JP2556411B2 - Method for producing high-strength hot-rolled steel sheet with good workability and weldability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-rolled steel sheet having good workability and weldability, and particularly, to meet the recent weight reduction of construction machinery, yield point 890 N / mm ² or more,
The present invention relates to a method for producing a high-strength hot-rolled steel sheet having a tensile strength of 950 N / mm ² or more, which is suitable for a construction machine and has good workability and weldability.

[0002]

2. Description of the Related Art With the recent increase in the height of buildings, construction machines such as trucks and cranes have become large in size, and there has been a demand for a model capable of suspending a heavy object on a high level. Along with that, it is necessary to reduce the weight of the crane itself, and steel sheets with high strength are required. Bending and arc welding are performed in the manufacturing process of construction machinery, and steel sheets having excellent workability are required.

As a high-strength hot-rolled steel sheet used in conventional construction machines, Japanese Patent Publication No. 55-49147 and Japanese Patent Laid-Open No. 59-2.
There is one disclosed in Japanese Patent No. 29464 (Japanese Patent Publication No. 3-65425).

Japanese Patent Publication No. 55-49147 discloses C:
0.05% to 0.20% or less, Si: 1.5% or less,
Mn: 0.3% to 2.0% and Ti 0.03%
From 0.30%, the coiling temperature after hot rolling is 550 ° C.
The present invention relates to a TiC precipitation-strengthened ferrite-pearlite steel sheet aiming at good bending workability at a temperature of from 750 ° C to 750 ° C. However, the tensile strength is at best below 800 N / mm ² grade, is not satisfactory tensile strength 950 N / mm ² or more that is required in recent years.

JP-A-59-229464 (Japanese Patent Publication No. 3-65425) discloses C: 0.05% to 0.20.
% Or less, Si: 1.2% or less, Mn: 0.5% to 2.
It is an invention of a steel sheet containing 0% or less of Ti and 0.04% to 0.20% of Ti, in which the precipitation form of Ti is limited and the amount of bainite structure is limited. Its tensile strength is 950 N / m
m ² or less.

On the other hand, as a method for producing a high-strength hot-rolled steel sheet having a high magnetic flux density as a high-strength hot-rolled steel sheet of 1000 N / mm ² grade or higher, as disclosed in JP-A-63-166931. There is. The present invention satisfies both strength and magnetic flux density at the same time, but does not simultaneously satisfy workability and weldability suitable for construction machinery.

[0007]

The present invention has a yield point 89.
It is an object of the present invention to provide a new method capable of accurately producing a high-strength hot-rolled steel sheet suitable for a construction machine having a tensile strength of 0 N / mm ² or more and a tensile strength of 950 N / mm ² or more. For construction machinery, it is necessary to use a base steel sheet that satisfies strength and toughness, a steel sheet that has good bending workability, and good weldability.

[0008]

The gist of the present invention is as follows. C by weight%: 0.05 to 0.1
5%, Si: 1.50% or less, Mn: 0.70 to 2.5
0%, Ni: 0.25 to 1.5%, Ti: 0.12
0.30%, B: 0.0005 to 0.0015%, P:
0.020% or less, S: 0.010% or less, sol. A
1: 0.010% to 0.10%, N: 0.0050% or less, and C + Si / 30 + Mn / 20 + Ni / 6
A steel slab satisfying 0 + 5B ≦ 0.27 and the balance being Fe and unavoidable impurities is heated to a heating temperature of 1250 ° C. or higher, and the total rolling reduction is 80% at a hot rolling finish temperature of Ar ₃ transformation point or higher and 950 ° C. or lower. Hot rolling is performed as above, and the cooling rate from 800 ° C to 500 ° C is 30 ° C / sec or more and 80 ° C.
A method for producing a high-strength hot-rolled steel sheet having good workability and weldability, which comprises cooling at a rate of not higher than / sec and winding at a temperature of not higher than 500 ° C.

