JP3898842B2 - Steel sheet with excellent low temperature toughness in the heat affected zone - Google Patents

Description

【００３２】
【表２】

【００３３】
【発明の効果】
本発明により溶接熱影響部の低温靭性の優れた鋼がより安価に作成することが可能となった。
【図面の簡単な説明】
【図１】ＨＡＺ部でのシャルピー試験の結果を添加Ｎｉ量に対してプロットした図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a low-cost steel sheet excellent in low-temperature toughness of a weld heat affected zone (HAZ) from small heat input welding to medium heat input welding.
[0002]
[Prior art]
As a technique for improving the HAZ toughness of a steel sheet, a technique in which Ti oxide or TiN is finely dispersed in coarse austenite and fine intragranular transformed ferrite having these as nuclei is generated (for example, JP-A-63-210235). )It has been known. In this steel, ferrite is generated from the Ti oxide in the austenite grains in the cooling process after welding to refine the microstructure and improve toughness. However, in this technique, the CTOD characteristic and the propagation stop characteristic are limited to about −30 ° C. As a countermeasure, Japanese Patent Laid-Open No. 06-075599 has further developed a steel capable of obtaining good HAZ toughness even at -40 ° C. or lower by controlling the Ti amount, controlling the hardened structure, adding a considerable amount of Ni, and the like. .
[0003]
[Problems to be solved by the invention]
However, in the conventional material, in order to improve the low temperature HAZ toughness, it is necessary to add 1 to 4% of Ni in order to increase the toughness of the matrix. Since Ni is an expensive metal, adding such a large amount of Ni causes an increase in the manufacturing cost of the steel sheet. Therefore, cost reduction by low Ni has been strongly desired.
[0004]
[Means for Solving the Problems]
The gist of the present invention is as follows.
[0005]
(1) By weight%
C: 0.04 to 0.12%,
Si: 0.4% or less,
Mn: 0.8 to 2.0% or less,
P: 0.02% or less,
S: 0.003% or less,
Al: 0.001 to 0.005%,
Ni: less than 1.0%,
Ti: 0.005 to 0.02%,
N: 0.002 to 0.006%,
O: 0.001 to 0.004%,
Mg: 0.0001 to 0.003%
Di ^* = 0.316C ^1/2 (1 + 0.7Si) (0.35 + 4.1Mn) (1 + 0.36Ni) (1 + 0.37Cu), with the chemical component consisting of iron and inevitable impurities Is limited in the range of 0.7 to 0.851 , and the effective Ti amount defined by the formula: Ti-2 = O-2 (O-0.66Mg-0.89Al) -3.4N is -0 0.10% to + 0.005%, and 0.01 to 0.3 μm of TiN containing 0.01 to 0.1 μm of an oxide composed of Mg and Al is 30000 pieces / mm ² or more, In addition, a low Ni and a welding heat-affected zone characterized in that there are 30 / mm ² or more oxides having a size of 0.5 to 5 μm, with the sum of the average contents of Mg and Ti being 15% by weight or more. Steel sheet with excellent low temperature toughness.
[0006]
(2) By weight%,
Cu: 0.05 to 0.5%,
Nb: 0.05% or less of 1 type or 2 types, steel plate excellent in low temperature toughness of weld heat affected zone as described in (1) above.
[0007]
In HAZ, the closer to the melting line, the higher the heating temperature during welding. In particular, the heated austenite grains become extremely coarse in the vicinity of the melting line, and the crystal grains of the HAZ structure after cooling become coarse and the toughness deteriorates. Such HAZ toughness includes crystal grain size, high carbon martensite (MA), dispersed state of hardened phase such as upper bainite (Bu), precipitation hardened state, presence or absence of grain boundary embrittlement, elemental microsegregation, matrix Dominated by various metallurgical factors such as toughening of the ground.
