KR101096991B1 - Steel sheet with less weld buckling deformation, and process for producing the same - Google Patents

Abstract

By controlling the composition of steel and its strength characteristics, especially the balance between the strength of the base material and the strength after being subjected to the heat of welding, the out-of-plane buckling deformation, which is called "dry-end deformation" at the welding site, is especially possible when welding and drying the hull structure. Provided is a steel sheet with a small weld buckling deformation that can be achieved. When the yield stress of the steel sheet is (YP ₀ ) and the thermal stress of the steel sheet is simulated and the thermal stress described in the text is given (YP ₁ ), YP ₀ is 400 MPa or more and YP ₀ / YP ₁ is 1 or more.

Description

Steel sheet with less weld buckling deformation and its manufacturing method {Steel sheet with less weld buckling deformation, and process for producing the same}

The present invention is a steel sheet mainly used in a hull structure, etc., despite being relatively thin, the steel sheet has a low buckling deformation during welding, and can obtain high buildability without the need for correction after welding drying. And to a method of manufacturing the same.

When constructing the outer shell portion forming the upper portion of the hull structure, it is common to weld and fix the reinforcement ribs to the steel sheet in order to increase the structural strength. In this case, the welded joint and the welded heat affected zone are melted or transformed by the heat at the time of welding (reverse transformation from? →?), And then heat shrinkage occurs when the temperature is lowered (cooled) to room temperature and then solidified. However, when the four sides are constrained by the reinforcing rib, the tensile residual stress corresponding to the yield stress of the welded portion is generated because it cannot contract freely. At this time, the welded structure causes out-of-plane buckling deformation due to the residual stress, which is also called "dry horse deformation" in a part of the welding work site.

By the way, when such buckling deformation occurs in the outer portion of the hull structure or the like, the appearance deteriorates. Therefore, in order to correct such out-of-plane buckling deformation, press calibration, spot heating calibration, or the like has been performed. However, this calibration work is not only complicated and requires a lot of work, but also causes a great prolongation of the construction period. Therefore, there has been a demand for the development of a steel sheet to prevent external buckling deformation caused by such welding.

However, Japanese Unexamined Patent Application Publication No. 6-172921 discloses low heat input welding of relatively thick steel plates (about 10 mm or more) for structural steel sheets used in marine structures, buildings, bridges, and the like. An improved technique for the purpose of reducing the welding angle deformation which is a problem at the time is disclosed. The present invention is intended to prevent the welding deformation by increasing the yield stress of the steel sheet affected by the welding heat, and in particular, to specify the composition of the steel as a method for increasing the yield stress of the steel sheet subjected to the welding heat, At least 30 area% or more of the microstructure of the steel cross section is made of bainite structure in which fine carbides are dispersed, and the yield strength is increased to 360 MPa or more to yield a yield strength of 400 ° C. or higher in the middle temperature range where welding deformation is likely to occur. It is intended to reduce the so-called angular deflection at the time of edge welding generally employed when constructing the steel structure as described above to below 1/2 level.

In Japanese Unexamined Patent Publication No. 2003-268484, the welding angle deformation, which is a problem when a relatively thick steel sheet (about 10 mm or more) is used for edge welding, is applied to structural steel sheets used in marine structures, buildings, bridges, and the like. An improved technique for the purpose of reducing the number of components is disclosed. This invention is also intended to prevent weld deformation by increasing the yield stress of steels affected by welding heat. Specifically, it is a method for increasing the yield stress of the steel sheet subjected to the welding heat effect, and the composition of the steel material is specified, and the microstructure has bainite and / or martensite, ferrite and / or pearlite having a small average particle diameter. By increasing the yield strength in the middle temperature zone where welding deformation is likely to occur, a large amount of fine carbonitride is present, thereby suppressing angular deformation caused by edge welding.

However, these inventions are aimed at suppressing angular deformation by increasing the yield stress of the welded heat affected zone as described above, and by suppressing the increase in strength of the welded section as described later in the present invention. The technical idea is essentially different from the present invention which prevents "dry phenomenon".

(Initiation of invention)

(Tasks to be solved by the invention)

The out-of-plane buckling deformation (so-called "dry deformation") to be improved in the present invention is a strain deformation when the reinforcing rib is welded to a steel plate of relatively thin thickness (typically less than about 10 mm) and is reinforced, for example, as shown in FIG. 1) The reinforcing ribs 2 having the same thickness are welded on one side of the joint, and joint shrinkage due to heat of welding caused by welding heat and subsequent cooling of the joint. The residual stress caused by the heat affected zone is complicated, and the flat part of the welded structure causes the buckling deformation to have the same shape as the "back side of the dry horse" as shown in the sectional view A-A 'of FIG. .

The cause of the buckling deformation will be described later, but the present invention provides a steel sheet that suppresses such dry horse deformation as much as possible by controlling the composition of the steel sheet and the strength characteristics, in particular, the balance between the base material strength and the strength after being subjected to heat influence. In addition, the present invention provides a production method capable of reliably obtaining such a steel sheet.

(Means to solve the task)

The steel sheet with less weld buckling deformation of the present invention which can solve the above problems is simulated by the yield stress of the steel sheet (YP ₀ ), tensile strength (TS ₀ ), and the thermal effect during welding to the steel sheet. When the yield stress after the heat history is given as (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 YP ₀ / YP ₁ is 1 or more It is characterized by.

