JP2000303147A - Tapered steel sheet and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the weldability of a steel sheet by allowing the steel sheet to have a specified elemental compsn. and allowing the each element of the compsn. to satisfy specified relation. SOLUTION: This steel sheet contains., by mass, <=0.20% C, <=1.00% Si, <=3.0% Mn, 0.005 to 0.10% Al and 0.001 to 0.007% N and moreover contains one or more kinds among 0.05 to 0.60% Cu, 0.05 to 1.00% Ni, 0.05 to 1.00% Cr, 0.05 to 1.00% Mo, 0.005 to 0.014% Nb, 0.005 to 0.014% V, 0.005 to 0.050% Ti and 0.0005 to 0.0030% B. Furthermore, the contents of P and S in impurities are respectively controlled to <=0.03% and <=0.015%. Moreover, the value of Pcm expressed by formula I is controlled to 0.10 to 0.21 in the case the content of B is <0.0003% and to 0.10 to 0.19 in the case the content of B is >=0.0003%, and also, the value of H20-50 expressed by formula II is controlled to <=15. The elemental symbols in each formula denote the contents (mass %) of the elements.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a tapered steel plate and a method for manufacturing the same. More specifically, even in an environment with a temperature of 25 ° C, there is no need for preheating during welding, the amount of taper,
That is, the present invention relates to a tapered steel sheet excellent in weldability, in which the difference between the thickest part and the thinnest part of the steel sheet is 10 mm or more, and a method of manufacturing the same.

[0002]

2. Description of the Related Art The thickness of a normal thick steel plate is constant in the width direction and the length direction, but recently, for example, in the field of bridges, in order to rationalize the design and reduce the mass of the structure and the number of construction steps. There is a great demand for a tapered steel sheet in which the thickness of the steel sheet changes continuously. In addition, as a taper steel plate,
Various shapes are required, such as a single-tapered steel plate in which the thickness monotonically increases from one end to the other end of the steel plate, and a chevron-shaped tapered steel plate having a large thickness at the center of the steel plate.

[0003] Generally, the mechanical properties of a thick steel plate depend on the amount of rolling and the cooling rate after rolling, so that if the thickness of the steel plate changes, the mechanical properties of each part of the steel plate will change. . Therefore, unlike the case of manufacturing a normal thick steel plate having a constant thickness, when a tapered steel plate is manufactured, it is necessary to take care to make the mechanical properties in the steel plate uniform. For example, for a rolled steel material for a welded structure, the tensile strength (hereinafter, also referred to as TS) depends on its strength level.
Is defined as 110 to 150 MPa (see JIS G 3106). In order to stably produce a large number of tapered steel sheets within the above range, it is necessary to reduce the variation in TS in each steel sheet. Required.

[0004] When a so-called "accelerated cooling" is performed after rolling to produce a thick steel plate, "cooling unevenness" occurs, and the "cooling unevenness" causes residual stress in the steel sheet.
When a so-called cutting is performed, bending or warping may occur. Particularly in the case of a tapered steel plate, since the plate thickness is different in the length direction or width direction of the steel plate, when the above-described accelerated cooling is performed after rolling, "cooling unevenness" occurs remarkably, and bending after cutting is performed. It is difficult to avoid that warpage occurs.

Further, in the case of a steel structure such as a bridge, welding is performed at the construction site. For this reason, there is a demand for a steel material that does not cause welding cracks even if preheating is not performed before welding. In order to improve the weldability, P _cm represented by the following equation, which is known as an index indicating the weld cracking susceptibility, is used.
It is necessary to keep the content of elements low according to
Lowering the P _cm value generally results in a decrease in strength and the desired T
It becomes difficult to secure S. The symbol of the element in the formula indicates the content of the element in mass%.

P _cm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B · .. Also, after welding, the presence or absence of welding defects is investigated by ultrasonic waves. However, if there is acoustic anisotropy in the thick steel plate as a material, it becomes difficult to diagnose welding defects. For this reason, the ratio (V _L / V _C ) of a thick steel plate having a small acoustic anisotropy, in other words, the shear wave velocity (V _L ) in the length direction of the steel sheet and the shear wave velocity (V _C ) in the width direction of the steel sheet. There is a demand for a thick steel plate having a value close to 1.00.

[0007] Techniques relating to the production of tapered steel sheets are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 8-92636 and 9-1554.
No. 06 is disclosed.

Among them, Japanese Patent Application Laid-Open No. Hei 8-92636 proposes "a method for producing a tapered steel sheet having a very low sensitivity to cooling conditions and capable of maintaining constant material properties". However, when accelerated cooling is performed by the manufacturing method disclosed in this publication, uneven cooling may occur, and the uneven cooling may cause residual stress, which may cause bending or warping after cutting. Furthermore, since Cu is contained as an essential component in an amount of 0.7 to 2.0% by mass, there has been a problem that surface cracks are likely to occur during hot working.

