenching temperature : Ac_3 points or more and below 950℃ Normalizing temperature: Ac_1 point or more and Ac_less than 3 points Tempering temperature: 500℃ or more and Ac_less than 1 point Pcm=C+S
i/30+Mn/20+Cu/20+Ni/60+Cr
/20+Mo/15+V/10+5B(%) (6) C: 0.07-0.15%, Si: 0.05-0
．． 50%, Mn: 0.30-1.80%, Cr: 0.1
0-1.20%, Mo: 0.10-1.00%, Al:
0.01-0.10%, V: 0.02-0.08%, N
b: 0.005 to 0.020%, the following Pcm is 0.26% or less, and the balance consists of Fe and unavoidable impurities, hot rolled at a rolling finishing temperature of 950 ° C. or higher,
By performing direct quenching and then applying the following heat treatment, a low yield ratio of 80% or less in the base material and a 70kg
Manufacturing method of low yield ratio 70kgf/mm^2 class steel plate with excellent weldability characterized by base metal strength of f/mm^2 class Heat treatment method: Normalizing + tempering Normalizing temperature : Ac_1 point or more Ac_less than 3 points Tempering temperature: 500℃ or more Ac_less than 1 point Pcm=C+S
i/30+Mn/20+Cu/20+Ni/60+Cr
/20+Mo/15+V/10+5B(%) (7) Cu: 0.05~0.30%, Ni: 0.20~
3.00%, B: 0.0003-0.0020%, Ti
:0.003~0.020%, Ca:0.001~0.
Claims (1), (2), (3), (4) characterized in that it contains one or more types selected from 0.01%.
, (5) or (6) low yield ratio 70k with excellent weldability
Method for producing gf/mm^2 grade steel plate.