(History grant condition)

Thermal history pattern: as shown in Figure 1,

Thermal history giving device: A 50-kilowatt heat cycle reproducing device manufactured by Fuji Electronic Industrial Co., Ltd. is used.

칭성지수(DI값)를 만족하는 것이다. A more preferred first embodiment of the steel of the present invention is characterized in that the steel has a chemical composition such as a) or b)

It satisfies the characterization index (DI value).

a) chemical composition;

C: 0.005 to 0.12%,

Si: 0.05-0.5%,

Mn: contains 0.05 to 1.2%,

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) × (3.0 × Mo + 1) × (1.75 × V + 1) × (200 × B + 1) ≤0.38

(The symbol in the formula represents the content rate (mass%) of each element)

b) chemical composition;

C: 0.005 to 0.12%,

Si: 0.05-0.5%,

Mn: 0.05 to 1.2%,

N: 0.002% to 0.007%, and others

Nb: 0.005 to 0.03%, V: 0.005 to 0.075%, Ti: 0.005 to 0.03% at least one selected from the group consisting of,

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) × (3.0 × Mo + 1) × (1.75 × V + 1) × (200 × B + 1) ≤0.38

(The symbol in the formula represents the content rate (mass%) of each element)

Moreover, in 2nd more preferable embodiment, this steel material satisfy | fills the following chemical component and said DI value.

Chemical composition;

C: 0.005 to 0.12%,

Si: 0.05-0.5%,

Mn: 0.05 to 1.2%,

N: 0.002% to 0.007%, and others

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 satisfies the relationship of the following formula (I)

Nb / 6.63N + V / 3.64N + Ti / 3.41N ＞ 1 ‥‥ (Ⅰ)

Balance: Fe and inevitable impurities.

The steel of the present invention may further contain 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% as another element, or other elements. It may contain at least one selected from the group consisting of Ni: 0.2% or less, Cu: 0.2% or less, Cr: 0.2% or less, and Mo: 0.1% or less.

In addition, the method for producing a steel sheet with low weld buckling deformation according to the present invention is a method applied when a steel sheet satisfying the requirements described in the first embodiment is used, and is heated to 950 ° C. or higher and then rolled to a target plate thickness. At this time, the above characteristics are imparted by rolling so that the cumulative reduction ratio in the temperature range below the Ar ₃ transformation point calculated by the following equation 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 (mass%) of each element.)

In addition, when using the steel piece which satisfy | fills the requirements of the said 2nd embodiment, when it heats to 950 degreeC or more and rolls to a target plate thickness, it accumulates in the temperature range of the average temperature of 850-950 degreeC in plate thickness direction. The said characteristic is provided by making a ratio into 50% or more, rolling to target plate thickness, and finishing rolling.

(Effects of the Invention)

According to the present invention, for example, a steel sheet for a hull structure or the like maintains sufficient structural strength, has excellent weldability, and can suppress out-of-plane buckling deformation (so-called dry deformation) accompanying welding as much as possible. As a result, it is not necessary to substantially perform the post-weld correction process, which can greatly increase work efficiency and significantly shorten the construction period. Moreover, according to the manufacturing method of this invention, the steel plate which has a target characteristic can be provided at low cost by using the steel material which suppressed the compounding quantity of expensive alloy elements etc. to the minimum.

Best Mode for Carrying Out the Invention [

The present inventors have paid attention to the out-of-plane buckling deformation called "dry phenomena" occurring in welding construction under the above circumstances, and have studied the mechanism of occurrence of "dry phenomena" in order to develop an effective prevention method. The same thing could be confirmed.

In welding drying, the portion melted at the time of welding and its vicinity (hereinafter referred to as the vicinity of the weld line) cause thermal contraction when the temperature is lowered to room temperature.

At the time of thermal contraction, since the vicinity of the weld line is confined in all directions by the reinforcing ribs, it cannot freely contract, so that the deficiency in deformation caused by the contraction is compensated for by plastic deformation. As a result, a tensile residual stress corresponding to the yield stress at that temperature occurs. In the state where the vicinity of the weld line of the steel sheet is lowered to room temperature, the tensile residual stress corresponding to the yield stress at the ambient temperature near the weld line is generated.

In addition to these phenomena, compressive residual stresses occur in the area away from the weld line to balance the tensile residual stresses generated near the weld line.

In addition, when the compressive residual stress exceeds the buckling critical strength of the steel sheet, an out-of-plane buckling deformation, that is, "dry speech phenomenon" occurs.

(2) of the above mechanism means that the residual stress generated near the weld line depends on the yield stress level of the part affected by the welding heat. Therefore, it is necessary to consider the following points in suppressing the out-of-plane buckling deformation. There is.

That is, `` When the area heated above the Ar ₃ transformation point of the steel sheet due to the heat effect during welding is cooled (cooled) to room temperature, the yield stress of the corresponding area is extremely low. It also reduces the compressive residual stress in the distant part, making it difficult to produce out-of-plane buckling deformation.

