JP2562964B2 - Manufacturing method of hot rolled high strength steel sheet for heavy working - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention has a tensile strength of 45 kgf / mm ² or more and is excellent in press workability, particularly tensile ductility and hole expandability, and is suitable for use in automobile structural steel sheets. The present invention relates to a method for producing a suitable hot-rolled high-strength steel sheet for strong working.

(Prior Art and Problems to Be Solved) In recent years, “complexity of automobile body structure” and “need for cost reduction by integral molding of parts” have been strengthened, and a steel sheet having good press workability has been demanded. Further, the higher the strength of the steel plate, the more the weight of the vehicle body can be reduced. Therefore, there is a need for higher tension for the purpose of reducing fuel consumption during driving. Further, in order to reduce costs, there is a movement to use hot rolled steel sheets instead of cold rolled steel sheets. For these reasons, a hot rolled steel sheet having excellent press workability is required as a steel sheet for automobile structures.

However, SAPH45 has hitherto been used as a high-strength hot-rolled steel sheet for processing.
Etc. were used, but a large amount of C was formed and a large amount of lamellar cementite that hinders local ductility was formed, and the hole expandability was not always sufficient.

In order to improve the hole expansibility, (1) control the morphology of inclusions and precipitates, (2) reduce the amount of C and reduce the strength by solid solution strengthening and Nb by adding Mn, Si and P. It is effective to supplement by a combination of NbC precipitation strengthening and crystal grain refining effect by addition of etc.

As a proposal regarding such a method, for example, Japanese Patent Laid-Open No.
In 54-36124, MnS, which is an inclusion that inhibits hole expansibility
By using TiS to control the morphology of inclusions, a method for producing a hot-rolled steel sheet having excellent press workability is disclosed. However, the press workability is not sufficient for use in place of the cold rolled steel sheet simply by controlling the inclusions.

Further, Japanese Patent Laid-Open No. 62-23975 discloses a method for producing a hot-rolled steel sheet using low C steel and having excellent hole expandability. With this technology, the strength decreases as much as the amount of C lowered, so in order to increase the strength, a larger amount of Mn,
It is necessary to add alloying elements such as P and Nb. But,
For steel with a C content of 0.03% or less, when a material with a tensile strength of 45 kgf / mm ² or more is made by adding Mn, Nb, etc., the structure becomes acicular ferrite during the transformation process after finish rolling in the hot rolling process. . This acicular ferrite has poor tensile ductility and deteriorates the workability of the steel sheet. Therefore, it is necessary to make the structure polygonal ferrite with excellent tensile ductility,
High-temperature winding is effective as a method for obtaining a polygonal ferrite structure in a steel containing a large amount of Mn, Nb, etc., but a steel containing a large amount of P or Mn has a problem of embrittlement.

As another method, in JP-A-61-48520, α-
By temporarily holding the alloy in the γ2 phase region during cooling, a fine polygonal ferrite structure can be obtained even when a large amount of alloying elements are added and the coil is rolled at low temperature, and hot rolling with excellent hole expandability and tensile ductility is achieved. A method of manufacturing a steel sheet is disclosed. However, it is difficult to maintain the temperature in the two-phase region during cooling because the cooling time is limited in actual operation, and sufficient time cannot be taken for cooling.

As described above, it is effective to use a low C-Nb-added steel containing a large amount of Mn and P to produce a 45 kg class hot-rolled steel sheet having excellent hole expansibility, but the C content is 0.03% or less. In that case, good tensile ductility cannot be obtained without embrittlement by the usual method.

An object of the present invention is to solve the above problems and to provide a method capable of producing a hot-rolled high-tensile steel sheet for high strength, which has high tensile strength and is excellent in press workability, particularly tensile ductility, and hole expandability. It is what

(Means for Solving the Problem) In order to achieve the above-mentioned object, the present inventors have a method for obtaining a polygonal ferrite structure without embrittlement from a steel sheet having a low C content and a high Mn content and a P content from the above viewpoints. I studied about it.

 The experimental results will be described in detail below.

Fig. 1 shows 0.018C-0.21Si-1.21Mn-0.018P-0.005S
Rolling end temperature (FD) that affects tensile properties, hole expandability, and percentage (PF (%)) of polygonal ferrite (PF) in the entire structure using -0.020Al-0.017Nb-containing steel
It shows the effect of T). The slab heating temperature is 1200 ° C and the winding temperature is 450 ° C.

