JPH04337037A - Production of hot rolled steel plate having superior formability and excellent in fatigue strength and crack propagation resistance - Google Patents

Abstract

PURPOSE:To obtain a hot rolled steel plate excellent simultaneously in workability, fatigue endurance limit, and fatigue crack propagation resistance which are the characterisitcs conflicting with each other. CONSTITUTION:A steel having a composition consisting of 0.0010-0.03% C, 0.10-1.5% Si, 0.10-2.0% Mn, 0.04-0.15% P, 0.3-1.5% Cu, 0.03-0.5% Ni, 0.01-0.10% Al, and the balance Fe with inevitable impurities is finish-rolled at a temp. not lower than the Ar3 point. Subsequently, the resulting plate is cooled at a rate of >=20 deg.C/sec and further coiled at 300-500 deg.C, by which a structure where the final structure is controlled to 15-35mum ferrite grain size can be obtained. This steel plate can also be used for a surface treated steel sheet. This steel plate is particularly suitable for use in automobile under carriage parts, car body structural member, etc.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for producing a hot-rolled steel sheet with excellent formability and excellent durability fatigue limit and crack propagation resistance.
In particular, 35-55 kgf/mm2 grade hot-rolled steel sheets with excellent fatigue durability, fatigue crack propagation resistance, and cold workability are suitable for applications such as automobile suspension parts and car body structural members. Relating to a manufacturing method.

0002

[Prior art and problems to be solved by the invention] In recent years,
In the automobile industry, the use of high-strength thin steel sheets is increasing in order to improve fuel efficiency, and there is an extremely high demand for thinner walls due to higher tensile strength, especially in suspension parts and various structural members of car bodies.

[0003] However, since high tension deteriorates workability, it is a drawback for automobile parts that require extensive cold working. In addition, even if static strengths such as tensile strength and yield strength are improved, fatigue strength, which is the most problematic issue in automobiles, will not be sufficiently improved, and higher tensile strength will cause structural and organizational defects such as notches and welds. There were problems such as lowering the resistance to fatigue crack propagation from continuous parts.

Conventionally, as seen in JP-A No. 55-107732, efforts have been made to reduce the amount of carbon in order to improve workability, but in order to reduce carbon, the tensile strength was 35 kgf in the first place. /mm2 or less, and fatigue strength cannot be ensured.

[0005] On the other hand, as a technique for improving fatigue strength, Japanese Patent Application Laid-Open Nos. 58-123823 and 63-2822
As seen in No. 40, the ferrite grains in the entire steel sheet or in the surface layer are refined, but C
Since the amount is high, there are problems in workability, and there are also other problems such as restrictions on rolling conditions such as rolling temperature, processing rate, and cooling rate. Further, even if the grains are made finer, the grains become coarser due to welding, and the effect of improving fatigue strength due to grain refinement is lost. Furthermore, if the fatigue durability limit is improved by grain refinement, there is a drawback that the crack propagation resistance deteriorates when macroscopic initial defects such as cracks are present.

[0006] Even in other prior art techniques, there has been no technique that makes it possible to improve both the fatigue durability limit and fatigue crack propagation characteristics while maintaining good workability.

As described above, the conventional techniques have had various problems, such as improving workability leading to a decrease in fatigue strength, and improving fatigue strength decreasing fatigue crack propagation resistance.

[0008] The present invention solves the above-mentioned drawbacks of the prior art and provides a method for obtaining a high-strength hot-rolled steel sheet that is excellent in workability, fatigue durability limit, and fatigue crack propagation resistance, which are contradictory properties. The purpose is to

[0009]

[Means for Solving the Problems] In order to solve the above problems, the present inventors obtained a high-tensile hot-rolled steel sheet that is excellent in workability, fatigue durability limit, and fatigue crack propagation resistance, which are contradictory characteristics. The present invention has now been completed as a result of comprehensive and intensive research into the component composition, structure, and manufacturing conditions.

[0010] That is, the present invention provides C: 0.0010 to
0.03%, Si: 0.10-1.5%, Mn: 0.1
0-2.0%, P: 0.04-0.15%, Cu: 0.
03-1.5%, Ni: 0.03-0.5% and Al:
A steel containing 0.01 to 0.10% and the balance consisting of Fe and unavoidable impurities is finish rolled at a temperature of Ar3 or higher, then cooled at a rate of 20°C/second or higher, and then further rolled at a temperature of 300 to 5
By winding at a temperature of 50°C, the final structure is
The summary is a method for producing a 35-55 kgf/mm2 class good formability hot-rolled steel sheet with excellent fatigue durability limit and crack propagation resistance, which is characterized by obtaining a structure controlled to 15 μm≦ferrite grain size≦35 μm. It is something to do.