[0008]
For this reason, the present invention (a) suppresses the growth of heated austenite grains by adding Mg and dispersing fine oxides in the steel, and (b) transforms oxides mainly composed of Ti and Mg into transformation nuclei. (C) Suppression and dispersion of hardened phase by controlling hardenability (Di ^* limitation) , (d) Controlling proper Ti amount , (e) Grain by controlling impurity amount avoidance of the field embrittlement and microscopic segregation, possible to adjust the proper ingredients for the five items or more (a) ~ (e), to improve the low temperature toughness by the addition of appropriate amounts of particular Mg among them, expensive The amount of Ni was reduced.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
First, suppression of heated austenite grain growth (a) will be described. The HAZ in the vicinity of the melting line is heated to 1400 ° C. or higher, and carbides and nitrides are dissolved and coarsened, so that the force for pinning the movement of the austenite grain boundaries is remarkably reduced. Therefore, the growth of austenite grains cannot be avoided in the conventional steel. Therefore, it was studied to suppress austenite grain growth by pinning with an oxide that exists stably even at a high temperature of 1400 ° C. or higher. As a result, it was found that a very large amount of very fine (Mg, Al) oxide having a size of 0.01 to 0.1 μm was formed by adding a small amount of Mg and Al. Furthermore, fine TiN of 0.01 to 0.3 μm in size is deposited on this (Mg, Al) oxide and exhibits a strong pinning effect even at high temperatures of ˜1400 ° C., suppressing austenite grain growth. I found it to be. Since this (Mg, Al) oxide has good lattice matching with TiN, it effectively acts as a TiN precipitation nucleus site. In addition, since the (Mg, Al) oxide is very finely dispersed, TiN that is compositely precipitated also precipitates in a fine size, and exhibits a stronger pinning effect. Here, when TiN having a size of 0.01 to 0.3 μm effective for pinning is 30000 pieces / mm ² or less, a sufficient pinning effect cannot be obtained, and austenite grains become coarse. The number of TiN having a size of 01 to 0.3 μm was set to 30000 / mm ² .
[0010]
Next, the promotion of the intragranular transformation of (b) will be described. When the ferrite transformation in the austenite grains is promoted, a HAZ structure covered with fine transformation ferrite can be generated. In order to promote the formation of intragranular transformation ferrite, it is effective to increase the number of oxides that become the core sites of intragranular transformation. As a result of intensive studies, the inclusion of Ti and a very small amount of Mg under a very small amount of Al results in fine dispersion of oxides mainly composed of Ti and Mg, and the intragranular transformation in HAZ using this as a nucleus. Has been found to be remarkably promoted and to improve the HAZ toughness. At that time, what is effective as a nucleus of intragranular transformation is a material having a sum of average contents of Ti and Mg of 15% by weight or more. A minimum of 30 pieces / mm ^{2 is} required.
[0011]
Described below is the suppression and dispersion of the cured phase by controlling the hardenability of (c) . In HAZ, since it is cooled rapidly after welding, it is easy to form a hardened structure called island martensite. Such a hardened structure is more likely to occur as the hardenability is higher, and this hardenability changes depending on the particle diameter and additive elements, and generally increases as the additive element amount increases. However, a certain amount of additive elements are indispensable for securing the strength, and therefore Di ^* = 0.316C ^1/2 (1 + 0.7Si) (0.35 + 4.1Mn) (1 + 0.36Ni) which is an index of hardenability. ) By controlling the components so that (1 + 0.37Cu) satisfies 0.7 to 0.851 , the strength is ensured and the precipitation of the hardened phase is avoided and the HAZ toughness is improved.
[0012]
The control of the appropriate Ti amount in (d) will be described. In the present application, the Ti amount is further limited from the relationship with other alloy elements. This is derived from the balance between Ti, O, and N content, but Al and Mg have stronger bonding strength with O than Ti. Combines with Ti to form an oxide. The excess Ti combined with oxygen combines with N to form TiN effective in suppressing the growth of austenite grains and improves the toughness of the steel sheet. At this time, if both Ti and N are too small, sufficient TiN is not generated, and austenite grains are coarsened, resulting in a decrease in toughness. On the other hand, if Ti is present excessively, TiC is formed and the toughness is significantly deteriorated. Moreover, when N exists excessively beyond the range of the effective Ti amount, the solute N increases, and the toughness deteriorates. For this reason, by controlling the effective Ti amount within a range of -0.010 to + 0.005%, it becomes possible to suppress austenite grain growth while avoiding TiC embrittlement and to obtain excellent toughness. I found out.
[0013]
Next, the reasons for limiting each chemical component will be described.
[0014]
C needs to be at least 0.04% in order to ensure the strength of the base material and the weld. However, if there is too much C, the low temperature toughness, weldability, and HAZ toughness of the base material are also lowered, so the upper limit was made 0.12%.
[0015]
Si is an element added for deoxidation, but if added in a large amount, the weldability and HAZ toughness deteriorate, so the upper limit was made 0.4%. From the viewpoint of improving the HAZ toughness, 0.15% or less is desirable.
[0016]
Mn is an element indispensable for ensuring the strength and toughness of the base material and the welded part. Also, Mn is bonded to S and precipitated as MnS on an oxide mainly composed of Ti and Mg, thereby causing intragranular transformation. In order to promote the production, the lower limit is made 0.8%. However, too much Mn promotes center segregation of the slab and degrades the HAZ toughness and weldability, so the upper limit is made 2.0%.
