JP3399983B2 - Method for producing high-strength steel sheet with excellent weldability and low yield ratio of 570 N / mm2 or more - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tempered high-strength steel sheet of 570 N / mm ² grade or higher, which is mainly used for building structures, and more specifically, 570 N / mm ² grade with excellent weldability and low yield ratio. The present invention relates to the method for manufacturing a high-strength steel plate described above.

[0002]

^2. Description of the Related Art Heat-treated high-strength steel sheets with a tensile strength of 570 N / mm ² grade or higher have been used for tanks, bridges, penstocks, etc., but by quenching and tempering, high hardness micro steel such as martensite and bainite. Since the generation of structure is used, the yield ratio (yield strength / tensile strength) is usually as high as 90% or more, the plastic deformability is not sufficient, and sufficient safety cannot be ensured in the event of a large earthquake. It was rarely used for business.

In recent years, there has been an increasing demand for building structures to have a higher layer and a larger span, and it has been attempted to use high-strength steel plates of 570 N / mm ² class or higher, which are stronger than the conventional 490 N / mm ² class steel plate. This has led to the demand for high-strength steel sheets with a yield ratio reduced to around 85% or less.

As a steel sheet satisfying this requirement, reheating quenching (Q) from a temperature of Ac ₃ point or higher (or direct quenching from a temperature of Ar ₃ point or higher (DQ)) and a temperature of less than Ac ₁ point Different from the conventional heat treatment method consisting of a combination with tempering (T), a new quenching (Q ') from the two-phase region temperature (Ac ₁ point or more and Ac ₃ points or less) is performed between these two heat treatments. Heat treatment method (Q-Q'-T and DQ-Q'-
T method) was developed. According to this method, a low yield ratio is obtained because ferrite having low hardness and excellent ductility is generated in the structure due to Q '.

The low yield ratio 570N / mm ² class steel sheet obtained by such heat treatment has come to be used for high-rise buildings, but since it utilizes the formation of ferrite, it has a predetermined strength. In order to secure it, it is necessary to make the carbon equivalent higher than that of the conventional 570N / mm ² grade steel sheet, and there is a problem that the weldability is slightly inferior, and improvement is required.

[0006] On the other hand, as the number of buildings increases, the amount of welding work increases, and in order to prevent this, it is necessary to use higher strength steel sheets in order to reduce the thickness of the steel sheets. Is being considered. That is, tensile strength
There is an increasing demand for development of steel plates with a high yield ratio of 780 N / mm ² or higher.

However, even in the case of the 780 N / mm ² grade steel sheet, in order to secure high strength and low yield ratio, the above-mentioned Q-Q'-T method also produces ferrite and at the same time produces martensite. Since the hardness and the fraction of bainite must be increased, the carbon equivalent must be higher than that of the conventional 780N / mm ² grade steel sheet, and there was the problem of poor weldability.

[0008]

The present invention has a tensile strength of 57.
For tempered high-strength steel sheets of 0 N / mm ² grade or higher, while maintaining a low yield ratio of 85% or less, yield yielding excellent weldability equivalent to or higher than the high yield ratio type high-strength steel sheets by the conventional QT method. It is an object of the present invention to provide a method for producing a high-strength steel sheet having a low ratio of 570 N / mm ² or higher.

[0009]

The present inventors have found that the tensile strength
In order to secure a high strength of 570 N / mm ² class or higher, a low yield ratio of 85% or less and a good weldability, intensive research was conducted. As a result, a new type that replaces Q '(quenching from the two-phase region), which is important for realizing a low yield ratio, with Modified Q' (heats to the two-phase region temperature and then air-cools to a predetermined temperature, then quenches) By applying heat treatment, while maintaining a low yield ratio, Q
The present invention has been completed based on the finding that higher strength can be realized than in the case of the -Q'-T method.

By utilizing the Q-Modified Q'-T method, it is possible to reduce the carbon equivalent of the steel sheet by an amount capable of increasing the strength, so that it is possible to improve the weldability of the low yield ratio type high strength steel sheet. It will be.

