JPH06228637A - Production of high ductility hot rolled high tensile strength steel plate - Google Patents

Abstract

PURPOSE:To secure means capable of easily and stably manufacturing a hot rolled high tensile strength steel plate combining excellent strength, ductility, weldability and bore expandability. CONSTITUTION:A hot rolled steel plate contg. 0.05 to 0.25% C, >0.05 to 1.0% Si, 0.8 to 2.5% Mn and 0.8 to 2.5% sol.Al or furthermore contg., at need, one or more kinds among 0.0002 to 0.0100% Ca, 0.01 to 0.10% Zr, 0.002 to 0.10% rare earth elements, 0.005 to 0.10% Nb, 0.005 to 0.10% Ti and 0.005 to 0.20% V, and the balance Fe with inevitable impurities is heated to >=850 deg.C. In the subsequent cooling, it is held to 650 to 800 deg.C for 15sec to 15min, is furthermore held to 300 to 500 deg.C for 30sec to 40min and is then cooled to room temp. at a cooling rate of air cooling or above, by which the high ductility hot rolled high tensile strength steel plate having a structure essentially consisting of ferrite and bainite including, by volume, >=5% retained austenite and excellent in ductility and bore expandability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-rolled steel sheet for processing, which is suitable as a steel sheet for high-strength members in automobiles, industrial machines and the like and which is excellent in ductility and hole expandability.

[0002]

2. Description of the Related Art Today, so-called "hot rolled steel sheet" manufactured by continuous hot rolling has been widely applied to various industrial equipment such as automobiles as a relatively inexpensive structural material. However, there are many members that are molded by press processing for their applications, and therefore there is a strong demand for "combination of high strength and high ductility".

As a steel material excellent in both strength and ductility, for example, DP steel (Dual Phase steel: ferrite + martensite dual phase steel) as described in JP-A-55-44551 is known. . This feature of the DP steel is that it "ductile low yield ratio", nevertheless the tensile strength: material that growth in the 60 kgf / mm ² is the current situation is of about 30%, ductility In view of the above, there has been a demand for a further improved material.

By the way, a method utilizing TRIP (transformation induced plasticity) of retained austenite has been developed as a means for improving the ductility of high strength steel sheets (for example, Japanese Patent Laid-Open No. 55-1).
45121). And according to this method
It is possible to manufacture a high-ductile high-strength steel sheet having a tensile strength of 110 kgf / mm ² or more and an elongation of 22% or more, and having a “value of [tensile strength] × [elongation]” exceeding 2400. However, with this method, it is necessary to adjust the C content to a high value of 0.35 to 0.85% (hereinafter,% representing the component ratio is taken as% by weight), so that the steel sheet obtained is inferior in weldability and is not suitable for automobiles. The range of application as a steel sheet was narrow.

As a means for securing retained austenite under a low C content to obtain high ductility in the steel, a method has been proposed in which a high Si content steel is largely reduced in the low temperature austenite region ( JP-A-63-4017). As described above, the addition of Si is effective for obtaining residual austenite, and therefore it is possible to secure high ductility in the steel material by adding an appropriate amount of Si, but on the other hand, coarse bainite is easily generated, and
If the amount of Si added is high, hard martensite is also likely to be formed, so hole expansibility, which is one of the important workability for steel sheets for automobiles, was low. In particular, when a large reduction is applied in the low temperature austenite region as described above, coarse bainite is likely to be formed into a band shape, which significantly deteriorates the hole expandability.

Therefore, the present inventors previously developed a [Al + Si] -added hot-rolled high-strength steel sheet containing low austenite and containing retained austenite, which is provided with excellent ductility and hole expansibility.
It was proposed as Japanese Patent Application No. 3-299825. However, in a subsequent study, "This hot rolled steel sheet requires a large rolling load because it is manufactured by performing finish rolling in the low temperature austenite region. It is very difficult to obtain products with a wide width, etc. due to load constraints. "

In view of the above, the object of the present invention is to provide a hot-rolled high-strength steel sheet having excellent strength, ductility, weldability and hole expandability, which is thin and wide. Was to establish a means that can be easily and stably manufactured.

