JPH0873985A - High strength hot rolled steel plate excellent in workability and weldability - Google Patents

Abstract

PURPOSE: To produce a high strength hot rolled steel plate excellent in bendability, weldability, and tensile strength by constituting a hot rolled steel plate, as a steel plate for construction machinery, of a low alloy steel of specific composition. CONSTITUTION: A slab of a steel, which has a composition consisting of, by weight, 0.05-0.15% C, <1.50% Si, 0.70-2.50% Mn, 0.25-1.5% Ni, 0.12-0.30% Ti, 0.0005-0.0030% B, <0.020% P, <0.010% S, 0.010-0.10% sol.Al, <0.0050% N, and the balance Fe, further containing, if necessary, <1.00% Cr, and satisfying C+Si/30+Mn/20+Ni/60+5B <=0.27, is hot-rolled and the resulting hot rolled steel plate is coiled. By adding B and controlling cooling velocity after hot rolling, fine ferrite crystals of <=10μm grain size, TiC of <=10nm, and Fe carbides of <=10/μm grain size are precipitated. By this method, the high strength hot rolled steel plate for construction, excellent in bendability and weldability, having >=95ON/mm<2> tensile strength, and particularly having >=850N/mm<2> proportional limit, can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet having good workability and weldability, and is particularly suitable for construction machinery having a tensile strength of 950 N / mm ² or more, which can quickly respond to the recent weight reduction of construction machinery. It relates to a high-strength hot-rolled steel sheet having excellent 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 a model capable of hoisting heavy objects in high layers has been required. 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.

Conventionally, as a high-strength hot-rolled steel sheet of 950 N / mm ² grade or higher that meets the above-mentioned demands from customers, there is JP-A-5-271865, and as a method thereof, JP-A-5-230529. There is something shown in. In order to obtain high strength, this invention cools at a cooling rate of 30 ° C./s or more after hot rolling to obtain fine ferrite crystal grains and fine T
It is a technology to obtain iC and achieve a predetermined strength,
With this steel sheet and method, good workability and weldability, which are certainly the original purpose, are sufficiently secured.

However, when designing an actual structure, what is essentially important for safety such as buckling resistance is the proportional limit rather than the yield point, and the proportional limit is 850 N / mm ² from the customer.
The above steel sheets are now required. Actually, JP-A-5
When investigating the steel -271865 discloses, yield point is defined by a 0.2% proof stress is proportional limit is 890N / mm ² or more is as low as 800 N / mm ² approximately, is hardly meet the requirements of customers Not a thing.

[0005]

DISCLOSURE OF THE INVENTION The present invention is a high-strength hot-rolled steel sheet having a tensile strength of 950 N / mm ² or more, which has good bending workability and weldability, and particularly a steel sheet having a proportional limit of 850 N / mm ² or more. Is to provide.

[0006]

The gist of the present invention is as follows. C: 0.05 to 0.15% by weight, 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 0.0030%, P: 0.020%
Below, S: 0.010% or less, sol.Al: 0.010
0.10%, N: 0.0050% or less, C as required
r: contains 1.00% or less, and C + Si / 30 + Mn
/20+Ni/60+5B≦0.27, the balance is steel consisting of Fe and unavoidable impurities, the ferrite crystal grain size is 10 μm or less, and TiC with a size of 10 nm or less and 10 μm or less. It is a high-strength hot-rolled steel sheet in which iron carbide of a size is precipitated. Hereinafter, details of the present invention will be described. For the purpose of the present invention, it is necessary to satisfy the four characteristics of strength, toughness, bending workability and weldability. Regarding bending workability and weldability, JP-A-5-27
As described in Japanese Patent No. 1865. The bending workability is deteriorated by the presence of MnS which is an A-based inclusion, but by adding Ti to the steel, it becomes a C-based inclusion T
Change to iS to improve bendability. Weldability evaluation items include weldability, weld joint strength, and toughness. Regarding the workability of welding, it is desirable that the preheating temperature before welding is low, but preheating at 50 ° C. is necessary in order to remove the dew on the steel sheet.

