JP2001279377A - Hot dip galvanized steel sheet excellent in spot weldability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip galvanized steel sheet with a composite structure in which the formation of a composite structure is attained without adding a large amount of Si and P harmful to plating properties and spot weldability. SOLUTION: This hot dip galvanized steel sheet excellent in spot weldability has a composition containing, by mass, 0.02 to 0.1% C, <=0.1% Si, 1.5 to 3.0% Mn, <=0.020% P and <=0.06% Al, and the balance iron with inevitable impurities and has a structure of the main phase composed of polygonal ferrite and a hard second phase in which the volume ratio of martensite is >=90%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled composite structure hot-dip steel sheet which is suitable for use mainly as a strength member of an automobile and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of global environmental protection, there has been a demand for improvement of fuel efficiency and reduction of CO ₂ emission of automobiles. In order to meet these demands, weight reduction of automobile bodies has been promoted. To reduce the weight, it is effective to reduce the thickness of the steel sheet by increasing the strength, and therefore, it is intended to use a high-strength steel sheet. On the other hand, when processing a steel sheet, it is required that the steel sheet can be easily formed. Therefore, a steel sheet having high strength and excellent workability has been desired.

As a steel sheet having such high strength and excellent workability, a steel sheet having a composite structure of a ferrite phase and a low-temperature transformation phase is known. For example, JP-A-60-52528 discloses There has been proposed a method for producing a high-strength steel sheet having excellent ductility by annealing a low-carbon steel at a high temperature and precipitating a martensite phase after cooling. In addition, JP-A-10-1955
No. 88 contains components such as Si and P and has an average particle size of 10 μm.
m or less of the ferrite phase occupies 80 to 97% by volume, and the balance consists of a second phase mainly composed of martensite having an average grain size of 0.2 to 1.5 times the average grain size of ferrite. A hot-rolled high-tensile steel sheet having excellent properties has been proposed.

[0004] Such a ferrite and martensite dual phase steel sheet generally contains Si or P positively in order to promote ferrite transformation during cooling, and is hot-rolled at a temperature not lower than the Ar ₃ transformation point. Finished, quenched to the temperature at which ferrite precipitates, then air-cooled, fully precipitate the ferrite, concentrate carbon in the remaining austenite phase, and then quenched to transform austenite to martensite It has both high strength by the hard second phase and excellent moldability by the ferrite phase.

[0005]

However, in the above-described conventional composite structure steel sheet, Si or P is added in order to promote ferrite transformation during cooling.
In some cases, surface treatment properties or weldability were not sufficiently satisfactory. For example, steel to which a large amount of Si is added is inferior in hot-dip galvanizing properties, and steel to which P is added may be inferior in spot weldability.

[0006] Steel sheets for automobiles are sometimes subjected to hot-dip galvanizing in order to improve rust prevention and appearance, and spot welding is performed during assembling processing. It is more desirable that the property is good. Accordingly, an object of the present invention is to propose a composite structure steel sheet which is excellent in hot-dip galvanizing property and spot weldability and aims at a composite structure without adding a large amount of Si and P, together with its advantageous production method. I do.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted research and development to improve the plating property and the spot weldability of a composite structure steel sheet, and as a result, a conventional composite structure forming method,
By adding Si and P, thereby promoting ferrite transformation after hot rolling, securing a predetermined amount of ferrite through grain growth of ferrite, and enriching C in austenite, thereby transforming the second phase into martensite. The present inventors have found that the improvement of the plating property and the spot weldability of a composite structure steel sheet can be achieved by forming a composite structure by a completely different method, not by a method of forming a composite structure.

[0008] The present invention relates to a method for producing C: 0.02-0.1 mass%, Si:
0.1 mass% or less, Mn: 1.5 to 3.0 mass%, P: 0.020 ma
ss% or less and Al: 0.06 mass% or less, the balance being iron and unavoidable impurities, and a main phase composed of polygonal ferrite and a hard second phase having a martensite volume ratio of 90% or more. This is a hot-dip galvanized steel sheet having a structure consisting of phases and having excellent spot weldability.

