JP3043518B2 - Manufacturing method of high strength hot rolled steel sheet - 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 excellent toughness and workability suitable for increasing the size and weight of construction machines in recent years.

[0002]

2. Description of the Related Art In recent years, as a high-strength hot-rolled steel sheet having excellent strength and workability, a non-heat-treated steel sheet which combines controlled rolling and controlled cooling with the addition of Ti has been remarkably advanced in recent years. Japanese Patent Publication No. 55-49147. Japanese Patent Publication No. 55-49147 discloses that Ti is 0.0
Regarding the low carbon steel added with 4 to 0.20%, the heating conditions in the production are as follows: high temperature heating is performed for solution of Ti carbide, hot rolling is completed at the Ar ₃ transformation point or higher, and 550 is obtained. Winding is performed in the range of ℃ to 750 ℃. This method is an excellent method for obtaining a steel sheet having good strength and workability. Further, there have been known methods for obtaining a steel sheet having excellent toughness, such as limiting the rolling reduction during hot rolling and limiting the cooling rate after rolling. However, this method usually has a thickness of 200 to 300 m.
The slab of m is heated to a temperature higher than the solution temperature of Ti carbide to the center by using a heating furnace for burning gas or heavy oil. For this reason, the crystal grain size of the slab after heating becomes extremely large, and there is a limit to the refinement of the crystal grain even if the slab is refined by controlled rolling.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a new production method for refining the crystal grains of a non-heat treated high-strength hot-rolled steel sheet to which Ti is added. It is intended to provide a high-strength hot-rolled steel sheet for construction machinery and the like.

[0004]

The gist of the present invention is as follows. C: 0.05 to 0.20%, Si: ≦ 0.60%, M
n: 0.10 to 2.50%, Sol. Al: 0.004
To 0.10%, Ti: 0.04 to 0.30%, and a heating temperature of at least 1100 ° C. to a solution temperature of TiC and 1400 ° C. or less when heating a continuous cast slab composed of the balance of Fe and unavoidable impurities. A high-temperature hot-rolled steel sheet characterized by heating at a heating rate of 150 ° C. or more per hour to a holding temperature of 5 minutes or more and 30 minutes or less and then hot rolling. It is in.

Hereinafter, the present invention will be described in detail. In order to produce a non-heat treated, high-strength, hot-rolled steel sheet with excellent toughness, a small amount of Ti is used as a precipitation hardening element, and a crystal is formed by a hot rolling process that involves heating, hot rolling, and cooling. It is necessary to refine the grains. Usually, steel adjusted to the target component is continuously cast and cooled to an Ar ₃ transformation point or less. The cooling speed is 200 to 300 m.
m, so that large precipitates of 0.05 μm or more of added elements such as TiC and TiN are precipitated on the slab. It is said that Ti contributing to the improvement in strength is extremely fine Ti that is coherently deposited. Therefore,
The added small amount of Ti needs to be solutionized in a heating step before hot rolling and precipitated after rolling.

[0006] The heating temperature for solution treatment is determined by the temperature T ° of the relationship between the solubility product of the precipitate and the temperature (T: ° K) shown below.
K or more is required. log ₁₀ [% Ti]. [% C] = 2.75-7,000 / T (1) However, when steel containing a large amount of Ti and C is heated at this temperature, the crystal grains are usually coarse due to the high temperature. In order to reduce the size, various measures are taken in the subsequent hot rolling and cooling steps for miniaturization. In the hot rolling process, various rolling temperatures and rolling reductions have been devised in order to refine austenite grains by hot rolling. In the cooling step, the cooling rate is controlled to increase the rate of grain refinement during transformation from austenite to ferrite.

[0007] Incidentally, if the austenite crystal grains before hot rolling can be refined by devising not only the heating and cooling steps but also the heating step, the austenite crystal grains of the steel sheet after hot rolling can be refined, and after cooling, The ferrite crystal grains can be made fine, which is extremely effective in improving toughness. In particular, if the product thickness is large, the rolling reduction at a low temperature during hot rolling cannot be sufficiently obtained, and the cooling rate is also limited, so it is effective to make the austenite crystal grains before hot rolling fine. . The present inventors have determined that at least 11
Heating the temperature range from 00 ° C to the heating temperature not lower than the solution temperature of TiC and not higher than 1400 ° C at a heating rate of 150 ° C or higher per hour, and limiting the holding time at the heating temperature to 5 minutes or more and 30 minutes or less. It is newly found that precipitates can be solutionized while preventing coarsening of crystal grains.