칭경화작용을 갖는 첨가합금원소를 극력저감할 필요가 있다.However, in steel materials, when the amount of added alloying element 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 yield stress of the weld heat affected zone is It is often higher than that of the base metal. In this case, if the mechanism 2 is used, the compressive residual stress level generated at the portion away from the weld line of the steel sheet becomes high, so that the reduction of out-of-plane buckling deformation cannot be realized. Therefore, in view of the mechanism (2) to suppress the increase in the yield stress of the weld heat affected zone

It is necessary to reduce the addition alloy element having a hardening action as much as possible.

In addition, as described above, the out-of-plane buckling deformation is caused by the compressive residual stress caused by the heat shrinkage of the weld heat affected zone, and it is confirmed that the buckling deformation is related to the yield stress of the steel sheet (base metal) as well as the residual stress at that time. . That is, when residual stress of the same level exists, the less yield stress of the base material itself, the more likely to cause buckling deformation. As a result of examining this point, it was confirmed that if the yield stress of the steel sheet (base material) is higher than the yield stress of the heat affected zone of the steel sheet, dry powder phenomenon can be suppressed as much as possible.

In addition, the strength of the weld heat affected zone is higher than the Ac ₃ transformation point (the temperature at which α → γ reverse transformation is completed). Since it is determined by the tissue transformation formed at the time, it is determined almost exclusively by chemical components. On the other hand, since the base material strength fluctuates with the change of rolling conditions in addition to the chemical composition, it was confirmed that the base material strength and the strength of the weld heat affected zone can be controlled by controlling the chemical composition and the rolling conditions.

Under this knowledge, the present invention provides 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 as the first essential requirement, that is, the yield stress of the steel sheet / (the corresponding steel sheet). The yield stress of the heat affected zone when welding is greater than or equal to 1, that is, the yield stress of the steel sheet is greater than the yield stress of the heat affected zone when the steel sheet is welded. It is revealed.

Therefore, in the present invention, the yield stress of the steel sheet (base material) is referred to as (YP ₀ ), and in order to standardize the yield stress at the time of receiving the heat history simulating the heat effect at the time of welding, after applying the above-described heat history to the steel sheet. The yield stress was referred to as (YP ₁ ), and the value of these (YP ₀ / YP ₁ ) was 1 or more as the first essential requirement. The more preferable value of (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 the hull structure is insufficient, and the required structural strength cannot be secured. Therefore, the lower limit values of the yield stress and the tensile stress as the steel plate base material are respectively `` 250 MPa or more ''. It was set as "400 MPa or more."

In addition, Figure 3 summarizes the effect of the ratio of (yield stress of the steel plate base material: YP ₀ ) / (yield stress of the weld heat affected zone: YP ₁ ) of the out-of-plane buckling strain among many experimental data including an embodiment to be described later In the graph, when the ratio of (YP ₀ / YP ₁ ) was 1.0 and less than that, the out-of-plane buckling strain exceeded 4.0. When the ratio became 1.0 or more, the out-of-plane buckling strain became a low value of 4.0 or less.

4 is a graph showing the relationship between the yield stress and the out-of-plane buckling strain of the welded heat affected zone in the same way, and in this graph, when the yield stress of the welded heat affected zone is 400 MPa or less, On the other hand, if it exceeds 400 MPa, the out-of-plane buckling strain clearly exceeds 4.0 mm. From this, it can be also seen that the yield stress of the weld heat affected zone is less than 400 MPa, which is effective for suppressing dry powder phenomenon.

칭성 지수(DI값)에 대하여 검토를 거듭하였다.Next, when deriving a desirable requirement for obtaining the above characteristics, the elements that have a significant effect on the yield stress of the steel plate base material and the weld heat affected zone, and a comprehensive index of these elements

The title property (DI value) was examined repeatedly.

As a result, it is very important to adjust the content of member elements so that the DI value calculated by the above formula is 0.38 (unit: inch) or less depending on the chemical composition of the steel used as the first preferred requirement for obtaining the above characteristics. Could know.

칭경화성을 저감하고, 용접부 및 그 열영향부가 고온으로 가열된 후 상온부근까지 강온했을 때

칭경화에 의해 강도상승을 일으키는 것을 방지하고, 마른말현상을 일으키는 최대 원인인 용접부 및 열영향부의 용접후 항복응력을 가급적 낮게 억제하기 위한 것이다.The upper limit of these DI values is determined by the steel material itself.

When the hardening property is reduced, and the welded part and its heat affected part are heated to high temperature, then the temperature is lowered to near room temperature.

It is to prevent the increase in strength by hardening and to suppress the yield stress after welding, which is the biggest cause of dry horse phenomenon, as low as possible.

칭성지수가 0.38을 넘으면 강소재의

칭경화성이 높아지고, 이에 따라 용접후 냉각과정에서 이 용접부와 열영향부가

칭경화를 일으켜 이 부위의 항복응력이 상승한다. 이와 더불어 상기 메가니즘(2)에서 설명한 바와 같이 용접선근방부의 인장잔류응력이 높아지고, 따라서 이 인장잔류응력과 균형을 이루도록 용접선에서 떨어진 부분에 발생하는 압축잔류응력도 증대하여 마른말현상을 촉진시키는 원인이 된다. 그러므로 이러한 현상을 억제하기 위해서는 그 근원이 되는 DI값을 0.38 이하로 억제하는 것이 필수적이다. 이들의

칭성지수의 보다 바람직한 값은 0.37 이하, 더욱 바람직하게는 0.36 이하이지만, 강재의

칭성지수가 너무 낮아지면 용접열영향부의 강도가 불충분하게 되어 구조용 강으로서의 필요강도를 확보하기 어려우므로 그 하한을 0.22정도 이상, 보다 바람직하게는 0.24정도 이상으로 하는 것이 좋다.In addition, the DI value, namely

If the Tsingsheng Index exceeds 0.38,

In this case, the welded portion and the heat-affected portion are cooled during the post-weld cooling process.

