JP3954607B2 - Steel plate with less weld buckling deformation and its manufacturing method - Google Patents

Description

  The present invention is a steel sheet mainly used for a hull structure and the like, and is relatively thin and has little buckling deformation at the time of welding, and does not require correction after welding construction and is high. It is related with the steel plate which can obtain construction workability, and its manufacturing method.

  When constructing the outer shell part constituting the upper part of the hull structure, it is common to weld and fix a reinforcing rib to the steel plate in order to increase the structural strength. At that time, the welded joint and weld heat affected zone melt or undergo structural transformation (reverse transformation from α to γ) by the heat during welding, and then heat shrink when the temperature is lowered (cooled) to normal temperature and solidified. Wake up. However, when the four perimeters are constrained by the reinforcing ribs, they cannot be freely contracted, so that a tensile residual stress corresponding to the yield stress of the weld is generated. At that time, the welded structure may cause out-of-plane buckling deformation due to the residual stress, and this deformation is called “skin deformation” in a part of the welding work site and causes a problem.

  By the way, when such buckling deformation occurs in the outer shell of the ship's hull structure, the appearance deteriorates. Conventionally, in order to correct such out-of-plane buckling deformation (skin horse deformation), press correction or spot heating correction Etc. are done. However, the correction work is not only complicated and time-consuming, but also a major cause of extending the work period. Therefore, it is necessary to develop a steel plate that does not cause such out-of-plane buckling deformation as much as possible.

  By the way, for example, Patent Document 1 discloses a welding that becomes a problem when a relatively thick (about 10 mm or more) steel plate is subjected to low heat input welding for structural steel plates used for offshore structures, buildings, bridges, and the like. An improved technique for reducing angular distortion has been disclosed. The present invention is intended to prevent welding deformation by increasing the yield stress of a steel plate affected by welding heat, and specifically, as a technique for increasing the yield stress of a steel plate affected by welding heat. The component composition of the steel material is specified, and at least 30% by area or more of the microstructure of the steel cross section is a bainite structure in which fine carbides are dispersed, and the yield strength is increased to 360 MPa or more, so that welding deformation is likely to occur at 400 ° C. or more. It is intended to increase the yield strength in the middle temperature range and reduce the so-called angular deformation during fillet welding generally employed when constructing a steel structure as described above to ½ level or less.

  Similarly, Patent Document 2 also has a problem of welding angle when a relatively thick (about 10 mm or more) steel plate is fillet welded for structural steel plates used for offshore structures, buildings, bridges, and the like. An improved technique for the purpose of reducing deformation is disclosed. This invention also prevents welding deformation by increasing the yield stress of a steel material affected by welding heat. Specifically, as a method for increasing the yield stress of a steel sheet affected by welding heat, the component composition of the steel material is specified, and the microstructure is bainite and / or martensite and ferrite and / or pearlite having a small average particle diameter. In addition, the presence of a large amount of fine carbonitride increases the yield strength in the intermediate temperature range where welding deformation occurs, and suppresses angular deformation due to fillet welding.

However, these inventions are intended for relatively thick steel plates as described above, and are intended to suppress angular distortion by increasing the yield stress of the weld heat affected zone. The technical idea is essentially different from the present invention that prevents the “salting horse phenomenon” by suppressing the rise.
JP-A-6-172921 JP 2003-268484 A

  In the present invention, out-of-plane buckling deformation (so-called “lean horse deformation”) intended to improve the distortion is a strain observed when reinforcing ribs are welded to a relatively thin steel plate (usually less than about 10 mm) and strengthened. For example, as shown in FIG. 2, when the reinforcing rib 2 having the same thickness is welded to one side of the steel plate 1 to give structural strength, the joint is melted by welding heat and then cooled. The solidification shrinkage of the steel, and the residual stress generated in the base metal and the heat affected zone of the weld at that time have a complicated effect, and the flat part of the welded structure is thin as shown in FIG. It is a phenomenon that causes buckling deformation like a “back of a horse”.

  The cause of the occurrence of such buckling deformation will be described later, but in the present invention, by controlling the balance between the composition and strength characteristics of the steel sheet, particularly the strength of the base metal and the strength after being affected by heat, An object of the present invention is to provide a steel plate that can suppress the deformation of a thin horse as much as possible, and to provide a manufacturing method that can reliably obtain such a steel plate.

The steel plate with less welded buckling deformation according to the present invention that has solved the above-mentioned problems is that the yield stress of the steel plate is (YP ₀ ), the tensile strength is (TS ₀ ), and the steel plate has a thermal effect during welding. When the simulated yield stress after giving the following thermal history is (YP ₁ ), YP ₀ (base material strength) is 250 MPa or more, TS ₀ is 400 MPa or more, YP ₁ is 400 MPa or less, and It is characterized in that YP ₀ / YP ₁ is 1 or more.

(Heat history provision conditions)
Thermal history pattern:
Thermal history imparting device: A 50 kilowatt thermal cycle reproduction device manufactured by Fuji Radio Engineering Co., Ltd. is used.

  A more preferable first embodiment of the steel material according to the present invention is one in which the steel material satisfies a chemical component and a hardenability index (DI value) indicated as a) or b) below.