Hereinafter, the present invention will be described in detail. Various experiments were conducted to obtain the strength and toughness, which are one of the objects of the present invention, and the following findings were obtained. Addition of Ti is an extremely effective method to improve the tensile strength, but 0.20%
Under the conventional hot rolling conditions of the heating temperature of 1280 ° C. for the Ti-added steel of No. 3, a tensile strength of only about 800 N / mm ² can be obtained. Therefore, it is necessary to devise to further increase the strength. As a result of many experiments, by adding a trace amount of B to the Ti-added steel and increasing the cooling rate after hot rolling,
It was newly found that a fine structure can be obtained, deterioration of toughness can be reduced, and a tensile strength of 950 N / mm ² or more can be obtained.

That is, FIG. 1 shows the effects of the addition of B and the cooling rate after hot rolling on the tensile strength. according to this,
Without adding B, the cooling rate was from 17 ℃ / sec to 70 ℃ /
Even if it is increased to sec, the increase in tensile strength is slight and the desired strength cannot be obtained. However, B was added and the cooling rate was 17 ° C /
When the temperature is increased from sec to 80 ° C./sec, the tensile strength is significantly increased, and when the cooling rate is 30 ° C./sec or more, the target tensile strength of 950 N / mm ² or more is obtained.

FIG. 2 shows the effect of B addition and cooling rate after hot rolling on the Charpy test fracture transition temperature. According to this, without adding B, the cooling rate was 17 ° C / sec.
To 70 ° C / sec, the fracture surface transition temperature is improved, and B is added to increase the cooling rate from 17 ° C / sec to 80 ° C / sec.
Even if it is set to sec, the deterioration of the fracture surface transition temperature is slight. As described above, it was clarified that a steel sheet having good strength and toughness can be obtained by adding B and cooling at 30 ° C./sec or more. Further, since refinement of crystal grains is effective for improving the fracture surface transition temperature, it is also necessary to set the total reduction rate of finish rolling in the hot strip mill to 80% or more.

A second object of the present invention is to improve bending workability. This object is achieved by replacing MnS which becomes an A type inclusion by TiS by adding Ti to form a C type inclusion.

The third object of the present invention is to have good weldability. That is, it is necessary that the workability of welding is good and the strength and toughness of the welded joint are good. For the workability of welding, it is desirable that the preheating temperature before welding is low, but a preheating temperature of 50 ° C. is required to remove the dew on the steel sheet. Furthermore, since the steel sheet of the present invention is premised on being manufactured by a hot strip mill, the applicable sheet thickness is 9 mm or less. Considering this, the carbon equivalent C + Si / 30 + Mn / 20 + Ni / 60 + 5B for preventing the occurrence of welding cracks at a preheating temperature of 50 ° C. is 0.
It must be 27 or less.

The strength of the welded joint is determined by the softening of the heat affected zone. Therefore, it is necessary to lower the heat input of welding and increase the cooling rate of the heat affected zone. From the viewpoint of increasing the cooling rate of the heat-affected zone, it is effective that the welding preheating temperature is low. Further, it is necessary that the toughness of the welded joint be good, which is greatly affected by the welding material and welding conditions, but it is also necessary to take measures from the base metal. That is, Ni was added to improve the toughness of the heat affected zone of the welded joint.

The reasons for limiting the above steel components in the present invention are as follows. C: C is the most effective element for obtaining high tensile strength and requires at least 0.05% for this purpose. However, since the workability, toughness, and weld cracking susceptibility deteriorate with the increase of C, the upper limit was made 0.15% and was limited to the range of 0.05 to 0.15%.

Si: Si is useful as a strengthening element, but in order to produce steel economically, 1.50% is added as the upper limit.

Mn: Mn is also an element effective for improving the strength, but it deteriorates the weld crack susceptibility. Therefore, at least 0.70% is required as a strengthening element. However, if it exceeds 2.50%, the deterioration of the weld crack susceptibility becomes large, so the upper limit was made 2.50% and was limited to the range of 0.70 to 2.50%.