Yielding hardening causes the yield stress in this area to rise. In addition, as described in the mechanism (2), the tensile residual stress in the vicinity of the weld line is increased, and thus, the compressive residual stress occurring at the part away from the weld line is increased to balance the tensile residual stress, thereby promoting dry speech. do. Therefore, in order to suppress this phenomenon, it is essential to suppress the DI value, which is the source thereof, to 0.38 or less. Of these

The more preferable value of the Tingsing index is 0.37 or less, more preferably 0.36 or less.

If the Tingling index is too low, the strength of the weld heat affected zone becomes insufficient, so that it is difficult to secure the required strength as structural steel, so the lower limit is preferably about 0.22 or more, more preferably about 0.24 or more.

In addition, when steels contain an appropriate amount of Nb, V, and Ti as precipitation hardening elements, the base metal strength is increased by precipitation hardening of carbides and carbonitrides of those elements, so the DI value of the steels may be about 0.09, but it is preferable. 0.16 or more.

In addition, if the carbon equivalent of the steel is too low, as will be described later in the manufacturing method, the base material strength is insufficient to secure the required strength as structural steel, regardless of the treatment for improving the base material strength, so the lower limit is about 0.15 or more, More preferably, it is good to set it as about 0.16 or more.

Next, the preferable chemical component of the steel material used by this invention is demonstrated. The steel sheet of this invention is classified into two types of steel materials (steel material 1, steel material 2) as demonstrated below.

Preferred chemical constituents of Steel 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%, and the balance consists of Fe and inevitable impurities, or N in addition to these elements. A steel material containing: 0.002% to 0.007%, and containing at least one member selected from the group consisting of Nb: 0.005% to 0.03%, V: 0.005% to 0.075%, and Ti: 0.005% to 0.03%, and the content of these components The reasons for this are as follows.

C: 0.005 ~ 0.12%

칭경화특성을 억제함으로써 마른말현상을 저감하기 때문에, 강재의 DI값을 제어할 필요가 있고, 이 DI값은 C의 영향이 크기 때문에 C 함량은 많아도 0.12% 이하, 바람직하게는 0.11% 이하, 더욱 바람직하게는 0.10% 이하로 제어하는 것이 좋다.C is an element indispensable because it is the most effective and inexpensive material to secure the necessary structural strength as a steel plate base material, and its content is more necessary because strength is insufficient at less than 0.005%. More preferable C content is 0.01% or more, More preferably, it is 0.03% or more. On the other hand, in the present invention, as described above, the weld heat affected zone

Since dry phenomena are reduced by suppressing the hardening property, it is necessary to control the DI value of the steel, and since the DI value has a large influence of C, the C content is 0.12% or less, preferably 0.11% or less, More preferably, it is controlled to 0.10% or less.

Si  0.05 to 0.5%

칭경화성을 상승시켜 해당영역에 발생하는 잔류응력을 크게함과 아울러 이 영역의 저온인성을 열화시키기 때문에 0.5%를 상한으로 한다. 바람직하게는 0.4% 이하, 더욱 바람직하게는 0.3% 이하로 제어하는 것이 좋다.Since Si is an element which serves as a deoxidizer of molten steel and raises DI value and contributes to the improvement of base material strength, it is preferable to contain Si at least about 0.05%. Preferably it is 0.10% or more. However, excessive addition of the weld heat affected zone

The upper limit of stress hardening in the area is increased by increasing the hardening hardness, and the low temperature toughness in the area is deteriorated. Preferably it is 0.4% or less, More preferably, it is good to control to 0.3% or less.

Mn  0.05 to 1.2%

칭경화성을 상승시켜 해당 영역에 발생하는 잔류응력을 크게함과 아울러 이 영역의 저온인성을 열화시키기 때문에, 많아도 1.2% 이하를 상한으로 한다. 바람직하게는 1.0% 이하, 더욱 바람직하게는 0.8% 이하로 제어하는 것이 좋다.Since Mn plays a role of increasing the base material strength and increases the DI value, it is preferable to contain Mn at least 0.05%. Preferably it is 0.10% or more, More preferably, it is 0.20% or more. However, excessive addition may cause

Since the hardening hardening property is raised to increase the residual stress generated in the region, and the low temperature toughness of the region is degraded, the upper limit is 1.2% or less at most. Preferably it is 1.0% or less, More preferably, it is good to control to 0.8% or less.

In the case where the element contains at least one of N, Nb, V, and Ti,

N: 0.002% to 0.007%, and at least one of Nb: 0.005% to 0.03%, V: 0.005% to 0.075%, and Ti: 0.005% to 0.03%;

In the present invention, N combines with Nb, V and Ti to form nitrides, and these nitrides effectively act to suppress the coarsening of the austenitic structure of the weld heat affected zone, thus contributing to the improvement in the toughness of the weld heat affected zone. In order to exert such an effect effectively, it is preferable to make N and Nb, V, Ti into the said range.