Japanese Patent Application Laid-Open No. Hei 9-155406 discloses that “a taper amount of 10 mm or more in the longitudinal direction with a tensile strength of 490 MPa or more, in which a difference in strength in a steel sheet is small, and bending and warping after stripping are small. Manufacturing method of a tapered plate having the same ”. However, since the steel plate proposed in this publication contains 0.015 to 0.06% by mass of Nb as an essential element, it has a large acoustic anisotropy, and it is difficult to diagnose welding defects by ultrasonic after welding. There was a case. Further, since there is no restriction on P _cm represented by the above equation, welding cracks may occur unless preheating is performed before welding.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has as its object to prevent surface cracks from occurring during hot working, to have a tensile strength (TS) of 490 MPa or more,
Fracture appearance transition temperature in impact testing using JIS4 No. Charpy impact test specimens _(V T _S) is 0 ℃ or less, it is easy to diagnose weld defects by ultrasonic, moreover, preheating during welding at 25 ℃ ambient temperature of the environment Not required, the taper amount is 10m
It is an object of the present invention to provide a tapered steel plate having a diameter of not less than m and a method for producing the same.

[0011]

SUMMARY OF THE INVENTION The gist of the present invention is to provide tapered steel sheets (4) and (5) shown in the following (1) to (3).
The manufacturing method shown in FIG.

(1) In mass%, C: 0.20% or less, S
i: 1.00% or less, Mn: 3.0% or less, Al: 0.
005-0.10%, N: 0.001-0.007%, Cu: 0.05-0.60%, N
i: 0.05 to 1.00%, Cr: 0.05 to 1.00
%, Mo: 0.05 to 1.00%, Nb: 0.005 to
0.014%, V: 0.005 to 0.100%, Ti:
0.005 to 0.050% and B: 0.0005 to 0.5%.
One or more of 0030%, the balance being Fe and impurities, P in the impurities is 0.03% or less, S is 0.015% or less, and further represented by the following formula: that when the value of P _cm is the content of B is less than 0.0003% is from 0.10 to 0.21, when the content of B is 0.0003% or more is 0.10 to 0.19 And a taper steel sheet having a Hv _20-50 value represented by the following formula of 15 or less and a taper amount of 10 mm or more.

P _cm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B · Hv _20-50 = -110 + 460C + 44Si + 39Mn-31Cu-9Ni + 11Cr + 22Mo + 180V + 9600B-23000Mo × B Here, the taper amount refers to the difference in thickness between the thickest part and the thinnest part of the steel plate. And the symbol of the element in the formula indicates the content of the element in mass%.

(2) In mass%, C: 0.05 to 0.20
%, Si: 0.05-0.55%, Mn: 0.3-1.
6%, Al: 0.01 to 0.10%, N: 0.001 to
0.007%, Cu: 0.05-
0.60%, Ni: 0.05 to 1.00%, Cr: 0.
05-1.00%, Mo: 0.05-1.00%, N
b: 0.005 to 0.014%, V: 0.005 to 0.
100%, Ti: 0.005 to 0.030% and B:
One or more of 0.0005 to 0.0030% is contained, and the balance is composed of Fe and impurities, P in the impurities is 0.02% or less, S is 0.005% or less, and , The value of P _cm represented by the above formula is B content of 0.0
When it is less than 003%, it is 0.21 or less, when the content of B is 0.0003% or more, it is 0.19 or less, and the value of Hv _20-50 represented by the above formula is 15 or less. Is a tapered steel plate having a taper amount of 10 mm or more.

(3) 0.20% or less by mass%, Si:
1.00% or less, Mn: 3.0% or less, Mo: 0.05
_{~1.00%, B: 0.0005~0.0030%,} A
l: 0.005 to 0.10%, N: 0.001 to 0.0
And Cu: 0.05-0.60
%, Ni: 0.05 to 1.00%, Cr: 0.05 to
1.00%, Nb: 0.005 to 0.045%, V:
0.005 to 0.100% and Ti: 0.005 to 0.
One or more of 050%, and the balance is F
e and impurities, P in the impurities is 0.03% or less, S is 0.015% or less, and the value of P _cm represented by the above formula is 0.10 to 0.19; and A tapered steel sheet having a taper amount of 10 mm or more, wherein the value of Hv _20-50 represented by the above formula is 15 or less.