B) Chemical composition;
C: 0.005-0.12%,
Si: 0.05 to 0.5%,
Mn: 0.05-1.2% included,
Balance: Fe and inevitable impurities,
DI = 1.16 × [√ (C / 10)] × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1) x (3.0 x Mo + 1) x (1.75 x V + 1) x (200 x B + 1) ≤ 0.38
[The symbols in the formula represent the content (% by mass) of each element],

B) Chemical composition;
C: 0.005-0.12%,
Si: 0.05 to 0.5%,
Mn: 0.05 to 1.2%,
N: other than 0.002 to 0.007%,
Including at least one selected from the group consisting of Nb: 0.005-0.03%, V: 0.005-0.075%, Ti: 0.005-0.03%,
The remainder: Fe and inevitable impurities.
DI = 1.16 × [√ (C / 10)] × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1) x (3.0 x Mo + 1) x (1.75 x V + 1) x (200 x B + 1) ≤ 0.38
[The symbol in the formula represents the content (% by mass) of each element].

  In a more preferred second embodiment, the steel material satisfies the following chemical components and the DI value.

Chemical composition;
C: 0.005-0.12%,
Si: 0.05 to 0.5%,
Mn: 0.05 to 1.2%,
N: Besides satisfying 0.002 to 0.007%,
It contains at least one selected from the group consisting of Nb: 0.005 to 0.03%, V: 0.005 to 0.075%, Ti: 0.005 to 0.03%, and the following formula (I )
Nb / 6.63N + V / 3.64N + Ti / 3.41N> 1 …… (I)
The remainder: Fe and inevitable impurities.

  The steel material of the present invention further includes, as another element, Ca: 0.0005-0.003%, Zr: 0.0005-0.004%, REM: 0.0005-0.005%. It may contain at least one selected, or as other elements, Ni: 0.2% or less, Cu: 0.2% or less, Cr: 0.2% or less, Mo: 0.1 It may contain at least one selected from the group consisting of% or less.

Moreover, the manufacturing method of the steel plate with little welding buckling deformation which concerns on this invention is a method applied when using the steel piece which satisfy | fills the requirements described as the said 1st embodiment, It heated at 950 degreeC or more Later, when rolling to the target plate thickness, the above-mentioned characteristics are given by rolling so that the cumulative reduction ratio in the temperature region below the Ar ₃ transformation point calculated by the following formula is 30% or more,
Ar ₃ (° C.) = 910−310 × C−80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo
[Chemical symbols in the formula represent (mass%) of each element],
Moreover, when using the steel piece which satisfy | fills the requirements described as said 2nd embodiment, after heating to 950 degreeC or more, when rolling to target plate | board thickness, average temperature 850-950 degreeC in a plate | board thickness direction is used. The cumulative reduction ratio in the temperature range is set to 50% or more, and the above-mentioned characteristics are given by rolling to the target plate thickness and finishing the rolling.

  According to the present invention, while maintaining a sufficient structural strength, for example, as a steel plate for a hull structure, the weldability is excellent and out-of-plane buckling deformation (so-called “skinning phenomenon”) associated with welding is possible. Can be suppressed. As a result, straightening treatment after welding can be substantially eliminated, the work efficiency can be greatly increased, and the work period can be significantly shortened. Further, according to the manufacturing method of the present invention, a steel plate having target characteristics can be provided at low cost by using a steel material in which the amount of an expensive alloy element or the like is minimized.

  Under the circumstances as described above, the present inventors pay attention to out-of-plane buckling deformation called “skin horse phenomenon” that occurs during welding, and to develop an effective prevention method of “skin horse phenomenon”. When the generation mechanism was examined, the following was confirmed.

  (1) In welding construction, the part melted during welding and the vicinity thereof (hereinafter sometimes referred to as the vicinity of the weld line) cause thermal contraction when the temperature is lowered to room temperature.

  (2) At the time of the above heat shrinkage, the weld line vicinity is restrained by reinforcing ribs around the four sides, so it cannot be freely shrunk. As a result, a tensile residual stress corresponding to the yield stress at that temperature is generated in the region. Then, in the state where the vicinity of the weld line of the steel sheet is cooled to room temperature, a tensile residual stress corresponding to the yield stress at room temperature in the vicinity of the weld line is generated.

  (3) In addition to these phenomena, compressive residual stress is generated in the part away from the weld line so as to balance with the tensile residual stress generated in the vicinity of the weld line.

  (4) When the compressive residual stress exceeds the critical buckling strength of the steel sheet, out-of-plane buckling deformation, that is, “skin horse phenomenon” occurs.

  Among the above mechanisms, (2) means that “the residual stress generated in the vicinity of the weld line depends on the yield stress level of the part affected by the welding heat”, and thus suppresses out-of-plane buckling deformation. It is considered effective to pay attention to the following points.

That is, it is important that the yield stress of the region portion is as low as possible when the region heated to the Ar ₃ transformation point or more of the steel sheet due to the thermal effect during welding is lowered (cooled) to room temperature. In that case, it is considered that the compressive residual stress generated in a part away from the weld line is also reduced, and accordingly, it is difficult to cause out-of-plane buckling deformation.

