JPS5914092B2 - Manufacturing method of composite structure type high tensile strength hot rolled steel sheet with high ductility and low yield ratio - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a composite structure type high tensile strength hot rolled steel sheet having high ductility and low yield ratio with high efficiency and at low cost.

In recent years, for example, in the automotive industry, there has been a strong desire to reduce the thickness of plates without changing the design strength as part of weight reduction from the perspective of energy conservation and resource conservation. There is a tendency.

However, conventional high-strength steel plates have disadvantages in price, press formability, and spot weldability, and have had considerable problems in practical application.

On the other hand, recently, a composite structure type high-strength hot-rolled steel sheet has been proposed, which has the characteristics of being soft and easy to form during press processing, and hardening after forming to increase the strength of the product. It has started to be provided.

This composite structure type high-strength hot-rolled steel sheet has a mixed structure of a low-temperature transformation product consisting of either or both of a martensitic phase and a bainite phase, and a ferrite phase, and the low-temperature transformation product strengthens the steel. The ferrite phase plays a role in imparting ductility to the steel.

Conventionally, many methods have been proposed for manufacturing the above-mentioned composite structure type hot rolled steel sheets, but among them, A is a method for manufacturing composite structure type high tensile strength hot rolled steel sheets having high ductility and low yield ratio.
There is a known method in which a steel plate that has been rolled at a final finishing temperature above the r3 transformation point is immediately rapidly cooled to a temperature below the Ar' transformation point at a cooling rate of 16° C./sec or above.

However, in hot-rolled steel sheets manufactured by the above-mentioned conventional method, the solid solution carbon and nitrogen in the steel sheet increase due to rapid cooling after rolling, and after hot rolling, skin pass, pickling, and As the process of shearing (cutting into plates with predetermined dimensions) progresses, strain aging occurs, resulting in a significant deterioration of ductility and a significant increase in yield point, resulting in the desired high ductility and low yield ratio. In other words, it has been extremely difficult to obtain a hot rolled steel sheet with excellent cold workability.

From the above-mentioned viewpoint, the present inventors conducted research to obtain a composite structure type high-strength hot-rolled steel sheet that does not reduce ductility or increase yield point due to strain aging. Immediately after rolling, the steel plate containing a trace amount of Ti is heated to 300°C to 15°C at a cooling rate exceeding 20°C/SeC.
When the steel sheet is rapidly cooled to a temperature within the temperature range of 0°C and wound into a coil, the solid solution nitrogen in the steel sheet is fixed as TiN due to the Ti component, and the steel sheet is wound at a temperature within the temperature range of 300 to 150°C. As a result, solute carbon precipitates as carbide, so the solute nitrogen and solute carbon in the steel sheet are significantly reduced. As a result, ductility deterioration and yield point increase due to strain aging are eliminated, resulting in high ductility and low yield point. They obtained the knowledge that the yield point could be secured.

Therefore, this invention has been made based on the above knowledge, and the target steel is C: 0.01 to 0 in terms of weight.
．． 15%, Si: 1.5% or less, Mn: 0.5-3
．． 0% 5otJ-t: 0.10% or less, Ti:
Contains 0.01 to 0.047%, and if necessary, Ca: 0.002 to 0.01% and rare earth element: 0.00
Inclusion shape adjusting element group consisting of 2 to 10%, Cu: 0.05 to 0.5%, Ni: 0.0
5-0.5%, Cr: 0Ω5-0.5%, and Mo
: Contains one or more elements from either of the strength improving element groups consisting of 0.03 to 0.5%, or two or more elements selected from both element groups,
A steel with the remainder consisting of Fe and unavoidable impurities was identified, and this steel was hot-rolled under conditions such that the final finishing temperature was equal to or higher than the Ar s transformation point, and then immediately rolled at a cooling rate of over 20°C/sec to 300°C. It is characterized by producing a composite structure type high-strength hot-rolled steel sheet with high ductility and low yield ratio, that is, excellent cold workability, by rapidly cooling it to a temperature within the temperature range of ~150°C and winding it into a coil. It has the following.

Next, in the method of this invention, the composition range of steel,
The reason why the hot rolling conditions and cooling conditions were limited as described above will be explained.

A Component composition range (a) C The C component has the effect of increasing the volume fraction of low-temperature transformation products in the composite structure and increasing the strength of the steel plate to have a tensile strength of 50 kgAt or more. Content is o, o1
If it is less than 0.0%, the desired effect cannot be obtained.
If the content exceeds 15%, weldability will deteriorate, and especially when used for automobiles and mandatory spot welding is performed, the hardness of martensite generated at these spot welds will increase significantly. Therefore, the content should be set at 0.01 to 0.
．． It was set at 151%.