(4) A steel slab having the chemical composition described in the above (1) to (3) is heated to a temperature in a temperature range of 950 to 1300 ° C., so that a hot rolling finish temperature becomes 950 to 700 ° C. A method for producing a tapered steel sheet having a taper amount of 10 mm or more, which is hot-rolled and air-cooled to room temperature after completion of the hot rolling.

(5) The steel slab having the chemical composition described in the above (1) to (3) is heated to a temperature in a temperature range of 1000 to 1300 ° C. so that the hot rolling finish temperature becomes 850 to 750 ° C. A method for producing a tapered steel sheet having a taper amount of 10 mm or more, which is hot-rolled and air-cooled to room temperature after completion of the hot rolling.

Here, as described above, the taper amount refers to a difference in thickness between the thickest part and the thinnest part of the steel sheet. The above formulas and the element symbols in the formulas indicate the content of the element in mass%.

Hereinafter, the inventions described in the above (1) to (5) will be referred to as inventions (1) to (5), respectively.

The present inventors did not require preheating at the time of welding even in an environment at a temperature of 25 ° C., and required a TS of 490 MPa or more.
When the base material toughness (fracture appearance transition temperature in impact testing using JIS4 No. Charpy impact test specimens _(V T _S) is 0 ℃ or less) required for use in structures such as bridges and has, moreover, Various studies were conducted on a tapered steel sheet having a taper amount of 10 mm or more, which facilitates diagnosis of welding defects by ultrasonic waves, and a method for manufacturing the same, and the following findings were obtained. Note that in the following description for the sake of simplicity, the _"V T _S in impact test using JIS4 No. Charpy impact test piece", simply referred to as _"V T _S".

(A) In order to obtain a steel sheet that does not require preheating during welding even in an environment at a temperature of 25 ° C., it is necessary to keep the value of P _cm represented by the above equation low.

(B) The allowable value of the P _cm varies depending on the B content. When the B content is less than 0.0003%, the allowable value is 0.21 or less, and when the B content is 0.0003% or more. In this case, it may be set to 0.19 or less.

(C) In order to make the TS of the steel sheet 490 MPa or more, the value of P _cm represented by the above equation is preferably 0.10 or more, more preferably 0.15 or more.

The acoustic anisotropy can be reduced by (d) at least (a) keeping the Nb content low or (b) increasing the hot rolling finishing temperature.

(E) In order to facilitate the diagnosis of welding defects by ultrasonic waves, the ratio (V _C ) of the shear wave velocity (V _L ) in the length direction of the steel sheet to the shear wave velocity (V _C ) in the width direction of the steel sheet. _L / V _C )
(Hereinafter, the above (V _L / V _C ) is simply referred to as the transverse wave sound speed ratio)
May be set to 0.98 to 1.02.

(F) Even if the steel sheet is air-cooled to room temperature without accelerated cooling after the completion of hot rolling, uneven cooling may occur in the steel sheet depending on the chemical composition of the steel sheet, and a large difference in strength may occur in portions having different thicknesses. However, when air-cooled to room temperature after the completion of hot rolling, the dispersion of TS in the steel sheet is reduced by 50MP.
If it is kept below a, no bending or warping will occur even if the steel sheet is cut.

(G) In the case where the steel sheet is air-cooled to room temperature after completion of the hot rolling, the chemical composition of the steel sheet may be strictly controlled in order to suppress the variation of TS in the steel sheet to 50 MPa or less.

Then, a small amount of steel having various contents of various alloying elements was melted on a laboratory scale, and a cylindrical test piece having a diameter of 3 mm and a length of 10 mm was collected from the steel piece. These test pieces were converted to 95 using a fully automatic transformation measurement device.
Heated to 0 ° C and 1150 ° C, the plate thickness was 20mm and 50 ° C.
The steel sheet having a thickness of 0.5 mm was cooled to room temperature at a cooling rate corresponding to an air cooling rate of 800 to 500 ° C., that is, 0.55 ° C./min and 0.15 ° C./min. By measuring the change in the length of the test piece during this cooling, the starting temperature and the ending temperature of the phase transformation were determined. Here, the thickness of the steel plate is 20 mm and 5 mm.
The reason for choosing 0 mm is that the thickness range required for a tapered steel plate having a tensile strength of 490 MPa or more is usually 2 mm.
0 to 50 mm.

Then, the hardness at the center of two test pieces of the same steel cooled at the above-mentioned cooling rate was measured at a test force of 9.807 N (that is, a test load of 1 kgf) using a micro Vickers hardness tester. . As a result, the following findings were obtained.