The remainder component of the steel used in the present invention is an impurity that is inevitably mixed with iron (Fe), among which Al, P, S and the like are also included. That is, Al is an element used as a deoxidizer, and it is preferable to contain Al in an amount of 0.02% or more in order to sufficiently reduce the amount of solid solution oxygen in steel and to suppress the deterioration of toughness of the base material. However, excessive content may cause formation of nonmetallic inclusions and cause deterioration of the toughness of the base metal and the weld heat affected zone. Therefore, it is preferable to suppress the content to 0.05% or less, more preferably 0.04% or less.

P and S are both elements that have been inevitably incorporated in the steel, and also become sources of inclusions, which adversely affects the toughness of the base metal of the steel sheet and the toughness of the weld heat affected zone. Hereinafter, more preferably, the content is preferably suppressed to 0.02% or less, and S is preferably suppressed to 0.02% or less, more preferably 0.01% or less, and still more preferably 0.005% or less.

In addition, the DI value of the steel material 1 that satisfies the component requirement is 0.38 or less as a value calculated by the above formula.

Next, the preferred chemical composition of steel 2 used in the present invention satisfies the N content and at least one content selected from Nb, V and Ti in addition to the C, Si and Mn content ranges defined in Steel 1 above. In addition, the relationship between the N content and the contents of Nb, V, and Ti satisfies "Nb / 6.63N + V / 3.64N + Ti / 3.41N> 1", and the balance consists of Fe and an unavoidable impurity.

That is, N forms nitride with Nb, V and Ti as described above, thereby contributing to the toughness of the weld heat affected zone. Steel 2 according to the second aspect of the present invention is characterized in that Nb, V, Ti is carbide (or carbonitride). Requirement to increase the strength by precipitation hardening by precipitation), and even if they form nitrides in the relationship with the mass ratio of Nb, V, and Ti, and also produce carbides and exhibit the precipitation hardening effect. It is necessary to satisfy "Nb / 6.63N + V / 3.64N + Ti / 3.41N> 1" as.

In addition, by effectively utilizing the precipitation hardening effect as carbides of Nb, V, and Ti as described above, the yield stress of the weld heat affected zone is not increased by appropriately controlling the temperature and the rolling reduction rate during the hot rolling of the steel sheet to be described later. The precipitation hardening of carbides (or carbonitrides) of these elements during rolling can effectively increase the yield stress and tensile stress of the steel plate base material.

The action of such Nb, V, Ti is at least Nb: 0.005% (more preferably 0.008% or more), V: 0.005% or more (more preferably 0.010% or more), or Ti: 0.005% or more (more preferably 0.008% or more), and it 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 the content is too high, the number or volume fraction of precipitated carbides (or carbonitrides) becomes excessive, which lowers the low temperature toughness and tensile ductility of the base material, Since the dissolved precipitates are easily precipitated again, the yield stress of the weld heat affected zone is increased. As a result, there is a problem that the dry horse strain also becomes large, so that 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, 0.050% or less), Ti should be suppressed to 0.030% or less (more preferably 0.025% or less, even more preferably 0.020% or less).

And the preferable DI value of the steel material 2 which satisfy | fills the said component requirement needs to be 0.38 or less by the value calculated by said formula.

In the case where the steel contains appropriate amounts of Nb, V, and Ti as precipitation hardening elements, the base metal strength is increased by precipitation hardening of carbides or carbon nitrides of these elements, so the lower limit of the DI value of the steel may be about 0.09. . However, preferred is about 0.16 or more.

Essential elements of the steels 1 and 2 preferably used in the present invention are as described above, the balance is Fe and inevitable impurities, in some cases additionally as Ca: 0.0005 ~ 0.003%, Zr: 0.0005 ~ At least one selected from the group consisting of 0.004%, REM: 0.0005 to 0.005%, or at least one selected from the group consisting of Ni: 0.2% or less, Cu: 0.2% or less, Cr: 0.2% or less, and Mo: 0.1% or less One type may be contained.

The Ca, Zr, and REM have the effect of suppressing the occurrence of cracks from internal cracks or weld heat affected zones by spheroidizing A-based inclusions (inclusions that tend to stretch in the rolling direction during rolling) such as MnS. In view of the same effect, these effects are effectively exhibited by individual addition or two or more types of compound additions. In order to effectively exert such effects, Ca is at least 0.0003% (more preferably at least 0.0007%), Zr is at least 0.0005% (more preferably at least 0.0010%), and REM is at least 0.0005% (more preferably at least 0.0010%). ) It is good to contain. However, if the content is too high, a large amount of oxides (CaO, etc.) of each element are generated, which may cause deterioration of base metal toughness and tensile ductility, so that Ca is 0.003% or less (more preferably 0.0025% or less), and Zr is 0.004% or less. (More preferably 0.004% or less), REM is preferably controlled to 0.005% or less (more preferably 0.0035% or less).