At FDT = 900 ° C, which is a normal FDT, PF is about 5%, and the rest is acicular ferrite. However,
If FDT = 850 or less, PF will be 50% or more, FDT = 900
The elongation is improved by 8% as compared with the case of ℃. Moreover, the decrease in strength accompanying this is as small as 1 kgf / mm ² . Therefore, strength −
Stretch balance (TS x El) is improved. The hole expandability (λb) is also improved.

As described above, it is considered that the strength-elongation balance can be improved by lowering the FDT to 850 ° C or lower for the following reasons.

That is, Nb (C, N) comes to precipitate during rolling, which hinders recrystallization of austenite and increases residual strain, so that the number of ferrite precipitation sites increases. When the number of ferrite precipitation sites increases, the temperature of ferrite formation rises. Since the ferrite becomes polygonal ferrite having a higher ductility as it is generated at a higher temperature, the tensile ductility is improved.

Figure 2 is a photograph of the microstructure, which shows that FDT is 900 ° C.
In the case of (a), almost all is acicular ferrite, whereas in the case of FDT of 800 ° C., about 80% of the whole is polygonal ferrite, and the rest is acicular ferrite. Polygonal ferrite is softer than acicular ferrite, but residual acicular ferrite suppresses the decrease in strength.

Fig. 3 shows the effect of the coiling temperature (CT) on the transition temperature (vTrs) using the same steel as the sample steel in Fig. 1. The hot rolling end temperature is 830 ° C. As is clear from the figure, the transition temperature rises significantly when the coiling temperature is 500 ° C or higher. This is 500
This is because P is segregated at the grain boundaries because it is gradually cooled between 500 and 600 ° C. when it is wound at a temperature of not less than 0 ° C.

From the above basic experiment results, low C high M with excellent hole expandability
In order to obtain good tensile ductility in n, P-Nb-added steel without embrittlement, FDT is 850 ° C or less and CT is 500 or less.
It has been found that it is necessary to keep the temperature below ℃. The present invention has been made based on such knowledge.

That is, the manufacturing method of the hot-rolled high-strength steel sheet for strong working according to the present invention, C: 0.0010 to less than 0.03%, Si: 0.01 to 0.5%, M
n: 1.0 to 2.5%, P: 0.002 to 0.10%, S: 0.001 to 0.015%, A
l: 0.01 to 0.10% and Nb: 0.010 to 0.060%, the balance of the composition of steel consisting of Fe and unavoidable impurities 1100 ℃
After heating at above, hot rolling, after finishing this hot rolling at a temperature of Ar ₃ points or more, 850 ° C or less, by winding at a temperature of 500 ° C or less, polygonal ferrite and acicular ferrite are obtained. It is characterized in that a hot-rolled steel sheet having a main structure is obtained.

 The present invention will be described in more detail below.

(Operation) First, the reasons for limiting the chemical components in the present invention are as follows.

C: The smaller the content of C, the better the hole expandability, and the content is limited to less than 0.03%. However, even if it is reduced to less than 0.0010%, not only the effect is saturated, but also decarburization treatment at the steelmaking stage takes a long time, which is expensive, which is not desirable. Therefore, the C content is 0.
The range is from 001 to less than 0.03%.

Si: Si is a solid solution strengthening element and is effective in increasing the strength of steel without impairing ductility, but if it is contained in excess of 0.5%, the surface properties and the plateability when used as the original plate of plated steel sheet deteriorate. Therefore, the upper limit is 0.5%. But 0.01
If it is less than%, desiliconization requires a long time and is expensive, which is not desirable. Therefore, the Si content is set to the range of 0.01 to 0.5%.

Mn: Mn is effective as a solid solution strengthening element, and it is necessary to add 1.0% or more to obtain a material of 45 kg class. However, if the content exceeds 2.5%, the strength becomes too high. Therefore, the Mn content is set to the range of 1.0 to 2.5%.

P: P is effective as a solid solution strengthening element, but if more than 0.10% is contained, weldability and workability deteriorate, so 0.10%
Is the upper limit. However, if the content is less than 0.002%, the workability is not improved even if the content is further decreased, and the dephosphorization treatment requires a long time, which is expensive. Therefore, the P content is in the range of 0.002 to 0.10%.

S: S is an element that reacts with Mn in the steel to form A-type inclusions and has a harmful effect on hole expansion, so the S content is preferably as small as possible, and is limited to 0.015% or less. However, even if it is reduced to less than 0.001%, not only the effect is saturated, but also desulfurization treatment requires a long time and becomes expensive, which is not desirable. Therefore, the S content is in the range of 0.001 to 0.015%.