By the way, in steel materials, when the amount of additive alloying elements is large, the region heated above the Ar ₃ transformation point by welding heat tends to form hard structures such as bainite and martensite in the subsequent cooling process, and the influence of welding heat The yield stress of the part is often higher than that of the base metal. In this case, according to the mechanism (2), the level of compressive residual stress generated in a portion away from the weld line of the steel plate becomes high, and thus reduction of out-of-plane buckling deformation cannot be realized. Therefore, in order to suppress the increase in the yield stress of the weld heat affected zone in consideration of the mechanism (2), it is necessary to reduce the additive alloy element having a quench hardening effect as much as possible.

  Further, as described above, the out-of-plane buckling deformation is caused by the compressive residual stress generated due to the thermal contraction of the weld heat affected zone, but the buckling deformation includes not only the residual stress at that time but also the steel plate ( It was confirmed that the yield stress of the base metal was also related. That is, when the residual stress of the same level exists, buckling deformation is more likely to occur as the yield stress of the base metal itself is smaller. As a result of studying this point, it was confirmed that if the yield stress of the steel sheet (base material) is made higher than the yield stress of the heat-affected zone of the steel sheet, the thin horse phenomenon can be suppressed as much as possible. .

Furthermore, the strength of the weld heat-affected zone is such that the region heated above the Ac ₃ transformation point (temperature at which α → γ reverse transformation is completed) reaches normal temperature due to heat transfer to the surrounding steel plate and heat dissipation from the steel plate surface to the air. It is determined by the tissue transformation formed when cooled and again undergoes the γ → α transformation, and is determined almost exclusively by chemical components. On the other hand, since the base metal strength fluctuates with changes in rolling conditions other than the chemical components, it was confirmed that the base metal strength and the strength of the weld heat affected zone can be controlled by controlling the chemical components and the rolling conditions. .

  Under such knowledge, in the present invention, as a first essential requirement, the relationship between the yield stress of the steel sheet and the yield stress of the heat-affected zone when the steel sheet is welded is the ratio between them, that is, (the yield stress of the steel sheet). ) / (Yield stress of the heat affected zone when the steel plate is welded) or more, in other words (yield stress of the steel plate) is greater than (yield stress of the heat affected zone when the steel plate is welded) They found out that the thin horse phenomenon can be prevented as much as possible.

Therefore, in the present invention, in order to standardize the yield stress when the yield stress of the steel sheet (base material) is (YP ₀ ) and the thermal stress that simulates the thermal effect during welding is received, The yield stress after giving is defined as (YP ₁ ), and the fact that (YP ₀ / YP ₁ ) is 1 or more is defined as the first essential requirement. A more preferable value (YP ₀ / YP ₁ ) is 1.2 or more.

  However, if the yield stress of the steel plate base material is too low, the strength of the structural steel plate such as a hull structure will be insufficient and the necessary structural strength cannot be secured. The lower limit values were defined as “250 MPa or more” and “400 MPa or more”, respectively.

FIG. 3 shows that the ratio of (yield stress of steel plate base material: YP ₀ ) / (yield stress of weld heat affected zone: YP ₁ ) out of many experimental data including examples to be described later is out-of-plane. FIG. 6 is a graph showing the effects on the amount of bending deformation, with the (YP ₀ / YP ₁ ) ratio being 1.0 as a boundary, and if it is less than that, the out-of-plane buckling deformation amount exceeds 4.0. On the other hand, when the ratio is 1.0 or more, the out-of-plane buckling deformation amount is a low value of 4.0 or less.

  Similarly, FIG. 4 is a graph showing the relationship between the yield stress and the amount of out-of-plane buckling deformation in the weld heat-affected zone from a lot of experimental data. When the yield stress is 400 MPa or less, the out-of-plane buckling deformation amount is suppressed to an allowable range of 4.0 mm or less, whereas when the yield stress exceeds 400 MPa, the out-of-plane buckling deformation amount clearly exceeds 4.0 mm. For this reason as well, it is effective to suppress the yield stress of the weld heat affected zone to 400 MPa or less in order to suppress the thin horse phenomenon.

  Next, in order to find preferable requirements for obtaining the above characteristics, the contained elements that have a considerable influence on the yield stress of the steel plate base metal and the weld heat-affected zone, and the hardenability index (DI) that is a comprehensive indicator of these elements Value).

  As a result, as a preferable first requirement for obtaining the above-mentioned characteristics, the constituent elements are adjusted so that the DI value calculated by the above formula is 0.38 (unit: inch) or less according to the chemical composition of the steel material to be used. It was found that adjusting the content is extremely important.

  The upper limit of these DI values was determined by reducing the hardenability of the steel material itself, and when the welded part and its heat-affected zone were heated to a high temperature and then cooled to near room temperature, the strength was increased by quenching hardening. This is because the yield stress after welding of the welded part and the heat-affected zone, which is the biggest cause of causing the skinnyness phenomenon, is suppressed as low as possible.

  Incidentally, when the DI value, that is, the hardenability index of the steel material exceeds 0.38, the hardenability of the steel material increases, and accordingly, the welded part and the heat-affected part undergo quench hardening in the cooling process after welding. The yield stress of the part increases. Accordingly, as described in the mechanism (2), the tensile residual stress in the vicinity of the weld line increases, and accordingly, the compressive residual stress generated in the part away from the weld line is balanced with the tensile residual stress. Increases and causes the skinnyness phenomenon. Therefore, in order to suppress such a phenomenon, it is essential to suppress the underlying DI value to 0.38 or less. More preferable values of these hardenability indexes are 0.37 or less, more preferably 0.36 or less. However, if the hardenability index of the steel material becomes too low, the strength of the weld heat affected zone becomes insufficient, and the structural Since it becomes difficult to ensure the required strength as steel, the lower limit should be about 0.22 or more, more preferably about 0.24 or more.

