JPH06122923A - Production of high ductility hot rolled steel sheet excellent in longitudinal cracking resistance - Google Patents

Abstract

PURPOSE:To stably produce a hot rolled steel sheet having excellent ductility equal to that of the conventional extra-low C series IF steel and excellent low temp. toughness (longitudinal cracking resistance) equal to that of low C-Al killed steel. CONSTITUTION:At the time of executing hot rolling to steel contg. 0.001 to 0.005% C, 0.1 to 0.6% Mn, <=0.01% P, <=0.015% S and <=0.0060% N and furthermore contg. 0.02 to 0.08% Ti or 0.02 to 0.08% Ti and 0.01 to 0.03% Nb as well as satisfying the following relational formula and inequality of In [(48/93) Nb+Ti*].[P]<=-8.50 and ((48/93)Nb+Ti*)/C: 4 to 20; where Ti*= Ti-(TiasTiS+TiasTiN), and the balance inevitable impurities, the hot rolling is started from a high temp. at which the precipitation of carbides is not recognized as well as finish rolling is executed at >=850 deg.C, and after that, it is coiled at 600 to 710 deg.C. This steel sheet is suitable as the hot rolled steel sheet for parts relating to the automobile.

Description

Detailed Description of the Invention

[0001]

The present invention relates to the production of hot rolled steel sheet,
The present invention relates to a method for producing a hot-rolled steel sheet having excellent press formability, which is likely to be used in the production of suspension parts for automobiles in the future.

[0002]

2. Description of the Related Art Recently, a demand for improving fuel efficiency of automobiles has been rapidly increased from the viewpoint of saving natural energy and preventing environmental pollution, and the number of parts is reduced for the purpose of reducing the weight of a vehicle body. Switching to thin high-strength steel sheets is under consideration. However, the reduction of the number of parts brings about the complexity of the shape of individual parts.
For suspension parts that have not been studied so far, there is a high demand for improvement in press formability of hot-rolled steel sheets.

An extremely low CIF steel sheet having a C content of 0.0080% or less (IF steel sheet: a steel sheet in which the total amount of C in the steel is fixed by Nb and Ti) exhibits excellent elongation and stretch flangeability, and therefore, this type Has been studied and is currently in practical use in the lower arms of some automobiles.

However, from the viewpoint of imparting press formability, this steel sheet is usually coiled at a relatively high temperature of 650 ° C. or higher, and C in the steel is fixed as Ti or Nb precipitates. If the strength of the grain boundary is significantly reduced and the steel sheet is subjected to severe press forming, or if the part is used in cold regions, brittle fracture (called "vertical crack") may occur, which is a problem. . On the other hand, when the coil is wound at a relatively low temperature of 500 ° C. or less, most of C in the steel remains in the ferrite grains and in the grain boundaries in a non-precipitated state, so that the grain boundary strength becomes high. Although it can be prevented, it deteriorates the press formability, which is not preferable.

The press formability of the steel sheet can be maintained to some extent high even if some unprecipitated C (solid solution C) remains. Therefore, there is a report that a predetermined amount of C that predicts the amount of C fixed to Nb, Ti, etc. at the product stage and the surplus C amount is made at the stage of steelmaking, but it is not possible to use C, N, Nb, Ti, etc. In reality, it is extremely difficult to consistently manufacture the quality of the final product due to the variation in the yield amount.

Further, due to the recent development of steelmaking degassing technology, it has become possible to manufacture steel containing an extremely small amount of C of 0.0010% or less which does not significantly deteriorate ductility, but this method currently raises costs. Is too big. Furthermore,
Since this method does not require addition of Ti, Nb, etc., the crystal grains become remarkably coarse in the production of a thin steel sheet, defects such as rough skin occur in press forming, and the weld structure also becomes coarse and the toughness, Practical problems such as reduced fatigue strength occur.

As described above, the conventional manufacturing technique cannot stably manufacture a hot-rolled steel sheet having both high ductility and excellent longitudinal crack resistance.

Under these circumstances, the present invention provides excellent ductility (elongation) comparable to that of conventional ultra-low C type IF steel and excellent low temperature toughness (longitudinal cracking resistance comparable to that of low C-Al killed steel). It is an object of the present invention to provide a technique for stably producing a hot-rolled steel sheet having).

