JPH1017932A - Production method of tempered thick steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a thickness-changed tempered steel sheet having a tensile strength of 570 MPa or more and having a small difference in strength depending on the sheet thickness even when quenching and tempering are performed. SOLUTION: C, Si, Mn, and Cu, Ni, Cr, Mo,
V, B, Nb, After quenching the adjusted thickness variation thick steel and Ti, etc., and within 30min holding time of the thickest portion in the temperature range of (Ac ₁ transformation point -50 ° C.) to Ac ₁ transformation point Tempering. The optimum tempering temperature and the holding time of the thickest part and the thinnest part are respectively obtained, and P = (T + 273) × (20 + log t)
_a ) × 10 ⁻³ (T: tempering temperature (° C.), t _a : holding time (hr)), and the optimum P value and the holding time and thinnest thickness of the thickest part determined from the temperature rise curve The tempering temperature and the holding time of the thickest part are determined from the difference D from the holding time of the part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a steel plate having a variable thickness in a plate such as a taper plate or a differential thickness plate used in a bridge or shipbuilding, and more particularly to a method of manufacturing a plate having a thickness variation by quenching and tempering. It relates to a tempered steel plate.

[0002]

2. Description of the Related Art Thickness-variable thick steel plates whose thickness varies within a plate have recently been used in bridges and shipbuilding. Such a thickness-variable steel sheet is charged into a heat treatment furnace in the same manner as a conventional thick material, and is held for a certain period of time. Then, the thin part and the thick part are brought to a uniform temperature, and then water-cooled or cooled. It was manufactured by. However, in such a method, there is a difference in cooling rate between the thin part and the thick part, and the cooling rate is large in the thin part and the cooling rate is small in the thick part, so that the difference in structure between the thin part and the thick part is obtained. When tempering is performed under the same conditions,
A difference in mechanical properties, particularly a difference in strength, occurs between the thin portion and the thick portion. Further, for example, when the tempering condition is adjusted to the optimum condition of the thick portion, the thin portion deviates from the optimum tempering condition, and the strength difference in the steel sheet still remains. Further, when tempering conditions are selected in order to make the strength uniform, the strength may be lower than a predetermined strength of the steel sheet, and it is difficult to obtain a steel sheet having a difference in strength among sheet thickness variable steel sheets.

As a method for manufacturing a steel sheet having a variable thickness, for example, Japanese Patent Application Laid-Open No. 1-225720 discloses an induction heating device, in which input control is performed according to the thickness of the steel sheet, and heating is performed according to the thickness of the steel sheet. In addition, there has been proposed a heat treatment method for a variable thickness steel sheet that can obtain a variable thickness steel sheet having arbitrary mechanical properties irrespective of the fluctuation of the thickness. Also, JP-A-61-284524 discloses that
After hot rolling, a method for producing a deformed non-heat treated high-strength steel sheet in which the cooling rate of the steel sheet is adjusted according to a desired strength distribution of the steel sheet by using an accelerated cooling facility has been proposed.

[0004] However, none of these methods can be implemented only with existing equipment, and special equipment must be installed.
The problem remains that it cannot be implemented immediately.

[0005]

The present invention advantageously solves the above problems, has a tensile strength of 570 MPa or more, and
It is an object of the present invention to provide a method of manufacturing a thickness-changed tempered steel sheet having a small difference in strength due to the sheet thickness after quenching and tempering.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a method of manufacturing a tempered steel sheet in which the thickness varies within the sheet, wherein the tempering temperature is set to (Ac) ₁ Transformation point -50 ° C) to Ac ₁ Transformation point temperature range, and the holding time of the thickest part of the thick steel plate is the time when the center of the thickest part reaches (atmospheric temperature -10 ° C). From 30min
A method for producing a tempered steel sheet having a thickness variation, characterized by performing tempering within the range.

Further, in the present invention, the optimum tempering conditions for the thickest part and the thinnest part of the thickness-varying steel plate are determined by the tempering temperature and the holding time, respectively. ) equation P = (T + 273) × ( the _{20 + log t a) × 10} -3 ... (1) ( where, T: tempering temperature (℃), t _a: retention time (h
r)) is calculated, and the optimum P values are Pa (the thickest part) and Pu (the thinnest part), respectively. These Pa, Pu, the thickest part and the thinnest part are calculated. From the difference D between the holding time of the thickest part and the holding time of the thinnest part determined from the temperature rise curve, the following equations (2) and (3) are used. Pa = (T + 273) × (20 + log t _aa ) × 10 ^-3 (2) Pu = (T + 273) × (20 + log ( _taa + D)) × 10 ^-3 (3) (where, _taa : retention time (hr) of the thickest part, D: thickest part) It is preferable to determine the tempering temperature and the holding time of the thickest part so as to simultaneously satisfy the difference (hr) between the tempering temperature and the thinnest part.

