JPH0987802A - High tensile strength steel plate excellent in plating crack resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high tensile strength steel plate excellent in plating crack resistance. SOLUTION: This high tensile strength steel plate has a composition which consists of 0.05-0.2% C, <=0.35% Si, <=2% Mn, <=0.1% sol. Al, <=0.010% S, <=0.0002% B, and the balance Fe with inevitable impurities and in which the value of carbon equivalent Ceq, represented by Ceq=C+Si/24+Mn/6, is regulated to <=0.40%. Moreover, this steel plate has a structure composed essentially of fine acicular ferrite, and further, the volume percentage (P) of pearlitic structure and the volume percentage (B) of bainitic structure are regulated to <=6% and <=15%, respectively, and 2P+B<20% is satisfied. It is recommended that, as a method for its production, a method, in which a slab of steel of this composition is heated to 1150-1300 deg.C and subjected, after the completion of rolling at 740-880 deg.C, to cooling at >=30 deg.C/s average cooling rate and then to coiling at 400-600 deg.C, is adopted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet having excellent resistance to plating cracks, which is suitable as a welded structural member used in large buildings such as steel towers and bridges, and a method for producing the same.

[0002]

2. Description of the Related Art It is possible to easily increase the strength of a steel material by increasing the amount of C% and adding other strength improving elements. However, high-strength steel used as a welded structural member has a problem that when hot dip galvanizing is applied for the purpose of rust prevention, cracking occurs in the weld heat-affected zone when immersed in a plating bath. There is a problem in that the crack resistance to plating becomes poorer as the value becomes higher.

Therefore, various measures have been studied and reported so far for the purpose of improving the plating crack resistance of high-strength steel. For example, Japanese Patent Laid-Open No. 59-50157 discloses a low alloy steel containing S in the range of 0.03 to 0.06%, focusing on the effect of improving the plating crack resistance of S. However, the low-alloy steel is improved in hot-dip galvanizing cracking resistance, but at the same time, its low temperature toughness is lowered, so that its application is limited.

In addition to the above, a technique for improving plating crack resistance has been proposed from the viewpoint of component composition.
Japanese Patent Application Laid-Open No. Sho 59-59 in which the N content is restricted to 0.01 to 0.02%
-200741, JP-A-59-126754 in which the upper limit of the Al content is regulated to 0.1%, and the content of B is 0.0.
Japanese Patent Publication No. 6-86649, which is limited to 002% or less, and Japanese Patent Laid-Open Publication No. Sho 6-06
1-133363 and JP-A-62-50448 are known.

[0005] In recent years, the transmission tower has become larger in size with the increase in the supply voltage, and the bridge tends to become longer.
Structural members used in such large-scale constructions are required to have higher strength in order to reduce the weight of the welded structure. With the increase in size of the above welded structures, high tensile strength steel with a tensile strength of 55 kgf / mm ² which has been the mainstream until now,
High-strength steel of about kgf / mm ² is being put to practical use. Along with the increase in strength of steel materials, it is not possible to prevent plating cracks due to thermal stress during plating with techniques that rely on the limitation of specific components, such as the conventional techniques described above. A new technology has been required because the crackability cannot be obtained.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a high-strength steel sheet excellent in plating crack resistance and a method for manufacturing the same.

[0007]

The high-strength steel sheet of the present invention which has solved the above-mentioned problems is C: 0.05 to 0.2%, Si:
0.35% or less, Mn: 2% or less, sol. Al: 0.
1% or less, S: 0.010% or less, B: 0.0002%
Hereinafter, the balance: Fe and unavoidable impurities, and the carbon equivalent Ceq = C + Si / 24 + Mn / 6 given by the following formula is 0.40% or less, and fine and acicular ferrite is formed. It is composed of a main structure, and the pearlite structure volume ratio (P) is 6% or less, the bainite structure volume ratio (B) is 15% or less, and 2P + B <20% is satisfied. .