The present invention will be explained in more detail below.

0012

[Effect]

[0013] In short, the present invention reduces the amount of carbon to 0.03%.
The following low or extremely low content causes carbides to develop and creates a structure as close as possible to a single ferrite phase, improving workability, and further containing appropriate amounts of P and Cu, which bring about solid solution strengthening and precipitation strengthening. At the same time, by optimizing the hot rolling finishing temperature, cooling rate, and winding temperature, the ferrite crystal grains were controlled to 15 to 35 μm, and the static strength, durability fatigue limit, and crack propagation resistance were improved. be.

First, the reason for limiting the chemical composition of steel in the present invention will be explained.

C: C is an important element added in trace amounts to ensure grain boundary strength. However, if it is less than 0.0010%, intergranular fracture tends to occur and fatigue strength decreases. 0 again
．． If it is added in an amount exceeding 0.3%, the amount of precipitated second phase consisting of carbides such as cementite and pearlite increases more than necessary, resulting in deterioration of workability. Therefore, the C content is set in the range of 0.0010 to 0.03%.

Si: Si is an element necessary to increase static strength without deteriorating workability, and in order to exhibit its effectiveness, it must be added in an amount of 0.10% or more. However, when it is added in an amount exceeding 1.5%, red scale is generated and the surface quality is impaired, and as a result, the as-rolled fatigue strength is reduced. Therefore, the Si content is 0.1
The range is 0 to 1.5%.

Mn: Mn is added in an amount of 0.10% or more in order to ensure static strength and at the same time prevent hot embrittlement. However, if it exceeds 2.0%, the workability deteriorates, so M
The n content is in the range of 0.10 to 2.0%.

P: P is the most important solid solution strengthening element for imparting static strength and fatigue strength without impairing workability. With the above Si and Mn amounts, C is 0.03
% or less could not be sufficiently compensated for, and the target tensile strength of 35 to 55 kgf/mm2 could not be achieved. (durability limit and tensile strength ratio) cannot be given. P is an essential element for this purpose, and in order to exhibit effective solid solution strengthening, it is necessary to add 0.04% or more. However, adding more than 0.15% deteriorates hot workability and weldability. Therefore, P
The content is in the range of 0.04 to 0.15%.

Cu: Cu is also 35 to 55 like the above P.
It is an important element added to achieve a tensile strength of kgf/mm2 and a good durability limit ratio. However, while P uses solid solution strengthening to improve fatigue strength, Cu produces fine precipitates (ε-Cu) during winding after the hot rolling process.
) to improve fatigue strength. Since these precipitates are significantly smaller than carbides such as cementite in conventional low carbon steel, the dispersion strengthening effect is large, and the workability is not reduced much compared to the increase in strength. Although 0.03% or more is required to precipitate ε-Cu, adding more than 1.5% tends to cause hot embrittlement. Therefore, the Cu content is 0.03 to 1
．． The range shall be 5%.

Ni: Ni is an element added to prevent hot embrittlement due to Cu content, and the content is approximately equal to that of C.
About 1/3 of the u amount is necessary. Therefore, the Ni content is in the range of 0.03 to 0.5%.

Al: Al is added as a deoxidizing agent, but 0
．． If it is less than 0.1%, the deoxidizing effect will be small, and if it exceeds 0.10%, inclusions will increase and workability will deteriorate. Therefore,
The Al content is in the range of 0.01 to 0.10%.

Next, the reason for limiting the crystal grains in the hot rolled steel sheet obtained by the present invention will be described.

[0023] The lower limit of fatigue crack propagation (ΔKth) in ferritic steel is almost unaffected by additive elements, depends mainly on the ferrite crystal grain size, and increases as the grain size increases (Fig. 1). In conventional low-carbon hot-rolled steel sheets as hot-rolled, there is a limit to coarsening of crystal grains, which is about 15 μm at most, even if the hot-rolling conditions are changed significantly. ΔKth corresponding to this crystal grain is

[Equation 1] On the other hand, since the steel of the present invention is an ultra-low carbon steel,
It is possible to coarsen the grains through the normal hot rolling process,

[Math 2] The crystal grain size can be set to 15 μm or more.