The first invention is that C: 0.05 to 0.20%, Si: 0.
05-0.50%, Mn: 0.30-1.80%, Al: 0.005-0.10%, hot-rolled steel slab consisting of balance Fe and unavoidable impurities, and then heat-treated as follows. This is a method for producing high-strength steel sheets with a low ratio of 570 N / mm ² or higher. Heat treatment method: Quenching (Q) + Special quenching (Modified
Q ') + tempering (T) However, quenching temperature: Ac ₃ points or more and 980 ° C or less Special quenching: Ac ₁ point or more and less than Ac ₃ points, 400 to 70
After air cooling to a specified temperature between 0 ° C, quenching Tempering temperature: 400 ° C or more and less than Ac ₁ point

The second invention is C: 0.05-0.20%, Si: 0.05-
Direct quenching is performed by hot rolling a steel slab containing 0.50%, Mn: 0.30 to 1.80%, Al: 0.005 to 0.10% and the balance Fe and unavoidable impurities at a rolling finish temperature of Ar ₃ points or higher. After that, the following heat treatment is performed, which is a method for producing a high-strength steel sheet having a high yield ratio of 570 N / mm ² or higher and excellent weldability. Heat treatment method: Special quenching (Modified Q ') + tempering (T) However, special quenching: Heating at Ac ₁ point or more and less than Ac ₃ points,
After air cooling to a specified temperature between 400 and 700 ° C, quenching Tempering temperature: 400 ° C or higher and less than Ac ₁ point

A third aspect of the present invention further comprises Cu:
0.05 to 1.20%, Ni: 0.05 to 3.00%, Cr: 0.05 to 1.20%,
Mo: 0.05-1.00%, V: 0.005-0.08%, Nb: 0.005-0.05
%, B: 0.0003 to 0.0025%, Ti: 0.005 to 0.025%, Ca: 0.0
The method for producing a high-strength steel sheet having a high yield ratio of 570 N / mm ² or more, which has excellent weldability and contains at least one selected from the range of 01 to 0.010%.

[0014]

The manufacturing conditions of the present invention will be described below.
First, the reasons for limiting the heat treatment method will be described. The inventors of the present invention used a steel sheet having the chemical composition shown in Table 1 and performed some kinds of heat treatments to investigate the influence of the heat treatment method on the strength and the yield ratio. The conventional heat treatment is 780 N / m
Q-T method applied to m ² grade steel sheets, Q-Q'-T method including Q'which is a two-phase heat treatment, and improved Q-Modi
There are three types of fied Q'-T method. The heat treatment conditions are shown in FIG.

Here, Q: reheating and quenching from a temperature of Ac ₃ points or more, Q ': reheating and quenching from a two-phase region temperature (Ac ₁ point or more and less than Ac ₃ points), Modified Q': two-phase region temperature After heating to T, the material is air-cooled to a predetermined temperature, then quenched, and tempered from a temperature of T: Ac less than ₁ point. The tempering start temperature in Modified Q'heat treatment is 7
It was changed in the range of 00 to 280 ° C.

The results of the tensile test after the heat treatment are shown in FIG. From the figure, the conventional Q-T method has a yield ratio of 90% or more, while the other two methods including the two-phase heat treatment can obtain a low yield ratio of 85% or less. Among these two-phase heat treatment methods, in the case of the Q-Modified Q'-T method in which the quenching start temperature of Modified Q'is 400 to 700 ° C,
It can be seen that higher tensile strength is obtained than in the case of QQ'-T method.

As described above, the new heat treatment called the Q-Modified Q'-T method is very effective in increasing the strength while ensuring a low yield ratio.

The reason for this is considered to be due to the difference in microstructure as schematically shown in FIG. That is, at the stage where the first-stage heat treatment Q is followed by heating in the two-phase region,
The pre-structure of martensite and lower bainite partially reverse transforms to austenite. In the subsequent cooling,
In the Q'heat treatment, quenching is performed immediately, so that austenite transforms to high hardness martensite or lower bainite.

On the other hand, in the Modified Q'heat treatment, a small amount of ferrite is generated during the initial air cooling, and C is concentrated in austenite. Since quenching is performed from this stage, the C concentration is increased from the austenite enriched with C. It is considered that martensite with high hardness and higher hardness is generated. Therefore, the Q-Modified Q'-T method can obtain higher strength than the Q-Q'-T method. In this case as well, since the soft ferrite is dispersed in the hard layer, a low yield ratio can be obtained.

[0020]

[Table 1]

By utilizing the above new knowledge, the present inventors have tried to improve the weldability of a high yield strength steel sheet with a low yield ratio of 570 N / mm ² or higher. As shown in Table 2, the conventional Q-Q '
Carbon equivalent of about 0 low yield ratio 570N / mm ² class steel sheet by -T method.
For steel with carbon equivalent reduced to 0.39% against 44%, Q
-Modified Q'-T method and Q-Q'-T method were applied. Its mechanical properties are also shown in Table 2.

As is clear from Table 2, Q-Modified
When the Q'-T method is applied, sufficient strength and low yield ratio are obtained despite the low carbon equivalent.
On the other hand, when the QQ'-T method is applied, sufficient strength cannot be secured because the carbon equivalent is low.