In another embodiment of the present invention, C: 0.02-0.1 ma
ss%, Si: 0.1 mass% or less, Mn: 1.5 to 3.0 mass%,
P: 0.020 mass% or less, Al: 0.06 mass% or less, and Mo: 0.
A hard secondary alloy containing 0.4 to 0.4 mass%, the balance being iron and unavoidable impurities, and a main phase composed of polygonal ferrite and a martensite having a volume fraction of 90% or more.
This is a hot-dip galvanized steel sheet having a structure consisting of phases and having excellent spot weldability.

In another embodiment of the present invention, C: 0.02-0.1 ma
ss%, Si: 0.1 mass% or less, Mn: 1.5 to 3.0 mass%,
P: 0.020 mass% or less and Al: 0.06 mass% or less,
The rest consists of steel slabs with a composition of iron and unavoidable impurities.
Heat to a temperature of 00 ° C or less, then perform hot rolling under the condition that the rolling reduction in the final finishing stand is 30% or more, and start cooling within 0.1 second from the end of hot rolling to a cooling rate of 20 ° C / s.
After cooling to a temperature in the range of 700 ° C to Ar ₃ transformation point above, winding at 700 ° C or more, then heating to 750 to 800 ° C, and below the martensite transformation point at a cooling rate of 10 ° C / s or more This is a method for producing a hot-dip galvanized steel sheet having excellent spot weldability, wherein the hot-dip galvanized steel sheet is subjected to hot-dip galvanizing after cooling.

In another embodiment of the present invention, C: 0.02-0.1 ma
ss%, Si: 0.1 mass% or less, Mn: 1.5 to 3.0 mass%,
P: 0.020 mass% or less, Al: 0.06 mass% or less, and Mo: 0.
A steel slab containing 04 to 0.4 mass%, the balance being iron and unavoidable impurities, is heated to a temperature of 1100 ° C or less, and then hot-rolled to a final rolling stand with a rolling reduction of 30%.
Under the above conditions, start cooling within 0.1 seconds after the end of hot rolling, cool at a cooling rate of 20 ° C / s or more to a temperature in the range of 700 ° C to the Ar ₃ transformation point, and wind at 700 ° C or more. Hot-dip galvanized steel sheet with excellent spot weldability, which is then heated to 750 to 800 ° C, then cooled to a temperature below the martensitic transformation point at a cooling rate of 10 ° C / s or more, and then galvanized. It is a manufacturing method of.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, in order to achieve both good plating properties and spot weldability, the amounts of Si and P, which may deteriorate the plating properties and spot weldability, are reduced. In the manufacturing process, the slab heating temperature is lowered, the rolling reduction of the final finishing stand is increased, and by rapid cooling immediately after rolling, a large amount of ferrite transformation nuclei in austenite is introduced to promote ferrite transformation. At the same time, by setting the winding temperature to a high temperature, polygonal ferrite is finely and uniformly generated, and the enrichment of C in the second phase is promoted. Furthermore, rapid cooling is performed subsequent to the heating in the annealing step before the hot-dip galvanizing treatment, so that the second phase is transformed into martensite to obtain a predetermined structure.

[0013] The composition of the steel sheet of the present invention is as follows: C: 0.02-
0.1 mass%, Si: 0.1 mass% or less, Mn: 1.5 to 3.0 mass
%, P: 0.020 mass% or less and Al: 0.06 mass% or less, with the balance being iron and unavoidable impurities, or
C: 0.02 to 0.1 mass%, Si: 0.1 mass% or less, Mn: 1.5
3.0 mass%, P: 0.020 mass% or less, Al: 0.06 mass%
The following and Mo: 0.04 to 0.4 mass% are contained, and the rest is composed of iron and inevitable impurities.

Hereinafter, the reason why the composition range of the component is limited in the present invention will be described. C: 0.02 to 0.1 mass% C is a component necessary for increasing the strength and volume fraction of martensite in the two-phase structure. If the C content is less than 0.02 mass%, a sufficient amount of martensite cannot be obtained in the second phase. On the other hand, when the C content exceeds 0.1 mass%, the spot weldability deteriorates. For this reason, the content of C is 0.02-0.
1 mass%.