Ni: Ni is an element effective in improving the toughness of the welded joint. Fracture surface transition temperature of joint Charpy is 0 ℃
To be below, at least 0.25% addition is required. The more Ni is added, the more effective it is in improving the toughness of the welded joint. However, from the economical viewpoint, the upper limit is 1.
It was set to 5%.

Ti: Ti is at least 0.12% because it is inexpensive and combines with C to form TiC and strengthens the steel with a small addition. Since an increase in Ti causes surface defects, the upper limit was made 0.30%.

B: B has a cooling rate of 30 after completion of hot rolling.
Although it has the effect of stabilizing austenite under a rapid cooling rate of ℃ / sec or more and making it easy to obtain a bainite structure,
If the content is less than 0.0005%, the desired effect cannot be obtained. On the other hand, if the content is 0.0015% or more, the effect is saturated, and surface defects such as slab cracks are more likely to occur. The amount was limited to 0.0005-0.0015%.

Al: Al is required to be 0.010% or more for deoxidation, but if it exceeds 0.10%, the crystal grains become coarse and the strength is deteriorated, so it is limited to 0.10% or less.

P, S: P and S are all impurity elements and impair the ductility and toughness of steel, so the smaller the content, the better. P, 0.020% or less, S, 0.010% or less. There is a need.

N: N forms a BN by combining with B added especially in the present invention, and does not act effectively on the stability of austenite. Although Ti is added as a preventive measure, if the amount of N is large, BN is formed and the effect thereof is reduced, so the upper limit was limited to 0.0050%.

Within the range of the above chemical composition, C + Si / 30
+ Mn / 20 + Ni / 60 + 5B is 0.
The reason why it is limited to 27 or less is that if it exceeds this, the preheating temperature at the time of welding exceeds 50 ° C., which deteriorates workability.

Hot rolling conditions: The slab is heated once as a cold piece or as a hot piece is heated to 1250 ° C. or higher. It is usually heated to a temperature range of 1250 ° C to 1350 ° C and then rolled. The heating temperature is set to 1250 ° C. or higher in order to promote the solid solution of Ti, and Ti is not sufficiently solid-dissolved at a heating temperature lower than 1250 ° C. The upper limit is not particularly limited, but heating above 1350 ° C. is unnecessary from the viewpoint of preventing scale loss and saving energy.

[0026] The slabs were heated above the 1250 ° C. hot-rolled, to the finishing temperature was Ar ₃ transformation point or higher, there to prevent the securing strength and toughness degradation due to TiC precipitation, Ar ₃ This is because this object cannot be achieved at finish rolling temperatures below the transformation point. Further, at a finishing rolling temperature exceeding 950 ° C., the temperature is too high and the crystal grains of the steel sheet become coarse, so that the required toughness cannot be obtained.

Further, the total rolling reduction during finish rolling is set to 80% or more because if the rolling reduction is lower than this, austenite crystal grains are large, the structure after cooling is not refined, and the toughness of the steel sheet cannot be secured. .

Cooling and winding conditions: The cooling rate after the hot rolling is completed is an essential condition for controlling the structure of the steel sheet together with the components. In order to obtain a high-strength hot-rolled steel sheet having a tensile strength of 950 N / mm ² or more, solid solution strengthening alone is not possible, and it is necessary to combine precipitation strengthening or transformation structure strengthening, and cooling control aims at a fine structure. Cooling is controlled from 800 ° C, which is the transformation region from the austenite phase to the ferrite phase, to 5
It is specified by the cooling rate up to 00 ° C. When the cooling rate is lower than 30 ° C / sec, the structure becomes coarse and the necessary strength cannot be obtained, and when the cooling rate exceeds 80 ° C / sec, martens It becomes a site structure and its ductility deteriorates and it does not meet the purpose of use.