(H) The difference in Hv hardness between steel plates having a thickness of 20 mm and 50 mm when cooled to room temperature at a cooling rate corresponding to an air cooling rate of 800 to 500 ° C. is as follows: C, Si, M
It has a correlation with the above expression represented by the contents of n, Cu, Ni, Cr, Mo, V, and B. Therefore, steels with various values of the formulas are further melted, and the steel slab is heated to 1150 ° C. and hot-rolled to finish into a tapered steel plate having a thickness difference of 10 to 60 mm in the length direction. After hot rolling, it was air-cooled to room temperature. Tensile test specimens were taken from the central part of the thickness of each of the tapered steel sheets thus obtained in a direction perpendicular to the rolling direction, and the TS was measured. As a result, the following findings were obtained.

The value of Hv _20-50 represented by the equation (i) is set to 1
If it is 5 or less, TS of 20 mm thickness and 50 mm thickness
The difference from the TS of the portion can be suppressed to 50 MPa or less.

(J) When both the conditions (b) and (h) are satisfied, the TS variation in the tapered steel plate becomes extremely small.

(K) If a specific amount of Mo and B are contained in the steel in combination, the phase transformation start temperature at the time of cooling is greatly reduced,
This causes a large increase in TS and a decrease in acoustic anisotropy. For this reason, the restrictions on the Nb content and the hot rolling finishing temperature described in (d) above can be relaxed. That is, the content of Nb can be increased for the purpose of improving the strength, and the hot rolling finishing temperature can also be lowered for the purpose of improving the strength.

The present invention has been completed based on the above findings.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, "%" of the content of each element means "% by mass". (A) Chemical composition of tapered steel sheet C: C is an element effective for securing strength, but if its content exceeds 0.20%, weldability and toughness are reduced,
Variation in strength due to plate thickness increases. Therefore, C
Was 0.20% or less. If the value of P _cm represented by the above formula is 0.10 or more, the desired 490 MPa
Since the above-mentioned TS is secured, C may not be added, and its content may be a value of the impurity level. In order to ensure the desired TS of 490 MPa or more inexpensively, reliably and stably, the lower limit of the C content is set to 0.0.
The content of C is preferably set to 0.06 to 0.11% in order to improve the strength-toughness balance.

Si: Si is an element effective for securing the strength, but if it is contained in excess of 1.00%, the weldability and toughness decrease. Therefore, the content of Si is set to 1.0
0% or less. The value of P _cm represented by the above equation is 0.1
If the value is set to 0 or more, a desired TS of 490 MPa or more is secured, so that Si need not be added, and the content may be a value of the impurity level. Note that the desired 490
The lower limit of the Si content is preferably set to 0.05% in order to reliably and stably secure a TS of MPa or more at a low cost. On the other hand, to obtain good weldability and toughness, It is preferable that the upper limit of the content be 0.55%. In order to improve the strength-toughness balance, the Si content is more desirably 0.1 to 0.3%.

Mn: Mn is an element effective for securing strength, but if its content exceeds 3.0%, weldability and toughness are reduced. Therefore, the content of Mn is set to 3.0% or less. If the value of P _cm represented by the above formula is set to 0.10 or more, a desired TS of 490 MPa or more is secured, so that Mn may not be added, and the content is an impurity level value. You may. In order to ensure the desired TS of 490 MPa or more inexpensively, reliably and stably, it is preferable to set the lower limit of the Mn content to 0.3%, while obtaining good weldability and toughness. Therefore, it is preferable to set the upper limit of the Mn content to 1.6%. In order to improve the strength-toughness balance, the Mn content should be 0.9 to 0.9%.
More preferably, it is 1.5%, and 1.2 to 1.5%.
It is very preferable.

Al: Al is an element effective for deoxidation.
However, if the content is less than 0.005%, the effect of addition is poor, while if it exceeds 0.10%, toughness is impaired. Therefore, the content of Al is set to 0.005 to 0.10.
%. In order to make deoxidation more reliable, A
The content of 1 is preferably set to 0.01 to 0.10%.

N: N has the effect of forming nitrides, reducing austenite grains and increasing toughness. But,
If the content is less than 0.001%, the effect of addition is poor,
On the other hand, if it exceeds 0.007%, the toughness is rather reduced. Therefore, the content of N is set to 0.001 to 0.007.
%.

The chemical compositions of the tapered steel sheets according to the inventions (1) and (2) include the following amounts of Cu, Ni, Cr, Mo, Nb, V, Ti and B in order to increase the strength. One or more of them are contained. In addition, the chemical composition of the tapered steel sheet according to the invention of (3) includes the following amounts of Mo and B in order to greatly lower the phase transformation start temperature upon cooling, increase strength, and lower acoustic anisotropy. And in order to enhance the strength, one or more of the following amounts of Cu, Ni, Cr, Nb, V and Ti are contained.