[0009]

In order to solve the above-mentioned problems, the present inventor has conducted various studies on the precipitation behavior of carbides in conventional ultra-low C type IF hot-rolled steel sheets. As a result, conventional IF steels usually contain 0.01% or more of P, but based on the finding that precipitation of Ti carbide is significantly delayed when the amount of P is greatly reduced, IF hot rolling for automobiles is performed. It was found that high ductility and excellent vertical cracking resistance can be realized by specifying the appropriate chemical composition and hot rolling conditions for the steel sheet,
The present invention is completed here.

That is, according to the present invention, C: 0.001 to 0.
005%, Mn: 0.1 to 0.6%, P ≦ 0.01%, S ≦
0.015%, N ≦ 0.0060%, and Ti:
0.02 to 0.08% or Ti: 0.02 to 0.08% and Nb: 0.01 to 0.03%, and the following relational expression: ln [(48/93) Nb + Ti *] [P] ≦ −8.50, ((48/93) Nb + Ti *) / C: 4 to 20, where Ti * = Ti− (TiasTiS + TiasTiN) is satisfied, and the balance is steel of inevitable impurities. During hot rolling, hot rolling is started from a high temperature at which no precipitation of carbide is observed, and finish rolling is performed at 850 ° C. or higher.
The gist is a method for producing a high-ductility hot-rolled steel sheet having excellent longitudinal crack resistance, which is characterized by winding a coil at 0 ° C.

The present invention will be described in more detail below.

[0012]

[Action]

The technical point of the present invention is that the amount of P is controlled to be relatively low even when coiled at a high temperature by Ti, which is capable of sufficiently fixing C in the steel stoichiometrically, or steel containing a combination of Ti and Nb. It was found that the precipitation of Ti or Nb carbides was suppressed by doing so, and this was linked to the impartation of high ductility and resistance to longitudinal cracking of the IF type hot rolled steel sheet.

First, the results of the experiment on which the present invention is based will be described.

The test materials were those in which the P content was changed with the chemical components shown in Table 1, and were dissolved in a laboratory to obtain an extremely low C-T.
i-type steel. After forging and rough rolling, a small sample for thermomechanical treatment was produced. In the thermo-mechanical treatment, solution treatment was performed at 1200 ° C, 30% processing was applied at 930 ° C, and after 10 seconds, 65% was applied.
Cooling and coil winding after finish rolling of the hot-rolled sheet were simulated by quenching to 0 ° C. and holding at the same temperature for 2000 seconds. The amount of solid solution C in steel, that is, the amount of C not precipitated as carbides was investigated as the strain aging amount by changing the holding time at the coiling temperature (650 ° C.).

[0016]

[Table 1]

The results of the investigation are shown in FIG. After being kept at 650 ° C for 1 second, regardless of the P content of the steel, both were about 3.7 kgf /
The strain aging amount in mm ² (difference in yield point before and after aging heat treatment at 170 ° C. × 10 minutes) is shown, and the strain aging amount varies as the holding time increases.

That is, steel A having a P content of 0.010% or more
In B and B, the strain aging amount is greatly reduced by holding for 300 seconds or more, whereas in the case of steel C in which the P amount is 0.002%.
Then, the strain aging amount does not decrease so much even if it is held for 2000 seconds. In this experiment, it is considered that all N in the steel is fixed by Ti, that is, precipitated as TiN,
The change in the strain aging amount is considered to be the change in the amount of solute C in the steel. In other words, a decrease in the strain aging amount means an increase in the TiC precipitation amount.

Conventionally, when a steel added with a Ti content that stoichiometrically sufficiently fixes C in steel is held at 650 ° C. for a long time of 1000 seconds or more, a strain aging amount of ² kgf / mm ² or more remains. There is no report example. However, stoichiometrically I
When a large amount of P (for example, 0.05% or more) is added to steel that becomes F to form a hot-rolled steel sheet, when the coil is wound at a temperature of 600 ° C or higher, Fe / P compound is formed in the ferrite grain boundaries and in the grains. There is a report that a large amount is deposited. Paradoxically, this is P
When the amount is small, the precipitation of Fe / P compound can be suppressed even when coiled at a high temperature of 600 ° C. or higher. TiC
If Fe / P compound in which carbides such as the above have been precipitated as a precipitation site, TiC precipitation is promoted when the P content is high in the above steel, while TiC precipitation is promoted when the P content is low. It can be said that the results of this experiment are not particularly novel because TiC precipitation is suppressed due to the reduction of sites.