칭성을 높여 모재강도를 높이는 작용을 갖는 점에서 동효원소이고, 이들 효과는 각각의 단독첨가 혹은 2종 이상의 복합첨가에 의해 유효히 발휘된다. 그러나 이들 원소가 너무 많으면

칭성이 너무 높아져 용접열영향부의 항복응력이 높아지고 그 결과 마른말변형량도 커지게 되며, 또한 원료비가 높이 올라 제조코스트가 높아지게 되는 문제가 생기므로, Ni은 0.2% 이하(보다 바람직하게는 0.1% 이하), Cu는 0.2% 이하(보다 바람직하게는 0.1% 이하), Cr은 0.2% 이하(보다 바람직하게는 0.1% 이하), Mo은 0.1% 이하(보다 바람직하게는 0.05% 이하)로 억제하는 것이 좋다.Ni, Cu, Cr, Mo are all

It is a copper effective element in that it has the effect | action which raises a toughness, and raises a base material strength, and these effects are exhibited effectively by each individual addition or 2 or more types of complex additions. But if there are too many of these elements

Niing is so high that the yield stress of the weld heat affected zone becomes high, and as a result, the amount of dry horse strain increases, and the raw material cost increases, resulting in a high manufacturing cost. Therefore, Ni is 0.2% or less (more preferably 0.1% or less). ), Cu is 0.2% or less (more preferably 0.1% or less), Cr is 0.2% or less (more preferably 0.1% or less), and Mo is 0.1% or less (more preferably 0.05% or less). good.

Next, since the specific steels 1 and 2 of the chemical composition are both low in carbon equivalent and low in reinforcing element content, if the steel sheet manufacturing conditions are normally applied as it is, sufficient strength as a steel sheet base material cannot be ensured, and the strength as structural steel Will be lacking. Therefore, in order to put this into practical use, it is necessary to study to ensure the strength required as a hull structure steel plate while at the vicinity of the weld seam using the steel plate in order to satisfy the characteristics of the resistive stress. Therefore, the manufacturing conditions for this were examined.

The hot-rolled structure of the low carbon and low alloy steel intended in the present invention is usually composed of a ferrite phase, and as a means for increasing the strength of the steel sheet of the ferrite main structure,

1) strengthening by miniaturization of ferrite grains,

2) reinforcement using work hardening of ferrite phase by rolling in the temperature range below Ar ₃ transformation point,

3) solid solution strengthening by adding alloying elements,

4) strengthening by utilizing precipitation strengthening of metal carbides,

Etc. can be mentioned. Among these methods, the method of reinforcing without adding an alloying element is 1) and 2) above, and in order to perform 1), the rolling must be performed at a very large one-pass reduction ratio, and the capability of a very large rolling mill is required. Or it can be realized only when the conditions, such as rolling size (a narrow rolling width and thin rolling thickness) are satisfied, and it is difficult to implement | achieve it stably in reality. In addition, the reinforcement method of 3) can only expect reinforcement of about 30 ~ 50MPa. On the other hand, the reinforcing method of 2) is a technique that can be realized by strictly managing the rolling temperature, and the method of 4) contains Nb, V, Ti, which are precipitation strengthening elements, and is represented by "Nb / 6.63N + V / 3.64N + Ti / Applicable for steel 2 which satisfies 3.41 N≥1 ".

Therefore, in the present invention, the yield stress (YP ₀₎ of the steel material if one uses a billet satisfying the composition requirements of the predetermined base material strength (YP ₀₎ yield stress (YP ₁₎ the heat affected zone of weld while maintaining a the base material In order to ensure that the ratio (YP ₀ / YP ₁ ) is 1 or more, the cumulative reduction in the temperature range below the Ar ₃ transformation point calculated by the following formula when the steel sheet is heated to 950 ° C. or more and then rolled to the target plate thickness. Rolling is carried out so that (累 積壓 下 率) is 30% or more.

Ar ₃ (° C.) = 910-310 × C-80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo

(In the formula, the chemical symbol indicates the content rate (mass%) of each element.)

At this time, as the reduction ratio in the temperature range below the Ar ₃ transformation point is increased, the processed ferrite structure increases, thereby increasing the yield strength of the base material. In particular, yield strength increases significantly compared to tensile strength. On the other hand, when the welding heat affected zone is heated above the Ar ₃ transformation point, the ferrite (α) is transformed into austenite (γ), so that the processed ferrite structure existing before the heating is reset, and then, according to the ferrite structure generated in the cooling process. Yield stress is shown. Since the yield stress is almost determined by the ratio of the second phase (mainly pearlite) fraction and the solid solution strengthened ferrite structure, the yield stress is determined by the amount of the added alloy element. Therefore, when manufacturing the steel sheet, when the rolling reduction in the temperature range of less than Ar ₃ transformation point at the time of rolling can be increased significantly increase the yield stress and tensile stress, in particular the yield stress of the steel sheet base material by work-hardening.

As a result of repeated experiments, when the reduction ratio in the temperature range below the Ar ₃ transformation point is 30% or more, the yield stress (YP ₀ ) of the steel sheet is secured to 250 MPa or more and the tensile strength of 400 MPa or more, It was confirmed that the ratio of yield stress (YP ₀ ) to yield stress (YP ₁ ) of the weld heat affected zone (YP ₀ / YP ₁ ) was more than 1. For this reason, in the case of using a steel sheet that satisfies the component requirements of Steel 1, when the steel sheet is heated to 950 ° C or higher and then rolled to a target plate thickness, the cumulative reduction ratio to the temperature range below the Ar ₃ transformation point is 30% or more. It is necessary to set it as it, More preferably, it is good to set it as 40% or more.