Note that Ca: 0.0005 to 0.01% may be contained, and in addition to the above component composition, Ti: 0.
1% or less, Nb: 0.1% or less, V: 0.1% or less, C
r: 0.5% or less, Mo: 0.5% or less, Cu: 1.0
% Or less, Ni: 1.0% or less, and at least one selected from the group consisting of carbon equivalent Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / It is desirable that the value of 40 + V / 14 + Mo / 4 is 0.40% or less.

As a manufacturing method, a steel slab having the above-mentioned composition is heated to 1150 to 1300 ° C. and then 740 to 740.
It is recommended to adopt a method in which after rolling is completed in the temperature range of 880 ° C., it is cooled at an average cooling rate of 30 ° C./s or more and wound in the temperature range of 400 to 600 ° C.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have earnestly studied how the composition of high-strength steel and the manufacturing conditions such as hot rolling conditions and cooling conditions influence the resistance to plating cracking. As a result, by controlling the microstructure of the high-strength steel sheet by limiting the component composition and manufacturing conditions, it was found that the plating crack resistance in the weld heat-affected zone of the high-strength steel sheet can be greatly improved. Is.

Specifically, by limiting the volume ratio of pearlite and the volume ratio of bainite to make the structure mainly composed of fine and acicular ferrite, it has been possible to obtain a high-strength steel excellent in plating crack resistance. It was done. First, the reasons for limiting the chemical components in the present invention will be described.

C is an element that contributes to the increase in strength,
If it is less than 0.05%, it is difficult to secure the strength,
Addition of a large amount exceeding 0.2% deteriorates weldability and toughness. Therefore, the amount of C added is 0.05 to 0.2%.
Limited to the range.

Si is an essential element for ensuring strength and deoxidizing, but if it exceeds 0.35%, it deteriorates the soundness and toughness of the hot dip galvanized surface, so the upper limit of the addition amount is 0.35%. And

Mn is an essential element for securing strength, weldability and toughness, but if added in excess of 2%, weldability, toughness and hot dip galvanizing crack resistance deteriorate. Therefore, the upper limit of the added amount is 2%.

Sol. Al originates from Al added as a deoxidizing element, but if added in excess of 0.1%, not only is it economically disadvantageous, but also many C-based inclusions are generated, causing surface scratches. Therefore, the upper limit of the amount added is 0.1%.
And

S is an element contained as an unavoidable impurity in steel and forms non-metallic inclusions to deteriorate toughness and promote anisotropy, so it is preferable to reduce S as much as possible. In particular, if added in excess of 0.010%, the toughness is greatly deteriorated, so the upper limit of addition is made 0.010%.

When B is contained in an amount exceeding 0.0002% of a high-strength steel sheet which deteriorates the hot-dip galvanizing cracking resistance, the degree of the deterioration becomes extremely large. Therefore, the upper limit of the added amount is 0.0002%.

Ca is an element that controls the morphology of inclusions and improves the toughness in the plate thickness direction in a small amount, but 0.0005%
If less than 0.01%, the effect is small, and if added in excess of 0.01%, non-metallic inclusions in the steel increase, causing internal defects. Therefore, the range is 0.0005-0.01.
%. Note that the rare earth element is an element that exhibits the same effect as Ca in the present invention, and it is also possible to use Y, La series elements, or the like as an alternative element to Ca.

Further, in order to prevent low temperature cracking during welding and toughness deterioration of the weld heat affected zone in high heat input welding,
It is necessary to control the carbon equivalent Ceq defined by the following equation to 0.40 or less. Ceq = C + Si / 24 + Mn / 6

According to the present invention, a steel sheet obtained by using a steel slab having the above-mentioned composition and then cooling it after hot rolling and winding it up has a microstructure at a depth of 1/4 of the sheet thickness, Perlite structure volume ratio (P) is 6% or less and bainite structure volume ratio (B) is 15% or less, and further 2P + B
By controlling the fine ferrite preferential structure to satisfy <20%, it is possible to obtain a high-strength steel sheet having excellent hot dip galvanizing crack resistance.