In order to prevent the occurrence of weld cracks at a preheating temperature of 50 ° C., it is necessary to control the hardenability, and for that purpose the carbon equivalent C + Si / 30 + Mn / 20 + Ni / 60 +
5B should be 0.27 or less, and the steel composition is limited to satisfy this. A low welding preheating temperature is also effective for the strength of the welded joint. The toughness of the welded joint is affected by the welding material and welding conditions, but it is necessary to take measures from the base metal. In the case of the steel of the present invention, Ni is added.

Next, the tensile strength will be described. Metallurgically, there are several methods for increasing strength, such as solid solution strengthening, transformation structure strengthening, precipitation strengthening, and refinement of structure.
It is difficult to increase the tensile strength to 0 N / mm ² class with a single strengthening mechanism. Here, as a result of various studies, it was achieved by combining precipitation strengthening and the effect of refining the ferrite structure.

First, the effect of adding Ti will be described.
In JP-A-5-271865, the fine TiC amount is
TOTAL Ti was defined as the effective Ti excluding those bound with O, N, and S, minus the amount of TiC measured by the acid dissolution method analysis. Direct observation of Fig. 5 (a),
An attempt was made to define TiC that succeeds as in (b) and newly contributes to strength.

The size of TiC can be changed by the heating temperature of hot rolling, the finishing temperature, etc., but the influence on the tensile strength was investigated by changing the average precipitate diameter of TiC by such a method. . The result is shown in FIG. The diameter and density of the precipitate change at the same time, so it is not possible to make a simple evaluation.
It can be seen that a tensile strength of 950 N / mm ² or more can be obtained with an average precipitate diameter of nm or less.

Next, the effect of increasing the tensile strength by making the ferrite crystal grains fine will be described. The crystal grain size changes depending on the addition of B and the cooling rate after hot rolling. B is added to change the cooling rate after hot rolling to change the crystal grain size,
The effect of grain size on tensile strength was investigated. The result is shown in FIG. Tensile strength increases with the refinement of ferrite crystal grain size.
It can be seen that a tensile strength of 950 N / mm ² or more can be obtained with an average ferrite crystal grain size of μm or less.

The target value of the tensile strength can be secured by the method as described above. However, in the invention of JP-A-5-271865, the yield point defined by 0.2% proof stress is 890 N /
Although it is secured at mm ² or more, it has been found that the proportional limit, which is a problem when actually designing a structure, is reduced to less than 850 N / mm ² . To solve this problem, the present inventors impair the weldability and the Charpy impact property by precipitating the residual solid solution carbon in the iron carbide in addition to the fine precipitates of TiC and the fine ferrite crystal grains. Without yield point 890 N / mm ² and proportional limit 850 N
We have newly found that it is possible to achieve a value of / mm ² or more.

FIG. 3 shows an enlarged stress-strain diagram in the vicinity of the yield point before and after the precipitation of iron carbide. The solid line shows that when iron carbide is not precipitated, but the yield point is 9
The target value of 10 N / mm ² is achieved, but the proportional limit is 80
It can be seen that the deformation rate is reduced to 0 N / mm ² , and plastic deformation is likely to occur. On the other hand, when iron carbide is precipitated, it is found that the proportional limit increases up to 940 N / mm ² together with the yield point. This is because when iron carbide is not precipitated, there is a trace amount of martensite phase or retained austenite phase, and the stress-strain diagram near the yield point becomes round due to the same mechanism as the composite structure steel. , It is considered that the proportional limit has decreased.

On the other hand, when iron carbide is precipitated, the heat input at that time decomposes the martensite phase and the retained austenite phase, and since iron carbide is also precipitated, the proportional limit greatly increases. Conceivable. The size of the iron carbide can be controlled by, for example, the reheating temperature after hot rolling and winding. In addition, it is possible to accelerate the precipitation of iron carbide by adding a light process with a leveler after hot rolling and winding,
Applying some strain is an effective method for raising the proportional limit by reheating for a short time.