Cu: If the Cu content is less than 0.05%, the effect of addition is poor. On the other hand, if the content exceeds 0.60%, surface cracks are likely to occur during slab heating to hot rolling. Therefore, to increase the strength,
The content when u is added is set to 0.05 to 0.60%. In order to suppress the surface cracks, the Cu content is desirably 0.05 to 0.40%. Note that C
When u is contained, the effect of improving weather resistance can be obtained at the same time.

Ni: If the Ni content is less than 0.05%, the effect of addition is poor. On the other hand, when the content exceeds 1.00%, scale flaws are easily generated. Therefore, the content when Ni is added to increase the strength is set to 0.0
5 to 1.00%. In order to suppress the occurrence of the scale flaw, the Ni content is desirably 0.05 to 0.30%. In addition, by adding Ni, it is possible to prevent surface cracking during slab heating and hot rolling caused by the addition of Cu. Therefore, when Cu is added, the amount of Cu should be １ ／ or more of the Cu content. Of Ni is desirably contained at the same time.

Cr: If the Cr content is less than 0.05%, the effect of addition is poor. On the other hand, if the content exceeds 1.00%, the toughness and weldability are significantly deteriorated. Therefore,
When Cr is added to increase the strength, the content is set to 0.1.
05 to 1.00%. In order to suppress deterioration of toughness and weldability, the Cr content is desirably 0.05 to 0.60%.

Mo: The chemical composition of the tapered steel sheet according to the invention of (1) and the invention of (2) is as follows:
Mo of 0.05 to 1.00% may be contained. Mo
If the content is less than 0.05%, the effect of addition is poor, while if the content exceeds 1.00%, the toughness and weldability are significantly deteriorated. In addition, in order to further improve the strength-toughness balance, the Mo content is 0.05 to
It is desirable to set it to 0.25%. The chemical composition of the tapered steel sheet according to the invention of (3) has a chemical composition of 0.05 to 1.00% in order to greatly lower the phase transformation start temperature at the time of cooling, increase the strength, and lower the acoustic anisotropy. Mo to 0.0
It must be contained in a complex with 005 to 0.0030% B. If the content of Mo is less than 0.05%, the effect of addition is poor, while if the content exceeds 1.00%, the toughness and weldability are significantly deteriorated. In addition, in order to further improve the strength-toughness balance, the Mo content is 0.05 to
It is desirable to set it to 0.25%.

B: The chemical composition of the tapered steel sheet according to the invention of (1) and the invention of (2) is as follows:
0.0005 to 0.0030% of B may be contained. If the content of B is less than 0.0005%, the effect of addition is poor. On the other hand, if the content of B exceeds 0.0030%, the toughness and weldability deteriorate significantly. In order to further improve the strength-toughness balance, the B content is desirably 0.0005 to 0.0015.
In the chemical composition of the tapered steel sheet according to the invention (3), 0.0005 to 0.005 to significantly lower the phase transformation start temperature at the time of cooling, increase the strength and lower the acoustic anisotropy.
It is necessary to contain 0.0030% of B in combination with 0.05 to 1.00% of Mo. B content 0.000
If the content is less than 5%, the effect of addition is poor, while if the content exceeds 0.0030%, the toughness and weldability deteriorate significantly. In order to further improve the strength-toughness balance, the B content is desirably 0.0005 to 0.0015%.

Nb: If the Nb content is less than 0.005%, the effect of addition is poor. On the other hand, the content is 0.014%
Is exceeded, the transverse wave speed ratio is often out of the range of 0.98 to 1.02, and it becomes difficult to diagnose welding defects by ultrasonic waves. Therefore, the content when Nb is added to increase the strength is set to 0.005 to 0.014%.

The chemical composition of the tapered steel sheet according to the invention (3) includes 0.05 to 1.00% of Mo and 0.00%.
Since 0.5 to 0.0030% of B is contained in a complex form, the phase transformation onset temperature at the time of cooling is greatly reduced and the acoustic anisotropy is reduced, so that the Nb content is 0.045% or less. For example, the transverse wave speed ratio can be in the range of 0.98 to 1.02. Therefore, in the case of the invention of (3),
To increase the strength, 0.005 to 0.045% Nb
May be contained.

V: If the V content is less than 0.005%, the effect of addition is poor. On the other hand, if the content exceeds 0.100%, the toughness and weldability are significantly deteriorated. Therefore,
The content when V is added to increase the strength is 0.0
05 to 0.100%. In addition, in order to further improve the strength-toughness balance, the V content is 0.005 to 0.5.
060% is desirable.