[0008]

Therefore, the present inventors have
In order to achieve the above-mentioned object, "a TRI which has a sufficiently high strength even for an automobile or the like with a C content range in which weldability is obtained, and which leads to excellent ductility and hole expandability.
In view of the following matters previously found by the present inventors, a "simple method capable of realizing a high ductility hot rolled high tensile strength steel sheet containing a sufficient amount of austenite to utilize the P effect" was sought. Further research was repeated.

A) In order to improve the hole expandability, residual
Even in steels with a low Si content, which is effective for securing austenite, it is possible to retain austenite by containing Al in a proportion of 0.8% or more, which results in excellent tensile strength / elongation balance. can get. b) In the conventional Si-added retained austenitic high-strength steel,
Although a high elongation is obtained in the tensile test, only a hole expansion ratio similar to DP steel is obtained in the hole expansion test.
In the case of Al-added retained austenitic high-strength steel, excellent elongation and high hole expansion ratio can be satisfied at the same time. c) Since Al is a ferrite stabilizing element, it was considered that addition of Al was disadvantageous for allowing austenite to remain, but addition of an appropriate amount of Al improves the elongation of the product, Are evenly dispersed, which is also effective for improving hole expandability. d) Therefore, if Al and Si are contained in steel having a C content range in which weldability is not significantly deteriorated and a structure containing 5% or more by volume of retained austenite is realized, steel sheets for automobiles and the like are required. A hot-rolled high-strength steel sheet that fully satisfies various properties such as strength, ductility, and hole expandability can be obtained.

Then, from the consideration of the research results for the method, the above-mentioned "hot-rolled high-tensile steel sheet having excellent strength, ductility, hole expandability, etc." can be manufactured more easily.
The present inventors have come to know the following new facts.

That is, as described above, the knowledge so far shows that the above-mentioned "hot-rolled high-strength steel sheet having excellent strength, ductility, hole expandability, etc." can be used in the low temperature austenite region. Finishing rolling was required, and thin and wide steel sheets could not be manufactured due to high rolling load. However, since the deformation resistance of steel decreases as the deformation temperature increases, the finishing temperature during rolling is increased to produce thin and wide steel sheets with the above composition under low deformation resistance, and then this "high temperature finish rolling" is performed. When the heat-treated hot-rolled steel sheet obtained by performing the heat treatment under the specific conditions, a structure containing an appropriate amount of retained austenite appears, and the retained austenite type high-strength hot-rolled steel sheet having the above-mentioned excellent properties is obtained. be able to.

That is, not only the retained austenite cannot be sufficiently obtained only by hot rolling of high-temperature finishing, but also a structure containing a large amount of martensite cannot be obtained and high ductility cannot be obtained. It has been found that when a specific heat treatment is further applied, a hot-rolled high-strength steel sheet containing an appropriate amount of retained austenite and having excellent ductility and hole expandability can be stably produced.

The present invention has been completed by further research based on the above findings and the like. "C: 0.05 to 0.25%, Si: more than 0.05 to 1.0%, Mn:
0.8 to 2.5%, sol.Al: 0.8 to 2.5%, or further Ca: 0.0002 to 0.0100%, Zr: 0.01 to 0.10%, rare earth element: 0.002 to 0.10%, Nb: 0.005 to 0.10%, Ti : 0.005 ~
0.10%, V: 0.005 to 0.20%, including at least one kind, and the balance consisting of Fe and unavoidable impurities, the hot rolled steel sheet is heated to 850 ° C or higher, and then cooled to a temperature of 650 to 800 ° C for 15 After holding for 15 seconds to 15 minutes, the temperature of 300 to 500 ° C for 30 seconds
Hold for 40 minutes and then cool to room temperature with air cooling or higher cooling rate to obtain ductility and holes with "structure mainly composed of ferrite and bainite containing 5% or more of retained austenite by volume". It is a feature that enables stable production of high-ductility hot-rolled high-strength steel sheets with excellent expandability. "

[0014]