[0008] In the present invention, the thick steel plate may be selected from the group consisting of:
And C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.30 to
2.50%, Al: 0.01 to 0.10%, N: 0.007% or less,
Further, Cu: 0.05 to 1.30%, Ni: 0.1 to 10.0%, Cr: 0.1%
05-1.50%, Mo: 0.03-0.50%, V: 0.01-0.15%,
B: One or two selected from 0.0003 to 0.0020%
It is preferable that the steel plate be a thick steel plate containing at least one or more species and the balance being Fe and inevitable impurities.

[0009] In the present invention, the steel plate may be selected from the group consisting of:
And C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.30 to
2.50%, Al: 0.01 to 0.10%, N: 0.007% or less,
Further, Cu: 0.05 to 1.30%, Ni: 0.1 to 10.0%, Cr: 0.1%
05-1.50%, Mo: 0.03-0.50%, V: 0.01-0.15%,
B: 0.0003 to 0.0020% group, Nb: 0.005 to 0.06%, Ti:
0.005-0.06% group and Ca: 0.005-0.0040%, RE
It is preferable that the steel sheet be a thick steel sheet containing one or more of each group selected from at least two of the groups of M: 0.001 to 0.020%, the balance being Fe and unavoidable impurities.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The variable thickness steel plate of the present invention means a thick steel plate having a different thickness in a plate such as a taper plate or a difference thickness plate. In the present invention, there is no problem even if the plate thickness fluctuates not only when the plate thickness fluctuates at a constant gradient in the longitudinal direction or the width direction of the steel plate but also when it is non-uniform.

According to the present invention, a slab is formed into a predetermined thickness by the hot rolling of a slab. The hot rolling conditions are not particularly limited, but the slab heating temperature for hot rolling is preferably from 1000 to 1300 ° C. If the slab heating temperature is lower than 1000 ° C., uniform solid solution of the added alloy element cannot be achieved. On the other hand, when the slab heating temperature exceeds 1300 ° C., austenite crystal grains are coarsened, and are not refined by subsequent rolling, and the toughness of the steel material is deteriorated.

[0012] The hot-rolled thick steel sheet of the present invention, after hot rolling,
Harden directly or after reheating. The quenching temperature is A
If c ₃ transformation point or higher is not particularly a problem, and it is preferably in the range of 800 to 1000 ° C.. If the temperature is lower than 800 ° C, it is difficult to obtain sufficient quenching strength. On the other hand, when the temperature exceeds 1000 ° C., the crystal grains become coarse and the toughness deteriorates. The temperature is more preferably 850 to 950 ° C.

In the case of direct quenching after hot rolling, if the steel sheet temperature is lower than the above-mentioned Ac ₃ transformation point, the steel sheet may be reheated to the Ac ₃ transformation point or higher. The variable thickness steel plate of the present invention is tempered after quenching. The tempering temperature is (Ac ₁
A temperature range of transformation temperature -50 ° C.) to Ac ₁ transformation point, holding time of outermost thick portion, 30min from the time it reaches the center of the thickest portion of the thick steel plate (ambient temperature -10 ° C.) Within.

If the tempering temperature is lower than (Ac ₁ transformation point −50 ° C.), the strength difference between the thick portion and the thin portion is large, and if it exceeds the Ac ₁ transformation point, the strength is sharply reduced. Therefore, tempering temperature was the temperature range of (Ac ₁ transformation point -50 ° C.) to Ac ₁ transformation point. Further, in the present invention, a thick steel plate is charged into an atmosphere at the above temperature, and the steel plate is kept for a short time, that is, 30 minutes in the holding time of the thickest part.
Tempering within min. For long time over 30min,
The strength is significantly reduced.

The tempering temperature is set within the above-mentioned temperature range.
Further, the optimum temperature is determined as follows. For each of the thickest portions (thick portions) and the thinnest portions (thin portions), the temperature and the holding time of the optimal tempering conditions for obtaining a predetermined strength are obtained by the following equation (1). The determined optimum P value is determined.

