JP2006051534A - Steel plate line-heating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel plate line-heating method capable of obtaining a large angular deformation and consistently obtaining the target angular deformation. <P>SOLUTION: The line heating of a steel plate is performed by selecting T<SB>ms</SB>and T<SB>mr</SB>so as to satisfy conditions and inequalities of T<SB>ms</SB>: 600-1,000°C, T<SB>mr</SB>: 300-600°C, T<SB>ms</SB>-T<SB>mr</SB>≥ 200°C, where T<SB>ms</SB>(°C) is the maximum temperature of a face side of the steel plate reached during the line heating, and T<SB>mr</SB>(°C) is the maximum temperature of a back side of the steel plate, and [YS(T<SB>mr</SB>)-YS(T<SB>ms</SB>)]/(T<SB>ms</SB>-T<SB>mr</SB>)≥ 0.7[N/mm<SP>2</SP>×°C], where YS(T) is the yield point [N/mm<SP>2</SP>] at the temperature T(°C). Thereafter, the surface of the steel plate is cooled so as to satisfy the condition and the inequality of T<SB>fs</SB>: 200-400°C, and T<SB>fr</SB>-T<SB>fs</SB>≥ 50°C, where T<SB>fs</SB>is the temperature of the face side of the steel plate, and T<SB>fr</SB>is the temperature of the back side of the steel plate when cooling is completed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a linear heating method in which a steel plate is linearly heated to be deformed into a predetermined shape.

  When manufacturing a steel plate having a final target shape by deforming a flat steel plate, a linear heating method is known in which a steel plate is linearly heated using a gas burner or the like to be deformed into a predetermined shape. Compared with breath processing and mechanical processing, the linear heating method does not require expensive processing equipment, and is still widely used today.

  When plastic deformation occurs due to thermal expansion when the steel plate is heated, the steel plate is deformed if it remains even after cooling to room temperature. If the thermal stress caused by the temperature rise is greater than the yield strength of the steel plate at that temperature, plastic deformation occurs. Thermal stress in one part is generated by restraint from other parts, but if the heated part is not restrained from the surroundings, it becomes free thermal expansion and no thermal stress is generated. That is, plastic deformation does not occur. On the other hand, when the constraint from the surroundings is large, the amount of final deformation is increased by introducing a large plastic deformation. In the case of linear heating, as compared with welding, heating is performed relatively slowly by a burner, so that heat is transmitted to a wide area other than the heated portion. That is, as viewed from the heated portion, the constrained portion is also usually accompanied by a significant temperature rise. Therefore, in order to obtain a larger deformation, it is effective to use a steel material in which the surrounding restraint is increased, that is, the yield strength does not become so low even if the temperature rises.

  In the linear heating method described in Patent Document 1, a steel sheet is used in which the decrease in yield strength accompanying temperature rise is small in the temperature range from room temperature to 600 ° C. This is because even if the temperature rises, the yield strength does not decrease so much, and even if the temperature of the restraining portion rises, the maximum temperature portion can be restrained sufficiently and a large deformation can be obtained. Further, in Patent Document 1, the steel plate maximum temperature on the heating side is set to 600 ° C. or more, the temperature gradient inside the steel plate is increased, and the temperature gradient is increased by water cooling to introduce plastic strain, and the surface temperature is 200 ° C. Stop water cooling at the following. Thereby, the angular deformation that occurs when the steel plate is linearly heated can be increased. In order to obtain a steel sheet with a low decrease in yield strength due to a temperature rise, it is a component containing Nb and Mo, the heating temperature during hot rolling is set to 1100 ° C. or more, and a sufficient amount of Nb and Mo is dissolved to secure the wire. Precipitation strengthening in the heat history of the shape heating is made possible, and the lower limit temperature of rolling is set to 850 ° C. to suppress precipitation during rolling.

JP-A-8-103824

  By using the method described in Patent Document 1, it becomes possible to increase angular deformation in linear heating. However, the amount of angular deformation due to linear heating is not constant, and in some cases, a sufficient amount of deformation cannot be obtained, and it has been difficult to stably shorten the working time.

  An object of this invention is to provide the linear heating method of the steel plate which can obtain the target amount of angular deformation stably while obtaining a large amount of angular deformation.

  In the linear heating method described in Patent Document 1, a steel plate having a small decrease in yield strength due to temperature rise in a temperature range from room temperature to 600 ° C. is used, and the steel plate surface temperature after heating and cooling is within a predetermined temperature range. The amount of bending deformation was increased by controlling to. However, it has been found that a stable amount of angular deformation cannot be obtained only by defining the steel sheet surface temperature range after heating and cooling. This is because the high-temperature strength of the steel plate varies depending on the material of the steel plate, and the temperature difference between the front and back surfaces may not be ensured sufficiently.