However, in reality, the precipitation behavior of Fe / P precipitates in the range of a trace amount of P as in the present experiment, Fe / P
In addition to the speed of precipitation between precipitation and TiC precipitation, Fe ・
How does the P precipitate become a TiC precipitation site, and how does TiN, MnS, etc., which may exist during the hot rolling process, work?
There are many unclear points, and therefore, the above-mentioned experimental results that are the basis of the present invention cannot be denied novelty by existing reports or known facts.

Next, it will be explained what the experimental result of increasing the strain aging amount by reducing P means for imparting longitudinal crack resistance to the steel sheet for automobiles.

Generally, the vertical crack resistance of a steel sheet is determined by the solid solution C in the steel.
It is greatly affected by the amount, and it is said that it is effective to leave solid solution C of several ppm or more in the steel for imparting vertical cracking resistance. Actually, it is extremely difficult to measure a minute amount of the solute C amount, and therefore, as a simple evaluation method, the amount of the solute C amount is judged by the magnitude of the strain aging amount (ΔYP).

FIG. 2 shows the relationship between the ductility-brittleness transition temperature (longitudinal cracking transition temperature) and the strain aging amount of a deep drawn product of a thin steel sheet. When the strain aging amount becomes small, the vertical crack transition temperature rises, and it becomes brittle and easily breaks. It is generally understood that it is necessary to use a steel sheet having a strain aging amount of ² kgf / mm ² or more in order to secure a transition temperature comparable to that of a low C-Al killed steel in which longitudinal cracking defects have not been reported.

Considering the above facts, when considering the above-mentioned investigation results regarding the P content in terms of resistance to longitudinal cracking, steel with a P content of 0.01% or more is still observed at 650 ° C. for 2000 seconds. Since it has a slight amount of strain aging, it can be seen that a small amount of solid solution C remains in the steel, but the amount of strain aging is about 1 kgf / mm ^2, which is to ensure the vertical cracking resistance of the steel sheet. Is not enough for. On the other hand, 0.002% P steel has a strain aging of about 3 kgf / mm ² at the same time, and it can be said that it exhibits excellent vertical cracking resistance.

As described above, it is understood that the results of this experiment have great significance in the production of automobile steel sheets used for press forming.

Regarding the influence of precipitates other than carbides on the decrease in the amount of solid solution C in steel, a clear idea has not been established as described above, but according to the subsequent experiments,
In the coil winding temperature range of 710 ° C., there is a general correlation with the value of the formula represented by the amount of P and the carbide forming elements such as Nb and Ti. FIG. 3 shows the relationship. In order to secure a transition temperature equivalent to that of a low C-Al killed steel, that is, a strain aging amount of ² kgf / mm ² or more, a value calculated as an effective Nb amount and an effective Ti amount that can be combined with C and a P amount are required. Product, [(48/93) Nb
It is necessary to set the natural logarithm of + Ti *) · P] to −8.50 or less.

As described above, the characteristics of steel change greatly depending on the amount of P even in the region of a small amount of P, which was conventionally regarded as an impurity. Moreover, considering that the reduction of P can be relatively easily reduced to about 0.005% P in the current steelmaking technology, the application of the present idea to the production of hot-rolled steel sheet is highly feasible.

Next, ensuring high ductility will be described.

As described above, the steel C having reduced P (Table 1) can stoichiometrically fix all the C in the steel with Ti, but after hot rolling at a relatively high temperature. Since some solid solution C remains even after coiling, the ductility (elongation) of steel
Seems to deteriorate. Using a 20 mm thick rough rolling machine for steel C, hot rolling described below was carried out to investigate the elongation of the hot rolled steel sheet.