P = (T + 273) × (20 + log t)_a) × 10^-3 (1) where, T: tempering temperature (° C.), t_a: Holding time (hr) Next, the optimum P value of the thick part is Pa, and the optimum P value of the thin part is P.
u, the holding time of the thick part is t _aaAnd the temperature rise curve as shown in FIG.
Where D is the difference between the holding time of the thick part and the thin part obtained from
In order to obtain the optimum tempering of each part,
Return condition temperature T, thick part holding time t_aaIs Pa = (T + 273) × (20 + log t)_aa) × 10^-3 ... (2) Pu = (T + 273) × (20 + log (t_aa+ D)) × 10^-3 (3) This is because the temperature T is lower in the thin part than in the thick part.
This is because the holding time is long by D. D is thick and thin
Because of the difference in the holding time of each part,
And therefore, equations (2) and (3) are
Optimum tempering of both thin and thick parts
T, t_aaCan be requested.

That is, as shown in FIG. 1, the optimum tempering temperature Pa is plotted between the optimum tempering parameter Pa of the thick part and the optimum tempering parameter Pu of the thin part in relation to the tempering temperature and the holding time. Can be easily determined by determining the tempering temperature that results in the difference D in the holding time. In the present invention, and in the temperature range of the tempering temperature (Ac ₁ transformation point -50 ° C.) to Ac ₁ transformation point (2), to a temperature satisfying the expression (3),
In addition, the holding time is set to satisfy the expressions (2) and (3) within 30 minutes.

[0018] This makes it possible to provide a variable thickness steel plate having both a thick part and a thin part, which satisfies the optimum tempering conditions having predetermined strength and toughness, and which has excellent toughness. The steel plate of the present invention preferably has a tensile strength of 570 MPa or more after quenching and tempering.

Next, the reasons for limiting the composition of the thick steel plate will be described. C: 0.03 to 0.20% C is an element that increases the hardenability, and is added for the purpose of securing strength. For this purpose, 0.03% or more is required, but if it exceeds 0.20%, toughness and weldability deteriorate. Therefore, C is set in the range of 0.03 to 0.20.

Si: 0.05 to 0.50% Si has the effect of promoting deoxidation and further increasing the strength. This effect is observed with the addition of 0.05% or more.
If it exceeds 50%, toughness deteriorates. For this reason, Si is 0.05-
The range was 0.50%. Mn: 0.30 to 2.50% Mn has the effect of increasing strength without impairing toughness. If it is less than 0.30%, the increase in strength is small, but if it exceeds 2.50%, workability deteriorates. For this reason, Mn
Was in the range of 0.30 to 2.50%.

Al: 0.01 to 0.10% Al is added as a deoxidizing agent, and has an effect of further refining crystal grains. For the refinement of crystal grains,
It is necessary to add 0.01% or more. However, when the content exceeds 0.10%, the amount of oxide-based inclusions increases and the toughness is deteriorated. For this reason, Al was made into the range of 0.01 to 0.10%.

N: 0.007% or less N combines with Al to prevent the crystal grains from becoming coarse during heating. However, if contained in a large amount, the toughness of the heat affected zone is deteriorated. Therefore, N is set to 0.007% or less. Cu: 0.05 to 1.30%, Ni: 0.1 to 10.0%, Cr: 0.05 to 1.50
%, Mo: 0.03 to 0.50%, V: 0.01 to 0.15%, B: 0.0003
One or more kinds of Cu selected from 0.0020% or more increase the strength by solid solution or precipitation in steel. In order to increase the strength by solid solution strengthening, it is necessary to add 0.05% or more. Further, in order to increase the strength by precipitation strengthening, it is necessary to add 0.50% or more. However, when the content exceeds 1.3%, the effect is saturated and, in addition, the hot workability deteriorates. Therefore, Cu is set in the range of 0.05 to 1.30%.

Ni increases the strength and further improves the toughness. For this, 0.10% or more is required,
If it exceeds 10.0%, it becomes economically expensive. For these reasons, Ni is set in the range of 0.1% to 10.0%. Cr improves the hardenability and increases the strength. For this purpose, 0.05% or more must be added, but if it exceeds 1.50%, the weldability deteriorates. Therefore, Cr is set in the range of 0.05 to 1.50%.

Mo improves quenching properties, increases tempering softening resistance, and increases strength. For this purpose, an addition of 0.03% or more is required, but if it exceeds 0.50%, weldability is deteriorated and the cost becomes high. For this reason, Mo
The range was 0.03 to 0.50%. V forms nitrides or carbides and increases the strength by precipitation strengthening. For this purpose, 0.01% or more must be added, but if it exceeds 0.15%, the toughness deteriorates. Therefore, V is 0.01
The range was about 0.15%.

