Hot-rolled steel sheet

Abstract

A hot-rolled steel sheet that contains prescribed components, that satisfies formula (1) with regard to added amount of Cr and Al, and that has a metallographic structure that has a volume fraction of ferrite of more than 90% but 98% or less, a volume fraction of martensite of 2% or more but less than 10%, and a volume fraction of less than 1% of a remainder structure that comprises one or more substance from among pearlite, bainite, and retained austenite, the ferrite having an average circle-equivalent diameter of 4 [mu]m or more and a maximum circle-equivalent diameter of 30 [mu]m or less, and the martensite having an average circle-equivalent diameter of 10 [mu]m or less and a maximum circle-equivalent diameter of 20 [mu]m or less. Formula (1): [Cr] x 5 + [Al] >= 0.50. In formula (1), [Cr] is Cr content (mass%), and [Al] is Al content (mass%).

Description

technical field [0001] The present invention relates to hot-rolled steel sheets. In particular, the present invention relates to a high-strength hot-rolled steel sheet suitable for running parts of automobiles and the like, having excellent shape freezing properties, hole expandability, and fatigue resistance. [0002] this application claims priority based on Japanese Patent Application No. 2014-188845 for which it applied to Japan on September 17, 2014, and uses the content here. Background technique [0003] In order to suppress the emission of carbon dioxide gas from automobiles, weight reduction of automobile bodies by using high-strength steel sheets has been promoted. Such high-strength requirements also involve structural members and running members accounting for about 20% of the weight of the vehicle body. High-strength hot-rolled steel sheets are also being used for these members. [0004] However, increasing the strength of steel sheets generally degrades mate...

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Problems solved by technology

[0010] However, in the above method, there is a problem that as the temperature of the slab after rough rolling rises, the temperature after finish rolling also rises, resulting in coarsening of the grain size and deteriorating properties such as strength, toughness, and fatigue properties.

However, since the former increases the yield strength, there is a possibility that the shape freezing property during press molding may be deteriorated.

The latter is difficult to temper in the existing hot rolling process, and additional special devices such as heating devices are required. Therefore, the latter has low feasibility and is not preferable from the viewpoint of production efficiency and production cost.

In addition, even if the installation of special devices such as heating devices can be realized, in the latter case, the fatigue characteristics may be deteriorated by softening of the hard phase

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