Ti: If the content of Ti is less than 0.005%, the effect of addition is poor. On the other hand, the content is 0.050%
If it exceeds, the toughness deteriorates remarkably. Therefore, the content when Ti is added to increase the strength is 0.00
5 to 0.050%. In addition, when the above amount of Ti is contained, the effect that the austenite grains become fine and the toughness is increased can be obtained at the same time. In order to further improve the strength-toughness balance, the content of Ti is set to 0.005 to 0.005.
The content is preferably set to 0.030%.

In the present invention, the contents of P and S as impurity elements are limited as follows.

P: P reduces toughness. In particular, when the content exceeds 0.03%, the toughness is significantly reduced.
Therefore, the content of P is set to 0.03% or less. The P content is 0.02% in order to further improve the toughness.
It is preferable to set the following.

S: S reduces toughness. In particular, when the content exceeds 0.015%, the toughness is significantly deteriorated. Therefore, the content of S is set to 0.015% or less. In order to further improve the toughness, the S content is set to 0.1.
005% or less is preferable.

[0053] P _cm: P _cm is an index representing the weld crack susceptibility, the value of P _cm represented by the formula, B content is 0.
If it is less than 0003%, it is 0.21 or less, and if the B content is 0.0003% or more, it is 0.19 or less.
Neither of them requires preheating at the time of welding even in an environment of a temperature of 25 ° C. Therefore, the upper limit of P _cm is set to 0.21 when the B content is less than 0.0003%, and is set to 0.19 when the B content is 0.0003% or more. on the other hand,
In order to secure a desired TS of 490 MPa or more, B
Irrespective of the content of, the value of P _cm needs to be 0.10 or more. In order to ensure reliably and stably the above TS 490 MPa is preferably set to a value of P _cm 0.15 or more.

Hv _20-50 : Hv _20-50 is an index which is a measure of the TS difference between the thin portion and the thick portion of the tapered steel plate. When the value of Hv _20-50 represented by the above equation is reduced to 15 or less. Thickness 2
The difference between the 0 mm TS and the 50 mm thick TS is 50M
Pa or less, and no bending or warping occurs even when the steel sheet is cut. Therefore, Hv
The value of _20-50 was set to 15 or less. In order to make the difference between the TS having a thickness of 20 mm and the TS having a thickness of 50 mm 30 MPa or less, it is desirable to set the value of Hv _20-50 to 10 or less. 50mm thick T
In order to make the difference from S 15 MPa or less, Hv _20-50
Is desirably set to 0 or less. The smaller the value of this Hv _20-50 (the larger the negative value and the larger its absolute value), the TS of the 20 mm thick part and the 50 m thick
The difference from the TS in the m section can be reduced. (B) Manufacturing conditions of tapered steel plate (B-1) Heating temperature before hot rolling In order to obtain a homogeneous structure, the heating temperature is preferably set to 950 ° C or higher. When the heating temperature is 1000 ° C. or higher, a more homogeneous structure can be obtained. However, the heating temperature is 1300 ° C
Exceeding the fuel cost increases. Further, the scale generation is increased, the yield is reduced, and the production efficiency is reduced.
Further, there are cases where the crystal grains become coarse and good toughness cannot be obtained. Therefore, the heating temperature of the steel slab having the chemical composition described in (A) before hot rolling is 950 to 1300.
℃ is good. In order to obtain a good structure and toughness, the heating temperature is desirably 1000 to 1300 ° C, and more desirably 1000 to 1200 ° C.

(B-2) Hot Rolling Finish Temperature In order to secure a TS of 490 MPa or more by air cooling to a room temperature after hot rolling, the hot rolling finish temperature is preferably 950 ° C. or less. In order to reliably and stably secure TS of 490 MPa or more, the hot rolling finish temperature is more preferably 850 ° C. or less. However, the hot rolling finish temperature is 70
When the temperature is lower than 0 ° C., it is difficult to secure an excellent acoustic isotropy with a transverse sound velocity ratio of 0.98 to 1.02, so that it may be difficult to diagnose welding defects by ultrasonic waves. In order to reliably and stably secure an excellent acoustic isotropy with a transverse sound speed ratio of 0.98 to 1.02, the lower limit of the hot rolling finishing temperature is desirably 750 ° C. Therefore, the hot rolling finish temperature is preferably 950 ° C to 700 ° C, and more preferably 850 to 750 ° C.

In order to provide a good strength-toughness balance, it is desirable to set the cumulative draft at 900 ° C. or less to 30% or more during hot rolling.