  When the steel material contains appropriate amounts of Nb, V, and Ti as precipitation hardening elements, the strength of the base material is increased by precipitation hardening of carbides and carbonitrides of these elements, so the DI value of the steel material is 0.09. However, it is preferably about 0.16 or more.

  Moreover, if the carbon equivalent of the steel material becomes too low, the base material strength becomes insufficient in spite of the treatment for improving the base material strength as described in the manufacturing method described later, and it becomes difficult to secure the necessary strength as structural steel. Therefore, the lower limit should be about 0.15 or more, more preferably about 0.16 or more.

  Next, preferable chemical components of the steel material used in the present invention will be described. The steel plates according to the present invention are classified into two types of steel materials 1 and 2 as described below.

  The preferable chemical components of the steel material 1 used in the present invention are C: 0.005 to 0.12%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.2%, with the balance being Fe and Inevitable impurities, or, in addition to these elements, N: 0.002 to 0.007%, Nb: 0.005 to 0.03%, V: 0.005 to 0.075% , Ti: a steel material containing at least one selected from the group consisting of 0.005 to 0.03%, and the reason why the contents of these components are specified is as follows.

C: 0.005-0.12%
C is the most effective and inexpensive element for securing the structural strength necessary for a steel plate base material, so addition is an indispensable element. Is required. A more preferable C content is 0.01% or more, and further preferably 0.03% or more. On the other hand, in the present invention, as described above, it is necessary to suppress the DI value of the steel material in order to reduce the thinning phenomenon by suppressing the quench hardening characteristics of the weld heat affected zone, and the DI value is affected by C. Because of its large size, the C content is at most 0.12% or less, preferably 0.11% or less, more preferably 0.10% or less.

Si: 0.05-0.5%
Si has a role as a deoxidizer for molten steel and is an element that increases the DI value and contributes to the improvement of the strength of the base metal. Therefore, it is desirable to contain at least about 0.05% or more. Preferably it is 0.10% or more. However, excessive addition increases the quenching hardenability of the weld heat affected zone to increase the residual stress generated in the region, and deteriorates the low temperature toughness of the region, so 0.5% is made the upper limit. Preferably it is 0.4% or less, more preferably 0.3% or less.

Mn: 0.05-1.2%
Mn plays a role of increasing the strength of the base material and increases the DI value to contribute to the improvement of the strength of the base material. Therefore, it is desirable to contain Mn at least 0.05% or more. Preferably it is 0.10% or more, more preferably 0.20% or more. However, excessive addition increases the hardenability of the heat affected zone and increases the residual stress generated in the region, and deteriorates the low temperature toughness of the region. To do. Preferably it is 1.0% or less, more preferably 0.8% or less.

In the case of containing N and one or more of Nb, V and Ti in addition to the above elements,
N: 0.002 to 0.007%, Nb: 0.005 to 0.03%, V: 0.005 to 0.075%, Ti: 0.005 to 0.03% or more ;
In the present invention, N combines with Nb, V, and Ti to form nitrides, and these nitrides effectively act to suppress coarsening of the austenite structure in the weld heat affected zone, thereby reducing the weld heat affected zone. Contributes to improved toughness. In order to effectively exhibit such an action, it is preferable to set N and Nb, V, and Ti within the above ranges.

  The remaining component of the steel material used in the present invention is substantially unavoidably mixed with iron and includes impurities such as Al, P, and S. That is, Al is an element used as a deoxidizer, and is desirably contained in an amount of 0.02% or more in order to sufficiently reduce the amount of dissolved oxygen in the steel and suppress the deterioration of the toughness of the base material. However, excessive inclusion becomes a source of formation of non-metallic inclusions and causes deterioration of the toughness of the base metal and the weld heat affected zone, so 0.05% or less, more preferably 0.04% or less. It is good to suppress.

  Further, P and S are elements that are inevitably mixed in the steel, and serve as inclusions, which adversely affect the base material toughness of the steel sheet and the toughness of the weld heat affected zone. 0.05% or less, more preferably 0.03% or less, still more preferably 0.02% or less, and S is 0.02% or less, more preferably 0.01% or less, still more preferably 0. It is good to keep it at 0.005% or less.

  And DI value of the steel material 1 which satisfies the said component requirements needs to be 0.38 or less by the value calculated by the said Formula.

  Next, a preferable chemical component of the steel material 2 used in the present invention is at least one selected from the N content, and further Nb, V, and Ti, in addition to the content range of C, Si, and Mn defined in the steel material 1. In addition to satisfying the seed content, the relationship between the N content and the content of Nb, V, Ti satisfies “Nb / 6.63N + V / 3.64N + Ti / 3.41N> 1” with the balance being Fe and inevitable impurities Is.