  On the other hand, it is understood that a stable angular deformation amount can always be obtained only when the yield strength temperature gradient between the front and back surfaces after heating is set to a predetermined value or more and the temperature difference between the front and back surfaces of the steel sheet is specified. It was.

The present invention has been made based on the above findings, that is, the gist of the present invention is as follows.
(1) When the maximum reached temperature on the steel sheet surface during linear heating is T _ms (° C) and the maximum reached temperature on the back surface of the steel plate is T _mr (° C),
T _ms : 600 to 1000 ° C., T _mr : 300 to 600 ° C., T _ms −T _mr ≧ 200 ° C.
When the yield stress [N / mm ² ] at temperature T (° C.) is YS (T),
[YS (T _mr ) −YS (T _ms )] / (T _ms −T _mr ) ≧ 0.7 [N / mm ² · ° C.]
Become as T _ms, select the T _mr performs linear heating, then the steel sheet surface is cooled, when the cooling at the end of the steel plate surface temperature T _fs, the steel plate back surface temperature was T _fr,
T _fs : 200 to 400 ° C., T _fr −T _fs ≧ 50 ° C.
A method of linearly heating a steel sheet, wherein cooling is performed so that
(2) As a steel sheet, in mass%, C: 0.05 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.6 to 2.0%, P ≦ 0.025%, S ≦ 0.010%, Al: 0.005-0.10%, N: 0.0010-0.0080%, Nb: 0.003-0.050%, Mo: 0.05-0. 50%, V: 0.005 to 0.10%, W: 0.05 to 0.50%, Ta: 0.05 to 0.50%, and the balance is Fe and inevitable impurities A method for linearly heating a steel sheet according to (1) above, wherein a steel sheet comprising:
(3) The steel sheet is further mass%, Cu: 0.05 to 1.5%, Ni: 0.05 to 3.5%, Cr: 0.05 to 1.0%, Ti: 0.005. 0.10%, B: 0.0002 to 0.0030%, Ca: 0.0003 to 0.0050%, REM: 0.0005 to 0.0060%, or one or more types The method for linearly heating a steel sheet according to (2) above.

  In the linear heating method of the steel sheet of the present invention, the yield strength temperature gradient between the front and back surfaces of the steel sheet during heating is set to a predetermined value or more, and the temperature difference between the front and back surfaces after cooling is set to a predetermined value or more. Therefore, it is possible to always obtain a large amount of angular deformation, and to stably obtain the target amount of angular deformation.

  In the present invention, the surface side of the steel sheet refers to the side that is heated by linear heating. Moreover, the back surface side of a steel plate means the opposite side to the said surface side. As shown in FIG. 1, heating is performed from the surface side of the steel plate 1 with a burner 2 or the like, and the heating position is sequentially moved in the heating direction 4 to be heated linearly. After the heating by the burner 2, cooling is performed by supplying cooling water to the heating part using the cooling water nozzle 3. The cooling water nozzle 3 also moves sequentially so as to keep a constant distance between the burner 2 and the cooling water nozzle 3. As a result, the steel plate undergoes angular deformation as shown in FIG. Δ shown in FIG. 2 is an angular deformation amount.

  When a steel sheet is heated in the linear heating method, compressive plastic strain occurs on the surface side of the steel sheet that has reached the maximum temperature, and angular deformation may occur due to tensile stress generated when the surface side is contracted by subsequent water cooling. it can.

  In order to increase the amount of angular deformation, first, it is necessary to increase the compression plastic strain during heating as much as possible. For this purpose, it is effective to increase the yield strength difference between the front and back surfaces of the steel sheet during heating. By keeping the yield strength on the back side during heating high, the front side can be effectively restrained. At the same time, by reducing the yield strength on the front surface side during heating, compressive plastic strain can be generated on the front surface side in combination with the restraint on the back surface side.

  To increase the amount of angular deformation, secondly, it is necessary to reverse the temperature distribution on the front and back surfaces during cooling after heating to increase the tensile stress on the front surface side.

  By having the above first and second requirements, it is possible to stably secure a large amount of angular deformation in linear heating. Hereinafter, the description will be made sequentially.

  First, the point of increasing the yield strength difference between the front and back surfaces of the steel sheet during heating, which is the first point, will be described in detail.

  In the present invention, as means for increasing the yield strength difference between the front and back surfaces of the steel sheet during heating, the temperature gradient between the front and back surfaces of the steel sheet during heating is increased and the temperature difference between the front and back surfaces of the steel sheet is set to a predetermined value. Secure. Thereby, as a result, the yield strength difference between the front and back surfaces of the steel sheet during heating can be increased.