[0017]
When P is present in a large amount, it promotes center segregation, and further causes grain boundary embrittlement by segregating at the grain boundaries, leading to a significant decrease in toughness. For this reason, P is made 0.02% or less.
[0018]
If S is also present in a large amount, the center segregation is promoted, and a large amount of elongated MnS is generated to deteriorate the toughness of the base material and the HAZ, so the upper limit is made 0.003%.
[0019]
Since Al forms (Mg, Al) oxide which is a precipitation nucleus of TiN which is a pinning particle for suppressing austenite grain growth, 0.001% is necessary as a lower limit. However, if Al is present in excess, the content of Al in the oxide serving as an intragranular transformation nucleus site increases, the content of Ti and Mg decreases, and the intragranular transformation ability from the oxide decreases. Therefore, the upper limit is made 0.005%.
[0020]
Conventionally, Ni was added in an amount of 1.0 to 4.0% for the purpose of increasing the toughness of the matrix. However, since Ni steel is expensive, adding a large amount of Ni leads to high costs. In the present application, by adding a small amount of Mg and regulating the amount of alloying elements to appropriate amounts, it is possible to achieve high toughness of HAZ at low temperatures without relying on expensive Ni. By reducing the Ni content defined in the present application to less than 1.0%, a large alloy cost can be reduced.
[0021]
Ti is an element to be controlled in a narrow concentration range by an appropriate effective Ti amount. Further, Ti is an element indispensable for the formation of TiN as pinning particles and an oxide serving as a nucleus of intragranular transformation, and in order to precipitate a sufficient amount thereof, the lower limit is made 0.005%. On the other hand, when Ti exceeds 0.02%, even if it is within the range of the appropriate effective Ti amount, a substantial amount of TiC is generated and the HAZ toughness is lowered.
[0022]
N is necessary for securing the number of composite precipitated TiN as pinning particles, and must be limited to a narrow range in combination with an appropriate range of effective Ti amount. When N is less than 0.002%, the number of TiN cannot be secured. On the other hand, if N exceeds 0.006%, even if the effective Ti amount is within an appropriate range, the solid solution N is substantially excessive and the HAZ toughness is lowered.
[0023]
Even if O is in an appropriate effective Ti amount region, if there is too much O, the size of the oxide becomes large, and the number of oxides having a size of ≧ 5 μm that becomes the starting point of destruction increases. On the other hand, if the amount is small, the number of oxides of 0.5 to 5 μm that become the transformation nuclei of intragranular ferrite is insufficient, so that sufficient toughness cannot be improved. Therefore, O is 0.001 to 0.004%.
[0024]
Mg is the most important element for toughening in the present application, and forms a 0.01 to 0.1 μm (Mg, Al) oxide which is a precipitation nucleus site of TiN. In addition, an oxide of 0.5 to 5 μm contains Ti and Mg on average 15% by weight or more, and Mg contains 3% by weight or more, so that intragranular transformation is generated to be equal to or higher than Ti-based oxides. To promote. In order to sufficiently generate these two kinds of oxides, Mg needs to be added in an amount of 0.0001% or more. On the other hand, 0.003% of Mg consumed as an oxide is sufficient, and Mg beyond this has no metallurgical effect, leading to high costs, which is contrary to the spirit of the present application.
[0025]
Cu has substantially the same effect as Ni and is 0.05% or more, and is also effective in corrosion resistance, hydrogen-induced cracking resistance, etc., but if it exceeds 0.5%, Cu cracking occurs during hot rolling, and production It becomes difficult. For this reason, the upper limit was made 0.5%.
[0026]
Nb suppresses ferrite generated at the austenite grain boundaries, is an element effective for refining the base material structure, and improves the mechanical properties of the base material. However, if the amount is too large, the HAZ toughness deteriorates, so the content was made 0.05% or less.
[0027]
The dispersion state of inclusions defined in the present invention is quantitatively measured by, for example, the following method. The dispersion state of 0.01 to 0.5 μm inclusions in which (Ng, Al) oxide (an oxide composed of Mg and Al) and TiN are combined is extracted from an arbitrary place on the base steel plate. Are observed over an area of at least 1000 μm ² at a magnification of 10,000 to 50,000 times using a transmission electron microscope (TEM), and the number of composite inclusions of a target size is measured per unit area. Convert to the number of items. At this time, the (Mg, Al) oxide and TiN are identified by composition analysis by energy dispersive X-ray spectroscopy (EDS) attached to TEM and crystal structure analysis of electron diffraction image by TEM. When it is complicated to perform such identification on all the complex inclusions to be measured, the following means are simply used. First, square inclusions are regarded as TiN, and at least 10 or more inclusions are identified in the target size of TiN and identified in the manner described above, and (Mg, Al) oxidation is performed. The ratio in which the product and TiN exist in a complex manner is calculated. Then, this ratio is multiplied by the number of composite inclusions measured first. When carbides in the steel interfere with the above TEM observation, the carbides can be aggregated and coarsened by a heat treatment at 500 ° C. or less to facilitate observation of the target composite inclusions.