Si: 0.1 mass% or less Si promotes ferrite transformation, and also enriches solid solution C in ferrite in austenite to improve the hardenability of steel. Has been positively added to. However, Si precipitates as an oxide on the surface of the steel sheet during the annealing step, which is a pretreatment step of the hot-dip galvanizing treatment, so that the wettability of the hot-dip zinc is significantly reduced, and as a result, poor plating is induced. Is an undesirable component. Therefore, in the present invention, the Si content is reduced as much as possible, and the upper limit of the Si content is set to 0.1 mass%. More preferably, 0.02 mass%
The following is assumed.

Mn: 1.5 to 3.0 mass% Mn is an austenite-forming component and is a component necessary for ensuring sufficient hardenability of steel. In the present invention, it is necessary to increase the amount of Mn in part because the amount of Si is reduced, and therefore, Mn is contained in an amount of 1.5 mass% or more. However, if the Mn content exceeds 3.0 mass%, the steel sheet hardens and the formability decreases. Therefore, the Mn content is 1.
The range is 5 to 3.0 mass%.

P: 0.020 mass% or less P promotes ferrite transformation like Si, and has the effect of concentrating solid solution C in ferrite in austenite to improve the hardenability of steel. Therefore, it is a component that has been positively added to the conventional composite structure steel sheet. However, since P embrittles the button portion during spot welding and lowers the welding strength, in the present invention, P is reduced as much as possible like Si, and the upper limit of the P content is set to 0.02 mass%. More preferably, 0.015 ma
ss% or less.

Al: 0.06 mass% or less Al is a component usually added to steel as a deoxidizing component. However, if the addition amount exceeds 0.06 mass%, a large amount of Al ₂ O ₃ is generated, and the continuous casting process is performed. In this case, the upper limit was set to 0.06 mass% because nozzle clogging and a decrease in cleanliness were caused.

In addition to the above components, Mo can be added as required in the present invention. Mo: 0.04 to 0.4 mass% Mo is a desirable component because it has the effect of lowering the critical cooling rate for producing martensite. When adding Mo, if the amount of addition is less than 0.04 mass%, the intended effect cannot be sufficiently obtained.
The effect saturates even if it exceeds ss%. Therefore, 0.
The range is from 04 to 0.4 mass%.

Next, the structure of the steel sheet of the present invention will be described. The structure is composed of a main phase composed of polygonal ferrite and a hard second phase having a martensite content of at least 90% by volume. The hot-rolled steel sheet targeted in the present invention is a composite structure steel, and it is necessary to use polygonal ferrite as a main phase similarly to a normal composite structure steel. Here, the main phase means that the volume fraction occupies 60% or more, and if the volume fraction of the polygonal ferrite is less than 60% of the entire structure, the moldability deteriorates.

The method for producing a steel sheet according to the present invention will be described later. However, in the present invention, a large amount of fine ferrite is introduced into austenite by introducing a large amount of ferrite transformation nuclei into austenite by limiting rolling conditions and cooling conditions after rolling. Thus, the required volume fraction of ferrite is obtained and the enrichment of C in the second phase is promoted. Therefore, polygonal ferrite produced in the cooling process after hot rolling is a fine ferrite having an average crystal grain size of 10 μm or less, and this polygonal ferrite does not grow in the subsequent annealing step or plating step. Therefore, the steel sheet of the present invention has an average crystal grain size of polygonal ferrite as a main phase of 10 μm or less, and thus has excellent formability and impact resistance.

On the other hand, the second phase is composed of a hard low-temperature transformation phase, and it is necessary that most of the second phase be martensite as in the case of ordinary composite structure steel. If the volume fraction of martensite in the second phase is less than 90%, the strength is reduced and the advantage of the composite structure steel is impaired.

Next, a method for manufacturing a steel sheet according to the present invention will be described. The steel slab adjusted to the component composition range described above is heated to a temperature of 1100 ° C. or less, and then hot rolling is performed at a reduction ratio of 30% or more in the final finishing stand,
Start cooling within 0.1 seconds after the end of hot rolling
After cooling to a temperature in the range of 700 ° C to Ar ₃ transformation point at 20 ° C / s or more, winding at 700 ° C or more, then heating to 750 to 800 ° C, then cooling at a cooling rate of 10 ° C / s or more. After cooling to below the site transformation point, hot-dip galvanizing is applied.