FIG. 1 shows 0.10% C-0.30% Si-
1.60% Mn-0.15% Ti-0.030% So
l. A 250 mm slab of Al steel was heated under different heating conditions and maintained at that temperature for 15 minutes. Then, rolling was started, and the thickness of the finished steel sheet was 40 mm, the hot rolling finishing temperature was 880 ° C., and the sheet thickness was 9 m.
m shows the tensile strength of a steel sheet hot-rolled at a cooling rate of 15 ° C./sec and a winding temperature of 600 ° C. after hot rolling after hot rolling. The experiment was conducted by changing the slab heating conditions at this time by changing the average slab cross-sectional heating temperature and heating rate. according to this,
The tensile strength increases as the heating temperature increases from 1100 ° C. to 1280 ° C., and shows that there is almost no change at 1280 ° C. or higher, and that the effect of the heating rate is hardly observed. I have. That is, the Ti
Since the calculated solution temperature of C is 1257 ° C, Ti
This indicates that the tensile strength hardly changes as long as C is in solution.

FIG. 2 shows the effects of the heating temperature and the heating rate on the Charpy test fracture surface transition temperature (vTrs) of a steel sheet hot-rolled under the same conditions as in FIG. According to this, when the heating rate is 70 ° C./hr, the fracture surface transition temperature increases as the heating temperature increases (the toughness deteriorates),
Despite the same tensile strength at 1280 ° C. or higher, the fracture surface transition temperature sharply increases. This is 1
When heated at 280 ° C. or higher, TiC is dissolved, precipitates that suppress the growth of crystal grains disappear, and austenite crystal grains rapidly increase, and growth of ferrite crystal grains of the steel sheet is suppressed even after rolling, so that the fracture surface transition temperature decreases. Is getting higher. On the other hand, when the heating rate is 150 ° C./hr or more, the breaking surface transition temperature increases as the heating temperature increases up to 1280 ° C., but hardly changes from 1280 ° C. to 1390 ° C. This is because the growth of austenite grains is suppressed due to rapid heating, so that the ferrite grains of the steel sheet heated under these conditions are fine, and the fracture surface transition temperature in the Charpy test does not increase.

FIG. 3 shows the effect of the heating rate on the fracture surface transition temperature of a steel sheet. This is because the heating rate is 150 ° C.
If it is less than / hr, the fracture surface transition temperature increases. These facts show that setting the heating rate to 150 ° C./hr or more is an effective method for producing a steel sheet having high tensile strength and low fracture surface transition temperature (good toughness). ing.

The reasons for limiting the steel components in the present invention are as follows. C: C is the most effective element for obtaining high tensile strength, and at least 0.05% must be added for this purpose. However, the workability, toughness, and weldability decrease with an increase in C, so the upper limit was limited to 0.20%, and was set to 0.05 to 0.20%. Si: Although Si is useful as a strengthening element, it is decided to add 0.60% as an upper limit in order to produce steel economically. Mn: Mn is also an effective element for improving strength, and at least 0.10% must be added for this purpose. However, if it exceeds 2.50%, it becomes difficult to produce molten steel, so the upper limit is set to 2.50%. Al: Al needs to be 0.004% or more for deoxidation.
If it exceeds 0.10%, the crystal grains become coarse and the strength deteriorates. Therefore, the content is limited to 0.10% or less. Ti: Ti combines with C, N, O, S with a small addition to form carbides, nitrides, oxides and sulfides. At least 0.04% is required to form carbides and effectively act to improve strength. Since an increase in Ti causes surface flaws, the upper limit is set to 0.30%,
It was limited to the range of 4 to 0.30%.

Next, the reasons for limiting the heating conditions are as follows. In order to improve the toughness of the steel sheet, it is necessary to make the crystal grains of the steel sheet fine. By making the austenite crystal grains of the slab at the time of heating fine, the ferrite crystal grains of the steel sheet can also be made fine. The austenite grain size of the slab is affected by the temperature and time maintained and the presence or absence of precipitates that stop grain growth. Accordingly, the heating rate during heating affects the temperature and time to be maintained, and the heating rate 15
If the temperature is less than 0 ° C./hr, the austenite crystal grains of the slab after heating become large, and as a result, the ferrite crystal grains of the steel sheet also become large and the toughness deteriorates. Therefore, the heating rate 150
C / hr or more. In addition, a heating rate of 150 ° C./h
The reason why the heating temperature range to be r or more is limited to 1100 ° C. or more is that, at the temperature up to that point, the growth of crystal grains is relatively small even if the rate of temperature rise is low.