Next, the reason why the component composition of the steel sheet and the production conditions thereof are limited as described above in the present invention will be described in detail together with its action. A) The chemical composition C C of the steel sheet contributes to the strengthening of the steel sheet while stabilizing the austenite by concentrating in untransformed austenite with the progress of ferrite transformation in the heat treatment process after hot rolling and winding. However, if its content is less than 0.05%, the strength is not secured and the austenite stabilizing effect is not sufficient. On the other hand, if the content of C exceeds 0.25%, the weldability becomes remarkable. In addition to deterioration, the amount of ferrite decreases and the amount of bainite increases too much, so the hole expandability also deteriorates. Therefore, C
The content was set to 0.05 to 0.25%.

Si In general, Si has the effect of promoting the formation of ferrite to assist the concentration of C in untransformed austenite, and delaying the precipitation of cementite. Although it is considered to be an effective element, in the present invention, it is mainly added in order to strengthen the solid solution of ferrite and enhance the strength of the steel sheet. In this case, the Si content is 0.05
%, The strengthening effect is insufficient and the desired strength cannot be secured in the steel sheet. On the other hand, if the content exceeds 1.0%, the effect due to the above effect is saturated and hard martensite is formed. As a result, the hole expandability is deteriorated. Therefore, the Si content is determined to be in the range of more than 0.05% and 1.0% or less.

Mn Mn is an austenite stabilizing element, and has an action of lowering the Ms point of untransformed austenite, improving hardenability, and suppressing pearlite transformation of untransformed austenite. Exert. However, if the Mn content is less than 0.8%, the desired effect due to the above-mentioned action cannot be obtained.
%, It becomes difficult to generate a sufficient amount of ferrite by heat treatment, so that the concentration of C in untransformed austenite becomes insufficient and the austenite cannot be stabilized. . Therefore, the Mn content is set to 0.8 to 2.5%.

Sol.Al Al is a particularly important component in the present invention. That is, Al
Like Si, it is a ferrite stabilizing element, promotes the formation of ferrite to promote the concentration of C in untransformed austenite, and promotes the retention of austenite through the action of delaying the precipitation of cementite. Moreover, the effect is more remarkable than when Si is added at the same weight ratio. In addition, it also has the effect of promoting the uniform and fine formation of ferrite and suppressing the formation of coarse bainite that deteriorates the hole expandability. However, Al content as sol.Al
If it is less than 0.8%, the desired effect due to the above action cannot be obtained. On the other hand, if it exceeds 2.5%, the effect is saturated and the formation of inclusions is promoted to deteriorate ductility and hole expansibility. Therefore, the sol.Al content was set to 0.8-2.5%.
In order to stably generate the retained austenite, it is desirable that the total amount of [sol.Al + Si] is 1.0% or more.

Ca, Zr, and rare earth elements All of these components have the effect of adjusting the shape of inclusions to improve cold workability, and therefore, if necessary, one or more of them may be added. However, if the content of Ca is less than 0.0002%, the content of Zr is less than 0.01%, and the content of rare earth element is less than 0.002%, the desired effect due to the above action is obtained. Not obtained, while Ca
0.0100%, Zr 0.10%, and rare earth elements 0.10%
%, The inclusions in the steel become too large and the workability deteriorates. Therefore, the Ca content is 0.
0002 to 0.0100%, Zr content was 0.01 to 0.10%, and rare earth element content was 0.002 to 0.10%.

Nb, Ti, and V Nb, Ti, and V are all elements that are precipitated as carbonitrides on the ferrite ground and are effective for further strengthening the steel sheet. Therefore, if necessary, one or more of them may be used. However, in any case, if the content is less than 0.005%, the desired effect cannot be obtained. On the other hand, in the case of Nb and Ti, the content exceeds 0.10%, and in the case of V, the content exceeds 0.20%. However, the effect is saturated and it is not economical. Therefore, the Nb content is 0.0
05-0.10%, Ti content was 0.005-0.10%, and V content was 0.005-0.20%.