(B-3) Cooling Method After Rolling In order to suppress the variation in TS in the steel sheet to 50 MPa or less and to prevent the occurrence of bending or warpage when the steel sheet is cut, hot rolling is performed. It is preferable to perform air cooling to room temperature without performing accelerated cooling after completion.

The method of manufacturing a tapered steel sheet according to the present invention may be applied to a single-tapered steel sheet in which the thickness monotonically increases from one end to the other end of the steel sheet, or a steel sheet in which the thickness at the center in the longitudinal direction is at both ends in the longitudinal direction. The present invention can be applied to the production of tapered steel plates of various shapes such as a tapered steel plate having an angle or trapezoid larger than the thickness.

Further, the tapered steel sheet according to the present invention can be used not only for bridges but also for buildings, tanks and other uses.

Hereinafter, the present invention will be described in detail with reference to examples.

[0061]

EXAMPLE A steel having the chemical composition shown in Tables 1 and 2 was prepared as follows.
An 80 kg vacuum furnace was produced. Steel 1 in Table 1 and Table 2
14 is a steel of the present invention example whose chemical composition is within the range specified by the present invention, and steels 15 to 18 in Table 2 are steels of comparative examples in which any of the components is out of the range of the content specified by the present invention. is there.

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[Table 1]

[Table 2] The steels are then subjected to normal hot forging to a thickness of 18
After making the slab of 0 mm, the steel 5, steel 6, and steel 12 were 960 ° C.
, And the other steels were heated to 1150 ° C. and then hot-rolled. Rolled shape is a single taper steel plate whose thickness changes linearly in the length direction, and the thickness at the thin end is 10m
m, the plate thickness at the thick end was set to 60 mm. After the completion of the hot rolling, it was air-cooled to room temperature. In addition, after hot rolling, the presence or absence of a surface crack was investigated. Tables 3 and 4 show the details of the cumulative rolling reduction at 900 ° C. or less and the rolling finishing temperature.

After air cooling, the transverse sound velocity ratio at the center of the plate width at positions where the plate thickness was 20 mm and 50 mm, respectively, was examined in accordance with JIS Z 3060. Also, from the center of the sheet width at the above position, in the sheet width direction, that is, in the direction perpendicular to the rolling direction, JIS
A No. 4 tensile test piece and a JIS No. 4 Charpy test piece were collected, and the mechanical properties of the base material were investigated.

Further, using the central portion in the longitudinal direction of each steel plate,
An oblique y-type weld crack test based on JIS Z 3158 was performed to evaluate weld crack susceptibility. In addition, all of the welding crack tests were performed using a low hydrogen type welding material (trade name: L53 (manufactured by Sumikin Welding Industry Co., Ltd.)) for a 500 MPa class steel in an atmosphere at a temperature of 25 ° C. and a humidity of 60% at an initial temperature of the specimen. 25 ° C
It carried out on condition of.

Tables 3 and 4 also show the results of various tests.

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[Table 3]

[Table 4] Test Nos. 1 to 14 in Tables 3 and 4 are tapered steel sheets manufactured under the conditions specified in the present invention from the steels of the examples of the present invention whose chemical composition is within the range specified in the present invention. In each case, no surface cracks were observed after hot rolling, and TS was 490.
MPa or more, TS difference between the thin portion and the thick portion in the following tensile properties 50 MPa, the _V T _S 0 ° C. or less, the shear wave velocity ratio 0.9
8 to 1.02, and no crack has occurred in the y-type crack test.

On the other hand, test numbers 15 to 18 in Table 4
Is a taper steel plate manufactured by manufacturing a steel of a comparative example in which any of the components is out of the range specified by the present invention under the conditions specified by the present invention.

Of the test No. 15, the Hv _20-50 value of Steel 15 as the test steel was out of the range specified in the present invention. Although the weldability satisfies the target, the TS difference between the thin portion and the thick portion exceeds 50 MPa, and the TS of the thick portion does not reach 490 MPa.

Test No. 16 indicates the P of steel 16 as the test steel.
_Since the value of _cm is out of the range specified in the present invention, the hot workability, tensile properties, impact properties, and acoustic anisotropy satisfy the targets, but cracks occur in the y-type crack test. ing.

Test No. 17 shows the C of steel 17 as the test steel.
Since the u content is out of the range specified in the present invention,
Although the target was satisfied with respect to tensile properties, impact properties, acoustic anisotropy, and weldability, surface cracks occurred during hot working.

Test No. 18 indicates the N of steel 18 as the test steel.
b content is out of the range specified in the present invention,
Although the hot workability, tensile properties, impact properties, and weldability satisfies the targets, the transverse wave velocity ratio in the thin portion is 1.03 and the acoustic anisotropy is large.