  That is, N forms a nitride with Nb, V, Ti as described above and contributes to the improvement of the toughness of the weld heat affected zone. However, in the steel material 2 according to the second aspect of the present invention, Nb, V, Ti By precipitating as carbide (or carbonitride), it is intended to increase strength by precipitation hardening, and even if they form nitrides in relation to the mass ratio of Nb, V, Ti, It is necessary to satisfy “Nb / 6.63N + V / 3.64N + Ti / 3.41N> 1” as a requirement for exhibiting the precipitation hardening effect while securing the amount of carbide produced.

  As described above, by effectively utilizing the precipitation hardening action as carbides of Nb, V, and Ti, the temperature and the rolling reduction during hot rolling when manufacturing a steel sheet as described in detail later are appropriately controlled. As a result, the yield stress and tensile stress of the steel base metal can be effectively increased by precipitation hardening of carbides (or carbonitrides) of these elements during rolling without increasing the yield stress of the weld heat affected zone. it can.

  Such action of Nb, V, Ti is Nb: 0.005% or more (more preferably 0.008% or more), V: 0.005% or more (more preferably 0.010% or more), or Ti: 0. 0.005% or more (more preferably 0.008% or more) is contained, and this is effectively exhibited when “Nb / 6.63N + V / 3.64N + Ti / 3.41N> 1” is satisfied. However, these elements are expensive and cause the material cost to increase, and if their content is too large, the number of precipitated carbides (or carbonitrides) and the volume fraction will be excessive, and In addition to lowering the low temperature toughness and tensile ductility, there is a problem that precipitates melted by welding heat are easily reprecipitated and the yield stress of the weld heat affected zone is increased, resulting in an increase in deformation of the thin horse. Therefore, Nb is 0.03% or less (more preferably 0.025% or less, more preferably 0.020% or less), and V is 0.075% or less (more preferably 0.060% or less, more preferably Is 0.050% or less), and Ti is 0.030% or less (more preferably 0.025% or less, and still more preferably 0.020% or less).

  And the preferable DI value of the steel material 2 which satisfies the said component requirements needs to be 0.38 or less by the value calculated by the said Formula.

  When the steel material contains appropriate amounts of Nb, V, and Ti as precipitation hardening elements, the base material strength is increased by precipitation hardening of carbides and carbonitrides of these elements, so the lower limit of the DI value of the steel material is 0. .09 or so. However, it is preferably about 0.16 or more.

  The essential constituent elements of the steel materials 1 and 2 that are preferably used in the present invention are as described above, and the balance is Fe and inevitable impurities, but in some cases, as other elements, Ca: 0.0005 to 0.00. At least one selected from the group consisting of 003%, Zr: 0.0005-0.004%, REM: 0.0005-0.005%, or Ni: 0.2% or less, Cu: 0.2 % Or less, Cr: 0.2% or less, Mo: at least one selected from the group consisting of 0.1% or less may be included.

  The above Ca, Zr, and REM have the effect of suppressing the occurrence of internal cracks and cracks from the heat affected zone by spheroidizing A-based inclusions (inclusions that are easy to extend in the rolling direction during rolling) such as MnS. These effects are effectively exerted by each single addition or two or more combined additions. In order to exhibit such an effect effectively, Ca is 0.0003% or more (more preferably 0.0007% or more), Zr is 0.0005% or more (more preferably 0.0010% or more), and REM is 0.00. It is good to make it contain 0005% or more (more preferably 0.0010% or more). However, if the content of these elements is too large, a large amount of oxides of each element (CaO, etc.) are generated, and the base material toughness and tensile ductility are deteriorated. Therefore, Ca is 0.003% or less (more Preferably, 0.0025% or less), Zr is 0.004% or less (more preferably 0.004% or less), and REM is 0.005% or less (more preferably 0.0035% or less). .

  Ni, Cu, Cr, and Mo are all effective elements in that they have the effect of increasing the hardenability and increasing the strength of the base material, and their effects are effective by adding each of them individually or in combination of two or more. To be demonstrated. However, if there are too many of these elements, the hardenability increases too much and the yield stress of the weld heat affected zone increases, resulting in an increase in the amount of deformation of the lean horse and, in addition, the raw material cost increases further and the manufacturing cost increases. Since a problem such as an increase occurs, Ni is 0.2% or less (more preferably 0.1% or less), Cu is 0.2% or less (more preferably 0.1% or less), and Cr is 0.2% or less. It is good to keep 2% or less (more preferably 0.1% or less) and Mo 0.1% or less (more preferably 0.05% or less).

  Next, since the steel materials 1 and 2 with the above specified chemical components are both low in carbon equivalent and low in the strengthening element content, applying the normal steel plate production conditions as they are, the strength sufficient as a steel plate base material is sufficient. Cannot be secured, and the strength of the structural steel is insufficient. Therefore, in order to put this to practical use, it is necessary to devise a technique to ensure the necessary strength as a steel plate for ship hull structure, while at the same time exhibiting low yield stress in the vicinity of the weld line using the steel plate. . Therefore, the manufacturing conditions for that purpose were examined.

