JPH03219012A - Production of high tensile steel reduced in yield ratio and having superior weldability - Google Patents

Abstract

PURPOSE:To produce a high tensile steel reduced in yield ratio and having superior weldability by hot rolling a steel in which components and PCM are specified, respectively, and successively subjecting this steel to rapid cooling, air cooling, heating and holding at two phase region temp., hardening, and tempering under respectively specified conditions. CONSTITUTION:A steel having a composition which consists of, by weight, 0.03-0.10% C, 0.05-0.60% Si, 0.60-2.00% Mn, 0.10-0.50% Mo, <=0.030% P, <=0.020% S, further one or more kinds among <=1.00% Ni, <=0.70% Ci, <=0.70% Cu, <=0.06% V, <=0.05% Nb, and <=0.0050% B, and the balance essentially Fe and in which PCM(%) represented by PCM(%)=C+Si/30+Mn/20+Ni/60+Cr/20+Cu/20+Mo/15 +V/10+5B is regulated to 0.16-0.21 is hot rolled, cooled rapidly down to 300-500 deg.C without delay, and air-cooled to room temp. Subsequently, the steel is heated and held at a two phase region temp. between the Ac3 and the Ac1 transformation point, hardened at a cooling velocity not lower than air cooling velocity, and further tempered at 450-600 deg.C. By this method, the high tensile steel reduced in yield ratio as well as in welding crack sensitivity can be obtained.

Description

Detailed Description of the Invention <Industrial Application Field> The present invention is used for steel structures such as buildings, bridges, and tanks, and has low weld crack susceptibility, a yield ratio of 80% or less, and a tensile strength of 60 kgf. The present invention relates to a method for producing high tensile strength steel with a low yield ratio of /lj or more.

<Conventional Technology> As steel structures such as buildings, bridges, and tanks become larger, steel materials used are required to have higher strength and thicker walls.

- On the other hand, from the viewpoint of structural safety, that is, prevention of brittle fracture, a low yield ratio and good weldability are required.

However, in general, as strength increases, yield ratio increases and weldability tends to decrease, and tensile strength of 60 kgf
/m11 or higher, it is not easy to achieve both a low yield ratio of 80% or less and good weldability. In other words, conventional low yield ratio 60 kg steel has a high weld cracking susceptibility (PCM-
0.24%) is high, the temperature is 100℃ during welding work.
Requires some preheating.

As a conventional method for producing high-strength steel with a low yield ratio, a method is known in which the steel is reheated to a temperature in the two-phase region of ferrite and austenite and then quenched. This method is effective for lowering the yield ratio.
However, since the carbon content is high at 0.35 to 0.50%, weldability is not very good.

An example of low yield ratio steel manufactured using this method is published in Nippon Kokan Giho No.
o, 122 (1988), page 9, but F.
eq, 0.45% (PCl30.24%), tensile strength 60kgf/*j or more, yield ratio 80%
Although the following was obtained, the weldability was high at 100°C at the preheating temperature to prevent Y cracking. As described above, with the conventional method, it was not possible to obtain a low yield ratio 60 kg steel that did not require preheating during welding.

<Problems to be solved by the invention> In view of the current situation, the present invention was developed in 'Xi'i. Below, the tensile strength is 60k
The purpose of the present invention is to propose a method for producing high-strength steel with a low yield ratio of gf/- or more.

The present inventors have conducted intensive research into a method for manufacturing high-strength steel that has low weld cracking susceptibility and a low yield ratio. With the added ingredients,
(2) Achieved by making the structure before quenching slightly coarser and subjecting it to quenching and tempering.

That is, in the present invention, in terms of weight ratio, C: 0.03 to 0
．． 10%, Si: 0.05-0.60%, Mn:
0.60-2.00%, Mo: 0.10-0.5
0%, P: 0.030% or less, S: 0.020
% or less, and further includes Ni: 1.00% or less, Cr
: o, 7% or less, Cu: 0.70% or less, V
: 0.06% or less, Nb: 0.05% or less and B
: Contains one or more selected from 0.0050% or less, and further includes Ti: 0, 003 to 0.05 as necessary.
%, the remainder consists essentially of Pe, and PcM (
%) = C+5i/30+Mn/20+Ni/60
lCr / 20 + Cu / 20← Gate o / 15
-V/10+5 After hot rolling steel with a B content of 0.16 to 0.21%, it is immediately subjected to elephant cooling to a temperature of 300 to 550 °C, or after air cooling, it is reheated to a temperature of Ac3 or higher and then heated to a temperature of 300 °C or higher. After cooling to ~550 °C for 2 times, air cooling to room temperature, and further heating and holding at a temperature in the two-phase region between Ac and 3~C1 transformation points, quenching at a cooling rate higher than air cooling, and then cooling at 450 to 600 °C. This is a method for producing a low yield ratio, high tensile strength steel with good weldability, which is characterized by performing tempering at a high temperature.