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Tapered steel sheet of the present invention exhibits surface cracking does not occur during hot working, excellent balance of strength and toughness have more TS and 0 ℃ less _V T _S 490 MPa, furthermore,
Since the dispersion of TS in the steel sheet is 50 MPa or less, no bending or warping occurs when the steel sheet is cut, and the transverse wave velocity ratio is 0.98 to 1.02, so that ultrasonic welding defects can be easily diagnosed. In addition, even in an environment at a temperature of 25 ° C., preheating at the time of welding is not required. Therefore, it can be used for steel structures such as bridges. This tapered steel plate can be manufactured relatively easily by the method of the present invention.

Claims (5)

[Claims] (1) In mass%, C: 0.20% or less, Si:
1.00% or less, Mn: 3.0% or less, Al: 0.00
5 to 0.10%, N: 0.001 to 0.007%, Cu: 0.05 to 0.60%, Ni:
0.05-1.00%, Cr: 0.05-1.00%,
Mo: 0.05 to 1.00%, Nb: 0.005 to 0.
014%, V: 0.005 to 0.100%, Ti: 0.
005 to 0.050% and B: 0.0005 to 0.00
One or more of the above 30%, with the balance being Fe
And P in the impurities is 0.03% or less,
S is 0.015% or less, and P is represented by the following formula.
The value of _cm is 0.10 to 0.21 when the content of B is less than 0.0003%, and 0.10 to 0.19 when the content of B is 0.0003% or more, Further, a tapered steel plate having a taper amount of 10 mm or more and a value of Hv _20-50 represented by the following formula of 15 or less. P _cm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B ... Hv _20-50 = -110 + 460C + 44Si + 39Mn-31Cu-9Ni + 11Cr + 22Mo + 180V + 9600B-23000Mo.times.B Here, the taper amount refers to a difference in thickness between the thickest part and the thinnest part of the steel plate. The element symbol indicates the content in mass% of the element. 2. C: 0.05 to 0.20% by mass%, S:
i: 0.05 to 0.55%, Mn: 0.3 to 1.6%,
Al: 0.01 to 0.10%, N: 0.001 to 0.0
And Cu: 0.05-0.60
%, Ni: 0.05 to 1.00%, Cr: 0.05 to
1.00%, Mo: 0.05 to 1.00%, Nb: 0.
005 to 0.014%, V: 0.005 to 0.100
%, Ti: 0.005 to 0.030% and B: 0.00
One or two or more of the following components are contained: Fe and impurities, and P in the impurities is 0.02% or less, S is 0.005% or less, and The value of P _cm represented by the formula is B content of 0.0003%.
If less than 0.21, the content of B is 0.000 or less.
A tapered steel sheet having a taper amount of 10 mm or more, which is 0.19 or less when it is 3% or more, and whose Hv _20-50 value represented by the following formula is 15 or less. P _cm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B ... Hv _20-50 = -110 + 460C + 44Si + 39Mn-31Cu-9Ni + 11Cr + 22Mo + 180V + 9600B-23000Mo × B ... 3. A mass% of not more than 0.20%, Si: 1.0
0% or less, Mn: 3.0% or less, Mo: 0.05-1.
00%, B: 0.0005 to 0.0030% _, Al:
0.005 to 0.10%, N: 0.001 to 0.007
%, And Cu: 0.05 to 0.60%,
Ni: 0.05 to 1.00%, Cr: 0.05 to 1.0
0%, Nb: 0.005 to 0.045%, V: 0.00
5 to 0.100% and Ti: 0.005 to 0.050%
And the balance is composed of Fe and impurities, P in the impurities is 0.03% or less, S is 0.015% or less, and P is represented by the following formula. The value of _cm is 0.10 to 0.19, and the value of Hv _20-50 represented by the following formula is 15 or less.
mm or more tapered steel plate. P _cm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B ... Hv _20-50 = -110 + 460C + 44Si + 39Mn-31Cu-9Ni + 11Cr + 22Mo + 180V + 9600B-23000Mo × B ... 4. A slab having the chemical composition according to claim 1 is heated to a temperature in a temperature range of 950 to 1300 ° C., and hot-rolled so that a hot-rolling finishing temperature is 950 to 700 ° C. And a method for producing a tapered steel sheet having a taper amount of 10 mm or more for air cooling to room temperature after completion of hot rolling. 5. A slab having the chemical composition according to claim 1 is heated to a temperature in a temperature range of 1000 to 1300 ° C., and hot-rolled so that a hot-rolling finishing temperature is 850 to 750 ° C. And a method for producing a tapered steel sheet having a taper amount of 10 mm or more for air cooling to room temperature after completion of hot rolling.