The hot-rolled structure of the low carbon / low alloy steel as intended in the present invention is mainly composed of a ferrite phase, and as a means for increasing the strength of the steel sheet having such a ferrite-based structure,
1) Strengthening by refinement of ferrite crystal grains,
2) Strengthening utilizing work hardening of ferrite phase by rolling in temperature range below Ar ₃ transformation point,
3) Solid solution strengthening by adding alloying elements,
4) Strengthening using precipitation strengthening of metal carbide, etc.
Etc. Of these, the methods that can be strengthened without adding alloying elements are the above 1), 2), but in order to carry out 1), they must be rolled at a very large one-pass reduction, which is very large. It is difficult to achieve stably at present, for example, it can be realized only when the capacity of the rolling mill is required or the conditions such as the rolling size (the rolling width is narrow and the rolling thickness is thin) are met. Further, in the strengthening method of 3), only a strengthening of about 30 to 50 MPa can be expected. On the other hand, the strengthening method 2) is a technique that can be realized by strictly controlling the rolling temperature, and the method 4) includes Nb, V, Ti, which are precipitation strengthening elements, and is “Nb / 6.63”. It is applicable to the steel material 2 that satisfies “N + V / 3.64N + Ti / 3.41N ≧ 1”.

Therefore, in the present invention, when using a steel piece that satisfies the component requirements of the steel material ₁ , while ensuring a predetermined base material strength (YP ₀ ), the yield stress (YP ₁ ) of the weld heat affected zone and the base material In order to ensure that the yield stress (YP ₀ ) ratio (YP ₀ / YP ₁ ) is 1 or more, the steel slab is heated to 950 ° C. or higher and then rolled to the target plate thickness. Rolling is performed so that the cumulative rolling reduction in the temperature range below the Ar ₃ transformation point is 30% or more.
Ar ₃ (° C.) = 910−310 × C−80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo
[The chemical symbol in the formula represents the content (% by mass) of each element]

At this time, as the rolling reduction in the temperature range below the Ar ₃ transformation point is increased, the processed ferrite structure increases, and the yield strength of the base material increases accordingly. In particular, the yield strength is greatly increased compared to the tensile strength. On the other hand, if the weld heat affected zone is heated above the Ar ₃ transformation point, it transforms from ferrite (α) to austenite (γ), so the processed ferrite structure that existed before heating is reset, and in the subsequent cooling process It shows the yield stress according to the generated ferrite structure. Since the yield stress is almost determined by the fraction of the second phase (mainly pearlite) and the solid solution strengthened ferrite structure, the yield stress is determined according to the amount of the added alloy element. Therefore, increasing the reduction ratio in the temperature range below A ₃ transformation point at the time of rolling in the production of the steel sheet, the yield stress and tensile stress of the plate matrix by work hardening, in particular the yield stress can be enhanced greatly.

As a result of repeated experiments from this point of view, if the reduction rate in the temperature range below the Ar ₃ transformation point is 30% or more, the yield stress (YP ₀ ) of the steel sheet is 250 MPa or more and the tensile strength is 400 MPa or more. while, it has been found that can secure one or a ratio of the yield stress of the steel plate (YP ₀₎ and weld heat affected zone of the yield stress _{(YP 1) (YP 0 /} YP 1). For this reason, when using a steel slab that satisfies the component requirements of the steel material 1, when the steel slab is heated to 950 ° C. or higher and rolled to the target plate thickness, the temperature range below the Ar ₃ transformation point. Up to 30% or more, more preferably 40% or more.

Incidentally, FIG. 5 shows the influence of the rolling reduction below the Ar ₃ transformation point on the tensile strength (TS ₀ ) of the base metal from various experimental data using the steel material with no carbide forming element added (the steel material 1). This graph is organized and shows that in order to ensure a tensile strength of 400 MPa or higher, it is necessary to ensure 30% or more at a rolling reduction below the Ar ₃ transformation point.

Next, when using a steel piece that satisfies the component requirements of the steel material 2, the yield stress (YP ₁ ) of the weld heat-affected zone and the yield stress of the base material (YP ₀ ) while securing a predetermined base material strength (YP ₀ ) to ensure the least ratio of _{YP 0) (YP 0 / YP} 1), a steel piece, when rolled to a target thickness after heating above 950 ° C., in the temperature range of 850 to 950 ° C. It is necessary to effectively exhibit the action of the precipitation strengthening elements Nb, V, and Ti by finishing the rolling at a cumulative reduction ratio of 50% or more.

  Incidentally, the precipitation temperature range of carbides (or carbonitrides) of Nb, V, and Ti is about 900 ° C. or less, but when left without being rolled, it is not completely precipitated, and precipitation strengthening is effectively utilized. It is necessary to temper after rolling. On the other hand, when rolling is performed immediately above the precipitation temperature range of such carbides, defects such as dislocations introduced by rolling become accumulation sites of precipitate forming elements (Nb, V, Ti) or carbide generation sites, Alternatively, dislocation diffusion [diffusion at a rate of about 10 times or more of normal diffusion (referred to as body diffusion)] promotes the accumulation of precipitate forming elements to promote precipitation of carbides, and without tempering after rolling. It has been found that 70-80% strengthening is possible when tempering is performed.

However, it is not necessary to simply roll just above the precipitation temperature range, while ensuring the above-mentioned base material strength (yield stress; YP ₀ of 250 MPa or more, tensile strength: TS ₀ of 400 MPa or more) intended in the present invention, In order to set (YP ₀ / YP ₁ ) to 1 or more, when the raw steel slab is heated to 950 ° C. or higher and then rolled to the target plate thickness, the cumulative reduction ratio in the temperature range of 850 to 950 ° C. is set. It was found that it should be 50% or more.

