JPH01176029A - Manufacture of high-tensile steel plate with low yield ratio by accelerated cooling method - Google Patents

Abstract

PURPOSE:To provide the desired low yield ratio, high strength, and high toughness by hot-rolling an Nb-containing steel with a specific composition under specific conditions and subjecting the resulting hot-rolled steel plate to accelerated cooling and then to tempering treatment. CONSTITUTION:A steel having a composition consisting of, by weight, 0.03-0.2% C, 0.03-0.5% Si, 0.4-2.3% Mn, 0.01-0.1% Al, 0.1-0.5% Mo, 0.01-0.05% Nb, 0.3-1.5% Ni, and the balance Fe with inevitable impurities is cast. This steel is hot-rolled so that draft in an uncrystallized austenite region and finish rolling-finishing temp. are regulated to >=30% and >=Ar3 point, respectively. Directly after rolling, accelerated cooling is applied from >=Ar3 point down to <=750 deg.C at 2-40 deg.C/sec cooling rate. Then, tempering treatment is applied at 300-700 deg.C. If necessary, one or more kinds among 0.02-0.15% V, 0.3-0.05% Cr, 0.2-1.3% Cu, 0.0003-0.003% B, and 0.005-0.03% Ti are incorporated to the above steel. By using this high-tensile steel plate with low yield ratio, the safety of welding construction can be improved.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a method for manufacturing high-strength steel sheets with a low yield ratio by an accelerated cooling method, and more specifically,
The present invention relates to a method for producing a thick steel plate having a tensile strength of Kgr/mm' or more by an accelerated cooling method.

[Prior art] Conventional 70 Kgf/mm" class, 80 Kgf/mif
Heavy steel plates for bridges are manufactured by cooling to room temperature without forced cooling, followed by quenching and tempering, and the yield ratio was about 95%.

In recent years, a method has been proposed to produce steel plates with a yield ratio as low as 85% by controlled rolling, accelerated cooling, quenching, and tempering (Proceedings of the Welding Society of Japan, Vol.

5.1985. No, 3. P2S5), no method has been proposed for producing thick steel plates having a yield ratio of around 80%.

In recent years, high-strength steel plates for welded structures have made remarkable progress, and are now being put into practical use with tensile strength of 100 Kgf/mm'' class.
For bridges, the conventional rate is 70 Kgf/+nn+”.

80Kgf/+nm" plate steel has a high yield ratio and a low load capacity after yielding until it breaks, so there is a concern from the perspective of hidden safety. Until now, 80Kgf/+nm
``The use of high-strength steel in this field has been extremely limited.In this field, too, there is a strong demand for weight reduction, and a low yield ratio of 70Kgf/mm1, which has a hidden safety feature,
The emergence of high-strength steel sheets of grade 1 or higher was desired.

[Problems to be Solved by the Invention] In view of the above-mentioned problems in the conventional manufacturing method of high-strength steel plates with a low yield ratio, the present inventor has conducted intensive research, and as a result, the present invention has been developed to solve problems such as bridges, etc. In order to improve the hidden safety of welded structures and reduce the weight of the structures, we have developed a method for manufacturing high-strength steel plates with low yield ratios using an accelerated cooling method. In the field of inter-rolled steel plates, methods to lower the yield ratio of steel have been developed and put into practical use.
These steel sheets have a structure that includes ferrite, 5 to 30% martensite, and depending on the situation, bainite or retained austenite, and the presence of martensite is said to be effective in lowering the yield ratio. However, although a low yield ratio can be obtained with this method, it has been found that there are fatal disadvantages in the low transition temperature, high upper shelf energy, hardness distribution of the welded part, toughness, etc., which are required for bridge materials. discovered that the cause of toughness deterioration was the coarse dispersion of soft ferrite and hard martensite, and by incorporating Nb and adopting an appropriate accelerated cooling rate, ferrite could be acceleratedly cooled. By an accelerated cooling method, the desired low yield ratio, high strength, and high toughness can be achieved by making extremely fine precipitates in the inside and making the remaining part into a fine bainite + martensite structure, and then tempering it under appropriate conditions. They developed a method for manufacturing high-strength steel plates with low yield ratios.

