JP2543459B2 - High strength hot rolled steel sheet with good workability and weldability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has good workability and weldability.
High-strength hot-rolled steel sheet, especially the weight reduction of construction machinery in recent years
The yield point 890N / mm²Above, tensile strength 9
50 N / mm ²Suitable workability and melting for the above construction machines
The present invention relates to a high-strength hot-rolled steel sheet having good contact.

[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 Publication No. 59-294464.
Japanese Patent Publication (JP-B-3-65425).

Japanese Examined Patent Publication No. 55-49147 discloses C0.
05% to 0.20% or less, Si 1.5% or less, Mn
Ti was added in the range of 0.3% to 2.0% and 0.03% to 0.30%. The present invention relates to a TiC precipitation-strengthened ferrite / pearlite steel sheet aiming at a good bending workability in cold by setting a winding temperature after hot rolling to 550 ° C to 750 ° C. However, its tensile strength is at most 8
00N / mm ² class following ones, do not satisfy the tensile strength of 950 N / mm ² or more that is required in recent years. JP 59
-229464 publication (Japanese Patent Publication No. 3-65425) is C
0.05% to 0.20% or less, Si 1.2% or less, M
It is an invention of a steel sheet in which n is 0.5% to 2.0% and Ti is added from 0.04% to 0.20%, the precipitation form of Ti is limited, and the amount of bainite structure is limited. Its tensile strength is 950 N / mm ² or less. On the other hand, 10
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 having a strength of 00 N / mm ² or higher, JP-A-6-2006
There is one such as that disclosed in Japanese Patent Publication No. 3-166931. 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.

[0004]

The present invention has a yield point 89.
It is an object of the present invention to provide a new 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.

[0005]

The gist of the present invention is as follows. 1) C: 0.05 to 0.15%, Si: 1.50% or less, Mn: 0.70 to 2.50%, Ni: 0.25 to 0.25
1.5%, Ti: 0.12 to 0.30%, B: 0.00
05-0.0015%, 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≦0.27, the balance is Fe and inevitable impurities, the Ti content as fine TiC is 0.10% or more, and the grain size of the ferrite crystal grains is A high-strength hot-rolled steel sheet having a workability of 10 μ or less and good weldability. 2) C: 0.05 to 0.15%, Si: 1.50% or less, Mn: 0.70 to 2.50%, Ni: 0.25 to 0.25
1.5%, Ti: 0.12 to 0.30%, B: 0.00
05-0.0015%, Cr: 1.00% or less, 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
0 + 5B ≦ 0.27 is satisfied, the balance is steel consisting of Fe and unavoidable impurities, and Ti as fine TiC.
It is a high-strength hot-rolled steel sheet having a content of 0.10% or more and a ferrite crystal grain size of 10 μm or less and good workability and weldability.

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. Although Ti addition is an extremely effective method for improving the yield point and the tensile strength, the tensile strength is increased under the conventional hot rolling conditions of 0.20% Ti-added steel and a heating temperature of 1280 ° C. 800 N / m
Only about m ² can be obtained. Therefore, it is necessary to devise to further increase the strength. Many experiments have revealed that increasing the amount of fine TiC is necessary to improve the strength of the Ti-added steel. Fine TiC here
Is an amount obtained by removing the amount of TiC measured by acid dissolution method analysis from effective Ti, and is defined by the equation (1). Fine TiC = effective Ti (eff [Ti])-TiC (measured by acid dissolution analysis) (1) Effective Ti (eff [Ti]) is a combination of total Ti, O, N, and S. It is excluded and defined by the equation (2). eff [Ti (%)] = Total Ti (%)-3.4 [N (%)]-1.5 [O (%)]-3.0 [S (%)] (2)

Further, the strength is increased, and at the same time, the toughness is improved.
Ferrite crystals when fine TiC is deposited
It became clear that it was effective to make grains fine
Was. In order to increase the precipitation amount of fine TiC,
The precipitation of TiC is suppressed and the cooling rate after hot rolling is increased.
It is necessary to prevent coarsening of TiC during cooling. Ferrai
To make the crystal grains ultra-fine, a small amount of B is added and hot rolling is performed.
It is necessary to increase the cooling rate afterwards. This gives toughness
With less deterioration and tensile strength of 950 N / mm ²that's all
It was newly found that

0.10% C-0.30% Si-1.85
% Mn-0.30% Ni-0.0010% B in P,
S, N, sol. For Al, steels within the scope of the invention were used, and slabs of steel with various amounts of addition of Ti were used to produce steel sheets with various hot rolling conditions. The relationship between the Ti as fine TiC amount and the ferrite crystal grain size and the tensile strength of these steel sheets is shown in FIG. According to this, the tensile strength becomes higher as the amount of Tias fine TiC increases, and becomes higher as the ferrite crystal grain size becomes smaller. That is, the Ti as fine TiC amount is 0.10% or more, and the average ferrite crystal grain size is 1
At 0 μm or less, a tensile strength of 950 N / mm ² or more is obtained.