The maximum reached temperature on the steel plate surface during linear heating is T _ms (° C.), and the highest reached temperature on the back surface of the steel plate is T _mr (° C.). Specifically, in the present invention, T _ms : 600 to 1000 ° C., T _mr : 300 to 600 ° C., T _ms −T _mr ≧ 200 ° C., and at the same time, yield stress at temperature T (° C.) [N / mm ² ] Is YS (T),
[YS (T _mr ) −YS (T _ms )] / (T _ms −T _mr ) ≧ 0.7 [N / mm ² · ° C.]
T _ms and T _mr are selected so that The left side of the above formula is hereinafter referred to as “heating front / back surface yield strength temperature gradient G”.

FIG. 3 shows the results of angular deformation δ when T _{ms is} 600 to 1000 ° C., T _{mr is} 300 to 600 ° C., T _ms −T _mr ≧ 200 ° C., and the front and back surface yield strength temperature gradient G during heating is variously changed. Shown in As apparent from FIG. 3, the value of the angular deformation amount δ is increased by setting the heating front and back surface yield strength gradient G to 0.7 N / mm ² · ° C. or more. That is, in the present invention, when the heating front / back surface yield strength gradient G is set to 0.7 N / mm ² · ° C. or higher, and the front / back surface temperature difference during heating is set to 200 ° C. or higher, a large yield is generated between the front and rear surfaces. An intensity difference can be produced. As a result, when the surface side of the steel sheet is about to thermally expand, a sufficient restraining force is generated on the back side, so that the surface side cannot sufficiently expand, and compressive plastic strain is generated on the surface side.

When the steel sheet surface maximum temperature _Tms is less than 600 ° C., the steel sheet has sufficient rigidity, and therefore hardly deforms. In order to set the front and back surface yield strength temperature gradient G during heating to 0.7 or more, it is necessary to set _Tms to 600 ° C. or more. On the other hand, if T _ms exceeds 1000 ° C., excessive baking occurs on the surface side by subsequent water cooling, and ductility and toughness deteriorate. Therefore, in the present invention, the steel sheet surface maximum temperature T _{ms is set} to 600 to 1000 ° C.

On the other hand, when the maximum temperature T _{mr at the} rear surface of the steel sheet is less than 300 ° C., the angular deformation hardly occurs. On the other hand, if T _mr exceeds 600 ° C., the temperature distribution on the front and back surfaces becomes too uniform, and it becomes difficult to ensure a temperature difference of 200 ° C. or more on the front and back surfaces. In order to set the front and back surface yield strength temperature gradient G during heating to 0.7 or more, it is necessary to set T _mr to 600 ° C. or less. Therefore, in the present invention, the maximum temperature T _mr on the rear surface of the steel sheet is set to 300 to 600 ° C.

In the linear heating method of the present invention, in order to make the front and back surface yield strength temperature gradient G during heating to be 0.7 or more, it is important to first select a steel material having a large temperature dependence of yield strength. Specifically, a steel material having a high yield strength is selected in a temperature range of 300 to 600 ° C. that is a temperature range of T _mr . Suitable steel components having such characteristics will be described later.

Next, data is collected on the temperature dependence of the yield strength of the steel used. Based on the collected data, T _ms : 600 to 1000 ° C., T _mr : 300 to 600 ° C. and T _ms −T _mr ≧ 200 ° C.
[YS (T _mr ) −YS (T _ms )] / (T _ms −T _mr ) ≧ 0.7 [N / mm ² · ° C.]
T _ms and T _mr are selected such that

By using the steel material selected as described above and performing linear heating so as to realize the selected T _ms and T _mr , it becomes possible to generate sufficient compressive plastic strain on the heated steel plate surface.

  Next, the second point, that is, the point of increasing the tensile stress on the front surface side by reversing the temperature distribution on the front and back surfaces during cooling after heating will be described in detail.

In the present invention, the steel plate surface temperature at the end of cooling is T _fs and the steel plate back surface temperature is T _fr . Then, T _fs: 200~400 ℃, to cool such that the T _fr -T _fs ≧ 50 ℃. By reversing the temperature distribution on the front and back surfaces so that T _fr −T _fs ≧ 50 ° C., the tensile stress on the front side can be sufficiently increased, and the angular deformation can be stably increased to a large value. Become.