FIG. 4 shows the result of investigating the influence on the proportional limit by changing the size of the iron carbide by changing the reheating temperature by lightly working with a leveler. Since the shape of the iron carbide is uneven as shown in FIG. 5 (d) later, the major axis is used as the size here, but when the iron carbide having a size of 10 μm or less is deposited, , Proportional limit is 850N /
Although the target value has been achieved with mm ² or more, when a large iron carbide having a size of 10 μm or more is deposited, the proportional limit decreases with strength and becomes 850 N / mm ² or less.

As a method for depositing an iron carbide having an appropriate size of 10 μm or less, for example, Japanese Patent Application No. 5-286729.
As disclosed in the above-mentioned invention, it is conceivable that after hot rolling, light processing is performed with a leveler or the like, and then reheating is performed. Fine ferrite crystal grains and fine Ti as described above
By obtaining C and iron carbide, a predetermined strength can be obtained, and because of having such a structure, the impact value of this steel sheet is also good, and a Charpy impact test fracture surface transition temperature of -20 ° C or less is obtained. .

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 strength, but it deteriorates weld crack susceptibility. At least 0.70% is required as a strengthening element, but if it exceeds 2.50%, the weld crack susceptibility deteriorates significantly, so the upper limit is 2.
It was set to 50% and limited to the range of 0.70 to 2.50%. Ni: Ni is an element effective in improving the toughness of the welded joint. In order to keep the fracture surface transition temperature of the joint Charpy at 0 ° C. or lower, addition of at least 0.25% is necessary.
The more Ni is added, the more effective it is in improving the toughness of the welded joint portion, but the upper limit was made 1.5% from the viewpoint of economy.

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 function of stabilizing austenite and facilitating obtaining a fine structure under a rapid cooling rate of 20 ° C./s or more after completion of hot rolling, but if it is less than 0.0005%, it is desirable for the above function. If the content is 0.0030% or more, the effect is not saturated, and the effect is saturated, and surface defects such as slab cracking are more likely to occur, so the content is set to 0.0005 to 0.0030%. Limited

Al: Al (sol.Al) is 0.01 after deoxidation.
It is necessary to be 0% or more, but if it exceeds 0.10%, the ferrite crystal grains are coarsened and the strength is deteriorated.
% Or less. P, S: P and S are all impurity elements and impair the ductility and toughness of the steel, so that the smaller the content, the better. P is 0.020% or less and S must be 0.010% or less. .

N: N forms a BN by combining with B particularly added 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%. Cr: Cr is an element effective for strengthening steel, similar to Mn. It is desirable to add 0.1% or more for toughening steel, but if the content exceeds 1.0%, no further effect can be obtained, so the content is 1.0%. Below.

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 becomes high and the workability deteriorates. Ferrite crystal grains: Making the ferrite crystal grains fine is effective in increasing the strength and at the same time improving the toughness. In order to obtain this effect, it is necessary to reduce the ferrite crystal grain size to 10 μm or less in the state where fine TiC is deposited.

Fine TiC: TiC having a size of 10 nm or less is necessary to obtain a tensile strength of 950 N / mm ² or more. When the size is changed, the amount of Ti added is constant within a certain range, so that the density of the precipitate also changes at the same time.
The density for obtaining strength needs to be about 10 ¹³ particles / m ² or more on the photograph from an electron microscope observation by a replica method in which precipitates are directly extracted from a sample.

Iron Carbide: It is necessary to precipitate iron carbide in order to raise the proportional limit. It is difficult to define the size because the shape of the precipitated iron carbide is eccentric, but the proportional limit is set to 850 N / mm ² or more by the electron microscope observation by the ordinary extraction replica method by setting the major axis to 10 μm or less. be able to. The density at that time is about 10 ⁸ pieces / m ² or more on an electron micrograph.