FIG. 2 shows the effects of fine TiC content and ferrite crystal grain size on the Charpy impact test fracture surface transition temperature. According to this, as for the steel plate with an average ferrite crystal grain size of 20 μ, the Charpy impact test fracture surface transition temperature rapidly increases as the Ti as fine TiC amount increases and the yield point becomes higher, and the Ti as fine TiC amount becomes 0.15%. -1
It reaches about 0 ℃. However, the same Ti as fine T
Even in the amount of iC, in the steel sheet having an average ferrite crystal grain size of 10 μ, the Charpy impact test fracture surface transition temperature decreases to −40 ° C. In this way, by making the ferrite crystal grain size ultrafine grains of 10 μm or less, it is possible to obtain a practical Charpy impact test fracture surface transition temperature of −20 ° C. or less even if the strength is improved by fine TiC. Also, the effect of fine TiC and ferrite crystal grain size on strength and toughness is C-Si-Mn-Ni-Ti-B even if Cr is added.
It is similar to steel, and the strength can be improved without lowering the toughness by solid solution strengthening of Cr.

Such a steel sheet can be manufactured by controlling the heating and hot rolling conditions in the following ranges. That is,
If the slab is heated once as a cold piece or as a hot piece, the solution temperature of TiC (T
Heat above ° K). log ₁₀ [% Ti] · [% C] = 2.75-7000 / T (3) The slab heated above the solution temperature is hot-rolled, and the finishing temperature is set to the Ar ₃ transformation point or higher. To do. Also, 950 ℃
If the finishing rolling temperature is over the range, the temperature will be too high and the ferrite crystal grains of the steel sheet will become coarse, so that the required toughness cannot be obtained. Further, the total rolling reduction during finish rolling reduces the austenite crystal grains,
The ferrite crystal grains after cooling are also desirably 80% or more in order to make them finer.

The cooling rate after the completion of hot rolling is an essential condition for controlling the ferrite grain size of the steel sheet together with the components. In order to obtain a high-strength hot-rolled steel sheet with 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 strengthening with finer ferrite grain size. Aim. On this occasion,
By adding B, the effect of refining the structure is extremely large. The cooling control is defined by the cooling rate between 800 ° C. and 500 ° C., which is the transformation region from the austenite phase to the ferrite phase. Cooling rate is 30 ℃ / sec-80 ℃ / s
ec is desirable. If the coiling temperature is high, the coiling is carried out before the transformation is completed, and after coiling, it is gradually cooled and transformed during that time, so that ferrite crystal grains become coarse and the desired strength cannot be obtained. Is preferred.

A second object of the present invention is to improve bending workability. This object is achieved by replacing MnS, which is an A-type inclusion, with 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 in order to remove the dew on the steel plate, 50 ° C
Preheating temperature of is required. Further, since the steel sheet of the present invention is considered to be manufactured on a hot strip mill, the applicable sheet thickness is 9 mm or less. Considering this, 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 required to be 0.27 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. In addition, it is necessary that the toughness of the welded joint is 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% was added as the upper limit. Mn: Mn is also an element effective for improving the strength, but deteriorates the weld crack susceptibility. Therefore, at least 0.70% is required as a strengthening element. However, 2.50%
If it exceeds the range, 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. 0 because it is inexpensive, and when added in a small amount, it combines with C to form TiC and strengthens the steel.
Requires 12%. Since an increase in Ti causes surface defects, the upper limit was made 0.30%. B: B has the effect of stabilizing austenite and facilitating the formation of a fine structure under rapid cooling at a cooling rate of 30 ° C./sec or more after the completion of hot rolling. No effect, while 0.001
Even if it is contained in an amount of 5% or more, the effect is saturated, and surface defects such as slab cracks are more likely to occur, so the content was limited to 0.0005 to 0.0015%.

Al: Al needs to be 0.010% or more for deoxidation, but if it exceeds 0.10%, the ferrite crystal grains become coarse and the strength deteriorates, so the content 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 better, the more preferable P is 0.020% and S must be 0.010%. . N: N combines with B added particularly in the present invention to form BN, which does not act effectively on the stability of austenite. To prevent this, Ti is added.
If it is present in a large amount, BN will be formed and its effect will be reduced, so the upper limit was made 0.0050%.