[Means for Solving the Problems] The present method for manufacturing a low yield ratio high tensile strength steel plate by the accelerated cooling method according to the present invention is as follows: (1) G 0.03 to 0.2 vt%, Si 0.03
~0.5wt%, Mn 0.4~2Jwt%, AI 0
．． 01-0.1wt%, Mo 0.1-0.5vt%,
Nb 0.01-0.05wt%, Ni 0.3-1.
5 wt%, with the remainder consisting of Fe and unavoidable impurities.
The rolling reduction in the unrecrystallized austenite region is 30% or more,
And, immediately after hot rolling so that the finish rolling end temperature is Ar or higher, the temperature is reduced to 2-40°C from a temperature of Ar3 or higher.
Accelerated cooling to below 750°C at a cooling rate of °C/sec,
The first invention is a method for manufacturing a low yield ratio high tensile strength steel plate by an accelerated cooling method characterized by performing tempering treatment in a temperature range of 300 to 700°C, (2) G 0.03 to 0.2 wt. %, Si0.03
~0.5vt%, Mn 0.4~2.3wt%, Al
0.01-0.1wt%, Mo 0.1-0.5wt%
, Nb 0.01-0.05wt%, Ni 0.3-1
．． 5 wt%, and further contains V 0.02-0.15 wt%, Cr OJ ~1.5
wt%, Cu 0.2-1.3wt%, B 0.000
3-0.003wt%, Ti 0.005-0.03w
t% of one kind or two or more kinds, and the balance consists of Fe and unavoidable impurities,
The rolling reduction in the unrecrystallized austenite region is 30% or more,
And, after hot rolling so that the finish rolling end temperature becomes Ar3 or higher, immediately accelerated cooling is performed from a temperature of Ar3 or higher to 2-b 750°C or lower, and then 300-700°C.
This invention consists of two inventions, the second invention being a method for producing a low yield ratio high tensile strength steel plate by an accelerated cooling method characterized by performing a tempering treatment in a temperature range of .degree.

A method for manufacturing a low yield ratio high tensile strength steel sheet using an accelerated cooling method according to the present invention will be described in detail below.

First, a method for manufacturing a low yield ratio high tensile strength steel plate by an accelerated cooling method according to the present invention (hereinafter sometimes simply referred to as the manufacturing method of the present invention).

) The contained components and content ratio of the steel used in the above will be explained.

C is an effective element for increasing strength, and the content is 0°03v
If it is less than t%, the strength increasing effect is small, and if it is less than 0.2wt%,
If the content exceeds %, weldability deteriorates.

Therefore, the C content is set to 0.03 to 0.2 wt%.

Si is an effective element for structure control, and the content is 0.03
If it is less than 0.0 wt%, no tissue control effect can be achieved, and if it is less than 0.
If the content exceeds 5vt%, the toughness will deteriorate.

Therefore, the St content is set to 0.03 to 0.5 wt%.

Like Si, Mn is an element effective in controlling the structure, and if the content is less than 0.4 wt%, the effect of controlling the structure is small, and if the content exceeds 2 Jvt%, a band-like structure is generated and the C direction , leading to deterioration of toughness in the Z direction. Therefore, the Mn content is set to 0.4 to 2.3 wt%.

Al is an element necessary as a deoxidizing agent, and the content h < 0
．． If the content is less than 0.1 wt%, there is no effect as a deoxidizing agent, and if the content exceeds 0.1 wt%, this effect is saturated. Therefore, the AI content is set to 0.01 to 0.1 wt%.

Mo is an element that is effective in generating a bainite structure and improving toughness.If the content is less than 0.1 wt%, these effects will be small, and if the content exceeds 0.5 wt%, These effects become saturated. Therefore, M
The OC content is set to 0.1 to 0.5 wt%.

Nb is an element that refines the γ grain size, expands the non-recrystallized rolling area, and contributes to the refinement of the bainite structure and increase in strength, and these effects are exhibited when the content is less than 0.01 vt%. It is not possible and 0.05v
If the content exceeds t%, the effect will be saturated. Therefore, the Nb content is set to 0.01 to 0.05 wt%.

Ni is an element effective in improving weldability and toughness, and if the content is less than 0.3 wt%, this effect will be small;
If the content exceeds 5 wt%, the effect will be saturated. Therefore, the Ni content is set to 0.3 to 1.5 vt%.