[0028]
An example of measuring the number of oxides of 0.5 to 5 μm mainly composed of Ti and Mg is shown below. A small piece sample is cut out from an arbitrary place on the base metal steel plate, and this is held at 1400 to 1450 ° C. for 10 minutes or more to solubilize 0.5 to 5 μm inclusions other than oxides, and then cooled with water. This is mirror-polished and observed over an area of at least 1 mm ² with an optical microscope at a magnification of 1000 times, and at least 10 oxides of the target size are attached to an X-ray microanalyzer (EPMA). The wavelength dispersion type spectroscopy (WDS) is used to analyze the composition, and the sum of the Ti and Mg contents in the average composition of the oxide is determined by weight%, and the sum thereof is determined. At this time, in the case where Fe of the iron is detected in the analysis value of the oxide composition, the average composition of the oxide is obtained by excluding Fe from the analysis value.
[0029]
【Example】
Table 1 shows the chemical composition and the effective Ti amount and Di ^* values used as component control indices, and Table 2 shows the manufacturing conditions of the steel sheet and the dispersion state and mechanical properties of inclusions effective for toughening. Moreover, the figure which plotted the result of the Charpy test in HAZ part with respect to this invention steel and conventional steel with respect to the amount of addition Ni is shown in FIG.
[0030]
As can be seen from FIG. 1, the steel obtained in the present application is as good as the conventional steel having a Ni content of 1.0 to 4.0% although the Ni content is less than 1.0%. It can be seen that it exhibits excellent HAZ toughness. Steels 19 and 20 were obtained by reducing the amount of Ni without adding Mg, but Charpy absorbed energy was 100 kJ / cm or less, and sufficient HAZ toughness was not obtained. In Steel 21, Di ^* is small and sufficient strength of 377 MPa is not obtained in YS. On the other hand, in Steel 22, since Di ^* is too high, a hardened phase appears and good HAZ toughness is not obtained. In Steel 23, since the amount of added aluminum is too large, O associated with Ti decreases and the effective Ti amount increases too much, so TiC is generated and good HAZ toughness is not obtained. Since a sufficient amount of pinning particles could not be obtained in steel 24, austenite grains were coarsened, leading to a reduction in HAZ toughness. In Steel 25, oxides mainly composed of Ti and Mg that promote intragranular transformation are not present in proper amounts in both composition and quantity, and sufficient intragranular ferrite is not obtained, and good HAZ toughness is obtained. It is not done.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
【The invention's effect】
According to the present invention, steel having excellent low temperature toughness of the heat affected zone can be produced at a lower cost.
[Brief description of the drawings]
FIG. 1 is a diagram in which the result of a Charpy test in a HAZ part is plotted against the amount of added Ni.

Claims (2)

% By weight
C: 0.04 to 0.12%,
Si: 0.4% or less,
Mn: 0.8 to 2.0% or less,
P: 0.02% or less,
S: 0.003% or less,
Al: 0.001 to 0.005%,
Ni: less than 1.0%,
Ti: 0.005 to 0.02%,
N: 0.002 to 0.006%,
O: 0.001 to 0.004%,
Mg: 0.0001 to 0.003%
Di ^* = 0.316C ^1/2 (1 + 0.7Si) (0.35 + 4.1Mn) (1 + 0.36Ni) (1 + 0.37Cu), with the chemical component consisting of iron and inevitable impurities Is limited in the range of 0.7 to 0.851 , and the effective Ti amount defined by the formula: Ti-2 = O-2 (O-0.66Mg-0.89Al) -3.4N is -0 0.10% to + 0.005%, and 0.01 to 0.3 μm of TiN containing 0.01 to 0.1 μm of an oxide composed of Mg and Al is 30000 pieces / mm ² or more, In addition, a low Ni and a welding heat-affected zone characterized in that there are 30 / mm ² or more oxides having a size of 0.5 to 5 μm, with the sum of the average contents of Mg and Ti being 15% by weight or more. Steel sheet with excellent low temperature toughness. In addition by weight%
Cu: 0.05 to 0.5%,
Nb: 0.05% or less of 1 type or 2 types, The steel plate excellent in the low-temperature toughness of the low Ni and welding heat affected zone of Claim 1 characterized by the above-mentioned.

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