Cr: Cr, like Mn, is an element effective for toughening steel. It is desirable to add 0.1% or more for the toughening of steel, but if the content exceeds 1.0%, no further effect can be obtained, so the content is 1.0%. Below. Within the above chemical composition range, C +
The reason why the value represented by the formula Si / 30 + Mn / 20 + Ni / 60 + 5B is limited to 0.27 or less is that the preheating temperature at the time of welding exceeds 50 ° C. and the workability is deteriorated when the value is exceeded. .

Fine TiC: The amount of fine TiC is extremely effective in improving the strength, and is required to be 0.10% or more in order to obtain a tensile strength of 950 N / mm ² or more. However, as a matter of course, the Ti addition amount is not exceeded. Ferrite crystal grains: Finer ferrite crystal grains are effective for increasing strength and at the same time improving toughness. In order to obtain this effect, it is necessary to make the ferrite crystal grains ultrafine with a size of 10 μm or less in the state where fine TiC is deposited.

[0019]

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, H, and I steels satisfy the composition conditions of the present invention. K, L, M, N steels are for comparison. Table 2 shows the hot rolling conditions and Ti.
The as fine TiC and the ferrite crystal grain size of the steel sheet are shown. The other hot rolling conditions were constant as shown below. That is, the hot rolling finishing temperature was 870 ° C., the total rolling reduction was 85%, and the winding temperature was room temperature. 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. In the welded joint test, MAG welding with a V groove shape of 60 degrees and a heat input of 10 KJ / cm was performed, the weld bead was deleted, a joint tensile test was performed, and a joint Charpy test of the bond portion was performed.

According to this, the steel plate No. 1 to 13 satisfy the chemical composition of the present invention, Ti as fine TiC and the ferrite crystal grain size of the steel sheet, and the target strength, bending workability, toughness and weldability are obtained. But N
o. No. 14 has a satisfactory chemical composition, but the heating temperature is low and T
The solution of i was not sufficiently solutionized, and the amount of Ti as fine TiC acting for strengthening was not sufficient, so that the desired strength was not obtained.
No. Sample No. 15 has a satisfactory chemical composition, but the cooling rate after hot rolling is slow, and the ferrite crystal grains are as large as 20 μm, and the desired strength cannot be obtained. No. No. 16 also satisfies the chemical composition, but the amount of Ti as fine TiC is 0.09%, which is not sufficient, and the ferrite crystal grains are as large as 20 μ, and the desired strength cannot be obtained. No. No. 17 also satisfies the chemical composition, but the ferrite grain size is as large as 20 μ and the toughness is poor. N
o. No. 18 also satisfies the chemical composition, but has a large ferrite crystal grain of 22 μ and poor toughness. No. No. 19 cannot obtain the intended strength because Comparative Steel K containing a small amount of Ti was used. No. Since No. 20 used Comparative Steel L to which B was not added, the intended strength was not obtained. No. 21 is N
Since comparative steel M containing a small amount of i was used, the toughness of the welded joint was poor. No. Since No. 22 used comparative steel N having a high C equivalent, the preheating temperature during welding becomes high.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

The present invention as described above has a yield point of 8
90 N / mm ² or more, has a tensile strength of 950 N / mm ² or more and bending property, is extremely useful as a high-strength hot-rolled steel sheet for construction machinery because it is high-strength hot-rolled steel sheet having excellent weldability .

[Brief description of drawings]

1] Effect of Ti as on the tensile strength of Ti and B added steels
Diagram showing the influence of fine TiS and ferrite crystal grain size,

FIG. 2 is a diagram showing the influence of Tias fine TiS and ferrite crystal grain size on the fracture surface transition temperature (vTrs) in the Charpy test of Ti and B-added steel.

Claims (2)

(57) [Claims] 1. C: 0.05 to 0.15%, Si: 1.50% or less, Mn: 0.70 to 2.50%, Ni: 0.25 to 1.5%, Ti: 0. 12 to 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 ≦
0.27, the balance being Fe and inevitable impurities, the Ti content as fine TiC is 0.10% or more and the grain size of ferrite crystal grains is 10 μm.
The following high-strength hot-rolled steel sheets with good workability and weldability. 2. C: 0.05 to 0.15%, Si: 1.50% or less, Mn: 0.70 to 2.50%, Ni: 0.25 to 1.5%, Ti: 0. 12 to 0.30%, B: 0.0005 to 0.0015%, Cr: 1.00% or less, 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 ≦
0.27, the balance being Fe and inevitable impurities, the Ti content as fine TiC is 0.10% or more and the grain size of ferrite crystal grains is 10 μm.
The following high-strength hot-rolled steel sheets with good workability and weldability.