■ is an element that is effective in refining the γ grain size, refining the bainite structure, contributing to an increase in strength, and contributing to an increase in strength through precipitation strengthening, and when the content is less than 0.01 wt%, The effects are small, and these effects are saturated if the content exceeds 15 vt% of G. Therefore, ■ content is 0. CI2~Q, I shall be 5wt%.

Cr is an element that is effective in controlling the structure and contributes to increasing strength. If the content is less than 0.3 wt%, this effect will be small, and if the content exceeds 1.5 wt%, the effect will be reduced. No further increase can be expected.

Therefore, the Cr content is set to 0.3 to 1.5 wt%.

Cu is an element effective in improving weldability and toughness, and if the content is less than 0.2 wt%, this effect is small, and
If it exceeds 1.3 wt%, the effect is saturated.

Therefore, the Cu content is set to 0.2 to 1.3 wt%.

B is an element that is effective in controlling the structure and contributes to increasing strength, and if the content is less than 0.0003 wt%, this effect is small (and if the content exceeds 0.003 wt%, the effect is saturated. Therefore, B content is 0.0003 to 0.003
Let it be wt%.

Ti is an element that fixes N and contributes to effectively utilizing the effect of B. If the content is less than 0.005wt%, this effect is small, and if the content is less than 0.03wt%
If the content exceeds this amount, this effect will be saturated. Therefore, the Ti content is set to 0.005 to 0.03 wt%.

Next, the manufacturing method according to the present invention will be explained.

The heating temperature of steel with the above-mentioned components and content ratios is preferably 900 to 1150°C. In particular, heating in a low temperature range results in finer γ grains, a finer structure obtained after transformation, and improved toughness. Therefore, it is desirable to heat to a low temperature within the allowable range of rolling mill capacity and finishing temperature.

The hot working conditions are such that the refinement of the γ grains and the introduction of deformation bands within the γ grains are effective in refining the structure after transformation and improving toughness. For example, hot rolling is required.

Regarding the cooling conditions, if the accelerated cooling start temperature is less than Ar3, coarse pro-eutectoid ferrite will precipitate during air cooling and the toughness will deteriorate. Therefore, it is necessary to start accelerated cooling immediately after the end of hot rolling. If the cooling rate is less than 2°C/Sec, a large amount of pro-eutectoid ferrite will precipitate coarsely, resulting in low strength and toughness, and if it exceeds 40°C/sec, the structure will be 100% bainite + martensite. Therefore, the yield ratio becomes too high. Furthermore, the appropriate cooling rate to precipitate an appropriate amount of pro-eutectoid ferrite in the structure depends on the content and content ratio of the steel, and the range of this appropriate cooling rate can be determined using, for example, the following formula: can.

・Boron-free steel-17.5-08g+10.2<1nR(℃/S)<
-17,5ceg+12.6 - Steel containing steel -17,5-Ceg+9.5< 1nR (℃/s)<
-17.5ceg+11.9 However, Ceg=C+Si/24+Mn/6+Ni/40+Cr
15+MO/4+V/14 In addition, when the stop temperature becomes higher than 750℃,
The tensile strength becomes low, below 70 Kgf/mm'. Figure 2 shows the cooling stop temperature and TS.

The relationship between YR is shown.

If the tempering temperature is less than 300° C., the YR will be 75% or less, especially for materials with a low yield ratio before treatment, and desired characteristics will not be obtained. For example, steel 2 shown in Table 1 below is heated to a temperature of 1000°C, the reduction rate is 50% at a temperature of 850°C or less, the finishing temperature is 780°C, and the cooling rate is 10°C.
/sec, TS, YRSYP when a 30++u++t thick steel plate is manufactured at a stopping temperature of 550℃ and the tempering temperature is changed.
The relationship between EI is shown in FIG. When the tempering temperature is less than 300℃, YR is a low value that is almost the same as YR after accelerated cooling, but in the temperature range of 300 to 700℃, the YR is a low value.
The R is in the range of 75 to 90%, and elongation at yield point has also occurred. However, in this temperature range, low-temperature transformation products that lower the yield ratio and contribute to strengthening, such as martensite, self-tempered martensite, and lower bainite, which constitute part of the microstructure as-is, are tempered. Transmission electron microscopy reveals clear cementite precipitation. Therefore, this phenomenon is TS
The desired strength, YR
In order to obtain this, it is necessary to fully understand the relationship between the mechanical properties obtained during accelerated cooling and the mechanical properties associated with the tempering temperature. In addition, when the tempering temperature exceeds 700℃, it enters the α+72 phase region, and depending on the cooling conditions and component system, it becomes a ferrite-martensitic structure after tempering.
With current technology, there is a decrease in YR and an increase in TS, and depending on the component system, it may remain unchanged, or furthermore, YR may increase and TS may decrease. Not suitable for industrial production.