By the way, as the rolling reduction ratio just above the precipitation temperature range of carbides etc. increases, the amount of carbides, etc. that precipitate during cooling after the end of rolling increases, and accordingly the yield strength and tensile strength of the steel plate base material increase. To do. On the other hand, when the weld heat affected zone is heated to the Ar ₃ transformation point or higher, transformation from α (ferrite) to γ (austenite) occurs, and carbides and the like precipitated during cooling after rolling are dissolved. The precipitation strengthened ferrite structure present before heating is reset. For this reason, the generation sites for precipitation in the subsequent cooling process are insufficient, and sufficient strengthening cannot be performed.

  Therefore, the yield stress and tensile stress of the part affected by the heat of welding seem to show a strength obtained by adding some precipitation strengthening (about 40-50% during tempering) to the strength corresponding to the ferrite structure generated after cooling. become. On the other hand, when the rolling reduction ratio just above the precipitation temperature region of carbide or the like is increased, the strength of the steel sheet base metal, particularly the yield stress, can be increased efficiently. As a result, it is possible to increase only the yield stress of the steel plate base material while minimizing the yield stress of the weld heat affected zone.

  As a result of repeated experiments from such a viewpoint, when heated to 950 ° C. or higher and then rolled to the target plate thickness, the cumulative rolling reduction in the temperature range of 850 to 950 ° C. is 50% or higher, more preferably 55% or higher. As it turns out, rolling is good to finish.

Incidentally, FIG. 6 shows that the cumulative rolling reduction in the temperature range of 850 to 950 ° C. is the tensile strength (TS ₀ ) of the base material among various experimental data using the steel material added with the carbide forming element (the steel material ₂ ). In order to secure a tensile strength of 400 MPa or more, it is understood that 50% or more should be secured in the cumulative rolling reduction in the temperature range of 850 to 950 ° C. .

  Further, FIG. 7 is a graph summarizing the relationship between the DI value and the yield stress of the weld heat-affected zone from the experimental data including examples described later. From this figure, the DI value (inch) is shown. It can be seen that the yield stress of the weld heat-affected zone can be suppressed to a low value of 400 MPa or less by suppressing it to 0.38 or less.

  The plate thickness of the steel plate according to the present invention is not particularly limited and can be applied to steel plates having various thicknesses, but the effect of the present invention is more effectively exhibited when the thickness is about 4.5 mm or more. It is a steel plate. The upper limit of the plate thickness is not particularly limited, but is usually about 10 mm or less.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and both are included in the technical scope of the present invention. The test methods employed in the following experimental examples are as follows.

[Measurement of yield stress (YP ₀ ), (YP ₁ )]
Test piece shape; see FIG.
Thermal history imparting device: A 50 kilowatt thermal cycle reproduction device manufactured by Fuji Radio Engineering Co., Ltd. is used.

[Measurement of both amounts of out-of-plane buckling deformation (decreased leanness)]
After welding the rib material cut out from the same steel plate as shown in FIG. 2 to one side of each test steel plate (thickness is 6 mm), the out-of-plane appearance as shown in the end view of line AA in FIG. The buckling deformation amount (a) is measured for each of the sections (1) to (12), and the average value is obtained.

(Welding conditions)
Welding current; 280 A,
Welding voltage: 32V
Welding speed: 58-62 cm / min,
Welding heat input: about 9 kJ / cm,
Leg length: 5mm,
Melting material: “MG-50” (diameter: 1.2 mm) manufactured by Kobe Steel, Ltd.

Experimental example 1
Steel slabs obtained by melting and casting steels having the chemical components shown in Table 1 were controlled and rolled under the conditions shown in Tables 2 and 3, and test plates of the predetermined dimensions (Japan Maritime Association; No. U1) from the obtained steel plates. A test piece) was cut out and subjected to a tensile test. Further, the same test plate was subjected to the heat treatment simulating the influence of welding heat and then subjected to a tensile test. The results are also shown in Tables 2 and 3.

  It can analyze as follows from Tables 1-3.

  In Table 1, steel types A to G are steel materials in which the component composition and DI value specified in the present invention all satisfy the specified requirements of the present invention, and steel types H to N are any of the component composition and DI value specified in the present invention. Kaga is a comparative requirement for each comparative material.

  And in Table 2, the code | symbols 4-6, 13-21 are Examples which all satisfy | fill the prescription | regulation requirements of this invention for a component composition, DI value, and manufacturing conditions, and all of a thin horse deformation amount are 4.0 mm or less. Indicates a small value.

  On the other hand, Table 3 is a comparative example in which any one of the component composition, DI value, and manufacturing conditions lacks the requirement of the present invention, and the deformation amount of the lean horse exceeds the allowable range of 4.0 mm, or the mother The tensile strength of the material does not reach the 400 MPa level and does not meet the object of the present invention.