5 is a graph showing the effect of the reduction ratio below the Ar ₃ transformation point on the tensile strength (TS ₀ ) of the base metal among various experimental data using steels without addition of carbide forming elements (400 MPa). In order to secure the tensile strength above the level, it can be seen that the reduction ratio below the Ar ₃ transformation point must be secured to 30% or more.

Next, the yield stress (YP ₀₎ of the steel material if the two use a billet satisfying the composition requirements of the predetermined base material strength of the heat affected zone of weld yield stress while maintaining the (YP ₀₎ (YP ₁₎ and the base material In order to ensure that the ratio (YP ₀ / YP ₁ ) is 1 or more, the steel sheet is heated to 950 ° C or higher, and when the steel sheet is rolled to a target plate thickness, the cumulative reduction ratio in the temperature range of 850 to 950 ° C is 50% or more. It is necessary to effectively exhibit the action of Nb, V, and Ti which are precipitation strengthening elements by finishing rolling.

In addition, the precipitation temperature range of carbides (or carbonitrides) of Nb, V, and Ti is about 900 ° C. or lower, but when left unrolled, the precipitates are not completely precipitated. Needs to be. On the other hand, when rolling is performed directly above the precipitation temperature range of these carbides or the like, defect portions such as dislocations introduced by rolling become the accumulation size of the precipitate forming elements Nb, V and Ti or the generation size of carbides. Or dislocation diffusion (usually diffusion at a rate of about 10 times or more than diffusion), which promotes the accumulation of precipitate-forming elements, thereby facilitating precipitation of carbides, and tempering without the need for tempering after rolling. It can be seen that it can be strengthened to 70 ~ 80% of cases.

However, it is not only good to simply roll the temperature above the precipitation temperature range, while securing the above-described base material strength (yield stress; YP ₀ 250 MPa or more, tensile strength; TS ₀ 400 MPa or more), and (YP ₀ / YP). _In order to make ₁ ) 1 or more, it can be seen that the cumulative reduction ratio at the temperature range of 850 to 950 ° C. should be 50% or more when the steel sheet is heated at 950 ° C. or higher and then rolled to a target plate thickness.

In addition, as the reduction ratio increases immediately above the precipitation temperature range of the carbide and the like, the amount of carbide and the like precipitated upon cooling after the end of rolling increases, and thus the yield strength and tensile strength of the steel plate base material increase. On the other hand, when the weld heat affected zone is heated above the Ar3 transformation point, transformation occurs from α (ferrite) to γ (austenite), and carbides precipitated during cooling after rolling are solid-dissolved. The tissue is reset. Therefore, since there is a lack of a production site for precipitation in the cooling process thereafter, there is no sufficient strengthening.

Therefore, the yield stress and the tensile stress of the portion subjected to the welding heat show the strength obtained by the addition of a little precipitation strengthening (about 40-50% in the tempering process) to the strength according to the ferrite structure generated after cooling. . On the other hand, when the rolling reduction ratio directly above the precipitation temperature range of carbides or the like can be increased, the strength of the steel plate base material, in particular, the yield stress can be efficiently increased. As a result, it is possible to increase only the yield stress of the steel plate base material while the yield stress of the weld heat affected zone is minimized.

As a result of repeated experiments, when rolling to the target plate thickness after heating to 950 degreeC or more, the rolling reduction is made 50% or more, more preferably 55% or more in the temperature range of 850-950 degreeC. I found it good to quit.

In addition, Figure 6 summarizes the effect of the cumulative pressure reduction rate in the temperature range of 850 ~ 950 ℃ of the base material tensile strength (TS ₀ ) of the various experimental data using the addition of carbide forming element (steel 2) As shown in the graph, it can be seen that in order to secure a tensile strength of 400 MPa or more, a cumulative reduction ratio of 50% or more at a temperature range of 850 ° C to 950 ° C must be secured.

7 is a graph showing the relationship between the DI value and the yield stress of the weld heat affected zone in the experimental data including the examples to be described later. From this figure, the breakdown of the weld heat affected zone is controlled by controlling the DI value (inch) to 0.38 or less. It can be seen that the stress can be suppressed to a low value of 400 MPa or less.

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

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the heat pattern of the heat history applied to the test steel plate which simulated the thermal effect at the time of welding.

FIG. 2 is an explanatory view of "drying phenomenon" seen when welding and drying a steel sheet. FIG.

3 is a graph showing the effect of the ratio of the yield stress (YP ₀ ) / yield stress (YP ₁ ) of the welded heat affected zone on the out-of-plane buckling strain according to "dry speech phenomenon".

Fig. 4 is a graph showing the relationship between the yield stress of the weld heat affected zone and the out-of-plane buckling strain caused by the " dry " phenomenon.

5 is a graph showing the effect of the reduction ratio below the Ar3 transformation point on the tensile strength (TS ₀ ) of the base metal among various experimental data using the steel (additional steel 1) having no carbide-forming element.

