JPH01176030A - 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 subjecting an Nb-containing steel with a specific composition to hot rolling under specific conditions and then to accelerated cooling down to a specific temp. 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 unrecrystallized austenite region and finish rolling-finishing temp. are regulated to >=30% and >=Ar3 point, respectively. After rolling, cooling is applied to the above weight delay from >=Ar3 point at 2-40 deg.C/sec cooling rate, and cooling is stopped at a temp. between 300 and 700 deg.C. If necessary, one or more kinds among 0.3-1.5% Cr, 0.2-1.3% Cu, 0.0003-0.0030% 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 increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] 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 of producing a thick steel plate having a tensile strength of Kgr/me" or more by an accelerated cooling method.

[Prior art] Conventional c7) 70 Kgf/mm”ffl, 80 K
gf/mm'' class thick steel plates for bridges were manufactured by cooling to room temperature without forced cooling, followed by quenching and tempering, and the yield ratio was 95% a degree.

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.

3.1985. No, 3. P2S5), also 60kg
f/mra" plate steel, a method has been proposed that after rolling is finished, the steel is air-cooled to an Ar point of 3 or less to precipitate a predetermined amount of pro-eutectoid ferrite, and then accelerated cooling is performed to obtain a low-yield ratio steel with a yield ratio of less than 70%. (Japanese Unexamined Patent Publication No. 59-211528).

However, the yield ratio is extremely low at 70kgf/m, around 70%.
No method has been proposed for producing high-strength steel sheets with low yield ratios of m'' class or higher.

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 lOOKgf/mm'' class.
For bridges, conventional 70 Kg4/mm”, 80 Kgf
/a+r class steel has a high yield ratio and a low load capacity after yielding until it breaks, so there is a cause for concern from the perspective of hidden safety. Its use was extremely limited, and
In this field as well, there is a strong demand for weight reduction, and the emergence of high-strength steel plates of 70 Kgr/am'' class or higher with a low yield ratio and hidden safety has been desired.

[Problems to be Solved by the Invention] In view of the above-described problems in the conventional manufacturing method of high-strength steel sheets with a low yield ratio, the present inventor has conducted extensive research, and as a result, the present invention has been developed to Developed a method for manufacturing high-strength steel plates with a low yield ratio using an accelerated cooling method to increase the hidden safety of welded structures and reduce the weight of the structures.
In the fields of cold-rolled steel sheets and hot-rolled steel sheets, methods to lower the yield ratio of steel have been developed and put into practical use, but these steel sheets contain ferrite and 5 to 30% martensite, and depending on the situation, bainite and residual It has a structure containing austenite, and the presence of this martensite is said to be effective in lowering the yield ratio.
Although a low yield ratio can be obtained even when applied to thick steel plates of Im'' grade or higher, it has fatal drawbacks such as low transition temperature, high upper shelf energy, hardness distribution of welded parts, toughness, etc. that must be provided for bridge materials. In particular, it was found that the cause of the deterioration of toughness value was the coarse dispersion of soft ferrite and hard martensite.
By using b content and an appropriate accelerated cooling rate, ferrite can be precipitated extremely finely during accelerated cooling, and
By making the remaining part a fine bainite + martensite structure, we developed a method for manufacturing high-strength steel sheets with a low yield ratio using an accelerated cooling method that achieves the desired low yield ratio, high strength, and high toughness.

[Means for Solving the Problems] This manufacturing method of a low yield ratio high tensile strength steel plate by the accelerated cooling method according to the present invention is as follows: (1) C0.03 to 0.2wt%, Si 0.0
3-0.5wt%, Mn 0.4-2.3wt%, Al
0.01-0.1wt%, Mo 0.1-0.5wt
%, Nb 0.01-0.05wt%, Ni 0.3
~1.5wt%, with the balance consisting 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 Ar8 or higher, it is immediately cooled from a temperature of Ar3 or higher by 2-b, and cooling is stopped at a temperature between 300 and 700°C. The first invention is a method for manufacturing a high-strength steel plate with a low yield ratio by an accelerated cooling method, and (2) C0.03 to 0.
, 2wt%, 5iO103~0.5wt%, Mn 0.
4 to 2.3 wt%, Al 0.01 to 0. iwt%, M
o 0.1-0.5wt%, Nb 0.01-0.05
wt%, Ni 0.3-1.5wt%, and further contains Or, 0.3-1.5wt%, Cu 0.2-1.3w
t%, B 0.0003-0.003wt%, T i
A wire containing one or more of 0.005 to 0.03 wt%, with the balance consisting of Fe and unavoidable impurities,
At a reduction rate of 30% or more in the unrecrystallized austenite region,
And, after hot rolling so that the finish rolling end temperature is Ar or higher, it is immediately cooled from a temperature of Ars or higher by 2 to 150°C, and a cooling stop is performed at a temperature of 300 to 700°C. This invention consists of two inventions, the second invention being a method for manufacturing a low yield ratio high tensile strength steel plate by an accelerated cooling method.

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 element effective in increasing strength, and the content is 0.03w
Below t%, the strength upper limit effect is small, and 0.2wt
If the content exceeds %, weldability deteriorates.

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

St 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 5 wt%, the toughness will deteriorate.

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

Like Sl, Mn is an element effective in controlling the structure, and if the content is less than 0.4 vt%, the effect of controlling the structure is small, and if the content exceeds 2.3 wL%, a band-like structure is generated. This leads to deterioration of toughness in the C direction and 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 is 0.0
If it is less than 1 wt%, it has no effect as a deoxidizing agent, and 0
．． This effect is saturated when the content exceeds 1 wt%.

In addition, the Al content is Q, QL ~ 0.1 wt%.