Al: Al is added for the purpose of deoxidation, but if it is less than 0.01%, the effect is not sufficient, and if it exceeds 0.10%, there is no effect even if it is added further. Therefore, the Al content is set to the range of 0.01 to 0.10%.

Nb: Nb has the effect of refining ferrite and forming a high-strength structure such as bainite, and is effective in increasing strength. Further, as described above, it has an effect of helping the structure to be polygonal ferrite. However, if it is less than 0.010%, these effects are not sufficient, and if it exceeds 0.060%, not only the effect is saturated, but also Nb itself is expensive, so the material becomes expensive, which is not desirable. Therefore, the Nb content is set to the range of 0.010 to 0.060%.

 Next, the manufacturing conditions of the present invention will be described.

Heating temperature: Since Nb does not have the above effect unless it is in solid solution, the lower limit of heating temperature is 1100 ° C at which Nb carbonitrides are almost melted in the slab heating stage.

Rolling end temperature: When the rolling end temperature is higher than 850 ° C, precipitation of Nb (C, N) is small, and as described above, the effect of making the structure of Nb into polygonal ferrite is not sufficient, so the structure of steel becomes acicular ferrite. And the tensile ductility deteriorates. On the other hand, when the rolling end temperature is less than Ar ₃ point, work strain remains in the steel structure after cooling and the tensile ductility deteriorates. Therefore, the rolling end temperature is set to Ar ₃ points or higher (desirably 720 ° C) and 850 ° C or lower.

Winding temperature: In high Mn steel, even if the P content normally contained is gradually cooled between 50 and 600 ° C, embrittlement occurs due to segregation of P at grain boundaries, so the winding temperature is set to 500 ° C or less in the present invention.

Needless to say, other manufacturing conditions are not particularly limited.

Next, examples of the present invention will be described. Needless to say, the above-mentioned basic experiment example may be sufficient as the embodiment of the present invention.

(Example) Steels having the chemical compositions shown in Table 1 were melted in the atmosphere, and then rolled under the rolling conditions shown in Table 2 to obtain hot rolled steel sheets having a thickness of 2.0 mm.

The mechanical properties and hole expansibility (λ
b), the impact value and the result of examining the surface condition are also shown in Table 2.

From Table 2, all of the examples of the present invention are TS> 45 kgf / mm
² , it is clear that hot-rolled steel sheets with TS × El> 1600, λb> 160% and vTrs <-100 ° C can be easily obtained. The surface condition is also good.

On the other hand, in the hot-rolled steel sheet obtained in the comparative example in which either the chemical composition of steel or the manufacturing conditions is outside the scope of the present invention, it is impossible to satisfy any of TS, TS × El, and λb. Recognize.

(Effects of the Invention) As described in detail above, according to the present invention, low C, high Mn, P
-With Nb-added steel, a polygonal ferrite structure can be obtained without embrittlement, so TS> 45 kgf / mm ² , TS x El> 160
The characteristics of 0, λb> 160%, vTrs <-100 ° C are obtained, and therefore, a hot-rolled high-tensile steel sheet excellent in press workability, particularly tensile ductility and hole expandability can be easily manufactured. The steel of the present invention can also be used as an original plate of plated steel sheet.

[Brief Description of Drawings] FIG. 1 is a diagram showing the influence of the rolling end temperature (FDT) on the tensile properties, hole expandability, and the proportion (PF) of polygonal ferrite in the entire structure, and FIG. 2 is In the micrograph showing the metal structure of the hot rolled steel sheet,
(A) shows the case where the rolling end temperature (FDT) is 900 ° C, (b) shows the case where the rolling end temperature (FDT) is 800 ° C, and Fig. 3 shows the coiling temperature (CT) that affects the transition temperature (vTrs). )
It is a figure which shows the influence of.

Claims (1)

(57) [Claims] 1. C: 0.0010 to 0.03 by weight% (hereinafter the same)
%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.5%, P: 0.002 to 0.
10%, S: 0.001 to 0.015%, Al: 0.01 to 0.10% and Nb: 0.01
Steel containing 0 to 0.060%, the balance of which is composed of Fe and unavoidable impurities, is heated at 1100 ° C or higher and then hot-rolled, and this hot-rolling is performed at a temperature of Ar ₃ points or higher and 850 ° C or lower. After that, by winding at a temperature of 500 ° C. or less, a hot rolled high tensile strength steel sheet for strong working characterized by obtaining a hot rolled steel sheet having a structure mainly composed of polygonal ferrite and acicular ferrite .