As the "unavoidable impurities" which are inevitably mixed in the steel, P, S, Cu, Ni, Cr, Mo, etc. can be mentioned. It is desirable to regulate it as follows. Since Pp is an impurity element that adversely affects weldability, it is desirable to keep its content to 0.05% or less, but 0.010% from the viewpoint of more evenly dispersing ferrite.
The following restrictions are recommended. Since S S is an impurity element that forms sulfide-based inclusions and deteriorates the workability, it is desirable to keep its content to 0.05% or less, but from the viewpoint of securing more excellent workability, It is more preferably 0.003% or less.

By the way, the steel having the composition as described above is melted by, for example, a converter, an electric furnace, an open furnace or the like. The steel type may be any of rimmed steel, capped steel, semi-killed steel or killed steel. Further, the production of the steel slab to be subjected to hot rolling may be performed by any means of "ingot-segmentation rolling" or "continuous casting".

B) Manufacturing Conditions for Hot Rolled Steel Sheet Now, a hot rolled high tensile steel sheet having the composition as described above and having a structure mainly composed of ferrite and bainite containing 5% or more by volume of retained austenite. First, a steel sheet obtained by hot rolling a steel slab having the above-mentioned composition is heated to 850 ° C. or higher, then kept at a temperature of 650 to 800 ° C. for 15 seconds to 15 minutes, and then at a temperature of 300 to 500 ° C. It can be produced by holding for 30 seconds to 40 minutes, and then cooling to room temperature at a cooling rate of air cooling or higher.

Here, the hot-rolled steel sheet is first heated to a temperature of 850 ° C. or higher in order to prepare for the next treatment by eliminating hard martensite phase and the like to generate austenite. If the temperature is lower than 850 ° C, austenitization is not sufficiently performed.

In the cooling process following the above heating, holding is carried out at a temperature of 650 to 800 ° C. for 15 seconds to 15 minutes. The holding in the first step is for producing ferrite, and with it The concentration of C in untransformed austenite is promoted. The holding temperature of the first stage is 80
If the temperature exceeds 0 ° C, ferrite will not be sufficiently generated, and 65
When the temperature is lower than 0 ° C, pearlite is formed, and in each case, a sufficient amount of retained austenite cannot be finally obtained. Further, if the holding time is less than 15 seconds, the formation of ferrite is insufficient, and finally a sufficient amount of residual alloy is formed.
Stenite cannot be obtained, on the other hand, even if it is held for more than 15 minutes, no particular effect can be expected, and only energy is wasted.

After holding the first stage, 300 to 500
The holding at the temperature of 30 ° C. for 30 seconds to 40 minutes is performed. In the holding in the second step, the austenite that has not been transformed in the holding in the first step is transformed into bainite, thereby forming retained austenite. It is carried out to When the holding temperature in the second stage exceeds 500 ° C, pearlite is formed and residual austenite does not remain, while at temperatures below 300 ° C, martensite formation is promoted and ductility and hole expandability are promoted. Deteriorates. Further, when the holding time is less than 30 seconds, the bainite transformation is insufficiently progressed and the concentration of C in the untransformed austenite is insufficient, and most of the untransformed austenite does not reach the room temperature where it is continuously performed. During the cooling, it transforms to martensite and high ductility cannot be obtained. On the contrary,
If the holding temperature exceeds 40 minutes, cementite is formed and a sufficient amount of retained austenite cannot be obtained.

Cooling from the second stage holding temperature to room temperature is performed by air cooling or at a cooling rate higher than that. If the cooling rate at this time is extremely slower than air cooling, cementite is generated during cooling and sufficient cooling occurs. No residual austenite can be obtained. The upper limit of the cooling rate is not particularly limited.

The above-described ferrite formation or the progress of bainite transformation is governed by the components, temperature and time,
It goes without saying that the optimum time varies depending on the components and the holding temperature.