Figure 6 summarizes the effect of the cumulative pressure reduction rate in the temperature range of 850 ~ 950 ℃ influence the tensile strength (TS ₀ ) of the base metal of the various experimental data using the addition of carbide forming element (steel 2) It is a graph.

7 is a graph showing the relationship between the DI value and the yield stress of the weld heat affected zone.

Fig. 8 shows the dimensions and sizes of tensile test pieces of test steel sheets used in the experiment.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following Examples, and of course, the present invention may be appropriately changed and carried out in a range suitable for the purpose of the preceding and the following. It is included in the technical scope.

In addition, the experimental method employed in the following examples is as follows.

[Measurement of yield stress (YP ₀ ), (YP ₁ )]

Specimen shape; See FIG. 8,

A thermal hysteresis device; A 50-kilowatt thermal cycle reproducing device manufactured by Fuji Electronic Industrial Co., Ltd. is used.

[Measurement of Out-of-plane Buckling Strain (Dry Decreased)]

After welding the rib material cut out from the steel plate as shown in Fig. 2 to one side of each test steel plate (plate thickness is 6 mm) under the following conditions, the out-of-plane buckling strain a shown in the sectional view taken along the line A-A 'of Fig. Zones (1) to (12) were measured and averaged.

(Welding condition)

Welding current; 280A,

Welding voltage; 32V,

Welding speed; 58-62 cm / min,

Welding heat input; About 9kJ / cm,

Angular length; 5 mm,

Yongjae; `` MG-50 '' (1.2 mm in diameter) manufactured by Kobe Steel Co., Ltd.

(Example 1)

The steel pieces obtained by melting and casting the steels of the chemical components shown in Table 1 were subjected to controlled rolling under the conditions of Tables 2 and 3, and a test plate (Japan Maritime Association; U1 Test Piece) of a predetermined dimension was cut out from the steel sheet thus obtained and subjected to a tensile test. . In addition, the same test plate was subjected to the above-mentioned heat treatment simulating the welding heat effect, and then subjected to a tensile test, and the results are listed in Tables 2 and 3.

TABLE 1

TABLE 2

TABLE 3

The following interpretation is possible from Tables 1-3.

In Table 1, steel grades A-G are steel materials which fully satisfy the component composition and DI value prescribed | regulated by this invention, and steel grades H-N are comparative materials which do not satisfy any of the component composition and DI value prescribed | regulated by this invention. to be.

Table 2 is an embodiment in which all of the composition, the DI value, and the manufacturing conditions satisfy the requirements of the present invention, and the dry horse strain shows a small value of 4.0 mm or less.

On the other hand, Table 3 is a comparative material which is out of the requirements of the present invention, either the composition of the composition, the DI value, or the manufacturing conditions, and the dry horse strain exceeds the allowable range of 4.0 mm, or the tensile strength of the base material is 400 MPa level. It does not reach the purpose of the present invention.

Claims (8)

C: 0.005 to 0.12% (mass%, the same below), Si: 0.05-0.5%, Mn: 0.05 to 1.2%, Ni: 0.2% or less, Cu: 0.2% or less, Cr: 0.1% or less, Mo: 0.1% or less, 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 Balance: Iron (Fe) and unavoidable impurities As a steel plate with less weld buckling deformation comprising: Grater

Satisfies the title index (DI value), YP ₀ is 250 MPa or more when the yield stress (YP ₀ ), tensile strength (TS ₀ ) of the steel sheet is simulated, and the thermal stress after welding is given to the steel sheet to give the following thermal history (YP ₁ ). Steel sheet having less weld buckling strain, characterized in that TS ₀ is 400 MPa or more, YP ₁ is 400 MPa or less, and the ratio of YP ₀ / YP ₁ is 1 or more: 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) × (3.0 × Mo + 1) × (1.75 × V + 1) ≤ 0.38 (Symbol in the formula represents the content (mass%) of each element), (Heat history grant condition) Heat history pattern: heating up to the maximum heating temperature of 1350 ℃ at a heating rate of 100 ℃ / s, held at 1350 ℃ for 5 seconds, then cooled to 800 ℃, cooled to 500 ℃ at a rate of 15 ℃ / s. delete The method of claim 1, A steel sheet with little weld buckling strain, characterized in that the steel sheet further comprises N: 0.002 to 0.007%. The method of claim 1, The steel sheet further contains N: 0.002% to 0.007%, and satisfies the following formula (I). Nb / 6.63N + V / 3.64N + Ti / 3.41N ＞ 1 ‥‥ (Ⅰ) The method according to any one of claims 1, 3 and 4, The steel sheet further contains at least one member 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% as another element. . delete The temperature below the Ar ₃ transformation point calculated by the following formula when the steel sheet satisfying the component requirements according to any one of claims 1, 3 and 4 is rolled to a target plate thickness after heating to 950 ° C or more. The steel sheet with few weld buckling deformations is provided by rolling so that the cumulative reduction in pressure may be 30% or more. Ar ₃ (° C.) = 910-310 × C-80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo (In the formula, the chemical symbol indicates the content rate (mass%) of each element.) When the steel sheet satisfying the ingredient requirement of claim 4 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 in the plate thickness direction is 50% or more, and the target plate The steel sheet with few weld buckling deformations is provided by rolling to thickness and finishing rolling, and providing the characteristic of Claim 1.

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