That is, after slab heating was performed at 1200 ° C., hot rolling was performed and finish rolling was performed at 900 ° C. 3.
After shower cooling the mm thick steel plate at an average cooling rate of 40 ° C / s, it is inserted into a furnace set to a temperature of 500 to 700 ° C for 3
It was held for 0 minutes and then cooled in the furnace. This experiment is carried out in a laboratory, and the temperature of 500 to 700 ° C. assumes the coil winding temperature, and the subsequent furnace cooling simulates the coil cooling.

The steel sheet cooled to room temperature was ground and subjected to a tensile test to examine its elongation. The survey results are shown in FIG.
The figure also shows the results of a survey on steel B, which is a normal IF steel component, and steel D, which does not become IF IF stoichiometrically. From the figure, it is understood that Steel C has excellent elongation comparable to that of conventional IF steel by coiling at 600 ° C. or higher. Steel D has a lower elongation than those two steel types. It is not clear why Steel C exhibits relatively excellent elongation, but precipitates coarsen due to high temperature winding, the amount of residual solid solution C is not as large as that of Steel D, and low P. It is considered that, etc. do not significantly deteriorate the ductility of steel.

The present invention was made based on the above experimental results, and the reasons for limiting each condition are shown below.

C: Solid solution C needs to remain in order to improve the vertical cracking resistance of the steel sheet. C addition amount is 0.005
%, It is necessary to add a large amount of Ti and Nb, which deteriorates the paintability and the adhesion of the plating when this steel sheet is plated with Zn, and there is a large variation in the amount remaining as solid solution C in the end. In particular, this is not preferable because the defect in ductility becomes serious. On the other hand, reducing the C content to less than 0.001% is not preferable because the load at the steelmaking stage becomes large and the manufacturing cost increases. Therefore, the amount of C is 0.001-
The range is 0.005%.

Mn: Mn is preferably added in an amount of 0.1% or more from the viewpoint of preventing hot embrittlement of steel. However, addition of a large amount exceeding 0.6% causes a decrease in the ductility of the steel, so 0.6%
Is the upper limit.

For the above reason, the smaller the P: P amount, the better. Although depending on the amounts of Ti and Nb, if the P amount is 0.01% or less, the effect of the present invention can be expected.

Ti, Nb: Addition of Ti or Ti and Nb is indispensable in terms of press formability of steel, welded portion characteristics, etc., as described above. In order to effectively bring out the effect of the present invention by lowering the P content, a Ti content of 0.02 to 0.08% and 0.0
A Nb content of 1-0.03% is preferred. If the amount is too small, it becomes impossible to control the carbide precipitation behavior, and if the amount is too large, the production cost rises, and the adverse effects described for the reason for limiting the amount of C occur.

Further, in the present invention, the amount of effective Ti capable of binding to C of steel (the amount of Ti excluding the amount of Ti capable of binding to N and S) is
(This is indicated by "Ti *") and the amount of Nb and the amount of P have the following relational expression, ln [(48/93) Nb + Ti *]. [P] ≤-8.50. Need to be satisfied. Here, Ti * = Ti- (T
iasTiS + TiasTiN).

The purpose of this limitation is to secure vertical cracking resistance as already shown in FIG. The lower limit of the value of this relational expression is not particularly specified, but is practically about -10.5.

Further, it is necessary to satisfy the following relational expression ((48/93) Nb + Ti *) / C: 4 to 20 between the effective Ti amount and the Nb amount and the C amount. If the value of this relational expression is smaller than 4, excellent ductility cannot be obtained even when the coil is wound at a high temperature as shown in FIG. On the other hand, if the value exceeds 20, the amount of precipitates in the steel increases and the ductility rather deteriorates.

S, N: S and N easily bind to Ti, and if they are present in a large amount, it is necessary to increase the amount of Ti added to secure the effective Ti amount. In this case, the ductility of the steel deteriorates due to the increase of non-metallic inclusions in the steel, and the manufacturing cost increases, which is not preferable. Therefore, S ≦ 0.015% and N ≦ 0.0060% are specified. The lower limits of the amounts of S and N are not specified, but considering practical manufacturing costs, S ≈ 0.0
01% and N≈0.0015%.