(b) Si The Si component has the effect of improving the strength and ductility of the steel plate through solid solution hardening, but if it is contained in an amount exceeding 1.5, it not only deteriorates the surface properties of the steel plate, but also Since steel making work becomes difficult, the content was reduced to 1.5.
Section below.

(C) Mn The Mn component has the effect of improving the hardenability of the austenite part after hot rolling and making it easier to obtain martensite and lower bainite structures, but when the content is 0.5
If the amount is less than 3%, the desired effect cannot be obtained;
．． If the content exceeds 0, it not only becomes difficult to make steel, but it is also uneconomical.

Its content was determined to be 0.5 to 3.0%.

(d) Sol, 1'-1 At is added to deoxidize molten steel, and is usually So1. Ai contains about 0.02~o, os%,
Even if 5ot-At is contained in an amount exceeding 10%, no further deoxidizing effect can be expected, and the deoxidizing effect becomes saturated, so the upper limit was set at 0.10.

(e) Ti The Ti component easily combines with solid solution nitrogen in the steel sheet in a high temperature range to form TiN'E.
It has the effect of not increasing solid solution nitrogen even when rolled at a very low temperature within the temperature range of 0 to 150°C, but if the content is less than 0.01%, the desired effect cannot be obtained. , o1% or more, but since the nitrogen content in the steel of this invention is usually 0.010% or less, the upper limit value is set to 0.01% or more as the Ti content corresponding to this.
It was set at 0.4%.

(f) Unavoidable impurities Among the unavoidable impurities, S in particular forms non-metallic inclusions and deteriorates the cold workability of steel sheets, so the lower the content, the better, but considering economic efficiency. ,0
．． It is desirable that the content not exceed 0.015%.

(g) Ca and rare earth elements Both of these components have the same effect of adjusting the shape of inclusions and improving cold workability, but their content is
If Ca: less than 0.002% and rare earth element: less than 0.002%, respectively, the desired effect cannot be obtained. If Ca is contained, on the contrary, inclusions in the steel will increase and cold workability will deteriorate.
0.01%, rare earth elements 20,002-0.10%.

(h) Cu, Ni, Cr and M.

These components have an equal effect of improving the strength of steel, but their contents are Cu: less than 0.05%, Ni: less than 0.05%, Cr: less than 0.05%, and Mo: If the content is less than 0.03%, the desired effect cannot be obtained, and on the other hand, Cu: 0.5% and Ni: 0.5%, respectively.
%, cr: o, 5%, and Mo: even if the content exceeds 0.5%, the action and effect will be saturated and it is not economical, so the respective contents are set to Cu: 0.05 to 0. .5
%, Ni: 0.05-0.5%, Cr: 0.05
~0.5%, and Mo: 0.03~0.5%.

B. Hot rolling conditions When hot rolling, rolling may be carried out after heating in a normal slab heating furnace, or the blooming rolled material may be directly rolled, and there is no particular restriction on the rolling start temperature. When rolling is performed under conditions where the final finishing temperature is lower than the Ar3 transformation point,
This rolling includes rolling in the ferrite region, and a structure in which pro-eutectoid ferrite has been processed is created, and even with such a processed structure, even if recovery treatment is performed on it, the yield point will not be lowered. Since this results in significant deterioration of workability, the final finishing temperature in hot rolling was set at the Ar3 transformation point or higher.

Note that preferably low-temperature hot rolling, low-temperature rough rolling, and Ar
Controlled rolling such as strong reduction directly above the J-state blind spot is performed to promote ferrite transformation and refine the composite structure,
It is desirable to improve ductility.

C0 cooling condition When cooling from a temperature above the Ar3 transformation point at a cooling rate of 20°C/SeC or less, ferrite-pearlite transformation occurs, resulting in the formation of a low-temperature transformation consisting of either or both of the desired martensite phase and bainite phase. On the other hand, from a metallurgical point of view, the faster the cooling rate, the easier it is to obtain a harder low-temperature transformed phase. Since the cooling rate changes depending on the thickness of the steel plate, the upper limit of the cooling rate is not particularly limited.

Furthermore, even if cooling is completed at a high temperature exceeding 300°C and wound into a coil, pearlite appears and the above-mentioned composite structure cannot be obtained.On the other hand, if cooling is performed at a low temperature below 150°C, Solute carbon is difficult to precipitate as carbide, and a considerable amount of solute carbon remains, making it impossible to avoid strain aging. Therefore, a composite structure with high ductility and low yield ratio is created. In order to obtain high-strength hot-rolled steel sheets, it is necessary to
300-150℃ at a cooling rate exceeding 08C/5ecf
It is necessary to rapidly cool the material to a temperature within the temperature range of 1, and then wind it into a coil.