[0028]

The hot-rolled steel sheet according to the present invention has a C content of 0.05 to 0.25% and thus has a weldability of a level required for a steel sheet for high-strength members, and further utilizes TRIP. A sufficient amount of austenite. Moreover, by adjusting the contents of Al and Si, the hole expandability, which was a weak point of residual steel with Si added, can be sufficiently improved. Therefore, according to the present invention, although the weldability is good, residual austenite is present, the "tensile strength-elongation balance" is good, and the thin and wide heat treatment is excellent in hole expandability. A rolled steel sheet can be realized. By subjecting the steel sheet of the present invention to surface treatment such as hot dip galvanizing, hot dip galvanizing, and electroplating, a surface-treated steel sheet having excellent ductility and hole expandability can be obtained.

Next, the effects of the present invention will be described more specifically by way of examples.

EXAMPLES First, steels A to Y having the chemical composition shown in Table 1
Was hot-rolled to obtain a hot-rolled steel sheet having a plate thickness of 2.3 mm. Next, each of these hot-rolled steel sheets was subjected to heat treatment under various conditions shown in Tables 2 and 3.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

Then, JIS No. 5 tensile test pieces were sampled from each of the steel sheets thus obtained, and their mechanical properties were investigated. Also, the dimensions are 2.3mm thickness x 120mm width x 120mm.
Clearance of 14mmφ hole in long steel plate specimen: 15%
Then, the hole expandability was investigated using a conical punch. Further, a test piece for X-ray test was sampled from the central portion of the steel sheet, and the amount of retained austenite was also investigated. The results are also shown in Tables 2 and 3.

As is clear from the results shown in Tables 2 and 3, the hot-rolled steel sheet manufactured according to the present invention has a high tensile strength of 70 kgf / mm ² or more and an elongation of 30% or more. It can be seen that at the same time as having ductility, it exhibits excellent punching workability with a hole expansion ratio of 90% or more, and satisfies both high strength and high workability at the same time.

On the other hand, in the test number 28 using steel type U having a C content exceeding the specified range of the present invention, the elongation is relatively high but the hole expansibility is poor. In the test number 29 using steel type V whose sol.Al content is below the specified range of the present invention, residual austenite is not formed and elongation and hole expansibility are both poor. In the test number 30 using the steel type W in which the C content is below the specified range of the present invention, the strength is significantly reduced. Test number using steel type X whose Mn content is below the specified range of the present invention
31 and the test number 32 using the steel type Y whose sol.Al content exceeds the specified range of the present invention, both elongation and hole expansibility are inferior.

Further, in the test No. 21 manufactured under the condition that the first stage holding temperature is out of the lower limit of the range specified by the present invention,
When the first stage is held, pearlite is generated and residual
Stenite is not formed and strength and elongation are inferior. Test No. 22 manufactured under the condition that the first stage holding temperature is outside the upper limit of the range specified by the present invention, or test produced under the condition that the first stage holding temperature is outside the lower limit of the range specified by the present invention number
In the case of No. 23, since the formation of ferrite by holding the first stage was insufficient, martensite was formed during the holding of the second stage and the residual o
Not enough stenite is formed. For this reason, the strength is high, but the elongation is low, and the hole expandability is not good.

In the test No. 24 manufactured under the condition that the second stage holding temperature was outside the upper limit of the range specified in the present invention, residual austenite was produced because pearlite was produced during the second stage holding. No, the growth is inferior. In the test number 25 manufactured under the condition where the second stage holding time was out of the lower limit of the range specified in the present invention, the retained austenite was not sufficiently produced because bainite was not sufficiently produced by the second stage holding. The elongation is poor and the hole expandability is not good.

In Test No. 26, which was manufactured under the condition that the second-stage holding time was out of the upper limit of the range defined by the present invention, cementite was generated due to the long second-stage holding time, and a sufficient amount of residual o Stenite cannot be obtained and elongation is poor. Two
In Test No. 27 manufactured under the condition that the stage holding temperature was out of the lower limit of the range specified in the present invention, the austenite which was not transformed in the first stage holding due to the low second stage holding temperature was Martensite transformation and insufficient generation of retained austenite are insufficient. Therefore, although the strength is high, the elongation is low and the hole expandability is not good.