Therefore, in the tempering temperature range of 300 to 700°C, TS is approximately △T, although the change tendency varies slightly depending on the component system.
S (Kgf/ ++unly 0, 35 (Kgf/
mm'') x T (tempering temperature), and YR increases.

Therefore, it is necessary to ensure mechanical properties that take this into consideration in the state of the accelerated coolant.

[Example] An example of a method for manufacturing a low yield ratio high tensile strength steel plate by an accelerated cooling method according to the present invention will be described.

Example Steel with the ingredients and content ratios shown in Table 1 was melted and cast using a normal manufacturing method, and a 100 mmt* (slab) was heated at various heating temperatures between 900°C and 1150°C.
is heated and rolled into a 30mm thick steel plate, and the finishing temperature is 8.
The target temperature was 00°C, and it was finished around that temperature. Both are 90
The rolling reduction ratio at 0° C. or lower was set to 50% or higher. After completion of hot rolling, accelerated cooling was performed under the conditions shown in Table 2. Thereafter, tempering treatments were performed at various temperatures as necessary. The tempering conditions were held for 1 hour and then air cooled.

This Table 2 shows the mechanical properties of each steel type, and the steel manufactured by the manufacturing method of the present invention has a yield ratio of 75 to 90%,
Other mechanical properties are equal to or better than those of the comparative example.
It turns out that it is excellent.

[Effects of the Invention] As explained above, since the method for producing a low yield ratio high tensile strength steel plate by the accelerated cooling method according to the present invention has the above configuration, the safety of the welded structure is high and the tensile strength is 70 Kgf. /+n
This is an excellent manufacturing method that can effectively produce high-strength steel sheets having a tensile strength of +u'' or more and a yield ratio of 75 to 90%.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between tempering temperature, TS, and YRSYPEI, and FIG. 2 is a diagram showing the relationship between cooling stop temperature, TS, and YR. Figure 1 14ζ'c)

Claims (2)

[Claims] (1) C0.03-0.2wt%, Si0.03-0.
5wt%, Mn0.4~2.3wt%, Al0.01~
0.1wt%, Mo0.1-0.5wt%, Nb0.0
Steel containing 1 to 0.05 wt%, 0.3 to 1.5 wt% Ni, and the balance consisting of Fe and unavoidable impurities,
The rolling reduction in the unrecrystallized austenite region is 30% or more,
And, immediately after hot rolling so that the finish rolling end temperature becomes Ar_3 or more,
A method for producing a low-yield-ratio high-strength steel plate by an accelerated cooling method characterized by accelerated cooling to a temperature of 750°C or less at a cooling rate of 40°C/sec, and then tempering in a temperature range of 300 to 700°C. . (2) C0.03-0.2wt%, Si0.03-0.
5wt%, Mn0.4~2.3wt%, Al0.01~
0.1wt%, Mo0.1-0.5wt%, Nb0.0
Contains 1-1.5wt%, Ni0.3-1.5wt%, and further contains V0.02-0.15wt%, Cr0.3-0.05w
t%, Cu0.2-1.3wt%, B0.0003-0
．． 003 wt%, Ti 0.005 to 0.03 wt%, and the balance is Fe and inevitable impurities.
The rolling reduction in the unrecrystallized austenite region is 30% or more,
And, immediately after hot rolling so that the finish rolling end temperature becomes Ar_3 or more,
A method for producing a high tensile strength steel sheet with a low yield ratio by an accelerated cooling method, which comprises accelerated cooling to 750°C or less at a cooling rate of 40°C/sec, and then tempering in a temperature range of 300 to 700°C.