Next, the method of the present invention will be explained by way of examples.

In each of the examples, steel having the composition shown in Table 1 was melted in a converter, then made into a slab by continuous casting, and then the slab was subjected to hot rolling conditions and cooling conditions also shown in Table 1. Invention steel plates 1 to 5 and comparison steel plate 1 are obtained by rolling.
, 2 were manufactured, respectively.

Note that Comparative Steel Sheet 1 does not contain Ti, and Comparative Steel Sheet 2 has a coiling temperature lower than the range of the present invention.

Next, the resulting steel sheets 1 to 5 of the present invention and comparative steel sheets 1 and 2 were subjected to a tensile test after skin pass, pickling, and shearing, and the test results are shown in Table 1.

As shown in Table 1, comparative steel sheet 1, which does not contain Ti, has a high solute nitrogen content, and comparative steel sheet 2, whose coiling temperature is outside the range of the present invention, has a high solute carbon content, so strain aging is difficult. It has a) high yield ratio, low ductility, and (a) high yield ratio.

On the other hand, steel plates 1 to 6 of the present invention all have high ductility and low yield ratio, that is, excellent cold workability, and have an excellent strength-ductility balance.

As described above, according to the present invention, a composite structure type high tensile strength hot rolled steel sheet having high ductility and low yield ratio can be manufactured efficiently and at low cost.

Claims (1)

[Claims] IC: 0.01 to 0.15 ratio, Si: 1.5 ratio or less, M
n: 0.5-3.0%, Sot, A7: 0.1
Conditions in which the final finishing temperature of a steel containing 0.01 to 0.04% Ti and the remainder consisting of Fe and unavoidable impurities (weight %) is the final finishing temperature equal to or higher than the Ar3 transformation point. Immediately after hot rolling at 20°
A method for producing a composite structure type high-strength hot-rolled steel sheet having high ductility and a low yield ratio, characterized in that it is rapidly cooled to a temperature within a temperature range of 300 to 150° C. at a cooling rate exceeding Vsec, and then wound into a coil. 2C: 0.01 to 0.15%, Si: 1.5% or less. Mn: 0.5-3.0%, Sol, At: 0
．． 10% or less, Ti: 0.01 to 0.04%, Ca: 0.002 to 0.01%, and rare earth elements: 0.002 to 10%. Contains one or more of these, and the rest is Fe.
After hot rolling a steel having a composition (weight ratio) consisting of unavoidable impurities under conditions such that the final finishing temperature is equal to or higher than the Ar3 transformation point, immediately cool the steel at a cooling rate of over 20°C/Sec to 300°C or more. A method for producing a composite structure type high-strength hot-rolled steel sheet having high ductility and a low yield ratio, which comprises rapidly cooling to a temperature within a temperature range of 150° C. and winding it into a coil. 3C: 0.01 to 0.15 ratio, Si: 1.5 ratio or less, M
n: 0.5-3.0%, 5olJLt: 0.10
% or less, Ti: 0.01 to 0.04%,
Further, Cu: 0.05 to 0.51%, Ni: 0.
05-0.5%, Cr: 0.05-0.5%, and Mo: 0. O: A steel containing one or more of the strength improving element group consisting of 3-0.5%, with the remainder consisting of Fe and unavoidable impurities, with a composition of C or higher (by weight), at a final finishing temperature of After being hot-rolled at a temperature above the Ar temperature point, it is immediately quenched at a cooling rate exceeding 20°C/SeC to a temperature within the temperature range of 300 to 150°C, and then wound into a coil. A method for producing a composite structure type high tensile strength hot rolled steel sheet having high ductility and low yield ratio. 4C: 0.01 to 0.15 ratio, Si: 1.5 ratio or less, M
n: 0.5-3.0%, Sot, At: 0.1
0% or less, Ti: 0.01-0.04%, and further consisting of Ca: 0.002-0.01% and rare earth elements: 0.002-0.10%. One or more of the above and Cu: 0.05~
0.5%, Ni: 0.05-0.5 ratio, cr: 0
．． 05 to 0.5%, and one or more of the strength improving element group consisting of Mo 20 and 03 to 0.5, with the remainder consisting of Fe and inevitable impurities (
After hot-rolling the steel with a final finish temperature of Ar and a temperature of 3 transformation points or higher, immediately cool the steel at a cooling rate of over 20°C/sec to 300-150°C.
A method for producing a composite structure type high-tensile hot-rolled steel sheet having high ductility and a low yield ratio, the method comprising rapidly cooling the steel sheet to a temperature within the temperature range of and winding it into a coil.