It is a figure which shows the heat pattern of the heat history given to the test steel plate which simulated the thermal influence at the time of welding. It is explanatory drawing of the "skin horse phenomenon" seen when welding construction of a steel plate. Yield stress (YP ₀₎ / weld heat affected zone of the yield stress of the steel sheet base material (YP ₁₎ ratio is a graph showing the effect on out-of-plane seat屈変shape amount of "lean horse phenomenon". It is a graph which shows the relationship between the yield stress of a welding heat affected zone, and the amount of out-of-plane buckling deformation by a "skin horse phenomenon". From the various experimental data using steel materials with no carbide-forming element added (steel material ₁ ), the influence of the rolling reduction below the Ar ₃ transformation point on the tensile strength (TS ₀ ) of the base material is shown. It is a graph. From the various experimental data using steel material added with carbide-forming elements (steel material 2), the effect of the cumulative reduction ratio in the temperature range of 850 to 950 ° C. on the tensile strength (TS ₀ ) of the base material is arranged. It is the graph shown. It is the graph which showed collectively the relationship between the DI value and the yield stress of a welding heat affected zone. It is a figure which shows the dimension and size of the tensile test piece of the test steel plate used in experiment.

Claims (6)

When the yield stress of a steel sheet is (YP ₀ ), the tensile strength is (TS ₀ ), and the yield stress after applying the following thermal history to the steel sheet by simulating the thermal effect during welding is (YP ₁ ). a, YP ₀ or more 250 MPa, TS ₀ is more than 400 MPa, YP ₁ is less than or equal 400 MPa and YP ₀ / YP ₁ is Ri der 1 or more, the steel has a following chemical composition, and the following hardenability index ( welding seat屈変form less steel, characterized in der Rukoto satisfy the DI value).
(Heat history provision conditions)
Thermal history pattern: The temperature is raised at 100 ° C./s, held at a maximum heating temperature of 1350 ° C. for 5 seconds, and cooled from 800 ° C. to 500 ° C. in 20 seconds (15 ° C./s) (FIG. 1) .
(Chemical composition)
C: 0.005 to 0.12% (meaning mass%, the same applies hereinafter),
Si: 0.05 to 0.5%,
Mn: 0.05 to 1.2%,
N: Besides satisfying 0.002 to 0.007%,
Including at least one selected from the group consisting of Nb: 0.005-0.03%, V: 0.005-0.075%, Ti: 0.005-0.03%,
Balance: Fe and inevitable impurities,
DI = 1.16 × [√ (C / 10)] × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1) x (3.0 x Mo + 1) x (1.75 x V + 1) x (200 x B + 1) ≤ 0.38
[The symbol in the formula represents the content (% by mass) of each element]. When the yield stress of a steel sheet is (YP ₀ ), the tensile strength is (TS ₀ ), and the yield stress after applying the following thermal history to the steel sheet by simulating the thermal effect during welding is (YP ₁ ). a, YP ₀ or more 250 MPa, TS ₀ is more than 400 MPa, YP ₁ is less than or equal 400 MPa and YP ₀ / YP ₁ is Ri der 1 or more, the steel has a following chemical composition, and the following hardenability index ( welding seat屈変form less steel, characterized in der Rukoto satisfy the DI value).
(Heat history provision conditions)
Thermal history pattern: The temperature is raised at 100 ° C./s, held at a maximum heating temperature of 1350 ° C. for 5 seconds, and cooled from 800 ° C. to 500 ° C. in 20 seconds (15 ° C./s) (FIG. 1) .
(Chemical composition)
C: 0.005-0.12%,
Si: 0.05 to 0.5%,
Mn: 0.05 to 1.2%,
N: Besides satisfying 0.002 to 0.007%,
It contains at least one selected from the group consisting of Nb: 0.005 to 0.03%, V: 0.005 to 0.075%, Ti: 0.005 to 0.03%, and the following formula (I )
Nb / 6.63N + V / 3.64N + Ti / 3.41N> 1 …… (I)
Balance: Fe and inevitable impurities,
DI = 1.16 × [√ (C / 10)] × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1) x (3.0 x Mo + 1) x (1.75 x V + 1) x (200 x B + 1) ≤ 0.38
[The symbol in the formula represents the content (% by mass) of each element]. The steel material is still another element,
3. At least one selected from the group consisting of Ca: 0.0005 to 0.003%, Zr: 0.0005 to 0.004%, and REM: 0.0005 to 0.005%. The steel sheet described. The steel material is at least one selected from the group consisting of Ni: 0.2% or less, Cu: 0.2% or less, Cr: 0.10 % or less, and Mo: 0.1% or less as another element. The steel plate according to any one of claims 1 to 3 , comprising: When a steel slab that satisfies the component requirements described in any one of claims 1 to 4 is heated to 950 ° C or higher and then rolled to a target plate thickness, an Ar ₃ transformation point or less calculated by the following formula: A method for producing a steel sheet with less weld buckling deformation, characterized by giving the characteristics according to claim 1 by rolling so that the cumulative rolling reduction in the temperature range is 30% or more.
Ar ₃ (° C.) = 910−310 × C−80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo
[The chemical symbol in the formula represents the content (% by mass) of each element]. After heating the steel slab satisfying the component requirements described in any one of claims 2 to 4 to 950 ° C or higher and then rolling the steel slab to a target plate thickness, a temperature range of a plate thickness direction average temperature of 850 to 900 ° C. A method for producing a steel plate with less weld buckling deformation, characterized in that the characteristic reduction according to claim 2 is given by rolling the steel sheet to a target plate thickness after rolling up to a target sheet thickness of 50% or more.

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