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

In the hot rolling of the above steel, the rolling is started at a temperature at which precipitation of TiC is not substantially recognized. In addition, a desirable upper limit of the rolling start temperature is 1200 ° C. due to restrictions on the furnace equipment or the transportation of high temperature slabs continuously cast, and a desirable lower limit depends on the chemical composition of the steel and the processing conditions of the slab, but it is substantially T.
The temperature is about 1050 ° C at which iC precipitation is not observed.

The finish rolling is carried out at 850 ° C. or higher, which is considered to be mostly in the austenite range, in order to secure dimensional accuracy.

The coil winding temperature is 600 ° C. or higher from the viewpoint of maintaining high ductility. However, if the coiling temperature is too high, the pickling property is significantly deteriorated, and coiling at a temperature higher than 710 ° C. should be avoided.

Although the present invention relates to a hot rolled steel sheet, high ductility and excellent vertical cracking resistance are not impaired even if the hot rolling steel sheet is used as a raw sheet for electroplating and hot dip coating.

Next, examples of the present invention will be described.

[0047]

【Example】

Steels having different chemical compositions shown in Table 2 were melted in a laboratory to prepare a 20 mm thick hot rolling slab. Hot rolling was performed at a slab heating temperature of 1150 ° C., and rolling was performed at a finishing temperature of 880 ° C. to a thickness of 3 mm. After that, shower cooling was performed at an average cooling rate of 30 ° C./s, and coil winding processing was performed at 670 ° C. The front and back surfaces of the rolled plate cooled to normal temperature by furnace cooling are ground, and a JIS No. 5 tensile test piece with a thickness of 2 mm is processed.
It was submitted to the test. The amount of strain aging was also investigated. The results are shown in Table 3.

In Table 3, steels manufactured under the conditions of the present invention
(Inventive steel) has a strain aging amount of ² kgf / mm ² or more and exhibits excellent elongation.

On the other hand, Comparative Steel No. 2 has a strain aging amount of less than ² kgf / mm ² because the balance between the amounts of P and Ti is outside the range of the conditions of the present invention. Further, the comparative steels No. 4 and No. 5 have deteriorated elongation because the balance between C and Ti or C, Ti and Nb is not stoichiometrically IF. Comparative steel No. 6 has a balance of Ti and C content and Ti and P.
The amount balance satisfies the conditions of the present invention, but the amount of P is too large, so the strain aging amount of the steel sheet is reduced, and there is a risk of vertical cracking.

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

As described in detail above, according to the present invention,
Excellent ductility (elongation) comparable to conventional ultra-low C IF steel and low C-
A hot rolled steel sheet having excellent low temperature toughness (longitudinal cracking resistance) comparable to that of Al killed steel can be stably produced, and is particularly suitable for suspension parts for automobiles.

[Brief description of drawings]

[Fig.1] Holding time at winding temperature (650 ℃) and strain aging amount
It is a figure which shows the relationship of (amount of solid solution C in steel: amount of C which has not precipitated as carbide).

FIG. 2 is a diagram showing a relationship between a transition temperature of vertical cracking of a deep-formed thin steel sheet and utilization of strain aging.

FIG. 3 is a diagram showing the influence of the amounts of Nb, Ti and P of steel on the strain aging amount.

FIG. 4 is a diagram showing the effect of coil winding temperature on the elongation of steel.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Goro Anan 1 Kanazawa-cho, Kakogawa-shi, Hyogo Kobe Steel Works Kakogawa Works

Claims (1)

[Claims] 1. In weight% (hereinafter, the same), C: 0.001
Up to 0.005%, Mn: 0.1 to 0.6%, P ≦ 0.01
%, S ≦ 0.015%, N ≦ 0.0060%, and Ti: 0.02 to 0.08% or Ti: 0.02 to 0.08%.
% And Nb: 0.01 to 0.03%, and the following relational expression: ln [(48/93) Nb + Ti *] [P] ≦ −8.50, ((48/93) Nb + Ti *) ) / C: 4 to 20, where Ti * = Ti- (TiasTiS + TiasTiN) is satisfied, and the balance is unavoidable impurities during the hot rolling of steel, the hot rolling is started from a high temperature at which no carbide precipitation is observed. Then, after finish rolling at 850 ° C. or higher, 600 to 71
A method for producing a high-ductility hot-rolled steel sheet excellent in longitudinal crack resistance, which comprises winding a coil at 0 ° C.