[0025]

EXAMPLES Steels having the components included in the scope of the present invention shown in Table 1 were hot-rolled under the conditions shown in Table 2, and then steel plates were prepared according to the thermomechanical treatment conditions shown in Table 3 with or without the iron carbide precipitation treatment. We investigated the material and structure.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

[Table 5]

Table 4 shows the average ferrite crystal grain size of the obtained steel sheet, the average grain size of TiC, the size of iron carbide, the mechanical properties of the steel sheet, and the weldability test results. 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 groove shape and heat input is 10kJ /
cm MAG welding was performed, the welding 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. Nos. 1 and 2 were lightly worked after hot rolling, reheated and subjected to iron carbide precipitation treatment, but satisfied the TiC size, ferrite crystal grain size, and iron carbide size. The desired yield point, proportional limit, tensile strength, bending workability, toughness and weldability are obtained. No. Observations of TiC and iron carbide of the steel plate of No. 1 are shown in FIGS. 5 (b) and 5 (d). It can be observed that extremely fine TiC having a size of about 2 nm and iron carbide having a size of about 1 μm are deposited.

No. No. 3 was not subjected to iron carbide precipitation treatment, but fine TiC and ferrite crystal grain size were included in a predetermined range, but iron carbide was not precipitated, so the proportional limit was 800 N / mm ² . It is falling. Observation of TiC and iron carbide at this time is shown in FIG.
It is (c). TiC is basically the same as that after reheating, and fine particles of about 2 nm are observed. Concerning the iron carbide, the contrast due to the unevenness of the sample surface can be seen, but the iron carbide cannot be observed. No. No. 4 is No. Two
Although it is the same steel as the above, the fine TiC and ferrite crystal grain sizes are included in a predetermined range because the iron carbide precipitation treatment is not performed, but the proportional limit is lowered to 750 N / mm ² .

[0034]

According to the present invention, the yield point is 890 N / mm ² or more,
A high-strength hot-rolled steel sheet with a proportional limit of 850 N / mm ² or more and a tensile strength of 950 N / mm ² or more and excellent bending workability and weldability, so it is extremely useful as a high-strength hot-rolled steel sheet for construction machinery. Is.

[Brief description of drawings]

FIG. 1 is a chart showing the effect of the average grain size of TiC on tensile strength.

FIG. 2 is a chart showing the effect of average ferrite crystal grain size on tensile strength.

FIG. 3 is a chart showing the influence of iron carbide on the behavior of the yield point.

FIG. 4 is a chart showing the effect of the size of iron carbide on the proportional limit.

5 (a) to (d) are fine TiC of invention steel and comparative steel.
3 is an electron micrograph showing a crystal structure of iron carbide.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naoki Shiomi 1 Fuji-machi, Hirohata-ku, Himeji City Nippon Steel Corporation Hirohata Works

Claims (2)

[Claims] 1. C: 0.05 to 0.15% by weight, Si: 1.50% or less, Mn: 0.70 to 2.50%, Ni: 0.25 to 1.5%, Ti : 0.12-0.30%, B: 0.0005-0.0030%, P: 0.020% or less, S: 0.010% or less, sol.Al: 0.010-0.10%, N: 0.0050% or less, and C + Si / 30 + Mn / 20 + Ni / 60 + 5B ≦ 0.
27, the balance being steel consisting of Fe and inevitable impurities,
The grain size of ferrite crystal grains is 10 μm or less, and 1
A high-strength hot-rolled steel sheet having good workability and weldability, characterized in that TiC having a size of 0 nm or less and iron carbide having a size of 10 μm or less are precipitated. 2. C: 0.05 to 0.15% by weight, Si: 1.50% or less, Mn: 0.70 to 2.50%, Ni: 0.25 to 1.5%, Ti : 0.12-0.30%, B: 0.0005-0.0030%, Cr: 1.00% or less, P: 0.020% or less, S: 0.010% or less, sol.Al:0 0.010 to 0.10%, N: 0.0050% or less, and C + Si / 30 + Mn / 20 + Ni / 60 + 5B ≦ 0.
27, the balance being steel consisting of Fe and inevitable impurities,
The grain size of ferrite crystal grains is 10 μm or less, and 1
A high-strength hot-rolled steel sheet having good workability and weldability, characterized in that TiC having a size of 0 nm or less and iron carbide having a size of 10 μm or less are precipitated.