"decrease steadily" is correct and usable in written English.
You can use this phrase when you need to describe a decrease that happens gradually over a period of time. For example, "The number of viewers for the television show decreased steadily over the last year."
As countries improve public-health standards and increase their socioeconomic level, the illness and mortality from this disease decrease steadily.
The U.N. assumes this rate will decrease steadily from five to three children per woman, which is considered the medium.
Under equilibrium conditions — which is to say, when the climate is stable — the very warmest temperatures in a borehole will be found at the bottom and they will decrease steadily as you go higher.
Unemployment in the US is forecast to fall from its current 7.5% level and its budget deficit to decrease steadily while the UK unemployment rate is predicted to remain stuck on 8% and its deficit to fall only slowly.
The stability of the generation can then be quantified in terms of the skewness of the power marginal, (Fig. 3c), which here is seen to decrease steadily towards zero with increasing pump power (also see Supplementary Fig. 5).
The temperature started to decrease steadily at 20 30 – when the actual cooling started – until it reached 4 °C at 01 30 in the next day when it rose again.
While for PC treatment, the mineralization percentages decreased steadily.
Because the Westchester County Department of Social Services budget has decreased steadily over the last several years.
The number of Spads has decreased steadily since the Paddington rail crash in 1999.
It fell to just over $4bn£2.65bnbn) in 2011 and is decreasing steadily.
The peak output of the heart decreases steadily from the age of thirty.