The _{reason why} T _{fs is set} to 200 ° C. or more is that if it is less than 200 ° C., not only the surface side but also the temperature on the back side is lowered, and a sufficient amount of angular deformation cannot be obtained. The reason why T _{fs is set} to 400 ° C. or less is that when the temperature exceeds 400 ° C., the reversal of the temperature distribution is insufficient, and the amount of angular deformation becomes small. In addition, it is also an important point in achieving T _fr −T _fs ≧ 50 ° C. that the maximum temperature T _mr at the rear surface of the steel plate during heating is 300 ° C. or higher.

In the linear heating method of the present invention, T _fr −T _fs ≧ 50 ° C. can be realized by cooling the surface of the steel sheet with water after completion of heating and ending water cooling at T _fs : 200 to 400 ° C.

  Next, the component composition suitable as a steel plate used in the linear heating method of the present invention and having a high temperature dependence of yield strength will be described.

  C is added for bainite or martensite organization and securing strength, and the lower limit is set to 0.05% from the limit of the effect, and adverse effects on the base metal toughness, deterioration of weldability, high carbon island martensite In order to prevent deterioration of the toughness of welded joints due to the generation of 0.20%, the upper limit was made.

  Si is an element necessary for deoxidation and is an element effective for further increasing the strength, so 0.05% is set as the lower limit, and 1.0% is added to prevent deterioration of weldability and weld joint toughness. The upper limit.

  Mn is added to the bainite or martensite structure and 0.6% as a lower limit to ensure strength and toughness, and a large amount of addition increases hardenability and produces a hardened structure, and also deteriorates weldability. The upper limit is 2.0%.

  Al is an element necessary for deoxidation, so 0.005% is set as the lower limit, and addition of a large amount impairs the cleanliness of the steel, so 0.10% was set as the upper limit.

  N binds to Al, refines the crystal grains of the steel material, and lowers 0.0010%, which is effective for increasing the toughness. If added in a large amount, the toughness of the steel material is impaired, so 0.0080% was made the upper limit.

  Impurities P and S are respectively set to P ≦ 0.025% and S ≦ 0.010% in order to maintain the base metal and weld joint toughness at desired levels, respectively.

  Furthermore, in addition to the above elements, one or more of Nb, Mo, V, W, Ta are contained, and by effectively utilizing the solid solution strengthening by these elements, the yield strength is high in the temperature range of 300 to 600 ° C., A steel sheet having high temperature dependency of yield strength can be obtained. The reason for the addition of each additive element will be described.

  Nb is an element effective for increasing the yield strength at room temperature and medium temperature by bainite or martensite organization, structure refinement, and solid solution strengthening, so 0.003% is the lower limit, and a large amount of addition is a welded joint. Since the toughness is impaired, the upper limit was made 0.050%.

  Mo, V, W, and Ta are elements effective for increasing the yield strength at room temperature and medium temperature by bainite or martensite organization and solid solution strengthening, so about 0.05% for Mo, W, and Ta, For V, 0.005% is set as the lower limit, and addition of a large amount impairs weldability and weld joint toughness, so the upper limit is set to 0.50% for Mo, W, and Ta, and V is set to 0.10%. .

  The present invention may further contain one or more of Cu, Ni, Cr, Ti, B, Ca, and REM in addition to the above elements. The reason for the addition of each additive element will be described.

  Since Cu and Cr are effective elements for increasing the strength together with bainite or martensite organization, 0.05% is set as the lower limit, and a large amount of addition impairs the weld joint toughness. For% and Cr, the upper limit was 1.0%.

  Ni is an element effective for bainite or martensite organization without impairing toughness, but is an expensive element, so it is added in the range of 0.05 to 3.5% from the viewpoint of economy. .

  Ti is an element effective for securing the toughness of the weld heat affected zone, so 0.005% is set as the lower limit, and 0.10% is set as the upper limit in order to prevent toughness deterioration due to excessive addition.

  B is an element effective for improving the strength of steel materials together with bainite or martensite organization and for refining the crystal grains in the weld heat-affected zone, but excessive addition deteriorates toughness, so 0.0002 to 0.0030% It was limited to the range.

  Ca is an effective element for controlling the form of sulfides, but addition of a large amount impairs the cleanliness of the steel, so it was limited to the range of 0.0003 to 0.0050%. REM is an element effective for refining the structure of the heat affected zone and enhancing toughness, but adding a large amount impairs the cleanliness of the steel, so it is limited to a range of 0.0005 to 0.0060%. did.

By performing linear heating using a steel sheet containing the above components, a high yield strength can be realized particularly in the temperature range of 300 to 600 ° C., so that the front and back surface yield strength temperature gradient G during heating is It is possible to find a combination of T _ms and T _mr that is 0.7 or more. As a result, the yield strength difference between the front and back surfaces can be increased.