If necessary, Ti, Nb, V, C
It is recommended to contain at least one selected from the group consisting of r, Mo, Cu and Ni from the viewpoint of improving strength and toughness. The range of the added amount of each element and the reason for limiting it are
It is shown below.

Ti is an element effective for improving the toughness of the weld joint by refining the austenite grains in the heat affected zone of the welding and promoting the generation of ferrite. However, if added in excess of 0.1%, the weldability will deteriorate, so the upper limit of the addition is preferably 0.1%.

Nb is an element effective for increasing strength by precipitation hardening and improving toughness by refining crystal grains. However, if added in excess of 0.1%, the toughness of the welded joint will deteriorate, so the upper limit of the addition is preferably 0.1%.

V is an element effective in increasing the strength by precipitation hardening, but if added in excess of 0.1%, the weldability deteriorates. Therefore, when adding V, 0.1
It is desirable that the upper limit be%.

Cr and Mo are both effective elements for securing strength and improving corrosion resistance, but if added in excess of 0.5%, toughness and weldability deteriorate. Therefore, the upper limit of the added amount is preferably 0.5%.

Cu is an element that contributes to the strength increase by solid solution strengthening and precipitation hardening, and is also an element effective in improving the corrosion resistance. However, if it is added in excess of 1.0%, hot work cracking tends to occur, so 1.0% is preferable as the upper limit.

Ni is an element effective in increasing the strength and improving the toughness, but even if added in a large amount exceeding 1.0%, the effect is saturated, and the addition thereof beyond that is economically disadvantageous. . Therefore, it is desirable to set the upper limit of the amount of Ni added to 1.0%.

In the case of containing Cr, Ni, V and Mo, the carbon equivalent Ceq is set to the lower limit in order to prevent low temperature cracking during welding and toughness deterioration of the weld heat affected zone in high heat input welding. It is necessary to limit the carbon equivalent Ceq defined by the formula to 0.40 or less. Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + V / 14 + Mo / 4

Next, the reasons for limiting the range of manufacturing conditions will be described. Rolling end temperature of billet is 740-880 ° C
By this, the austenite grains are sufficiently refined and a fine ferrite structure is obtained. If it is lower than 740 ° C., rolling resistance remarkably increases, which is not preferable. On the other hand, if it exceeds 880 ° C., the austenite grains are not sufficiently refined and fine ferrite grains cannot be obtained. Therefore,
The rolling end temperature is in the range of 740 to 880 ° C.

Immediately after rolling, the average cooling rate was 30 ° C. /
It is necessary to cool to 400 to 600 ° C. for s or more. When the average cooling rate is 30 ° C./s or more, the structure mainly consists of fine and acicular ferrite, and excellent plating crack resistance is obtained. If it is less than 30 ° C./s, a fine and acicular ferrite structure cannot be obtained, and the pearlite volume ratio, which hinders plating crack resistance, increases.

If the coiling temperature is lower than 400 ° C., island-like martensite, which is a low-temperature transformation product, is formed, which lowers the low-temperature toughness. On the other hand, when the coiling temperature exceeds 600 ° C., the volume ratio of pearlite increases and the predetermined strength cannot be secured. Therefore, the winding temperature is in the range of 400 to 600 ° C.

In the present invention, by limiting the production conditions as described above, that is, a steel piece satisfying the above-mentioned composition of ingredients is heated to 1150 to 1300 ° C, and 740 to 880 ° C.
After the rolling is completed in the temperature range of 1), it is cooled at an average cooling rate of 30 ° C / s or more and wound in the temperature range of 400 to 600 ° C.
In the microstructure at 4 depths, the pearlite structure volume ratio (P) is 6% or less and the bainite structure volume ratio (B) is 1 or less.
When the content is 5% or less, and 2P + B <20% is further satisfied, a high-strength steel sheet having a structure mainly composed of fine and acicular ferrite can be produced, and excellent plating crack resistance can be obtained.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design of the present invention can be made in view of the gist of the preceding and the following. It is included in the technical scope.