B improves the hardenability by adding a small amount,
Increase strength. For this purpose, 0.0003% or more must be added, but if it exceeds 0.0020%, toughness deteriorates. Therefore, B is set in the range of 0.0003 to 0.0020%. Further, Cu: 0.05 to 1.30%, Ni: 0.1 to 10.0%,
Cr: 0.05-1.50%, Mo: 0.03-0.50%, V: 0.01-0.15
%, B: 0.0003 to 0.0020% group, Nb: 0.005 to 0.06%,
Ti: 0.005 to 0.06% group, and Ca: 0.005 to 0.0040
%, REM: 0.001 to 0.020%. Each group may contain one or more members selected from at least two of the groups.

Nb: 0.005 to 0.06%, Ti: 0.005 to 0.06%
Nb is an element that forms nitrides, suppresses austenite grain coarsening, or contributes to grain refinement during hot rolling and precipitation strengthening during tempering. is there. To obtain such an effect, 0.005% or more is necessary, but 0.06%
If it exceeds, the toughness of the weld is degraded. For these reasons, Nb is set in the range of 0.005 to 0.06%.

Ti forms a nitride like Nb and suppresses austenite grain coarsening. For this purpose, 0.005% or more must be added, but if it exceeds 0.06%, toughness deteriorates. Therefore, the content of Ti is set in the range of 0.005 to 0.06%. One or two kinds of Ca selected from Ca: 0.005 to 0.0040% and REM: 0.001 to 0.020% form spherical sulfides and have an effect of improving toughness. For this purpose, 0.005% or more must be added. However, if it exceeds 0.0040%, oxide-based inclusions increase and the toughness deteriorates. Therefore, Ca is 0.005 ~
The range was 0.0040%.

REM makes sulfides spherical like Ca,
Improve toughness. To get this effect, 0.001%
The above addition is necessary, but if it exceeds 0.020%, the amount of inclusions increases and the toughness is deteriorated. From such a thing
REM was set in the range of 0.001 to 0.020%. In the present invention, the balance consists of Fe and inevitable impurities. Inevitable impurities can be tolerated up to P: 0.025% or less and S: 0.015% or less.

[0029]

【Example】

(Example 1) Steel having the composition shown in Table 1 was melted in a converter, and a slab having a thickness of 260 mm was formed by continuous casting. The slab was rolled into a tapered plate having the shape shown in FIG.
The thickest part (thick part) is 75 mm thick, and the thinnest part (thin part) is 55 mm thick. The tapered plate was heated to 930 ° C. and then cooled with water.

After quenching, the optimum tempering conditions were determined for each of the thick and thin portions, and the optimum P value was calculated from equation (1).

[0031]

[Table 1]

[0032]

[Table 2]

The difference D in the retention time between the thick part and the thin part was determined from the temperature rise curve of the thick part and the thin part as shown in FIG. In this case, the D value was 34 min. The optimum tempering P value of the thick portion and the thin portion of the tapered plate is calculated by the following equation. 18.2 = (T + 273) × (20 + log t _aa ) × 10 ^-3 (2A) 18.9 = (T + 273) × (20 + log (t _aa + D)) × 10 ^-3 (3A) FIG. 4 shows equations (2A) and (3A). Here, when T (tempering temperature) that satisfies D = 34 min is obtained, T = 680 ° C. is obtained.

The taper plate is tempered at the temperature of 680 ° C. with the above quenched, and the holding time of the thick part is 8 min (128 min in the furnace).
Table 3 shows the strength of each part when tempering was performed.

[0035]

[Table 3]

From Table 3, the strength difference between the thick part and the thin part is 10 MPa.
Within and very small. As a comparative example, Table 3 also shows an example in which the same taper plate as that of the example was quenched under the same quenching conditions, and then tempered at 640 ° C. × 175 min (in a furnace) under the optimum tempering conditions for a thick portion. In this comparative example,
Naturally, the strength of the thin part is too high, and the strength difference between the thick part and the thin part is 70 MPa or more.

Example 2 Steel having the composition shown in Table 4 was melted in a converter, slabs having a thickness of 215 mm were formed by continuous casting, and tapered plates having a predetermined shape were formed by rolling a thick plate. After quenching the tapered plate, the optimum tempering condition is obtained for each of the thick part and the thin part, the optimum P value is calculated from equation (1), and then the optimum P value and the temperature rise curves of the thick part and the thin part are calculated. The difference D in the holding time between the thick part and the thin part was determined, the tempering temperature was determined from the optimum P value and the D value using the equations (2) and (3), and the tapered plate was tempered at the tempering temperature. Table 5 shows the strength after tempering.