However, in steel having the lower limit amounts of P and Cu specified in the present invention, if the grain size exceeds 35 μm, the tensile strength becomes less than 35 kg/mm 2 , making it impossible to secure the required strength (FIG. 1). In addition, in any component range, if the crystal grain size is 35 μm or more (particle size number 7 or less), roughness will occur in the press-molded product and the product value will be impaired. Therefore, the ferrite crystal grain size is 15-3
It is set to 5 μm.

[0025] The manufacturing conditions in the present invention will be explained.

[0026] Steel having the above chemical composition is melted, cast, and subjected to hot rolling by a conventional method, but the finishing temperature of the hot rolling varies depending on the grain size and texture, thereby affecting workability,
This affects fatigue properties, and if the Ar point is less than 3, the processed structure of ferrite crystal grains remains, resulting in deterioration of fatigue properties and workability. Therefore, the finishing temperature should be set to Ar3 points or higher,
860-980°C is preferred.

After finish rolling, the material is cooled, and the cooling rate is set to 20° C./second or more in order to maintain an appropriate grain size after hot rolling. Under these conditions, a decrease in strength due to coarsening of crystal grains and segregation of P can be prevented.

After cooling, the coiling temperature is set to ε-C to prevent embrittlement due to P segregation and coarsening of crystal grains.
The temperature is set at 550° C. or lower to maximize the precipitation of u. Further, sufficient precipitation of ε-Cu does not occur unless the temperature is 300°C or higher. Therefore, the winding temperature is in the range of 300 to 550°C.

[0029] In the present invention, as-rolled (as ro)
Although the case of manufacturing by the hot rolling process of ll) has been specified, surface treatment such as hot-dip galvanizing or electroplating on the obtained hot-rolled steel sheet will not impair the characteristics of the present invention in any way, so the surface treatment It can also be used as a steel plate.

Next, examples of the present invention will be shown.

[0031]

【Example】

[Table 1] Hot-rolled steel sheets were manufactured using the steel having the chemical composition shown in the table under the hot rolling conditions shown in the same table. In addition, No. 1~No. No. 6 is an example of the present invention. 7~No. No. 10 is an example of low carbon steel with conventional composition. 11~No. No. 12 is an example in which the hot rolling conditions are outside the scope of the present invention.

Hole expansion test [(
Hole diameter after molding - initial hole diameter) / (initial older diameter) x 100],
A tensile test, a plane bending fatigue test, a fatigue crack propagation test at a stress ratio of 0.1 were conducted, and the ferrite grain size was measured. those results

[Table 2] Also shown in FIGS. 2 and 3.

[0033] From Table 2 and the figures, the hot rolled steel sheet of the example of the present invention has the following properties:
It satisfies the tensile strength of 35 to 55 kg/mm2 or more, the hole expansion rate is 150% or more, and the durability limit ratio (fatigue limit/tensile strength) is also higher than the same conventional strength level. Also,
It can be seen that since the crystal grains are controlled to 15 to 35 μm, the lower limit of fatigue crack propagation (ΔKth) is also higher than that of conventional steel.

[0034]

[Effects of the Invention] As detailed above, according to the present invention,
It is possible to obtain a hot-rolled steel sheet that has high strength, excellent workability, and significantly improved fatigue durability and fatigue crack propagation resistance, and can also be used as a surface-treated steel sheet, so the effects are great.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between ferrite grain size, tensile strength (σв), and lower limit of fatigue crack propagation (ΔKth).

FIG. 2 is a diagram showing the relationship between tensile strength (σв) and hole expansion rate.

[Figure 3] Tensile strength (σв) and fatigue durability limit ratio (σw/σ
в) is a diagram showing the relationship between

Claims (1)

[Claims] [Claim 1] In weight% (hereinafter the same), C: 0.0
010-0.03%, Si: 0.10-1.5%, Mn
:0.10~2.0%, P:0.04~0.15%, C
A steel containing u: 0.03 to 1.5%, Ni: 0.03 to 0.5%, and Al: 0.01 to 0.10%, with the balance consisting of Fe and unavoidable impurities, was heated at three Ar points. Finish rolling at the above temperature, then cooling at 20℃/second or more, and further 3
By winding at a temperature of 00 to 550℃, the final structure is controlled to 15μm≦ferrite grain size≦35μm.35 to 55kgf with excellent fatigue durability limit and crack propagation resistance. /mm2 grade hot rolled steel sheet with good formability.