[0034]

Example: Steels having the components shown in Table 1 were melted, and the temperature was 1250 ° C.
After heating to, the plate thickness was finished to 12 mm by controlled rolling. Test pieces were taken from these steel sheets and subjected to a room temperature tensile test, a Charpy impact test, and a hot dip galvanizing cracking test. Table 2 shows tensile strength, fracture surface transition temperature, and plating crack resistance.

FIG. 1 shows a test piece used in the hot-dip galvanizing crack test. In this test, stress is applied to the test bead toe 2 by the restraining bead 3, and then the test piece is immersed in a hot dip galvanizing bath to determine if the test bead toe 2 is cracked or not. The sex is evaluated.

The welding condition for the test bead was a rod diameter of 4 m.
m, current 170 A, voltage 24 V, welding speed 150 mm /
The welding conditions for the restraining bead are rod diameter 5 mm and current 2
20 A, voltage 26 V, welding speed 150 mm / min.

[0037]

[Table 1]

[0038]

[Table 2]

No. Nos. 1 to 10 are examples of the present invention having a structure mainly composed of fine and acicular ferrite, and the tensile strength was high, plating cracks did not occur, and excellent plating crack resistance was exhibited. On the other hand, No. 11 to 17 are comparative examples that do not satisfy one or more of the conditions according to the present invention, and plating cracks have occurred. In addition, No. No. 16 is a comparative example in the case where the S content is too large, and plating cracks did not occur, but it is understood that the fracture surface transition temperature is high and the low temperature toughness is poor.

FIG. 2 is a graph showing the relationship between the pearlite volume ratio (P) and the bainite volume ratio (B) and the presence or absence of plating cracks from the results of Table 2. FIG.
As is clear from the above, when the pearlite volume ratio (P) is 6% or less and the bainite volume ratio (B) is 15%, an additional 2P + is obtained.
It was confirmed that plating cracks did not occur in the range of B <20%.

[0041]

EFFECTS OF THE INVENTION Since the present invention is constituted as described above, it is possible to provide a high-strength steel sheet excellent in plating crack resistance and a manufacturing method thereof.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a test piece used for hot-dip galvanizing cracking.

FIG. 2 is a graph showing the relationship between the microstructure volume ratio of a steel sheet and the presence or absence of hot dip galvanizing cracking.

[Explanation of symbols]

 1 Test plate 2 Test bead 3 Restraint bead

Claims (4)

[Claims] 1. C: 0.05 to 0.2% by weight (hereinafter simply referred to as%), Si: 0.35% or less, Mn: 2% or less, sol. Al: 0.1% or less, S: 0.010%
Below, B: 0.0002% or less, balance: Fe and unavoidable impurities, and the carbon equivalent Ceq = C + Si / 24 + Mn / 6 given by the following formula is 0.40% or less. , A fine and acicular ferrite-based structure as a main component, a pearlite structure volume ratio (P) of 6% or less, and a bainite structure volume ratio (B)
Is 15% or less and satisfies 2P + B <20%. A high-strength steel sheet having excellent plating crack resistance. 2. Further, Ca: 0.0005 to 0.01%
The high-strength steel sheet according to claim 1, which comprises: 3. Further, Ti: 0.1% or less, Nb: 0.
1% or less, V: 0.1% or less, Cr: 0.5% or less,
Mo: 0.5% or less, Cu: 1.0% or less, Ni: 1.
0% or less, one or more selected from the group consisting of, and the carbon equivalent given by the following formula Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + V / 14 + The high-tensile steel plate according to claim 1 or 2, wherein the value of Mo / 4 is 0.40% or less. 4. A steel slab having the composition according to claim 1 is heated to 1150 to 1300 ° C.,
After finishing the rolling in the temperature range of 40 to 880 ℃, 30 ℃
A method for producing a high-tensile steel sheet, which comprises cooling at an average cooling rate of / s or more and winding in a temperature range of 400 to 600 ° C.