  A steel plate was produced from a steel piece having the components shown in Table 1, and the linear heating method of the present invention was applied in the manner shown in FIG. The calculation method of the angular deformation amount δ is as shown in FIG. Table 2 shows the plate thickness, mechanical properties, linear heating conditions, amount of angular deformation, and toughness of the heated portion of the steel plate base material. No. 1-8 are examples of the present invention, No.1. 9 to 16 are comparative examples. Table 3 shows the outline of the linear heating test.

As is apparent from Table 2, Example No. In Nos. 1 to 8, since the components and the strength at the temperature during linear heating satisfy predetermined conditions, the amount of angular deformation due to linear heating is as extremely large as 3 × 10 ⁻² radian or more.

  On the other hand, Comparative Example No. In Nos. 9 to 16, the amount of angular deformation is small because any of the steel plate components, medium / high temperature strength, and heating conditions deviates from the predetermined range.

Comparative Example No. In No. 9, since the maximum surface temperature _Tms during heating was lower than the lower limit of 600 ° C. of the present invention, the steel sheet had sufficient rigidity and could not give sufficient compressive plastic strain. No. At the same time, the maximum back surface temperature T _mr at the time of heating was low, and the steel plate back surface temperature T _fr at the end of cooling was also low, so that the temperature difference between the front and back surfaces was insufficient. For this reason, the amount of angular deformation was small.

Comparative Example No. For Nos. 10 and 12, the selection of T _ms and T _mr was not appropriate, so the heating front and back surface yield strength temperature gradient G was less than 0.7, and the amount of angular deformation was small. Comparative Example No. No. 11 had a small amount of angular deformation because the difference between T _ms and T _mr was less than 200 ° C.

Comparative Example No. No. 13 shows that the steel plate surface temperature _Tfs at the end of cooling is too low. _No. 14 had too high T _fs , and as a result, the temperature difference between the front and back surfaces was less than 50 ° C., and the amount of angular deformation was small.

Comparative Example No. _No. 15 had both T _ms and T _mr too high. As a result, the heating front and back surface yield strength temperature gradient G was less than 0.7, and the amount of angular deformation was small.

No. No. 16 has low C and low Mn as the component composition of the steel sheet, so that the yield strength at medium and high temperatures cannot be sufficiently increased. For this reason, the front and back surface yield strength temperature gradient G during heating is set to 0.7 or more. The combination of T _ms and T _mr that could be found was not found, and the amount of angular deformation was small.

It is a figure which shows the point of the linear heating method. It is a figure which shows the calculation method of angular deformation amount (delta). It is a figure which shows the relationship between the front and back surface yield strength temperature gradient at the time of heating, and the amount of angular deformation δ.

Explanation of symbols

1 Steel plate 2 Burner 3 Cooling water nozzle 4 Heating direction

Claims (3)

When the maximum temperature reached on the steel sheet surface during linear heating is T _ms (° C) and the maximum temperature reached on the back surface of the steel sheet is T _mr (° C),
T _ms : 600 to 1000 ° C., T _mr : 300 to 600 ° C., T _ms −T _mr ≧ 200 ° C.
When the yield stress [N / mm ² ] at temperature T (° C.) is YS (T),
[YS (T _mr ) −YS (T _ms )] / (T _ms −T _mr ) ≧ 0.7 [N / mm ² · ° C.]
Become as T _ms, select the T _mr performs linear heating, then the steel sheet surface is cooled, when the cooling at the end of the steel plate surface temperature T _fs, the steel plate back surface temperature was T _fr,
T _fs : 200 to 400 ° C., T _fr −T _fs ≧ 50 ° C.
A method of linearly heating a steel sheet, wherein cooling is performed so that As the steel sheet,
C: 0.05-0.20%, Si: 0.05-1.0%, Mn: 0.6-2.0%, P ≦ 0.025%, S ≦ 0.010%, Al: 0 0.005 to 0.10%, N: 0.0010 to 0.0080%,
Nb: 0.003 to 0.050%, Mo: 0.05 to 0.50%, V: 0.005 to 0.10%, W: 0.05 to 0.50%, Ta: 0.05 to The steel sheet linear heating method according to claim 1, wherein the steel sheet contains at least one of 0.50% and the balance is Fe and inevitable impurities. The steel sheet is further in mass%,
Cu: 0.05-1.5%, Ni: 0.05-3.5%, Cr: 0.05-1.0%, Ti: 0.005-0.10%, B: 0.0002- The steel wire according to claim 2, comprising one or more of 0.0030%, Ca: 0.0003 to 0.0050%, and REM: 0.0005 to 0.0060%. Heating method.

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