<For production> The present invention will be explained in detail below.

First, we will discuss the reasons for limiting the composition of steel.

c: o, o3~0.10% C is 0.03% (wt%) to obtain high strength and low YR.
The same applies hereinafter) or more is necessary, but if it exceeds 0.10%, the susceptibility to weld cracking increases, so it is set at 0.03 to 0.10%.

Si: 0.05-0.60% Si is required as a deoxidizing agent in an amount of 0.05% or more, but 0.05% or more is required as a deoxidizing agent.
If it exceeds 65%, the low-temperature toughness of the weld heat-affected zone deteriorates, so it is set at 0.05 to 0.60%.

Mn: 0.60-2.00% Mn is required at 0.60% or more to ensure hardenability and strength, but excessive addition of more than 2.00% will reduce weldability, so .60 to 2.00%.

P: 0.030% or less, S: 0.020%
Hereinafter, P and S are unavoidable as impurities mixed in steel, but since both impair weldability, toughness, and ductility, they are limited to 0.010% or less and 0.020% or less, respectively.

Mo: O, IO ~ 0.50% Mo is an element that solidifies in austenite and improves the hardenability of austenite, and also precipitates during tempering to increase tempering softening resistance and contribute to increased strength. and
0.10% or more is required, but addition of more than 0.50% reduces weldability and ductility, so 0.10 to 0.5
It was set to 0%.

Furthermore, in addition to the above-mentioned component system, one or more of the following components can be added in order to obtain a predetermined strength.

Ni: 1.00% or less, Cr: 0.70% or less, Cu: 0.70% or more, L V: 0.06% or less, Nb: 0.05% or less, and T3: 0.005
Less than 0%.

Both elements are effective in increasing strength, but excessive addition deteriorates weldability and ductility, so the upper limits for each element were set as above. Although Ni and Cu do not significantly reduce weldability and ductility, Ni is an expensive element and if it exceeds 1% there is a problem in economic efficiency, and if Cu exceeds 0.7% it deteriorates hot workability.

Furthermore, in the present invention, T1 can be added in the following range if necessary.

Ti: 0.003 to 0.05% Ti finely disperses γ dispersed in the ferrite base during heating in the (α1 to T) dual range, thereby increasing the strength and yield ratio. To ensure the specified strength, TiO,00
Ti is required to be 3% or more, but since adding more than 0.05% increases the yield ratio, it is set to 0.003 to 0.05%.

Furthermore, since the steel of the present invention has good hardenability, P
cM (%) -C+5i/30tMn/20+
Cu/20]-Cr/20+Ni/60+V
/IQ÷5B was limited to a range of 0.16 to 0.21%. The smaller the PCH, the lower the susceptibility to weld cracking, but 0.21% or less is required for free preheating, but 0.21% or less is required.
If it is less than 16%, strength cannot be ensured, so
4 was set in the range of 0.16 to 0.21%.

After forming the steel consisting of the above composition system by normal ingot making or continuous casting, hot rolling is performed to a predetermined thickness, and immediately after rolling, the steel is rapidly cooled to 300 to 550°C, or
Or, after rolling, it was first air cooled and then reheated to a temperature of Ac3 point or above? After cooling for 9 days from B00 to 550℃,
Perform pretreatment by air cooling to room temperature.

The reason why the rapid cooling is stopped at 300 to 550° C. in this pretreatment is to obtain a low yield ratio. Low-pert steels with reduced C content and increased alloying elements tend to have a low yield ratio.
As shown in Figure 1, the pretreatment is rapidly cooled to 300-550℃.
By stopping at this point, the yield ratio can be lowered to 80% or less. Therefore, the stopping temperature for pretreatment is 30
Limited to 0-550°C.

Ru.

Figure 1 shows steels with the compositions shown in Table 1 that were quenched immediately after rolling (Δ type), and those that were air-cooled after rolling and then reheated and quenched at an intended temperature of 900°C (Ac).
For the two types of pretreatment (type B), the cooling rate to the cooling stop temperature was set to 6℃/s, and the material was quenched and then air cooled, then reheated to 800℃ within the range of Ac and ~Ac1 points, and then water cooled. This figure shows the relationship between the tensile properties when tempered at 550°C and the cooling stop temperature of the pretreatment.

Next, heating is performed to a temperature in the two-phase region between Ac+ and Ac=transformation points in order to obtain a soft ferrite phase effective for low yield ratio and a hard hardened phase (austenite phase) necessary for high strength.
When heated above the c3 point, it is difficult to obtain a two-phase structure containing soft ferrite, and a low yield ratio cannot be obtained. The quenching cooling rate from the two-phase region temperature is desirable in order to obtain high strength and high toughness.