● Slab heating temperature 1100 ° C. or lower By lowering the slab heating temperature,
Al remains, and coarsening of austenite grains is suppressed, and the crystal grain size after finish rolling can be refined. Therefore, the slab heating temperature is set to 1100 ° C. or less. The lower limit of the slab heating temperature is not particularly limited, but needs to be higher than the temperature at which hot rolling can be completed at the Ar ₃ transformation point or higher.

In order to obtain polygonal ferrite having an average crystal grain size of 10 μm or less, the rolling reduction of the final rolling final stand must be 30% or more. By performing high-pressure rolling at the final finish rolling stand in this manner, high-density strain is introduced into the steel, which serves as a core of ferrite transformation and contributes to refinement of polygonal ferrite crystal grains. If the rolling reduction of the final finishing stand is less than 30%, the introduction of strain becomes insufficient and the required volume ratio of ferrite cannot be obtained, and the average crystal grain size of polygonal ferrite does not become 10 μm or less. The finishing rolling temperature is about 830 to 930 ° C.

● Cooling is started within 0.1 second after finishing rolling, and cooling is performed at a cooling rate of 20 ° C./s or more. After strain is introduced at the final finishing rolling stand, in order to suppress recovery of austenite grains and suppress grain growth. It is necessary to start cooling immediately after the end of rolling to lower the temperature to the Ar ₃ transformation point or lower. If the cooling is started more than 0.1 second after the end of the rolling, or if the cooling rate is less than 20 ° C./s, the recovery of austenite grains and the grain growth occur, making it difficult to obtain fine polygonal ferrite.

[0027] ● For cooling stop temperature to obtain a main phase consisting of 700 ° C. to Ar ₃ temperature uniformity fine polygonal ferrite in the range of the transformation point, it is necessary to stop the cooling below Ar ₃ transformation point.
On the other hand, if the cooling stop temperature is too low, polygonal ferrite cannot be obtained, and the winding temperature is 700
Since the temperature needs to be higher than ℃, the lower limit of the cooling stop temperature is set to 700 ℃.

If the winding temperature is 700 ° C. or higher and the winding temperature is lower than 700 ° C., the polygonal ferrite grains cannot be uniformly distributed.
It must be at least 700 ° C. In the present invention, Ar ₃
In order to stop cooling at 700 ° C or lower below the transformation point, cooling may be completed in the first half of the run-out table provided on the exit side of the finishing mill. May be air-cooled. In this case, the cooling stop temperature needs to be adjusted to a temperature at which the winding temperature of 700 ° C. or more can be secured.

● After being heated to 750 to 800 ° C. and cooled to a temperature below the martensite transformation point at a cooling rate of 10 ° C./s or more, the structure of the hot-rolled steel sheet after winding has a main phase composed of polygonal ferrite, And a second phase consisting of
The second phase is austenitized by heating a steel sheet having such a structure to 750 to 800 ° C, and then 10 ° C / s
By cooling at the above cooling rate, martensite having a predetermined volume ratio is obtained in the second phase. Such heating and cooling treatments can also be performed as annealing which is usually performed as a pretreatment for continuous galvanizing. Heating temperature is 750
If the temperature is lower than ° C, the second phase cannot be sufficiently austenitized, and as a result, a required amount of martensite cannot be obtained.
On the other hand, when the temperature exceeds 800 ° C., the concentration of C in the second phase becomes insufficient, and as a result, it becomes difficult to obtain martensite. If the cooling rate is less than 10 ° C./s, the ratio of martensite in the second phase cannot be 90% or more. Needless to say, the cooling needs to be performed to a temperature lower than the martensite transformation point. Further, since the hot-rolled steel sheet of the present invention has a high Mn content, if there is a possibility that the plating property may be deteriorated, the hot-rolled steel sheet may be used prior to the heat treatment.
After heating to 0 ° C. or more to concentrate the Mn on the surface, it is desirable to perform acid washing to remove the Mn-enriched phase. After cooling, hot dip galvanizing is applied. This hot-dip galvanizing may be performed according to a conventional method.