[0038]

[Table 4]

[0039]

[Table 5]

If the composition falls within the range of the present invention and the tempering temperature falls within the range of the present invention, the tensile strength 570 MPa or more is satisfied. The tapered plate has a small difference in strength between the parts and uniform strength.

[0041]

According to the present invention, it is possible to make the strength of a thickness-variable thick steel plate whose thickness fluctuates in the plate uniform within the plate despite the difference in the thickness, and to improve the design. The use of a variable thickness steel plate having an increased degree of freedom and a uniform strength has a great industrial effect, such as improving the safety of a building or a structure.

[Brief description of the drawings]

FIG. 1 is a schematic relationship diagram showing a relationship between a tempering temperature at which Pa and Pu values are obtained and a holding time.

FIG. 2 is a schematic diagram showing a temperature rise curve at the time of tempering of a thick portion (thick portion) and a thin portion (thin portion) of a thickness-variable thick steel plate.

FIG. 3 is a cross-sectional view showing the dimensions and shape of a tapered plate.

FIG. 4 is a graph showing a relationship between a tempering temperature at which Pa and Pu values are obtained and a holding time.

Claims (4)

[Claims] 1. A method of manufacturing a tempered steel sheet having a thickness varied within a sheet, wherein the tempered steel sheet is quenched and then tempered.
After quenching, the tempering temperature is set to (Ac ₁ transformation point-50 ° C).
To the temperature range of the Ac ₁ transformation point, and the holding time of the thickest part of the thick steel plate is set at the center of the thickest part (atmospheric temperature −10 ° C.).
A method for producing a tempered thick steel sheet, wherein the tempering is performed within 30 minutes from the time when the steel sheet reaches the thickness. 2. The optimum tempering conditions of the thickest part and the thinnest part of the thickness-variable thick steel plate are respectively determined by a tempering temperature and a holding time, and then defined by the following equation (1) from the tempering temperature and the holding time. The optimum P value is calculated as Pa (the thickest portion) and Pu (the thinnest portion), respectively, and determined from these Pa and Pu and the temperature rise curves of the thickest portion and the thinnest portion. From the difference D between the holding time of the thickest part and the holding time of the thinnest part, the tempering temperature and the holding of the thickest part are determined so as to simultaneously satisfy the following expressions (2) and (3). The method for producing a sheet steel sheet with a thickness-changed temper according to claim 1, wherein the time is determined. P = (T + 273) × (20 + log t _a ) × 10 ^-3 (1) Pa = (T + 273) × (20 + log t _aa ) × 10 ^-3 (2) Pu = (T + 273) × (20 + log (t _aa) a ^{+ D)) × 10 -3 ...} (3) where, T: tempering temperature (℃), t _a: retention time (hr), t _aa: retention time of the thickest portion (hr) D: top and the thickest portion Difference in retention time of thin part (hr) 3. The steel plate according to claim 1, wherein the steel sheet contains, by weight%, C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.30 to 2.50%, Al: 0.01 to 0.10%, N: 0.007% or less; Further, Cu: 0.05 to 1.30%, Ni: 0.1 to 10.0%, Cr: 0.05 to 1.50%, Mo: 0.03 to 0.50%, V: 0.01 to 0.15%, B: 0.0003 to 0.0020% Seed or 2
The method for producing a tempered thick steel sheet according to claim 1, wherein the steel sheet contains at least one species, and the balance is Fe and unavoidable impurities. 4. The steel sheet according to claim 1, wherein the steel sheet contains, by weight: C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.30 to 2.50%, Al: 0.01 to 0.10%, N: 0.007% or less; Further, Cu: 0.05 to 1.30%, Ni: 0.1 to 10.0%, Cr: 0.05 to 1.50%, Mo: 0.03 to 0.50%, V: 0.01 to 0.15%, B: 0.0003 to 0.0020%, Nb: 0.005 to One or more of each group selected from at least two of the group of 0.06%, Ti: 0.005 to 0.06% and Ca: 0.005 to 0.0040%, REM: 0.001 to 0.020% , Balance Fe
3. The method for producing a tempered steel sheet having a variable thickness according to claim 1 or 2, wherein the method comprises an unavoidable impurity.