[0039]

[Summary of Effects] As described above, according to the present invention, it is possible to stably provide a high-strength hot-rolled steel sheet which is excellent in strength, ductility, hole expandability, and weldability and is excellent in workability. It can be applied to industrial equipment members such as automobile undercarriage parts to further improve the performance and life of those products, which brings extremely useful effects in industry.

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[Procedure amendment]

[Submission date] July 30, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Title of invention

[Correction method] Change

[Correction content]

Title: Method for manufacturing high-ductility hot-rolled high-strength steel sheet

Claims (4)

[Claims] 1. A weight ratio of C: 0.05 to 0.25%, Si: more than 0.05 to 1.0%, Mn:
0.8-2.5%, sol.Al: 0.8-2.5%, the balance is Fe and unavoidable impurities
In addition to heating to 850 ° C or higher, in the subsequent cooling,
It is characterized by holding at a temperature of 650 to 800 ° C. for 15 seconds to 15 minutes, further holding at a temperature of 300 to 500 ° C. for 30 seconds to 40 minutes, and then cooling to room temperature at a cooling rate of air cooling or higher. , Residual volume of 5% or more by volume
A method for producing a high-ductility hot-rolled high-strength steel sheet having a structure mainly composed of ferrite and bainite containing stenite and excellent in ductility and hole expansion. 2. A weight ratio of C: 0.05 to 0.25%, Si: more than 0.05 to 1.0%, Mn:
0.8 to 2.5%, sol.Al: 0.8 to 2.5%, Ca: 0.0002 to 0.0100%, Zr: 0.01 to 0.10%, rare earth element: 0.002 to 0.10%, and the balance Fe and The hot-rolled steel sheet made of unavoidable impurities is heated to 850 ° C. or higher and, during cooling thereafter, held at a temperature of 650 to 800 ° C. for 15 seconds to 15 minutes, and then at a temperature of 300 to 500 ° C. for 30 seconds to
Ductility having a structure mainly composed of ferrite and bainite containing 5% or more of retained austenite by volume, characterized by holding for 40 minutes and then cooling to room temperature at a cooling rate of air cooling or higher. And a method for producing a high-ductility hot-rolled high-strength steel sheet with excellent hole expansion. 3. By weight ratio, C: 0.05 to 0.25%, Si: more than 0.05 to 1.0%, Mn:
0.8-2.5%, sol.Al: 0.8-2.5%, Nb: 0.005-0.10%, Ti: 0.005-0.10%, V: 0.005-
After heating a hot-rolled steel sheet containing 0.20% of one or more and the balance being Fe and unavoidable impurities to 850 ° C. or higher and holding it at a temperature of 650 to 800 ° C. for 15 seconds to 15 minutes during the subsequent cooling. Further, at a temperature of 300 to 500 ° C for 30 seconds to
Ductility having a structure mainly composed of ferrite and bainite containing 5% or more of retained austenite by volume, characterized by holding for 40 minutes and then cooling to room temperature at a cooling rate of air cooling or higher. And a method for producing a high-ductility hot-rolled high-strength steel sheet with excellent hole expansion. 4. By weight ratio, C: 0.05 to 0.25%, Si: more than 0.05 to 1.0%, Mn:
0.8 to 2.5%, sol.Al: 0.8 to 2.5%, Ca: 0.0002 to 0.0100%, Zr: 0.01 to 0.10%, rare earth element: 0.002 to 0.10%, and Nb: 0.005 to 0.10 %, Ti: 0.005 to 0.10%, V: 0.005 to
After heating a hot-rolled steel sheet containing 0.20% of one or more and the balance being Fe and unavoidable impurities to 850 ° C. or higher and holding it at a temperature of 650 to 800 ° C. for 15 seconds to 15 minutes during the subsequent cooling. Further, at a temperature of 300 to 500 ° C for 30 seconds to
Ductility having a structure mainly composed of ferrite and bainite containing 5% or more of retained austenite by volume, characterized by holding for 40 minutes and then cooling to room temperature at a cooling rate of air cooling or higher. And a method for producing a high-ductility hot-rolled high-strength steel sheet with excellent hole expansion.