[0030]

EXAMPLES Steel slabs having various component compositions shown in Table 1 were prepared, and these slabs were subjected to slab heating, hot rolling, cooling, and winding under the conditions shown in Table 2. Next, after performing an annealing treatment in a continuous hot-dip galvanizing line, the hot-dip galvanizing treatment was performed at a basis weight of 50 g / m ² per one surface.

[0031]

[Table 1]

[0032]

[Table 2]

Table 3 shows the results of examining the structure, mechanical properties, plating properties and spot weldability of the hot-dip galvanized steel sheet thus obtained. In the table, the plating property is evaluated by the amount of zinc adhering to the tape after applying a tape to the inside of the bent portion after bending the steel sheet and bending back, and peeling off the tape.
Those that hardly adhered were evaluated as ○. In addition, the spot weldability is such that an overlapped portion is provided on two steel plates, the overlapped portion is spot-welded, and the presence or absence of dust at this time is evaluated. It was evaluated based on whether or not the base material portion was broken, and those having no dust and broken at the base material portion were evaluated as ○. Further, in the evaluation of material properties, those having a yield ratio (yield strength / tensile strength) of 0.60 or less and a martensite ratio in the second phase of 90% or more were evaluated as ○ by tensile properties and microstructure examination. Table 3
Therefore, it is clear that the steel sheet having the composition range of the present invention has both excellent plating property and spot weldability.

[0034]

[Table 3]

[0035]

EFFECT OF THE INVENTION The composite structure galvanized steel sheet of the present invention can achieve both the plating property and the spot weldability, so that the use of the steel sheet can be expanded. It is.

Claims (4)

[Claims] C: 0.02 to 0.1 mass%, Si: 0.1 mass% or less, Mn: 1.5 to 3.0 mass%, P: 0.020 mass% or less;
Al: contains 0.06 mass% or less, the balance is composed of iron and unavoidable impurities, and consists of a main phase composed of polygonal ferrite and a hard second phase in which martensite has a volume fraction of 90% or more. Hot-dip galvanized steel sheet with excellent spot weldability with a fine structure. 2. C: 0.02 to 0.1 mass%, Si: 0.1 mass% or less, Mn: 1.5 to 3.0 mass%, P: 0.020 mass% or less, A
l: 0.06 mass% or less and Mo: 0.04 to 0.4 mass%, the balance being iron and unavoidable impurities, and
A hot-dip galvanized steel sheet having an excellent spot weldability and having a structure comprising a main phase made of polygonal ferrite and a hard second phase having a martensite content of 90% or more by volume. 3. C: 0.02 to 0.1 mass%, Si: 0.1 mass% or less, Mn: 1.5 to 3.0 mass%, P: 0.020 mass% or less;
Al: Contain 0.06 mass% or less, the remainder is heated to a temperature of 1100 ° C or less, a steel slab having a composition of iron and inevitable impurities,
Next, hot rolling is performed, and the rolling reduction in the final stand is 30.
%, Start cooling within 0.1 seconds after the end of hot rolling, cool at a cooling rate of 20 ° C / s or more to a temperature in the range of 700 ° C to the Ar ₃ transformation point, and then roll at 700 ° C or more. Hot-dip galvanizing with excellent spot weldability, after heating to 750 to 800 ° C, cooling to a temperature below the martensite transformation point at a cooling rate of 10 ° C / s or more, and then applying hot-dip galvanizing Steel plate manufacturing method. 4. C: 0.02 to 0.1 mass%, Si: 0.1 mass% or less, Mn: 1.5 to 3.0 mass%, P: 0.020 mass% or less, A
l: 0.06 mass% or less and Mo: 0.04 to 0.4 mass%, the balance is steel and the composition of iron and unavoidable impurities is heated to a temperature of 1100 ° C or less, and then hot rolling is performed at the finishing stand. Cooling was started within 0.1 seconds from the end of hot rolling, and the cooling rate was 20%.
After cooling to a temperature in the range of 700 ° C to Ar ₃ transformation point at a temperature of ℃ / s or more, winding at 700 ° C or more, then heating to 750 to 800 ° C and then a martensite at a cooling rate of 10 ° C / s or more A method for producing a hot-dip galvanized steel sheet having excellent spot weldability, characterized in that hot-dip galvanizing is performed after cooling to a temperature below the transformation point.