Mo is an element that is effective in generating a bainite structure and improving toughness.If the content is less than 0.1vt%, these effects will be small, and if the content exceeds 0.5wj%, These effects become saturated. Therefore, M
When the OC content is 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. If m is less than 0.01 wt%, these effects cannot be exhibited, and if m is contained in excess of 0.05 wt%, the effects are saturated. Therefore, the Nb content is set to 0.01-0.05 wt%.

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

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-containing rIt is set to 0.2 to 1.3 t%.

B is an element that is effective in controlling the structure and contributes to increasing strength. 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, the B content is 0.0003 to 0.003
wj%.

Ti is an element that fixes N and contributes to effectively utilizing the effect of B, and its content is (1,005wt5
If it is less than 0.03wt, this effect is small, and if it is less than 0.03wt.
If the content exceeds %, this effect will be saturated. Therefore, the Ti content is set to o, oos to 0.03 wt%.

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

The heating temperature of the 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 Ars, coarse pro-eutectoid ferrite will precipitate during air cooling, and the toughness will deteriorate.
Therefore, it is important to start accelerated cooling immediately after hot rolling.

In addition, if the cooling rate is less than 2°C/sec, pro-eutectoid ferrite will precipitate in large quantities and coarsely, resulting in low strength and toughness, and if it exceeds 40°C/sec, the
It becomes a bainite + martensite structure, and the yield ratio becomes too high.

The appropriate cooling rate for precipitating an appropriate amount of pro-eutectoid ferrite in the structure depends on the components and content ratio of the steel. This appropriate cooling rate range can be determined using the following equation.

・Boron-free steel-17,5-Ceq+10.2<1nR("C/s)<
-17,5Ceq+ 12.6 ・Boron-containing steel -17,5・Ceq+9.5<1nR("C/s)<-
17.5Ceq+11.9 However, Ceq=C+Si/24+Mn/6+Nt/40+Cr
15+Mo/4+V/14 However, if the cooling stop temperature is less than 300°C, not only will it be difficult to control the temperature, but the rolling pressure in the leveler process will be large, which is undesirable.
l The structure is close to Phase 4, and the yield ratio is approximately 60.
% and at a stopping temperature exceeding 300°C, a strength of 80 Kgr/mQ+” is obtained, and the yield ratio is around 70% of the required value, and when the stopping temperature exceeds 700°C. At high temperatures, it becomes a ferrite-pearlite structure with a tensile strength of 70 Kgf/aus'
It will become below. To find out this, the relationship between the cooling stop temperature, TS, and YR was investigated using the steel shown in Table 1 as an example, and the results are shown in FIG. From this figure 2, it can be seen that the cooling stop temperature is 300~
It can be seen that a temperature range of 700°C is suitable.

In addition, in the state of accelerated cooling, that is, in the state of hot rolling and cooling, the elongation at yield point is zero in most cases, and it is a continuous yielding phenomenon.Especially when designing bridges, etc.
If you want to give yield point elongation, it is necessary to perform tempering treatment at a temperature of 100 to 300°C, but as shown in Figure 3, if the tempering temperature exceeds 300°C, YR will be 75% of the upper limit.
As a result, the desired low yield ratio cannot be obtained, and if the tempering temperature is less than 100° C., no substantial effect can be obtained.

[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 having the components and content ratios shown in Table 1 was melted and cast using a normal manufacturing method, and a steel (slab) of 100 s + mt was heated at various heating temperatures between 900°C and 1100°C, and then heated for 20 times. was rolled into a thick steel plate, and the rolling reduction at 900°C or less was set to 50%. After the hot rolling was completed, the specimens were cooled at various cooling rates. A tempering treatment was performed as necessary, and the sample was kept at a predetermined temperature for 1 hour and then cooled in air.

Table 2 shows the mechanical properties of each steel type.

From Table 2, the steel manufactured by the manufacturing method of the present invention has a lower yield ratio of 65 to 75% and higher toughness than the comparative example, and other mechanical properties are equivalent to the comparative example. It's more than that, and you can see that it's superior.

[Effects of R-light] As explained above, the method for manufacturing a low yield ratio high-strength steel plate by the accelerated cooling method according to the present invention has the above-mentioned configuration, so the welded groove structure can be produced with high safety of 70 Kgf/mm. This is an excellent manufacturing method that can effectively manufacture a high tensile strength plate with a low yield ratio of 65 to 75%.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between accelerated cooling start temperature and T, S, vTrs, Fig. 2 is a diagram showing the relationship between cooling stop temperature and TSSYR, and Fig. 3 is a diagram showing the relationship between tempering temperature and TS. It is a figure showing the relationship between Yrt and YPEI. 3rd figure

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, the temperature is reduced to 2 to 4 from Ar_3 or more.
Accelerated cooling at a cooling rate of 0°C/sec, 300-700
A method for manufacturing high-strength steel sheets with a low yield ratio by an accelerated cooling method characterized by stopping cooling at a temperature between ℃. (2) C0.03-0.2wt%, Si0.03-0.
5wt%, Mn0.4~2.3wt%, Al0.01~
0.1wt%, Mo0.1-0.5wt%, Nb0.0
1 to 1.5 wt%, Ni0.3 to 1.5 wt%, and further contains Cr0.3 to 1.5 wt%, Cu0.2 to 1.3 wt%.
, B0.0003~0.003wt%, Ti0.005~0.03wt%, and the balance is 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, the temperature is reduced to 2 to 4 from Ar_3 or more.
Accelerated cooling at a cooling rate of 0°C/sec, 300-700
A method for manufacturing high-strength steel sheets with a low yield ratio by an accelerated cooling method characterized by stopping cooling at a temperature between ℃.