Since sufficient hardenability could not be obtained in the hardened phase (austener phase, austenite phase) with gradual cooling less than air cooling, cooling more than air cooling was used.

Next, the brittle hardened phase that has been quenched and hardened needs to be tempered at 450° C. or higher to improve its toughness.

On the other hand, if tempering is performed at a high temperature exceeding 600°C, the hardened phase will become significantly softened, leading to a decrease in tensile strength and an increase in yield ratio, so the upper limit of the tempering temperature was set at 600°C.

Table 1 (Example) Table 2 shows the chemical composition of the test materials. Test materials A to L are steels within the composition range of the present invention, and M-0 steel is a comparison steel.

After subjecting these steels to the pretreatment methods A and B shown in Table 3, they were heated and maintained at a two-phase region temperature within the Ac+ to Ac+ point range, and then quenched at a cooling rate higher than air cooling to a temperature of 500 to 56
Tempering treatment was performed at 0°C.

These mechanical properties are shown in Table 3. According to the method of the present invention, a tensile strength (T S ) of 60 kgf/- or more and a low yield ratio (YR) of 77% or less are obtained. The preheating temperature for preventing cracking in the Y-type weld cracking test was 25° C. or lower in all cases, and no preheating was required. Also, compared to Ti-free steels (A to I), T
YR is somewhat higher in i-added @ (J-L), but YS,
TS is high.

On the other hand, in the comparative example, TS≧60kgf/lJ, YR
Either ≦80% or Y crack prevention preheating temperature≦25°C is not satisfied.

For example, A2 and A3 steels are within the range of the invention in terms of composition and have good weldability, but cannot satisfy YR≦80% because the heat treatment conditions are not appropriate. M, N, and 0 steels all have high Pc and Y crack prevention preheating temperatures of 75°C or higher.

Furthermore, since the Ti content of P steel exceeds the invention range, YR exceeds 80%.

<Effects of the invention> By reducing the C content and increasing the content of alloying elements such as MO, PC
Copper with 9 adjusted to 0.16 to 0.21% is subjected to the pretreatment of the present invention in advance, and then reheated and held at a temperature in the two-phase region between Ac and Ac3 transformation points, then quenched and tempered. Through this treatment, it has become possible to produce a low-yield-ratio, high-strength steel with low weld cracking susceptibility, no need for preheating, a yield ratio of 80% or less, and a tensile strength of 60 kgf/i or more.

[Brief explanation of drawings]

FIG. 1 is a graph showing the influence of the cooling stop temperature during the if① treatment on the tensile properties.

Claims (2)

[Claims] (1) In terms of weight ratio, C: 0.03 to 0.10%, Si:
0.05-0.60%, Mn: 0.60-2.00%,
Mo: 0.10 to 0.50%, P: 0.030% or less,
Contains S: 0.020% or less, and further Ni: 1.00%
Below, Cr: 0.70% or less, Cu: 0.70% or less,
V: 0.06% or less, Nb: 0.05% or less, and B: 0
．． Including one or more types selected from 0.0050% or less,
The remainder essentially consists of Fe, and P_C_M(%)=
C+Si/30+Mn/20+Ni/60+Cr/20
+Cu/20+Mo/15+V/10+5B is 0.16
~0.21% steel immediately after hot rolling.
It was rapidly cooled to 50°C, or after air cooling, it was reheated to a temperature of Ac_3 point or higher, then rapidly cooled to 300 to 550°C, air cooled to room temperature, and then heated and maintained at a two-phase region temperature between Ac_3 and Ac_1 transformation points. A method for producing a low yield ratio, high tensile strength steel with good weldability, characterized in that it is then quenched at a cooling rate higher than air cooling, and then tempered at a temperature of 450 to 600°C. (2) In weight ratio, C: 0.03 to 0.10%, Si:
0.05-0.60%, Mn: 0.60-2.00%,
Mo: 0.10-0.50%, Ti: 0.003-0.
05%, P: 0.030% or less, S: 0.020% or less, Ni: 1.00% or less, Cr: 0.70
% or less, Cu: 0.70% or less, V: 0.06% or less,
Contains at least one type selected from Nb: 0.05% or less and B: 0.0050% or less, the remainder substantially consists of Fe, and P_C_M (%) = C + Si / 30 + Mn
/20+Ni/60+Cr/20+Cu/20+Mo/
After hot rolling steel with 15+V/10+5B of 0.16 to 0.21%, it is immediately quenched to 300 to 550℃, or after air cooling, it is reheated to a temperature of Ac_3 or higher and then quenched to 300 to 550℃. After that, air cool to room temperature,
Furthermore, after heating and holding at a temperature in the two-phase region between Ac_3 and Ac_1 transformation points, quenching is performed at a cooling rate higher than air cooling, and then 4
A method for producing a low yield ratio, high tensile strength steel with good weldability, which comprises tempering at a temperature of 50 to 600°C.