For the above reasons, in the present invention, the heat treatment method is limited to the Q-Modified Q'-T method. The Q heat treatment may be direct quenching after rolling, which is equivalent in terms of quenching from a complete austenite region.

[0024]

[Table 2]

Next, the reason for limiting the temperature range in each of the above heat treatments will be described. For the quenching temperature,
In order to generate a high-hardness microstructure such as martensite and bainite and secure sufficient strength, it is necessary to make it a complete austenite region, and Ac is set to ₃ or more. However, if the temperature is too high, the structure becomes coarse and the ductility and toughness deteriorate, so the upper limit is made 980 ° C.

Further, the reason why the rolling finishing temperature in the case of direct quenching is set to Ar ₃ point or higher is that when the rolling finishing temperature is lower than this, the martensite and the fully tempered before the two-phase region heat treatment are This is because the bainite phase cannot be secured and it becomes difficult to secure the strength.

Regarding the quenching start temperature of Modified Q ', C in austenite accompanying the formation of ferrite
In order to generate high-C martensite by utilizing the above-mentioned concentration and achieve high strength, it is necessary to set the quenching start temperature in the range of 400 to 700 ° C as shown in Fig. 2 described above.

The tempering temperature is set to 400 ° C. or higher because it is not preferable at a too low temperature for improving the toughness and ensuring the safety of the structure. On the other hand, if it exceeds the Ac ₁ point, the strength is lowered, so the value is set to less than the Ac ₁ point.

Next, the reasons for limiting the chemical components in the present invention will be explained. C is an element necessary to secure the strength as a high-strength steel sheet, and if the content is less than 0.05%, it is difficult to obtain a tensile strength of 570 N / mm ² grade or higher. Also, 0.
If it is added in excess of 20%, the weld crack resistance is impaired, which is not preferable. Therefore, the C content is in the range of 0.05 to 0.20%.

Si is an element necessary for deoxidation, but its content is
If it is less than 0.05%, this effect is small, and if it is added in excess of 0.50%, the weldability and toughness are deteriorated, which is not preferable. Therefore, the Si content is set to the range of 0.05 to 0.50%.

Mn is an element necessary for improving the hardenability and ensuring the strength inside the plate thickness, but its content is 0.30%.
If it is less than 1, this effect is not sufficiently obtained, and 1.80
If it is added in excess, the weldability and toughness are deteriorated, which is not preferable. Therefore, the Mn content is 0.30 to 1.
The range is 80%.

Al is a deoxidizing element, and if the content is less than 0.005%, such an effect is small, and if it exceeds 0.10%, toughness is deteriorated. Therefore, the Al content should be in the range of 0.005 to 0.10%.

Besides this, Cu, Ni, Cr, Mo, V, Nb, B,
Ti, Ca, etc. are added in one kind or two or more kinds depending on the plate thickness and the target toughness level.

Cu is an element effective for increasing strength by solid solution strengthening and precipitation strengthening, but if the content is less than 0.05%, such effects cannot be sufficiently exhibited, and if it exceeds 1.20%. If added, hot workability deteriorates and cracks easily occur on the steel sheet surface. Therefore, the Cu content is in the range of 0.05 to 1.20%.

Ni has an effect of improving the toughness, but if the content is less than 0.05%, the sufficient effect cannot be obtained.
If added in excess of 3.00%, scale defects are likely to occur and the cost will increase. Therefore, the Ni content is in the range of 0.05 to 3.00%.

Although Cr is an element effective for improving hardenability,
If the content is less than 0.05%, such an effect is not sufficiently exhibited, and if it exceeds 1.20%, the weldability is impaired. Therefore, the Cr content is in the range of 0.05 to 1.20%.

Mo is an element that enhances the hardenability and increases the resistance to temper softening, but if the content is less than 0.05%, a sufficient effect cannot be obtained, and if it is added in excess of 1.00%,
It also deteriorates the weldability and increases the cost. Therefore, the Mo content is set to 0.05 to 1.00%.

By adding a small amount of V, the hardenability is increased,
It is an element that enhances temper softening resistance. To obtain its effect, it is necessary to add 0.005% or more, but 0.08%
If added in excess of 1, the weldability will be impaired. Therefore, the V content should be in the range of 0.005 to 0.08%.

Nb is an element which has a crystal grain refining action, and also exerts a precipitation strengthening action when directly quenching and tempering. To obtain this effect, addition of 0.005% or more is necessary, and addition in excess of 0.05% tends to deteriorate weldability and toughness. Therefore, the Nb content should be in the range of 0.005 to 0.05%.

Although B is an element that improves the hardenability in a small amount, its effect cannot be obtained if the content is less than 0.0003%, and the toughness deteriorates if it is added in excess of 0.0025%. Therefore, the B content should be in the range of 0.0003 to 0.0025%.

Ti has a deoxidizing effect and an effect of promoting the hardenability improving effect of B by immobilizing N, but the content is 0.005%.
If it is less than 0.025%, these effects cannot be obtained, and if it exceeds 0.025%, the toughness deteriorates due to the increase of inclusions. Therefore, the Ti content should be in the range of 0.005 to 0.025%.

Ca has a spheroidizing action of non-metallic inclusions and is effective in reducing anisotropy, but if the content is less than 0.001%, the sufficient effect cannot be obtained, and if it exceeds 0.010%. If added as an additive, the toughness deteriorates due to the increase of inclusions. Therefore, the Ca content should be in the range of 0.001 to 0.010%.

[0043]

[Examples] Hereinafter, examples of a method for producing a high-strength steel sheet having a high yield ratio and a low yield ratio of 570 N / mm ² or more according to the present invention will be described, but the present invention is limited to the present embodiment. It is not something that will be done.

The test steel sheet is obtained by rolling a steel piece having the chemical composition shown in Table 3 into a steel sheet having a plate thickness of 25 to 80 mm shown in Table 4 and then heat treating it under the heat treatment conditions shown in Table 4. Test pieces were sampled from these steel plates and a tensile test of the base material was performed. The results are also shown in Table 4 together with the heat treatment conditions.

As is clear from Table 4, the present invention methods A to I
Have a tensile strength of 570 N / mm ² or higher and a low yield ratio of less than 85%.

On the other hand, in Comparative Examples J to N, since the heat treatment method is not the Q-Modified Q'-T method, the strength is too low or the yield ratio is too high.

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

As described above, the method for producing a high-strength steel sheet having a low yield ratio of 570 N / mm ² or more, which has excellent weldability according to the present invention, controls the chemical composition, and after rolling, quenching, After further heating to the two-phase region temperature, air cooling to a predetermined temperature, quenching, and then tempering are performed, so good weldability and low yield ratio of 85% or less are obtained. It has an excellent effect that a steel sheet of 570 N / mm ² grade or more can be manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing heat treatment conditions.

FIG. 2 is a diagram showing a tensile test result after heat treatment.

FIG. 3 is a diagram schematically showing a difference in microstructure at each heat treatment stage.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shigeo Okano 1 Kanazawa-machi, Kakogawa-shi, Hyogo Kobe Steel Works Kakogawa Steel Works (56) Reference JP-A-63-286517 (JP, A) JP-A-64 -55335 (JP, A) JP 4-110423 (JP, A) JP 4-318 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/02 C22C 38/00 301

Claims (3)

(57) [Claims] 1. C: 0.05 to 0.20%, Si: 0.05 to 0.50%, M
n: 0.30 to 1.80%, Al: 0.005 to 0.10%, hot-rolled steel slab consisting of the balance Fe and unavoidable impurities, and then subjected to the following heat treatment yielding excellent weldability A method for manufacturing high-strength steel sheets with a low ratio of 570 N / mm ² or higher. Heat treatment method: Quenching (Q) + Special quenching (Modified
Q ') + tempering (T) However, quenching temperature: Ac ₃ points or more and 980 ° C or less Special quenching: Ac ₁ point or more and less than Ac ₃ points, 400 to 70
After air cooling to a specified temperature between 0 ° C, quenching Tempering temperature: 400 ° C or more and less than Ac ₁ point 2. C: 0.05 to 0.20%, Si: 0.05 to 0.50%, M
A steel slab containing n: 0.30 to 1.80% and Al: 0.005 to 0.10% and the balance Fe and unavoidable impurities has a rolling finish temperature of Ar ₃
A method for producing a high-strength steel sheet having a high yield ratio of 570 N / mm ² or higher, which is excellent in weldability, which is obtained by hot rolling at a temperature equal to or higher than a point, direct quenching, and then performing the heat treatment described below. Heat treatment method: Special quenching (Modified Q ') + tempering (T) However, special quenching: Heating at Ac ₁ point or more and less than Ac ₃ points,
After air cooling to a specified temperature between 400 and 700 ° C, quenching Tempering temperature: 400 ° C or higher and less than Ac ₁ point 3. The chemical composition further contains Cu: 0.05 to 1.20.
%, Ni: 0.05 to 3.00%, Cr: 0.05 to 1.20%, Mo: 0.05 to
1.00%, V: 0.005-0.08%, Nb: 0.005-0.05%, B: 0.00
03-0.0025%, Ti: 0.005-0.025%, Ca: 0.001-0.010
%, 570 N / mm ² with excellent weldability and low yield ratio, containing one or more selected from the group
High-strength steel sheet of higher grade or higher.