If the coiling temperature is high, the coiling is carried out before the transformation is completed, and after the coiling is gradually cooled and transformed during that time, the crystal grains become coarse and the desired strength cannot be obtained.
Limited to below ℃. A high-strength hot-rolled steel sheet having good workability and weldability can be obtained by such a manufacturing method.

[0030]

EXAMPLE Steels having the chemical composition shown in Table 1 were melted in a converter and cast into slabs by continuous casting. Regarding the chemical composition, the A, B, C, D, E, F, G and H steels satisfy the compositional conditions of the present invention. K, L, M, N steels are for comparison. Table 2 shows the hot rolling conditions, and Table 3 shows the mechanical properties and weldability test results of the steel sheets obtained at that time. Tensile test pieces and Charpy test pieces were taken in parallel from the steel sheet in the rolling direction, and wide bending test pieces were taken at right angles to the rolling direction. Welded joint test is 60 degree V
MAG welding was performed with a groove shape and a heat input of 10 KJ / cm, the welding beads were deleted, a joint tensile test was performed, and a joint Charpy test of the bond portion was performed.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

According to this, the steel plate No. 1 to 13 satisfy the manufacturing conditions of the present invention. However, no. 1
No. 4 is satisfied with the chemical composition, but the heating temperature is low, the solution of Ti is not sufficiently performed, the amount of TiC that works for strengthening is not sufficient, and the desired strength is not obtained. No. No. 15 has a satisfactory chemical composition, but the cooling rate after hot rolling is slow and the crystal grains are large, so that the desired strength cannot be obtained.

No. No. 16 also satisfies the chemical composition, but the coiling temperature is high, the crystal grains are large, and the desired strength is not obtained. No. No. 17 also satisfies the chemical composition, but has a high rolling finishing temperature, large crystal grains, and the desired toughness is not obtained. No. No. 18 also satisfies the chemical composition, but the rolling reduction during rolling is small, the crystal grains are large, and the target toughness is not obtained. No. No. 19 cannot obtain the intended strength because Comparative Steel K containing a small amount of Ti was used. No. 20-22 is B
Since the comparative steel L to which was not added was used, the intended strength was not obtained in any case. No. In No. 23, since comparative steel M containing a small amount of Ni was used, the toughness of the welded joint was poor. No. For No. 24, since the comparative steel N having a high C equivalent was used, the preheating temperature during welding becomes high.

[0036]

According to the present invention as described above, the yield point is 890 N / mm ² or more and the tensile strength is 950 N / mm ^2.
Since it is possible to accurately manufacture a high-strength hot-rolled steel sheet having a mm ² or more and excellent bending workability and weldability, its effect is industrially great.

[Brief description of drawings]

FIG. 1 shows the relationship between the presence or absence of B addition and the cooling rate and the tensile strength of Ti-added steel.

FIG. 2 shows the relationship between the presence or absence of addition of B and the cooling rate of Ti-added steel and the fracture surface transition temperature (vTrs) of the Charpy test.

Claims (1)

(57) [Claims] 1. C: 0.05 to 0.15%, Si: 1.
50% or less, Mn: 0.70 to 2.50%, Ni: 0.
25-1.5%, Ti: 0.12-0.30%, B:
0.0005 to 0.0015%, P: 0.020% or less, S: 0.010% or less, sol. Al: 0.010
% To 0.10%, N: 0.0050% or less, and C + Si / 30 + Mn / 20 + Ni / 60 + 5B ≦ 0.
A steel slab satisfying No. 27, the balance consisting of Fe and unavoidable impurities is heated to a heating temperature of 1250 ° C. or higher, and hot rolling is performed at a hot rolling finish temperature of Ar ₃ transformation point or higher and 950 ° C. or lower at a total finishing reduction rate of 80% or higher. A high-strength hot-rolled steel sheet having good workability and weldability, which is rolled, cooled at a cooling rate from 800 ° C to 500 ° C at 30 ° C / sec or more and 80 ° C / sec or less, and wound at 500 ° C or less. Manufacturing method.