The reason why the heating temperature is set to be equal to or higher than the solution solution temperature of TiC is that coarse Ti precipitated by slow cooling during casting of the slab is formed.
The upper limit is set to 1400 ° C. at a temperature above which the surface scale melts and deteriorates the surface properties of the steel sheet. That's why. The holding time at the heating temperature is set to 5 minutes or more and 30 minutes or less.
This is because austenite crystal grains are coarsened during the retention time if the solution solution is insufficiently longer than 30 minutes.

The heating method for increasing the heating rate of the slab to 150 ° C./hr or more in the present invention includes, but is not limited to, a method using induction heating and a method using direct energization. In addition, the heating rate is not specified.
Furnace heating by fuel heating is performed in the temperature range up to 100 ° C.
Induction heating or direct energization may be used only at 1100 ° C. or higher where the heating rate is 150 ° C./hr or higher. In addition, as a method for manufacturing a steel sheet, any method using a hot plate mill or a thick plate rolling machine in which reverse rolling is performed may be used even when manufacturing with a hot strip mill.

[0015]

EXAMPLE Steel having the chemical components shown in Table 1 was melted in a converter and cast into a 250 mm thick slab by continuous casting.
Looking at the chemical composition, A, B, C, D, and E
The added low-carbon steel satisfies the component conditions of the present invention. Table 2 shows the slab heating conditions and the material test results of the steel sheet hot-rolled by the hot strip mill. As the heating method, heating method I: a method in which the temperature is raised to 1100 ° C. or more by gas heating up to 1100 ° C., and a heating rate of 150 ° C./hr or more by induction heating;
Method of heating at a rate of 50 ° C./hr or more, heating method III:
The three methods of performing a heating rate of 70 ° C./hr at 1100 ° C. or higher by gas heating from room temperature to a target temperature were compared.
The hot-rolling conditions were such that the hot-rolling finishing temperature was 880 ° C., the winding temperature was 600 ° C. for the Ti-added low-carbon steel, and the same steel type had the same hot-rolling conditions but different heating conditions.

[0016]

[Table 1]

[0017]

[Table 2]

According to this, the steel plates A-1, A-2, A-
3, A-4, A-5, A-6, and A-7 satisfy the production conditions of the present invention using steel type A. However, although the steel sheet A-8 uses the steel type A, it is a comparative example in which the holding time at the heating temperature is short and the required strength is not obtained. Steel plate A-9 uses steel type A, but the heating rate is 70 ° C / hr.
This is a comparative example in which vTrs is higher than that of steel sheet A-2. Steel plate A-10 also uses steel type A, but the heating rate is 1
Although it is 50 ° C./hr, this is a comparative example in which the retention time during heating is as long as 40 minutes and vTrs is high. Steel plates B-1, C-1,
D-1 and E-1 satisfy the production conditions of the present invention using steel types B, C, D and E. However, steel plate B-2,
C-2, D-2, and E-2 have a slow heating rate of 70 ° C./hr, and have a vT higher than steel plates B-1, C-1, D-1, and E-1.
This is a comparative example having a high rs.

[0019]

As described above, according to the present invention, it is possible to produce a steel plate having excellent toughness under the same steel type and the same hot rolling conditions as well as obtaining the desired strength. Therefore,
A steel sheet excellent in strength and toughness can be economically manufactured, and its effect is industrially large.

[Brief description of the drawings]

FIG. 1: 0.10% C-0.30% Si-1.60% M
n-0.15% Ti-0.030% Sol. Diagram showing the effect of heating temperature and heating rate on the tensile strength of a steel sheet using Al steel,

FIG. 2 is a diagram showing the influence of the heating temperature and the heating rate on the Charpy test fracture surface transition temperature (vTrs).

FIG. 3 is a diagram showing the influence of the heating rate on the Charpy test fracture surface transition temperature (vTrs).

Continuation of the front page (51) Int.Cl. ⁷ identification code FI C22C 38/14 C22C 38/14 (56) References JP-A-10-46258 (JP, A) JP-A-6-271936 (JP, A) JP-A-57-79116 (JP, A) JP-B-7-35540 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02-8/04 C21D 9/46 -9/48 C21D 6/00, 9/00 C22C 38/00-38/14

Claims (1)

(57) [Claims] 1. C: 0.05 to 0.20% Si: ≦ 0.60% Mn: 0.10 to 2.50% Sol. Al: 0.004 to 0.10% Ti: 0.04 to 0.30%, and at least 1100 ° C. when heating a continuous cast slab comprising the balance of Fe and unavoidable impurities,
The temperature range from the solution temperature of TiC to the heating temperature of 1400 ° C or less is heated at a heating rate of 150 ° C or more per hour, the holding time at the heating temperature is 5 minutes or more and 30 minutes or less, and then hot rolling is performed. A method for producing a high-strength hot-rolled steel sheet, comprising: